Spar-type Floating Wind Turbine Research Articles

Design and Dynamic Analysis of a Co-Existence Solution Between a Semi-Submersible Offshore Fish Cage and a Spar-Type Floating Offshore Wind Turbine

Abstract Clean energy captured by offshore wind turbines has been widely used for supporting onshore activities. In the near future, facilities such as offshore wind turbines can also play an important role in the energy transition of offshore activities. Offshore wind energy can be employed for electrifying operations in offshore fish farms, which are traditionally supplied by diesel-engine barges/generators. Based on this motivation, this study focuses on the design of a shared mooring system between a semi-submersible offshore fish cage and a spar-type floating wind turbine. A numerical model of the proposed shared mooring system is implemented in a global response analysis software sima for performing fully coupled time-domain simulations. The configuration of the shared mooring line is determined using an engineering approach, which comprises Irvine’s formulation, system eigenvalue analysis, and cost estimation. Moreover, relevant case studies by altering the environmental conditions are performed. Extreme operational conditions that may give large relative motions are investigated thoroughly. The dynamic performance of the integrated system is compared with that of individual structures. The global motion of the floating wind turbine and its mooring line’s tension behavior are obviously influenced by the existence of the shared line. In general, the present work investigates the feasibility of a shared mooring system for these types of offshore structures and further gives insights into the engineering design procedure.

Hurricane Wave Loads on Spar-Type Floating Wind Turbines: A Comparison of Simulation Schemes

Floating wind turbines are sensitive to hurricane events. Since the turbine rotors are parked and the blades are feathered during hurricanes, the aerodynamic loads due to boundary-layer winds are relatively small compared to the hydrodynamic loads due to sea surface elevations. Hence, accurate modeling of the hurricane wave loads is crucial to ensure the safety of floating wind turbines. During a hurricane, large wave heights with severe flow separation make it inaccurate to use either linear panel method-based models (without nonlinear consideration associated with fluid viscosity) or Morison equation-based models (without unsteady consideration associated with fluid memory). Efforts have been made to advance simulation schemes of hurricane wave loads on spar-type floating wind turbines. This study systematically compares and assesses the efficacy of six hydrodynamic models available in the literature along with a newly proposed model. The ability of these seven hydrodynamic models to capture nonlinear and/or unsteady effects is investigated. As a demonstration example, the wave loads on a spar-type wind turbine are calculated using these seven models to highlight the underlying role of each simulation scheme in accurately acquiring the dynamic responses of this type of offshore floating structure in severe hurricane seas. It is found that the nonlinear viscous term in the Morison equation and hybrid model serves as an important nonlinear damping mechanism. The reduction of the low-frequency wave load and added mass in the modified hybrid model collectively leads to larger displacements compared to those based on the hybrid model. While the displacements based on the stretching method and Rainey’s equation are similarly larger than those based on the Morison equation, their nonlinear wave loads are much smaller than those in FNV theory.

The dynamic response of a Spar-type floating wind turbine under freak waves with different properties

In the present work, the dynamic responses of a Spar-type floating offshore wind turbine (SFWT) under freak waves with different properties are simulated and analyzed. The properties of freak wave are represented by changing different decision parameters a1, a2, a3 and a4, corresponding to the ratios of the wave elevations around the freak wave crest (FWC). The improved phase modulation method is adopted to generate the original freak wave (OFW). On the basis, the freak wave elevation is further modulated to increase the decision parameters with different degrees, and the modulated freak waves (MFWs) are obtained. The SFWT motions under the OFW and MFWs are simulated in the time domain. We firstly investigated the influence of freak wave on the SFWT transient dynamic performances, including the surge, heave, pitch motions, the turbine output power and the mooring tensions. Then, the SFWT dynamic response under different MFWs corresponding to the specific factors were analyzed. The results show that the freak wave fluctuation represented by the factor a1 significantly affects the surge, heave, pitch transient motion, output power and extreme mooring tension of the SFWT. The output power of the SFWT decreases less after the FWC with the wave fluctuation increasing. At the same time, the wave fluctuation significantly affects the mooring tension, which greatly increases both near the focus time and the extreme mooring tension time. The height of the previous wave (PW) related to a2 affects the heave motion at the trough after PW impact, which in turn affects the transient heave. a3 represents the height of the next wave (NW) and has little effect on the dynamic performance. The impact height of freak waves represented by a4 has a particularly significant effect on heave. It not only affects the transient motion at the focus time, but also has a significant effect on heave after the freak wave subsides.

Comparative study of the mating process for a spar-type floating wind turbine using two alternative installation vessels

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.

Fault detection of offshore wind turbine drivetrains in different environmental conditions through optimal selection of vibration measurements

In this study, a vibration-based fault detection method is proposed for offshore wind turbine drivetrain based on the optimal selection of the acceleration measurements. The main aim is to find the sensor positions mounted on the drivetrain that provides the most fault-related information. In fact, this study tries to optimize the vibration sensors suggested by ISO standards in terms of their position and number in order to get accurate fault detection results. The faults are considered in a set of bearings with a high probability of failure in a 5-MW reference drivetrain high-fidelity model installed on a spar-type floating wind turbine. Different simulated shaft acceleration measurements are examined under three environmental conditions. Correlation analysis is first performed on the measurements to see how the different faults and environmental conditions affect the correlation between the measurements. Then, a combined principal component analysis (PCA) and convolutional neural network (CNN) is employed as the fault detection method through the optimal vibration measurements. The prediction findings demonstrate that only two vibration sensors, one near the main shaft and another near the intermediate-speed shaft, can fully detect the considered faulty bearings. Also, it will be shown that the axial vibration data give more promising results than the radial ones which can be used in virtual digital twin models.

Dynamic Response of a SPAR-Type Floating Wind Turbine Foundation with Taut Mooring System

Compared with the traditional catenary or semi-taut mooring lines, the taut mooring system is more advantageous in many aspects, such as reduction of mooring line loads, erosion and fatigue damage during the powering productions of the floating wind turbines. This paper presents a taut mooring system made of synthetic fiber mooring lines, which can experience large elongations for a spar-type floating wind turbine. A finite element method (FEM)-based tensile mooring line model is proposed to study the mooring statics and dynamics of the floating wind turbine. A time domain modelling method coupled with the developed mooring line model is adopted to study the dynamics of a spar-type floating wind turbine foundation moored by the taut mooring system under regular waves. A systematic dynamic response and structural analysis are conducted based on variations in the mooring length and pretension. Additionally, comparative performance analyses are investigated for two mooring configurations with different numbers of mooring lines: two-point and three-point taut mooring system. It is found that factors, such as mooring length, pretension and the number of mooring lines, have significant impact on the in-plane and out-of-plane motion responses of the foundation.

Bilinear tuned mass damper for spar-type floating wind turbines

As the dynamic stability of a spar-type floating wind turbine (SFWT) must be ensured for a secure and robust operation, tuned mass dampers (TMDs) have been proposed to reduce the vibration of the SFWT platform. However, the TMDs in previous studies have several limitations. First, the previously used linear TMDs successfully reduced platform vibration in the absence of wind thrust forces. However, their performance is not satisfactory in the presence of strong winds; if the SFWT structure tilts due to strong winds, the mass of the TMD remains close to the inner wall of the platform. This behavior undermines its vibration reduction performance. To address this issue, a bilinear TMD is introduced in this work. The bilinear TMD significantly reduces vibration even when a strong wind thrust force is applied. Second, previous studies determined the optimized parameters of the TMD based on the free response of the SFWT structure. However, this optimization leads to a low performance TMD when dynamic and stochastic loads were imposed. To address these limitations, this study introduces a bilinear TMD and optimizes its parameters for a variety of operational wind conditions. The bilinear TMD outperforms previous TMDs in every evaluated wind condition by 10%–15%.

Optimal model reference adaptive control of spar-type floating wind turbine based on simulated annealing algorithm

Carbon neutrality has led to an explosion in renewable energy, such as floating wind turbine (FWT). However, stability and safety become the main bottleneck in the development of FWTs. In this paper, an active vibration control system is designed to improve the stability of Spar-type FWT. The dynamic model of Spar-type FWT was established, and the model was linearized according to the actual situation of wind turbine. The vibration control of wind turbine structure is analyzed by using tuned mass damper (TMD) active control and the designed active linear quadratic regulator (LQR) algorithm. The weight of LQR controller is designed by simulated annealing (SA) algorithm to eliminate the subjectivity of parameter selection. The analysis shows that compared with the wind turbine system under the passive control system, the deflection angular displacement of the tower of the wind turbine decreases by 80.4% and the deflection angular velocity decreases by 67.6%. The deflection angular displacement of Spar platform decreased by 47.2% and the deflection angular velocity decreased by 37.7%. The optimal model reference adaptive control (MRAC) is designed to solve the uncertainty of structural parameters and loads of wind turbine system. Simulation results show that the proposed method can accurately and quickly track the optimal trajectory with SA-LQR control as the reference model, and realize the real-time optimal control of Spar-type FWT vibration system. All the results show that the method reported here can greatly improve the stability and safety of Spar-type FWTs.

A data-driven approach for fault diagnosis of drivetrain system in a spar-type floating wind turbine based on the multi-point acceleration measurements

This paper deals with the condition monitoring of a floating wind turbine drivetrain using multi-point acceleration measurements. Single sensor data obtained from drivetrain system may provide insufficient information about the health condition due to the complicated structure and applied loading on this system. As a result, multi-point measurements are required to be employed for reliable fault diagnosis. The shared information between the multi-point measurements can be used for identifying the system’s condition. In this study, the fault diagnosis of the floating wind turbine drivetrain system is performed using a data-driven approach. Fault cases are considered in bearings most likely to damage. A combined principal component analysis (PCA) and deep convolutional neural network (CNN) is proposed to extract common and fault-related information between the measurements on the one hand and to classify different health conditions of the drivetrain on the other. It will be demonstrated that PCA-based information provides more satisfactory fault diagnosis results than individual sensor data. The method is numerically validated using the acceleration responses obtained from a 5-MW reference drivetrain model installed on a spar-type floating wind turbine.

Long-term rogue wave occurrence probability from historical wave data on a spatial scale relevant for spar-type floating wind turbines

Very large waves, such as rogue waves, can be dangerous for ships and offshore structures. There is no consensus on the theoretical occurrence probability of these rogues and ocean measurements containing rogue waves are rare. This paper addresses the long-term occurrence probability of rogue waves using time-extreme (TE) and space–time extreme (STE) statistical wave models, evaluated with a significant amount of historical directional wave data from Monterey Bay, California, USA. A novel understanding of these models is obtained by exploring the area size for which the STE models estimate a higher rogue wave occurrence probability compared to the TE models. For four rogue wave and crest heights the return periods until a 95% rogue wave occurrence probability are estimated. To illustrate an application of this statistical analysis, the risk of rogue waves to a spar-type floating wind turbine is evaluated. Based on both the dynamic response and the long-term occurrence probability of the rogue wave, this research presents a new and advanced approach for a risk analysis of rogue waves to floating offshore structures.

An aero-hydro coupled method for investigating ship collision against a floating offshore wind turbine

Ship collision hazards with floating offshore wind turbines (FOWTs) has been identified and the dynamic responses of FOWT under ship collision scenarios are necessary to be investigated. This paper develops a numerical solver to address the challenge of considering wind-wave loads in the ship-FOWT collision analysis. The user-defined load subroutine, LOADUD, in LS-DYNA is utilized to model the wind, wave and mooring loads. A simplified aerodynamic model considering blade pitch strategy is used and the hydrodynamic loads are calculated based on potential-flow theory with a linear wave kinematic model. The mooring forces are considered with a linearized model. The implementation of the coupled solver is verified by conducting free decay test, wave-only and wind-only analysis, respectively and comparing them with the reference values. Good agreement has been achieved, indicating that the aero-hydro coupling effects can be accurately simulated by the numerical solver. With the developed method and solver, cases of ship collision with a spar-type floating wind turbine are simulated and both local structural deformation and global motions in 6DOF are obtained. The influence of velocity, flexibility of tower, deformability of the ship, wind-wave loads are further discussed. It is found that the impact velocity can greatly affect the structural responses and a high-speed impact can directly lead to tower collapse. By comparing the cases of ship collision with FOWT, and those with fixed-support wind turbine, the flexibility of wind turbine tower is found to rarely influence the energy dissipation under collision scenario of FOWT, while it is important for the collision scenario of fixed-support wind turbine. The wind-wave condition can be more hazardous as a smaller impact velocity can lead to structures collapsing. This is because, even though the structure can withstand the ship collision, the residual strength may not be enough for the following wind loads, which can result in the structure to finally collapse. Additionally, the proposed fully coupled method is used to validate one of the authors’ previous studies, which is a decoupled method regarding the ship-FOWT collision analysis [1]. The external dynamic results from two methods are compared and discussed. Good agreement is reached between two methods for low-velocity impact but for high-speed collision or collisions with serious deformation, limitations of decoupled method can be found because some basic assumptions of them are violated. The proposed method and solver can be used to evaluate the crashworthiness of FOWT during the engineering design phase.

Proposal of a Novel Mooring System Using Three-Bifurcated Mooring Lines for Spar-Type Off-Shore Wind Turbines

Floating wind turbine vibration controlling becomes more and more important with the increase in wind turbine size. Thus, a novel three-bifurcated mooring system is proposed for Spar-type floating wind turbines. Compared with the original mooring system using three mooring lines, three-bifurcated sub-mooring-lines are added into the novel mooring system. Specifically, each three-bifurcated sub-mooring-line is first connected to a Spar-type platform using three fairleads, then it is connected to the anchor using the main mooring line. Six fairleads are involved in the proposed mooring system, theoretically resulting in larger overturning and torsional stiffness. For further improvement, a clump mass is attached onto the main mooring lines of the proposed mooring system. The wind turbine surge, pitch, and yaw movements under regular and irregular waves are calculated to quantitatively examine the mooring system performances. A recommended configuration for the proposed mooring system is presented: the three-bifurcated sub-mooring-line and main mooring line lengths should be (0.0166, 0.0111, 0.0166) and 0.9723 times the total mooring line length in the traditional mooring system. The proposed mooring system can at most reduce the wind turbine surge movement 37.15% and 54.5% when under regular and irregular waves, respectively, and can at most reduce the yaw movement 30.1% and 40% when under regular and irregular waves, respectively.

On the aerodynamic loading effect of a model Spar-type floating wind turbine: An experimental study

Aerodynamic loading is one of the most dominating environmental excitations of Floating Wind Turbines (FWTs) and plays an important role in the FWT dynamics. In this study, we developed a model Spar-type FWT and then constructed a dedicated experiment apparatus to reveal the aerodynamic loading effects. As for the floater motion, the wind loading serves as an external exciting force, as well as potential damping source and equivalent added mass item. To take all these roles into account, we proposed a concept of aerodynamic loading effect. The presence of aerodynamic loading effect is validated by free decay tests and white noise wave tests. Results show that the aerodynamic loading effect alters the natural frequencies and damping ratios of the FWT system. We suggest the FWT designers refer to the altered natural frequencies when designing the floater and the FWT controllers. We experimentally observed that the increased aerodynamic loading seems to suppress the pitch resonance vibration while amplifies the resonance vibration at surge frequency. Besides, the nacelle motions, blade loads, and the tower dynamics, are all significantly impacted by the aerodynamic loading effect. The presented results are potentially helpful for optimizing FWTs and developing advanced FWT controllers.

Coupled dynamic analysis of spar-type floating wind turbine under different wind and wave loading

Coupled dynamic analysis of spar-type floating wind turbine under different wind and wave loading

Fault Classification of a Blade Pitch System in a Floating Wind Turbine Based on a Recurrent Neural Network

This paper describes a recurrent neural network (RNN) for the fault classification of a blade pitch system of a spar-type floating wind turbine. An artificial neural network (ANN) can effectively recognize multiple faults of a system and build a training model with training data for decision-making. The ANN comprises an encoder and a decoder. The encoder uses a gated recurrent unit, which is a recurrent neural network, for dimensionality reduction of the input data. The decoder uses a multilayer perceptron (MLP) for diagnosis decision-making. To create data, we use a wind turbine simulator that enables fully coupled nonlinear time-domain numerical simulations of offshore wind turbines considering six fault types including biases and fixed outputs in pitch sensors and excessive friction, slit lock, incorrect voltage, and short circuits in actuators. The input data are time-series data collected by two sensors and two control inputs under the condition that of one fault of the six types occurs. A gated recurrent unit (GRU) that is one of the RNNs classifies the suggested faults of the blade pitch system. The performance of fault classification based on the gate recurrent unit is evaluated by a test procedure, and the results indicate that the proposed scheme works effectively. The proposed ANN shows a 1.4% improvement in its performance compared to an MLP-based approach.

Feasibility studies of a novel spar-type floating wind turbine for moderate water depths: Hydrodynamic perspective with model test

Spar-type platforms have been widely applied in ocean engineering and first get commercial applications in the offshore wind industry. The spar-type platform is typically deployed in deep waters whereas shallow and moderate water depths are regarded as more ideal regions for deploying floating wind turbines (FWT). However, the deep draft of spar-type platform limits its use in these regions, encountering great challenges. This paper aims to seek a solution for deploying spar-type FWTs in moderate water depths of about 100 m. To this end, a novel Stepped Short Spar for Shallow water is developed by Shanghai Jiao Tong University (SJTU) with a draft of 50 m, abbreviated as SJTU-S4. Then, a model system is designed and manufactured at a scaling ratio of 1:50. An integrated experimental apparatus is constructed and comprehensive model tests are carried out. The hydrodynamic characteristics of the proposed SJTU-S4 FWT are objectively evaluated referring to those of the typical OC3 Hywind FWT. The model test results indicate that the SJTU-S4 FWT has reasonable hydrodynamic performance in water depth of 100 m. This study suggests that the application of SJTU-S4 FWT concept is feasible and can be an alternative in offshore wind exploitation in moderate water depths.

Detection of Damages in Mooring Lines of Spar Type Floating Offshore Wind Turbines Using Fuzzy Classification and Arma Parametric Modeling

Floating wind turbines may encounter severe situations because of harsh environments. Higher cost of repair and maintenance of floating wind turbines have led researches to focus on damage detection methods that can prevent sudden failures. This paper presents an applicable method of damage detection and structural health monitoring for floating wind turbines based on the autoregressive moving average (ARMA) model and fuzzy classification. First, the dynamic model of a spar type floating wind turbine is constructed, by which the time responses of each degree of freedom of the system are acquired. With the system’s nonlinearity included, the intrinsic mode functions are obtained for the response signal. The Hilbert–Huang transform is applied and the appropriate measured signal for each degree of freedom is chosen for the ARMA modeling. In order to evaluate the proposed method, the ARMA parameters are first estimated for the undamaged condition then assumed damages are injected to the model and the ARMA parameters are once again estimated for the damaged condition. These parameters are considered as inputs for the fuzzy classification method. After training the system using the assumed damaged and undamaged conditions, the proposed method is simulated. Furthermore, the effect of measurement noise on the success rate is investigated. The results show that, in the presence of noise, the proposed method is able to identify the damage location and severity of mooring lines with acceptable success rate.

Fault detection and diagnosis of a blade pitch system in a floating wind turbine based on Kalman filters and artificial neural networks

This paper describes the development of a fault detection and diagnosis method to automatically identify different fault conditions of a hydraulic blade pitch system in a spar-type floating wind turbine. For fault detection, a Kalman filter is employed to estimate the blade pitch angle and valve spool position of the blade pitch system. The fault diagnosis scheme is based on an artificial neural network method with supervised learning that is capable of diagnosing a predetermined fault type. The neural network algorithm produces a predictive model with training, validation and test procedures after the final performance evaluation. The validation and test procedures of the artificial neural network model are conducted with the training model to prove the model performance. The proposed method is demonstrated in case studies of a spar floating wind turbine with stochastic wind and wave conditions and with consideration of six different types of faults, such as biases and fixed outputs in pitch sensors and excessive friction, slit-lock, wrong voltage, and circuit shortage in actuators. The fault diagnosis results from the final performance evaluation show that the proposed methods work effectively with good performance.

A study on the structural response of a partially prestressed concrete allowing cracks of a spar-type floating wind turbine under extreme environment

A study on the structural response of a partially prestressed concrete allowing cracks of a spar-type floating wind turbine under extreme environment

Sway vibration control of floating horizontal axis wind turbine by modified spar-torus combination

Spar type floating wind turbines exposed to marine environment experience significant vibration, which is more prominent under wind-wave misalignment. This unavoidable vibration, particularly in the sway direction, affects overall performance and can induce significant damage to sensitive electro-mechanical components leading to downtime for maintenance. With this in view, the present study aims to propose a modified spar-torus combination by introducing spring and dashpot in between, so that it can work as an isolator. First, a comprehensive mathematical model for this multi-body system is developed using Kane's approach with proper aero-elastic and hydrodynamic simulation. For this purpose, 3D wind fields are generated in TurbSim and the waves are simulated from Pierson-Moskovitch spectrum considering wind-wave correlation and misalignment. Aerodynamic loads are computed using modified Blade Element Momentum theory while and hydrodynamic loads are generated using Morison's equation. Using these inputs, the response of the modified spar-torus combination is solved, which demonstrates the efficiency and advantage of the proposed vibration isolation. Different sea states and wind conditions are simulated replicating the actual scenario to investigate the performance envelope of the proposed controller for spar-type floating wind turbines.