Experimental study on dynamic response of a floating offshore wind turbine under various freak wave profiles

This study performed experimental investigation on the dynamic response of an in-place floating offshore wind turbine (FOWT) under freak wave actions. Based on the method of wave profile modulation, various freak wave profiles embedded in unidirectional Gaussian seas were generated in wave basin and the action of these waves on the FOWT was measured and analyzed, which has not been done before. The motions of FOWT were analyzed in time domain as well as time-frequency domain. The effect of freak wave parameters on FOWT motions was addressed, i.e., freak wave height, freak wave period, large crest, and deep trough. The dynamic response of FOWT was observed as a spike at the occurrence of freak wave in a conventional random wave, where the impact of freak wave can last for 17 spectral peak periods of wave. Data analysis shows that the motions of FOWT increased linearly with the freak wave height. In addition, the occurrence of freak wave induced the coupled effect on surge and pith, which was strengthen with the increase of freak wave height and wave period. Compared to a large crest, a deep trough of freak wave led to stronger motions and was supposed to be a key concern on the safety of the FOWT. The novel findings in this study provided a reference for the design of survival load on a FOWT and benchmarks for validating numerical models.     

浮式海上风力机运动性能和锚泊系统(英文)

The development of offshore wind farms was originally carried out in shallow water areas with fixed(seabed mounted) structures.However,countries with limited shallow water areas require innovative floating platforms to deploy wind turbines offshore in order to harness wind energy to generate electricity in deep seas.The performances of motion and mooring system dynamics are vital to designing a cost effective and durable floating platform.This paper describes a numerical model to simulate dynamic behavior of a new semi-submersible type floating offshore wind turbine(FOWT) system.The wind turbine was modeled as a wind block with a certain thrust coefficient,and the hydrodynamics and mooring system dynamics of the platform were calculated by SESAM software.The effect of change in environmental conditions on the dynamic response of the system under wave and wind loading was examined.The results indicate that the semi-submersible concept has excellent performance and SESAM could be an effective tool for floating wind turbine design and analysis.     

近海单桩风机在波浪地震联合作用下的动力特性分析

针对典型单桩海上风机结构,利用ANSYS建立有限元模型,并根据实际情况选取3种地震加速度激励和两个水深的波浪荷载,对结构进行地震单独作用动力分析以及地震与波浪联合作用下动力分析.对比分析结果表明:地震发生时,在水位较低、波浪较小的情况下,结构主要以地震作用引起的响应为主,波浪引起的响应相对较小;若地震发生的同时伴有极端恶劣海况,则结构响应较大;波浪对结构下部响应有较大的影响;由于地震加速度峰值与波浪力峰值之间存在相位差,所以两者联合作用时,结构的响应也会有减小的情况发生.由此可见,地震发生时波浪的存在将对结构动力响应造成不同程度的影响,在进行海上风机设计时,有必要考虑地震与波浪联合作用的情况.     

Dynamic responses analysis of a 5 MW spar-type floating wind turbine under accidental ship-impact scenario

Ship impact against offshore floating wind turbine (OFWT) has been identified as one of the major hazards with the development of OFWTs. The dynamic responses of OFWTs under ship impact should be taken into consideration during the design phase. This paper addresses a study on the dynamic responses of an OFWT in ship collision scenarios. Firstly, a mathematical model for external mechanism of ship-OFWT collision scenario is developed. Secondly, this model is combined with an in-house programme, DARwind, which can be used to predict nonlinear dynamic responses of whole OFWT system in time-domain. With the newly combined analysis tool, simulation cases for different scenarios are conducted to investigate the nonlinear dynamic responses of OFWT system, including the cases of still water condition, wave-only condition and wind-wave condition. It is shown that in still water condition, the ship impact will more obviously change the responses of motions and mooring system, compared with those in wave and wave-wind conditions. In the wave-only condition, these motions responses of platform are suppressed by wave effect, but the tower vibration and tower top deformation are sensitive to ship collision. For the wave-wind combined condition, the motions increment in surge and pitch due to ship collision becomes smaller than that of wave-only condition, but yaw motion has a considerable variation compared with those of the other two conditions. Additionally, the blade tip deformation increment due to ship collision are analyzed and it is found that the edgewise tip deformation got more obvious increment than that of flapwise. To further asses the safety of OFWT, the acceleration at nacelle are analyzed because some equipment might be sensitive to acceleration. The analysis results indicate that even though the OFWT structure doesn't get critical damage by ship impact, the equipment inside may still fail to work due to the high value of acceleration induced by ship impact. The research outcomes can benefit the safety design of OFWT in the engineering practice.     

Truss Spar结构浮式风机基础总体强度分析

海上浮式风机基础目前有半潜式,重力式,导管架式,Spar式很多种。目前国内对于桁架式浮式风机基础的结构分析比较少。本文采用Truss spar型海上浮式风机基础,基于三维势流理论和直接计算法,对其在深海环境中进行结构强度分析。分析表明,结构在垂荡板和桁架结构连接处容易发生较大应力集中,易产生破坏,在后续的设计修改中可对这一部位进行重点研究。     

海上风力发电机组支撑结构动力响应特性研究

支撑结构设计是大型海上风电机组设计的重要部分。文章分析了海上风电机组的各种环境载荷,并以3MW风力机组为例计算其所受环境载荷,包括作用在支撑结构顶端的由风机叶轮转动引起的水平轴向力、作用在塔筒上的风载荷以及作用在基础上的海流、海浪载荷,并采用非线性弹簧来模拟基础与海底土层之间的相互作用。在考虑风轮影响情况下,利用有限元法对支撑结构进行了模态分析。最后,分析了环境载荷作用下支撑结构的动态响应。计算结果表明,在对海上风力发电机组进行动态响应计算时,环境载荷之间的相互耦合作用不能忽略。     

Dynamic analysis of a dual-spar floating offshore wind farm with shared moorings in extreme environmental conditions

The concept of a shared mooring system was proposed to reduce mooring and anchoring costs. Shared moorings also add complexity to the floating offshore wind farm system and pose design challenges. To understand the system dynamics, this paper presents a dynamic analysis for a dual-spar floating offshore wind farm with a shared mooring system in extreme environmental conditions. First, a numerical model of the floating offshore wind farm was established in a commercial simulation tool. Then, time-domain simulations were performed for the parked wind farm under extreme wind and wave conditions. A sensitivity study was carried out to investigate the influence of loading directions and shared line mooring properties. To highlight the influence of the shared line, the results were compared to those of a single spar floating wind turbine, and larger platform motions and higher tension loads in single lines are observed for the wind farm with shared moorings. The loading direction affects the platform motions and mooring response of the floating offshore wind farm. Comparing the investigated loading directions to the 0-deg loading direction, the variation of mean mooring tension at the fairlead is up to 84% for single lines and 16% for the shared line. The influence of the shared line properties in the platform motions and the structural responses is limited. These findings improve understanding of the dynamic characteristics of floating offshore wind farms with a shared mooring system.     

海上风电安装船的发展趋势研究

随着海上风力发电产业的迅速发展,风电安装船需求越来越大,并且风电安装船是高附加值工程船,因此这一市场的吸引力将越来越受到造船界重视,竞争将越来越激烈,但海上风电场施工成本高、海上作业时间长及工期长等问题的存在延缓了海力发电产业的发展。因此,本文将着重对海上风电结构物施工、安装过程的单一海工设备发展进行浅析,为探索深水化、大型化、专业化、集成化的海上风电安装船来完成深海域基础施工及风机安装问题提供设计参考。     

Development of interlink wear estimation method for mooring chain of floating structures: Validation and new approach using three-dimensional contact response

Long-term operation of mooring systems is one of the challenging issues of floating structures such as floating offshore wind turbines (FOWTs). For integrity assessment, fatigue and its affecting factors have generated considerable recent research interest as the occurrence of a large number of mooring chain failures at a high rate has been reported. By contrast, only few studies on the effect of nonuniform volume loss of mooring chain links due to wear can be found because of difficulties to estimate wear amounts quantitatively. Considering this issue, in this paper, validation of the quantitative interlink wear estimation method is investigated by applying to a spar-type floating structure. Firstly, the method is presented which consists of the material test, derivation of an interlink wear estimation formula with FE analysis, and calculation of mooring chain response with coupled dynamic analysis using a mass-spring model. To improve insufficient accuracy due to the mass-spring model around a clump weight and the touchdown point, the method is further modified by using a 3-D rigid-body link model. The estimation results and comparison show that the modified method distinguishing between rolling and sliding can calculate the interlink wear amount closer to the chain diameter measurements and more reasonable than the method using the conventional mass-spring model.     

Design,modelling, and analysis of a large floating dock for spar floating wind turbine installation

Installation of floating wind turbines at the offshore site is a challenging task. A significant part of the time efficiency and costs are related to the installation methods which are sensitive to weather conditions. This study investigates a large floating dock concept, which can be used to shield a floating wind turbine during installation of tower, nacelle, and rotor onto a spar foundation. In this paper, the concept is described in detail, and a design optimisation is carried out using simple design constraints. Hydrodynamic analysis and dynamic response analysis of the coupled system of the optimum dock and spar are conducted. Two spars of different sizes are considered, and the motion responses of the spars with and without the dock in irregular waves are compared. Through analysis of the motion spectra and response statistics, dynamic characteristics of the coupled system is revealed. The present design of the dock reduces the platform-pitch responses of the spars and potentially facilitates blade mating, but may deteriorate the heave velocity of the spars in swell conditions. Finally, future design aspects of the floating dock are discussed.     

南海风浪条件下Spar式浮式风机平台的构型及参数

使用WAMIT软件建立了OC3-Hywind Spar式浮式风机模型,联合FAST软件计算了浮式风机在不同工况下的幅值响应算子;结合为南海海域波浪条件优化后的JONSWAP谱,对浮式风机进行了不同工况下的响应谱分析及最大响应值计算;进一步阐述了浮式平台结构构型参数的变化对整体结构在南海相应工况下的运动影响。     

Fluid-structure interaction and stress analysis of a floating wind turbine

In this paper, we describe a method to investigate the fluid-structure interaction of a floating wind turbine and to analyze the global deformations and the corresponding stresses with a detailed finite element model. To solve the fluid-structure interaction problem, a partitioned approach is chosen. The in-house C++ library comana, which was developed to solve multi-physic problems by coupling existing solvers, is extended to couple the fluid solver panMARE and the structural solver ANSYS. The significance of the interaction of structural deformations and the fluid loads is pointed out for the rotor of the wind turbine. In order to enable the use of a detailed finite element model in the fluid-structure interaction simulation, a model reduction method is applied in ANSYS. As a result, an efficient stress analysis can be performed under consideration of the fluid-structure interaction.     

Validation and application of nonlinear hydrodynamics from CFD in an engineering model of a semi-submersible floating wind turbine

Nonlinear hydrodynamics play a significant role in accurate prediction of the dynamic responses of floating wind turbines (FWTs), especially near the resonance frequencies. This study investigates the use of computational fluid dynamics (CFD) simulations to improve an engineering model (based on potential flow theory with Morison-type drag) by modifying the second-order difference-frequency quadratic transfer functions (QTFs) and frequency-dependent added mass and damping for a semi-submersible FWT. The results from the original and modified engineering models are compared to experimental data from decay tests and irregular wave tests. In general, the CFD results based on forced oscillation tests suggest increasing the frequency-depending added mass and damping at low frequencies compared to first order potential flow theory. The modified engineering model predicts natural periods close to the experimental results in decay tests (within 5%), and the underprediction of the damping is reduced compared to the original engineering model. The motions, mooring line tensions and tower-base loads in the low-frequency response to an irregular wave are underestimated using the original engineering model. The additional linear damping increases this underestimation, while the modified QTFs based on CFD simulations of a fixed floater in bichromatic waves result in larger difference-frequency wave loads. The combined modifications give improved agreement with experimental data in terms of damage equivalent loads for the mooring lines and tower base.     

Active heave compensation of floating wind turbine installation using a catamaran construction vessel

The application of floating wind turbines is limited by the high cost that increases with the water depth. Offshore installation and maintenance continue to consume a high percentage of the project budget. To improve the installation efficiency of the floating offshore wind turbine, a novel concept is proposed by the SFI MOVE project. Several wind turbine superstructure components are preassembled onshore and carried to the installation site by a catamaran construction vessel. Each assembly can then be installed using only one lift, and the concept is less sensitive to weather conditions. In this paper, a control algorithm of the proposed hydraulic active heave compensator system is developed using singular perturbation theory to cancel the relative motion between the spar top and gripped preassembly bottom. Closed-loop stability is proven, and the simulation results show that the installation efficiency is improved with an increase in the acceptable weather conditions.     

海上风力发电平台概念设计及系泊系统特性研究

为实现海上风力发电平台工作状态的准确预报,本文采用有限元软件建立浮式风力发电平台三维模型,完全时域耦合分析在不规则波作用下,浮式风力发电平台动力响应特性.通过时域结果对比分析可知,在风浪流同向作用下,浮式风力发电平台的运动响应幅值及系泊缆索顶端张力最大.通过运动响应时间历程曲线可知,横荡运动和横摇运动表现为低频特性,垂荡运动表现为波频特性,以上研究成果可为浮式风力发电机的优化设计提供一定的指导,并可为将来的相关试验提供一定参考.     

Experimental study of the state-of-the-art offshore system integrating a floating offshore wind turbine with a steel fish farming cage

An innovative offshore system integrating a floating offshore wind turbine with a steel fish farming cage (FOWT-SFFC) has recently been developed by the two leading authors for offshore wind and aquaculture industry. The purpose of this paper is to investigate the dynamic responses of FOWT-SFFC subjected to simultaneous wind and wave actions in the harsh South China Sea environment by a series of model tests. The tests are conducted at the Tsinghua Ocean Engineering Basin with Froude scale of 1:30. In this paper, the similarity law and setup of model tests are given first. Then a series of calibration tests and identification tests are carried out to validate the capacity of wind generator and wave maker, and to identify the vibration frequencies of tower, the stiffness of mooring system, natural periods and system damping, motion response amplitude operators (RAOs) of FOWT-SFFC, and thrust-speed performance of the turbine in wave basin. After that, seakeeping tests are implemented for random waves, followed by a sequence of load cases including normal operating and extreme conditions. Constant wind speeds and random wind speeds are respectively considered in load combinations. The experimental results affirm the excellent seakeeping and dynamic performance of FOWT-SFFC. Existence of metal fish nets increases the damping of foundation's 6 degree-of-freedoms motions. Generally, the influence of nets on the dynamic responses is insignificant in wind sea states.     

Dynamic response of a SWATH vessel for installing pre-assembled floating wind turbines

New and efficient installation concepts which can reduce the cost of developing an offshore wind farm are of particular interest. This paper explores a promising concept using the small water-plane area twin-hull vessel (SWATH) to install pre-assembled wind turbines (OWT) onto floating spar foundations. A focus is placed on the hydrodynamic performance of the SWATH and the response analysis of the coupled SWATH-spar system. Firstly, the numerically calculated difference-frequency wave force effect and damping forces of the original SWATH were verified with experimental data. Secondly, the original SWATH was modified to satisfy the criteria of weight-carrying capacity and hydrostatic stability. Thirdly, a multibody numerical model for the SWATH-spar system was developed, in which the hydrodynamic and mechanical couplings between the SWATH and a spar were considered. The SWATH is equipped with a dynamic positioning system to counteract the slow-drift wave force effects. The nonlinear time-domain simulations were carried out for the mating stage when a wind turbine is lifted above the spar foundation. Based on the analysis of statistics of the relative displacement and velocity of the tower bottom and the spar top, the installation concept with SWATH is found to be of decent performance. Finally, recommendations are provided for future research on this concept, which contributes to developing next-generation installation concepts for bottom-fixed and floating wind farms.     

不同结构形式浮式防波堤湿拖过程中的运动响应研究

浮式防波堤是当前海洋工程装备研发的热点,由于浮式防波堤主尺度大,采用半潜船等工程船运输成本高。本文考虑三种不同类型浮式防波堤采用湿拖方式进行运输,采用AQWA软件,利用数值模拟方法,研究不同海况下浮式防波堤各自由度运动响应。研究发现：圆筒型和板架型浮式防波堤适用于不同海况下湿拖;网笼型浮式防波堤湿拖过程中运动响应幅值较大,建议使用半潜船运输。本文通过探究不同浮式防波堤湿拖过程中的运动响应,可为工程实际中浮式防波堤运输提供一定的借鉴和参考。     

非线性混合海况下近海风机塔筒底部载荷的动力响应

本文主要研究在非线性混合海况(即风浪和涌浪组合海况)下,以NREL 5MW_Baseline Monopile近海风机为研究对象,对其塔筒底部(基线)所受到的剪力和弯矩载荷的动力响应进行仿真。在近海风机的时域仿真中,选用了Ochi-Hubble六参数波浪谱,并编制了该谱的程序嵌入到FAST中进行编译。计算过程中,共进行了20次10 min的仿真分析。对于得到的短期载荷,给出了波高程,塔筒底部首尾向剪力和弯矩在线性与非线性不规则波作用下的时程曲线对比图。采用分块最大值法对每一次的短期载荷提取极值,并基于20次仿真所得的极值,给出了塔筒底部首尾向剪力与弯矩在线性与非线性不规则波作用下的超越概率曲线对比图。研究表明,在非线性混合海况下进行近海风机塔筒底部载荷的动力响应研究,计算结果对工程实际应用具有指导意义。