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.     

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.     

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.     

Corrosion fatigue load frequency sensitivity analysis

This paper presents experimental assessment of crack growth rates of S355J2+N steel in a corrosion fatigue environment similar to what is experienced on offshore wind farm monopile structures under various cyclic load frequencies in order to assess the effect of cyclic frequency of the applied loading within a frequency range pertinent to the structure. Fatigue crack propagation behaviour in this test programme is evaluated through fatigue tests on six compact tension test specimens in air and in laboratory simulated seawater under free corrosion condition. Fatigue crack lengths were monitored by back face strain (BFS), DCPD and ACPD. A regression model was derived through the BFS method to express strain values as a function of crack length to width ratio. The effectiveness of BFS method is particularly demonstrated in the simulated marine environment. Within the range of test frequencies, crack growth rates in simulated seawater when compared to the equivalent air test revealed environmental reduction factors of 2 and 4 at lower and higher values of stress intensity factors respectively. Significant difference in the results of the seawater test frequencies is discussed.     

Static and dynamic loading behavior of a hybrid foundation for offshore wind turbines

Considering the deficiencies of the traditional monopile foundation for offshore wind turbines (OWTs) in severe marine environments, an innovative hybrid foundation is developed in the present study. The hybrid foundation consists of a traditional monopile and a wide–shallow bucket. A series of numerical analyses are conducted to investigate its behavior under the static and dynamic loading, considering various loading eccentricities. A traditional monopile with the same steel volume is used as a benchmark. Although the monopile outperforms the hybrid foundation in terms of the ultimate moment capacity under each loading eccentricity, the latter can achieve superior or the same performance with nearly half of the pile length in the design loading range. Moreover, the horizontal load and moment are mainly resisted by the bucket and the single pile in the hybrid foundation respectively. The failure mechanism of both the hybrid foundation and the monopile is excessive rotation. In the rotation angle of 0.05 rad, the rotation center is located at the depth of approximately 0.6–0.75 times and 0.65–0.75 times the pile length for the hybrid foundation and the monopile respectively. The increasing loading eccentricities can lead to increasing moment bearing capacity, increasing initial stiffness and upward movement of the rotation center of the two foundations, while decreasing load sharing ratio of the single pile in the hybrid foundation. Three scenarios are considered in investigating the dynamic loading behavior of the hybrid foundation. Dynamic response results reveal that addition of the bucket to the foundation can restrain the rotation and lateral displacement effectively. The superiority of the hybrid foundation is more obvious under the combined wave and current loading.     

基于雨流计数法的高低频复合疲劳试验研究

船体固有频率与遭遇波浪频率及其倍频相接近时,波浪载荷极易引起船体结构产生持续的波激振动现象,对大型船舶结构疲劳损伤的影响达到40 %以上。因此,有必要针对波激振动引起的非线性垂向弯矩载荷特点,开展高低频复合工况下典型切口试件疲劳试验及累计损伤分析研究。基于非线性随机载荷的分析方法,采用闭合雨流计数法提取叠加应力历程中的多级循环载荷,并结合平均应力修正方法和疲劳极限以下SN曲线局部修正法,对叠加应力历程中小载荷的损伤效应展开研究。分析结果表明,高低频载荷叠加产生的附加损伤效应对疲劳寿命的影响显著,在恒定应力比下随平均应力的变化近似成二次函数关系。考虑高频小载荷的叠加应力历程疲劳寿命明显降低,可以通过修正累计损伤临界值或引入应力放大因子,近似的利用线性累积损伤理论预测实际叠加应力历程的寿命范围。     

圆筒型FPSO上部模块支撑结构疲劳分析

与船型FPSO相比,圆筒型FPSO没有明显的总纵弯曲,上部模块与船体结构之间通常采用刚性支墩来连接,水平运动所产生的弯矩和装/卸载引起的船体垂向变形对模块支撑结构的影响较为显著。因此,以“希望6号”圆筒型FPSO上部模块支撑结构为研究对象,基于DNVGL船级社规范,介绍一种简化疲劳分析方法。以FPSO运动加速度和船体变形载荷作为载荷输入条件,利用SESAM/GeniE软件进行有限元分析,得到结构在所有组合工况下应力的扫描计算结果。根据作业海域各个方向波浪发生的概率,运用简化疲劳分析方法计算得到所关注节点的疲劳损伤和各个工况对结构节点疲劳损伤度的贡献。结果表明,所关注节点的疲劳强度均满足设计疲劳强度要求;同一节点的疲劳损伤对不同浪向的敏感度不一样。该简化疲劳分析方法同样适用于承受周期性载荷的FPSO上部模块主结构和其他型式海洋结构物的疲劳分析。     

Permanent accumulated rotation of offshore wind turbine monopile due to typhoon-induced cyclic loading

In order to study the effect of typhoons on the accumulated deformation of monopile foundations for offshore wind turbines, a series of 1-g laboratory model tests with a geometrical scale of 1:100 were carried out. Through the horizontal static and cyclic loading tests of a stiff pile embedded in a medium dense sand deposit, the relationship between the accumulated rotation of the pile and the number of loading cycles under different loading conditions was obtained. The results show that the final accumulated rotation is mainly caused by the typhoon load series and is not affected by the loading sequence. Based on these results, a method is presented to predict the accumulated rotation of the monopile foundation during its service life, and a case study of a 6 MW wind turbine supported by a monopile at a water depth of 30 m in sand is conducted by using the method. The results show that the permanent accumulated rotation of the monopile throughout the design life is mainly contributed by cyclic loading induced by typhoons and the contribution of cyclic loading with small amplitudes can be ignored.     

Hysteretic damping model for laterally loaded piles

Fatigue assessment is a critical design aspect for many offshore structures. Soil-foundation interaction has a direct impact on the system dynamic response of these structures. While the stiffness of the soil-foundation interaction influences the system's natural frequency, the damping influences the amplification of the structural response to environmental excitations. This paper presents a simplified model for estimating the soil damping due to nonlinear soil response for pile foundations, which have wide applications in the offshore industry, such as for supporting jacket platforms, wind turbines and wellhead facilities. The proposed model is fundamentally linked to the damping response of the soil measured at element level therefore it offers design engineers an efficient and accurate way to estimate soil-pile interaction damping based on site-specific soil data. Approaches to include the suggested model for structural analysis are also proposed.     

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.     

Foundation damping for monopile supported offshore wind turbines: A review

Today, an important challenge for offshore wind energy is to design efficient and reliable offshore wind turbines (OWTs). The overall damping of OWTs plays an important role in the design process as it limits the amplitude of the OWT dynamic response at frequencies near resonance. Therefore, an accurate estimation of OWTs damping is necessary for the efficient design of these systems. The foundation damping is one of the main sources overall turbine damping and is the least well understood. This paper presents a critical review of recently published studies on foundation damping for OWTs on monopiles and explains how soil damping contributes to the total damping of OWTs. It also reviews the main methods that have been used for the estimation of foundation damping in numerical and experimental studies. In addition, the importance of damping to the OWTs fatigue life is discussed. Finally, a discussion is provided on the challenges to be overcome and recommendations for the accurate estimation of foundation damping.     

Dynamic ice loads for offshore wind support structure design

For offshore wind farms which are planned in sub-arctic regions like the Baltic Sea and Bohai Bay, support structure design has to account for load effects from dynamic ice-structure interaction. There is relatively high uncertainty related to dynamic ice loads as little to no load- and response data of offshore wind turbines exposed to drifting ice exists. In the present study the potential for the development of ice-induced vibrations for an offshore wind turbine on monopile foundation is experimentally investigated. The experiments aimed to reproduce at scale the interaction of an idling and operational 14 MW turbine with ice representative of 50-year return period Southern Baltic Sea conditions. A real-time hybrid test setup was used to allow the incorporation of the specific modal properties of an offshore wind turbine at the ice action point, as well as virtual wind loading. The experiments showed that all known regimes of ice-induced vibrations develop depending on the magnitude of the ice drift speed. At low speed this is intermittent crushing and at intermediate speeds is ‘frequency lock-in’ in the second global bending mode of the turbine. For high ice speeds continuous brittle crushing was found. A new finding is the development of an interaction regime with a strongly amplified non-harmonic first-mode response of the structure, combined with higher modes after moments of global ice failure. The regime develops between speeds where intermittent crushing and frequency lock-in in the second global bending mode develop. The development of this regime can be related to the specific modal properties of the wind turbine, for which the second and third global bending mode can be easily excited at the ice action point. Preliminary numerical simulations with a phenomenological ice model coupled to a full wind turbine model show that intermittent crushing and the new regime result in the largest bending moments for a large part of the support structure. Frequency lock-in and continuous brittle crushing result in significantly smaller bending moments throughout the structure.     

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 impedances and load carrying mechanism for skirted foundations

Dynamic impedances of foundations include dynamic stiffness and damping which have important effect on the internal forces in the structure. In some cases, such as offshore wind turbines, the influence of the foundation impedances on the system's natural frequency and overall damping could potentially have a significant effect on the fatigue life of the structure. The vertical, horizontal and rocking impedances of a skirted foundation (also termed bucket foundation in offshore wind industry) embedded in a fully saturated poroelastic seabed are addressed in this paper. The vertical impedance is most relevant for jacket foundations supported on three or four bucket foundations, while horizontal and rocking impedances are applicable for mono-bucket foundations. The dynamic vibration problems are solved semi-analytically with the help of dual integral equations and Green's functions. Numerical results for dynamic impedances are obtained; damping ratio are also obtained to show the importance of radiation damping for bucket foundations, even at very small excitation frequencies. The influence of length-to-radius ratio, Poisson's ratio, permeability of soil, excitation frequency and thickness-to-radius ratio on the impedances are also studied. Besides, the dynamic load sharing among the top plate, bucket shaft and bucket tip is obtained for vertical load, horizontal load and moment to shed light on the carrying mechanism of bucket foundation at dynamic working loads. It is found that for a rigid bucket foundation, even when the length-to-radius ratio is small (e.g. l/a = 1.0), most of the loads are carried by the shaft, while the top plate and tip of the bucket take only a small portion of the loads. The results of this study will be helpful for understanding the load-carrying mechanism of offshore bucket foundations for normal operation conditions.     

Fatigue damage calculation for oil tanker and container ship structures

The fatigue behaviour of longitudinal stiffeners of oil tankers and container ships, subjected to dynamic loads, is analysed. The following dynamic load components are considered: hull girder vertical wave bending moment, alone and combined with the horizontal wave bending moment, hydrodynamic pressure and inertial forces caused by cargo acceleration.

The spectral method was selected to calculate the fatigue damage, based on S—N curves and Miner's rule. Following this approach, the fatigue damage may be calculated as a function of a stress parameter Ω_p, which represents the cumulative effect of wave induced loads in the unit of time and incorporates the combined effects of stress level and its occurring frequency.

Simple formulas for Ω_p of oil tankers and container ships are given, obtained from the results of hydrodynamic analyses performed on several ships, in different wave environments.

Several examples show the applicability of the methods to real ship structures. The method, however, still needs to be calibrated because of the simplifying hypotheses introduced in the loading conditions.     

超大型 VLCC 波激振动和砰击振动模型试验研究

船舶的波激振动和砰击振动对船舶结构的安全性有较大影响。文章以一艘超大型 VLCC 为研究对象,通过变截面梁分段模型试验方法对船舶在规则波和不规则波中波激振动和砰击振动响应进行了比较分析,介绍了船模波浪载荷试验中模型的设计原则,通过静水试验得到了船体梁垂向振动频率特性、振动阻尼和静水兴波弯矩等参数,通过规则波和不规则波试验分析了波高、波浪周期和装载状态等因素对波激振动和砰击振动的影响。该文的研究结果对大型船舶的结构设计具有一定的指导意义。     

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.     

含月池开孔的FPSO水动力性能及波浪载荷研究

为研究月池开孔对FPSO水动力性能和波浪载荷的影响,基于三维势流理论和波浪诱导载荷理论,分析了不同的月池尺寸、不同的月池纵向位置以及不同的船体吃水时附加质量和附加阻尼的变化趋势。并进行了含月池开孔的FPSO波浪诱导载荷的长期预报,得到了截面垂向剪力和弯矩沿船长方向的分布规律,分析了月池尺寸对波浪诱导载荷的影响。结果表明含有月池的FPSO船体的垂向附加质量和附加阻尼会出现一个峰值,随月池尺寸增加而逐渐减小;月池处垂向剪力和垂向弯矩随月池尺寸增大有小幅度减小。     

Time domain analysis procedures for fatigue assessment of a semi-submersible wind turbine

Long term time domain analysis of the nominal stress for fatigue assessment of the tower and platform members of a three-column semi-submersible was performed by fully coupled time domain analyses in Simo-Riflex-AeroDyn. By combining the nominal stress ranges with stress concentration factors, hot spot stresses for fatigue damage calculation can be obtained. The aim of the study was to investigate the necessary simulation duration, number of random realisations and bin sizes for the discretisation of the joint wind and wave distribution. A total of 2316 3-h time domain simulations, were performed.In mild sea states with wind speeds between 7 and 9 m/s, the tower and pontoon experienced high fatigue damage due to resonance in the first bending frequency of the tower from the tower wake blade passing frequency (3P).Important fatigue effects seemed to be captured by 1 h simulations, and the sensitivity to number of random realisations was low when running simulations of more than 1 h. Fatigue damage for the tower base converged faster with simulation duration and number of random realisations than it did for the platform members.Bin sizes of 2 m/s for wind, 1 s for wave periods and 1 m for wave heights seemed to give acceptable estimates of total fatigue damage. It is, however, important that wind speeds that give coinciding 3P and tower resonance are included and that wave periods that give the largest pitch motion are included in the analysis.