Zhiyu Jiang

• Shi, Wei; Yan, Chaojun; Jiang, Zhiyu; Soares, C. Guedes; Han, Xu & Li, Xin (2026). Efficient fatigue assessment framework for floating wind turbine mooring systems using Bayesian quadrature adaptive kriging surrogate. Marine Structures. ISSN 0951-8339. 110. doi: 10.1016/j.marstruc.2026.104147. Fulltekst i vitenarkiv Vis sammendrag

  Long-term fatigue assessment of floating wind turbine mooring systems is a critical yet computationally demanding task, owing to the need for high-fidelity simulations across a wide range of stochastic environmental conditions. An efficient fatigue assessment framework for floating wind turbine mooring systems is developed based on a Bayesian quadrature-driven adaptive Kriging surrogate. The framework combines Gaussian process modelling with a probabilistic integration–oriented sampling strategy, aiming to reduce the uncertainty associated with long-term fatigue damage estimation efficiently. Its performance is first examined through one- and two-dimensional analytic examples, where the integration-driven variance reduction mechanism is shown to accelerate convergence compared with the classical Adaptive Kriging Damage Assessment method. The framework is then extended to the fatigue assessment of mooring lines in floating wind turbines. In a three-dimensional environmental space defined by wind speed, significant wave height, and wave period, accurate fatigue damage predictions with substantially fewer simulations than required by the Adaptive Kriging approach. The robustness of the framework is further investigated under different turbulence intensity levels, demonstrating stable performance in the presence of turbulence-induced uncertainty. Finally, an industrial-scale four-dimensional case including wind-wave misalignment is analyzed, confirming that the framework can reduce the number of required simulations by more than two orders of magnitude while maintaining prediction errors below 1.0%. Overall, the proposed Bayesian Quadrature-based framework provides an efficient approach for fatigue assessment of floating wind turbine mooring systems. It can support rapid fatigue evaluation in early-stage design processes and offers practical value for engineering analyses under realistic environmental uncertainties.

• Leventopoulou, Theano; Jiang, Zhiyu & Dai, Jian (2026). Cost estimation of mooring systems for a floating photovoltaic concept, Innovations in Maritime Technology and Engineering. CRC Press. ISSN 9781042001194. doi: 10.1201/9781042001194-141. Fulltekst i vitenarkiv Vis sammendrag

  Floating photovoltaic (FPV) systems are rapidly gaining attention as a viable solution for large scale renewable energy deployment in offshore environments, where mooring systems represent a significant part of the overall installation cost. This study develops a weight-based cost estimation method to evaluate the economic feasibility of alternative mooring designs for FPV applications. This study develops a weight-based cost estimation framework to evaluate the economic feasibility of alternative mooring designs for FPV applications. The proposed methodology estimates total mooring cost as the sum of line material cost, additional component cost, and installation cost, with material cost calculated directly from the total mass of the mooring line. By relating cost to physical properties such as line length, diameter, and linear density, this approach provides a more realistic and transparent representation of manufacturing and material expenses than conventional cost-per-meter methods. Three different mooring configurations are examined: a baseline catenary system, a configuration with clump weight, and an anchorline system. Each configuration is assessed across multiple line diameters to capture variations in material usage and deployment requirements. Previous hydroelastic analysis showed that the clump-weight configuration leads to reduced mooring loads while still satisfying prescribed motion responses. These lower loads enable more efficient use of line material and components, which in turn reduces total system cost. A sensitivity study on the installation coefficient is also carried out to represent different offshore deployment scenarios. Results indicate that line material remains the primary cost driver, with component and installation costs also contributing significantly to total Capital Expenditures (CAPEX). Within the examined diameter range of 90–130 mm, the 110 mm diameter mooring line with clump weights achieves the most cost-efficient balance, offering minimal additional requirements and substantially lower total cost compared to anchor line systems. The findings demonstrate that a weight-based cost framework provides a practical and transparent tool for comparing mooring designs and can guide economical decision-making in future FPV developments.

• Leventopoulou, Theano; Dai, Jian & Jiang, Zhiyu (2026). Influence of mooring configurations on the hydroelastic responses of a floating photovoltaic array. Marine Structures. ISSN 0951-8339. 111. doi: 10.1016/j.marstruc.2026.104155. Fulltekst i vitenarkiv Vis sammendrag

  Floating photovoltaic (FPV) systems have gained increasing attention for offshore renewable energy generation. However, the deployment of these systems in open seas is associated with significant challenges due to wave-induced deformations and structural failures analysis accounts for the fluid–structure interaction and is essential for evaluating FPV systems. This study assesses the influence of mooring configurations on the hydroelastic responses of a prototype offshore FPV array, which consists of six interconnected floats. The three mooring configurations include tendon leg buoys with shared mooring, tendon leg buoys with shared mooring and clump weights, and catenary-shape anchor lines. The numerical study is carried out using a previously developed hybrid finite element and boundary element method. The floats are modeled as rectangular solid plates using Mindlin plate theory, and the modal expansion method is adopted to reduce computational time of the FPV array with 2724 degrees of freedom. We apply Irvine’s equations to analytically model the mooring lines connected to the fairleads, and then linearize the mooring stiffness. The modified stiffness matrices including hydrostatic stiffness, connection stiffness, and mooring stiffness are used in the hydroelastic analysis. A detailed sensitivity study is performed for the FPV array considering various mooring properties and wave periods. Among the three evaluated mooring configurations, the clump weight proved to be the most balanced solution. It offers a moderate increase in vertical mooring stiffness while keeping pretension loads relatively low, namely less than 150 kN and maintaining hydroelastic bending moments and plate deformations across the FPV array at modest levels, with the percentage difference in hydroelastic response remaining under 5%. This can be ascribed as a practical and cost-effective option for improving the structural integrity and motion performance of offshore FPVs.

• Ameer, Shoaib; Re, Carlo Lo; Jiang, Zhiyu & Cappietti, Lorenzo (2026). Comparative analysis of hindcast data against wave buoy measurement for a site in the mediterranean sea. Ships and Offshore Structures. ISSN 1744-5302. doi: 10.1080/17445302.2026.2658742. Fulltekst i vitenarkiv
• Wang, Xinru; Jiang, Zhiyu; Liu, Zihan; Wang, Shuaishuai & Cappietti, Lorenzo (2026). Preliminary design and analysis of a modular floating energy island concept. Ships and Offshore Structures. ISSN 1744-5302. doi: 10.1080/17445302.2026.2659868. Fulltekst i vitenarkiv Vis sammendrag

  With rapid population growth and the shortage of land resources, ocean space utilization has become a critical area of research. While multi-use offshore platforms have been extensively studied, challenges remain in ensuring their operability and survivability under offshore conditions. This work aims to develop an innovative floating island concept designed to facilitate multi-purpose ocean space utilization in Europe. First, hydrodynamic numerical analysis is carried out for a previously proposed single hexagonal floating platform. The numerical results show good agreement with the reference data from literature. Then, a floating renewable energy platform is proposed to host a 10-megawatt (MW) wind turbine and photovoltaic (PV) panels by the Pareto multi-objective optimization method. Key geometric parameters, including the side length (20–50 m), height (5–15 m), and panel thickness (0.5–2 m), are considered as the main optimization variables. The optimization objectives include its energy performance and construction cost, and the criteria include freeboard, initial stability, and the natural periods of heave and pitch motions. The dynamic responses of the designed floating energy island are also evaluated. This concept will be further developed in the future by combining more considerations, such as the configurations of PV arrays and the share of wind and solar power.

• Fu, Yushun; Shi, Wei; Jiang, Zhiyu; Yan, Chaojun; Han, Xu & Li, Xin [Vis alle 7 forfattere av denne artikkelen] (2026). Comparative study on mooring models for large floating offshore wind turbine considering effects of wave kinematics and current dynamics. Ocean Engineering. ISSN 0029-8018. 343(Part 3). doi: 10.1016/j.oceaneng.2025.123349. Fulltekst i vitenarkiv Vis sammendrag

  With the advancement of offshore wind technology, mooring design and modelling of large Floating Offshore Wind Turbines (FOWT) have become crucial for ensuring their stability and cost-effectiveness under complex design conditions. However, the applicability of conventional mooring models to these next-generation, large-scale systems, particularly under harsh sea states where nonlinear effects become prominent, requires systematic investigation. This study presents a comprehensive framework for mooring system design for the IEA 15 MW large semi-submersible wind turbine, employing both a Multi-Segmented Quasi-Static (MSQS) model and a Lumped Mass (LM) dynamic model for in-depth analysis. This work not only quantifies the distinct contributions of wave kinematics and current dynamics on the mooring lines, but also further evaluates the critical interplay between mooring model fidelity and the high-order wave forces on the platform. Based on the full spectrum of IEC design load conditions, this study assessed the performance of the FOWT under normal, severe, and extreme environments. It was found that, while quasi-static models are adequate for platform motion and associated loads under normal conditions, dynamic models offer more precise predictions for platform motion, structural loads, and mooring tensions under severe and extreme conditions. A key finding is that, regardless of the design state, dynamic models consistently outperform quasi-static models in predicting mooring behaviour. Furthermore, our analysis provides an actionable guideline for designers: the effects of wave kinematics on mooring lines can often be neglected in favour of focusing on current effects. Crucially, the study reveals that, when high-order wave forces on the platform are considered, a hydrodynamic-inclusive dynamic mooring model becomes essential for accurate prediction. The selection of an appropriate mooring calculation model based on these findings not only optimizes the trade-off between computational efficiency and accuracy, but also ensures accuracy, thereby providing a more robust scientific foundation for the planning, construction, and operation of offshore wind farms.

• Yan, Chaojun; Shi, Wei; Jiang, Zhiyu; Li, Lin; Han, Xu & Li, Xin (2026). Reliability-based design optimization of a mooring system for a floating wind turbine. Renewable Energy. ISSN 0960-1481. 260. doi: 10.1016/j.renene.2025.125116. Fulltekst i vitenarkiv Vis sammendrag

  Despite the rapid development of the floating wind industry, mooring systems optimization remains challenging and critical for reducing cost. The process involves a large design space, nonlinear constraints, and high computational demands. Moreover, uncertainties in marine environments and construction processes significantly affect system responses, yet their impact on mooring optimization remains unclear. To address these gaps, this paper proposes an integrated framework for accurate and efficient reliability-based design optimization (RBDO) of mooring systems. First, a stepwise constraint screening method was employed to obtain the coupled dynamic response of floating offshore wind turbines (FOWT), including platform motion and mooring tensions. The Kriging surrogate model was utilized to establish correlations between mooring design variables and system output responses. Subsequently, the most sensitive design variables were selected as random variables, and the Kriging-based subset simulation (KSS) method was proposed to estimate the mooring system's reliability index. Compared to conventional Monte Carlo simulation (MCS), the proposed method significantly improves computational efficiency. With the well-trained Kriging model, the gradient-based algorithm was employed to explore the design space and quickly identify optimal mooring solutions. Validation demonstrates excellent consistency between Kriging predictions and the simulation results. Furthermore, a comparison was made between the proposed RBDO framework and deterministic optimization, which considers mooring strength safety using partial safety factors. The cost and mooring strength safety performance of the optimal solutions were analyzed. Results demonstrate that the proposed integrated RBDO framework efficiently yields reliable, cost-effective mooring solutions for FOWTs under extreme conditions.

• Kashyap, Subham; Saha, Nilanjan; Jiang, Zhiyu & Poursina, Mohammad (2026). Dynamic response analysis of an offshore wind turbine under seismic loads and vibration-induced shutdown. Ships and Offshore Structures. ISSN 1744-5302. doi: 10.1080/17445302.2025.2611327. Fulltekst i vitenarkiv
• Tan, Songyuan; Liu, Zhongxiang; Jiang, Zhiyu; Guo, Tong; Sun, Zhen & Jia, Dongdong (2026). Investigation on scour evolution of bridge groups under tidal currents. Ocean Engineering. ISSN 0029-8018. 353. doi: 10.1016/j.oceaneng.2026.124683. Fulltekst i vitenarkiv
• Liu, Zihan; Jiang, Zhiyu & Cappietti, Lorenzo (2026). Hydrodynamics-based geometric optimisation and parametric analysis of a wing-type floating breakwater. Ocean Engineering. ISSN 0029-8018. 351. doi: 10.1016/j.oceaneng.2026.124422. Fulltekst i vitenarkiv
• Song, Ming; Chen, Jiyuan; Zhang, Wenzhe; Calderon-Sanchez, Javier & Jiang, Zhiyu (2025). Crashworthiness study of a monopile OWT with metamaterial-based fenders under vessel collisions. Engineering structures. ISSN 0141-0296. 345(Part B). doi: 10.1016/j.engstruct.2025.121559. Fulltekst i vitenarkiv Vis sammendrag

  The growing risk of vessel collisions with offshore wind turbines (OWTs) poses significant threats to both maritime safety and wind farm economics. To mitigate the consequences upon accidental collisions, this study proposes a metamaterial-based fender concept that effectively protects both OWT structures and impacting vessels. Key variables such as impact velocities and operational wind loads are considered. Special emphasis is placed on the unique characteristics of OWT-vessel collisions, distinguishing them from traditional waterway structure impacts. A case study is carried out for a 22-MW OWT and a 5000 deadweight tonnage vessel with impact velocities ranging from 2 to 6 m/s. Nonlinear finite element simulations are performed for the impact scenarios. Through the simulations, we demonstrate the effectiveness of the metamaterial-based fender design. The structural damage of the OWT is reduced to negligible levels and nacelle accelerations remain within the safety threshold of 6 . For the striking vessel, the proposed fender design decreases the crushing depths of the bow by 14.53%, thereby improving post-impact survivability. Additionally, the influence of the wind speed and its relative angle to the impact direction of the vessel proves to be a critical factor in collision responses. Moreover, varying metamaterial geometric configurations are examined, revealing a nonlinear relationship between cell density, structural stiffness, and overall OWT protection performance. Future research will focus on optimizing fender designs for specific scenarios to balance risk, efficiency, and economic feasibility.

• Yan, Chaojun; Shi, Wei; Jiang, Zhiyu; Han, Xu; Li, Xin & Soares, C. Guedes (2025). A domain-adaptive deep neural network for real-time mooring tension prediction in floating wind turbines. Ocean Engineering. ISSN 0029-8018. 343, Part 2. doi: 10.1016/j.oceaneng.2025.123259. Fulltekst i vitenarkiv Vis sammendrag

  A domain-adaptive deep learning framework that learns sea-state-invariant features under varying operational domains is proposed. This allows real-time prediction of mooring line tension in floating offshore wind turbines, which is essential for health monitoring under uncertain sea states and is an improvement over traditional physics-based approaches, which are limited to generalising across different environmental conditions. Specifically, a Domain-Adaptive Convolutional Neural Network–Long Short-Term Memory–Attention (DACLA) model is developed, integrating convolutional layers for spatial feature extraction, recurrent units for temporal modelling, and an attention mechanism for dynamic feature weighting. To enhance transferability, a gradient reversal layer-based adversarial training strategy is incorporated, aligning feature distributions between source and target domains. The IEA 15 MW wind turbine serves as the case study. Extensive experiments demonstrate that the DACLA model significantly improves prediction accuracy across wave and wind variations compared to conventional architectures. Furthermore, frequency-domain power spectral density analysis is introduced to interpret the cross-domain behaviour, offering insights into spectral fidelity and dynamic consistency. Finally, robustness tests under Gaussian noise and wind-wave misalignment validate the model's resilience to sensor-level disturbances. These findings confirm that the proposed DACLA framework offers a scalable and adaptive solution for real-time mooring tension forecasting in offshore floating wind applications.

• Hugon, Mathieu; Eriksen, Amalie Borlaug; Schlanbusch, Rune & Jiang, Zhiyu (2025). Dynamic Analysis of Anchor Loads for a Floating Offshore Wind Farm. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 3131(1). doi: 10.1088/1742-6596/3131/1/012023. Fulltekst i vitenarkiv Vis sammendrag

  Floating offshore wind turbines are subjected to hydrodynamic and aerodynamic loads, influencing the design of the anchor and mooring systems. A well-designed shared anchor system can reduce the needed number of anchors in a wind farm, potentially lowering costs. To facilitate a better understanding of the anchor loads, this study presents the dynamic simulation results of a prototype wind farm that consists of three 15 MW VolturnUS-S semi-submersible floating wind turbines in the North Sea. In the simulations, the wind turbines are parked, and collinear and misaligned wind and wave directions are considered. Compared with the anchors of a single turbine, the shared anchors in the wind farm experience a reduction in the peak anchor loads, and wind-wave misalignment may significantly increase the anchor loads. This study highlights the importance of considering multiple wind and wave directions and the misalignment in anchor load analysis, contributing to a better understanding of load distributions in floating wind farms aiding shared anchor system design.

• Salahshour, Soheil; Ong, Muk Chen & Jiang, Zhiyu (2025). Operability Analysis of the Vibratory Extraction Process of an Offshore Monopile Foundation Using a Crane Barge. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 148(2). doi: 10.1115/1.4069867. Fulltekst i vitenarkiv Vis sammendrag

  Analysis of the operability under environmental uncertainty is important for on-board decision-making and will impact the safety and economy of complex marine operations. To this end, this study examines the operability of a crane barge during the vibratory extraction of a monopile foundation. A numerical model has been developed to couple the dynamic behavior of the monopile with that of the crane barge during vibratory monopile extraction. The model incorporates the centrifugal force generated by the vibro-hammer and the reduced soil resistance caused by cyclic loading, implemented via the external dynamic link library. This model is used in the present study to conduct operability analysis. The dynamic response of the coupled system is analyzed by investigating the effects of modeling the vibro-hammer mass as either a distributed mass or a lumped mass. An extreme value analysis is conducted to evaluate the operability of the crane barge and establish the allowable sea states for safe operation. The findings reveal that the modeled crane barge exhibits low operability in the examined zone, primarily due to its limited freeboard, large weight of the monopile and vibro-hammer, and the harsh environmental conditions of the considered site.

• Hosamo, Haidar; Hosamo, Mohsen; Dai, Jian & Jiang, Zhiyu (2025). Efficient monitoring and prediction of the motion responses of compliant floating photovoltaic arrays: Case study of a hydrodynamic model test with erroneous sensor data. Results in Engineering (RINENG). 28. doi: 10.1016/j.rineng.2025.107625. Fulltekst i vitenarkiv Vis sammendrag

  Floating photovoltaic (FPV) arrays under irregular waves and harsh offshore conditions face monitoring failures from simultaneous multi-float dropouts and optical switching between neighboring floats. This study introduces a two-layer strategy that combines advanced preprocessing, low-pass filtering, Z-score outlier removal, and cross-correlation correction of switched signals with an optimized Long Short-Term Memory (LSTM) model capable of reconstructing both isolated and multiple simultaneous gaps. Unlike prior reconstruction studies on single-platform marine measurements, this work addresses compliant multibody FPV arrays where irregular waves create complex patterns of sensor faults. Validation on a scaled 216-float array under regular and storm-like irregular seas shows 95–100% fidelity for isolated gaps and superior performance to spline, autoregressive, and persistence baselines in multi-gap conditions. The framework preserves reliability-critical statistics such as upcrossing rates and extremes, reduces intensive simulation and testing time, and improves monitoring robustness for FPV systems under harsh marine environments.

• Ma, Yu; Ong, Muk Chen; Jiang, Zhiyu & Li, Lin (2025). A Relaxation Scheme for Solving Convolutional Forces in Adaptive Time Step ODE Solvers. International Conference on Offshore Mechanics and Arctic Engineering (OMAE) [proceedings]. ISSN 1523-651X. doi: 10.1115/omae2025-156353. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Efficiently solving Ordinary Differential Equations (ODEs) is of importance in simulation-based digital-twin solutions for different marine and offshore industrial applications. Many existing engineering codes in this field adopt a uniform time-step approach in solving ODEs. For local instantaneous variations existing in the solutions, a fine time-step setup is necessary. This leads to redundant computations in global computations. An adaptive-time-step ODE solver offers potential improvement by adjusting time steps dynamically and minimizing the number of total steps required. However, this solution can still encounter inefficiencies, especially when common operations like convolution integrals are repeatedly computed within each major time step for evaluating the next state. To address this challenge, this study proposes a relaxation scheme within adaptive-time-step ODE solvers. The relaxation scheme dynamically smooth over repeated calculations with a below-threshold filtering mechanism on time consuming parts such as convolutions. This enables efficient solving within any major step. Applied to hydrodynamic problems, this method offers reduced computational complexity and improved solving speed. In addition, numerical results demonstrate that this approach maintains good accuracy for numerical emulations to marine dynamic systems. This framework could work as a foundation for further advancements in ODE solving optimization, particularly for applications where different categories of environmental loads are involved. This numerical scheme may also enable efficient time-domain simulations with multiple rigid floaters.

• Ma, Yu; Ong, Muk Chen; Jiang, Zhiyu & Li, Lin (2025). A Relaxation Scheme for Solving Convolutional Forces in Adaptive Time Step ODE Solvers. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. doi: 10.1115/1.4070622. Fulltekst i vitenarkiv Vis sammendrag

  Efficiently solving Ordinary Differential Equations (ODEs) is of importance in simulation-based digital-twin solutions for different marine and offshore industrial applications. Many existing simulation codes in this field adopt a uniform time-step approach in solving ODEs. The complexity of a simulation is influenced by the number of time steps. To minimize the required number of total steps within a simulation, an adaptive-time-step explicit ODE solver can offer potential improvement by adjusting time stepping dynamically. However, this solution can still encounter inefficiencies, especially when operations like convolution integrals are repeatedly computed within each major time step for evaluating the next state. To address this challenge, a relaxation scheme within adaptive-time-step explicit ODE solvers is proposed in this study. The relaxation scheme dynamically smooths over repeated calculations with a below-threshold filtering mechanism on time consuming parts such as convolutions. This enables efficient solving within any major step. A case study with a floating vessel for aqua cultural cultivation is performed to guide the choice of threshold value. The improved performance in calculation speed is demonstrated by comparing to results using uniform-time-step solvers. The proposed relaxation scheme offers reduced computational complexity and improved solving speed. In addition, numerical results demonstrate that this approach maintains good accuracy for numerical simulations of marine dynamic systems.This study serves as a foundation for further advancements in improvement of ODE solving, particularly for applications where different categories of environmental loads are involved. This numerical scheme may also enable efficient time-domain simulations for multiple-floater dynamics.

• Ma, Yu; Jiang, Zhiyu; Ong, Muk Chen & Li, Lin (2025). Surrogate-based global sensitivity analysis of a floating fish cage array with shared mooring. Ocean Engineering. ISSN 0029-8018. 342(3). doi: 10.1016/j.oceaneng.2025.122993. Fulltekst i vitenarkiv Vis sammendrag

  The deployment of large steel-framed cages for salmon farming has yielded successful trial harvests offshore. Following this technologic trend, an innovative shared-mooring solution is proposed in this study to accommodate multiple semi-submersible cages within a single array for cost-effective harvesting. However, directional ocean currents during harvesting seasons can induce excessive horizontal displacements of the fish cages, posing risks to critical infrastructure, such as import power cables or refill pipes. This study introduces an interpretable surrogate-based Sobol global sensitivity analysis (GSA) framework to identify both critical mooring design variables and worst-case current headings. The surrogate GSA framework is built upon a simplified numerical scheme, employing catenary equations to efficiently calculate the mooring forces at 23 mooring lines. Drag forces at both the hull and nettings of each fish cage structure are also accounted for. Using the training data generated from the numerical model, Kriging surrogates are constructed for fish-cage offset prediction. Approximately 95 % accuracy and an 80-fold improvement in efficiency are achieved compared to direct full simulations. The computed Sobol indices identify the current directions that lead to significant displacements across all possible mooring configurations. Further interpretation of the indices reveals that increasing mooring line diameters offers the most effective stiffness gains. These insights inform design recommendations for both array orientation and local reinforcements. The proposed surrogate based Sobol GSA framework thus enables fast and interpretable assessment for design of offshore shared-mooring systems.

• Kashyap, Subham; Saha, Nilanjan & Jiang, Zhiyu (2025). Parametric Scoring Framework for Site Selection of Floating Offshore Wind Farm in India. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 148(3). doi: 10.1115/1.4070555. Fulltekst i vitenarkiv Vis sammendrag

  A comprehensive feasibility study for floating offshore wind projects requires consensus among various stakeholders through iterative dialogs and data exchanges. The present study proposes a framework for evaluating the suitability of floating offshore wind sites, incorporating multicriteria decision-making based on parametric scoring functions. The methodology utilizes the open-source ERA-5 reanalysis hindcast dataset for wind resource assessment in conjunction with GEBCO Gridded Bathymetry data. A weighted multiplicative score, termed as suitability score, is formulated by combining wind speed, water depth, and proximity to ports. The suitability score is quantified using hypothetical weights representing the relative importance of the chosen key parameters. Gaussian and logistic penalty scoring functions are applied for water depth and distance from ports. The framework is further utilized to estimate optimal sites for installing floating offshore wind turbines in India’s exclusive economic zone (EEZ). The study uses the offshore regions of Tamil Nadu and Gujarat as a case study. Results indicate a high suitability in the deeper waters of Gujarat and Tamil Nadu, with the Gujarat EEZ exhibiting favorable conditions due to higher mean windspeeds (⁠m/s) and appropriate depths (⁠m) closer to the chosen ports within the EEZ. The assumed floating wind farms in these zones yield up to 12% higher annual energy production and 10% improved capacity factor compared to bottom-fixed offshore wind farms located in shallower waters. This work contributes to early-phase feasibility evaluation of offshore wind farms.

• Payenda, Mohammad Arif; Wang, Shuaishuai; Prinz, Andreas; Hussiny, Mohammad Ali & Jiang, Zhiyu (2025). Predicting Mooring Tensions of a Floating Wind Turbine Using Motion Sensors and Neural Networks. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 3131. doi: 10.1088/1742-6596/3131/1/012032. Fulltekst i vitenarkiv Vis sammendrag

  In floating wind turbines, mooring systems are vital for station-keeping and operational safety. Ensuring the safe and cost-effective maintenance of these systems is of great importance. In this research, we have investigated the performance of nacelle motion accelerometers in predicting mooring line tensions under various operational conditions using deep learning algorithms. An accelerometer with varying degrees of freedom (DOFs) is assumed to be placed inside the nacelle to measure platform vibrations. To predict dynamic mooring tensions, we propose a hybrid model (CNN-BiLSTM) that combines Convolutional Neural Networks (CNN) and Bidirectional Long Short-Term Memory (BiLSTM) networks, leveraging the strengths of both algorithms to improve prediction accuracy. Our findings show that including the surge motion accelerometer has the greatest effect in predicting mooring tensions, with an R 2 value of 0.9941, compared to an R 2 of 0.9907 for the 6-DOF model without accelerometers, representing a 0.34% increase. However, the sway motion shows a 0.15% increase with an R 2 value of 0.9956, and the heave motion shows a 0.17% increase with an R 2 value of 0.9956, both relative to the surge motion accelerometer. The robustness of the model under varying dynamic conditions suggests its potential utility in other predictive maintenance tasks in the marine energy sector.

• Baniya, Jenish; Dai, Jian; Jiang, Zhiyu & Zhang, Chi (2025). Numerical study of soft-connected modular offshore floating photovoltaic array. Marine Structures. ISSN 0951-8339. 104. doi: 10.1016/j.marstruc.2025.103893. Fulltekst i vitenarkiv Vis sammendrag

  Offshore floating photovoltaics (FPV) has emerged as a viable technology due to abundant space and efficiency. However, deploying FPV in offshore environments is challenging due to harsh environmental conditions. This paper studies a recently developed FPV concept with a lightweight, semi-submersible float with soft connections for offshore conditions to address the technical and economic challenges. The study aims to obtain insight into the dynamic behaviour of the proposed FPV design with a numerical model. The hydrodynamic model is based on the potential solver WADAM. The rigid body motions of floats with the dynamics of flexible structural elements subjected to wave excitations are studied in time-domain software SIMO-RIFLEX. The time domain numerical model is calibrated and validated by using available experimental results. The results from the numerical simulation are found to match the experimental results on the float motions and mooring tensions satisfactorily under regular and irregular wave conditions. A parametric study is carried out to examine the effect of mooring line pretension and stiffness on the motion response of the FPV floats. The findings from the numerical study provide valuable insight for further design optimisation and development of similar systems.

• Leventopoulou, Theano; Dai, Jian & Jiang, Zhiyu (2025). Hydroelastic analysis of interconnected offshore floating photovoltaic floats. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 147(6). doi: 10.1115/1.4068495. Fulltekst i vitenarkiv Vis sammendrag

  Floating photovoltaic (FPV) concepts have recently emerged as a promising solution for sustainable energy generation, gaining increasing market interest. Despite their potential, FPV systems face significant design challenges related to cost-effectiveness and structural integrity. For FPV arrays in offshore conditions, the structural and hydrodynamic performance of interconnected modules under wave action is a critical consideration, yet research in this area remains limited. This numerical study focuses on the frequency-domain hydro-elastic analysis of a novel FPV concept with semi-submersible floats and rope connections. Each float is simplified as a rectangular plate and modeled using the Mindlin plate theory. A hybrid boundary element-finite element method code is modified and verified to account for the connection stiffness between the floats. Subsequently, a case study is conducted for two and three interconnected plates in two orientations, considering realistic material properties for the connections. The analysis examines the bending moments, deformations, and stresses of the plates under various wave periods and headings. Additionally, the effect of connection stiffness on the responses of the floats is evaluated under varying wave periods. The findings indicate that softer connections mitigate adverse effects, and the differences in structural responses remain below 5% for connections with two material properties. While the system exhibits sensitivity to shorter wave periods, the maximum von Mises stress is well below the allowable yield stress. Overall, the hydroelastic response confirms the good structural integrity of the configurations. This study contributes to a fundamental understanding of modular floating systems under wave effects.

• Wang, Yipin; Yao, Tiancheng; Zhao, Yongsheng & Jiang, Zhiyu (2025). Review of tension leg platform floating wind turbines: Concepts, design methods, and future development trends. Ocean Engineering. ISSN 0029-8018. 324. doi: 10.1016/j.oceaneng.2025.120587. Fulltekst i vitenarkiv Vis sammendrag

  Tension leg platform (TLP) floating wind turbines (FWTs) represent a promising frontier in the research and development of the offshore wind industry. The recent completion of the first TLP floating wind farm, Provence Grand Large, marks a significant milestone. Compared to other FWT designs, TLP FWTs have advantages in motion dynamics and structural utilization, leading to improved performance and reduced cost. However, their unique physical characteristics require specialized design considerations and innovative solutions for practical implementation. To facilitate the development of next-generation TLP FWTs, this paper presents a state-of-the-art review focused on various design aspects: development, technical challenges, requirements, innovative concepts, and research methods. An overview is provided, detailing key design parameters of existing TLP FWTs, such as weight, displacement, installed capacity, and the natural period of motion. Additionally, the design challenges of TLP FWTs are summarized. Afterwards, the TLP FWT designs are classified and critically reviewed, highlighting the design features and installation methods of innovative concepts while identifying development trends in the field. Then, numerical and experimental methods for design are discussed, along with current research gaps. Finally, this paper presents four key areas for future research on TLP FWTs.

• Ma, Can; Zhang, Taiyu; Jiang, Zhiyu & Ren, Zhengru (2025). Dynamic analysis of lowering operations during floating offshore wind turbine assembly mating. Renewable Energy. ISSN 0960-1481. 243(2025). doi: 10.1016/j.renene.2025.122528. Fulltekst i vitenarkiv Vis sammendrag

  An integrated method for tower–nacelle–rotor assemblies has been proposed as an innovative approach to the transportation and installation of floating offshore wind turbines. This efficient approach offers potential value to the industry with increasing turbine sizes. During the mating phase at an offshore site, the installation system becomes a complex multibody system that involves a vessel, a wind turbine assembly, a crane, and a floating foundation. While much existing research focuses on the steady-state dynamic analysis of the lifted turbine assembly in fixed positions, the lowering operation of the assembly is an unsteady process with potential risks. To this end, this paper develops a fully coupled multibody model for the lowering scenario, accounting for the effects of environmental loads on the overall dynamic responses and the couplings between the multibodies. The study reveals the occurrence of re-impact phenomena between the foundation and the lifted structure under environmental loads, and the backward motion of the installation vessel during lowering. The selection of lowering speed and time instant for starting the operation both influence the occurrence of re-impact. Numerical simulation results offer valuable insights for heavy payload lowering operations and contribute to further decision making of assembly transportation and installation processes.

• Bru, Jakob; Seland, Tom Stian; Dai, Jian & Jiang, Zhiyu (2025). Life cycle cost analysis of an offshore floating photovoltaic concept in the North Sea. Renewable Energy. ISSN 0960-1481. 249. doi: 10.1016/j.renene.2025.122981. Fulltekst i vitenarkiv Vis sammendrag

  Offshore floating photovoltaic (FPV) technologies offer a promising solution for clean solar energy generation in remote offshore environments. However, the key factors affecting life cycle costs remain poorly understood. This study presents a comprehensive economic assessment of a novel offshore FPV concept featuring lightweight semi-submersible floats connected by soft ropes, which has demonstrated strong resilience in harsh wave conditions. The analysis focuses on two Norwegian offshore sites, Utsira Nord and Sørlige Nordsjø II, as potential deployment locations for a reference 6-megawatt (MW) FPV system. The levelised cost of energy (LCOE) is estimated at 256.7 EUR/MWh for Sørlige Nordsjø II and 287 EUR/MWh for Utsira Nord, with floats accounting for 51% of capital expenditure. Key factors influencing the LCOE include the discount rate, lifespan, energy production, and grid distance. Monte Carlo simulations reveal a 90% probability of LCOE ranging from 203 to 407 EUR/MWh. Locating the FPV system near existing subsea power infrastructure is recommended to improve cost efficiency. This paper provides valuable insights into the techno-economic viability of innovative FPV systems for offshore applications, paving the way for optimising deployment strategies and reducing costs in the future.

• Jiang, Zhiyu (2025). Mooring design for floating wind turbines: A review. Renewable and Sustainable Energy Reviews. ISSN 1364-0321. 212. doi: 10.1016/j.rser.2024.115231. Fulltekst i vitenarkiv Vis sammendrag

  Mooring systems are essential for ensuring the safety and station-keeping of floating offshore structures. Despite advances in mooring technologies over the past two decades, several design challenges remain. The design of mooring systems is a multidisciplinary task that requires expertise in marine hydrodynamics, structural mechanics, materials engineering, and mechanical design. This paper reviews state-of-the-art mooring designs for floating wind turbines, addressing key aspects such as floater-mooring interactions, material and component selection, design methods and guidelines, simulation tools, and innovative solutions. This review critically examines existing mooring designs for three types of floating wind turbines from 20 projects, presents eight mooring materials, and compares three design guidelines. Additionally, three innovative mooring solutions are highlighted at the farm level. Finally, six areas for future research are identified. This review aims to contribute to the development of safe, robust, and cost-effective mooring systems for the floating wind industry.

• Salahshour, Soheil; Jiang, Zhiyu; Ong, Muk Chen & Skaare, Bjørn (2025). Dynamic analysis of crane barge during vibratory extraction of offshore monopile foundation. Ocean Engineering. ISSN 0029-8018. 319. doi: 10.1016/j.oceaneng.2024.120262. Fulltekst i vitenarkiv
• Marino, Enzo; Gkantou, Michaela; Malekjafarian, Abdollah; Bali, Seevani; Baniotopoulos, Charalampos & Beeck, Jeroen van [Vis alle 31 forfattere av denne artikkelen] (2025). Offshore renewable energies: exploring floating modular energy islands—materials, construction technologies, and life cycle assessment. Journal of Ocean Engineering and Marine Energy. ISSN 2198-6444. 11, s. 1157–1182. doi: 10.1007/s40722-025-00403-y. Fulltekst i vitenarkiv Vis sammendrag

  Floating modular energy islands (FMEIs) are modular, interconnected floating structures designed to collectively produce, store, convert, and transport renewable energy. This review aims to establish a foundation for developing innovative approaches to sustainably harness multi-energy sources in offshore environments. It leverages existing technological expertise while exploring new solutions to address specific challenges associated with FMEIs. The review initially presents existing technologies for floating energy structures and assesses their applicability to FMEI. The structural materials that could be utilised for the construction of a floating energy island are subsequently reviewed. Next, the offshore construction technologies suitable for FMEI are reviewed. Finally, studies on the life cycle assessment of hybrid energy systems are examined, highlighting the environmental advantages of integrating multiple renewable energy sources, thereby underscoring the potential of FMEIs.

• Baniya, Jenish; Dai, Jian; Jiang, Zhiyu; Zhang, Chi & Keprate, Arvind (2025). Effect of Mooring Pretension Force of a Soft-Connected Floating Photovoltaic System Under Regular Waves. I Zhao, Xiao Lin; Dai, Jian-Guo; Liu, Siwei & Lim, Soon Heng (Red.), Proceedings of the Fourth World Conference on Floating Solutions: WCFS 2024; 2–4 December; Hong Kong. Springer Nature. ISSN 9789819645688. doi: 10.1007/978-981-96-4569-5_35. Fulltekst i vitenarkiv Vis sammendrag

  The rise in global temperature has accelerated the demand for replacing fossil fuels with sustainable renewable energy. Alongside wind, waves and current energies in the oceans, there has been huge interest in developing large floating photovoltaic (FPV) plants in offshore environments. Applying FPV structures in offshore environments remains challenging due to harsh environmental conditions. This paper numerically studies a recently developed soft-connected FPV concept for harsh offshore conditions. The case of a small array consisting of 6 modules interconnected by soft ropes that were experimentally examined is considered. A numerical model for the time domain analysis is built using the commercial solver SIMO. The numerical model is calibrated and validated according to the available experimental results. Next, a parametric study is conducted to examine the effect of different mooring line pretensions on the global responses of the FPV array under regular waves. The study will provide a better understanding of the effect of mooring pretension on the global FPV design. Moreover, recommendations are presented for further optimisation and development of such FPV systems.

• Hosamo, Haidar; Hosamo, Mohsen; Dai, Jian & Jiang, Zhiyu (2025). Bridging Data Gaps with Machine Learning for Reliable Monitoring of a Floating Solar Array Under Extreme Marine Environments. ISOPE - International Offshore and Polar Engineering Conference. Proceedings. ISSN 1098-6189. Fulltekst i vitenarkiv Vis sammendrag

  An experimental campaign was conducted at Universidad Politécnica de Madrid using a scaled floating photovoltaic (FPV) array subjected to various wave conditions to examine the technical feasibility of the concept. Optical sensors were instrumented to track the motions of PV panels at different locations in an array of 216 interconnected panels. A challenge of sensor data discontinuity in the FPV array is observed due to wave-induced motions under harsh marine environments. To deal with these challenges, machine learning (ML) models, including Long Short-Term Memory (LSTM) networks, Artificial Neural Networks, Extreme Gradient Boosting, and Decision Trees, were implemented to predict and reconstruct missing sensor data collected during these tests. Among these, LSTM networks showed the highest predictive accuracy, achieving a Root Mean Squared Error (RMSE) of 0.001. These results highlight the potential of advanced ML methods in enhancing the resilience, reliability, and operational efficiency of renewable energy infrastructures in dynamic marine environments.

• Saitov, Khamzat & Jiang, Zhiyu (2025). Advancing offshore solar energy generation: Numerical modelling and calibration of a floating solar array against model tests. Solar Energy. ISSN 0038-092X. 288. doi: 10.1016/j.solener.2024.113225. Fulltekst i vitenarkiv Vis sammendrag

  Offshore areas present significant opportunities for sustainable solar energy production; however, most existing floating photovoltaic (FPV) solutions are designed for mild environmental conditions. To overcome the technical and economic challenges posed by harsh offshore environments, an innovative FPV concept, RECsolar, has been proposed. This concept is characterised by lightweight, semi-submersible floats with soft connections. To gain insight into the dynamic behaviour, this paper numerically studies an array with six modules (2-by-3) connected by ropes and moorings. The numerical model is first calibrated against 1:60 scaled model tests under various regular and irregular wave conditions, focusing on the float motion and mooring responses. Subsequently, statistics of the rope responses are analysed and presented. Findings demonstrate a good match of the motion spectra for surge and heave motion under operational and survival wave conditions. Comparing the numerical and experimental models, the maximum difference in heave motion ranges from 0.25% to 8.36%, while the surge difference is between 2.17% and 6.53%. Tension analysis of the connecting ropes identified snap load events, which could be mitigated by increasing the rope stiffness. This study provides valuable insights for the robust design and analysis of next-generation FPV systems.

• Leventopoulou, Theano; Dai, Jian & Jiang, Zhiyu (2024). Hydroelastic analysis of floating photovoltaic floater under regular waves, Proceedings of ASME 2024 43rd International Conference on Ocean, Offshore and Arctic Engineering. Volume 4: Ocean Space Utilization.. The American Society of Mechanical Engineers (ASME). ISSN 9780791887813. s. 1–9. doi: 10.1115/OMAE2024-124497. Fulltekst i vitenarkiv Vis sammendrag

  The need for renewable energy is increasing and limited land space for installations has led to the rise of floating photovoltaic (FPV) systems, a promising and relatively new technology for harnessing solar power on water surfaces. Apart from the design and the construction of the FPVs, the hydroelastic problem posed by water waves on plate-like floating structures has a considerable value in the offshore application. This study focuses on the frequency-domain hydroelastic analysis of a recently proposed FPV concept with rope connection. In numerical modelling, each FPV float is simplified as a rectangular plate and modelled by using Mindlin plate elements considering only the vertical degrees of freedom. After a mesh convergence study, the FPV systems with one float and two interconnected floats are studied. Through parametric studies considering different wave headings, periods, and connector stiffness, we compare the distributions of displacement, and bending moments along the float for each system. This study serves as a reference for understanding the hydroelasticity effect of simple FPV systems under different environmental conditions and mechanical connections.

• Cheynet, Etienne; Li, Lin & Jiang, Zhiyu (2024). Metocean conditions at two Norwegian sites for development of offshore wind farms. Renewable Energy. ISSN 0960-1481. 224. doi: 10.1016/j.renene.2024.120184. Fulltekst i vitenarkiv Vis sammendrag

  This paper examines metocean data from NORA3, a state-of-the-art wind and wave hindcast dataset for Northern Europe. Two offshore Norwegian areas, Utsira Nord (UN) and Sørlige Nordsjø II (SN2), are investigated. Both areas offer significant potential for the offshore wind sector. UN is situated in deep-sea water, suitable for floating offshore wind turbines. In contrast, SN2 lies in intermediate waters and ranks among the North Sea's most promising regions allocated for offshore wind. Data from NORA3, originally on a 3-km resolution grid, are resampled into unstructured grids spanning from 1982 to 2022. This refined dataset offers a climatology time scale with superior spatial and temporal resolution compared to most other hindcast and reanalysis databases. The study examines mean wind speed and direction across seven levels, ranging from 10~m to 750~m above the surface. Analyses of extreme wind and wave conditions have been conducted. Results reveal that UN experiences higher extreme wave heights than SN2 while the extreme wind speeds may be substantially larger at SN2 than UN. Moreover, this study establishes joint distribution models that encompass several parameters, including mean wind speed, significant wave height, wave spectral peak period, and direction difference between wind and waves. Thus, this metocean data is valuable for designing and analyzing floating wind farms over their lifecycles.

• Liang, Guodong; Hanssen, Finn-Christian Wickmann; Merz, Karl Otto & Jiang, Zhiyu (2024). Numerical analysis of a tethered-buoy mooring system for a prototype floating wind farm. Wind Energy. ISSN 1095-4244. 27(5), s. 500–529. doi: 10.1002/we.2898. Fulltekst i vitenarkiv
• Ma, Yu; Li, Lin; Ong, Muk Chen; Jin, Jingzhe & Jiang, Zhiyu (2024). Influence of Environmental Load Directions on the Design of Shared Mooring for an Integrated Floating Fish Cage-Wind Turbine System, Proceedings of the Thirty-fourth (2024) International Ocean and Polar Engineering Conference - ISOPE 2024. International Society of Offshore and Polar Engineers. ISSN 9781880653784. s. 887–896. Fulltekst i vitenarkiv
• Ma, Yu; Li, Lin; Ong, Muk Chen & Jiang, Zhiyu (2024). Numerical modeling of a steel-framed offshore fish cage: Code development and verification. I Soares, Carlos Guedes & Santos, Tiago A. (Red.), Advances in maritime technology and engineering: Volume 2. CRC Press. ISSN 9781032831077. s. 555–563. doi: 10.1201/9781003508779-61. Fulltekst i vitenarkiv
• Zhou, Yiming; Yan, Chaojun; Shi, Wei; Han, Xu; Jiang, Zhiyu & Michailides, Constantine (2024). A Surrogate Optimization Modeling Method for Floating Wind Turbine Mooring Line Design, Proceedings of the Thirty-fourth (2024) International Ocean and Polar Engineering Conference - ISOPE 2024. International Society of Offshore and Polar Engineers. ISSN 9781880653784. Fulltekst i vitenarkiv
• Hu, Weifei; Jiao, Qing; Liu, Hongwei; Wang, Kai; Jiang, Zhiyu & Wu, Jianwei [Vis alle 8 forfattere av denne artikkelen] (2024). A transferable diagnosis method with incipient fault detection for a digital twin of wind turbine. Digital Engineering. doi: 10.1016/j.dte.2024.100001. Fulltekst i vitenarkiv
• He, Yutao; Guo, Jiamin; Zhao, Yu; Zhu, Mingliang & Jiang, Zhiyu (2024). Prestress analysis and geometry optimization for conical cable domes with zero Gaussian curvature. Thin-walled structures. ISSN 0263-8231. 196. doi: 10.1016/j.tws.2023.111555. Fulltekst i vitenarkiv
• Bertozzi, Andrea; Niosi, Francesco; Jiang, Xiaoli & Jiang, Zhiyu (2024). Numerical Calibration of the Mooring System for a Semi-Submersible Floating Wind Turbine Model. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 146(6). doi: 10.1115/1.4065551. Fulltekst i vitenarkiv
• Liang, Guodong; Lopez-Olocco, Tomas; Medina-Manuel, Antonio; Saavedra-Ynocente, Leandro Antonio; Souto-Iglesias, Antonio & Jiang, Zhiyu (2024). Experimental investigation of two shared mooring configurations for a dual-spar floating offshore wind farm in irregular waves. Marine Structures. ISSN 0951-8339. 95. doi: 10.1016/j.marstruc.2024.103579. Fulltekst i vitenarkiv Vis sammendrag

  Shared mooring brings potential cost benefits to a floating wind farm. In such a wind farm, adjacent floaters are connected by shared lines and the total number of mooring lines is reduced. This paper presents an experimental study on a dual-spar floating offshore wind farm. In the first configuration, two spars are connected by a shared line. In the second configuration, a clump weight is added to the shared line. Irregular wave tests are performed for both configurations under operational and extreme wave conditions. The platform motions of one wind turbine and the mooring tension are measured during the tests. The influence of the added clump weight is found to be insignificant in the operational wave conditions. Under extreme waves, the added clump weight results in smaller platform motions in the wave direction and reduced tension oscillations in all mooring lines. With the additional clump weight, the extreme mooring tension in the shared line can decrease by 30% and fewer snap loading events occur, but the extreme tension in single lines can increase by 6%. This study contributes to an improved understanding of shared mooring systems and facilitates the development of model test methods for floating wind farms.

• Tian, Zhongmei; Shi, Wei; Li, Xin; Park, Yonghui; Jiang, Zhiyu & Wu, Ji (2024). Numerical simulations of floating offshore wind turbines with shared mooring under current-only conditions. Renewable Energy. ISSN 0960-1481. 238. doi: 10.1016/j.renene.2024.121918. Fulltekst i vitenarkiv Vis sammendrag

  In recent years, with the continuous development of offshore wind power, reducing construction costs has become one of the key issues. Among them, the mooring system cost accounts for about 20%–30 % of the entire floating offshore wind turbine investment. The concept of shared mooring systems can potentially reduce the cost of floating offshore wind farms (FOWF). In this paper, research is carried out on the dynamic response of an FOWF under representative current conditions of the South China Sea. The comparative analysis focuses on the motion response of a single-spar Floating Offshore Wind Turbine (FOWT)and a dual-spar FOWF. A sensitivity analysis was performed on the length of the shared mooring line. The NREL 5-MW wind turbine with a spar platform is taken as a basis FOWT model for this work. The frequency-domain hydrodynamics is computed based on potential flow theory. The time domain analysis was simulated in a software for marine operations: SIMA(DNV). The viscous effect on the floater is modeled by the drag term in Morison's equation and the hydrodynamic force acting on the mooring line is computed using Morison's equation. Based on the finite element method, the mooring line is defined by a sequence of segments with homogeneous cross-sectional properties. The result shows that with the increase of the current return period, the two platforms move synchronously, and the movement increases almost linearly. Moreover, the motion response of the shared mooring platform is smaller than that of the single mooring platform under the same conditions, especially in the vertical direction. This paper contributes to improved fundamental understanding of shared mooring systems under complex marine environmental loads.

• Liu, Yingzhou; Li, Xin; Shi, Wei; Wang, Wenhua & Jiang, Zhiyu (2024). Vibration control of a monopile offshore wind turbines under recorded seismic waves. Renewable Energy. ISSN 0960-1481. 226. doi: 10.1016/j.renene.2024.120455. Fulltekst i vitenarkiv
• Jiang, Zhiyu; Dai, Jian; Saettone, Simone & Souto-Iglesias, Antonio (2024). Model Tests of a 12 by 18 Floating Solar Array Under Regular Wave Conditions, ASME 2024 43rd International Conference on Ocean, Offshore and Arctic Engineering - Volume 7: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 9780791887851. doi: 10.1115/OMAE2024-125822. Fulltekst i vitenarkiv Vis sammendrag

  Floating solar has appeared as an emerging technology in the global renewable energy portfolio. Although several floating solar concepts have been proposed in the past, most of them are well-suited for deployment in inland or near-shore water bodies with benign environmental conditions. To address the need for sustainable and cost-effective floating solar energy production in offshore areas, a novel modular floating photovoltaic concept suitable for offshore environmental conditions has very recently been proposed in (Jiang et al., 2023). The concept proposes the use of semi-submersible floats for individual modules and crisscrossed rope connections to join neighbouring modules. Based on the conceptual design, this paper presents the experimental investigation of a 12 by 18 array with 216 floats in regular waves under both operational and survival conditions. First, we describe the model test set-up, the physical models of the simplified floats, the environmental conditions, and the sensor instrumentation and measurement. Then, selected time histories of the array motions are analysed. It is observed that the tested large array has good motion performance with limited wave overtopping. Finally, limitations of and lessons learned from the model tests are summarised. This paper paves the way for the next-stage research and development of the floating solar concept for harsh offshore conditions.

• Marino, Enzo; Gkantou, Michaela; Malekjafarian, Abdollah; Bali, Seevani; Baniotopoulos, Charalampos & Beeck, Jeroen van [Vis alle 31 forfattere av denne artikkelen] (2024). Offshore renewable energies: A review towards Floating Modular Energy Islands—Monitoring, Loads, Modelling and Control. Ocean Engineering. ISSN 0029-8018. 313. doi: 10.1016/j.oceaneng.2024.119251. Fulltekst i vitenarkiv
• Payenda, Mohammad Arif; Wang, Shuaishuai; Jiang, Zhiyu & Prinz, Andreas (2024). Prediction of mooring dynamics for a semi-submersible floating wind turbine with recurrent neural network models. Ocean Engineering. ISSN 0029-8018. 313(3). doi: 10.1016/j.oceaneng.2024.119490. Fulltekst i vitenarkiv Vis sammendrag

  Offshore wind technology emerges as a promising solution to tap into wind resources in shallow and deep waters. For floating wind turbines, mooring systems provide station-keeping functionalities. Efficient monitoring of mooring loads is critical for safe and cost-effective operation and maintenance. As direct measurement of the dynamic mooring line tension is expensive, alternative means are needed. To this end, this article focuses on predicting mooring tensions based on accessible motion data of the floating platform and proposes utilizing deep learning algorithms, i.e., recurrent neural networks (RNNs). We selected three RNN algorithms: Gated Recurrent Units (GRU), Long Short-Term Memory (LSTM), and Bidirectional LSTM (BiLSTM). These algorithms were trained with various load cases of a 10-MW floating wind turbine and their efficacy was assessed using unseen data. Among the three algorithms, the BiLSTM algorithm performed best, achieving R2 of more than 0.97 under various load conditions. Further, a statistical assessment of the tension time series demonstrates an excellent agreement between the predicted and the measured mooring line tensions with a percentage difference in the peak tension of less than 0.5%. The outcomes of this paper contribute to real-time prediction and structural health monitoring of floating wind turbines.

• Song, Ming; Jiang, Zhiyu; Liu, Kun; Han, Yue & Liu, Renwei (2023). Dynamic response analysis of a monopile-supported offshore wind turbine under the combined effect of sea ice impact and wind load. Ocean Engineering. ISSN 0029-8018. 286(Part 2). doi: 10.1016/j.oceaneng.2023.115587. Fulltekst i vitenarkiv Vis sammendrag

  The existence of drifting ice is a key challenge for structural design of offshore wind turbines (OWTs) in cold regions. To better understand the structural behavior of OWT under combined sea ice impact and wind load effects, this paper investigates the dynamic response of a monopile-supported OWT using the nonlinear finite element method. The interaction between the OWT and ice impact, wind loads and soil contact are addressed in the developed numerical models in LS-DYNA. The coupling between the main program and the aerodynamic damping model is achieved by a user-defined load subroutine. To calibrate and verify the ice material model and the simulation technique, model test data are used of ice impacts with a nearly vertical monopile foundation. In the case study, numerical simulations of the interaction between a typical 5-MW monopile-supported OWT and an ice sheet are performed under various combined load scenarios. The dynamic response characteristics are presented and the effects of ice drifting speed and mean wind speed are elucidated by statistical methods. Finally, insights into the ice loads are obtained by comparing the simulation results against two international design standards. The present study contributes to an improved understanding of load effects of monopile OWTs in cold regions.

• RIPOLL, ÁLVARO ÚBEDA; Jiang, Zhiyu; Verma, Amrit Shankar & Zhou, Jing (2023). Active damage mitigation of the blade leading edge erosion for a wind turbine during rainfall events. IOP Conference Series: Materials Science and Engineering. ISSN 1757-8981. 1294(012001). doi: 10.1088/1757-899X/1294/1/012001. Fulltekst i vitenarkiv Vis sammendrag

  Leading edge erosion (LEE) of wind turbine blades is a major concern for the wind turbine industry. LEE leads to increased blade surface roughness and incurs significant repair and maintenance costs. To address this issue, the present paper proposes an active LEE mitigation method by operating wind turbines in an erosion-safe mode with reduced rotor speed during precipitation. The proposed method involves first numerically modelling the NREL 5-MW reference wind turbine in openFAST. Then, during a regular operation in the presence of rainfall, the erosion-safe mode is designed, implemented and initiated with a modified blade pitch controller in Matlab-Simulink. To evaluate the effectiveness of this mode, the long-term rainfall distribution of the De Kooy site in the Netherlands is examined. The analysis considers a comparison of power output, erosion lifetime, and levelized cost of energy (LCOE) with and without the proposed safe modes. The study shows that the erosion-safe mode can significantly extend the erosion lifetime at the cost of reduced power generation. The controller settings affect the wind turbine performance, and a reduced rotor speed of 11 revolutions per minute is recommended considering the LCOE (308.64€/MWh). While the proposed mitigation method shows promise, it requires further refinement for real-life implementation in wind turbines.

• Shi, Wei; Zhang, Lixian; Karimirad, Madjid; Michailides, Constantine; Jiang, Zhiyu & Li, Xin (2023). Combined effects of aerodynamic and second-order hydrodynamic loads for floating wind turbines at different water depths. Applied Ocean Research. ISSN 0141-1187. 130. doi: 10.1016/j.apor.2022.103416. Fulltekst i vitenarkiv Vis sammendrag

  In this paper, fully coupled time-domain analysis of three different semisubmersible floating offshore wind turbines (semi-FOWTs) is performed. The focus of the present paper is to investigate the impact of second-order wave hydrodynamic loads combined with aerodynamic loads on the dynamic responses of semi-FOWTs under normal and extreme sea conditions. Three water depths (50, 100, and 200 m) are considered for examining the depth effect. The simulation results show that the second-order hydrodynamic loads play an important role in the motion and structural responses under extreme sea conditions irrespective of the inclusion of the aerodynamic loads in the analysis. Meanwhile, the aerodynamic loads significantly affect the dynamic responses of all semi-FOWTs under normal operational conditions. Among the three semi-FOWTs, the V-shaped semi-FOWT has the largest pitch motion responses under extreme sea conditions. Compared to the normal operation condition, the water depth has a larger impact on the motion and force responses of the three examined platforms under extreme sea condition. The surge and pitch motions decrease with water depth increases. The tower base loads tend to be lower for deep water depth, while the mooring line tensions are opposite. This findings could benefit the offshore wind turbine industries.

• Vanova, Patricia; Sun, Zhen; Odinson, Odin-Eliott & Jiang, Zhiyu (2023). Dynamic response analysis of a model truss bridge considering damage scenarios. Engineering Failure Analysis. ISSN 1350-6307. 151. doi: 10.1016/j.engfailanal.2023.107389. Fulltekst i vitenarkiv Vis sammendrag

  Structural health monitoring plays a key role for maintenance and lifetime extension of bridges under complex loading conditions and damage scenarios. This paper applies vibration-based methods to monitor the structural health of a Warren-type truss bridge. First, acceleration measurements are used to obtain natural frequencies and modal shapes of the bridge by modal identification. Then, a high-fidelity numerical model in a finite element code is calibrated. Damage scenarios with various crack location and severity are considered for a member, and time-domain simulations are carried out to assess the dynamic responses of the bridge under random excitation. An analysis of the acceleration statistics shows that the damaged member displays a more significant increase in accelerations than the intact members, and locations near the supports of the damaged member experience more changes than other locations. This study contributes to a fundamental understanding of bridge dynamics and facilitates development of damage identification methods.

• Jiang, Zhiyu; Verma, Amrit Shankar & Márquez, Fausto Pedro García (2023). Numerical simulations of offshore wind farms considering accidental scenarios. I Márquez, Fausto Pedro García; Papaelias, Mayorkinos & Junior, Valter Luiz Jantara (Red.), Non-Destructive Testing and Condition Monitoring Techniques in Wind Energy. Elsevier. ISSN 9780323996662. s. 317–349. doi: 10.1016/B978-0-323-99666-2.00010-1. Fulltekst i vitenarkiv Vis sammendrag

  The development of offshore wind farms has entered a new era when new floating offshore wind farms are under construction or being planned. Offshore wind farms are subjected to risks of accidents, and accidental load scenarios must be considered for design, installation, operation, and decommissioning. This chapter provides an overview of the accidental load scenarios and introduces the internal and external accidents that may occur to an offshore wind farm. A focus is laid on the numerical simulations of selected scenarios taken from literature including the blade pitch system faults, mooring failures, wind turbine blade impacts, and ship collisions. Then, interesting physical problems, numerical methods, results, and observations are presented. Finally, four future research areas are identified.

• Bhaskaran, Saravanan; Verma, Amrit Shankar; Yuan, Shuai; Jiang, Zhiyu & Halse, Karl Henning (2023). A Code-to-Code Comparison for Dynamic Modeling and Response Analysis of Offshore Wind Turbine Blade Mating Process. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 145(6). doi: 10.1115/1.4056617. Fulltekst i vitenarkiv
• Jiang, Zhiyu & Manuel, Lance (2023). Special Section on Sustainable and Innovative Use of Emerging Ocean Structural Systems. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 145(3). doi: 10.1115/1.4062260. Fulltekst i vitenarkiv
• Liu, Ting; Halse, Karl Henning; Leira, Bernt Johan; Jiang, Zhiyu; Chai, Wei & Brathaug, Hans-P. [Vis alle 7 forfattere av denne artikkelen] (2023). Dynamic response of a SWATH vessel for installing pre-assembled floating wind turbines. Marine Structures. ISSN 0951-8339. 88. doi: 10.1016/j.marstruc.2022.103341. Fulltekst i vitenarkiv Vis sammendrag

  New and efficient installation concepts which can reduce the cost of developing an offshore wind farm are of particular interest. This paper explores a promising concept using the small water-plane area twin-hull vessel (SWATH) to install pre-assembled wind turbines (OWT) onto floating spar foundations. A focus is placed on the hydrodynamic performance of the SWATH and the response analysis of the coupled SWATH-spar system. Firstly, the numerically calculated difference-frequency wave force effect and damping forces of the original SWATH were verified with experimental data. Secondly, the original SWATH was modified to satisfy the criteria of weight-carrying capacity and hydrostatic stability. Thirdly, a multibody numerical model for the SWATH-spar system was developed, in which the hydrodynamic and mechanical couplings between the SWATH and a spar were considered. The SWATH is equipped with a dynamic positioning system to counteract the slow-drift wave force effects. The nonlinear time-domain simulations were carried out for the mating stage when a wind turbine is lifted above the spar foundation. Based on the analysis of statistics of the relative displacement and velocity of the tower bottom and the spar top, the installation concept with SWATH is found to be of decent performance. Finally, recommendations are provided for future research on this concept, which contributes to developing next-generation installation concepts for bottom-fixed and floating wind farms.

• Wang, Yipin; Lu, Qi; Yao, Tiancheng; Yin, Taofeng; Zhao, Yongsheng & Han, Zhaolong [Vis alle 8 forfattere av denne artikkelen] (2023). Global sensitivity analysis of a semi-submersible floating wind turbine using a neural network fitting method. Ocean Engineering. ISSN 0029-8018. 285(Part 1). doi: 10.1016/j.oceaneng.2023.115351. Fulltekst i vitenarkiv Vis sammendrag

  The stochastic platform motions of a floating offshore wind turbine (FOWT) significantly affect its load characteristics and power production. It is vital to understand the influence of various input parameters on the global performance of an FOWT to ensure structural safety and power efficiency. This study investigates the influence of key design parameters related to metocean conditions and natural periods of platform motions on wind turbine performance. A global sensitivity analysis (GSA) framework is proposed to map the input-output relation of a 4- MW semi-submersible FOWT. This framework uses fully-coupled numerical analysis and neural network fitting (NNF). Several important parameters are selected, including natural periods of the platform motions, mean wind speed, turbulence intensity, wave spectral peak period and significant wave height. Results demonstrate that natural periods of platform motions and wave conditions have a significant influence on the dynamic responses of the FOWT. Furthermore, statistical analysis reveals that severe sea states and low natural periods of platform motions can increase the operation cycles of the blade pitch system by 20%. The use of NNF method effectively increases the efficiency of the GSA by at least forty times. The proposed framework can be applied to other areas of FOWTs.

• Jiang, Zhiyu; Liang, Guodong; Lopez-Olocco, Tomas; Medina-Manuel, Antonio; Saavedra-Ynocente, L.A. & Souto-Iglesias, Antonio (2023). Model test of a dual-spar floating wind farm in regular waves. I Ringsberg, J. W. & Soares, C. Guedes (Red.), Advances in the analysis and design of marine structures : Proceedings of the 9th International Conference on Marine Structures (MARSTRUCT 2023), Gothenburg, Sweden, 3-5 April 2023. CRC Press. ISSN 9781003399759. doi: 10.1201/9781003399759. Fulltekst i vitenarkiv Vis sammendrag

  A floating wind farm with shared moorings has the potential to reduce capital expenditure but may face structural dynamics issues. We selected a prototype wind farm that consists of two spar floating wind turbines with shared moorings and carried out model tests with a scale factor of 1:47. Rigid-body motions of one spar and mooring line tensions were measured. In this paper, the test setup is described, and results from the decay and regular-wave tests are discussed. In regular waves, the spar motions in surge, heave, and pitch are dominated by wave frequencies and the extreme motion ranges are acceptable. Compared with the baseline, the clump weight affects the mean position of platform motion; it also reduces the dynamic tension of the shared line but causes higher mean and maximum tension in the single lines. This paper contributes to an improved understanding of complex floating systems in offshore environments.

• Lopez-Olocco, Tomas; Liang, Guodong; Medina-Manuel, Antonio; Ynocente, Leandro Saavedra; Jiang, Zhiyu & Souto-Iglesias, Antonio (2023). Experimental comparison of a dual-spar floating wind farm with shared mooring against a single floating wind turbine under wave conditions. Engineering structures. ISSN 0141-0296. 292. doi: 10.1016/j.engstruct.2023.116475. Fulltekst i vitenarkiv Vis sammendrag

  Shared mooring can potentially be a cost-reduction factor for future floating wind farms. To assess the effect of shared mooring on the response dynamics of floating wind turbines (FWTs), experiments have been conducted for two spar FWTs with a shared mooring configuration and for a single spar FWT with catenary mooring, both at a scale of 1:47. Various regular and irregular wave conditions were tested and only one wave heading was considered. From the test results, the response amplitude operators and response spectra of platform motions and statistics of mooring tensions are compared for the two configurations. In the extreme wave condition, the shared mooring configuration leads to a 40% increase in the platform surge motion compared with the single spar FWT. Still, the absolute offset is within allowable limits prescribed by power cables. The fairlead tensions of the anchor lines are not significantly increased in the shared mooring configurations, but the shared line shows extreme tension peak events that are seldom present in the anchor lines. This study demonstrates the technological promises and challenges of a shared mooring system from an experimental perspective. Results and videos are made available and can be used to validate numerical models.

• Jiang, Zhiyu; Dai, Jian; Saettone, Simone; Tørå, Glenn; He, Zhao & Bashir, Musa [Vis alle 7 forfattere av denne artikkelen] (2023). Design and model test of a soft-connected lattice-structured floating solar photovoltaic concept for harsh offshore conditions. Marine Structures. ISSN 0951-8339. 90. doi: 10.1016/j.marstruc.2023.103426. Fulltekst i vitenarkiv Vis sammendrag

  Various types of floating solar photovoltaic (FPV) devices have been previously proposed, designed and constructed with applications primarily limited to onshore water bodies or near-shore regions with benign environmental conditions. This paper proposes a novel FPV concept which can survive harsh environmental conditions with extreme wave heights above 10 m. This concept uses standardised lightweight semi-submersible floats made of circular materials as individual modules. The floating modules are soft connected with ropes to form an FPV array. We first present the conceptual design of the floats and the connection systems, including hydrostatic, hydrodynamic, and structural assessments of the floats. To verify the motion response performance, we carried out 1:60 scaled model tests for a 2 by 3 array under regular and irregular wave conditions. From the time series and response spectra, the motion characteristics of the array and the mooring responses are analysed in detail. The proposed concept exhibits excellent performances in terms of modular motions with limited wave overtopping and no contact is observed between adjacent modules under the extreme wave conditions. The findings of this study can serve as a valuable reference to developing reliable and cost-effective FPV technologies for offshore conditions.

• Amlashi, Hadi; Jiang, Zhiyu & Karimirad, Madjid (2023). Ultimate Limit State Safety Format Requirement for Mooring Lines of Floating Wind Turbines, ASME 2023 5th International Offshore Wind Technical Conference. The American Society of Mechanical Engineers (ASME). ISSN 9780791887578. s. 1–8. doi: 10.1115/IOWTC2023-119493. Fulltekst i vitenarkiv
• Shi, Wei; Fu, Jie; Ren, Zhengru; Jiang, Zhiyu; Wang, Tao & Cui, Liang [Vis alle 7 forfattere av denne artikkelen] (2023). Real-time hybrid model tests of floating offshore wind turbines: Status, challenges, and future trends. Applied Ocean Research. ISSN 0141-1187. 141. doi: 10.1016/j.apor.2023.103796. Fulltekst i vitenarkiv
• Liu, Ting; Halse, Karl Henning; Leira, Bernt Johan & Jiang, Zhiyu (2023). Comparative study of the mating process for a spar-type floating wind turbine using two alternative installation vessels. Applied Ocean Research. ISSN 0141-1187. 132. doi: 10.1016/j.apor.2022.103452. Fulltekst i vitenarkiv Vis sammendrag

  The offshore wind industry faces many challenges as the water depth of offshore sites increases. For installation of the spar floating offshore wind turbines (OWTs), cost-effective installation methods are needed to facilitate installations at moderate or deep-water depths. In this paper, a novel installation concept initially designed for installing pre-assembled wind turbines is further studied. Two alternative installation vessels (i.e., a catamaran and a small water-plane area twin hull vessel (SWATH)) equipped with a dynamic positioning (DP) system are considered for installing 10-MW OWTs. Hydrodynamic responses of the installation systems are obtained by the fully coupled nonlinear time domain method. Based on the numerical analysis in SIMO-Riflex, a comparative study of the two vessel concepts is conducted to study selected response variables of two mating points. Compared with the catamaran installation vessel, the SWATH substantially reduces dynamic motions during the critical mating phase. The findings of this paper provide references for industrial stakeholders selecting alternative installation vessels and serve as a basis for further optimizing the concept.

• Bjørni, Fredrik Andersen; Lien, Sverre; Midtgarden, Torjus Aasrum; Santos, Laura-Castro & Jiang, Zhiyu (2023). Life cycle cost analysis of a floating wind farm in the Norwegian Sea. IOP Conference Series: Materials Science and Engineering. ISSN 1757-8981. 1294(012006). doi: 10.1088/1757-899X/1294/1/012006. Fulltekst i vitenarkiv Vis sammendrag

  The offshore wind energy industry has witnessed rapid growth in the past decade. Still, there is a lack of commercial floating wind projects due to the relatively high development costs and other factors. To facilitate the holistic evaluation of floating wind farms in Norway, this article investigates the levelized cost of energy of a floating offshore wind farm and its economic feasibility. The Troll field west of Bergen, Norway, is assumed to be the target offshore site, and a farm size of 50 wind turbines with a lifespan of 25 years are considered. Each floating wind turbine has a 15-megawatt turbine mounted on a semi-submersible floater. Based on detailed analysis, the levelized cost of energy of the wind farm is estimated to be approximately 100.7 $/MWh. The capital expenditure is the most prominent cost and constitutes 63.1% of the total cost, and the operational expenditure constitutes the remaining 36.9%. Further, sensitivity analyses show the influence of the lifespan, capacity factor, and project discount rate on the levelized cost of energy. The present study contributes to techno-economic evaluation of floating wind projects at an early phase.

• Verma, Amrit Shankar; Yan, Jiquan; Hu, Weifei; Jiang, Zhiyu; Shi, Wei & Teuwen, Julie J.E. (2023). A review of impact loads on composite wind turbine blades: Impact threats and classification. Renewable and Sustainable Energy Reviews. ISSN 1364-0321. 178. doi: 10.1016/j.rser.2023.113261. Fulltekst i vitenarkiv Vis sammendrag

  A fiber-reinforced composite wind turbine blade (WTB) is exposed to numerous impact threats during its service life causing damages that can be detrimental to its structural integrity. Currently, impact loads are not considered during blade design, so high safety factors are introduced, which result in a conservative design. However, as wind turbine blades become stiffer and lighter and health monitoring systems become more sophisticated, the design process is shifting toward damage-tolerant approaches. The design philosophy accepts damages to the structure, but it also requires that the damaged blade still meet structural and functional requirements. This design procedure requires a comprehensive understanding of different impact threats and their characteristics, which is currently unavailable in the public domain. This paper is a first attempt to review the impact loads on composite wind turbine blades. The aim of the current paper is to (a) identify different sources of impact threats on wind turbine blades during different stages of their service life, (b) describe their qualitative (causes and vulnerable regions) as well as quantitative characteristics (size, mass, and velocity of impactor), and to (c) provide modeling guidelines by comparing these impact threats using five different criteria - (i) relative deformability of projectile and wind turbine blade, (ii) impact velocity, (iii) kinetic energy of impact, (iv) repeatability of impacts and (v) nature of the impact. The review paper will be of special interest to researchers working on wind turbine blades and will serve as a baseline report for designing damage-tolerant blades. Recommendations are also provided for future research.

• Gao, Liu; Liu, Ting; Halse, Karl Henning & Jiang, Zhiyu (2023). Numerical analysis of the offshore wind turbine pre-mating process using a low-height lifting system for a nonconventional installation vessel. Ocean Engineering. ISSN 0029-8018. 286(2). doi: 10.1016/j.oceaneng.2023.115555. Fulltekst i vitenarkiv Vis sammendrag

  The offshore wind industry is a growing business. New and alternative installation methods which have the potential to reduce the cost of developing offshore wind farms are urgently needed. In this paper, an innovative concept that is tentatively designed for median or large water-depth sites is studied. The conventional jacked-up installation vessel is replaced by the small waterplane area twin-hull vessel (SWATH) equipped with the dynamic position (DP) system, and the heavy lifting is replaced by the low-height lifting crane with lifting wires for lowering the assembled OWT to the floating foundation. The global dynamic responses of the installation system under environmental loads are investigated by the fully coupled nonlinear numerical models, and the corresponding strength capacity for the lifting crane is validated. Several sea states are selected to study the feasibility of the concept, and the response variables of the OWT tower mating point (i.e., the displacement and velocity) are analyzed under different loading conditions. The lifting wires and selected lifting crane components are also investigated to understand their force situations. Some interesting conclusions are drawn, which can provide references for introducing the control system to the low-height lifting system and further optimizing the concept.

• Liang, Guodong; Jiang, Zhiyu & Merz, Karl Otto (2023). Dynamic analysis of a dual-spar floating offshore wind farm with shared moorings in extreme environmental conditions. Marine Structures. ISSN 0951-8339. 90. doi: 10.1016/j.marstruc.2023.103441. Fulltekst i vitenarkiv Vis sammendrag

  The concept of a shared mooring system was proposed to reduce mooring and anchoring costs. Shared moorings also add complexity to the floating offshore wind farm system and pose design challenges. To understand the system dynamics, this paper presents a dynamic analysis for a dual-spar floating offshore wind farm with a shared mooring system in extreme environmental conditions. First, a numerical model of the floating offshore wind farm was established in a commercial simulation tool. Then, time-domain simulations were performed for the parked wind farm under extreme wind and wave conditions. A sensitivity study was carried out to investigate the influence of loading directions and shared line mooring properties. To highlight the influence of the shared line, the results were compared to those of a single spar floating wind turbine, and larger platform motions and higher tension loads in single lines are observed for the wind farm with shared moorings. The loading direction affects the platform motions and mooring response of the floating offshore wind farm. Comparing the investigated loading directions to the 0-deg loading direction, the variation of mean mooring tension at the fairlead is up to 84% for single lines and 16% for the shared line. The influence of the shared line properties in the platform motions and the structural responses is limited. These findings improve understanding of the dynamic characteristics of floating offshore wind farms with a shared mooring system.

• Langeroodi, Soheil Salahshour; Ong, Muk Chen; Skaare, Bjørn & Jiang, Zhiyu (2022). A Perspective of Decommissioning Methods for Bottom-Fixed Offshore Wind Turbines. I OMAE, 2022 41st (Red.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering Volume 8: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 9780791885932. doi: 10.1115/OMAE2022-80933. Fulltekst i vitenarkiv Vis sammendrag

  The number of decommissioning activities in the offshore wind industry will increase in the coming years as many wind turbines reach their planned service lifetimes. Cost-effective solutions for decommissioning are important considering the levelized cost of energy of offshore wind farms. In addition to the existing challenges in marine operations such as waiting for suitable weather window, logistics and availability of the vessels, the limited experience in offshore wind decommissioning makes the process more challenging regarding the fact that the size of current installations of offshore wind turbines is growing considerably and more preparation and competence are required in association with new projects. In this paper, a review of existing decommissioning methods for offshore bottom-fixed wind turbines is provided first. The removal of wind turbines’ foundation and transition piece, on which significant duration of decommissioning operation is spent, is identified as a critical element, and is focused on. The decommissioning challenges and the decommissioning techniques implemented in previous projects are studied. These include partial and complete removals of foundations. These techniques and methods will be investigated thoroughly.

• Hassani, Milad; Li, Lin; Kalofotias, Filippos & Jiang, Zhiyu (2022). Hydrodynamic Performance Evaluation of a Semi-Submersible Floater Using Open-Source Tools. I ASME, Digital Collection (Red.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering. Volume 4: Ocean Space Utilization. The American Society of Mechanical Engineers (ASME). ISSN 9780791885888. doi: 10.1115/OMAE2022-78823. Fulltekst i vitenarkiv Vis sammendrag

  Floating structures are viable alternatives for supporting wind turbines in deep water. Design of floating support structures is a challenging task, and a design evaluation requires considerations of costs and system performance. An approach for evaluation of semi-submersible floaters has been developed in this paper. A braceless semi-submersible floater is selected and this floater is designed to support a 5-MW wind turbine in the North Sea. The evaluation framework consists of automatic modeling and numerical simulations in open-source tools. A Python-SALOME-NEMOH interface is used to obtain the hydrodynamic properties for geometries defined by various variables. The geometries are subjected to three performance constraints related to the static platform pitch, metacentric height, nacelle acceleration and wind loads in operating and parked conditions. Finally, the geometries are evaluated using two combined objective functions related to material cost and nacelle acceleration, and the Pareto Fronts are discussed. This work contributes to developing efficient design optimization methods for floating structures.

• Ren, Zhengru; Zhen, Xingwei; Jiang, Zhiyu; Gao, Zhen; Li, Ye & Shi, Wei (2022). Underactuated control and analysis of single blade installation using a jackup installation vessel and active tugger line force control. Marine Structures. ISSN 0951-8339. 88. doi: 10.1016/j.marstruc.2022.103338. Fulltekst i vitenarkiv Vis sammendrag

  The assembly and installation costs account for a large share in the overall expenditures of an offshore wind farm project. Single blade installation is suitable for large scale wind turbines due to the lower crane capability requirement and lower transportation time. By introducing active tension control on the tugger lines, an automatic single blade installation approach can accomplish operations in higher sea states, reduce the waiting-on-weather time, and improve the operational efficiency. Compared to early research, a more complicated control objective is achieved in this paper, i.e., a two-tugger-line configuration is applied to stabilize the suspended blade in three degrees of freedom during crane rotation and blade root-hub mating processes. The pulleys on the crane boom, i.e., the ends of the tugger lines, are assumed to be fixedly placed, resulting in tugger line time-varying inclinations. A novel backstepping-like controller is designed and proved. It is able to stabilize the blade around its equilibrium and make it track the desired path. Sensitivity studies are conducted to evaluate the influence of the tugger line inclinations. In addition, the influence of the installed blades on a three-blade horizontal wind turbine with a monopile foundation is discussed. The proposed active control setup improves the installation success rate and reduces the risks for blade impacts that may occur during mating.

• Xiao, Zhongming; Wang, Kai; Liu, Tianhui & Jiang, Zhiyu (2022). Numerical Simulation of an Installation Process of a Wind Turbine Assembly Considering a Pre-Installed Monopile Foundation. I Chung, Jin S.; Buzin, Igor & Kawai, Hiroyasu (Red.), Proceedings of The Thirty-second (2022) International OCEAN AND POLAR ENGINEERING CONFERENCE ISOPE-2022 Shanghai, China (Hybrid) June 5–10, 2022. ISOPE. ISSN 9781880653814. Fulltekst i vitenarkiv Vis sammendrag

  The installation phase is a critical stage during the lifecycle of an offshore wind turbine. The technology development of bottom-fixed offshore wind turbines is more mature compared to that floating wind turbines. This paper focuses on the installation process of an offshore monopile wind turbine using a previously proposed catamaran installation vessel. The simulated installation process involves the catamaran installation vessel, a lifted 5-MW wind turbine assembly and a pre-installed monopile foundation. Time-domain numerical simulations are conducted for the coupled catamaran-monopile system with mechanical coupling. The forces on the monopile foundation and the motion responses of the catamaran during the process are analyzed. Finally, this study calculates the catamaran response motion and the forces on sliding grippers of the coupled installation model, exploring the influence of sea state parameters on the coupled installation model and comparing the forces on sliding grippers of two different coupled installation models.

• Pang, Zhengyang; Feng, Aichun; Jiang, Zhiyu; Verma, Amrit Shankar & Chen, Ke (2022). Nonlinear Hydrostatic Restoring Characteristics of a Spar Floating Wind Turbine. I ASME, . (Red.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering Volume 5A: Ocean Engineering. The American Society of Mechanical Engineers (ASME). ISSN 9780791885895. doi: 10.1115/OMAE2022-78799. Fulltekst i vitenarkiv Vis sammendrag

  Floating wind turbine technologies have worldwide applications. Regarding the wind turbine floater stability, there are three principal design philosophies: ballast-, buoyancy- and mooring-stabilized. Although linear hydrostatic stiffness coefficient has been applied in most hydro-aero-elastic codes, accurate calculation of the nonlinear hydrostatic restoring forces is important for the floating stability evaluation and load. This study selects a 5-megawatt spar floating wind turbine as a representative floater. The nonlinear hydrostatic stiffness coefficient for different heeling angles is analytically calculated and compared against those obtained by a hydrodynamic software, and an excellent match is shown. A sensitivity study is carried out to consider the uncertainties in the hydrostatic stiffness due to varying geometry and weight distribution. The present results can be applied in the time-domain simulations for floating wind turbines.

• Guo, Zhiqun; Zhong, Liangjing; Wang, Kai & Jiang, Zhiyu (2022). Nonlinear air dynamics of a surface effect ship in small-amplitude waves. Journal of Ocean Engineering and Science. ISSN 2468-0133. doi: 10.1016/j.joes.2022.11.001. Fulltekst i vitenarkiv Vis sammendrag

  In many existing works, the seakeeping motions and air dynamics of a surface effect ship (SES) were assumed to be linear under small-amplitude waves (wave amplitude to wave length ratio ≤ 5%) to enhance the computational efficiency. However, according to SES model test results, it was found that even in small-amplitude waves, the fluctuating air cushion pressure shows significantly nonlinear effects. To precisely reveal this distinctive feature, the origin of nonlinearity was carefully investigated and the air leakage was considered as the main source of nonlinearity based on mathematical analysis in this paper. The reason is that the variance of clearance height under seals is comparable to the clearance height at equilibrium state in small-amplitude waves, which makes the air leakage area intermittently equal to zero without any harmonic variance. Therefore, an efficient partial nonlinear numerical model for the SES dynamics was proposed by combining a linear frequency-domain hydrodynamic model based on the efficient 2.5D methods with a nonlinear time-domain air dynamic model. The nonlinear parts of numerical results from the partial nonlinear model, including the fluctuating air pressure and midship accelerations, agree well with experimental results. The results demonstrate the effectiveness of the partial nonlinear model on the SES seakeeping performance prediction, and confirm that its nonlinearity mainly originates from the air leakage.

• Liang, Guodong; Jiang, Zhiyu & Merz, Karl Otto (2022). Influence of Aerodynamic Loads on a Dual-Spar Floating Offshore Wind Farm With a Shared Line in Parked Conditions. I OMAE, 2022 41st (Red.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering Volume 8: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 9780791885932. doi: 10.1115/OMAE2022-78929. Fulltekst i vitenarkiv Vis sammendrag

  The concept of a shared mooring system, in which adjacent wind turbines are coupled by sharing mooring lines, has been proposed to reduce the mooring costs of floating offshore wind farms. This work investigates the influence of aerodynamic loads on a floating offshore wind farm of two spar wind turbines connected with a shared line in extreme environmental conditions. A case study is performed for the floating offshore wind farm under parked conditions using a numerical simulation tool. The environmental conditions are determined from environmental contours with a return period of 50 years. Turbulent wind and irregular waves are simulated in dynamic analyses. Wind and waves are aligned and two loading directions are considered. Floater motions and structural response are analyzed. The influence of aerodynamic loads is studied by comparing the simulation results under both wind and waves with those under wave-only. It is concluded that the aerodynamic loads affect the horizontal motions of floating offshore wind turbines, the mooring response and the tower-base bending moment significantly in extreme environmental conditions, especially when the loading direction is 90 deg. The findings from this study improve understanding of the design loads of shared mooring systems.

• Sanfilippo, Filippo; Thorstensen, Rein Terje; Jha, Ajit; Jiang, Zhiyu & Robbersmyr, Kjell Gunnar (2022). A Perspective Review on Digital Twins for Roads, Bridges, and Civil Infrastructures. I -, IEEE (Red.), IEEE 2022 3rd International Conference on Electrical, Computer, Communications and Mechatronics Engineering (ICECCME). IEEE (Institute of Electrical and Electronics Engineers). ISSN 9781665470957. doi: 10.1109/ICECCME55909.2022.9988693. Fulltekst i vitenarkiv
• Jiang, Zhiyu; Yang, Limin; Gao, Zhen & Moan, Torgeir (2022). Integrated dynamic analysis of a spar floating wind turbine with a hydraulic drivetrain. Renewable Energy. ISSN 0960-1481. 201(1), s. 608–623. doi: 10.1016/j.renene.2022.10.104. Fulltekst i vitenarkiv Vis sammendrag

  Conventional drivetrains with gearbox failures are associated with major downtime in offshore wind turbines. This impacts the maintenance cost and cost of energy. A hydraulic transmission eliminates the need for gearbox and potentially improves the turbine reliability. This paper explores the application of a novel high-pressure transmission machinery to a utility-scale spar floating wind turbine. We present a dynamic model of a hydraulic system consisting of a hydraulic pump, pipelines, a hydraulic motor, and an induction generator. The motor is placed inside the spar platform and operates at a fixed displacement. The hydraulic system is coupled with the aero-hydro-elastic code HAWC2 through an external dynamic link library. The dynamic responses of a 5 MW spar floating wind turbine and a land-based reference wind turbine with this hydraulic system were investigated under various wind and wave conditions. A comparison is made between the characteristics of the two wind turbines under operational conditions and both turbines show decent behaviors, and the statistics of power generation is at the same level compared to that of conventional wind turbines. Through simulation studies, we verify the feasibility and the engineering practice of the proposed hydraulic drive solution to floating wind turbines.

• Kaptan, Mert; Skaare, Bjørn; Jiang, Zhiyu & Ong, Muk Chen (2022). Analysis of spar and semi-submersible floating wind concepts with respect to human exposure to motion during maintenance operations. Marine Structures. ISSN 0951-8339. 83. doi: 10.1016/j.marstruc.2021.103145. Fulltekst i vitenarkiv Vis sammendrag

  Floating offshore wind turbines (FOWTs) are expected to experience onsite maintenances and inspections during their lifetimes. To carry out offshore maintenance activities, a crew will be transferred to an FOWT and spend several hours on board. A challenge may arise if the motions of a floating platform affect the crew's comfort level and further jeopardise their work performance or even health. To address this challenge, this paper analyses the motion characteristics and dynamic properties of a spar and two semi-submersible FOWTs, all exhibiting very different motion characteristics. The impact of the platform motions and accelerations on the workability of the FOWTs are investigated. We carry out hydrodynamic analysis in a potential-flow software for the FOWTs and estimate the relevant short-term root-mean-square values for relevant motions and accelerations of the parked FOWTs in the frequency domain. Hindcast data for two representative sites in Norway and South Korea are selected, and both single peaked and double peaked wave spectra are considered. Using the limiting motion response criteria from a NORDFORSK study, we calculate the workability index of the FOWTs for the two locations. It is found that both the spar and the semi-submersible floating wind concepts fulfil the limiting criteria for significant wave heights up to the maximum known significant wave height for crew transfers to FOWTs. The present study contributes to a better understanding of FOWTs during the maintenance phase.

• Han, Zhaolong; Zhao, Yongsheng; Su, Jie; He, Yanping; Xu, Yuwang & Wu, Feng [Vis alle 7 forfattere av denne artikkelen] (2022). On the hydrodynamic responses of a multi-column TLP floating offshore wind turbine model. Ocean Engineering. ISSN 0029-8018. 253. doi: 10.1016/j.oceaneng.2022.111262. Fulltekst i vitenarkiv Vis sammendrag

  The offshore floating wind turbine system is a key piece of equipment for harvesting wind energy from the ocean field. However, the structural safety analysis and design of such equipment largely require a suitable platform to accurately obtain the dynamic characteristics. In the present study, an attempt was made to investigate the dynamic responses of the proposed 5-MW WindStar multi-column tension-leg-platform (TLP) system under different ocean environment conditions. Typical motion responses of the WindStar system, including the surge and pitch, and a series of measurements obtained from wave basin tests, such as nacelle acceleration and tendon tension, were compared with those of simulated results from the FAST code under four hydrodynamic load conditions. Additionally, the second-order wave load was incorporated with the first-order wave load, so as to allow for accurate evaluation of the high-frequency response of the floating offshore wind turbine (FOWT). Further, the effect of high-order wave loads on the high-frequency responses of the TLP FOWT was elucidated. The second-order frequency wave load was found to be a significant factor in determining the motion responses of the WindStar TLP system, and the results from FAST were found to agree well with those of the experiment. The coupled method of experiments and numerical analysis can be effectively used to understand the high-frequency dynamic responses of TLP FOWT systems, such as the WindStar TLP.

• Fang, Jianhao; Hu, Weifei; Liu, Zhenyu; Chen, Weiyi; Tan, Jianrong & Jiang, Zhiyu [Vis alle 7 forfattere av denne artikkelen] (2022). Wind turbine rotor speed design optimization considering rain erosion based on deep reinforcement learning. Renewable and Sustainable Energy Reviews. ISSN 1364-0321. 168. doi: 10.1016/j.rser.2022.112788. Fulltekst i vitenarkiv Vis sammendrag

  Rain erosion is one of the most detrimental factors contributing to wind turbine blade (WTB) coating fatigue damage especially for utility-scale wind turbines (WTs). To prevent rain erosion induced WTB coating fatigue damage, this paper proposes a deep reinforcement learning (DRL)-based optimization method for finding the optimal rotor speed under different rain intensities and wind speeds. First, an efficient physics-based model for predicting WTB coating fatigue damage considering the comprehensive blade coating fatigue mechanism, rain intensity distribution, and wind speed distribution is presented. Then, a WT rotor speed design optimization problem is constructed to search for the optimal rotor speed under different rain intensity and wind speed conditions. To address the challenge of optimizing the efficiency, the original design optimization problem is converted into a DRL-based design optimization model. A hybrid reward is proposed to enhance the DRL agent trained by a deep deterministic policy gradient algorithm. Finally, the proposed DRL-based design optimization method is utilized to guide the optimal rotor speed scheduling of a 5-MW WT under given wind speed and rain intensity conditions. The results show that the proposed method could extend the predicted WTB blade coating fatigue life by 2.55 times with a minor reduction in the energy yield (0.027%) compared to the original rotor speed schedule that only considers maximum power capture. The computational time of the proposed method is reduced significantly compared to that of the traditional gradient and evolutional design optimization methods.

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• Jiang, Zhiyu (2025). Recent advances in floating offshore solar technologies. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2025). Shared mooring systems for prototype floating wind farms. Fulltekst i vitenarkiv
• Hugon, Mathieu; Eriksen, Amalie; Schlanbusch, Rune & Jiang, Zhiyu (2025). Dynamic analysis of anchor loads for a floating offshore wind farm. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2025). Utdanning og forskning på havvind. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2025). Mooring Design for Floating Wind Turbines: Challenges and Innovations. Fulltekst i vitenarkiv Vis sammendrag

  Mooring systems are essential for ensuring the safety and station-keeping of floating offshore structures. Despite advances in mooring technologies over the past two decades, several design challenges remain. The design of mooring systems is a multidisciplinary task that requires expertise in marine hydrodynamics, structural mechanics, materials engineering, and mechanical design. This seminar will focus on the challenges of mooring design for floating wind farms. Existing mooring designs for three types of floating wind turbines will be critically reviewed and technological innovations of various share mooring systems will be presented and discussed.

• Jiang, Zhiyu (2024). Major trends and technological innovations for offshore wind. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2023). Recent developments in offshore floating photovoltaic concepts. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2023). Floating solar tech for harsh offshore conditions. [Journal]. https://www.pv-magazine.com/2023/06/14/floating-solar-tech-f. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2023). Accidental scenarios of offshore wind farms: numerical simulations. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2022). Ulykkescenarioer for havvindparker. Fulltekst i vitenarkiv
• Cheynet, Etienne; Li, Lin & Jiang, Zhiyu (2022). Metocean Conditions at Two Norwegian Sites for Development of Offshore Wind Farms. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2022). Assessment of accidental scenarios for offshore wind farms. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2022). Utdanning og forskning på havvind. Fulltekst i vitenarkiv
• Jiang, Zhiyu (2022). Mooring system design challenges for modular floating energy islands. Fulltekst i vitenarkiv Vis sammendrag

  In this talk, possible mooring system configurations for modular offshore energy islands will be discussed. First, different types of environmental loads acting on a modular energy island will be presented. Then, existing mooring systems applied to offshore wind energy and wave energy systems will be reviewed. Last, novel concepts with potential cost-effectiveness for large modular floating energy islands will be presented. The presenter has been involved in projects related to design and analysis of floating wind turbines, offshore dynamics, metocean conditions, mooring system design, and marine operations.

• Pang, Zhengyang; Feng, Aichun; Chen, Ke; Verma, Amrit Shankar & Jiang, Zhiyu (2022). Nonlinear hydrostatic restoring characteristics of a spar floating wind turbine. Fulltekst i vitenarkiv Vis sammendrag

  Floating wind turbine technologies have worldwide applications. Regarding the wind turbine floater stability, there are three principal design philosophies: ballast-, buoyancy- and mooring-stabilized. Although linear hydrostatic stiffness coefficient has been applied in most hydro-aero-elastic codes, accurate calculation of the nonlinear hydrostatic restoring forces is important for the floating stability evaluation and load. This study selects a 5-megawatt spar floating wind turbine as a representative floater. The nonlinear hydrostatic stiffness coefficient for different heeling angles is analytically calculated and compared against those obtained by a hydrodynamic software, and an excellent match is shown. A sensitivity study is carried out to consider the uncertainties in the hydrostatic stiffness due to varying geometry and weight distribution. The present results can be applied in the time-domain simulations for floating wind turbines.

• Hassani, Milad; Li, Lin; Kalofotias, Filippos & Jiang, Zhiyu (2022). Hydrodynamic performance evaluation of a semi-submersible floater using open-source tools. Fulltekst i vitenarkiv Vis sammendrag

  Floating structures are viable alternatives for supporting wind turbines in deep water. Design of floating support structures is a challenging task, and a design evaluation requires considerations of costs and system performance. An approach for evaluation of semi-submersible floaters has been developed in this paper. A braceless semi-submersible floater is selected and this floater is designed to support a 5-MW wind turbine in the North Sea. The evaluation framework consists of automatic modeling and numerical simulations in open-source tools. A Python-SALOME-NEMOH interface is used to obtain the hydrodynamic properties for geometries defined by various variables. The geometries are subjected to three performance constraints related to the static platform pitch, metacentric height, nacelle acceleration and wind loads in operating and parked conditions. Finally, the geometries are evaluated using two combined objective functions related to material cost and nacelle acceleration, and the Pareto Fronts are discussed. This work contributes to developing efficient design optimization methods for floating structures.

• Dvorak, Ota; Bohan, Greg; Blomgren, Atle; O’Connell, Ross & Jiang, Zhiyu (2025). IEA Wind TCP Task 49 Supply Chain Capability Consultation Report. International Energy Agency (IEA). doi: https:/usercontent.one/wp/iea-wind.org/wp-content/uploads/2025/06/IEA-Wind-Task-49-WP4-Report-3-Supply-Chain-Consultation.pdf. Fulltekst i vitenarkiv
• Ong, Muk Chen; Jiang, Zhiyu; Skaare, Bjørn & Salahshour, Soheil (2025). Numerical Modeling and Analysis of Monopile Foundation Removal Process for Offshore Wind Turbines. Universitetet i Stavanger. ISSN 9788284393445. Fulltekst i vitenarkiv Vis sammendrag

  The rapid pace of technological advancements provides limited opportunities for the environment to adapt. The over-reliance on fossil fuels is placing immense strain on Earth's ecosystems. However, there has been a growing recognition of the importance of renewable energy sources, with wind energy being one of the most prominent. Wind energy is generally classified into two categories: onshore and offshore. While offshore wind energy offers several advantages over onshore systems, regarding higher wind speed and consistency, reduced noise pollution, less visual interference, possibility of larger installation, and less turbulence, it comes with a significant drawback—its high cost. The installation, maintenance, and decommissioning of offshore wind turbines are considerably more complex compared to their onshore counterparts, necessitating the development of innovative methods to offset these additional expenses. Although cost is a critical factor, it is not the most pressing concern. As energy demand continues to grow, larger offshore wind turbines are required, which in turn necessitates more robust foundation structures. The most commonly used bottom-fixed foundation type, monopile (MP), is also increasing in size to support these larger turbines. Monopiles are favoured due to their relatively low cost, simplicity, and ease of installation, making them the dominant choice in offshore wind energy infrastructure. Offshore wind turbines have a lifetime of 15 to 25 years and when they reach to their end of design lifetime, decommissioning of them becomes inevitable. Efficient removal of monopiles during the decommissioning phase is critical, offering both environmental and economic benefits. Developing methods to accelerate the removal process is essential, as shorter decommissioning times reduce costs for wind farm owners and operators. The use of vibro-hammers in offshore projects has grown significantly, particularly for monopile installations. Their application in real-world removal processes and field measurements has demonstrated their effectiveness, highlighting the need for further research to enhance understanding of their efficiency in offshore wind decommissioning. This study presents a numerical analysis of vibratory monopile extraction using a floating vessel, conducted in two phases: Phase 1: Soil Resistance Reduction Analysis The first phase investigates the reduction of soil resistance under high-frequency centrifugal forces generated by the vibro-hammer. Distinctions between cohesive and non-cohesive soils are implemented in the analysis. The pile-soil interaction is modeled using the commercial software ABAQUS, employing the Coupled Eulerian-Lagrangian (CEL) approach to address large deformation problems. For non-cohesive soils, dense sand is modeled with characteristics such as pre-peak hardening and post-peak softening incorporated into the simulations. For cohesive soils, both soft and stiff clays are analyzed, with simulations accounting for shear strain rate and strain softening effects. Phase 2: Floater Dynamics and Operability Analysis In the second phase, a crane barge is modeled as the floater for executing the vibratory monopile extraction. Simulations are performed using the software SIMO, with soil resistance data for sandy seabeds obtained from Phase 1 used as input. The dynamic response of the crane barge during the extraction process is first analyzed. Subsequently, the operability of the crane barge at a specific offshore site is evaluated using extreme value analysis. This study provides insights into the effectiveness of vibro-hammers in monopile removal, and the dynamic performance and operational limits of floating vessels involved in the decommissioning phase. These findings contribute to the optimization of offshore wind decommissioning strategies, balancing cost-effectiveness and operational safety.

• Dvorak, Ota; Bohan, Greg; Aoife, Kelly; David, Cahill; Malcolm, Bowie & Kami, Delivand Mitra [Vis alle 7 forfattere av denne artikkelen] (2024). The IEA Wind Task 49 MSP and Market Consultation Year 1 Report. IEA Wind. Fulltekst i vitenarkiv
• Hall, Matthew; Lozon, Ericka; McAuliffe, Fiona Devoy; Bessone, Matteo Baudino; Bayati, Ilmas & Bowie, Malcolm [Vis alle 28 forfattere av denne artikkelen] (2024). The IEA Wind Task 49 Reference Floating Wind Array Design Basis. IEA Wind TCP. Fulltekst i vitenarkiv
• Janocha, Marek Jan; Ong, Muk Chen & Jiang, Zhiyu (2024). The IEA Wind Task 49 Reference Floating Wind Array Design Basis. International Energy Agency (IEA). Fulltekst i vitenarkiv
• Zhou, Jing; RIPOLL, ÁLVARO ÚBEDA & Jiang, Zhiyu (2023). Active control of the blade leading edge erosion for an onshore wind turbine during rainfall events. Universitetet i Agder. Fulltekst i vitenarkiv
• Mahdavipour, Mohammad Ali; Vysochinskiy, Dmitry & Jiang, Zhiyu (2022). Failure prediction of steel components under the coupled effect of excessive plastic deformations and pitting corrosion. Universitetet i Agder. ISSN 9788284270739. Fulltekst i vitenarkiv Vis sammendrag

  Nowadays, modern computers have made advanced computational resources accessible to analysts engaged in the field of structural engineering. Through these advancements, the role of numerical modeling has increasingly become more significant in both research and engineering projects. In addition, improved testing facilities and measuring instruments like digital image correlation (DIC) have made material testing more informative than before, and engineers can calibrate more complicated material models to simulate complex phenomena such as plasticity and fracture. All developed testing and numerical modeling techniques come together to form a numerical failure assessment framework for steel structures. Due to the distinct advantages of numerical modeling over full-scale testing, this framework is welldeveloped in many applications (e.g., elastic analyses). However, in many other applications, numerous involved phenomena and their interactions with each other, as well as additional factors (e.g., corrosion), call for more development both in material testing and numerical techniques to customize the framework to better suit the final application. This thesis has focused on the failure assessment of steel components under the coupled effect of extensive plasticity and pitting corrosion. A framework was developed based on experimental material calibration and stochastic numerical modeling approaches.

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