Zhiyu Jiang

• 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.
• 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.
• Liang, Guodong; Wickmann Hanssen, Finn-Christian; Otto Merz, Karl & Jiang, Zhiyu (2024). Numerical analysis of a tethered-buoy mooring system for a prototype floating wind farm. Wind Energy. ISSN 1095-4244. 27(5), p. 500–529. doi: 10.1002/we.2898.
• 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, p. 1–17. doi: 10.1016/j.renene.2024.120184. Full text in Research Archive Show summary

  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; 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. Full text in Research Archive
• 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.
• 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 978-0-7918-8757-8. p. 1–8. doi: 10.1115/IOWTC2023-119493.
• 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. Full text in Research Archive Show summary

  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.

• 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. Full text in Research Archive Show summary

  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; Fu, Jie; Ren, Zhengru; Jiang, Zhiyu; Wang, Tao & Cui, Liang [Show all 7 contributors for this article] (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.
• Wang, Yipin; Lu, Qi; Yao, Tiancheng; Yin, Taofeng; Zhao, Yongsheng & Han, Zhaolong [Show all 8 contributors for this article] (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. Show summary

  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.

• 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. Full text in Research Archive Show summary

  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.

• 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. Show summary

  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.

• 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. Full text in Research Archive Show summary

  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.

• 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. Full text in Research Archive Show summary

  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; Verma, Amrit Shankar & Márquez, Fausto Pedro García (2023). Numerical simulations of offshore wind farms considering accidental scenarios. In Márquez, Fausto Pedro García; Papaelias, Mayorkinos & Junior, Valter Luiz Jantara (Ed.), Non-Destructive Testing and Condition Monitoring Techniques in Wind Energy. Elsevier. ISSN 978-0-323-99666-2. p. 317–349. doi: 10.1016/B978-0-323-99666-2.00010-1. Show summary

  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.
• 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.
• 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. Show summary

  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.

• 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. Full text in Research Archive Show summary

  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.

• 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. In Ringsberg, J. W. & Soares, C. Guedes (Ed.), 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. Full text in Research Archive Show summary

  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.

• Jiang, Zhiyu; Dai, Jian; Saettone, Simone; Tørå, Glenn; He, Zhao & Bashir, Musa [Show all 7 contributors for this article] (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. Full text in Research Archive Show summary

  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.

• 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. Show summary

  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.

• 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. Show summary

  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.

• Liu, Ting; Halse, Karl Henning; Leira, Bernt Johan; Jiang, Zhiyu; Chai, Wei & Brathaug, Hans-P. [Show all 7 contributors for this article] (2022). 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. Show summary

  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.

• 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. Show summary

  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.

• Salahshour Langeroodi, Soheil; Ong, Muk Chen; Skaare, Bjørn & Jiang, Zhiyu (2022). A Perspective of Decommissioning Methods for Bottom-Fixed Offshore Wind Turbines . In 41st OMAE, 2022 (Eds.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering Volume 8: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8593-2. doi: 10.1115/OMAE2022-80933. Show summary

  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.

• 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. In 41st OMAE, 2022 (Eds.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering Volume 8: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8593-2. doi: 10.1115/OMAE2022-78929. Full text in Research Archive Show summary

  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. In -, IEEE (Eds.), IEEE 2022 3rd International Conference on Electrical, Computer, Communications and Mechatronics Engineering (ICECCME). IEEE (Institute of Electrical and Electronics Engineers). ISSN 978-1-6654-7095-7. doi: 10.1109/ICECCME55909.2022.9988693.
• 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. In Chung, Jin S.; Buzin, Igor & Kawai, Hiroyasu (Ed.), Proceedings of The Thirty-second (2022) International OCEAN AND POLAR ENGINEERING CONFERENCE ISOPE-2022 Shanghai, China (Hybrid) June 5–10, 2022. ISOPE. ISSN 978-1-880653-81-4. Show summary

  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.

• 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. Full text in Research Archive Show summary

  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.

• Pang, Zhengyang; Feng, Aichun; Jiang, Zhiyu; Verma, Amrit Shankar & Chen, Ke (2022). Nonlinear Hydrostatic Restoring Characteristics of a Spar Floating Wind Turbine. In ASME, . (Eds.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering Volume 5A: Ocean Engineering. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8589-5. doi: 10.1115/OMAE2022-78799. Show summary

  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. In ASME, Digital Collection (Eds.), ASME 2022 41st International Conference on Ocean, Offshore and Arctic Engineering. Volume 4: Ocean Space Utilization. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8588-8. doi: 10.1115/OMAE2022-78823. Show summary

  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.

• 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), p. 608–623. doi: 10.1016/j.renene.2022.10.104. Full text in Research Archive
• Fang, Jianhao; Hu, Weifei; Liu, Zhenyu; Chen, Weiyi; Tan, Jianrong & Jiang, Zhiyu [Show all 7 contributors for this article] (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. Show summary

  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.

• Han, Zhaolong; Zhao, Yongsheng; Su, Jie; He, Yanping; Xu, Yuwang & Wu, Feng [Show all 7 contributors for this article] (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. Show summary

  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.

• 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. Full text in Research Archive Show summary

  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.

• Zhao, Yu; Guo, Jiamin; Jiang, Zhiyu; Chen, Weigang & Zhou, Guangen (2022). Control method for determining feasible pre-stresses of cable-struts structure. Thin-walled structures. ISSN 0263-8231. 174. doi: 10.1016/j.tws.2022.109159. Show summary

  The structural stiffness of a cable-struts structure is dependent on its feasible pre-stresses. The search for the pre-stresses is an inverse problem, and classical approaches for this problem include the singular value decomposition (SVD) method. Driven by the need for more efficient methods, this study proposes a control method to search for feasible pre-stresses of cable-struts structures. First, we formulate the governing equation for cable-struts structures under pre-stresses and external loads. Then, we apply feedback control and use a proportional–integral–derivative (PID) controller to solve for the feasible pre-stresses based on the condition of zero structural displacements. To verify the proposed method, the study selects a Geiger dome, a Kiewitt dome and a honeycomb-type cable dome under three different loading conditions, and compares the present results against those from literature. The comparison shows that the developed control method can efficiently and accurately determine the feasible pre-stresses under external loads. The relative errors in the pre-stresses determined using the PID controller and using another two methods (singular value decomposition and iterative method) are less than 0.020% and 0.015% for the Geiger dome and the honeycomb-type dome, respectively. This study contributes to developing efficient methods for design of cable-struts structures under complex service loads.

• Bjørni, Fredrik Andersen; Lien, Sverre; Midtgarden, Torjus Aasrum; Kulia, Geir; Verma, Amrit & Jiang, Zhiyu (2021). Prediction of dynamic mooring responses of a floating wind turbine using an artificial neural network. IOP Conference Series: Materials Science and Engineering. ISSN 1757-8981. doi: 10.1088/1757-899X/1201/1/012023. Full text in Research Archive Show summary

  Numerical simulations in coupled aero-hydro-servo-elastic codes are known to be a challenge for design and analysis of offshore wind turbine systems because of the large number of design load cases involved in checking the ultimate and fatigue limit states. To alleviate the simulation burden, machine learning methods can be useful. This article investigates the effect of machine learning methods on predicting the mooring line tension of a spar floating wind turbine. The OC3 Hywind wind turbine with a spar-buoy foundation and three mooring lines is selected and simulated with SIMA. A total of 32 sea states with irregular waves are considered. Artificial neural works with different constructions were applied to reproduce the time history of mooring tensions. The best performing network provides a strong average correlation of 71% and consists of two hidden layers with 35 neurons, using the Bayesian regularisation backpropagation algorithm. Sea states applied in the network training are predicted with greater accuracy than sea states used for validation of the network. The correlation coefficient is primarily higher for sea states with lower significant wave height and peak period. One sea state with a significant wave height of 5 meters and a peak period of 9 seconds has an average extreme value deviation for all mooring lines of 0.46%. Results from the study illustrate the potential of incorporating artificial neural networks in the mooring design process.

• Jiang, Zhiyu & Xing, Yihan (2021). Load mitigation method for wind turbines during emergency shutdowns. Renewable Energy. ISSN 0960-1481. 185, p. 978–995. doi: 10.1016/j.renene.2021.12.068. Full text in Research Archive Show summary

  Wind turbines experience countless shutdowns during their lifetimes. A shutdown is a transient process characterised by a pitch-to-feather manoeuvre of three blades. Such a pitch manoeuvre is often collective, open-loop, and can substantially slow the rotor speed within several seconds. However, undesirable structural responses may arise because of the imbalanced aerodynamic loads acting on the rotor. To address this issue, this paper proposes a method that actively adjusts the individual pitch rate of each blade during an emergency shutdown. This method is founded on a minimal intervention principle and uses the blade-root bending moment measurements as the only inputs. The control objective is to minimise the differences in the blade-root flapwise bending moment among the three blades during the shutdown. Using a high-fidelity aeroelastic model, we demonstrate the controller performance under representative steady wind conditions with vertical wind shear. Compared with the baseline shutdown strategy, the proposed method effectively reduces the maximum nontorque bending moment at the main shaft and the tower bottom bending moment; the reductions vary between 10% and 40% under the investigated conditions. The present work can be further extended to reduce structural fatigue damages or to handle complex loading scenarios of offshore wind turbines during shutdowns.

• Wang, Shuaishuai; Moan, Torgeir & Jiang, Zhiyu (2021). Influence of variability and uncertainty of wind and waves on fatigue damage of a floating wind turbine drivetrain. Renewable Energy. ISSN 0960-1481. 181, p. 870–897. doi: 10.1016/j.renene.2021.09.090. Full text in Research Archive
• Shah, Kamran Ali; Meng, Fantai; Li, Ye; Nagamune, Ryozo; Zhou, Yarong & Ren, Zhengru [Show all 7 contributors for this article] (2021). A synthesis of feasible control methods for floating offshore wind turbine system dynamics. Renewable and Sustainable Energy Reviews. ISSN 1364-0321. 151. doi: 10.1016/j.rser.2021.111525. Full text in Research Archive Show summary

  During the past decade, the development of offshore wind energy has transitioned from near shore with shallow water to offshore middle-depth water regions. Consequently, the energy conversion technology has shifted from bottom-fixed wind turbines to floating offshore wind turbines. Floating offshore wind turbines are considered more suitable, but their cost is still very high. One of the main reasons for this is that the system dynamics control method is not well-adapted, thereby affecting the performance and reliability of the wind turbine system. The additional motion of the platform tends to compromise the system’s performance in terms of power maximization, power regulation, and load mitigation. To provide a recommendation based on the advantages and disadvantages of different control methods, we systematically analyze feasible control methods for existing floating offshore wind turbine designs. Based on a brief overview of floating offshore wind turbine system dynamics, we present several promising control methods by classifying them as blade-pitch-based and mass–spring–damper-based. Furthermore, we emphasize on the incoming wind and wave forecasting associated with the control methods. We then compare different methods by evaluating a matrix involving platform motion minimization, load mitigation, and power regulation and identify the advantages and disadvantages. Finally, recommendations and suggestions for further research are provided by integrating the advantageous control algorithm and forecasting technologies to reduce costs.

• Verma, Amrit Shankar; Jiang, Zhiyu; Caboni, Marco; Verhoef, Hans; Meijer, Harald van der Mijle & Castro, Saullo G.P. [Show all 7 contributors for this article] (2021). A probabilistic rainfall model to estimate the leading-edge lifetime of wind turbine blade coating system. Renewable Energy. ISSN 0960-1481. 178, p. 1435–1455. doi: 10.1016/j.renene.2021.06.122. Full text in Research Archive Show summary

  Rain-induced leading-edge erosion of wind turbine blades is associated with high repair and maintenance costs. For efficient operation and maintenance, erosion models are required that provide estimates of blade coating lifetime at a real scale. In this study, a statistical rainfall model is established that describes probabilistic distributions of rain parameters that are critical for site-specific leading-edge erosion assessment. A new droplet size distribution (DSD) is determined based on two years’ onshore rainfall data of an inland site in the Netherlands and the obtained DSD is compared with those from the literature. Joint probability distribution functions of rain intensities and droplet sizes are also established for this site as well as for a coastal site in the Netherlands. Then, the application of the proposed model is presented for a 5 MW wind turbine, where the model is combined with wind statistics along with an analytical surface fatigue model that describes lab-scale coating degradation. The expected lifetime of the blade coating is found three to four times less for the wind turbine operating at the coastal site than for the inland site - primarily due to rainfall at higher wind speeds. Further, the robustness of the proposed model is found consistent with varying data periods used for the analyses.

• Verma, Amrit Shankar; Di Noi, Sandro; Ren, Zhengru; Jiang, Zhiyu & Teuwen, Julie J. E. (2021). Minimum leading edge protection application length to combat rain-induced erosion of wind turbine blades. Energies. ISSN 1996-1073. 14(6), p. 1–26. doi: 10.3390/en14061629. Full text in Research Archive Show summary

  Leading edge erosion (LEE) repairs of wind turbine blades (WTBs) involve infield application of leading edge protection (LEP) solutions. The industry is currently aiming to use factory based LEP coatings that can applied to the WTBs before they are shipped out for installation. However, one of the main challenges related to these solutions is the choice of a minimum LEP application length to be applied in the spanwise direction of the WTBs. Generally, coating suppliers apply 10–20 m of LEP onto the blades starting from the tip of the blade using the “rule of thumb”, and no studies in the literature exist that stipulate how these LEP lengths can be calculated. In this study, we extend the scope of a recently developed long-term probabilistic framework to determine the minimum LEP application length required for WTBs to combat rain-induced erosion. A parametric study is performed where different wind turbines with varying power ratings of 2.1 MW to 15 MW at different Dutch sites ranging from inland to coastal are considered. The results of the study show that the LEP application length is sensitive to the choice of the site, as well as the turbine attributes. Further, LEP lengths for WTBs are found to be the highest for turbines installed at coastal sites and turbines with higher power ratings. A detailed investigation is further performed to check the sensitivity of the LEP application length with the wind turbine parameters. The results of the study are expected to provide guidelines to the industry for efficient repair strategies for WTBs

• Wu, Haoyu; Zhao, Yongsheng; He, Yanping; Shao, Yanlin; Mao, Wengang & Han, Zhaolong [Show all 9 contributors for this article] (2021). Transient response of a TLP-type floating offshore wind turbine under tendon failure conditions. Ocean Engineering. ISSN 0029-8018. 220. doi: 10.1016/j.oceaneng.2020.108486. Show summary

  Among various types of floating offshore wind turbines (FOWTs), the tension leg platform (TLP) floating wind turbines have relatively small motions due to stiff tendons. Similar to TLP applications in the offshore industry, tendon failure may lead to deteriorated stability and large transient responses, which should be considered as part of accidental limit state (ALS) checks at the design stage of the TLP FOWTs. This paper takes the WindStar TLP system as a representative and investigates the transient effects of one-tendon failure on the system responses. A coupled numerical model is first established using the aero-hydro-servo-elastic simulation tool FAST. Subsequent numerical simulations of tendon failure are carried out to consider different tendon breakage and turbine shutdown scenarios in both operational and parked conditions. Response statistics of key design parameters including platform motion, nacelle acceleration, and tendon tension are analyzed. The results indicate that several transient responses of the FOWT under operational conditions may be even higher than the extreme values under 50-year conditions. Our analyses also reveal that the shutdown strategies, if not applied appropriately, may have limited effects in reducing the responses of the FOWT, thus in certain scenarios threaten the safety of the tendon closest to the broken tendon if no other actions are taken. For the considered TLP FOWT, the ALS design requirements dominate over the ultimate limit state requirements. These findings can be relevant to the ALS design of TLP FOWTs.

• Verma, Amrit Shankar; Jiang, Zhiyu; Ren, Zhengru; Caboni, Marco; Verhoef, Hans & Mijle-Meijer, Harald [Show all 8 contributors for this article] (2021). A probabilistic long-term framework for site-specific erosionanalysis of wind turbine blades: A case study of 31 Dutch sites. Wind Energy. ISSN 1095-4244. 24(11), p. 1315–1336. doi: 10.1002/we.2634. Full text in Research Archive Show summary

  Rain-induced leading-edge erosion (LEE) of wind turbine blades (WTBs) is associated with high repair and maintenance costs. The effects of LEE can be triggered in less than 1 to 2 years for some wind turbine sites, whereas it may take several years for others. In addition, the growth of erosion may also differ for different blades and turbines operating at the same site. Hence, LEE is a site- and turbine-specific problem. In this paper, we propose a probabilistic long-term framework for assessing site-specific lifetime of a WTB coating system. Case studies are presented for 1.5 and 10 MW wind turbines, where geographic bubble charts for the leading-edge lifetime and number of repairs expected over the blade's service life are established for 31 sites in the Netherlands. The proposed framework efficiently captures the effects of spatial and orographic features of the sites and wind turbine specifications on LEE calculations. For instance, the erosion is highest at the coastal sites and for sites located at higher altitudes. In addition, erosion is faster for turbines associated with higher tip speeds, and the effects are critical for such sites where the exceedance probability for rated wind conditions are high. The study will aid in the development of efficient operation and maintenance strategies for wind farms.

• Hu, Weifei; Chen, Weiyi; Wang, Xiaobo; Jiang, Zhiyu; Wang, Yeqing & Verma, Amrit [Show all 7 contributors for this article] (2021). A computational framework for coating fatigue analysis of wind turbine blades due to rain erosion. Renewable Energy. ISSN 0960-1481. 170, p. 236–250. doi: 10.1016/j.renene.2021.01.094. Full text in Research Archive
• Jiang, Zhiyu (2021). Installation of offshore wind turbines: A technical review. Renewable and Sustainable Energy Reviews. ISSN 1364-0321. 139, p. 1–21. doi: 10.1016/j.rser.2020.110576. Full text in Research Archive
• Liang, Guodong; Jiang, Zhiyu & Merz, Karl Otto (2021). Mooring Analysis of a Dual-Spar Floating Wind Farm With a Shared Line. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 143(6). doi: 10.1115/1.4050965. Full text in Research Archive
• Ren, Zhengru; Verma, Amrit Shankar; Li, Ye; Teuwen, Julie J.E. & Jiang, Zhiyu (2021). Offshore wind turbine operations and maintenance: A state-of-the-art review. Renewable and Sustainable Energy Reviews. ISSN 1364-0321. 144. doi: 10.1016/j.rser.2021.110886. Full text in Research Archive Show summary

  Operations and maintenance of offshore wind turbines (OWT) play an important role in the development of offshore wind farms. Compared with operations, maintenance is a critical element in the levelized cost of energy, given the practical constraints imposed by offshore operations and the relatively high costs. The effects of maintenance on the life cycle of an offshore wind farm have high degrees of complexity and uncertainty. The selection of maintenance strategies influences the overall efficiency, the profit margin, and the safety and sustainability of offshore wind farms. For an offshore wind project, after a maintenance strategy is selected, a schedule planning will be considered which is an optimization problem. Onsite maintenance will involve complex marine operations whose efficiency and safety depend on practical factors. Meanwhile, negative environmental impacts due to offshore maintenance deserve attention. To address these aspects, this paper reviews the state-of-the-art research on OWT maintenance covering strategy selection, schedule optimization, onsite operations, repair, assessment criteria, recycling, and environmental concerns. A large number of methods are summarized and compared. Limitations in research and shortcomings of industrial development in OWT operations and maintenance are revealed. Finally, promising areas are identified with regard to future studies of maintenance strategies.

• Verma, Amrit Shankar; Jiang, Zhiyu; Ren, Zhengru; Hu, Weifei & Teuwen, Julie J.E. (2021). Effects of onshore and offshore environmental parameters on the leading edge erosion of wind turbine blades: A comparative study . Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 143(4). doi: 10.1115/1.4049248. Full text in Research Archive Show summary

  The presence of rain-induced leading edge erosion of wind turbine blades necessitates the development of erosion models. One of the essential parameters for erosion modelling is the relative impact velocity between rain droplets and the rotating blade. Based on this parameter, the erosion damage rate of a wind turbine blade is calculated to estimate the expected leading edge lifetime. The environmental conditions that govern this parameter have site-specific variations, and thus, rain and wind loading on a turbine differ for onshore and offshore locations. In addition, there are wave loads present in the offshore environment. The present paper tries to provide guidelines for erosion modelling and investigates whether there are differences in erosion of blades due to (1) varying rainfall conditions modelled using different droplet size distributions for onshore and offshore locations in combination with (2) winds of varying turbulence intensities and (3) wave-induced loads. Aero-hydro-servo-elastic simulations are carried out for an onshore wind turbine and a monopile-supported offshore wind turbine. Furthermore, erosion variables such as the relative impact velocities and the associated erosion damage rate of a blade are analysed for various blade azimuth angles. The study shows that the rainfall intensity and turbulence intensity minorly influence the impact velocity and pressure but have a substantial effect on the overall erosion damage rate. Additionally, a significantly higher erosion damage rate is found for blades exposed to offshore rainfall conditions than for blades under onshore rainfall conditions. Furthermore, no substantial influence on erosion is found because of wave-induced loads.

• Ren, Zhengru; Skjetne, Roger; Verma, Amrit Shankar; Jiang, Zhiyu; Gao, Zhen & Halse, Karl Henning (2021). Active heave compensation of floating wind turbine installation using a catamaran construction vessel. Marine Structures. ISSN 0951-8339. 75. doi: 10.1016/j.marstruc.2020.102868. Show summary

  The application of floating wind turbines is limited by the high cost that increases with the water depth. Offshore installation and maintenance continue to consume a high percentage of the project budget. To improve the installation efficiency of the floating offshore wind turbine, a novel concept is proposed by the SFI MOVE project. Several wind turbine superstructure components are preassembled onshore and carried to the installation site by a catamaran construction vessel. Each assembly can then be installed using only one lift, and the concept is less sensitive to weather conditions. In this paper, a control algorithm of the proposed hydraulic active heave compensator system is developed using singular perturbation theory to cancel the relative motion between the spar top and gripped preassembly bottom. Closed-loop stability is proven, and the simulation results show that the installation efficiency is improved with an increase in the acceptable weather conditions.

• Wu, Haoyu; Zhao, Yongsheng; He, Yanping; Jiang, Zhiyu; Shao, Yanlin & Han, Zhaolong [Show all 8 contributors for this article] (2020). Transient response of a tlp-type floating offshore wind turbine under tendon failure conditions. ISOPE - International Offshore and Polar Engineering Conference. Proceedings. ISSN 1098-6189. 2020-, p. 317–325.
• Jiang, Zhiyu; Bjørnholm, Marius; Guo, Jiamin; Dong, Wenbin; Ren, Zhengru & Verma, Amrit Shankar (2020). Damage identification of a jacket support structure for offshore wind turbines, 2020 IEEE 15th Conference on Industrial Electronics and Applications (ICIEA). IEEE conference proceedings. ISSN 978-1-7281-5169-4. p. 995–1000. doi: 10.1109/ICIEA48937.2020.9248415. Full text in Research Archive
• Song, Ming; Jiang, Zhiyu & Yuan, Wei (2020). Numerical and analytical analysis of a monopile-supported offshore wind turbine under ship impacts. Renewable Energy. ISSN 0960-1481. 167, p. 457–472. doi: 10.1016/j.renene.2020.11.102. Full text in Research Archive
• Wang, Yipin; Zhao, Yongsheng; He, Yanping; Jiang, Zhiyu; Han, Zhaolong & Mao, Wengang [Show all 7 contributors for this article] (2020). Performance of Tuned Mass Dampers for Vibration Reduction in a TLP Floating Wind Turbine. In ISOPE, ISOPE (Eds.), Proceedings of the Thirtieth (2020) International Ocean and Polar Engineering Conference Shanghai, China, October 11-16, 2020. International Society of Offshore & Polar Engineers. ISSN 978-1-880653-84-5.
• Verma, Amrit Shankar; Castro, Saullo GP; Jiang, Zhiyu; Hu, Weifei & Teuwen, Julie JE (2020). Leading edge erosion of wind turbine blades: Effects of blade surface curvature on rain droplet impingement kinematics. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. 1618. doi: 10.1088/1742-6596/1618/5/052003. Full text in Research Archive
• Zhang, Lixian; Shi, Wei; Karimirad, Madjid; Michailides, Constantine & Jiang, Zhiyu (2020). Second-order hydrodynamic effects on the response of three semisubmersible floating offshore wind turbines. Ocean Engineering. ISSN 0029-8018. 207. doi: 10.1016/j.oceaneng.2020.107371. Full text in Research Archive
• Feng, Aichun; Kang, Hooi Sang; Zhao, Binbin & Jiang, Zhiyu (2020). Two-dimensional numerical modelling of a moored floating body under sloping seabed conditions. Journal of Marine Science and Engineering (JMSE). ISSN 2077-1312. 8(6). doi: 10.3390/JMSE8060389. Full text in Research Archive
• Verma, Amrit Shankar; Castro, Saullo GP; Jiang, Zhiyu & Teuwen, Julie (2020). Numerical investigation of rain droplet impact on offshore wind turbine blades under different rainfall conditions: A parametric study. Composite structures. ISSN 0263-8223. 241. doi: 10.1016/j.compstruct.2020.112096. Full text in Research Archive
• Chen, Xiaolu; Jiang, Zhiyu; Li, Qinyuan; Li, Ye & Ren, Nianxin (2020). Extended Environmental Contour Methods for Long-Term Extreme Response Analysis of Offshore Wind Turbines. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 142(5). doi: 10.1115/1.4046772. Full text in Research Archive Show summary

  Environmental contour method is an efficient method for predicting the long-term extreme response of offshore structures. The traditional environmental contour is obtained using the joint distribution of mean wind speed, significant wave height, and spectral peak period. To improve the accuracy of traditional environmental contour method, a modified method was proposed considering the non-monotonic aerodynamic behavior of offshore wind turbines. Still, the modified method assumes constant wind turbulence intensity. In this paper, we extend the existing environmental contour methods by considering the wind turbulence intensity as a stochastic variable. The 50-year extreme responses of a monopile-based offshore wind turbine are compared using the extended environmental contour methods and the full long-term method. It is found that both the environmental contour method and the modified environmental contour method, with the wind turbulence intensity included as an individual variable, give more accurate predictions compared with those without. Using the full long-term method as a benchmark, this extended approach could reduce the nonconservatism of the environmental contour method and conservatism of the modified environmental contour method. This approach is effective under wind-dominated or combined wind-wave loading conditions, but may not be as important for wave-dominated conditions.

• Liang, Guodong; Merz, Karl Otto & Jiang, Zhiyu (2020). Modeling of a Shared Mooring System for a Dual-Spar Configuration, ASME 2020 39th International Conference on Ocean, Offshore and Arctic Engineering - Volume 9: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8441-6. doi: 10.1115/OMAE2020-18467. Full text in Research Archive
• Gran, Viktor; Jiang, Zhiyu & Pan, Zhiyuan (2020). Hydrodynamic Analysis of Floating Docks With Alternative Geometries for Floating Wind Turbine Installation, ASME 2020 39th International Conference on Ocean, Offshore and Arctic Engineering - Volume 6A: Ocean Engineering. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8437-9. doi: 10.1115/OMAE2020-18756.
• Jiang, Zhiyu; Skrudland, Trond Kvia; Karimirad, Madjid; Constatine, Machiladies & Shi, Wei (2020). Effect of a Passive Tuned Mass Damper on Offshore Installation of a Wind Turbine Nacelle, ASME 2020 39th International Conference on Ocean, Offshore and Arctic Engineering - Volume 6A: Ocean Engineering. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8437-9. doi: https:/doi.org/10.1115/OMAE2020-18171. Full text in Research Archive
• Verma, Amrit; Jiang, Zhiyu; Ren, Zhengru & Teuwen, Julie J.E. (2020). Leading Edge Erosion of Wind Turbine Blades: Effects of Environmental Parameters on Impact Velocities and Erosion Damage Rate, ASME 2020 39th International Conference on Ocean, Offshore and Arctic Engineering - Volume 9: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-8441-6. doi: 10.1115/OMAE2020-18173. Full text in Research Archive
• Jiang, Zhiyu; Yttervik, Rune; Gao, Zhen & Sandvik, Peter Christian (2020). Design, Modelling and Analysis of a Large Floating Dock for Spar Floating Wind Turbine Installation. Marine Structures. ISSN 0951-8339. 72. doi: 10.1016/j.marstruc.2020.102781. Full text in Research Archive Show summary

  Installation of floating wind turbines at the offshore site is a challenging task. A significant part of the time efficiency and costs are related to the installation methods which are sensitive to weather conditions. This study investigates a large floating dock concept, which can be used to shield a floating wind turbine during installation of tower, nacelle, and rotor onto a spar foundation. In this paper, the concept is described in detail, and a design optimisation is carried out using simple design constraints. Hydrodynamic analysis and dynamic response analysis of the coupled system of the optimum dock and spar are conducted. Two spars of different sizes are considered, and the motion responses of the spars with and without the dock in irregular waves are compared. Through analysis of the motion spectra and response statistics, dynamic characteristics of the coupled system is revealed. The present design of the dock reduces the platform-pitch responses of the spars and potentially facilitates blade mating, but may deteriorate the heave velocity of the spars in swell conditions. Finally, future design aspects of the floating dock are discussed.

• Verma, Amrit Shankar; Jiang, Zhiyu; Gao, Zhen & Vedvik, Nils Petter (2020). Effects of a Passive Tuned Mass Damper on Blade Root Impacts During the Offshore Mating Process. Marine Structures. ISSN 0951-8339. 72. doi: 10.1016/j.marstruc.2020.102778. Full text in Research Archive
• Verma, Amrit Shankar; Jiang, Zhiyu; Ren, Zhengru; Gao, Zhen & Vedvik, Nils Petter (2020). Effects of Wind-Wave Misalignment on a Wind Turbine Blade Mating Process: Impact Velocities, Blade Root Damages and Structural Safety Assessment. Journal of Marine Science and Application. ISSN 1671-9433. 19, p. 218–233. doi: 10.1007/s11804-020-00141-7. Full text in Research Archive
• Guachamin-Acero, Wilson; Jiang, Zhiyu & Li, Lin (2020). Numerical study of a concept for major repair and replacement of offshore wind turbine blades. Wind Energy. ISSN 1095-4244. 23(8), p. 1673–1692. doi: 10.1002/we.2509. Full text in Research Archive
• Ren, Zhengru; Skjetne, Roger; Jiang, Zhiyu & Gao, Zhen (2020). Active Single-Blade Installation Using Tugger Line Tension Control and Optimal Control Allocation. International Journal of Offshore and Polar Engineering. ISSN 1053-5381. 30(2), p. 220–227. doi: 10.17736/ijope.2020.jc759. Show summary

  The single-blade installation is a common method for the installation of wind turbine blades. In an offshore installation, a jackup vessel is often involved, and a crane is used to lift, move, and bolt each blade onto the rotor hub at the tower top. To reduce the blade pendular motions, tugger lines are connected to the suspended blade. Active control of the tension force on the tugger lines has been recently investigated to reduce the blade motion. In this situation, a pre-tension is needed during the mating process, as only positive tension can be provided by the tugger lines. To further improve the effectiveness of active force control, we propose an active control strategy with a three-tugger-line configuration in this work. The placement of the third tugger line is examined. The proportional–integral–derivative (PID) control strategy is adopted, and allocation is achieved by convex programming. Aeroelastic simulations are carried out to verify the active control scheme under turbulent wind conditions. The results show that the proposed active control scheme is an effective means of reducing the translational motion of the blade root relative to the hub in the mean wind direction.

• Kuang, Limin; Su, Jie; Chen, Yaoran; Han, Zhaolong; Zhou, Dai & Zhao, Yongsheng [Show all 8 contributors for this article] (2019). Flow characteristics and dynamic responses of a parked straight‐bladed vertical axis wind turbine. Energy Science & Engineering. ISSN 2050-0505. 7(5), p. 1767–1783. doi: 10.1002/ese3.389. Full text in Research Archive
• Chen, Xiaolu; Jiang, Zhiyu; Li, Qinyuan & Li, Ye (2019). Effect of Wind Turbulence on Extreme Load Analysis of an Offshore Wind Turbine . In ASME, . (Eds.), ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering. Volume 1: Offshore Technology; Offshore Geotechnics. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-5876-9. doi: 10.1115/OMAE2019-95634. Full text in Research Archive
• Shi, Wei; Zhang, Lixian; Ning, Dezhi; Jiang, Zhiyu; Michailides, Constantine & Karimirad, Madjid (2019). A Comparative Study on the Dynamic Response of Three Semisubmersible Floating Offshore Wind Turbines. In ASME, . (Eds.), ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering. Volume 10: Ocean Renewable Energy. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-5889-9. doi: 10.1115/OMAE2019-96221. Full text in Research Archive
• Verma, Amrit Shankar; Gao, Zhen; Jiang, Zhiyu; Ren, Zhengru & Vedvik, Nils Petter (2019). Structural Safety Assessment of Marine Operations From a Long-Term Perspective: A Case Study of Offshore Wind Turbine Blade Installation. In ASME, . (Eds.), ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering. Volume 3: Structures, Safety, and Reliability. The American Society of Mechanical Engineers (ASME). ISSN 978-0-7918-5878-3. doi: 10.1115/OMAE2019-96686. Full text in Research Archive Show summary

  A marine operation is a complex non-routine activity of limited duration carried out in offshore environment. Due to safety reasons, these operations are normally performed within specific sea state limits, which are derived from numerical modelling and analysis of hazardous events. In view of the uncertainties in the assessment of structural responses under stochastic environmental conditions, these limiting curves correspond to a target structural failure probability recommended in offshore standards (for example, 10−4 per operation as specified by DNV-GL). However, one of the main limitations is that these curves do not reflect site-specific safety assessment. The current paper presents a novel methodology for assessing the structural safety level of marine operations from a long-term perspective. The methodology includes estimation of extreme response distribution under all possible operational sea states (i.e. the operational domain under the limiting sea states) for a given offshore site and is compared to the response limit to obtain an average failure probability. A case study is also presented for a blade root mating process onto preassembled hub using a jack-up crane vessel and risk of impact between root and hub is considered critical. Global time-domain simulations are performed using multibody dynamics, and extreme value distributions for impact velocities are derived for different wind-wave conditions. The allowable impact velocity between the blade root and the hub is determined by an explicit finite element analysis of the damage at the blade root. Finally, the average failure probabilities considering the operational domain are obtained for four different European offshore sites and are compared to the target level of structural failure probability considered for the limiting sea states.

• Li, Lin; Jiang, Zhiyu; Ong, Muk Chen & Hu, Weifei (2019). Design optimization of mooring system: An application to a vessel-shaped offshore fish farm. Engineering structures. ISSN 0141-0296. 197. doi: 10.1016/j.engstruct.2019.109363.
• Xu, Qianlong; Li, Ye; Yu, Yi-Hsiang; Ding, Boyin; Jiang, Zhiyu & Lin, Zhiliang [Show all 7 contributors for this article] (2019). Experimental and numerical investigations of a two-body floating-point absorber wave energy converter in regular waves. Journal of Fluids and Structures. ISSN 0889-9746. 91. doi: 10.1016/j.jfluidstructs.2019.03.006.
• Verma, Amrit Shankar; Jiang, Zhiyu; Ren, Zhengru; Gao, Zhen & Vedvik, Nils Petter (2019). Response-Based Assessment of Operational Limits for Mating Blades on Monopile-Type Offshore Wind Turbines. Energies. ISSN 1996-1073. 12(10). doi: 10.3390/en12101867. Full text in Research Archive Show summary

  Installation of wind-turbine blades on monopile-type offshore wind turbines is a demanding task. Typically, a jack-up crane vessel is used, and blades are individually lifted from the vessel deck and docked with the preinstalled hub. During the process of mating, large relative motions are developed between the hub and root due to combined effects of wind-generated blade-root responses and wave-generated monopile vibrations. This can cause impact loads at the blade root and induce severe damages at the blade-root connection. Such events are highly likely to cause the failure of the mating task, while affecting the subsequent activities, and thus require competent planning. The purpose of this paper is to present a probabilistic response-based methodology for estimating the allowable sea states for planning a wind-turbine blade-mating task, considering impact risks with the hub as the hazardous event. A case study is presented where the installation system consisting of blade-lift and monopile system are modelled using multibody formulations. Time-domain analyses are carried out for various sea states, and impact velocities between root and hub are analyzed. Finally, an extreme value analysis using the Gumbel fitting of response parameters is performed and limiting sea state curves are obtained by comparing characteristic extreme responses with allowable values. It is found that the limiting sea states for blade-root mating tasks are low for aligned wind–wave conditions, and further increase with increased wind–wave misalignment. The results of the study also show that the parameter Tp is essential for estimating limiting sea states given that this parameter significantly influences monopile vibrations during the blade-root mating task. Overall, the findings of the study can be used for a safer and more cost-effective mating of wind-turbine blades.

• Li, Lin; Jiang, Zhiyu; Wang, Jungao & Ong, Muk Chen (2019). Numerical study on the heading misalignment and current velocity reduction of a vessel-shaped offshore fish farm. Journal of Offshore Mechanics and Arctic Engineering. ISSN 0892-7219. 141(5), p. 1–9. doi: 10.1115/1.4042266. Full text in Research Archive
• Wu, Xiaoni; Hu, Yu; Li, Ye; Yang, Jian; Duan, Lei & Wang, Tongguang [Show all 12 contributors for this article] (2019). Foundations of offshore wind turbines: A review. Renewable and Sustainable Energy Reviews. ISSN 1364-0321. 104, p. 379–393. doi: 10.1016/j.rser.2019.01.012. Full text in Research Archive
• Ren, Zhengru; Skjetne, Roger; Jiang, Zhiyu; Gao, Zhen & Verma, Amrit Shankar (2019). Integrated GNSS/IMU Hub Motion Estimator for Offshore Wind Turbine Blade Installation. Mechanical systems and signal processing. ISSN 0888-3270. 123, p. 222–243. doi: 10.1016/j.ymssp.2019.01.008. Full text in Research Archive Show summary

  Offshore wind turbines (OWTs) have become increasingly popular for their ability to harvest clean offshore wind energy. Bottom-fixed foundations are the most used foundation type. Because of its large diameter, the foundation is sensitive to wave loads. For typical manually assisted blade-mating operations, the decision to perform the mating operation is based on the relative distance and velocity between the blade root center and the hub, and in accordance with the weather window. Hence, monitoring the hub real-time position and velocity is necessary, whether the blade installation is conducted manually or automatically. In this study, we design a hub motion estimation algorithm for the OWT with a bottom-fixed foundation using sensor fusion of a global navigation satellite system (GNSS) and an inertial measurement unit (IMU). Two schemes are proposed based on a moving horizon estimator, a multirate Kalman filter, an online smoother, and a predictor. The moving horizon estimator mitigates the slow GNSS sampling rate relative to the hub dynamics. The multirate Kalman filter estimates the position, velocity, and accelerometer bias with a constant GNSS measurement delay. The online smoothing algorithm filters the delayed estimated trajectory to remove sudden step changes. The predictor compensates the delayed estimate, resulting in real-time monitoring. HAWC2 and MATLAB are used to verify the performance of the estimation algorithms, showing that a sufficiently accurate real-time position and velocity estimate with a high sampling rate is achieved. A sensitivity study compares the accuracy of different algorithms applied in various conditions. By combining both proposed algorithms, a sufficiently accurate estimation can be achieved for a wider scope of practical applications.

• Verma, Amrit Shankar; Jiang, Zhiyu; Vedvik, Nils Petter; Gao, Zhen & Ren, Zhengru (2019). Impact assessment of a wind turbine blade root during an offshore mating process. Engineering structures. ISSN 0141-0296. 180, p. 205–222. doi: 10.1016/j.engstruct.2018.11.012. Full text in Research Archive
• Verma, Amrit Shankar; Vedvik, Nils Petter; Haselbach, Philipp Ulrich; Gao, Zhen & Jiang, Zhiyu (2019). Comparison of numerical modelling techniques for impact investigation on a wind turbine blade . Composite structures. ISSN 0263-8223. 209, p. 856–878. doi: 10.1016/j.compstruct.2018.11.001. Full text in Research Archive Show summary

  Wind turbine blades are exposed to numerous impact risks throughout their lifetimes. The impact risks range from bird collisions during operation to impacts with surrounding structures at the time of transportation and installation. Impact loads on the fibre composite blades can induce several complex, simultaneously interacting and visually undetectable damage modes and have a high potential to reduce the local and global blade stiffness. An assessment of such impact-induced damages is therefore necessary and usually involves high computational costs using numerical procedures, especially when analysing large composite components. To minimise this computational expense, different numerical impact modelling techniques are utilised, primarily shell-element-based approaches and multiscale-modelling-based global-local approaches. In this article, a comparison between (1) pure shell, (2) shell-to-solid coupling, and (3) submodelling finite element modelling techniques using Abaqus/Explicit is presented for a case where an impactor hits the leading edge of a blade. A high-fidelity local solid finite element model is developed for the leading edge of a DTU 10 MW blade at the region of impact and its stiffness is compared with baseline. A user material subroutine VUMAT for the intralaminar damage mode based on the Hashin failure criterion is formulated and then validated via an experiment from the literature. Finally, based on different numerical modelling techniques, impact investigations are performed, and the impact responses, damage to the blade and computational analysis durations are compared. It is found that the submodelling-based global-local approach is the most efficient analysis technique for this case, capturing failure modes including delamination, core crushing and local surface indentation in the blade. The findings of this study can be used to develop accurate and computationally efficient tools for modelling impact-induced damage to a blade.

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• Jiang, Zhiyu (2023). Accidental scenarios of offshore wind farms: numerical simulations.
• Jiang, Zhiyu (2023). Floating solar tech for harsh offshore conditions. [Business/trade/industry journal]. https://www.pv-magazine.com/2023/06/14/floating-solar-tech-f.
• Jiang, Zhiyu (2023). Recent developments in offshore floating photovoltaic concepts.
• Jiang, Zhiyu (2022). Ulykkescenarioer for havvindparker.
• Jiang, Zhiyu (2022). Assessment of accidental scenarios for offshore wind farms.
• Jiang, Zhiyu (2022). Utdanning og forskning på havvind.
• Cheynet, Etienne; Li, Lin & Jiang, Zhiyu (2022). Metocean Conditions at Two Norwegian Sites for Development of Offshore Wind Farms.
• Hassani, Milad; Li, Lin; Kalofotias, Filippos & Jiang, Zhiyu (2022). Hydrodynamic performance evaluation of a semi-submersible floater using open-source tools. Show summary

  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.

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

  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.

• Jiang, Zhiyu (2022). Mooring system design challenges for modular floating energy islands. Show summary

  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.

• Sanfilippo, Filippo; Thorstensen, Rein Terje; Jha, Ajit; Jiang, Zhiyu; Knausgård, Kristian Muri & Robbersmyr, Kjell Gunnar (2021). A Perspective of Road Infrastructure and Bridges Digital Twins.
• Jiang, Zhiyu (2021). Installation of offshore wind turbines: Challenges and Opportunities . Show summary

  In this talk, an overview is provided of various installation methods for offshore wind turbines. The concepts of single-blade installation, catamaran installation vessel and floating dock are respectively reviewed, and numerical simulations results are discussed. Finally, future research areas for offshore wind turbine installations are presented.

• Karimirad, Madjid & Jiang, Zhiyu (2020). Mechanical-Dynamic Loads. In Elias, Scott A. (Eds.), Reference Module in Earth Systems and Environmental Sciences. Elsevier. ISSN 978-0-12-409548-9. doi: https:/doi.org/10.1016/B978-0-12-819727-1.00075-3. Show summary

  Wind turbine technology has been growing rapidly in the past decade. The innovative concepts, larger onshore and offshore turbines as well as floating turbines require dynamic analysis considering comprehensive load modeling. Loads in the operational and survival conditions including fault cases, controller actions and response-induced forces govern the design and the limit states requirements associated with it. The wind turbine industry has good experience in bottom-fixed applications. However, when it comes to floating wind turbine technology, there are only a few commercial cases in operation. Further research is needed to fill the gap and enhance the floating wind turbine technology. This article provides an introduction to loads for both onshore and offshore wind turbines with a focus on the wave- and wind-induced loads as well as controller loads and load actions to assess the structural integrity and power performance of wind turbines.

• Jiang, Zhiyu & Shi, Wei (2020). Analysis of Renewable Energy Devices. In Cui, Weicheng; Fu, Shixiao & Hu, Zhiqiang (Ed.), Encyclopedia of Ocean Engineering. Springer. ISSN 978-981-10-6963-5. doi: https:/doi.org/10.1007/978-981-10-6963-5_192-1.
• Jiang, Zhiyu (2020). Design of Renewable Energy Devices. In Cui, Weicheng; Fu, Shixiao & Hu, Zhiqiang (Ed.), Encyclopedia of Ocean Engineering. Springer. ISSN 978-981-10-6963-5. doi: 10.1007/978-981-10-6963-5_198-1.
• Hall, Matthew; Lozon, Ericka; McAuliffe, Fiona Devoy; Bessone, Matteo Baudino; Bayati, Ilmas & Bowie, Malcolm [Show all 28 contributors for this article] (2024). The IEA Wind Task 49 Reference Floating Wind Array Design Basis. IEA Wind TCP.
• 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.
• Mahdavipour, Mohammad Ali; Vysochinskiy, Dmitry & Jiang, Zhiyu (2022). Failure prediction of steel components under the coupled effect of excessive plastic deformations and pitting corrosion. University of Agder. ISSN 978-82-8427-073-9.

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