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Collaborating Authors
Investigation of the Equivalent Rocking Stiffness for Pile Raft Foundation Systems of Wind Turbines
Cao, Shuhan (Institute for Geotechnical Engineering, Leibniz University Hannover) | Abdel-Rahman, Khalid (Institute for Geotechnical Engineering, Leibniz University Hannover) | Wörden, Florian tom (Institute for Geotechnical Engineering, Leibniz University Hannover) | Achmus, Martin (Institute for Geotechnical Engineering, Leibniz University Hannover)
ABSTRACT The foundation system for wind turbines can be a shallow concrete raft or a pile raft foundation; this depends on the loading conditions and on the sub-soil parameters. The economic design of the pile foundation depends mainly on the number of piles, pile diameter, pile arrangement, pile length and inclination of the piles. Since wind turbines are structures under high dynamic loading conditions, requirements of minimum dynamic stiffness for the overall operational behavior of the pile foundation system must be checked in addition to the behavior under static loading. For the simplified overall calculation, an approach for dynamic equivalent rocking spring stiffness is given in the literature for shallow foundations. It will be examined whether certain pile raft systems can possibly be considered as equivalent shallow foundations so that rocking stiffness approach for shallow foundations can be applied for predesign. In this paper, the equivalent rocking stiffness for a pile raft foundation is investigated using finite element method (FEM). Within the framework of a parametric study, the main factors affecting the equivalent rocking stiffness are identified and subsequently evaluated. Finally the applicability of the rocking spring stiffness for shallow foundations to be adopted for pile raft is checked. INTRODUCTION A wind turbine generally has five components; foundation, tower, nacelle, rotor blade, and hub. Generally, the foundation for wind turbines can be designed as gravity or pile raft foundation. The design methods and considerations for supporting structures with piles as foundation elements are given in several regulations [e.g. IEC (2019), DNV-GL (2018), API (2014)]. A pile raft foundation is used where the upper soil layers are of a weaker quality and the loads need to be transferred to larger depths where stiffer soil or bed rock is present. The load from the superstructure is transmitted to the foundation through the piles. In addition to transferring the load to the foundation, the pile cap also provides structural rigidness to the pile group and acts as rigid connection between the piles and wind turbine tower.
Effects of Scour on Stiffness of Wide Shallow Bucket Foundation and 1st Natural Frequency of Offshore Wind Turbine
Yuan, Yu (Tianjin University) | Liu, Run (Tianjin University) | Lian, Jijian (Tianjin University) | Fu, Dengfeng (Tianjin University) | Zhang, Haiyang (Tianjin University) | Wang, Yingchun (Tianjin University)
ABSTRACT The Wide shallow bucket foundation has been successfully used for offshore wind turbines in city of Xiangshui in Jiangsu Province, China, and has a promising application prospect. Stiffness of shallow bucket foundation and natural frequencies of wind turbine are key parameters in dynamic analyses of the offshore wind turbine. However, the effects of scour on the stiffness of wide shallow bucket foundation and the natural frequencies of offshore wind turbine are not studied sufficiently. Numerical models of the wind turbine considering bucket-soil-interaction by using finite element method are established. The influences of scouring depth on static stiffness of wide shallow bucket foundation and the first order natural frequency of wind turbine are analyzed. The results show that scouring depth has a more distinct impact on rocking static stiffness than horizontal stiffness. Due to the large rocking and horizontal stiffness of wide shallow bucket foundation, scouring has little influence on the 1 natural frequency of the offshore wind turbine. INTRODUCTION Due to the growing demand of resources, offshore wind turbine (OWT) developed rapidly in the past several decades for its advantages of clean and high efficiency to produce power. A series of policies have been established by Chinese government to encourage further developments of offshore wind energy in the coast of China. However, the cost of offshore wind turbine is still high especially in construction of foundation. Generally, the cost of the foundation accounts for nearly 30% of total costs (Thornley et al., 2009; Arshad and Kelly, 2013). Accordingly, optimizations of the foundation have raised concerns related to offshore wind farm investment (Wang et al., 2017). There are several typical types of the fixed foundations for offshore wind turbine including monopile, gravity base, suction bucket and jacket structures (Bhattacharya, 2014). Suction bucket has become better alternatives to driven piles because of technical challenges and costs associated with the installation equipment, the process of installation of suction buckets involve little noises and are friendly to environments. Besides, suction buckets also provide a greater resistance to lateral loads than driven piles because of the larger diameters typically used. In this background, wide-shallow composite bucket foundation (WSCBF) has been proposed and successfully examined (Lian et al., 2012; Liu et al., 2015; Zhang et al., 2016) (Fig.1). In fact, a fully operational wind turbine equipped with WSCBF was successfully applied in city of Xiangshui in Jiangsu Province, China.
- Research Report > New Finding (0.34)
- Research Report > Experimental Study (0.34)
ABSTRACT This paper proposes three kinds of SWA (Small Water-plane Area) trifloater foundations, aiming to study this new-type floating foundation for offshore wind turbines with excellent hydrodynamic response performance. The overall hydrodynamic model including foundationtower-wind turbine was established by using the SESAM software. Based on three-dimensional potential flow theory and Morrison formula, combined with the environmental loads of wind, wave and current in South China Sea, the hydrodynamic responses of these three SWA tri-floater foundations and constant section tri-floater foundation are calculated and compared in both frequency and time domain. The results show that the wave excitation loads acting on the foundation could be decreased effectively by reducing the water-plane area of columns. In addition, the motion responses of foundation are sensitive to the stiffness and water-plane area. The SWA scheme is possible to be adopted for FOWT (Floating Offshore Wind Turbine) by designing the foundation appropriately. INTRODUCTION With the development trend of offshore wind turbine from shallow water to deep water, the floating foundations are becoming increasingly popular compared with the traditional fixed foundations. Generally, the floating foundation can be roughly divided into three categories: Spar type, TLP (Tension Leg Platform) type and Semi-submersible (Zhang C, 2017). FOWT is a combination system. In general, it could be divided into two main parts: the wind turbine subassembly and floating foundation. The upper tower suffers from wind loads and the blades suffer from aerodynamic loads. The foundation suffers from wave and current loads simultaneously. The interaction of various loads due to wind, wave and current brings great challenges for the design of floating structure. The improvement of stability and reduction the response amplitude of floating structure when subject to environment loads is a reason of concern. Meng, He, Zhou, Zhao and Liu (2018) proposed a fully coupled aero-hydro-servo-elastic time domain model by FAST software to investigate the dynamic response characteristics of a 6MW spar-type FOWT under the multi interaction of wind & wave loads. The results show that the mean value of dynamic response was mainly dependent on the wind-induced action. Zhang and Kim (2018) presented a fully coupled numerical simulation method to analyze the dynamic response of a semi-submersible floating wind turbine under the combined wind-wave excitation environment loads. Ye and Ji (2019) studied on the dynamic response of a 5MW spar-type offshore wind turbine by taking into account the interaction of current, wave and wind. The effects of hydrodynamic & aerodynamic loads on dynamic response of floater is calculated by time history and its FFT (Fast Fourier Transformation) spectrum results. Bahramiasl, Abbaspour and Karimirad (2018) carried out a study on influence of floater responses by the gyroscopic effect of rotating rotor and wind heading angle. It was found that the peak of spectrums could be shifted to a higher frequency by increasing the rotor rotation velocity. In addition, the heave and pitch response amplitudes in time domain as well as heave, sway and surge motion in frequency domain could be changed with the changing of heading angle of wind. Huijs, Vlasveld, Gormand, Savenije, Caboni, LeBlanc, Ferreira, Lindenburg, Gueydon, Otto and Paillard (2018) integrated design a semi-submersible floating VAWT (Vertical Axis Wind Turbine) with active blade pitch. The result illustrated that the weight of floater can be reduced by 20% under the same rated power. Chen, Hu, Duan (2018) compared the dynamical characteristics of a 5MW spar-buoy with semi-submersible floating wind turbine by the model testing results. It was found that the sparbuoy floating wind turbine is more sensitive to wind loading due to the smaller water plane, whereas the semi-submersible floating wind turbine is more sensitive to wave loading, especially for the second order difference frequency wave loading. Liapis, Lu, Li and Peng (2015) used the model testing and CFD simulation methods to investigate a design of a semi-submersible utilizing heave plates. They concluded that heave motion can be significantly reduced by adopting heave plates. Jang, Park, Kim, Kim and Hong (2019) conducted a numerical and experimental study on a semi-submersible multi-unit offshore floater with four wind turbines. The study indicates that the heave and pitch motions of floaters had been reduced effectively due to the effect of heave plates, and their natural frequencies were also changed.
- Research Report > New Finding (0.89)
- Research Report > Experimental Study (0.75)
A Family of Practical Foundation Models for Dynamic Analyses of Offshore Wind Turbines
Page, Ana María (Norwegian Geotechnical Institute) | Løkke, Arnkjell (Norwegian Geotechnical Institute) | Skau, Kristoffer Skjolden (Norwegian Geotechnical Institute) | De Vaal, Jacobus Bernardus (Institute for Energy Technology)
Abstract The concept of macro-element modelling – which was first introduced almost 30 years ago – has proven to be a convenient and accurate technique for modelling offshore foundations, but historically these models have mainly been used for academic purposes. Recent developments in foundation modelling now allow for application of such models in engineering practise and design. One such example is the family of new macro-element models that have been developed in the research project REDWIN to represent the foundation behaviour in dynamic analyses of Offshore Wind Turbines (OWTs). These models exhibit characteristic foundation behaviour such as nonlinearity, coupling of the load from different load components and hysteretic load dependent damping. This paper presents two of the REDWIN models, one applicable for monopile foundations and one for skirted suction caisson foundations. Use of the models are demonstrated through two practical problems that reflect typical design analyses of OWTs: the first example shows a fatigue damage assessment for a monopile, and the second considers an extreme load event for a suction bucket jacket. The structural response is computed using the REDWIN foundation models and compared with the response based on distributed API p-y springs for the monopile and clamped legs at seabed for the jacket. Special emphasis is devoted to how the model input is obtained to guide readers on practical use of the models.
- Europe (0.94)
- North America > United States > Texas (0.28)
Effective Consideration of Soil Characteristics in Time Domain Simulations of Bottom Fixed Offshore Wind Turbines
Hübler, Clemens (Leibniz Universität Hannover) | Häfele, Jan (Leibniz Universität Hannover) | Ehrmann, Andreas (Leibniz Universität Hannover) | Rolfes, Raimund (Leibniz Universität Hannover)
Abstract An effective consideration of the soil characteristics is challenging. Sophisticated soil-structure interaction models have many degrees of freedom and are highly non-linear. Hence, they are not applicable for transient calculations of the design stage due to high computing times. Therefore in a first step, a six-directional, linear approach to consider soil effects is presented which assumes the turbine connected to the soil by inertial and elastic coupling terms. Results of jackets with piles and suction buckets are presented and show significant shifts of the eigen-frequencies compared to approaches with substructures clamped to the seabed. In a second step, a piecewise defined response surface has been developed in order to take the operating point into account. It correlates the previously known environmental conditions with the loads required for calculating the interaction matrices. This approximation has been proven to be accurate enough in this context and led to a further shift of the eigen-frequencies compared to results with no loads applied. Introduction The design and certification process of offshore wind turbines involves holistic time domain simulations because common standards and guidelines require ultimate and fatigue limit state verifications. One major challenge in this context is the consideration of soil effects. The spectrum of soil-structure interaction models reaches from complex and non-linear finite element models (Augustesen et al., 2009; Grabe et al., 2005) to simplified p-y-curves (API, 2002; Wiemann et al., 2004). Furthermore, other models based on CPTs (cone penetration tests; Achmus and Müller, 2010) or spring-damper combinations with varying complexity exist (Van Buren and Muskulus, 2012). However, all sophisticated soil-structure interaction models have many degrees of freedom. Hence, they are not suitable for transient calculations with demands on numerical efficiency due to high computational times. That is why these days offshore wind turbines are often modeled clamped to the seabed in transient simulations. In Häfele et al. (2016) a two-step approach is presented that allows an effective consideration of soil characteristics in transient simulations. However, this approach supposes the initial operation point as basis for the stiffness calculation. As the soil stiffness reduces with higher deflections due to increasing loads, the disregard of the operating point is a simplification, though it is much less serious than the clamped approach. The operating point in this context is defined as the turbine state, mainly the tip speed ratio, that is created by the controler. This leads to different load conditions for every operating point.
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems > Platform design (0.91)
- Facilities Design, Construction and Operation > Facilities and Construction Project Management > Offshore projects planning and execution (0.81)
- Data Science & Engineering Analytics > Information Management and Systems (0.66)