Nacelle Acceleration Research Articles

Numerical analysis and comparison study of the 1:60 scaled DTU 10 MW TLP floating wind turbine

This paper presents numerical investigations and comparison to experiments for a scaled DTU 10MW Tension Leg Platform (TLP) floating wind turbine. Two state-of-the-art aero-servo-hydro-elastic models, HAWC2 and FAST, are used and compared with experimental results. This study applies the same controller, turbulent wind, and hydrodynamic input as in the experiments to the numerical simulations. First, detailed numerical model calibration and validation have been performed. Free decay experimental test results are compared. Next, the damping ratio is calibrated. The rotor thrust is matched to the measurements by adjustment of the blade pitch angle in both numerical models. The same controller used in the experiments is applied to the numerical model via a Dynamic Link Library interface. Detailed comparisons for regular waves, irregular waves, and a focused wave group impact are presented. Results for surge motion, nacelle acceleration, front mooring line tension, blade pitch and rotor speed are included. In general, both numerical models predict the floating wind turbine responses very well compared to the experimental results both in time and frequency domains. Owing to the use of a linear wave forcing model and Morison based viscous loads, however, the low-frequency surge motion is found to be under-predicted in both models.

A parametric optimization approach for the initial design of FOWT’s substructure and moorings in Brazilian deep-water fields

As Floating Offshore Wind Turbines (FOWT) continue to demonstrate their technical feasibility in different projects, the conceivable scenarios for the application of this technology rapidly expand. Outside the mainstream perspective of floating farms connected to onshore grids, several projects currently study the technical and economic feasibility of adopting FOWTs to supply part of the electrical power required by the offshore production of oil and gas. Having this goal as a main target, the present work addresses initial results obtained in a comprehensive research project that aims to prospect potential uses for FOWT technology in the context of offshore oil and gas production in the Brazilian oil fields. In this context, a parametrical optimization procedure has been adopted to provide suitable candidates for the floating substructures and their mooring systems. The optimization framework was designed for automatic generation of geometrical concepts already considering the equilibrium involving hull weight, ballast and the tensions applied by the mooring lines. The latter are modeled in hybrid configuration, composed of sections of steel chains and polyester rope, considering both catenary and taut-leg configurations. The main goal is to minimize CAPEX costs but, knowing that the acceleration levels in the rotor hub are inevitably linked to indirect costs that are hardly predictable at this stage, a multi-objective approach is preferred, taking the minimization of the nacelle acceleration as one of the drives. Moreover, for a better resolution of the site-specific design, the optimization is done not only for a few pre-selected metocean conditions, but considering a long-term series of the simultaneous action of seas, swell waves, currents and winds. In this paper, two illustrative concepts with different floater geometries are presented, both of them optimized for a water depth of 600m. A discussion on the mooring configurations is made and the rationale involved in the optimization procedure that leads to the final floater dimensions is investigated by connecting the floater motion responses to the wave characteristics in the chosen field.

Dynamics of hybrid offshore renewable energy platforms: Heaving point absorbers connected to a semi-submersible floating offshore wind turbine

Hybrid platforms composed of a floating offshore wind turbine (FOWT) and wave energy converters (WECs) may possibly reduce the levelised cost of energy of both energy harvesting devices and improve the platform dynamics. This paper investigates the dynamics of a hybrid platform composed of a semi-submersible FOWT combined with three spherical heaving point absorbers (PAs), either floating or submerged, under turbulent wind and irregular waves conditions. Since the WECs are attached to the floating structure, complex hydrodynamic couplings occur between the floating bodies. In this regard, a parametric investigation of the power-take-off system is performed to understand the wave power absorption and motion characteristics, which are compared against their respective stand-alone configurations. At sea states with short wave peak period, both hybrid platforms (with floating or submerged PAs) were able to operate under maximum wave power absorption condition and extract nearly the same amount of wave energy as the PAs attached to the ground (stand-alone), while providing the additional benefit of reducing the horizontal nacelle accelerations of the FOWT. On the other hand, when operating at sea states with long wave periods, the hybrid platforms may need to restrict the wave power absorption capability to prevent additional nacelle horizontal acceleration.

Detection of magnitude and position of rotor aerodynamic imbalance of wind turbines using Convolutional Neural Network

Rotor imbalances present a serious problem for wind turbines. In particular, for offshore wind turbines, aerodynamic imbalance can have a severe impact because of the large rotor size. In this study, the impact of the aerodynamic imbalance is investigated. A novel framework for detecting aerodynamic imbalance is proposed. Firstly, a model of a 3MW direct-driven wind turbine was developed. The signals were acquired to test and verify the impact of aerodynamic imbalance. Secondly, a method based on optimized maximum correlated kurtosis deconvolution was proposed for the primary detection. The intrinsic mode functions of nacelle vibration were adopted as the input variable. The weak unbalanced signals could be discerned. Moreover, the azimuth of rotor allows the unbalanced blades to be obtained. Thirdly, a convolutional neural network with a new structure was used to determine the magnitudes of aerodynamic imbalances. The first layer of the convolutional neural network is sufficiently wide for improving feature extraction, it could make nacelle acceleration as the input. This structure exhibits accuracy and robustness satisfactorily. Finally, the framework was demonstrated in a high-fidelity simulation environment. Different scenarios of aerodynamic imbalance were tested, the results demonstrate the satisfactory performance of the proposed framework.

On the hydrodynamic responses of a multi-column TLP floating offshore wind turbine model

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.

Parametrisation Scheme for Multidisciplinary Design Analysis and Optimisation of a Floating Offshore Wind Turbine Substructure – OC3 5MW Case Study

The development of novel energy technologies is considered imperative in the provision of solutions to meet an increasing global demand for clean energy. Floating Offshore Wind Turbine (FOWT) is one of the emerging technologies to exploit the vast wind resources available in deeper waters. To lower the levelized cost of energy (LCOE) or optimise the performance response associated with a FOWT system, a detailed understanding of the different disciplines (Aero-Hydro-Servo-Elastic) within the system and the relationship between the FOWT system and the dynamics of the marine environment is required. This requires an efficient Multidisciplinary Design, Analysis and Optimisation (MDAO) framework for FOWT systems to reduce the capital cost and increase dynamic performance. A key component of any MDAO framework is the shape parameterisation scheme, as it enables the modelling of a large array of platform designs with different geometric shapes using limited number of parameters. This work focuses on the B-Spline parameterisation modelling technique of OC3 spar-buoy and the use pattern search optimization algorithm to select the optimal design variants. The parametrisation technique is implemented in an analysis framework, where a B-spline library from Sesam GeniE is used to model each design representation, and a potential flow frequency domain analysis solver (HydroD/Wadam) is used for the hydrodynamic analysis. Validation of the selected designs within the design space is conducted with a benchmark NREL5MW spar-buoy hydrodynamic response results in literature with the hydrodynamic response of the frequency domain modelling approach using Sesam GeniE and HydroD/Wadam. This analysis process shows a high accuracy in response results between the OC3 spar-buoy in literature and the OC3 spar-buoy model design using B-Spline parametrization technique. Key performance metrics like the cost of materials and root mean square (RMS) of the nacelle acceleration also show improvement with the design variants compared to estimation from OC3 design in literature.

Impact of pitch actuator fault on 10-MW semi-submersible floating wind turbine

The pitch actuator of the offshore wind turbine has a high failure rate, and this fault will lead to aerodynamic imbalance on the floating system, especially when the 10 MW wind turbine is applied. In this paper, the impact of actuator faults on the floating platform motion and turbine structure is investigated. The 10-MW OO-Star semi-submersible platform is used as the reference structure. The fault discussed in this study involved one blade seizing, followed by an emergency shutdown. Coupled non-linear aero-hydro-servo-elastic simulations were carried out in the time domain using OpenFAST. The platform motions and structural loads caused by the fault were compared with the normal operating conditions. The effects of the pitch rate at the initial fault on the loads of the critical components are presented. The results revealed that imbalanced loads due to blade seizing can lead to particularly large excitations of the dynamic responses of the wind turbine. The maximum lateral nacelle acceleration is increased by 27.9%. In addition, the parametric study on the effects of the pitch rate for feathering blade was carried out. It was found that the pitch rate of 8 deg/s is favorable to alleviate the structural loads.

Design and Techno-Economic Analysis of a Novel Hybrid Offshore Wind and Wave Energy System

In the past few years, advanced technologies such as floating offshore wind turbines (FOWT) and wave energy converters (WECs) have been developed. As demonstrated by the innovative hybrid platform Poseidon, the feasibility of combining floating wind turbines and wave energy converters has already been explored. Furthermore, diversification of offshore renewable energy technologies reduces power fluctuations and lowers investment costs. This paper focuses on the development of an integrated wind and wave platform and the creation of a numerical model to evaluate the system performance for the Belmullet site. The novel concept consists of the semi-submersible Nautilus platform, integrated with four-point absorbers. A hydro-servo-aero time-domain model, combining WEC-Sim with an in-house wind turbine model, simulated the device motion and estimated the power generated. The performance of the Wave Energy Converters (WECs) was optimised based on their Power Take Off (PTO) damping. Finally, the hybrid concept was compared with the simple FOWT concerning the energy produced, Levelized Cost of Energy (LCOE) and hydrodynamic stability. The hybrid configuration proved to be a promising solution with 10% lower LCOE and improved hydrodynamic stability evaluated in terms of nacelle acceleration and platform pitch motion. These results show that wind and wave could be one of the best solutions for the future of the marine energy sector and the energy transition.

Numerical modelling and dynamic response analysis of a 10 MW semi-submersible floating offshore wind turbine subjected to ship collision loads

The number of installed offshore wind turbines is continuously growing worldwide in recent years. Offshore wind farms are generally located near the coast close to traffic lanes and are exposed to the risk of collisions from visiting and passing ships. Potential consequences of collisions may vary from local structural damage to the detachment of turbine nacelles and rotors, and even tower collapse and capsizing of the turbine platform, causing significant economic loss and fatalities.This paper investigates ship collision responses of a semi-submersible floating offshore wind turbine (FOWT), i.e. the OO-STAR floater with the DTU 10 MW blades, using the nonlinear finite element code USFOS. The OO-STAR floater is made of post-tensioned concrete designed by Dr. techn. Olav Olsen. The striking ships are selected to be a modern supply vessel of 7500 tons and a shuttle tanker of 150,000 tons, representing respectively service/coastal merchant vessels and large passing vessels. Modelling of the FOWT in USFOS is described in detail including the OO-STAR floater, the DTU 10 MW turbine blade, the turbine tower, and the mooring system. The modelled hydrodynamic loads include buoyancy loads and motion induced radiation loads using the Morrison equation. The effects of external waves and currents are assumed to be small and ignored in all directions. Eigenmode analysis of the turbine model is performed to verify the established model.Global collision response analyses of the FOWT were performed in both parked and operating conditions. The ship resistance is modelled as nonlinear springs in USFOS containing force displacement curves simulated in LS-DYNA. In operating conditions, wind loads are introduced including wind thrust loads and wind induced torque to rotate the turbine blades. The changes of upstream wind speeds and rotor/wake interactions during collisions are neglected. The results are discussed with respect to energy absorption of the ship and the FOWT, and structural responses of the FOWT including global motions, nacelle accelerations, tower clearance, tower vibrations, and responses of the mooring system.

Analysis of Dynamic Response Characteristics for 5 MW Jacket-type Fixed Offshore Wind Turbine

This study aims to evaluate the dynamic responses of the jacket-type offshore wind turbine using FAST software (Fatigue, Aerodynamics, Structures, and Turbulence). A systematic series of simulation cases of a 5 MW jacket-type offshore wind turbine, including wind-only, wave-only, wind & wave load cases are conducted. The dynamic responses of the wind turbine structure are obtained, including the structure displacement, rotor speed, thrust force, nacelle acceleration, bending moment at the tower bottom, and shear force on the jacket leg. The calculated time-domain results are transformed to frequency domain results using FFT and the environmental load with more impact on each dynamic response is identified. It is confirmed that the dynamic displacements of the wind turbine are dominant in the wave frequency under the incident wave alone condition, and the rotor thrust, nacelle acceleration, and bending moment at the bottom of the tower exhibit high responses in the natural frequency band of the wind turbine. In the wind only condition, all responses except the vertical displacement of the wind turbine are dominant at three times the rotor rotation frequency (considering the number of blades) generated by the wind. In a combined external force with wind and waves, it was observed that the horizontal displacement is dominant by the wind load. Additionally, the bending moment on the tower base is highly affected by the wind. The shear force of the jacket leg is basically influenced by the wave loads, but it can be affected by both the wind and wave loads especially under the turbulent wind and irregular wave conditions.

Motion control studies of a barge mounted offshore dynamic wind turbine using gyrostabilizer

This paper is a sequel to our recently published article “Motion control of a barge for offshore wind turbine (OWT) using gyrostabilizer’, Ocean Eng. 209 (2020). https://doi.org/10.1016/j.oceaneng.2020.107500.The present investigation focuses on the performance of a gyrostabilizer on a barge mounted dynamic Offshore Wind Turbine (OWT) on reducing the angular motions with rotor operating at different speeds through experimental studies. The adjective ‘dynamic’ refers to model wind turbine with rotor that is in motion. To represent the operational scenario, different wind turbine rotor rotating conditions such as below rated power (60 rpm), rated power (85 rpm) and above rated power (114 rpm) were selected. Experiments were conducted in the wave basin with a 1:50 Froude-scaled model of ITI barge supported 5-MW NREL OWT. Forced vibration tests were conducted to evaluate the platform rotations and nacelle response under regular and irregular waves in parked and operating conditions. The platform rotations and nacelle accelerations during roll and pitch DOF for gyrostabilizer ‘off’ and ‘on’ condition with gyro flywheel spin velocity of 5700 rpm were measured and compared to evaluate the motion reduction rates. Using gyrostabilizer, under regular waves, roll/pitch motion reduction by 64/54% for parked, 57/39% for below rated, 67/45% for rated and 64/39% for above rated conditions were obtained. For irregular waves a better reduction of these motions was obtained. By reducing the roll and pitch motions, the nacelle responses are also reduced significantly.

Dynamics of a Y-shaped semi-submersible floating wind turbine: a comparison of concrete and steel support structures

ABSTRACT Concrete semi-submersible floating wind turbines (FWTs) have generated increasing interest in the offshore wind energy industry due to their superiority over steel structures in aspects of construction and maintenance. However, the influence of construction materials on the dynamics of FWTs was scarcely studied before. The dynamics of two FWTs, mounted on a concrete and a steel Y-shaped semi-submersible platform respectively, are investigated by means of 1:60 scale combined wind and wave model tests and coupled multi-body simulations. The results indicate that the steel structure with a lower centre of gravity exhibits advantages in platform pitch motion alleviation. The steel structure is also subjected to lower pitch-induced tower base loads and nacelle acceleration than its concrete counterpart. However, an opposite trend is found in the wave-frequency responses, leading to an insignificant difference of tower base loads and nacelle acceleration between the two structures. ABBREVIATIONS: FWTs: Floating wind turbines; CoG: Centre of gravity; CoB: Centre of buoyancy; LCP3: Lightweight prestressed concrete platform; HSP3: High-strength steel platform; WTWF: Wind tunnel and wave flume joint laboratory; ROC: Rated operational condition; JONSWAP; Joint North Sea Wave Observation Projection; RAO: Response amplitude operator; BEM: Blade element momentum theory; DOF: Degree of freedom; GDW: Generalised dynamic wake theory; PSD: Power spectral density; QTF: Quadratic transfer function

Vibration reduction methods of large-scale wind turbines based on system-TMD coupled algorithm

To relieve response of OWTs, this paper investigates the application of fore–aft tuned mass damper (TMD) in nacelle/tower for passive control. Considering flexibility of key components, system-TMD coupled algorithm is proposed based on the multi-body dynamics theory for a monopile offshore wind turbine. Hereby the improved artificial fish swarm algorithm is applied to optimize parameters and installation location of a TMD in terms of dynamic response and modal analysis. The results show that when TMD inherent frequency approximates the first-order inherent frequency of the tower front/back bending moment, vibration reduces to the maximum extent, so does the nacelle acceleration in the frequency domain; optimal installation location is within 0~1 m away from the center of the tower top-nacelle connection toward the front end of the nacelle; and the TMD after parameter optimization may function to reduce the tower top displacement by 68.30% under the shutdown condition.

Global sensitivity study on the semisubmersible substructure of a floating wind turbine: Manufacturing cost, structural properties and hydrodynamics

Trading off the capital expenditure and dynamic performance of the substructure through optimization techniques is the key to wide application of floating wind turbines (FWT) in the energy industry. The optimization of floating substructure is a high-dimensional problem. Sensitivity analysis can help reducing the dimensions of design space by highlighting the most relevant design parameters to potential objective functions or to constraints, and increasing the understanding in the interrelationship of various parameters throughout the design space. This paper evaluates the influence of the hull main dimensions and mooring system parameters on the manufacturing cost, structural properties and hydrodynamics of a four-column semi-submersible FWT. Two widely-used global sensitivity analysis methods, the variance-based method and the density-based method, are employed to measure the sensitivity contributions of 10 design parameters. The column radius is found to be one of the most influential factors for substructure manufacturing cost and surge natural frequency, while the platform pitch and tower fore-aft natural frequency are more sensitive to column spacing. Regarding the hydrodynamics, column spacing, column radius and platform draft are the three of the most critical factors for the wave-frequency response in surge, pitch and tower top acceleration. The impact of draft is less significant in the case of low-frequency (second-order) hydrodynamic response, column spacing and radius are much more dominant. These results are confirmed by the visual inspection of the input-output samples. This information provides a new perspective to the FWT substructure design space and serves as a basis for the further sensitivity investigation on the long-term dynamic performance such as the peak platform tilting angle, nacelle acceleration, fatigue damage at tower base and fairlead.

Transient response of a TLP-type floating offshore wind turbine under tendon failure conditions

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.

Operational and extreme responses of a new concept of 10MW semi-submersible wind turbine in intermediate water depth: An experimental study

A floating wind turbine concept, named as SPIC, is composed of the DTU 10MW reference wind turbine and a newly designed semi-submersible platform featured with three partially inclined side columns. The dynamic responses of the SPIC concept considering an intermediate water depth of 60 m are investigated through the basin model test approach. After calibrating the wind and wave conditions and identifying the model wind turbine system, the operational and short-term extreme responses of the floating wind turbine concept were studied. This innovative concept meets reasonable natural period requirements and achieves great stability under design environmental conditions. For operational responses, low-frequency turbulent wind increases the surge resonance response but provides aerodynamic damping to reduce pitch resonance and tower vibration. The wind also excites the responses at 1P, 2P, and 3P frequencies, but the current slightly suppress these responses. The extreme surge, mooring line tension and nacelle acceleration occur under extreme environments, while the extreme pitch, tower-top shear force and tower-base bending moment should be examined under operational conditions with rated wind as well. These extreme responses are proved to be smaller than the limited values recommended by standards, demonstrating the reasonability of the SPIC concept in intermediate water depth.

Reduction in the Fluctuating Load on Wind Turbines by Using a Combined Nacelle Acceleration Feedback and Lidar-Based Feedforward Control

An advanced pitch controller is proposed for the load mitigation of wind turbines. This study focuses on the nacelle acceleration feedback control and lidar-based feedforward control, and discusses how these controllers contribute to reduce the load on wind turbines. The nacelle acceleration feedback control increases the damping ratio of the first mode of wind turbines, but it also increases the fluctuation in the rotor speed and thrust force, which results in the optimum gain value. The lidar-based feedforward control reduces the fluctuation in the rotor speed and the thrust force by decreasing the fluctuating wind load on the rotor, which reduces the fluctuating load on the tower. The combination of the nacelle acceleration feedback control and the lidar-based feedforward control successfully reduces both the response of the tower first mode and the fluctuation in the rotor speed at the same time.

Design Optimization and Coupled Dynamics Analysis of an Offshore Wind Turbine with a Single Swivel Connected Tether

The increased interest in renewable wind energy has stimulated many offshore wind turbine concepts. This paper presents a design optimization and a coupled dynamics analysis of a platform with a single tether anchored to the seabed supported for a 5 MW baseline wind turbine. The design is based on a concept named SWAY. We conduct a parametric optimization process that accounts for important design considerations in the static and dynamic view, such as the stability, natural frequency, performance requirements, and cost feasibility. Through these optimization processes, we obtain and present the optimized model. We then establish the fully coupled aero-hydro-servo-elastic model by the time-domain simulation tool FAST (Fatigue, Aerodynamics, Structures, and Turbulence) with the hydrodynamic coefficients from an indoor program HydroGen. We conduct extensive time-domain simulations with various wind and wave conditions to explore the effects of wind speed and wave significant height on the dynamic performance of the optimized SWAY model in various water depths. The swivel connection between the platform and tether is the most special design for the SWAY model. Thus, we compare the performance of models with different tether connection designs, based on the platform motions, nacelle velocity, nacelle accelerations, resonant behaviors, and the damping of the coupled systems. The results of these comparisons demonstrate the advantage of the optimized SWAY model with the swivel connection. From these analyses, we prove that the optimized SWAY model is a good candidate for deep water deployment.

Analysis of a Gyroscopic-Stabilized Floating Offshore Hybrid Wind-Wave Platform

The energy innovation scenario sees hybrid wind-wave platforms as a promising technology for reducing the variability of the power output and for the minimization of the cost of offshore marine renewable installations. This article presents a model that describes the installation of a 5 MW wind turbine on a floating platform designed by Fincantieri and equipped with gyroscopic stabilization. The use of gyros allows for the delivery of platform stabilization by damping the wave and wind induced motion on the floater and at the same time producing extra power. Shetland Island was chosen as the reference site because of its particularly harsh weather. Final results show that the total production of power in moderate and medium climate conditions is considerable thanks to the installation of the gyro, together with a significant stabilization of the platform in terms of pitching angle and nacelle acceleration.

A New Conceptual Design and Dynamic Analysis of a Spar-Type Offshore Wind Turbine Combined with a Moonpool

Floating offshore wind turbines promise to provide an abundant source of energy. Currently, much attention is being paid to the efficient performance and the economics of floating wind systems. This paper aims to develop a spar-type platform to support a 5-MW reference wind turbine at a water depth of 150 m. The spar-type platform includes a moonpool at the center. The design optimization process is composed of three steps; the first step uses a spreadsheet to calculate the platform dimensions; the second step is a frequency domain analysis of the responses in wave conditions; and the final step is a fully coupled simulation time domain analysis to obtain the dynamic responses in combined wind, wave, and current conditions. By having a water column inside the open moonpool, the system’s dynamic responses to horizontal and rotating motions are significantly reduced. Reduction of these motions leads to a reduction in the nacelle acceleration and tower base bending moment. On the basic of optimization processes, a spar-type platform combined with a moonpool is suggested, which has good performance in both operational conditions and extreme conditions.