Offshore Wind Turbine System Research Articles

Control Structures for Combined H2/Electricity from Offshore Wind Turbines

Wind energy proves to be a highly favourable choice for electricity generation due to its clean and renewable nature, and is playing a significant role in reducing global greenhouse gas emissions. Offshore wind turbine systems have gained widespread popularity as they can capitalise on elevated and consistent wind speeds surpassing those found in onshore locations, resulting in increased energy efficiency. Furthermore, offshore wind power possesses the potential to emerge as a significant electricity source for the production of green hydrogen. As water electrolysis technology for hydrogen production continues to advance, utilizing offshore wind power for hydrogen generation is becoming more economically viable and practical. Offshore wind power with higher wind speeds in combination with efficient control structures presents an attractive option for electricity generation and hydrogen co-production. This paper aims to present and evaluate four different production structures for combined H2/energy generation from offshore wind turbines. Previous research studies in this area often overlook control structures and lack information on power converter operations. In contrast, this article studies control structures that enable proper functionality and ensure adequate interoperability, enhancing the reliability of renewable energy integration. Each structure, including both wind turbines and electrolyser, is described in detail, along with the corresponding controllers. Simulation results are presented for each structure and controller to demonstrate their effective operation.

The Method of the Natural Frequency of the Offshore Wind Turbine System Considering Pile–Soil Interaction

Accurately and efficiently evaluating the influence of pile–soil interaction on the overall natural frequency of wind turbines is one of the difficulties in current offshore wind power design. To improve the structural safety and reliability of the offshore wind turbine (OWT) systems, a new closed-form solution method of the overall natural frequency of OWTs considering pile–soil interactions with highly effective calculations is established. In this method, Hamilton’s principle and the equivalent coupled spring model (ECS model) were firstly combined. In Hamilton’s theory, the Timoshenko beam assumption and continuum element theory considering the three-dimensional displacement field of soil were used to simulate the large-diameter monopile–soil interaction under lateral load in multilayer soil. Case studies were used to validate the proposed method’s correctness and efficiency. The results show that when compared with the data of 13 offshore wind projects reported in existing research papers, the difference between the overall natural frequency calculated by the proposed method and that reported in this study is within ±10%. This calculation method achieves the goal of convenient, fast and accurate prediction of the overall natural frequency of offshore wind systems.

Effect of the transition piece on the natural frequencies of monopile-supported offshore wind turbines

The lateral natural frequency of an offshore wind turbine (OWT) is a crucial factor to consider in the design of OWT foundations. Eighty percent of currently installed OWTs are mounted on monopile foundations, which are connected to the superstructure via a transition piece (TP). To explore the differences in natural frequencies between grouted connection (GC) and TP-less designs, this study incorporates a transition piece into the calculation model of the OWT system. Based on the Timoshenko beam theory and transfer matrix method, the characteristic equations of the lateral natural frequency of the OWT are derived. The study found that the effect of the transition piece on the natural frequency is strongly influenced by the OWT type and the soil stiffness, and the influence of transition piece on different order natural frequencies is different for the same OWT. For the NREL offshore 5-MW baseline wind turbine, the TP-less model reduces the first natural frequency of the OWT by 1.31 % and elevates the second and third natural frequencies by 11.88 % and 2.72 %, respectively, compared to the GC model. The increase in soil shear wave velocity amplifies the influence of the transition piece on the first and third natural frequencies, while weakening its influence on the second natural frequency.

Analytical approach for lateral dynamic responses of offshore wind turbines supported by cement-soil composite steel pipe pile embedded in gassy marine sediment layer

This study aims to clarify and gain an insight into the lateral dynamic response of offshore wind turbines (OWTs) supported by cement-soil composite steel (CCS) pipe pile embedded in gassy marine sediment layer by developing an analytical approach. In the proposed approach, the CCS pipe pile-supported OWT system is divided into three parts including the tower part, the seawater-immersed steel pipe pile part, and the embedded CCS pipe pile part. The lateral dynamic behaviours of the CCS pipe pile-supported OWT system have been determined and then validated by solving the analytical solution of lateral displacement for the above three parts. Numerical discussions are finally conducted to analysis the influence of some physical parameters on the lateral displacement and the first-order natural frequency of the CCS pipe pile-supported OWT system. The main findings can be summarized as: (i) the offshore deep cement-soil mixing (ODCM) pile is an effective reinforcement method to reduce the lateral vibration of conventional steel pipe pile-supported OWTs; (ii) the increase in tower height and seawater layer thickness will have a serious negative impact on the lateral dynamic response of CCS pipe pile-supported OWTs; (iii) the increase in gas saturation of gassy marine soil will lead to a negative influence on the maximum lateral displacement of CCS pipe pile-supported OWTs.

Seismic response of the composite bucket foundation supported offshore wind turbines under marine environmental loadings

Many offshore wind turbine (OWT) foundations are situated in earthquake-prone areas, where complex environmental loads intensified the seismic responses of the OWTs. In response to this situation, the composite bucket foundations is proposed and extensively applied due to its reliable installation and high seismic resistance. To analyze the seismic response of composite bucket foundations subjected to environmental loads, refined numerical models of the 6.45 MW OWT system are established, and parametric analyses are conducted, considering the influence of seismic frequency contents and intensities, the soil layer distributions and wind velocities. The results show that the composite bucket foundations undergo cumulative rotational deformation around a point biased toward the windward side of the foundation under the seismic and environmental loads. The increasing mean wind velocity significantly enlarges the displacement response envelope, while has limited influence of the acceleration response envelope of OWTs. Increasing the bucket diameter enhances the deformation resistance of the OWT, while the efficiency of this improvement diminishes as the diameter increases. Greater bucket skirt length also enhances its deformation resistance, but may lead to increased lateral acceleration response of the OWT.

Simultaneous design optimisation methodology for floating offshore wind turbine substructure and feedback-based control strategy

This research article explores the application of control co-design methodologies for optimising floating offshore wind turbine systems concurrently. The primary objective is to offer insights into concurrent design approaches employing an advanced genetic optimisation algorithm. To achieve this, a reduced-order dynamic model is employed to minimise computational time requirements, complemented by a modified version of the levelized cost of energy equation serving as the cost function. Furthermore, various optimisation scenarios are investigated under diverse wind and wave conditions to assess the advantages and drawbacks of increasing the complexity of dynamic cases used in evaluating the cost function. The optimised system designs are then compared against baseline floating system designs to underscore the advantages of employing this approach to floating wind turbine design.

A Fixed-Time Robust Controller Based on Zeroing Neural Dynamics for Projective Synchronization of Offshore Wind Turbine Systems

A Fixed-Time Robust Controller Based on Zeroing Neural Dynamics for Projective Synchronization of Offshore Wind Turbine Systems

LTFM-net framework: Advanced intelligent diagnostics and interpretability of insulated bearing faults in offshore wind turbines under complex operational conditions

The rapid advancement of intelligent theories and models, exemplified by deep learning, has achieved remarkable success across numerous fields. However, given that the complexity and variability of offshore wind turbine systems, particularly in the context of high-power variable-frequency control of insulated bearings in offshore wind turbines, the problem of fault identification has become a recognized technical challenge. Additionally, fully exploiting the temporal characteristics of insulated bearing fault data poses a key problem that demands urgent resolution. To address these gaps, this paper pioneers a novel Lightweight Temporal Feature-focused framework, named LTFM-Net, aimed at solving the difficult problem of identifying insulated bearing faults in offshore wind turbines in practical engineering applications. Specifically, this framework enables the intelligent identification of insulated bearing faults under harsh operating conditions such as alternating voltage and variable loads, marking a first in this field. Furthermore, an innovative strategy named Weighted Diminish Recurrent Unit (WDRU) was developed, along with the derivation of its backpropagation formula, which is innovatively applied to the feature extraction module of the LTFM-Net framework for the first time. Thus, an efficient method for acquiring fault data from insulated bearings in offshore wind turbines was proposed. Based on a unified dataset, the diagnostic performance of the LTFM-Net framework was evaluated and compared with seven advanced methods. The results demonstrate that the LTFM-Net achieves precise identification of insulated bearing faults, confirming its excellent generalization, robustness, and superiority. The introduction of t-SNE for visualizing the fault characteristics of insulated bearings uncovered by the LTFM-Net framework further enhances its reliability, accuracy, and credibility.

Shaking table tests and numerical analysis of monopile-supported offshore wind turbines under combined wind, wave and seismic loads

Currently, large-diameter monopile-supported offshore wind turbines (OWTs) are widely used in offshore areas of China. However, considerable numbers of wind farms are located in seismically active areas with liquefiable soil layers. Therefore, shaking table tests and numerical simulation based on a scaled model of Siemens SWT-4.0-130 OWTs were carried out to investigate the failure mechanism of the OWT system in liquefiable sand under combined wind, wave and seismic loads. The scaling law focusing on the governing physical mechanism of the OWT system considering multi-field coupling are presented and verified. The dynamic responses of the OWT and the excess pore pressure fluctuation around the soil-monopile domain in different operating states are discussed and compared. The results show that the proposed scaling laws can well restore the dynamic characteristics of the prototype. The seismic excitation is the dominant load for inducing the structural response under combined wind, wave and seismic loads. Due to the coupling effect, the dynamic responses under combined wind and wave loads were slightly less than the superposition of structural responses under separated wind or wave loads. Liquefaction of the shallow soil layer within the range of four times the pile diameter was detected when the PGA reached 0.4 g (three-dimensional input) or 0.64 g (one-dimensional input), which induced obvious tilt and settlement of the OWT model after the test. The sudden increase in the pore pressure resulted in the liquefaction of soil, which further led to a significant decrease in the natural frequencies and decay of the acceleration responses at the tower top. The above findings are expected to provide valuable experience for the 1 g underwater shaking table tests on OWT systems and insights for the practical design of OWTs in seismically active areas.

Metaheuristic Airflow control for vibration mitigation of a hybrid oscillating water Column-Floating offshore wind turbine system

Unlike the fixed wind turbines, the structure of Floating Offshore Wind Turbines (FOWT) have the added motions of six degrees of freedom induced by the wind, wave and tidal loads. These motions lead to vibration and the degradation of the structure. This paper presents a novel approach to model and stabilize the FOWT by employing the Oscillating Water Columns (OWC) as active structural control system. The innovative concept involves designing a new floating barge-type platform with integrated OWCs on opposite sides of the platform to mitigate undesired oscillations of the system. These OWCs counteract the bending forces caused by wind on the tower and waves on the barge platform. To synchronize the opposing forces with the system’s tilting, a proposed Particle Swarm Optimization with Decreasing Inertia-based Adaptive Neuro-Fuzzy Inference System (PSODI-ANFIS) airflow control strategy is employed. Through manipulation of the barge platform’s pitch angle, the PSODI-ANFIS airflow control system adjusts the valves on either side, opening one and closing the other accordingly. Simulation results, compared with the standard FOWT as well as the Fuzzy-based airflow control system, demonstrate the effectiveness of the PSODI-ANFIS airflow control. It is shown to be superior in reducing platform pitching and the fore-aft translation of the top tower.

Conceptual design of a novel 10 MW T-Semi floating offshore wind turbine and mooring system for transitional water depths

ABSTRACT A novel semi-submersible concept called T-Semi, along with several mooring systems designs, for a 10-MW FOWT is proposed and applied at a water depth of 60 m, to address the challenges of deploying floating offshore wind turbines (FOWTs) in transitional water depths. Comprehensive analyses were performed to analyze the performance of this innovative concept, including (a) intact and damaged stability; (b) frequency-domain hydrodynamic behaviour of the T-Semi floater; (c) different mooring system design configurations tailored for the T-Semi FOWT. The results demonstrate robust stability characteristics of the T-Semi floater and its favourable motion dynamics in heave and pitch. Additionally, the mooring system well satisfies the requirements of motion natural periods and safefy based on relevant international standards. In terms of the mooring design, the nonlinearity of the horizontal restoring force could be weakened by increasing the clump mass and decreasing the angle between mooring lines in a redundant arrangement.

Static and dynamic analysis of a new offshore wind turbine system supported with outriggers

This article presents an innovative structural system for offshore wind turbines supported on monopiles. The novelty of this system lies in its enhancement of the lateral stiffness of the tubular tower through the incorporation of an outrigger system, widely employed in high-rise buildings. This novel system is modeled as a Timoshenko beam-column element under both static and dynamic loading scenarios, incorporating geometric nonlinearity and accounting for the effect of soil-structure interaction. A comprehensive numerical analysis was conducted to evaluate the system’s behavior under various static and dynamic load cases, utilizing soil data from the Red Sea region of Egypt. A modal analysis is also incorporated to investigate the eigenmodes of the proposed system. This study offers valuable insights for designers and researchers, providing guidelines and diagrams for the implementation of the proposed system.

A Comprehensive Review on Advanced Control Methods for Floating Offshore Wind Turbine Systems above the Rated Wind Speed

This paper presents a comprehensive review of advanced control methods specifically designed for floating offshore wind turbines (FOWTs) above the rated wind speed. Focusing on primary control objectives, including power regulation at rated values, platform pitch mitigation, and structural load reduction, this paper begins by outlining the requirements and challenges inherent in FOWT control systems. It delves into the fundamental aspects of the FOWT system control framework, thereby highlighting challenges, control objectives, and conventional methods derived from bottom-fixed wind turbines. Our review then categorizes advanced control methods above the rated wind speed into three distinct approaches: model-based control, data-driven model-based control, and data-driven model-free control. Each approach is examined in terms of its specific strengths and weaknesses in practical application. The insights provided in this review contribute to a deeper understanding of the dynamic landscape of control strategies for FOWTs, thus offering guidance for researchers and practitioners in the field.

Experimental study on monopile-supported offshore wind turbine subjected to long-term wind and wave cyclic loading

Offshore wind energy has recently gained much attention. During its service life, a monopile-supported offshore wind turbine (OWT) is subjected to long-term wind and wave lateral cyclic loads with different cycle characteristics, inducing accumulated deformation of the OWT system to exceed the serviceability limit state and causing safety accidents. In this study, a 1g scaled model test with a similarity ratio of 1:100 was conducted to investigate the lateral response of a monopile-supported OWT in sand under long-term wind and wave cyclic loads. Two sets of centrifugal gear cyclic loading devices applied stable long-term wind and wave cyclic loads and different cyclic load amplitudes and directions were achieved by adjusting the input voltage and counterweight masses. Four groups of long-term cyclic loading tests were conducted for the monopile-supported OWT, considering different wind load simplification methods and various wind-wave load contribution ratios. The lateral displacement and accumulated tilt of the OWT were monitored using two laser displacement transducers installed at different heights. The results show that a simplistic treatment of wind loads as static loads results in an overestimation of the cumulative rotational deformation, and an increase in the wind-wave load contribution ratio decreases the cumulative tilts of the OWT structures.

A sequential three-way risk sorting model with the cautionary principle under probabilistic linguistic environment

Risk assessment plays a crucial role in managing risks associated with systems, products and services. In practical risk assessment, the cautionary principle is widely adopted by individuals to prevent accidents, and the degree of caution varies with the risk level of potential hazardous activities (PHAs). Therefore, it is necessary to explore a sequential risk assessment model integrating the cautionary principle. To this end, we design an improved risk assessment model based on sequential three-way decision (STWD) model and cautionary principle. Firstly, a risk distance measure under probabilistic linguistic environment is proposed to effectively reflect varying degrees of caution when individuals analyzed PHAs of different risk levels. Then, a comprehensive risk weight optimization model is constructed on the consideration of the importance ranking and correlations of risk attributes. Subsequently, STWD and the as low as reasonably practicable (ALARP) principle are integrated to build the risk sorting model, and both the risk prioritization and classification of PHAs are obtained from the sequential risk sorting process. Finally, we verify the reliability of our proposed model through an example of a floating offshore wind turbine system. The findings validate the crucial role of cautionary principle in risk assessment.

Analysis of Offshore Wind Turbine by Considering Soil-Pile-Structure Interaction: Effect of Sea-Wave Load Duration

ABSTRACT Offshore Wind Turbines (OWTs) confront different types of environmental loads during their lifetime. One of the most significant loads is the cyclic sea-wave load which affects the OWT system during the approximate 25 years of design life. This lateral cyclic load can influence the response of the OWT system because of accumulated permanent displacement and excess pore water pressure generation in soil. This procedure can change pile-soil stiffness. However, the behavior of OWT and its foundations is mostly studied under short-term cyclic loading, and the effects of duration of the cyclic loads on pile-soil interaction and performance of the foundation are not well understood and documented. Therefore, there is a lack of guidance in codes for the duration effects of cyclic loads on the structural and geotechnical response of OWTs. In this regard, the current study considers the effects of duration of the cyclic loads on serviceability and performance of the OWT system by considering soil-foundation-structure interaction using a 2-D finite element method. The behavior of the OWT system is evaluated based on the internal forces and deformation of the monopile foundation, shear strain, and excess pore water pressure ratio in the surrounding soil. Besides, liquefaction susceptibility in the sandy soil layer at the vicinity of the monopile and its effect on the performance of the foundation is investigated. Finally, the results can provide guidance on estimation of the dynamic performance of the OWT system during long-term cyclic loads. They can be used to specify the need for consideration of the duration of cyclic lateral loads for the design of OWT structures.

Catenary Mooring Length Control for Motion Mitigation of Semi-Submersible Floating Wind Turbines

Besides improving the generator torque and blade pitch controller, incorporating additional control actuations, such as a vibration absorber and active ballast, into the floating offshore wind turbine (FOWT) system is also promising for the motion mitigation of FOWTs. This work aims to study the catenary mooring length re-configuration effect on the dynamic behaviours of semi-submersible FOWTs. The mooring length re-configuration mentioned here is achieved by altering the mooring length with winches mounted on the floating platform, which is in a period of minutes to hours, so that the mooring tensions could be adjusted to reduce the aerodynamic load induced platform mean pitch. Control designs for both single mooring line and multiple mooring lines have been described and studied comparatively. In order to assess the motion mitigation performance of the proposed mooring line length re-configuration methods, fully coupled numerical simulations under different environmental cases have been conducted. Results indicate that the catenary mooring length re-configuration is able to reduce the platform pitch motion by up to 15.8% under rated condition, while careful attention must be paid to the scenarios where the catenary moorings become taut, which may lead to large load variations.

Influence of pulse-like motions and extreme environmental loads on the seismic foundation response of offshore wind turbines on layered liquefiable soils

Monopile foundations are known as the most common foundation solution for offshore wind turbines (OWTs). However, the state of practice for designing monopile foundations in high seismicity areas is still limited. In particular, the impact of soil liquefaction on the seismic soil-foundation-OWT interaction is not yet well understood. In this paper, three-dimensional (3D), fully-coupled, nonlinear finite-element analyses performed in the OpenSees numerical platform were used to evaluate the seismic performance of a series of hypothetical 5 MW OWTs on monopile foundations in layered, liquefiable sites. A suite of earthquake recordings with and without strong velocity pulses (i.e., near fault, pulse-like and ordinary motions, respectively) was used to investigate the impact of ground motion characteristics on the seismic response of the OWT system. Also, the influence of soil-structure interaction and earthquake shaking coupled with extreme environmental loading (i.e., wind and wave loads) on the seismic performance of soil-OWT systems was evaluated. The numerical results showed pile movements induced by extreme climate loading led to a bias in permanent settlement accumulation across the foundation area and accumulation of soil deformations in the proximity of the pile. Ground motion velocity pulses increased the cyclic stress demand in soil and, therefore, the potential for the occurrence of soil liquefaction. A subsequent, limited numerical sensitivity study showed that the foundation rotations of the OWT system were influenced by ground motion characteristics such as polarity and velocity pulses, and the presence of the wind and wave loads. The cumulative absolute velocity (CAV) was identified as the optimum ground motion intensity measure for permanent foundation settlement and tilt as well as for peak transient foundation tilt of the OWT system under extreme environmental loadings. The net outcome of these factors determined the magnitude and orientation of the foundation rotations at the end of shaking. This study highlights the importance of considering the effects of extreme loadings and pulse-like motions in the design and performance of OWT systems.

Random vibration analysis of nonlinear floating offshore wind turbine system via statistical linearization process under coupled nonlinear wave excitation

The work presented in this paper investigates the possibility of using statistical linearization approach to predict the stochastic dynamics of nonlinear floating offshore wind turbines (FOWT) system under coupled nonlinear wave excitation. OC3-Hywind Spar floating platform equipped with a 5 MW reference wind turbine offered by NREL is taken as the discussed FOWT of the study. A coupled nonlinear analytical model subjected to coupled irregular wave excitation is established to describe the dynamic characteristics of the FOWT, and further, the governing equation of the model is transformed into the equivalent linear one through the statistical linearization approach. The error caused in the linearization process is minimized through an iterative process. The accuracy, adaptability and efficiency of the model and the proposed method built in this paper are verified by numerical examples and comparison with other methods, such as Monte Carlo method and FAST etc. The research shows that the proposed method based on statistical linearization process to predict the stochastic dynamic response of nonlinear FOWT system under coupled nonlinear wave excitation has not only high accuracy but also high computational efficiency. It is about 4–5 orders of magnitude faster than the deterministic analysis method, and is of great significance for the design and research of the FOWT. An application example conducted in this paper indicates that this method is effective and can be used extensively, such as structural design optimization, reliability analysis, determination of the optimal parameters of vibration control and fatigue analysis of components, etc.

Harnessing the synergy of grout and adhesive: Numerical prediction of load capacity in hybrid composite joints

This study examines grouted joints for offshore wind turbine systems, where gaps between steel pipes are filled with grout, relying on adhesion, friction and compression struts formed by shear keys. Concerns about the limited load carrying capacity of large diameter grouted joints in offshore structures have prompted investigation. Whilst adhesive bonds are recognized for load transfer in structural steel, the complete replacement of grouting with bonding alone is proving impractical for large gaps in offshore structures, posing challenges in terms of cost and handling. To address these challenges, the study presents an innovative hybrid grouted joint for steel structures that combines grout and adhesive layers. This novel approach replaces traditional large shear keys with distributed micro-shear keys — small granules embedded in the grout material. Under axial loading, the hybrid joint demonstrates robust performance, with a maximum nominal shear stress on the inner pipe of 30.1 MPa and consistent load capacity across tests. Average shear strength is in line with, and occasionally exceeds, expectations. Notably, the hybrid joint shows resilience in different configurations and maintains a low coefficient of variation of 5.5%, indicating consistent performance. When the influence of the adhesive material on the hybrid joint is examined, the effect on the joint stiffness is minimal, despite variations in adhesive stiffness. Sikadur 370, which has a higher modulus of elasticity than DuploTEC, shows similar deformation curves in the elastic region, emphasizing the robustness of the joint. Differences in the maximum loads are attributed to the thickness of the adhesive layer and the lower modulus of DuploTEC, resulting in different shear strengths. The proposed hybrid joint offers a promising solution to improve the performance of conventional grouts and adhesives in offshore structures. The study also formulates a failure criterion, performs numerical stress analysis, and models the effect of geometric dimensions, eccentricity, and misalignment on load capacity. The methodology’s accuracy in predicting load capacity, supported by numerical analysis and large-scale joint simulations, contributes to a comprehensive understanding of hybrid joint performance in engineering applications.