Tragverhalten von Stahlgussbauteilen in Offshore‐Windenergie‐Anlagen unter vorwiegend ruhender und nicht ruhender Beanspruchung

Load bearing behaviour of cast steel components in offshore wind turbines under fatigue and static loads. The installation of offshore wind turbines of the 5‐megawatt class in deep water of the North Sea makes it economically necessary to build foundations in high quantities. Constructions of this kind are a major technical challenge in terms of design, manufacturing and operation. In this paper the dynamic behaviour of jacket constructions in the North Sea are investigated. Currently jacket structures are constructed with steel pipes that are welded at the connecting nodes. This procedure implies several disadvantages. In contrast cast steel elements are universally applicable and designable. To verify the load bearing behaviour numerical simulations and laboratory tests are done. Laboratory tests include the testing of the ultimate load bearing capacity. In a large scale setup and fatigue tests at small cast steel specimen.     

海上风机的分部与整体结构设计现状

海上风机结构是一个相对复杂的结构系统,其动力响应受到多种环境因素影响,且多种荷载之间的相互作用是一种非线性行为。此外,风力机不同组成部分之间都有其独特特性,结构间的耦合作用决定着风机在非平稳环境力作用下的整体动力反应。分别综述了风机叶片的转动与弹性变形的耦合、塔架与叶片的耦合、塔架的优化设计、基础的模型以及结构随参数变化敏感性分析。最后对未来有待于进一步研究的问题进行介绍。     

Research into composite tubular construction for offshore jacket structures

This paper describes recent developments in the use of steel and concrete composite construction for offshore structures. The developments concern filling steel tubular members with cementitious materials to improve static and fatigue strength of memebers and joints. Applications are described for jacket to pile connections, nodal joints and repair situations.     

Einsparpotentiale bei Tragkonstruktionen von Offshore‐Windenergieanlagen

Savings potential for support structures of offshore wind turbines. Due to the high ratio of dynamic loads from wind and wave forces on offshore wind turbines, the fatigue design analysis proves to be the determining factor for the dimensioning of support structures. As a result, the current design practice is limited, particu‐larly with regard to the economic benefits of employing high strength steels. Especially from the aspect of mass production, however, the economic optimization of the components is a sub‐stantial economic advantage for the companies. This paper presents different options for savings in the design and assessment of offshore‐foundation structures and provides the example of a transition piece for a jacket foundation structure. Savings potential is realized through optimal construction design, directionally dependent load determination, a refined methodology of analysis, as well as through modern methods of post weld treatment.     

Tragstruktur und Installation der Offshore‐Windenergieanlage Repower 5M

Design, fabrication and installation of the offshore wind turbine REpower5M. In summer 2006 the first 5 MW offshore wind turbine has been installed in 45 m water depth in Scotland. The large water depth and other difficult site conditions, like harsh wave climate and unfavourable soil conditions, made numerous innovations for the support structures and installation necessary. This paper describes design, fabrication and installation of the jacket structures which have been developed. Furthermore the installation methodology for the turbine for which a floating crane instead of a fixed platform was used, is described. The knowledge gained from this project will enable realization of offshore windfarms in water depth beyond 20 m.     

Simulation of offshore wind turbine response for long-term extreme load prediction

When there is interest in estimating long-term extreme loads for an offshore wind turbine using simulation, statistical extrapolation is the method of choice. While the method itself is rather well-established, simulation effort can be intractable if uncertainty in predicted extreme loads and efficiency in the selected extrapolation procedure are not specifically addressed. Our aim in this study is to address these questions in predicting blade and tower extreme loads based on stochastic response simulations of a 5 MW offshore turbine. We illustrate the use of the peak-over-threshold method to predict long-term extreme loads. To derive these long-term loads, we employ an efficient inverse reliability approach which is shown to predict reasonably accurate long-term loads when compared to the more expensive direct integration of conditional load distributions for different environmental (wind and wave) conditions. Fundamental to the inverse reliability approach is the issue of whether turbine response variability conditional on environmental conditions is modeled in detail or whether only gross conditional statistics of this conditional response are included. We derive long-term loads for both these cases, and demonstrate that careful inclusion of response variability not only greatly influences such long-term load predictions but it also identifies different environmental conditions that bring about these long-term loads compared with when response variability is only approximately modeled. As we shall see, for this turbine, a major source of response variability for both the blade and tower arises from blade pitch control actions due to which a large number of simulations are required to obtain stable distribution tails for the turbine loads studied.     

Comparison of the dynamic responses of monopiles and gravity base foundations for offshore wind turbines in sand using centrifuge modelling

Monopiles and gravity base foundations (GBF) are two of the most commonly used foundations for offshore wind turbines. As resonance can cause damage and even failure of wind turbines, understanding the difference between the dynamic responses of monopiles and GBFs under free vibration is important. However there is little experimental data regarding their natural frequency, especially from model tests carried out at correct stress levels. This paper presents the results of novel monopile and GBF tests using a centrifuge to directly determine the natural frequency (f_n) of the foundation-soil system. The natural frequencies of wind turbine monopiles and GBFs in centrifuge models were measured during harmonic loading using a piezo-actuator, with the results confirming that soil-structure interaction must be considered to obtain the system’s natural frequency as this frequency reduces substantially from fixed-base values. These results will contribute in preventing resonance induced damage in wind-turbines.     

Fatigue crack growth analysis of a square hollow section T-joint

Stress concentration sites are always found at the brace and chord intersection corners of any rectangular welded tubular joint. As a result, fatigue cracks are liable to be initiated and propagated from these corners. In this paper, the 3D fatigue crack growth under the weld toe of a square hollow section welded T-joint is simulated using boundary element method. In accordance with the 3D analyses, fatigue crack growth is predicted using a model based on the Paris’ law and stress intensity factors. Good agreement between the experimental and predicted crack growth and crack shape development is obtained. Based on this crack growth analysis, the fatigue life of a specimen is predicted and compared with the standard S-N curve for hollow section joints. It is found that the standard S-N curve is safe and slightly conservative.     

Wind‐induced fatigue of large HAWT coupled tower–blade structures considering aeroelastic and yaw effects

An efficient approach for predicting wind‐induced fatigue in large horizontal axis wind turbine coupled tower–blade structures subject to aeroelastic and yaw effects is presented. First, aerodynamic loads under yaw conditions are simulated based on the harmonic superposition method and modified blade element momentum theory, in which wind shear, tower shadow, tower–blade interactions, aeroelastic, and rotational effects are taken into account. Then, a nonlinear time‐history of wind‐induced responses under simulated aerodynamic loads is obtained. Finally, based on these results, wind‐induced fatigue damage and lifespan are predicted according to linear cumulative damage theory. For completeness, the influences of mean wind speed, aeroelasticity, and yaw angle on horizontal axis wind turbine fatigue life are discussed. The results indicate that the aerodynamic loads and residual fatigue life can be estimated accurately by the proposed model, which can be used to simulate the 3D wind fields of wind turbines under given wind conditions. The wind energy of the wind turbine blade is mainly concentrated at its edge and is weaker at the hub. Estimation of wind turbine fatigue life is therefore suggested to be based on the component with the shortest life, being the blade root. Furthermore, yaw conditions significantly shorten fatigue life and should not be ignored. Fatigue life is also rather sensitive to mean wind speed.     

杭州湾观光塔铸钢节点疲劳性能试验研究

杭州湾跨海大桥观光塔处于近海,结构受到海洋波浪荷载的作用,需对塔中2个铸钢节点的疲劳性能进行试验研究。通过对塔中的2个铸钢上节点模型(缩尺比例为1∶4)的静载试验和抗弯疲劳试验,研究了GS-20Mn5V的铸钢节点的疲劳性能。试件在3种循环荷载作用下的滞回曲线的变化和应变随着循环次数的增加而降低的规律表明,GS-20Mn5V铸钢属于应力强化材料。分析试件在不同应力幅作用下的疲劳寿命变化,得到了GS-20Mn5V铸钢节点的疲劳极限及S-N曲线。研究结果表明,观光塔中的铸钢节点在波浪荷载作用下能够保证结构安全。     

海洋导管架结构的现状研究及应用前景

研究了各种结构形式的特点,并分别对三导管架、四导管架和八导管架结构进行了介绍,根据对导管架节点的连接形式的研究,总结出各种节点的结构优缺点,对导管架节点进行了分类,提出了深海石油平台和近海风力发电平台两种导管架发展前景。     

不同焊接方式下圆钢管节点力学性能的试验比较

对两个不同焊接方式的空间KTT型圆管搭接节点进行了静力试验,一个节点是所有腹杆沿节点相贯线全周焊接,另一个节点是被搭接的腹杆沿相贯线的部分周长焊接.综合比较了两种节点在破坏模式、荷载-位移曲线、荷载-应变曲线、Mises应力分布和节点承载力等方面的差异.研究结果显示不同的焊接方式没有引起节点承载力的显著差别,但引起节点破坏模式的不同.这初步表明对非直接承受动力荷载的圆管搭接节点,被搭接的腹杆可考虑采用部分周焊.     

考虑桨叶旋转效应的海上风力发电高塔系统随机风振响应与动力可靠度分析

研究了考虑桨叶旋转效应的海上风力发电高塔系统随机动力响应与风振可靠度分析.在风场模拟中,桨叶风荷载需要考虑旋转效应的影响.因此,对塔体风荷载,直接采用基于物理机制的随机Fourier谱,而对桨叶风荷载,则采用考虑桨叶旋转机制的随机Fourier谱概念.在此基础上,结合概率密度演化理论,对海上风力发电高塔系统进行了随机动力响应分析以及基于塔顶位移响应的风振动力可靠度分析.结果表明,上述方法能够有效地进行此类结构的随机动力响应及可靠度分析.     

Acoustic emission detection of fatigue damage in cruciform welded joints

Weld seams are critical points for the initiation of fatigue cracks in steel structures subjected to cyclic loads. Semi-elliptical surface cracking at the toes of a fillet weld is not easily found when it is partially through the thickness and subcritical. In this study the acoustic emission (AE) method is used to detect crack propagation in cruciform fillet welded joints that are representative of typical fatigue sensitive details in steel bridge superstructures. The effect of geometry and fatigue load on the AE data is investigated by varying the width of the base plate and the stress ratio. AE data filtering based on load pattern, source location, and waveform feature analysis was implemented to minimize noise-induced AE signals and false indications due to wave reflections. AE time domain features such as amplitude (b-value), counts, signal strength, and absolute energy are employed to study the influence of geometry and fatigue load on the AE data.     

An investigation into the use of push-in pile foundations by the offshore wind sector

This paper presents the results of a field test performed to study the effects of installation method on the load–displacement response of piles used to support the offshore wind turbines. An instrumented open-ended model pile was installed by jacking in a deposit of medium-dense sand. Pile jacking has environmental benefits over the traditional method of pile driving which can cause noise and vibration damage to the marine mammals. Pile installation by jacking was shown to enhance the pile-soil stiffness response during compression loading. Residual stresses, generated during the installation process, caused the pile to exhibit a relatively soft stiffness response during tension loading. Environmental loading caused by wind and waves which causes piles that support jacket structure to experience tension loading and the serviceability limit state of the foundation to these loads governs the design.     

Reliability of design approaches for axially loaded offshore piles and its consequences with respect to the North Sea

In the near future, several offshore wind farms are planned to be built in the North Sea. Therefore, jacket and tripod constructions with mainly axially loaded piles are suitable as support structures. The current design of axial bearing resistance of these piles leads to deviant results regarding the pile resistance when different design methods are adopted. Hence, a strong deviation regarding the required pile length must be addressed. The reliability of a design method can be evaluated based on a model error which describes the quality of the considered design method by comparing measured and predicted pile bearing resistances. However, only few pile load tests are reported with regard to the boundary conditions in the North Sea. This paper presents 6 large-scale axial pile load tests which were incorporated within a new model error approach for the current design methods used for the axial bearing resistance, namely API Main Text method and cone penetration test (CPT)-based design methods, such as simplified ICP-05, offshore UWA-05, Fugro-05 and NGI-05 methods. Based on these new model errors, a reliability-based study towards the safety was conducted by performing a Monte-Carlo simulation. In addition, consequences regarding the deterministic pile design in terms of quality factors were evaluated. It is shown that the current global safety factor (GSF) prescribed and the partial safety factors are only valid for the API Main Text and the offshore UWA-05 design methods; whereas for the simplified ICP-05, Fugro-05 and NGI-05 design methods, an increase in the required embedded pile length and thus in the GSF up to 2.69, 2.95 and 3.27, respectively, should be considered to satisfy the desired safety level according to DIN EN 1990 of β = 3.8. Further, quality factors for each design method on the basis of all reliability-based design results were derived. Hence, evaluation of each design method regarding the reliability of the pile capacity prediction is possible.     

Study on fire resistance of circular hollow section (CHS) T-joint stiffened with internal rings

This paper aims to study the fire resistant performance of circular hollow section (CHS) T-joint stiffened with internal rings under axial compression at elevated temperatures. Experimental study and finite element analysis are combined to carry out the investigation. Two full-scale tubular joints, one is un-stiffened and the other is stiffened with ring-stiffeners, are designed and tested. A constant axial loading is applied at brace end of the joints firstly, and then the joints are heated in accordance with ISO 834 standard heating curve uniformly in an electric heating furnace until failure. It is found that the internal rings enhance the fire resistant performance of CHS T-joint by decreasing the temperature of chord efficiently and prolonging the fire-resistant time. Sequentially, the corresponding finite element models of the two joints are built on basis of the experiments by employing sequentially coupled thermal-stress method. By comparing with the experimental data including temperatures of chord and brace and displacements of crown, saddle and brace end, the finite element models are verified to be accurate and reliable. And then numerical analysis is conducted to investigate the reinforcing efficiency of ring-stiffeners and impact of geometrical parameters (β and γ) on the fire resistant performance of the stiffened joints by adopting the verified modeling method. It is summarized that the failure mode converts with geometrical dimensions of ring-stiffener and geometrical parameters of tubular joint changing.     

FEMA approaches in seismic assessment of jacket platforms (case study: Ressalat jacket of Persian gulf)

The assessment of existing platforms under past environmental (wave, wind, current and etc.) loads and probable future loads (earthquake) is a relatively new process and has not yet been standardized as the design has. This lack of standardization creates some difficulty in establishing performance requirements which must be developed depending upon the risks (i.e., hazards, exposures and consequences) associated with the future operation of the platform. The present criteria of the offshore structure standards for seismic assessment can be improved using Building prestandards. Recently some documents such as FEMA-356 and ATC-40 are developed for seismic assessment of buildings. However there is advice in API documents (API RP 2A) in order for seismic assessment of jacket platforms, but because of brief existing comments in this field, it is necessary to use more appropriate (pre)standards for seismic assessment of these structures. In this paper rough and global comments of API are compared with a detailed method of FEMA. As an example seismic assessment of the existing 4 legged Service platform placed in the Ressalat Oil Field (Persian gulf) is presented. It is very useful and efficient to use of FEMA for seismic assessment of jacket platforms.     

Stress concentration factors induced by out-of-plane bending loads in ring-stiffened tubular KT-joints of jacket structures

In the present paper, data extracted from the finite element analysis of 118 models, which were validated against the test results obtained from an experimental study, were used to investigate the geometrical effect on the weld-toe stress concentration factors (SCFs) in internally ring-stiffened tubular KT-joints subjected to four different types of out-of-plane bending (OPB) loads. Although the tubular KT-joints are commonly found in offshore jacket structures and despite the crucial role of SCFs in the evaluation of the tubular joint׳s fatigue performance, the SCFs in internally ring-stiffened KT-joints subjected to OPB loads have not been investigated so far and no design equation is available to determine the SCFs for OPB-loaded joints of this type. In the present research, geometrically parametric study was followed by the nonlinear regression analysis to develop a new set of SCF parametric formulas for the fatigue design of ring-stiffened KT-joints under the OPB loadings.