Ultimate load analysis of a 10 MW offshore monopile wind turbine incorporating fully nonlinear irregular wave kinematics

Loads from storm waves can in some cases be dimensioning for offshore wind turbine substructures. Accurate determination of nonlinear wave loads is therefore important for a safe, yet economic design. In this paper, the fully nonlinear waves, realized by a fully nonlinear potential wave solver OceanWave3D, are incorporated into coupled aero-servo-hydro-elastic simulations for a reduced set of wave-sensitive design load cases, in comparison with the widely used linear and constrained waves. The coupled aero-elastic simulations are performed for the DTU 10 MW reference wind turbine on a large monopile at 33 m water depth using the aero-elastic code HAWC2. Effect of the wave nonlinearity is investigated in terms of the ultimate sectional moments at tower bottom and monopile mudline. Higher ultimate moments, 5% at tower bottom and 13% at monopile mudline as maximum, are predicated when the nonlinear waves are used. It could be explained by the fact that the extreme nonlinear waves, that are close to the breaking limit, can induce resonant ringing-type responses, and hereby dominate the ultimate load responses. However, the constrained wave approach shows marginal difference compared to the standard linear wave approach. It can be concluded at least for the present configuration that the industry standard approaches (linear and constrained wave approach) underestimate the ultimate load responses on offshore wind turbines in severe sea states.     

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。     

半潜型浮式风机运动特征及系泊系统的研究

近年来海上浮式风机的研究备受关注,安全可靠的系泊系统将保证风机在风、浪、流等复杂环境荷载作用下稳定运行,准确合理地描述风机运动将为评估风机发电效率提供支持。以半潜型浮式风机的系泊系统为研究对象,基于经典悬链线理论,采用准静态分析法提出一套系泊系统的设计方法。通过坐标变换,得到风轮真实的俯仰运动用于计算风机的动力效应及评定其发电效率。采用动力法分析了系泊系统锚链的导缆孔位置、预张力大小、锚链间夹角等参数对风机系统发电效率、浮式平台运动性能和系泊锚链张力的影响,得到了浮式平台迎风面俯仰倾角、水平偏移及锚链张力随参数的变化规律,为半潜型浮式风机系泊系统的设计提供了参考。     

The influence of fully nonlinear wave forces on aero-hydro-elastic calculations of monopile wind turbines

The response of an offshore wind turbine tower and its monopile foundation has been investigated when exposed to linear and fully nonlinear irregular waves on four different water depths. The investigation focuses on the consequences of including full nonlinearity in the wave kinematics. The linear and nonlinear irregular wave realizations are calculated using the fully nonlinear potential flow wave model OceanWave3D [1]. The linear and nonlinear wave realizations are compared using both a static analysis on a fixed monopile and dynamic calculations with the aeroelastic code Flex5 [2]. The conclusion from this analysis is that linear wave theory is generally sufficient for estimating the fatigue loading, but wave nonlinearity is important in determining the ultimate design loads.     

半潜驳改造为坐底式风电安装船的设计及应用研究

为解决现有海上风电安装设备的不足,将原“三航工5”半潜驳改造为坐底式风电安装船,并研制了一套船体结构应力监测系统,对国内某海上风场砂性地质条件引起的冲刷和船底掏空进行监控,实际工程应用表明改造后的船舶在安全性和工效上满足坐底安装风机的要求。     

Equivalent linear pseudostatic and dynamic modelling of vertically vibrating monopile

To optimize offshore wind turbine (OWT) design, an engineering tool has been developed allowing for a detailed investigation of the effects of nonlinear soil stiffness and damping on foundation dynamics. We have studied the response of a vertically oscillating offshore wind monopile foundation in a realistic soil profile subjected to loads between 1 and 200 MN in the frequency range 0–10 Hz with pseudo-static and equivalent linear dynamic model. The non-linear soil behaviour is modelled with an equivalent linear method with shear modulus reduction and damping curves as input. The tool is verified and validated by comparison with elasto-dynamic model and experiments. With increasing load amplitudes foundation stiffness increases and damping decreases. For large load amplitudes the lower part of the pile foundation contributes more to foundation damping. The results indicate the nonlinear foundation stiffness and damping can be modelled rationally by combining stiffness and hysteretic damping from nonlinear static tools with apparent mass and radiation damping from elasto-dynamic analysis. The tool can be used to compute soil springs and dampers based on laboratory-based soil stiffness and damping.     

Hydroelastic code-to-code comparison for a tension leg spar-type floating wind turbine

The development of robust design tools for offshore wind turbines requires knowledge of both wave and wind load models and response analysis. Verification of the numerical codes is required by the use of experiments and code-to-code comparisons. This paper presents a hydroelastic code-to-code comparison between the HAWC2 and USFOS/vpOne codes for a tension leg spar (TLS) wind turbine with a single tether. This concept is hence based on the TLP and Spar concepts. The comparison is performed using coupled hydroelastic time domain simulations. Several aspects of modelling, such as wave simulation, hydrodynamic and structural modelling, are addressed for the TLS. Wave-induced motions of the support structure affect the power performance of a wind turbine. Furthermore, overload of the tension leg should be avoided. In this paper, the motion and tension responses are compared. The tension leg introduces nonlinear effects on the spar motion. These nonlinear effects include combined-frequency effect such as double, difference and sum of wave, as well as natural pitch and surge frequencies. Hydrodynamic loads are based on a combination of the Morison formula and the pressure integration method. A comparison indicates that the motion and tension responses obtained in the two codes are in good agreement.     

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.     

风机运输甲板驳结构强度评估

风机运输甲板驳是用于海上风机运输的海洋工程辅助船,近年来随着海上风力发电的兴起而迅速发展。本文采用全船有限元直接计算和 CCS规范计算2种方法对某风机运输甲板驳的结构强度进行评估,计算表明该船结构强度满足设计要求。本文的研究成果和结论对同类型风电安装船舶的结构设计具有参考意义。     

1000t海上风电安装平台设计及稳性研究

以1 000 t海上风电安装平台为研究对象,从平台总体布置、关键技术参数和主要性能等方面对平台的设计特点进行介绍,并对平台稳性性能进行研究。结果表明,该平台具有较好的稳性性能,尤其是破舱稳性具有较大的安全裕量,总体布置合理。在此基础上,从稳性性能方面对平台设计提出建议,研究结论对海上风电安装平台的设计及优化具有一定的指导意义。     

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。     

Study on a new method for installing a monopile and a fully integrated offshore wind turbine structure

This paper presents a preliminary technical feasibility study on a new methodology proposed for installing a monopile-based bottom supported offshore wind turbine structure. The concept is developed to address the problem of “waiting for a suitable weather window” which is commonly faced by the existing installation methods that uses a typical jack-up platform. In the methodology, a floating vessel along with a floatable subsea structure fitted with a hull on the top, hereafter named SSIP (subsea structure for installing a pile), is proposed first to install a monopile. Then the same structure is used to carry an FIUS (fully integrated upper structure) of an offshore wind turbine, which is characterized by a telescopic tower, and install it over the monopile by using an FOP (float-over-pulling) arrangement. Here, the installation methodologies are first briefly described along with the critical load cases associated with them. These load cases are then numerically studied for a significant wave height (H_S) of 2.5 m, and the results are summarized. For installing a fully integrated offshore wind turbine upper structure on a monopile foundation by the FOP method, two installation schemes are presented, and their dynamic characteristics are compared. It is shown that the proposed methodologies have potential to provide installation solutions which can be environmentally more robust compared to the existing method for installing an offshore wind turbine.     

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.     

基于环境因素的海上风机支撑结构方案比选多级模糊决策

海上风电是最有前途的清洁能源之一,在概念设计时,海上风机的支撑结构选型优化非常重要。选型优化是一个多属性决策问题,本文将广泛应用的多因素、多层次模糊优选理论引入支撑结构的选型决策中。针对环境等影响因素复杂、确定隶属函数主观因素较强的问题, 成功引入因素的优先关系法来确定优选矩阵的隶属度,较好地减少了确定隶属函数的人为影响。通过此优选模型成功地将四种优选方案(单桩,三脚架,五桩,高桩门架)和影响支撑结构选型的8种因素(施工难度,维护成本,CO2排放量,海水浑浊度,珊瑚礁形成,强度,刚度,耐久性)进行了多级模糊综合优选决策,得到了比较科学的决策结果,本研究为海上风机支撑结构的方案选型决策提出了一种量化分析的手段。     

冰区海上风电基础的抗冰性能分析

海上风电基础属于典型的柔性结构。由于冰与柔性抗冰结构相互作用的复杂性,长期以来尚未形成基于动冰力响应分析的结构设计。结构抗冰设计中大都是从极端荷载出发,只考虑最大静冰力或最大倾覆力矩。基于对渤海辽东湾柔性抗冰结构的多年监测,发现强烈的冰激振动引起柔性结构的风险性要远大于极端静冰荷载下结构的整体安全问题。为了明确冰区风电基础结构的抗冰性能及抗冰设计的合理性,文章结合基于多年现场冰与结构作用观测及冰荷载的研究成果,明确该类柔性结构与海冰作用形式及其动力特性;提出了柔性抗冰结构设计中应考虑的主要失效模式及评价方法。最后,以渤海某典型风电基础为例,对其抗冰性能进行评价。该文的研究可为寒区风电基础的抗冰设计及安全保障提供合理依据。     

Experimental field study of floater motion effects on a main bearing in a full-scale spar floating wind turbine

In this paper we present a full-scale experimental field study of the effects of floater motion on a main bearing in a 6 MW turbine on a spar-type floating substructure. Floating wind turbines are necessary to access the full offshore wind power potential, but the characteristics of their operation leave a gap with respect to the rapidly developing empirical knowledge on operation of bottom-fixed turbines. Larger wind turbines are one of the most important contributions to reducing cost of energy, but challenge established drivetrain layouts, component size envelopes and analysis methods. We have used fibre optic strain sensor arrays to measure circumferential strain in the stationary ring in a main bearing. Strain data have been analysed in the time domain and the frequency domain and compared with data on environmental loads, floating turbine motion and turbine operation. The results show that the contribution to fluctuating strain from in-plane bending strain is two orders of magnitude larger than that from membrane strain. The fluctuating in-plane bending strain is the result of cyclic differences between blade bending moments, both in and out of the rotor plane, and is driven by wind loads and turbine rotation. The fluctuating membrane strain appears to be the result of both axial load from thrust, because of the bearing and roller geometry, and radial loads on the rotating bearing ring from total out-of-plane bending moments in the three blades. The membrane strain shows a contribution from slow-varying wind forces and floating turbine pitch motion. However, as the total fluctuating strain is dominated by the intrinsic effects of blade bending moments in these turbines, the relative effect of floater motion is very small. Mostly relevant for the intrinsic membrane strain, sum and difference frequencies appear in the measured responses as the result of nonlinear system behaviour. This is an important result with respect to turbine modelling and simulation, where global structural analyses and local drivetrain analyses are frequently decoupled.     

浮式风机气动-水动耦合特性数值模拟（英文）

The exploration for renewable and clean energies has become crucial due to environmental issues such as global warming and the energy crisis. In recent years,floating offshore wind turbines(FOWTs) have attracted a considerable amount of attention as a means to exploit steady and strong wind sources available in deep-sea areas. In this study, the coupled aero-hydrodynamic characteristics of a spar-type 5-MW wind turbine are analyzed. An unsteady actuator line model(UALM) coupled with a twophase computational fluid dynamics solver naoe-FOAM-SJTU is applied to solve three-dimensional Reynolds-averaged NavierStokes equations. Simulations with different complexities are performed. First, the wind turbine is parked. Second, the impact of the wind turbine is simplified into equivalent forces and moments. Third, fully coupled dynamic analysis with wind and wave excitation is conducted by utilizing the UALM. From the simulation, aerodynamic forces, including the unsteady aerodynamic power and thrust, can be obtained, and hydrodynamic responses such as the six-degrees-of-freedom motions of the floating platform and the mooring tensions are also available. The coupled responses of the FOWT for cases of different complexities are analyzed based on the simulation results. Findings indicate that the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform are obvious. The aerodynamic loads have a significant effect on the dynamic responses of the floating platform, and the aerodynamic performance of the wind turbine has highly unsteady characteristics due to the motions of the floating platform. A spar-type FOWT consisting of NREL-5-MW baseline wind turbine and OC3-Hywind platform system is investigated. The aerodynamic forces can be obtained by the UALM. The 6 DoF motions and mooring tensions are predicted by the naoe-FOAM-SJTU. To research the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform, simulations with different complexities are performed. Fully coupled aero-hydrodynamic characteristics of FOWTs, including aerodynamic loads, wake vortex, motion responses, and mooring tensions, are compared and analyzed.     

TLP型风力机的空气动力学影响及发电预测(英文)

This research proposes a new offshore wind energy generation system that uses a tension leg platform (TLP) and describes experiments performed on a TLP type wind turbine in both waves and wind. The following conclusions can be made from the results of this research. 1) In the case of coexisting wave-wind fields, the wind effect stabilizes the pitch motion. 2) The wind effect decreases vibration of the mooring lines when waves and wind coexist. In particular, the springing (2nd or 3rd order force) also decreases in this field. 3) It can be estimated that the reduction in the rate of generation of electrical power can be up to about 6% as a result of the heel angle. In addition, the annual amount of electricity generated was estimated along with the utilization factor based on the experimental results.     

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.     

A new time-frequency analysis method based on single mode function decomposition for offshore wind turbines

The Hilbert-Huang transform (HHT) has been widely applied and recognised as a powerful time-frequency analysis method for nonlinear and non-stationary signals in numerous engineering fields. One of its major challenges is that the HHT is frequently subject to mode mixing in the processing of practical signals such as those of offshore wind turbines, as the frequencies of offshore wind turbines are typically close and contaminated by noise. To address this issue, this paper proposes a new time-frequency analysis method based on single mode function (SMF) decomposition to overcome the mode mixing problem in the structural health monitoring (SHM) of offshore wind turbines. In this approach, the structural vibration signal is first decomposed into a set of window components using complex exponential decomposition. A state-space model is introduced in the signal decomposition to improve the numerical stability of the decomposition, and then a novel window-alignment strategy, named energy gridding, is proposed and the signals are constructed in the corresponding gridding. Furthermore, energy recollection is implemented in each gridding, and the reassembling of these components yields an SMF that is comparable to the intrinsic mode function (IMF) of the HHT, but with a significant improvement in terms of mode mixing. Four case studies are conducted to evaluate the performance of the proposed method. The first case attempts to detect three different frequencies in a simulated time-invariant signal. The second case attempts to test a synthesised signal with segmental time-varying frequencies (each segment contains three different frequencies components). The analysis results in these two cases indicate that mode mixing can be reduced by the proposed method. Furthermore, a synthesised signal with slowly varying frequencies is used. These analysis results demonstrate the effective suppression of non-relevant frequency components using SMF decomposition. In the third case, the experimental data from vortex-induced vibration (VIV) experiments sponsored by the Norwegian Deepwater Programme (NDP) are used to evaluate the proposed SMF decomposition for vibration mode identification. In the final case, field data acquired from an offshore wind turbine foundation and offshore wind turbine are analysed. The mode identification results obtained using SMF decomposition are compared with those produced by the HHT. The comparison demonstrates superior performance of the proposed method in identifying the vibration modes of the VIV experimental and field data.