Semi-submersible Wind Turbine Research Articles

Research on the Blades and Performance of Semi-Submersible Wind Turbines with Different Capacities

With the gradual increase in the maturity of wind energy technology, floating offshore wind turbines have progressively moved from small-capacity demonstrations to large-capacity commercial applications. As a direct component of wind turbines used to capture wind energy, an increase in the blade length directly leads to an increase in blade flexibility and a decrease in aerodynamic performance. Furthermore, if the floater has an additional six degrees of freedom, the movement and load of the blade under the combined action of wind and waves are more complicated. In this work, two types of semi-submersible wind turbines with different capacities are used as the research objects, and the load and motion characteristics of the blades of these floating offshore wind turbines are studied. Through the analysis of the simulation data, the following conclusions are drawn: with the increase in the capacity of the wind turbine, the flexible deformation of the blade increases, the movement range of the blade tip becomes larger, the blade root load increases, and the power fluctuation is more obvious. Compared with the bottom-fixed wind turbine, the flexible blade deformation of the floating offshore wind turbine is smaller; however, the blade root load is more dispersed, and the power output is more unstable and lower.

Power absorption and dynamic response analysis of a hybrid system with a semi-submersible wind turbine and a Salter's duck wave energy converter array

A multi-energy system that integrates a floating offshore wind turbine (FOWT) with wave energy converters (WEC) is an effective way to commercialize new offshore energy and the levelized cost of energy in the future. This paper proposes a FOWT-WEC hybrid system concept consisting of a semi-submersible FOWT with a Salter's duck (SD) WEC array. A coupling framework is established based on OpenFAST and ANSYS-AQWA. A preliminary feasibility study is conducted on the power absorption and motion response of the hybrid system to evaluate the interaction between the SD WEC array and the FOWT. Systematic studies demonstrate that the SD WEC array significantly reduces the motion response of the hybrid system in terms of pitch and heave. An increase in the equilibrium position of the platform in the surge direction does not have a large effect on the stability of the system. The WEC array improves the stability of wind power output and can be used as an effective supplement to power generation. The effect of wave direction shows that the SD WEC has a high sensitivity to the angle of incident waves, and the diffraction and radiation from the platform have a large impact on the power absorption of the SD WEC array.

Application of an active learning method for cumulative fatigue damage assessment of floating wind turbine mooring lines

Long-term cumulative fatigue damage of mooring lines is crucial for the design of floating wind turbine structures (FWTs). Although many efforts are carried out for the offshore floating platforms, there still needs to be an efficient approach for assessing the long-term fatigue damage of the floating wind turbine mooring lines due to the complex loading in FWTs. An active learning approach named AK-DA (Adaptive Kriging Damage Assessment) was recently proposed for the cumulative fatigue damage assessment of wind turbine structures. However, in the original work, the AK-DA approach was only tested on fatigue damage assessment of a 5MW wind turbine tower with monopile support structures. It is unclear whether it is applicable to other parts of the wind turbine system, especially considering the complex loading of FWTs. Therefore, in this work, the AK-DA approach is used to assess the cumulative fatigue damage of mooring lines. The Gaussian process regression (Kriging) model is used to predict the fatigue damage of the mooring line under different wind-wave cases. The cumulative fatigue damage of the mooring lines in the IEA 15MW semi-submersible wind turbines is assessed with the AK-DA approach. The numerical simulation results show that the AK-DA approach can efficiently and accurately estimate the cumulative fatigue damage of the mooring lines. Compared to the traditional simulation approach, the AK-DA approach can increase efficiency by more than 45 times, and the absolute error is less than 1%. This active learning approach could serve as a helpful tool for offshore mooring system designers, facilitating the cumulative fatigue damage assessment during the design process.

Effects of the second-order hydrodynamics on the dynamic behavior of the platform among the wind-wave hybrid systems

This paper presents three different wind-wave hybrid systems (HSs) with multiple point-absorption wave energy converters (WECs) integrated onto a floating semi-submersible wind turbine (WT). The focus of the present study is to explore the effects of the second-order wave forces on the dynamic behavior of the platform in the HS, including the platform motion and the mooring line tension, under both operational and extreme sea states. ANSYS/AQWA based on 3D diffraction/radiation potential theory is used to conduct numerical investigation on the second-order hydrodynamics of the HS. To ensure the reliability of the investigation, the validations of the semi-submersible platform and point-absorption WEC are done based on the available experimental data. Additionally, two models, "1st-order" and "1st&2nd-order", are set up for comparative analysis, to highlight the significance of the second-order forces. The simulation results show that the second-order hydrodynamics have a significant influence on the surge and pitch responses of the platform, but almost no effects on the heave response. In addition to this, ignoring the second-order hydrodynamics will significantly underestimate the mooring line tension response. Overall, this study suggests that the second-order hydrodynamics should be considered in the design of wind-wave HSs, in order to ensure their performance and safety, especially under the severe sea states.

Dynamic responses of a semi-submersible wind turbine platform subjected to focused waves with viscous effects

Dynamic responses of a semi-submersible wind turbine platform subjected to focused waves with viscous effects

Numerical Study of an Innovative Concept for a Multibody Anti-Pitching Semi-Submersible Floating Wind Turbine

The floating offshore wind turbine provides a feasible solution for the development of renewable ocean energy. However, the sizeable rotor diameter of the wind turbine results in large wind heeling moments and pitch amplitude. It will increase the structural loads and cause safety problems. Additionally, the contradictory nature between the stability and the sea-keeping of the floating structure requires that the more flexible method should be adopted to reduce the motion response of the floating offshore wind turbine. Therefore, an innovative concept of a multibody anti-pitching semi-submersible floating offshore wind turbine, named the MBAPSF, is proposed in this paper. The MBAPSF consists of a 5 MW braceless semi-submersible wind turbine and three wave energy converters. The multibody coupled numerical model is established by using an F2A tool, and the dynamic performance of the MBAPSF is compared with that of the traditional semi-submersible wind turbine named the TSSF. The results show that the innovative concept proposed in this paper can reduce pitch motion up to approximately 27% under different load cases, and the maximum bending moment and shearing force at the tower base are also reduced by more than 10%. Meanwhile, WECs are beneficial for increases in the total power generation capacity.

Effects of the yaw error and the fault conditions on the dynamic characteristics of the 15 MW offshore semi-submersible wind turbine

The floating offshore wind turbines (FOWTs) have a higher failure rate compared to onshore wind turbines, which could lead to aerodynamic imbalance on the floating systems. In addition, yaw error presents an inevitable challenge for FOWTs. This study aims to investigate the effects of the yaw error and the fault conditions on the dynamic properties of the IEA 15 MW semi-submersible FOWT using OpenFAST. The fault conditions involve blade seize without shutdown, blade seize with shutdown and grid loss. The platform motions and structure loads under different yaw errors (−20°, −8°, 0°, 8°, 20°) for different fault conditions were investigated. The dynamic response resulting from the fault was compared with the normal condition. Moreover, the optimal yaw angle under fault conditions was investigated in this paper. The results revealed that the fault shutdown conditions significantly impact the motions and loads of the wind turbine, with substantial reverse tower top and base bending moments. The yaw error, conversely, has less effect on the motions of the wind turbine, but can ameliorate the transient effect in the fault shutdown conditions. Additionally, it is recommended to operate the wind turbine at a yaw angle of 25–30° for wind speeds greater than 12 m/s.

Dynamic Analysis of Three Alternative Mooring Systems for a Semi-Submersible Wind Turbine in Intermediate Water Depth

Abstract Three forms of the mooring system in 60 m water depth are proposed for semi-submersible with partially inclined columns (SPIC) concept floating wind turbine (FWT). One is a simple form with only catenary lines, and the other two are hybrid forms including clump weights. The clumps are attached to the suspended section for Hybrid form1 and the bottom section for Hybrid form2. Hybrid form2 achieves the smallest line length and chain weight. Three alternative proposals can be evaluated through mooring line characteristics, dynamic responses, utilization factors, and simple cost analysis. Hybrid form2 allows for smallest pretension, and largest stiffness and nonlinearity only at large offsets. Under operational conditions, the mean surge for Hybrid form1 and Hybrid form2 is similar, but the fairlead tension is significantly smaller for Hybrid form2. Under the survival condition, the clumps of Hybrid form2 are lifted up and put down, leading to small mean offsets of FWT but large wave-frequency components of line tension. Among the three forms of the mooring system, the Hybrid form2 can limit the FWT to the smallest offset range while also controlling the mean mooring line tension to a level similar to the other two forms. Under normal working conditions and accidental conditions with single line broken, the maximal surge motions of FWT under the restraint of three mooring systems all meet the design requirements. The mooring line strength of the three mooring systems meets the requirements in ultimate limit state (ULS) and accidental limit state (ALS) analyses. Among them, the utilization coefficient of Hybrid form2 is closest to 1, demonstrating its best economic performance.

Bidirectional vibration control for fully-coupled floating offshore wind turbines

The vast and stable wind resources present in deep waters have made deep-sea floating wind power the mainstream trend for future development. However, the operating environment of offshore floating wind turbines is complex and variable. The joint action of wind and waves causes significant platform motion and turbine vibration, reducing power generation efficiency, causing structural damage, and shortening the service life of turbines. Hence, this study focuses on the NREL 5 MW DeepCwind semi-submersible wind turbine and establishes a fully-coupled floating wind turbine model utilizing the SIMPACK multi-body dynamics software. The model includes aero-, hydro-, flexible body structure, mooring, control, and transmission chain components. 3D pendulum tuned mass dampers (3D-PTMD) and bilinear tuned mass dampers (2TMDs) are designed to control the bidirectional vibration causes due to wind-wave misalignment in the platform motion and nacelle displacement of the floating wind turbine. The research results show that: For the roll and pitch of the platform, there is only one platform roll or pitch main frequency, and the amplitude at the main frequency can be effectively reduced by setting the frequency of 2TMDs near the main frequency, thereby suppressing the vibration of the platform; For the displacement of the nacelle, in addition to the main frequency of platform roll or pitch, there is also a secondary frequency of first-order bending of the tower, 2TMDs corresponding to the main frequency can effectively reduce the amplitude at the main frequency, and 3D-PTMD corresponding to the secondary frequency can effectively reduce the amplitude at the secondary frequency, it is better to configure 2TMDs alone than 3D-PTMD alone, and it is best to configure both at the same time.

Effects of the WEC shape on the performance of a novel hybrid WEC-FOWT system

A novel wind-wave energy hybrid concept is presented in this paper, consists of a floating semi-submersible wind turbine and multiple point absorption wave energy converters (WECs). Numerical simulations are conducted using the hydrodynamic analysis program AQWA, based on the three-dimensional (3D) potential theory. To ensure the reliability of the numerical model, the hydrodynamic responses of the semi-submersible platform and the WECs are validated using available experimental data. Finally, three WECs with different shapes are designed to investigate the shape on the performance of this hybrid system, including platform motion, mooring line tension and power captured by the WECs. The results demonstrate that the hybrid system with the circular truncated conical WECs has the best performance compared to the other two shaped WECs. In addition, the influence of the platform motion on the performance of the WEC array is negative. Overall, this study could provide some insights for the design of other wind-wave integrated systems.

Effects of Mooring Line with Different Materials on the Dynamic Response of Offshore Floating Wind Turbine

The influence of mooring systems with lines of different material on the dynamic response of a floating wind turbine is studied using a 5 MW OC4-DeepCwind semi-submersible wind turbine as a representative prototype in this study. Two types of mooring systems were designed using the MoorDyn module in OpenFAST software (v3.1.0): one uses chains, and the other uses a hybrid mooring line composed of chains and high-strength polyethylene (HMPE) ropes. A wind turbine with two types of mooring systems was simulated using the OpenFAST software. The results show that the floating wind turbine moored with the hybrid lines exhibited a larger heave and pitch motion than that moored using chains alone. At the same time, the surge displacement was smaller than that of the wind turbine using chains alone. In terms of mooring line tension, the mean and amplitude values of the hybrid mooring system at the location examined were smaller than those of the chain mooring system. Thus, using HMPE ropes in the mooring system can significantly reduce line loads. In addition, the HMPE ropes used in the floating wind turbine mooring system did not affect the power generation of the wind turbine. This study provides promising support data and observations for applying high-strength polyethylene (HMPE) ropes in mooring systems for floating wind turbines.

Research on dynamic response prediction of semi-submersible wind turbine platform in real sea test model based on machine learning

The motion response of floating offshore wind turbine (FOWT) platform is closely related to its safety, and model test is the most direct method to fully study its dynamic characteristic behavior. In this paper the dynamic response of intermediate-scale physical semi-submersible FOWT model test under real sea environmental loads is studied and compared with the numerical model dynamic response, demonstrating that some limitations of small-scale physical model tests can be overcome, such as the catenary effect of mooring lines. It is found that the nonlinear dynamic characteristics of the physical model under actual marine environmental loads can be captured by the numerical model, which can improve the fidelity of model test data for semi-submersible FOWT structures in the existing literature. The dynamic response database of semi-submersible FOWT under complex wind, wave and current loads was established by using a numerical model with high accuracy, and was used in the prediction research of different machine learning algorithm models such as Genetic Algorithm optimization Back Propagation neural network (GA-BP), Support Vector Machine (SVM) and Gaussian Process (GP). The research shows that the relationship between the environmental load and the dynamic response of FOWT can be established by using the machine learning model, in which the number of samples has a significant impact on the prediction of the mooring line tension, and the research on the three algorithm models shows that GP performs significantly better than GA-BP and SVM in predicting the mean value of surge, pitch and mooring line force. For the dynamic response prediction of mooring line tension, GP model can get better results. In the future, this method will become an important method to predict and monitor the dynamic response of FOWT.

Numerical modeling of a mooring line system for an offshore floating wind turbine in Vietnamese sea conditions using nonlinear materials

The offshore renewable energy industry has been developing farms of floating offshore wind turbines in water depths up to 100 m. In Vietnam, floating offshore wind turbines have been developed to increase the production of clean and sustainable energy. The mooring system, which is used to keep the turbine stable and ensure the safety and economic efficiency of wind power production, is an important part of a floating offshore wind turbine. Appropriate selection of the mooring type and mooring line material can reduce the risks arising from the motion of wind turbines. Different types of mooring line material have been simulated and compared in order to determine the optimal type with the minimum motion risk for a floating wind turbine. This study focused on numerical modeling of semi-taut mooring systems using nonlinear materials for a semi-submersible wind turbine. Several modeling approaches common to current practice were applied. Hydrodynamic analysis was performed to investigate the motion of the response amplitude operators of the floating wind turbine. Dynamic analysis of mooring systems was performed using a time domain to obtain the tension responses of mooring lines under the ultimate limit states and fatigue limit states in Vietnamese sea conditions. The results showed that the use of nonlinear materials (polyester and/or nylon) for mooring systems can minimize the movement of the turbine and save costs. The use of synthetic fibers can reduce the maximum tension in mooring lines and the length of mooring lines. However, synthetic fiber ropes showed highly nonlinear load elongation properties, which were difficult to simulate using numerical software. The comparison of the characteristics of polyester and nylon mooring lines showed that the maximum and mean tensions of the nylon line were less than those of the polyester line. In addition, the un-stretched length of the polyester line was greater than that of the nylon line under the same mean tension load. Therefore, nylon material is recommended for the mooring lines of a floating offshore wind turbine.

Optimization and evaluation of a semi-submersible wind turbine and oscillating body wave energy converters hybrid system

A hybrid system consisting of a floating offshore wind turbine and wave energy converters (WECs) is promising for multiple extractions of ocean renewable energy. Optimization of the layout and an evaluation of the performance of the hybrid systems are important before practical use but are not well done due to limited simulation tools. This study fills these gaps by proposing a complete toolkit based on the OpenFAST-AQWA framework together with a code that can deal with the full couplings in the problem. A novel DeepCWind-Wave Stars hybrid system is proposed as a representative to validate and practice the new tool. The key parameters of the WECs are optimized using an easily performed method based on the local wave contour disturbed by the platform. Motions, mooring loads, and power of the hybrid system are evaluated under a wide range of wind and wave conditions to examine the influence of the WECs. Results show that the integration of the WECs brings no harm to the stability of the system. The WECs reduce the fluctuation in the mooring force but slightly increase the average mooring force. The generated wave power is an adequate supplement to wind power, especially at a small wind speed.

Control-Oriented Modeling and Characteristic Analysis of Offshore Semisubmersible Wind Turbine Under Wind-Wave Joint Loads

A dynamic model is the basis of controller design. However, the existing floating wind turbine models often have complex structures and pay more attention to the fine description of detailed characteristics. They are more suitable for the performance test rather than the control system design. In this brief, a control-oriented nonlinear physical-based (CNO-NP) model of the offshore semisubmersible wind turbine (OSWT) consisting of the wind turbine, a floating platform, and a mooring system is built first based on mechanism analysis. Then, a control-oriented linear parameter varying (CNO-LPV) model is derived, and its parameters are achieved by analyzing the steady states and dynamic responses under various operating conditions. By nonlinearity analysis, the complexity of the CNO-LPV model can be further simplified. Take a high-fidelity benchmark model as the comparison model to verify the accuracy of the CNO-NP model, and a model predictive controller is designed and tested for wind-wave joint excitations to validate the effectiveness of the proposed CNO-LPV model.

Preliminary design and dynamic analysis of constant tension mooring system on a 15 MW semi-submersible wind turbine for extreme conditions in shallow water

Floating offshore wind in China faces the challenge of shallow water depths and frequent typhoons, which are very different from the environmental conditions in European waters. The performance of the mooring system directly affects the overall cost of the floating offshore wind turbine and its survivability in extreme conditions. Under extreme conditions in shallow water, line dragging or breaking can frequently occur when the floating platform restrained by a conventional chain catenary mooring system drifts with waves. A constant tension mooring system concept is proposed to solve the design challenges of shallow water mooring systems in harsh conditions for floating wind turbines. The newly proposed mooring system consists of clump weight and winch wires, which release kinetic energy by increasing the platform's moving distance and ensuring that the instantaneous maximum mooring force never exceeds the breaking strength of the chain. Based on the time-domain potential flow theory, the coupling dynamic response of the UMaine VolturnUS-S semi-submersible reference wind turbine moored by a constant tension mooring system is studied. The mooring line tension and the platform motion are analysed. By comparing with the traditional catenary mooring system, it is found that the constant tension mooring system can significantly reduce the length of the mooring line used and the maximum mooring line tension. It completely prevents the mooring chain from being tightened and the turbine from being damaged by excessive mooring loads, effectively improving the survivability of a floating offshore wind turbine in extreme conditions in shallow water areas.

Coupled dynamic and power generation characteristics of a hybrid system consisting of a semi-submersible wind turbine and an array of heaving wave energy converters

A wind-wave hybrid system consisting of a semi-submersible floating wind turbine and an array of heaving wave energy converters (HWECs) provides a solution for co-located marine renewable energy extraction. The knowledge of the coupled dynamic and power features of the hybrid system is important to the design and operation but is poorly understood. This study proposes a comprehensive numerical model that can tackle the full couplings by bridging the aerodynamic module of OpenFAST and the hydrodynamic simulator WAFDUT through an in-house code based on multi-body dynamics. The coupled dynamic and power generation characteristics of the hybrid system are investigated, with an emphasis on the influence of the HWECs on the wind-induced motion, mooring tension, and wind power generation of the turbine. Results show that the HWECs do not produce negative effects on the wind-induced surge, heave, and pitch motions. The HWECs can slightly reduce the mooring tension and slightly improve wind power generation. The array of HWECs can generate over 500 kW wave power in the waves with periods of 6 s–10 s, which can be a qualified supplement to wind power. The findings offer useful knowledge about the key properties of the hybrid system for practical use.

Coupled dynamic characteristics of 10 MW semi-submersible offshore wind turbine under typical operation, extreme, and fault conditions

The Technical University of Denmark (DTU) 10 MW wind turbine is selected as the object of this study. The braceless semi-submersible platform for the National Renewable Energy Laboratory 5 MW wind turbine is upscaled to support the DTU 10 MW wind turbine. The servo control system of the 10 MW semi-submersible wind turbine is redesigned. Furthermore, torque control is performed below the rated wind speed to achieve maximum power capture, and the gain scheduling proportional integral (GSPI) control method is performed above the rated wind speed to realize the design of the variable propeller control system. This ensures the safe operation of the floating wind turbine above the rated wind speed. The reliability of the designed servo control system is verified by a full coupling dynamic response analysis with the designed servo control system, aerodynamics, and hydrodynamics under the combined action of wind and waves. The coupled dynamic characteristics of the 10 MW semi-submersible wind turbine under different conditions are analyzed, including operational, extreme, and fault situations. The results show that the servo control system effectively controls the motion response of a 10 MW floating offshore wind turbine (FOWT). Turbulent wind has an obvious excitation effect on the surge and pitch of the FOWT, whereas the heave motion is primarily excited by wave loads. Different fault conditions can cause different motion and structural responses. Furthermore, the tower top bending moment is greatly affected by the imbalance of aerodynamic load, and the surge, pitch motion, tower base bending moment, and mooring tension of the FOWT are affected by different fault conditions to varying degrees. In addition, the heave motion is generally barely affected by the fault conditions.

Numerical investigations on nonlinear effects of catenary mooring systems for a 10-MW FOWT in shallow water

This paper presents a detailed investigation on the influences of different water depths and mooring parameters on the nonlinear responses of mooring systems and motion behaviors of a 10 MW semi-submersible wind turbine. To reduce computational effort, a developed mooring design and analysis program MOOR_SAD is used for quasi-static analysis, which is validated through comparing with the simulation results in OrcaFlex. The fully coupled dynamic numerical analysis is performed using OrcaFlex-FAST and validated through the comparison of numerical and experimental results of the NREL OC5 DeepCwind. Then, thirty mooring cases with different water depths and mooring parameters are studied for analyzing the nonlinear responses of chain mooring lines and motion performances of the 10 MW FOWT in shallow water. The results show that when the chain properties and pretension of the mooring lines are kept the same, the shorter mooring line leads to a stronger nonlinearity in mooring restoring force and stiffness. Besides, the weight of the mooring line has little effect on the mooring shape. This study could be expected to provide an engineering approach for mooring analysis and design of FOWTs in shallow water.

Design and Modeling of a Constant Tension Mooring System for a 5 MW Semisubmersible Wind Turbine

Design and Modeling of a Constant Tension Mooring System for a 5 MW Semisubmersible Wind Turbine