Floating Wind Turbine Concepts Research Articles

Numerical investigation of a new hybrid floating wind turbine concept

In today’s energy scenario dominated by the need to develop new methods of generating electricity, energy-intensive infrastructures are a promising solution towards energy self-sufficiency and environmental sustainability. Floating offshore wind turbine represents one of the major solution to exploit renewable energies. Currently, the increase of offshore wind market opens up the possibility of integrating different technologies to take advantage of marine energy potential, in particular wave energy.
 This work presents a new wave-wind hybrid floating platform with three Oscillating Water Columns (OWC) integrated in a floating offshore wind spar buoy. The design methodology is described showing the size of geometric characteristics of OWCs and the size of the energy conversion system. The hybrid system is investigated by a time-domain model that integrates the wind turbine model, the hydrodynamics of the floater, the thermodynamics of the OWC air chambers, and the damping effect induced by the OWC air turbine. The thermo-aero-hydro coupled numerical framework is described to highlight the implementation of the thermodynamic model in WEC-Sim/MATLAB environment. Specifically, the water column dynamics are solved as pistonrigid body representation, enabling the time-domain analysis of the OWC dynamic behaviour. Impulse turbines are considered as OWC Power Take-Offs (PTO). The resource scenario of Mediterranean Sea is considered as case study, in terms of both wind and wave conditions. The analysis focuses on the power extraction capabilities of the OWCs and on the impact of OWCs on the productivity of the wind turbine. Further developments on multi-objective design tools and on optimal PTO control design aiming power maximisation could be conducted for hybrid energy platforms based on this established numerical framework.

Platform yaw drift in upwind floating wind turbines with single-point-mooring system and its mitigation by individual pitch control

Abstract. This work demonstrates the feasibility of an individual pitch control strategy based on nacelle yaw misalignment measurements to mitigate the platform yaw drift in upwind floating offshore wind turbines, which is caused by the vertical moment produced by the rotor. This moment acts on the platform yaw degree of freedom, being of great importance in systems that have low yaw stiffness. Among them, single-point-mooring platforms are one of the most important ones. During recent years, several floating wind turbine concepts with single-point-mooring systems have been proposed, which can theoretically dispense with the yaw mechanism due to their ability to rotate and align with environmental conditions (weather-vaning). However, in this paper it is proven that the vertical moment overcomes the orienting ability, causing the yaw drift. With the intention of reducing the induced yaw response of a single-point-mooring floating wind turbine, an individual pitch control strategy based on nacelle yaw misalignment is applied, which introduces a counteracting moment. The control strategy is validated by numerical simulations using the 5 MW National Renewable Energy Laboratory (NREL) wind turbine mounted on a single-point-mooring version of the DeepCwind OC4 floating platform to demonstrate that it can mitigate the yaw drift and therefore maintain the alignment of the wind turbine rotor with the wind.

Development and performance evaluation of a novel wind generation system for the floating wind turbine model test

The Wind Generation System (WGS) is an essential piece of equipment for the model test of the Floating Wind Turbine (FWT) in integrated wind & wave environment. With the evolution of FWT techniques, the requirements for the comprehensive performance of WGS are improved. In this paper, we develop a novel WGS with modular design philosophy, aiming to improve the temporal & spatial quality and controllability of the generated wind fields. 68 individually-controlled carbon fiber propellers are deployed in a hexagonal configuration to form a 4.2 m × 4.2 m wind generation area. Steady/dynamic and uniform/non-uniform wind fields can be reproduced, with the wind speed varying from 0 to 12 m/s. Extensive tests are conducted to assess the feasibility and performance of the proposed WGS. The statistical characteristics of the generated wind fields coincide well with the targets, demonstrating a satisfying comprehensive performance of the present WGS. Finally, the WGS is applied to evaluate a novel FWT concept in an integrated model test in wave basin. The FWT dynamic performances agree well with the expected ones from numerical simulation, further demonstrating that the proposed WGS could contribute to improving the accuracy and reliability of FWT model tests.

Preliminary Analysis on the Hydrostatic Stability of a Self-Aligning Floating Offshore Wind Turbine

There exist vast areas of offshore wind resources with water depths greater than 100 m that require floating structures. This paper provides a detailed analysis on the hydrostatic stability characteristics of a novel floating wind turbine concept. The preliminary design supports an 8 MW horizontal-axis wind turbine with a custom self-aligning single-point mooring (SPM) floater, which is to be constructed within the existing shipyard facilities in the Maltese Islands, located in the Central Mediterranean Sea. The theoretical hydrostatic stability calculations used to find the parameters to create the model are validated using SESAM®. The hydrostatic stability analysis is carried out for different ballast capacities whilst also considering the maximum axial thrust induced by the rotor during operation. The results show that the entire floating structure exhibits hydrostatic stability characteristics for both the heeling and pitching axes that comply with the requirements set by the DNV ST-0119 standard. Numerical simulations using partial ballast are also presented.

Experimental and numerical analysis of wind field effects on the dynamic responses of the 10 MW SPIC floating wind turbine concept

Experimental methods are useful to study global responses of floating wind turbines under wind excitations. One experimental technique is based on the regeneration of wind fields in open space in a hydrodynamic laboratory and the use of the performance-matched rotor to produce the equivalent aerodynamic thrust force according to a target wind field. However, for wind generation systems, due to small variations in the output of each fan and differences among the fans, the obtained wind speed varies in space and time, making it difficult to generate the target ideal uniform, constant or time-varying wind fields. For investigating the quality of the wind field in the experiment and their effects on the global responses, numerical simulations under wind only conditions for the full-scale 10 MW SPIC wind turbine were compared with the upscaled responses from model tests. Because of the limitation in the generation and measurement of the target wind field, and the model scaling issue for wind turbine aerodynamics considering the TSR variation, there are uncertainties in interpreting the obtained responses. Numerical simulations were then performed to investigate the effects of spatial and temporal variations in wind fields. Wind shear influences 1P and 3P responses. Below the rated wind speed, when the turbulence intensity increases, the average responses decrease and the response spectra are dominated by low-frequency and 3P components. In Kaimal and Mann wind fields with coherent structures, the generated power and pitch motion show fewer fluctuations than in uniform time-varying wind field, while the nonuniform wind field excites greater 1P and 3P responses.

Optimization of a Lightweight Floating Offshore Wind Turbine with Water Ballast Motion Mitigation Technology

Floating offshore wind turbines are a promising technology for addressing energy needs by utilizing wind resources offshore. The current state of the art is based on heavy, expensive platforms to survive the ocean environment. Typical design techniques do not involve optimization because of the computationally expensive time domain solvers used to model motions and loads in the ocean environment. However, this design uses an efficient frequency domain solver with a genetic algorithm to rapidly optimize the design of a novel floating wind turbine concept. The concept utilizes a liquid ballast mass to mitigate motions on a lightweight post-tensioned concrete platform. The simple cruciform-shaped design of the platform made of post-tensioned concrete is less expensive than steel, reducing the raw material and manufacturing cost. The use of ballast water to behave as a tuned mass damper allows a smaller platform to achieve the same motions as a much larger platform, thus reducing the mass and cost. The optimization techniques applied with these design innovations resulted in a design with a levelized cost of energy of USD 0.0753/kWh, roughly half the cost of the current state of the art.

Prediction of long-term extreme response of two-rotor floating wind turbine concept using the modified environmental contour method

The modified environmental contour method (MECM) is assessed for the prediction of 50-year extreme response of a two-rotor floating wind turbine concept (2WT) deployed in two offshore sites in the northern North Sea (Norway 5) and the North Atlantic Ocean (Buoy Cabo Silleiro). The sites considered are in areas known for their floating wind development potential. The environmental contour method (ECM) is used to reduce the computational effort of full long-term analysis (FLTA) by only considering environmental conditions associated with a given return period. MECM is a modification of the ECM where additional environmental contours are included to account for discontinuous operation modes of dynamic structures. The results obtained in MECM are benchmarked against FLTA results and compared to ECM results. ECM leads to large underpredictions of responses governed by wind loads if compared to FLTA, as it is not capable of taking into account important operational modes of the 2WT. It is found that MECM, which includes the wind turbines cut-off contour, is able to reduce most response underpredictions within 15% difference compared to FLTA results. MECM may thus be considered as a sufficiently accurate and computationally efficient method for the long-term extreme analysis of 2WT concepts.

Comparative analysis of fatigue life of a wind turbine yaw bearing with different support foundations

The yaw bearing is a connection component which is located on the tower top of a wind turbine. The yaw bearing may be subjected to environmental dynamic loads and wave forces which contribute to the fatigue life of the yaw bearing. The fatigue life of a yaw bearing used in onshore wind turbines is well established, but less so for offshore wind turbines. In this work, fatigue life assessments of yaw bearings are compared in 5 MW wind turbines with six different support foundations: onshore, monopile, ITI barge, Tension Leg Platform (TLP), spar and semi-submersible. A time-domain method is applied to predict the wind and hydrodynamic loads on the yaw bearings using three wind velocities. A Gumbel distribution, rain flow counting algorithm, linear cumulative damage law and S–N curve theory are used to generate the life-time damage equivalent loads. The fatigue life of the yaw bearing is then calculated using a finite element method and a code-based approach. The results show the dynamics of the floating wind turbine concept are directly linked to the fatigue life of the yaw bearing and that the major components of the yaw bearing should be considered separately.

Dynamic responses of a 10 MW semi-submersible wind turbine at an intermediate water depth: A comprehensive numerical and experimental comparison

A series of experiments and numerical simulations were conducted to investigate the dynamic responses of a 10 MW floating wind turbine concept named SPIC, at an intermediate water depth of approximately 60 m. A fully coupled numerical model was established using the aero-hydro-servo-elastic-moor method. The experimental and numerical dynamic responses under operational conditions, including irregular waves, steady and turbulent winds, and combined wind–wave conditions, generally agree well. The mean values are well predicted, and the powers at motion resonant frequencies, wave frequency and tower vibration frequency are captured. Discrepancies between the numerical and experimental results are further addressed by in-depth numerical analyses. Under turbulent winds, the discrepancies of dynamic responses at 1P and 2P frequencies are induced mainly by individual blade installation error, while the discrepancies at 3P components are caused mainly by the nonuniformity of the wind field. In addition, in the turbulent wind case with variable blade pitch angles and rotor speed control system, the mean tower-base bending moment is smaller, and the low-frequency responses are larger than those in the case with fixed blade pitch angles and a constant rotor speed. The reveal of uncertainties is helpful for further guiding the model test of floating wind turbines.

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.

Recent Advances in Integrated Response Analysis of Floating Wind Turbines in a Reliability Perspective

AbstractOffshore wind provides an important source of renewable energy. While wind turbines fixed to the seabed in shallow water have already been industrialized, floating wind turbines are still at an early stage of development. The cost of wind power is decreasing fast. Yet, the main challenges, especially for novel floating wind turbine concepts, are to increase reliability and reduce costs. The reliability perspective here refers to the lifecycle integrity management of the system to ensure reliability by actions during design, fabrication, installation, operation, and decommissioning. The assessment should be based on response analysis that properly accounts for the effect of different sub-systems (rotor, drivetrain, tower, support structure, and mooring) on the system behavior. Moreover, the load effects should be determined so as to be proper input to the integrity check of these sub-systems. The response analysis should serve as the basis for design and managing inspections and monitoring, with due account of inherent uncertainties. In this paper, recent developments of methods for numerical and experimental response assessment of floating wind turbines are briefly described in view of their use to demonstrate system integrity in design as well as during operation to aid inspection and monitoring. Typical features of offshore wind turbine behavior are also illustrated through some numerical case studies.

Mooring System Design and Verification for a Floating Vertical Axis Wind Turbine

The aim of this study is to investigate the technical feasibility of an innovative vertical axis floating wind turbine concept with the main focus on the design and verification of the mooring system. The study is developed through iterative processes in order to identify the optimum design for the new floating wind turbine concept. The Ultimate Limit State (ULS) criteria have been considered to verify the integrity of the mooring system in the extreme environmental conditions with a 50-year return period. For this purpose, time domain dynamic analysis has been performed using the commercial software OrcaFlex [Orcina website, OrcaFlex software, https://www.orcina.com/ ]. Although the analysis is carried out for a specific site deemed suitable for the project, the results can be used as an input for any future application in other locations. The present study is intended to be a proof of concept with a proposed scientific framework for optimization of the mooring system which is considered to be a crucial part in the design of floating wind turbines due to their complex dynamic behavior.

Numerical modeling of the hydraulic blade pitch actuator in a spar‐type floating wind turbine considering fault conditions and their effects on global dynamic responses

AbstractThis paper deals with numerical modeling of the hydraulic blade pitch actuator and its effect on the dynamic responses of a floating spar‐type wind turbine under valve fault conditions. A spar‐type floating wind turbine concept is modeled and simulated using an aero‐hydro‐servo‐elastic simulation tool (Simo‐Riflex [SR]). Because the blade pitch system has the highest failure rate, a numerical model of the hydraulic blade pitch actuator with/without valve faults is developed and linked to SR to study the effects of faults on global responses of the spar‐type floating wind turbine for different faults, fault magnitudes, and environmental conditions. The consequence of valve faults in the pitch actuator is that the blade cannot be pitched to the desired angle, so there may be a delay in the response due to excessive friction and the wrong voltage, or slit lock may cause runaway blade pitch. A short circuit may cause the blade to get stuck at a particular pitch angle. These faults contribute to rotor imbalance, which result in different effects on the turbine structure and the platform motions. The proposed method for combining global and hydraulic actuator models is demonstrated in case studies with stochastic wind and wave conditions and different types of valve faults.

Sensitivity analysis on the levelized cost of energy for floating offshore wind farms

In this paper, a sensitivity analysis is performed on the levelized cost of energy (LCOE) for floating offshore wind farms (FOWFs). The analysis is carried out for three floating wind turbine concepts and three different offshore sites. At first, a methodology is presented for calculating the LCOE for a specific FOWF. Afterwards, the base LCOE values for each of the floating wind turbine concepts and sites are obtained. The sensitivity analysis includes over 325 input parameters that are studied in order to identify the ones that most influence the LCOE. Furthermore, a complementary sensitivity analysis is performed by varying the input parameters based on uncertainty ranges provided by each of the concept designers. This serves to obtain maximum and minimum LCOE variation limits and possible cost reduction potentials. It has been observed that the capital cost related parameters such as turbine, substructure and mooring system manufacturing cost as well as power cable cost are some of the most influencing parameters besides common parameters such as the discount rate and energy losses. The LCOE variation limits obtained in this study vary between 67 €/MWh and 135 €/MWh among the different concepts and offshore sites including offshore transmission costs.

Dynamic mooring simulation with Code_Aster with application to a floating wind turbine

The design of reliable station-keeping systems for permanent floating structures such as offshore renewable energy devices is vital to their lifelong integrity. In highly dynamic and/or deep-water applications, including hydrodynamics and structural dynamics in the mooring analysis is paramount for the accurate prediction of the loading on the lines and hence their dimensioning. This article presents a new workflow based on EDF R&D's open-source, finite-element analysis tool Code_Aster, enabling the dynamic analysis of catenary mooring systems, with application to a floating wind turbine concept. The University of Maine DeepCwind-OC4 basin test campaign is used for validation, showing that Code_Aster can satisfactorily predict the fairlead tensions in both regular and irregular waves. In the latter case, all of the three main spectral components of tension observed in the experiments are found numerically. Also, the dynamic line tension is systematically compared with that provided by the classic quasi-static approach, thereby confirming its limitations. Robust dynamic simulation of catenary moorings is shown to be possible using this generalist finite-element software, provided that the inputs be organised consistently with the physics of offshore hydromechanics.

Experimental study of a new multi-column tension leg-type floating wind turbine concept

Offshore deep-water region is rich in high-quality wind energy. Currently offshore floating wind turbine is the most potential equipment to exploit it. The relevant scaling laws need to be considered during the offshore floating wind turbine model test were analysed and a proper scaling methodology was established according to the loading conditions. The wave tank model test was conducted for the proposed new multi-column tension leg-type floating wind turbine concept in SKLOE’s wave basin with the geometric scaling factor of 50 according to the main dimensions and environmental conditions. By using a modified blade design based on the same wind turbine thrust coefficient, the aerodynamic Reynolds number scaling effect was dissimilated during the model test. Still water tests, white noise wave tests and combined wind wave and current tests were carried out. During the combined wind, wave and current tests, both the turbine operating (DLC01-05) and parked conditions (DLC06, DLC07) were investigated. To identify the dominate load of the dynamic responses, each design load case was further separated into four different sub-load cases: wind only case, wave only case, current only case and combined load case. Global response variables including platform motion, tendon tension, rotor thrust, and nacelle accelerations were examined. In summary, the proposed WindStar TLP system showed relatively small dynamic motion responses to environmental loads due to its natural frequencies are well kept away from the dominant wave periods and the tendons still holds a sufficient safety factor, and the minimum tendon tensions remain positive under all the selected DLCs. The model testing results could serve as a reference for the further research.

Non-linear dynamic analysis of the response of moored floating structures

The complexity of the dynamic response of offshore marine structures requires advanced simulations tools for the accurate assessment of the seakeeping behaviour of these devices. The aim of this work is to present a new time-domain model for solving the dynamics of moored floating marine devices, specifically offshore wind turbines, subjected to non-linear environmental loads. The paper first introduces the formulation of the second-order wave radiation-diffraction solver, designed for calculating the wave-floater interaction. Then, the solver of the mooring dynamics, based on a non-linear Finite Element Method (FEM) approach, is presented. Next, the procedure developed for coupling the floater dynamics model with the mooring model is described. Some validation examples of the developed models, and comparisons among different mooring approaches, are presented. Finally, a study of the OC3 floating wind turbine concept is performed to analyze the influence of the mooring model in the dynamics of the platform and the tension in the mooring lines. The work comes to the conclusion that the coupling of a dynamic mooring model along with a second-order wave radiation-diffraction solver can offer realistic predictions of the floating wind turbine performance.

Model test investigation of a spar floating wind turbine

Several floating wind turbine designs whose hull designs reflect those used in offshore petroleum industry have emerged as leading candidates for the future development of offshore wind farms. This article presents the research findings from a model basin test program that investigated the dynamic response of a 1:50 scale model OC3 spar floating wind turbine concept designed for a water depth of 200 m. In this study the rotor was allowed to rotate freely with the wind speed and this approach eliminated some of the undesirable effects of controlling wind turbine rotational speed that were observed in earlier studies. The quality of the wind field developed by an array of fans was investigated as to its uniformity and turbulence intensity. Additional calibration tests were performed to characterize various components that included establishing the baseline wind turbine tower frequencies, stiffness of the delta type mooring system and free decay response behaviour. The assembled system was then studied under a sequence of wind and irregular wave scenarios to reveal the nature of the coupled response behaviour. The wind loads were found to have an obvious influence on the surge, heave and pitch behaviour of the spar wind turbine system. It was observed from the experimental measurements that bending moment at the top of the support tower is dominated by the 1P oscillation component and somewhat influenced by the incoming wave. Further it was determined that the axial rotor thrust and tower-top shear force have similar dynamic characteristics both dominated by tower’s first mode of vibration under wind-only condition while dominated by the incident wave field when experiencing wind-wave loading. The tensions measured in the mooring lines resulting from either wave or wind-wave excitations were influenced by the surge/pitch and heave couplings and the wind loads were found to have a clear influence on the dynamic responses of the mooring system.

English

English

Dynamic Analysis of a Braceless Semisubmersible Offshore Wind Turbine in Operational Conditions

The development of cost effective floating type offshore wind turbines has been desired in deep sea areas. Semisubmersible platform can be considered as a very efficient design configuration among the different type proposed floating platforms. In the present paper, a preliminary assessment of the dynamic behavior of a 5-MW braceless semisubmersible offshore wind turbine with three columns and two fully submerged pontoons is presented for selected environmental operational conditions that correspond to a deep water offshore site with depth 200 m. Hydro-aero-servo-elastic time domain analysis is performed with the use of the coupled analysis tool Simo-Riflex-AeroDyn in order to calculate the dynamic behavior and response of the offshore floating wind turbine. Statistical values and spectra of time histories of response quantities are presented. The results demonstrate the feasibility of the presented deep sea floating wind turbine concept.