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。     

首外飘砰击对主船体结构响应的预报

假设在波浪砰击的条件下,线性“切片理论”仍适用于预报舰船的纵向运动。于是,将不规则海浪上的船首外飘砰击载荷及其对主船体的响应,简化为首部船一波相对垂向运动响应谱峰值频率的某个规则波问题,即船体响应的概率与船一波相对垂向速度的平方相联系,从而获得在各种不同海况下,首外飘砰击船体响应的概率分布可以用w。儿Ull分布来近似,即“短期预报”。 计算中对各种非线性水动力载荷成分作了分析,认为首外飘砰击的船体响应应考虑非线性惯性力及非线性浮力这两项载荷。文中把某船实测结果与本方法的短期预报作了比较,最后评述了长期预报。     

近海风电设备安装船波浪载荷预报

本文以自航自升式近海风电设备安装船为研究对象,基于其特殊的船体外形建立不同装载工况下的水动力模型,应用水动力分析软件AQWA计算船舶典型横剖面处的波浪载荷幅值响应算子,在此基础上分析目标海况下指定概率水平的弯矩和剪力的长期统计值,为近海风电设备安装船结构设计和强度校核提供外部载荷。     

Spar型海上浮式风机极端载荷预报

在风机极限强度校核问题中,通常采用传统的直接积分法外推求解构件所受的极端载荷。由于传统算法本身的缺陷,在求解海上风机时所需仿真数量急剧增加,给外推造成了困难。为了减少所需的仿真数据,引入反向一阶可靠度方法求解风机极限载荷。将其二维形式环境等值线法与1维最优化方法二分法结合起来,形成一种新的搜索算法。以某Spar型海上浮式风机为例,对其长期载荷进行求解,获得了叶片根部面外弯矩和塔筒基底首尾弯矩分别在1年和20年重现周期下的极端载荷。为了验证计算的准确性,采用计算量更大的直接积分法获取了这2项载荷的长期分布。发现计算结果与超越概率曲线具有较好的吻合度,由此体现出该方法在求解风机特定极限载荷时,不仅具有计算成本的优势同时也有较高的可靠性,为快速求解风机极限载荷提供了一种参考。     

半潜式浮式风机耦合动力响应特性试验研究

漂浮式风机顶部叶片水平平均气动载荷和叶片旋转运动诱发的高频载荷与塔筒结构和锚泊系统相互耦合，构成复杂的非线性系统。系统的耦合动力特性分析是方案总体设计的关键。在水池中开展不同浪向和海况条件下的试验。试验结果表明，在该系统模型的纵摇和横摇间存在显著的低频耦合；1倍和3倍叶片旋转频率分别与波浪频率和塔筒一阶频率接近，极易引起大幅谐振，而2倍频的影响较小。     

规则波和不规则波中船舶艉砰击及其振动响应的试验研究

在拖曳水池中对某船舶进行了艉砰击及其振动响应的试验研究.在规则波以及不规则波中的零航速、艉随浪情况下观察到了严重的艉砰击现象.试验数据分析表明,合成弯矩可以分成由波浪载荷引起的低频成分以及由砰击载荷引起的高频成分.由于严重艉砰击载荷的作用,发现在某次规则波试验中合成弯矩比波浪弯矩要大出44%,在3.24m不规则波中合成弯矩增加了43%.不规则波中的试验数据统计表明合成弯矩分布范围服从Weibull分布.推导了服从Weibull分布随机变量的短期概率极值预报公式,针对试验数据进行了预报.还讨论了试验数据分析中的不确定性问题.试验研究表明,对于艉部平坦肥大的船舶,在设计和使用中需要引起对艉砰击及其振动响应问题的重视.     

平台运动对深水钻井隔水管非线性动力响应的影响

文章建立了波浪、海流和船体运动等载荷综合作用下的深水钻井隔水管非线性时域动力响应模型,模型考虑了顶部张力和隔水管内部泥浆对隔水管动力响应的影响,计人了张力沿隔水管长度的变化,并采用有限差分法求解了四阶偏微分运动方程.计算了随机波浪和规则波浪下船体运动对深水钻井隔水管非线性动力响应的影响,研究分析了船体平均偏移以及波频响应和低频响应的周期和幅值对深水钻井隔水管动力响应的影响.结果表明,船体运动对深水钻井隔水管动力响应有较大的影响,在随机波浪和海流作用下,考虑船体运动时,隔水管上部最大弯矩增大了59%.船体的平均偏移主要影响底部最大弯矩.船体波频运动对上部最大弯矩有较大影响,随着波频响应的周期和幅值的变化,上部最大弯矩变化较快,而低频响应的则主要影响隔水管上部弯矩.     

冰区海上风机的动力响应及疲劳分析

本文针对冰区作业的海上风机进行动力响应分析与疲劳损伤计算。采用Kaimal风速谱进行风载荷计算。采用K?rn?冰力谱进行冰载荷计算。分别进行风载荷、冰载荷与风冰联合作用3种不同工况下的海上风机动力响应分析与疲劳损伤评估。结果表明,风载荷作用下塔筒顶端位移要远大于冰载荷作用。风冰联合作用下风机在泥面处的支座反力与弯矩均大于单一载荷的作用。冰载荷作用下风机的疲劳损伤小于风载荷所造成的疲劳损伤,但风冰联合作用下风机的疲劳损伤均大于任一载荷单独作用。采用DNV方法计算得到的疲劳损伤值较接近风冰载荷联合作用计算结果,且偏于保守。     

6750箱集装箱船非线性波激振动和颤振响应分析

本文采用三维时域非线性水弹性方法分析了一艘6750箱集装箱船的水弹性响应以及运动和垂向弯矩特征。通过考虑入射波力、静水恢复力、砰击效应的非线性,研究了在恶劣海况下船体的非线性运动和垂向弯矩响应,同时分析了波激振动及颤振对垂向弯矩的影响。数值计算结果表明:(1)非线性入射波力对运动的影响较小,但是对垂向弯矩的影响较大,使得其有明显的倍频成分,同时中垂弯矩显著大于中拱弯矩。另外,非线性入射波力也引起了明显的非线性波激振动;(2)非线性静水恢复力对运动和载荷的影响均较大,但是没有引起明显的非线性响应。非线性计算的垂荡响应小于线性结果,而纵摇和垂向弯矩响应大于线性结果;(3)砰击效应对运动的影响较小,但对垂向弯矩的影响较大,砰击效应引起了显著的船体弹性高频振动,增大了载荷幅值,但是其引起的合成中垂和中拱幅值相差不大;(4)非线性水动力的作用主要引起垂向弯矩的倍频响应,包括倍频可能引起的二节点垂向弯矩弹性共振,而砰击效应主要引起船体二节点垂向弯矩共振;(5)本文的非线性水弹性响应计算结果与Kim给出的数值计算结果吻合很好。     

海上风机半潜型浮式基础水动力性能研究

基于三维势流理论,利用AQWA软件对某海上千瓦级垂直轴风机浮式基础及其系泊系统进行了水动力分析,在频域中计算浮式基础在不同浪向规则波中的水动力及其运动响应,在时域中计算风浪流联合作用下,处于锚泊状态的浮式基础在福建近海生存海况下的非线性耦合运动响应,同时对浮式基础的系泊方式进行了安全校核计算,结果表明系泊方式安全有效。本文所得出的结论对于相关工程实践有一定的指导意义。     

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.     

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.     

Bottom effects on the tower base shear forces and bending moments of a shallow water offshore wind turbine

In this paper the tower base shear forces and bending moments of a shallow water offshore wind turbine have been rigorously calculated by using a nonlinear simulation method taking into account the bottom effects. It has been found that nonlinearly simulated realistic waves with bottom effects should be used as inputs in the stochastic time domain simulation in order to design an un-conservative support structure for the offshore wind turbine. In order to further improve the simulation efficiency, a transformed linear simulation method has been utilized in this paper for generating equivalent waves as those obtained from the nonlinear simulation method. The accuracy and efficiency of the transformed linear simulation method have been convincingly substantiated through the subsequent calculation examples in this article.     

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.     

Semi-submersible wind turbine hull shape design for a favorable system response behavior

Floating offshore wind turbines are a novel technology, which has reached, with the first wind farm in operation, an advanced state of development. The question of how floating wind systems can be optimized to operate smoothly in harsh wind and wave conditions is the subject of the present work. An integrated optimization was conducted, where the hull shape of a semi-submersible, as well as the wind turbine controller were varied with the goal of finding a cost-efficient design, which does not respond to wind and wave excitations, resulting in small structural fatigue and extreme loads.The optimum design was found to have a remarkably low tower-base fatigue load response and small rotor fore-aft amplitudes. Further investigations showed that the reason for the good dynamic behavior is a particularly favorable response to first-order wave loads: The floating wind turbine rotates in pitch-direction about a point close to the rotor hub and the rotor fore-aft motion is almost unaffected by the wave excitation. As a result, the power production and the blade loads are not influenced by the waves. A comparable effect was so far known for Tension Leg Platforms but not for semi-submersible wind turbines. The methodology builds on a low-order simulation model, coupled to a parametric panel code model, a detailed viscous drag model and an individually tuned blade pitch controller. The results are confirmed by the higher-fidelity model FAST. A new indicator to express the optimal behavior through a single design criterion has been developed.     

Numerical and experimental investigations into the application of response conditioned waves for long-term nonlinear analyses

The coefficient of contribution method, in which the extreme response is determined by considering only the few most important sea states, is an efficient way to do nonlinear long-term load analyses. To furthermore efficiently find the nonlinear short-term probability distributions of the vessel responses in these sea states, response conditioned wave methods can be used. Several researchers have studied the accuracy of response conditioned wave methods for this purpose. However, further investigations are necessary before these can become established tools. In this paper we investigate the accuracy by comparing the short-term probability distributions obtained from random irregular waves with those from response conditioned waves. We furthermore show how response conditioned wave methods can be fitted into a long-term response analysis. The numerical and experimental investigations were performed using a container vessel with a length between perpendiculars of 281 m. Numerical simulations were done with a nonlinear hydroelastic time domain code. Experiments were carried out with a flexible model of the vessel in the towing tank at the Marine Technology Centre in Trondheim. The focus was on the probability distributions of the midship vertical hogging bending moments in the sea states contributing most to the hogging moments with a mean return period of 20 years and 10 000 years. We found that the response conditioned wave methods can very efficiently be used to accurately determine the nonlinear short-term probability distributions for rigid hulls, but either accuracy or efficiency is to a large effect lost for flexible hulls, when slamming induced whipping responses are accounted for.     

Vertical and horizontal bending moments on the hydroelastic response of a large-scale segmented model in a seaway

Wave-induced vertical bending moment (VBM) and horizontal bending moment (HBM) on a large-scale segmented model with a box-type backbone beam in short-crested irregular seas are systematically analyzed using sea trial measurement data. New insights into the relationship between nonlinear VBM and HBM of the ship sailing in short-crested sea waves are explored and presented. The results indicate that the HBM significantly contributes to the total sectional loads when the ship is sailing in a seaway and the HBM has a strong correlation with VBM in both magnitude and tendency. Therefore, design loads of HBM and the corresponding lateral structural strength issues should also be concerned in addition to VBM at the ship design phase.     

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.     

Integrated design optimization of spar floating wind turbines

A linearized aero-hydro-servo-elastic floating wind turbine model is presented and used to perform integrated design optimization of the platform, tower, mooring system, and blade-pitch controller for a 10 MW spar floating wind turbine. Optimal design solutions are found using gradient-based optimization with analytic derivatives, considering both fatigue and extreme response constraints, where the objective function is a weighted combination of system cost and power quality. Optimization results show that local minima exist both in the soft-stiff and stiff-stiff range for the first tower bending mode and that a stiff-stiff tower design is needed to reach a solution that satisfies the fatigue constraints. The optimized platform has a relatively small diameter in the wave zone to limit the wave loads on the structure and an hourglass shape far below the waterline. The shape increases the restoring moment and natural frequency in pitch, which leads to improved behaviour in the low-frequency range. The importance of integrated optimization is shown in the solutions for the tower and blade-pitch control system, which are clearly affected by the simultaneous design of the platform. State-of-the-art nonlinear time-domain analyses show that the linearized model is conservative in general, but reasonably accurate in capturing trends, suggesting that the presented methodology is suitable for preliminary integrated design calculations.