Semi-submersible Floating Offshore Wind Turbine Research Articles

Dynamic Responses for WindFloat Floating Offshore Wind Turbine at Intermediate Water Depth Based on Local Conditions in China

Offshore wind energy, a clean energy resource, is considered to be a possible alternative to fossil energy. Floating offshore wind technology is considered to be a proper concept to develop abundant wind energy in deep water. Considering the reality of offshore wind energy development in China, the floating offshore wind turbine concept is expected to be developed at moderate water depths. In this paper, a mooring system of the WindFloat semisubmersible floating offshore wind turbine (SFOWT) at a water depth of 60 m is designed. The dynamic responses of the WindFloat SFOWT under different wind–wave combination conditions are investigated using the coupled method and the simplified method, which do not include the effect of the tower top motion in the aerodynamic calculation. The results show that the dynamic responses of the WindFloat SFOWT, including the platform motions, tower loads, and mooring line tensions, perform fairly well at a moderate water depth. A comparison between the coupled method and simplified method shows that the calculated results are slightly different between the different conditions for the time domain results, response spectra results, and fatigue results. In addition, mooring line 1 (ML 1) suffers higher fatigue damage than ML2, which should be paid more attention.

Risk assessment of floating offshore wind turbines based on fuzzy fault tree analysis

Risk assessment of Floating Offshore Wind Turbines (FOWTs) relies on failure statistics, which are usually collected from various source. In order to quantify the information uncertainties, this paper introduces the Fuzzy Fault Tree Analysis (FFTA) to assess the risk of semi-submersible FOWTs. A Fault Tree (FT) model is adopted to analyze the risk evolution mechanism of the FOWT and the fuzzy set theory is employed to process the failure statistics. As the results of quantitative analysis, the failure intervals of the FOWT and critical assemblies are calculated. The risk measure named Fuzzy Critical Importance (FCI) is utilized to compute the sequence of the basic fault events and identify critical components. A comparison analysis is performed and the results indicate that the present FFTA method is more in line with practical and has the merit in error reduction of failure rate figured out.

Effects of Dynamic Motion and Structural Response of a Semi-submersible Floating Offshore Wind Turbine Structure Under Waves Generated in a Hurricane Environment

This study aims to analyze the effects of dynamic motion and structural response of semi-submersible floating offshore wind turbine structures (FOWTs) in waves generated in hurricane environments. The extreme environment utilizes numerical simulation data of hurricanes and is divided into low-frequency and high-frequency components based on wave-age, and a load case combining wind and wave is applied. As the hurricane progresses, a spectrum divided into swell and wind sea components is generated in the ocean, and the swell component is distributed in the natural frequency range of the structure, increasing the dynamic response. Dynamic analysis compares the cumulative response by fitting the response for each wave component generated by the hurricane through the Gaussian mixture model. In the structural analysis, the hydrodynamic pressure calculated through diffraction analysis is mapped to a finite element model using the quasi-dynamics interaction method, and the von Mises stress per wave component is analyzed. Through this, it is possible to grasp the main factors of the dynamic and structural responses of semi-submersible FOWTs.

The influence of power-take-off control on the dynamic response and power output of combined semi-submersible floating wind turbine and point-absorber wave energy converters

Floating offshore wind turbines (FOWTs) have received extensive attention in recent years, particularly after the successful demonstration of several pilot projects, such as Hywind and WindFloat. Integrating wave energy converters (WECs) into FOWTs could potentially help reduce cost of energy by absorbing additional power from waves and introduce restoring moments and extra damping to the floating platform thus reducing motion responses and fatigue loads. In this work, we propose a hybrid floating wind and wave power generation platform, consisting of a semi-submersible FOWT and three point-absorber WECs. Preliminary feasibility study of this concept is performed with verified integrated aero-hydro-servo-mooring numerical simulations. Dynamic response and power output of this hybrid concept are evaluated under several typical environmental conditions. Particularly, different WEC power-take-off control strategies have been comparatively studied, which have shown considerable influences on the platform dynamics and power generation. More specifically, reactive control generally worsen the platform motion responses, while spring–damping control is able to mitigate the pitch motion to certain extent. Regarding power output, reactive control leads to the highest wave power generation, almost twice as much as that of spring–damping, which has been used in most existing works on hybrid power generation system. Moreover, it is found the optimal control design for point-absorber WEC attached to fixed structures is no longer optimal for the combined floating wind and wave energy production platform, which needs further investigations in the future.

Structural Control of an Ultra-Large Semi-Submersible Floating Offshore Wind Turbine

Abstract A braceless semi-submersible floating platform is proposed for a Technical University of Denmark (DTU) 10-MW wind turbine at moderate water depths with reference to an existing National Renewable Energy Laboratory (NREL) 5-MW braceless semi-submersible floating platform, and a servo control system for a 10-MW semi-submersible floating offshore wind turbine (FOWT) is introduced. To control the ultimate and fatigue loads of the FOWT, a fore-aft tuned mass damper (TMD) installed in the nacelle of the 10-MW semi-submersible FOWT was investigated for vibration alleviation and load reduction. Considering the hydrodynamic and mooring effect, a four degrees-of-freedom (DOFs) (platform surge and pitch motions, tower fore-aft bending, and TMD translation) simplified dynamic model for the 10-MW semi-submersible FOWT is established based on D’Alembert’s principle. Then, the parameter estimation is conducted based on the Levenberg–Marquardt (LM) algorithm, and the simplified dynamic model was further verified by comparing the output responses with FAST and the proposed model. Furthermore, the exhaustive search (ES) and genetic algorithm (GA) are embedded into the simplified dynamic model to optimize the TMD parameters. Finally, a fully coupled time-domain simulation for all the selected environmental conditions is conducted in FAST, and the vibration suppression performance of the optimized TMD design for the 10-W semi-submersible FOWT was further examined and analyzed.

Effects of second-order hydrodynamics on an ultra-large semi-submersible floating offshore wind turbine

Second-order wave loads on floating offshore wind turbine (FOWT) may excite severe resonance at the eigenfrequencies of the floating system and cause fatigue damage. The hydrodynamic coefficients including first- and second-order wave force transfer functions are calculated, and fully coupled time-domain simulation by FAST is conducted for selected environmental conditions. Each load case is simulated without second-order loads, and with difference-frequency full quadratic transfer functions (QTFs), sum-frequency full QTFs, and complete second-order solution. The effect of second-order difference- and sum-frequency wave loads on the dynamic behaviors of the 10 MW semi-submersible FOWT is discussed. Subsequently, the effect of second-order wave loads on the structural fatigue behavior of the 10 MW and 5 MW semi-submersible FOWTs is investigated and compared by computing ultimate and damage-equivalent loads (DELs). The results show that ignoring second-order difference-frequency wave loads may underestimate the low-frequency responses, and second-order sum-frequency wave loads significantly affect the structural dynamics of the 10 MW FOWT. In addition, the effect of second-order wave loads on the fatigue damage of the 5 MW and 10 MW semi-submersible FOWTs are significantly different.

Towards credible CFD simulations for floating offshore wind turbines

The credibility in the results needs to be demonstrated in order to be able to use computational fluid dynamics (CFD) as an engineering tool. This may be obtained by sufficient verification and validation studies involving error and uncertainty quantification. This study investigates how to perform credible CFD simulations of floating offshore wind turbines (FOWTs). Three methods to estimate discretisation errors were compared for three different problems related to FOWTs: wave propagation in 2D, wave loads on a circular cylinder, and surge decay of a semi-submersible FOWT. The three discretisation error estimation methods are a least-squares formulation using the observed order of convergence in combination with a data quality measure for different spatial and temporal refinement, another least-squares fit method using the theoretical order of convergence for constant Courant number grid refinement studies, and the factor of safety method applying a ratio between observed and theoretical order of convergence. We compared the final results to an analytical solution of the 2D wave signal, and to experimental data for the wave loads and surge decay motions, for validation purposes. The results of this work show the advantages and disadvantages of the three error estimation methods. The uncertainty bars for the discretisation uncertainty of the numerical simulations were mostly larger than the comparison error with the model test data and analytical solution.

Second-order hydrodynamic effects on the response of three semisubmersible floating offshore wind turbines

Floating structures have become the most feasible solution for supporting wind turbines when offshore wind project move to deeper water. In this paper, a hydrodynamic analysis of three different semisubmersible floating offshore wind turbines is carried out including second-order hydrodynamic effects. The three examined platforms are V-shaped semisubmersible, Braceless semisubmersible and OC4-DeepCwind semisubmersible and are used to support the NREL 5 MW reference wind turbine. The main objective of the present study is to investigate and compare the hydrodynamic response of the three different semisubmersible floaters in two water depths (100 m, and 200 m) under different load conditions. The effects of second-order wave loads on the platform motions and mooring tension are discussed and compared by using different methods including Newman's approximation and the full QTF (Quadratic transfer function) method. The drag effect on the structure motion response is also discussed in this paper. The comparison presented is based on statistical values and response spectra of floating platform motions as well as mooring tensions. The results show that the dynamic response of semisubmersible FOWTs (floating offshore wind turbines) is overestimated when ignoring the Morison drag effect on the columns of the semisubmersible FOWT. The second-order difference wave loads can excite the resonance of motion especially for the platform-pitch motion, which could cause structural failures. The full QTF method should be used to calculate the second-order wave force to better simulate the realistic dynamic response of semisubmersible FOWTs.

Hydrodynamic Response of a Combined Wind–Wave Marine Energy Structure

Due to the energy crisis and greenhouse effect, offshore renewable energy is attracting increasing attention worldwide. Various offshore renewable energy systems, such as floating offshore wind turbines (FOWTs), and wave energy converters (WECs), have been proposed and developed so far. To increase power output and reduce related costs, a combined marine energy structure using FOWT and WEC technologies has been designed, analyzed and presented in the present paper. The energy structure combines a 5-MW braceless semisubmersible FOWT and a heave-type WEC which is installed on the central column of the semisubmersible. Wave power is absorbed by a power take-off (PTO) system through the relative heave motion between the central column of the FOWT and the WEC. A numerical model has been developed and is used to determine rational size and draft of the combined structure. The effects of different PTO system parameters on the hydrodynamic performance and wave energy production of the WEC under typical wave conditions are investigated and a preliminary best value for the PTO’s damping coefficient is obtained. Additionally, the effects of viscous modeling used during the analysis and the hydrodynamic coupling on the response of the combined structure are studied.

Mechanism of mooring line breakage of floating offshore wind turbine under extreme coherent gust with direction change condition

With an extreme load condition, the mooring system of a floating offshore wind turbine (FOWT) will be led to failure, such as mooring line breakage. However, the induced FOWT mooring line breakage in extreme gust still requires further study for design optimization in the future. In this paper, an aero-hydro-cable-servo time domain coupled simulation have been carried out of a NREL’s 5 MW OC4-DeepCwind semi-submersible type FOWT for investigate the mooring system response under extreme coherent gust with direction change (ECD) condition. The platform is assumed to be installed at 50 m depth location in the South China Sea. The practical ECD is simulated by a fast conversion between two wind conditions with different mean wind speeds and wind direction. In addition, the gust characteristics that can generate snap tension of mooring lines were identified, and the consequence of the induced ECD accident is investigated. ECD condition with rise time of 10 s is prone to cause a snap tension of the mooring line, and it may eventually lead to cascading mooring line breakage and potential catastrophic collision events.

The influence of an unstable turbulent wind spectrum on the loads and motions on floating Offshore Wind Turbines

Floating offshore wind turbines (FOWT) are exposed to harsh environmental conditions and often experience unstable atmospheric conditions. Including the effects of atmospheric stability should improve the accuracy of fatigue load calculations, and subsequently, the design of the wind turbine. The current standards recommend two turbulence spectral models that are valid for neutral atmospheric conditions only. The objective of this study is to investigate the influence of Hejstrup’s 1981 Unstable Spectra Model on the loads and motions on a spar and a semi-submersible FOWT. This study focuses on the effect of unstable atmospheric conditions in the free stream wind and does not include the effect of wakes. The most significant differences observed in this study were for the tower top torsion loads, where very unstable conditions gave 47% larger loads than neutral conditions for OC3-Hywind turbine and 30.4% larger for OC4-DeepCwind turbine. Since very unstable conditions corresponded to the highest turbulence intensities and larger turbulent fluctuations, they also resulted in higher fatigue loads. The blade root flap-wise loads were also observed to be higher under unstable conditions compared to neutral conditions, but the differences were smaller with 7.5% for OC3-Hywind and 23% for OC4-DeepCwind.

Analysis of Dynamic Characteristics of an Ultra-Large Semi-Submersible Floating Wind Turbine

An initial design of the platform for the moderate water depth (100 m) is performed by upscaling of an existing 5 MW braceless semi-submersible platform design to support the DTU (Danish University of Science and Technology) 10 MW wind turbine. To investigate the dynamic characteristics of the ultra-large semi-submersible floating offshore wind turbine (FOWT), an aero-hydro-servo-elastic numerical modeling is applied to carry out the fully coupled time-domain simulation analysis. The motion responses of the ultra-large semi-submersible FOWT are presented and discussed for selected environmental conditions. Based on the quasi-static and dynamic analysis methods, the influence of the dynamic effects of the mooring lines on the platform motion responses and mooring line tension responses are discussed. Subsequently, the difference in the motion responses and structural dynamics of the DTU 10 MW and NREL (National Renewable Energy Laboratory) 5 MW FOWT is studied due to the difference in turbine properties. The simulation results reveal that the excitation of the low-frequency wind loads on the surge and pitch motions, the tower-base fore-aft bending moments and the mooring line tension response becomes more prominent when the size of the wind turbine increases, but the excitation action of the 3P effect on the structural dynamics of the 5 MW FOWT is more obvious than those of the 10 MW FOWT.

Wind spectral characteristics on fatigue responses of towerbase and moorings of a floating offshore wind turbine

The tower-platform interface and mooring system of floating offshore wind turbines (FOWTs) are some of the most critical components with significant influences on overall project costs. In addition to satisfying strength requirements, it is typical and vital to meet fatigue criteria for a service life of 25 years or more. Wind spectra characteristics considered in analysis can penalize fatigue designs, leading to unnecessary costs. The International Electrotechnical Commission (IEC, 2009) recommends the use of site-specific wind data (spectrum, turbulence intensity, etc.) in design of FOWTs, but for offshore sites it is often the case that such data is unavailable and land-based data are used as surrogates in design. For such scenarios, it is worth investigating whether such alternative approach is suitable and accurate, and understanding the consequence of the selection of wind spectral characteristics on fatigue design. This paper addresses the impact of the subsequent selection on fatigue responses of towerbase and mooring system in a FOWT, as a sequel to the paper by Udoh and Zou (2018) which focused on impacts on strength design. The 5 MW semi-submersible FOWT platform with six mooring lines implemented in the preceding study is applied in analysis. Results indicate significant variations in resulting fatigue life with considered wind parameters. Thus, it is critical to apply proper wind spectra characteristics for analysis and design of FOWTs to avoid unnecessary conservatism and costs. Based on the findings of this study, more explicit guidance on the application of turbulence intensities for IEC-recommended models in offshore sites could lead to more accurate load estimates in design of FOWTs.

Reducing Tower Fatigue through Blade Back Twist and Active Pitch-to-Stall Control Strategy for a Semi-Submersible Floating Offshore Wind Turbine

The necessity of producing more electricity from renewable sources has been driven predominantly by the need to prevent irreversible climate chance. Currently, industry is looking towards floating offshore wind turbine solutions to form part of their future renewable portfolio. However, wind turbine loads are often increased when mounted on a floating rather than fixed platform. Negative damping must also be avoided to prevent tower oscillations. By presenting a turbine actively pitching-to-stall, the impact on the tower fore–aft bending moment of a blade with back twist towards feather as it approaches the tip was explored, utilizing the time domain FAST v8 simulation tool. The turbine was coupled to a floating semisubmersible platform, as this type of floater suffers from increased fore–aft oscillations of the tower, and therefore could benefit from this alternative control approach. Correlation between the responses of the blade’s flapwise bending moment and the tower base’s fore–aft moment was observed with this back-twisted pitch-to-stall blade. Negative damping was also avoided by utilizing a pitch-to-stall control strategy. At 13 and 18 m/s mean turbulent winds, a 20% and 5.8% increase in the tower axial fatigue life was achieved, respectively. Overall, it was shown that the proposed approach seems to be effective in diminishing detrimental oscillations of the power output and in enhancing the tower axial fatigue life.

Development of a fully coupled aero-hydro-mooring-elastic tool for floating offshore wind turbines

A floating offshore wind turbine (FOWT) is a coupled system where a wind turbine with flexible blades interacts with a moored platform in wind and waves. This paper presents a high-fidelity aero-hydro-mooring-elastic analysis tool developed for FOWT applications. A fully coupled analysis is carried out for an OC4 semi-submersible FOWT under a combined wind/wave condition. Responses of the FOWT are investigated in terms of platform hydrodynamics, mooring dynamics, wind turbine aerodynamics and blade structural dynamics. Interactions between the FOWT and fluid flow are also analysed by visualising results obtained via the CFD approach. Through this work, the capabilities of the tool developed are demonstrated and impacts of different parts of the system on each other are investigated.

A CFD study of coupled aerodynamic‐hydrodynamic loads on a semisubmersible floating offshore wind turbine

AbstractThe prediction of dynamic characteristics for a floating offshore wind turbine (FOWT) is challenging because of the complex load coupling of aerodynamics, hydrodynamics, and structural dynamics. These loads should be accurately calculated to yield reliable analysis results in the design phase of a FOWT. In this study, a high‐fidelity fluid‐structure interaction simulation that simultaneously considers the influence of aero‐hydrodynamic coupling due to the dynamic motion of a FOWT has been conducted using computational fluid dynamics based on an overset grid technique. The DeepCwind semisubmersible floating platform with the NREL 5‐MW baseline wind turbine model is considered for objective numerical verification with the NREL FAST code. A state‐of‐the‐art computational model based on the coupled computational fluid dynamics and dynamic structure analysis is constructed and analyzed to solve multiphase flow, 6 degrees of freedom motions of OC4 semisubmersible FOWT. A quasi‐static mooring solver is also applied to resolve the constraint motion of floater because of a 3‐line mooring system. The influence of tower shadow on the unsteady aerodynamic performance and loads is also demonstrated. Finally, complex unsteady flow fields considering blade and tower interference effects among blade‐tip vortices, shedding vortices, and turbulent wakes are numerically visualized and investigated in detail.

Establishing a fully coupled CFD analysis tool for floating offshore wind turbines

An accurate study of a floating offshore wind turbine (FOWT) system requires interdisciplinary knowledge about wind turbine aerodynamics, floating platform hydrodynamics and mooring line dynamics, as well as interaction between these discipline areas. Computational Fluid Dynamics (CFD) provides a new means of analysing a fully coupled fluid-structure interaction (FSI) system in a detailed manner. In this paper, a numerical tool based on the open source CFD toolbox OpenFOAM for application to FOWTs will be described. Various benchmark cases are first modelled to demonstrate the capability of the tool. The OC4 DeepCWind semi-submersible FOWT model is then investigated under different operating conditions.With this tool, the effects of the dynamic motions of the floating platform on the wind turbine aerodynamic performance and the impact of the wind turbine aerodynamics on the behaviour of the floating platform and on the mooring system responses are examined. The present results provide quantitative information of three-dimensional FSI that may complement related experimental studies. In addition, CFD modelling enables the detailed quantitative analysis of the wind turbine flow field, the pressure distribution along blades and their effects on the wind turbine aerodynamics and the hydrodynamics of the floating structure, which is difficult to carry out experimentally.

The coupled dynamic response computation for a semi-submersible platform of floating offshore wind turbine

Many numerical methods based on potential-flow theory have been used to analyze the hydrodynamic effect of floating offshore wind turbine (FOWT). However, they cannot directly and fully consider the complex viscous interference effects for the motion of multi-structure platform geometry. To accurately account for this effect, it necessarily requires an additional numerical modification process based on experimental data or an implementation of Morison equation. This paper deals with an unsteady hydrodynamic simulation using CFD method with dynamic motion based on overset grid and potential based panel approach for the semi-submersible DeepCWind FOWT. Using the CFD approach, the coupled fluid flow and multi-body dynamic analysis is applied using the volume of fluid approach to investigate the hydrodynamic responses of the typical semi-submersible floater. Herein, the restoring force and moment of the catenary mooring lines are also considered in the time-domain. Additionally, a potential flow linear diffraction model extended with and without Morison elements to include viscous damping is performed. It is shown that the present results show good correlations with experimental data without using the adjusting parameters. In addition, numerical tests for major solver parameters of the CFD method are conducted, verified, and investigated in detail.

Influence of Wave Induced Second-Order Forces in Semisubmersible FOWT Mooring Design

AZIMUT project (Spanish CENIT R&D program) is designed to establish the technological groundwork for the subsequent development of a large-scale offshore wind turbine. The project (2010–2013) has analyzed different alternative configurations for the floating offshore wind turbines (FOWT): SPAR, tension leg platform (TLP), and semisubmersible platforms were studied. Acciona, as part of the consortium, was responsible of scale-testing a semisubmersible platform to support a 1.5 MW wind turbine. The geometry of the floating platform considered in this paper has been provided by the Hiprwind FP7 project and is composed by three buoyant columns connected by bracings. The main focus of this paper is on the hydrodynamic modeling of the floater, with especial emphasis on the estimation of the wave drift components and their effects on the design of the mooring system. Indeed, with natural periods of drift around 60 s, accurate computation of the low-frequency second-order components is not a straightforward task. Methods usually adopted when dealing with the slow-drifts of deep-water moored systems, such as the Newman's approximation, have their errors increased by the relatively low resonant periods of the floating system and, since the effects of depth cannot be ignored, the wave diffraction analysis must be based on full quadratic transfer functions (QTFs) computations. A discussion on the numerical aspects of performing such computations is presented, making use of the second-order module available with the seakeeping software wamit®. Finally, the paper also provides a preliminary verification of the accuracy of the numerical predictions based on the results obtained in a series of model tests with the structure fixed in bichromatic waves.

Study of the Effect of Water Depth on Potential Flow Solution of the OC4 Semisubmersible Floating Offshore Wind Turbine

This work aims at assessing the influence of water depth on the potential flow solution for a semisubersible floating offshore wind turbine. More specifically, the system developed for the Offshore Code Comparison Collaboration Continuation (OC4) of the Inter- national Energy Agency IEA was considered for this paper. This work has been inspired by previous studies concerning the effect of shallow water on Liquified Natural Gas Carriers (LNGC). The influence of water depth on the hydrodynamics of such systems is evident from measurements as well as from simulations, specifically when secondary effects in the wave and flow modelling are addressed. This scenario has motivated the comparative study for the Floating Wind Turbine herein reported, also taking into account second order hydrodynamics (Quadratic Transfer Functions, QTF) as well as low frequency contribution in the incoming wave, due to shallow water (Setdown effect). The simulations were conducted relying on the codes DIFFRAC and aNySIM, de- veloped at Maritime Research Institute of Netherlands (MARIN) and the results are presented for a range of water depth between the nominal value of 200 m and the extreme shallow water of 30 m.