Analysis Of Offshore Wind Turbine Research Articles

Tower Load Analysis of Offshore Wind Turbines and the Effects of Aerodynamic Damping

Abstract This study presents an analysis of the loads on offshore Horizontal Axis Wind Turbines (HAWTs). The aerodynamic loadsare estimatedusing the Blade Element-Momentum (BEM) theory, including the effects of dynamic inflow and dynamic stall. The wave loads are calculated using Morison's equation. Modelsare proposedto account for the effects of aerodynamic damping on the fatigue loading on the tower during its working lifetime. Load analysis of a 5 MW offshore HAWT is carried out and the influence of aerodynamic damping on the fatigue load is investigated.

Implementation of a non‐linear foundation model for soil‐structure interaction analysis of offshore wind turbines in FAST

AbstractBottom‐fixed offshore wind turbines (OWTs) involve a wide range of engineering fields. Of these, modelling of foundation flexibility has been given little priority. This paper investigates the modelling of bottom‐fixed OWTs in the non‐linear aero‐hydro‐servo‐elastic simulation tool FAST v7. The OWTs considered is supported on a monopile. The objective of this paper was to implement a non‐linear foundation model in this software. The National Renewable Energy Laboratory's idealized 5MW reference turbine was used as a base for the analyses.Default modelling of foundation in FAST v7 is by means of a rigid foundation. This implies that soil stiffness and damping is disregarded. Damping may lead to lower design loads. A softer foundation, on the other hand, will increase the natural periods of the system, shifting them closer to the frequencies of the environmental loads. This may in turn lead to amplified moments at the mudline. Therefore, it is important to include soil stiffness and damping in analyses.In this paper, a non‐linear foundation model is introduced in FAST v7 by means of uncoupled parallel springs. To verify that the implementation is successful, non‐linear load‐displacement curves of the foundation spring are presented. These show the typical hysteresis loops of an inelastic material, which confirms the implementation. Copyright © 2016 John Wiley & Sons, Ltd.

Reliability Analysis of Offshore Wind Turbines Considering Soil-Pile Interaction and Scouring Effect

최근 í•´ìƒí’ë ¥í„°ë¹ˆì— 대한 하부구조물로 재킷 또는 트라이포드 형태의 ê³ ì •ì‹ 하부구조물이 기존의 ëª¨ë ¸íŒŒì¼ì„ 대체하는 좋은 대안으로 ì œì‹œë˜ê³ ìžˆë‹¤. 이러한 재킷 또는 트라이포트 하부구조물은 이미 ê¸°ìˆ ì„±ìˆ™ë„ê°€ ë†’ê³ 25-50 m 사이의 중수심에서 ê²½ì œì„± 확보가 가능하다. 본 ë ¼ë¬¸ì—ì„œëŠ” ëª¨ë ¸íŒŒì¼ì„ 포함하여 트라이포드, 재킷 하부구조물을 채택한 ê³ ì •ì‹ í•´ìƒí’ë ¥í„°ë¹ˆì— 대하여 지반물성치 및 하중의 불확실성, ê·¸ë¦¬ê³ ì„¸êµ´ 깊이를 ê³ ë ¤í•˜ì—¬ ì‹ ë¢°ì„± 해석을 수행하였다. NREL 5 MW í’ë ¥í„°ë¹ˆ ì œì›ì„ 이용한 수치해석을 통하여, 지반물성치의 불확실성을 ê³ ë ¤í•œ ì‹ ë¢°ë„ 지수 분석 ê²°ê³¼ ëª¨ë ¸íŒŒì¼ 기초를 채택한 í•´ìƒí’ë ¥í„°ë¹ˆì˜ ì‹ ë¢°ë„ 지수가 세굴깊이가 증가함에 따라 크게 감소하는 것을 알 수 있었으며, 재킷 또는 트라이포드 기초를 채택한 경우 세굴깊이가 ì‹ ë¢°ë„ 지수에 미치는 영향이 크지 않음을 알 수 있었다. ê²°ë¡ ì ìœ¼ë¡œ 재킷 또는 트라이포드 기초를 채택한 í•´ìƒí’ë ¥í„°ë¹ˆì˜ 경우 지반-말뚝 상호작용을 ê³ ë ¤í•˜ì§€ 않아도 구조 ì‹ ë¢°ì„± 해석을 ìˆ˜í–‰í• ìˆ˜ 있으나, ëª¨ë ¸íŒŒì¼ì„ 채택한 경우, ì‹ ë¢°ì„± 해석 시 지반물성치 및 이에 포함되어 있는 불확실성의 ì •ë³´ê°€ ìƒëŒ€ì ìœ¼ë¡œ 중요함을 알 수 있다. 핵심용어: í•´ìƒí’ë ¥í„°ë¹ˆ, 지반-말뚝 상호작용, ì‹ ë¢°ì„± 해석, 응답면 기법, ëª¨ë ¸íŒŒì¼, 트라이포드, 재킷, 세굴깊이

The effect of the nonlinear velocity and history dependencies of the aerodynamic force on the dynamic response of a rotating wind turbine blade

Existing models for the analysis of offshore wind turbines account for the aerodynamic action on the turbine rotor in detail, requiring a high computational price. When considering the foundation of an offshore wind turbine, however, a reduced rotor model may be sufficient. To define such a model, the significance of the nonlinear velocity and history dependency of the aerodynamic force on a rotating blade should be known. Aerodynamic interaction renders the dynamics of a rotating blade in an ambient wind field nonlinear in terms of the dependency on the wind velocity relative to the structural motion. Moreover, the development in time of the aerodynamic force does not follow the flow velocity instantaneously, implying a history dependency. In addition, both the non-uniform blade geometry and the aerodynamic interaction couple the blade motions in and out of the rotational plane. Therefore, this study presents the Euler–Bernoulli formulation of a twisted rotating blade connected to a rigid hub, excited by either instantaneous or history-dependent aerodynamic forces. On this basis, the importance of the history dependency is determined. Moreover, to assess the nonlinear contributions, both models are linearized. The structural response is computed for a stand-still and a rotating blade, based on the NREL 5-MW turbine. To this end, the model is reduced on the basis of its first three free-vibration mode shapes. Blade tip response predictions, computed from turbulent excitation, correctly account for both modal and directional couplings, and the added damping resulting from the dependency of the aerodynamic force on the structural motion. Considering the deflection of the blade tip, the history-dependent and the instantaneous force models perform equally well, providing a basis for the potential use of the instantaneous model for the rotor reduction. The linearized instantaneous model provides similar results for the rotating blade, indicating its potential application for this scenario, and allowing for the definition of an added damping matrix, applicable for the dynamic analysis of rotating turbine blades.

An improved two-step soil-structure interaction modeling method for dynamical analyses of offshore wind turbines

The detailed modeling of soil-structure interaction is often neglected in simulation codes for offshore wind energy converters. This has several causes: On the one hand, soil models are in general sophisticated and have many degrees of freedom. On the other hand, for very stiff foundations the effect of soil-structure interaction could often be discounted. Therefore, very simple approaches are utilized or the whole structure is assumed to be clamped at the seabed. To improve the consideration of soil-structure interaction, a six-directional, coupled, linear approach is proposed, which contains an implementation of soil-structure interaction matrices in the system matrices of the whole substructure. The aero-hydro-servo-elastic simulation code FAST has been modified for this purpose. Subsequently, a 5MW offshore wind energy converter with pile foundation is regarded in two examples.

Multi-criteria decision analysis of offshore wind turbines support structures under stochastic inputs

Selection of the optimum support structural configuration for offshore wind turbines is a decision that should take into account a variety of both technical and non-technical criteria in order to aggregate cumulatively the performance of several concepts such as jackets, tripods, monopiles, spars, semi-subs, tri-floaters and others with varying suitability and relevance to offshore wind applications. In the preliminary phases of a project, consideration of qualitative attributes based on experts’ opinions is a common practice due to the limited availability of quantitative data for attribute scores and relevant weights, which are mainly based on past experience. This practice can introduce a bias in the considered scores reducing the confidence of the qualifying solution. This paper documents the extension of the widely used TOPSIS method (Technique for Order of Preference by Similarity to Ideal Solution) to explicitly consider stochastic inputs (statistical distributions) following a systematic gathering of data through surveys and questionnaires. The methodology proposed has been efficiently implemented in a numerical tool and the case study that is presented on the decision analysis of an offshore wind turbine supports structure under given deployment conditions, illustrates its applicability and allows for a sensitivity analysis of the effect of the resolution of simulation, selection of statistical distributions and weighting of experts’ opinions based on their perceived level of expertise.

Failure rate, repair time and unscheduled O&M cost analysis of offshore wind turbines

AbstractDetermining and understanding offshore wind turbine failure rates and resource requirement for repair are vital for modelling and reducing O&M costs and in turn reducing the cost of energy. While few offshore failure rates have been published in the past even less details on resource requirement for repair exist in the public domain. Based on ~350 offshore wind turbines throughout Europe this paper provides failure rates for the overall wind turbine and its sub‐assemblies. It also provides failure rates by year of operation, cost category and failure modes for the components/sub‐assemblies that are the highest contributor to the overall failure rate. Repair times, average repair costs and average number of technicians required for repair are also detailed in this paper. An onshore to offshore failure rate comparison is carried out for generators and converters based on this analysis and an analysis carried out in a past publication. The results of this paper will contribute to offshore wind O&M cost and resource modelling and aid in better decision making for O&M planners and managers. Copyright © 2015 John Wiley & Sons, Ltd.

Computer programming of free GUIs for the analysis of the behaviour of marine structures

This work presents the development of two free graphical user interfaces (GUIs), called FASTLognoter and MorisonForm, both focused on the analysis of the behaviour of offshore structures, especially of offshore wind turbines. The first one is related to the aeroelastic analysis of wind turbines, and the second is concerned with the seakeeping of marine structures. The development of these tools has been carried out using a powerful software called Lognoter. This tool is a free software for knowledge management in technology, which integrates computer programming for allowing the development of GUIs. These GUIs give an open platform for conducting a parametric study of the structural and dynamic behaviour of marine structures. Their coupling permits the user to set a suitable way to evaluate new concepts in marine structures. Finally, an application for the intensive analysis of offshore wind turbines is shown.

Seismic analysis of offshore wind turbines on bottom-fixed support structures.

This study investigates the seismic response of a horizontal axis wind turbine on two bottom-fixed support structures for transitional water depths (30-60 m), a tripod and a jacket, both resting on pile foundations. Fully coupled, nonlinear time-domain simulations on full system models are carried out under combined wind-wave-earthquake loadings, for different load cases, considering fixed and flexible foundation models. It is shown that earthquake loading may cause a significant increase of stress resultant demands, even for moderate peak ground accelerations, and that fully coupled nonlinear time-domain simulations on full system models are essential to capture relevant information on the moment demand in the rotor blades, which cannot be predicted by analyses on simplified models allowed by existing standards. A comparison with some typical design load cases substantiates the need for an accurate seismic assessment in sites at risk from earthquakes.

Seismic analysis of offshore wind turbine including fluid-structure-soil interaction

This paper presents the seismic response analysis of offshore wind turbines subjected to multi-support seismic excitation by using a three dimensional numerical finite element model considering viscous boundaries. The sea water-offshore wind turbine-soil interaction system is modeled by the Lagrangian (displacement-based) fluid and solid-quadrilateral-isoparametric finite elements. The research conducts a parametric study to estimate the effects of different foundation soil types on the seismic behavior of the offshore wind turbine coupled interaction system. The results obtained for different cases are compared with each other.

Relative Assessment of Fatigue Loads for Offshore Wind Turbine Support Structures

Time domain analyses of Offshore Wind Turbines involve a lot of computational effort. In this paper, we present a method for reducing this effort by using results from analyses with shorter simulation lengths to predict the results of the longer simulations lengths required by the standards. This involves using simple statistical treatment of the first 10 minutes of simulation data, and a subsequent linear regression, to predict the damage equivalent load of the full 60 minutes simulation. With some reservations about the general applicability of the method, the results are promising. Some suggestions for further investigations and developments of the presented methods are discussed.

Reliability analysis of offshore wind turbine support structures under extreme ocean environmental loads

Reliability analysis of jacket type offshore wind turbine (OWT) support structure under extreme ocean environmental loads was performed. Limit state function (LSF) of OWT support structure is defined by using structural dynamic response at mud-line. Then, the dynamic response is expressed as the static response multiplied by peak response factor (PRF). Probabilistic distribution of PRF is found from response time history under design significant wave load. Band limited beta distribution is used for internal friction angle of ground soil. Wind load is obtained in the form of thrust force from commercial code called Bladed and then, applied to tower hub as random load. In numerical example, response surface method (RSM) is used to express LSF of jacket type support structure for 5 MW OWT. Reliability index is found using first order reliability method (FORM).

Nonlinear wave and currents loads analysis of offshore wind turbines

In recent years, the offshore wind energy has increased in popularity due to the demand for renewable energy. Being built in a very dynamic environment, the nonlinear wave and current loads on offshore wind turbines are required to be investigated for design. The wave and current kinematics were calculated by the model that the wave propagates on a linear shear current which was developed by Dalrymple based on the stream function wave theory. The wave and current loads were calculated based on the Morison equation. For the engineering application purpose, some problems such that the wave and current kinematics models, the wave and current combination schemes in statistics, and the free surface’s influences on wave and current loads were discussed. At the end, the nonlinear results were compared with linear results to verify the calculation correctness.

Dynamic analysis of offshore wind turbine in clay considering soil–monopile–tower interaction

A comprehensive study is performed on the dynamic behavior of offshore wind turbine (OWT) structure supported on monopile foundation in clay. The system is modeled using a beam on nonlinear Winkler foundation model. Soil resistance is modeled using American Petroleum Institute based cyclic p–y and t–z curves. Dynamic analysis is carried out in time domain using finite element method considering wind and wave loads. Several parameters, such as soil–monopile–tower interaction, rotor and wave frequencies, wind and wave loading parameters, and length, diameter and thickness of monopile affecting the dynamic characteristics of OWT system and the responses are investigated. The study shows soil–monopile–tower interaction increases response of tower and monopile. Soil nonlinearity increases the system response at higher wind speed. Rotor frequency is found to have dominant role than blade passing frequency and wave frequency. Magnitude of wave load is important for design rather than resonance from wave frequency.

Seismic fragility analysis of 5 MW offshore wind turbine

Considering nonlinear soil–pile interaction, seismic fragility analysis of offshore wind turbine was performed. Interface between ground soils and piles were modeled as nonlinear spring elements. Ground excitation time histories were applied to spring boundaries. Two methods of applying ground motion were compared. Different time histories from free field analysis were applied to each boundary in the first loading plan (A). They were compared with the second loading plan (B) in which the same ground motion is applied to all boundaries. Critical displacement for wind turbine was proposed by using push-over analysis. Both the stress based and the displacement based fragility curves were obtained using dynamic responses for different peak ground accelerations (PGAs). In numerical example, it was shown that seismic responses from loading plan A are bigger than from plan B. It seems that the bigger ground motion at surface can cause less response at wind turbine due to phase difference between ground motions at various soil layers. Finally, it can be concluded that layer by layer ground motions from free field analysis should be used in seismic design of offshore wind turbine.

선박충돌에 의한 해상풍력발전기의 취약도 평가

Offshore wind turbines has to be proved against accidental events such as ship collision. In this study, ship collision fragility analysis of offshore wind turbine is done. Dynamic collision analysis is accomplished by considering soil foundation interaction and fluid structure interaction. Uncertainties due to ship weight and speed, angle are also considered. By analyzing dynamic response of offshore wind turbine, fragility curves are obtained for different damage levels. They can be used for restricting boat speed around the wind turbine and allowable size of the boat for inspection and for other purposes. Results of the fragility, it was confirmed fragility of collision speed of bulk ship of 30,000DWT and 850ton barge ship.

Stochastic seismic response analysis of offshore wind turbine including fluid‐structure‐soil interaction

SUMMARYThis paper presents the stochastic seismic response analysis of offshore wind turbines subjected to multi‐support seismic excitation by using a three‐dimensional numerical finite element model considering viscous boundaries. The seawater‐offshore wind turbine‐soil interaction system is modelled by the Lagrangian (displacement‐based) fluid and solid‐quadrilateral‐isoparametric finite elements. The random seismic excitation is described by the filtered white noise model and applied to each support point of the three‐dimensional finite element model of the coupled interaction system. The research conducts a parametric study to estimate the effects of variable seawater level, different foundation soil types and support site conditions on the stochastic behaviour of the offshore wind turbine coupled interaction system. The finite element model of coupled interaction system was also analyzed to examine the effect of the surrounding ice sheet on the stochastic response of the coupled system with and without ice sheet. The results obtained for different cases are compared with each other. Copyright © 2010 John Wiley & Sons, Ltd.

Structural Design and Analysis of Offshore Wind Turbines from a System Point of View

Offshore wind turbines are relatively complex structural and mechanical systems located in a highly demanding environment. In this study, the fundamental aspects and major issues related to the design of such structures are inquired. The system approach is proposed to carry out the design of the structural parts: in accordance with this philosophy, a decomposition of the system (environment, structure, actions/loads) and of the structural performance is carried out, in order to organize the qualitative and quantitative assessment in various sub-problems. These can be faced by sub-models of different complexity both for the structural behavior and for the load models. Numerical models are developed to assess the safety performance under aerodynamic and hydrodynamic actions. In the structural analyses, three types of turbine support structures have been considered and compared: a monopile, a tripod and a jacket.