Simulation Of Offshore Wind Turbines Research Articles

A novel python-based floating offshore wind turbine simulation framework

The expansion of floating offshore wind brings the industry closer to achieving commercial viability. However, the challenging environment characterised by strong winds, waves, and currents, along with the growing size of wind turbines and the dynamic behaviour of floaters, introduces concerns about power production efficiency and system durability due to increased fatigue loads, which subsequently impacts overall costs. In an attempt to mitigate the financial implications coming from alterations made to control strategies and structural elements during the initial design phase, this paper propounds an all-encompassing simulation framework for offshore wind turbines. The current study thoroughly explores the various capabilities of the tool, with a focus on its simulation models. Importantly, the paper highlights the complex interactions between tool models and different controllers. Carefully designed, this tool offers users a variety of functions to enhance system design, fine-tune control strategies, and thoroughly assess performance metrics. The paper elaborates on these aspects, providing an explanation of the tool's capabilities and enhancing the dynamic comparison between the models.

A new methodology for upscaling semi-submersible platforms for floating offshore wind turbines

Abstract. This paper presents a new upscaling methodology for semi-submersible floating offshore wind turbine platforms. The size and power rating of offshore wind turbines have been growing in recent years, with modern wind turbines rated at 10–18 MW in contrast with 2–5 MW in 2010. It is not apparent how much further wind turbine size can be increased before it is unjustified. Scaling relations are a useful method for analyzing wind turbine designs to understand the mass, load, and cost increases with size. Scaling relations currently do not exist but are needed for floating offshore platforms to understand how the technical and economic development of floating offshore wind energy may develop with increasing turbine size. In this paper, a hydrodynamic model has been developed to capture the key platform response in pitch. The hydrodynamic model is validated using OpenFAST, a high-fidelity offshore wind turbine simulation software. An upscaling methodology is then applied to two semi-submersible case studies of reference systems (the Offshore Code Comparison Collaboration Continuation (OC4) 5 MW and the International Energy Agency (IEA) 15 MW). For each case study, the platform pitch angle at rated wind turbine thrust is constrained to a specified value. The results show that platform dimensions scale to a factor of 0.75, and the platform steel mass scales to a factor of 1.5 when the wall thickness is kept constant. This study is the first to develop generalized upscaling relations that can be used for other triangular semi-submersible platforms that have three outer columns with the turbine mounted at the center of the system. This is in contrast with other studies that upscale a specific design to a larger power rating. This upscaling methodology provides new insight into trends for semi-submersible platform upscaling as turbine size increases.

Wind field reconstruction using nacelle based lidar measurements for floating wind turbines

In this work we investigate the influence of floater motions on nacelle based lidar wind speed measurements. The analysis is focused on wind field characteristics which are most relevant for performance analysis, namely rotor effective wind speed, shear and turbulence intensity. A numerical approach, coupling the publicly available in-house lidar simulation framework ViConDAR (Virtual Constrained turbulence and liDAR measurements) with an aeroelastic floating offshore wind turbine simulation is employed. Synthetic turbulent wind fields are generated with the open source turbulence generator TurbSim. The dynamics of the floating offshore wind turbine are simulated in the aeroelastic simulation code OpenFAST. Turbine dynamics and the corresponding synthetic wind field are then passed to the lidar simulation module of ViConDAR, which has been adapted for the consideration of turbine dynamics in all six degrees of freedom to simulate the lidar measurements under influence of motion. The simulation framework is demonstrated in a case study, simulating a lidar system on the nacelle of the IEA 15 MW turbine in combination with the WindCrete floater concept. Two different realistic lidar patterns are investigated under different metocean conditions. Different motion cases are examined individually and combined to evaluate the influence of rotational and translational degrees of freedom. Results show an increase of mean absolute error between lidar estimated and full wind field rotor effective wind speed of 0 to 25% depending on the environmental conditions. Observed overestimation of mean rotor effective wind speed was found to be in the region of up to 1%.

An integrated FEA-CFD simulation of offshore wind turbines with vibration control systems

The vibration mitigation of a monopile Offshore Wind Turbine (OWT) under wind and sea wave excitation is investigated, incorporating dynamic Vibration Control systems (VCS). The application of VCS to the OWTs has the potential to significantly improve the damping of the structure and its overall dynamic responses. A novel passive vibration absorption configuration is proposed, namely the Extended KDamper (EKD). Contrary to the conventional tuned mass dampers, EKD can increase its vibration absorption capability by introducing negative stiffness elements, instead of increasing the additional mass at the top of the towers. Therefore, EKD provides better isolation properties. The EKD optimum design and its realization are based on engineering criteria and realistic manufacturing limitations. The turbulence wind load time histories are determined stochastically while the mean velocity value is produced using the Blade Element Momentum theory. The influence of the sea wave excitation is studied using an integrated Computational Fluid Dynamics (CFD) approach. For the sea wave simulation, both fluids (water and air) are discretized using a non-uniform grid on which the Navier–Stokes equations are solved using the Double Control Volume Finite Element Method (DCVFEM). In order to render the large-scale physical problem computationally feasible, the Dynamic Adaptive Mesh Optimisation (DMO) and High Performance Computing (HPC) schemes are incorporated. The OWT tower is modelled using variable cross section beam elements accounting for geometrical nonlinearity (second order phenomena). The monopile soil–structure interaction (SSI) is modelled using a prismatic beam on elastic foundation together with the corresponding springs and dashpots along the pile’s length. An extensive case study is carried out on a monopile OWT providing insight to the structural dynamics and illustrating the viability of the VCS on the offshore wind industry. It is shown that vibration control can extend the lifetime of the structure, increasing the OWTs’ reliability and sustainability.

Comparative study of load simulation approaches used for the dynamic analysis on an offshore wind turbine jacket with different modeling techniques

In this paper, the influences of different load simulation approaches and jacket models on the loading and the structural responses of offshore wind turbine (OWT) jackets are investigated. In the first part, the effect of three load simulation approaches used for the dynamic analysis in OWT simulation is compared under different wind and wave conditions. One is the uncoupled method, the second is the sequentially coupled approach and the final one is the fully integrated (coupled) approach. The fully integrated method is with strong coupling and is considered as the reference. The comparison with the other two methods is conducted based on an OWT jacket, which is modeled with pure beam elements. For the comparison of load simulation approaches, the results show that the simulation results of the sequentially coupled approach are mostly in good agreement with those of the fully coupled approach. The uncoupled approach may lead to big errors in the extreme responses of dynamic analysis. Based on this numerical investigation, the sequentially coupled approach is chosen for the following study. In the second part, the influences of the different jacket models on the structural responses are also studied, because the OWT jackets are often modeling with beam elements, which will over/underestimate the responses of the jackets. Two numerical models of the jacket are developed. One is with pure beam elements (Beam model), while the second is with more advanced modeling by using super-elements (Super-element model). The sequentially coupled load simulation approach is applied for both the two jacket models. For the comparison between the two jacket models, the responses of the super-element model are different from those of the beam model, especially in the jacket displacement. Hence, from this study, the fully and sequentially coupled approaches are recommended for offshore load calculations and the super-element model is recommended for better modeling the jacket behaviors.

Research on influence of ice-induced vibration on offshore wind turbines

This paper presents the simulation of offshore wind turbines (OWTs) suffering from turbulent wind and ice-induced vibrations (IIVs). To ascertain the effectiveness of IIVs, the fully coupled simulation of the NREL 5 MW OWT is implemented. The OWT model, which is processed as a multibody system, takes the aerodynamic load and the IIV simultaneously. Firstly, the Kaimal spectrum is used to simulate the turbulent wind conditions. Then, the aerodynamic load acting on the blades is solved by the blade momentum theory. The IIV can be explained as the forced vibration or the self-excited vibration. The Matlock model and the Määttänen model are employed here to solve the forced ice-induced vibration and the self-excited ice-induced vibration, respectively. Finally, the kinetics and the kinematic computation are coupled with aerodynamic load calculations. The dynamic responses of some crucial parts of the OWT model reveal some important results. The results prove the great effectiveness of IIVs impacting the OWTs, especially for the tower. The frequency-domain responses note that the IIV affects the particular frequencies remarkably.

The artificial generation of the equilibrium marine atmospheric boundary layer for the CFD simulation of offshore wind turbines

Computational fluid dynamics (CFD) technique has been widely used in simulating dynamic responses of offshore floating wind turbines under excitations of prescribed wave and wind loads. In the simulation, the Volume of Fluid (VOF) method is generally employed to capture the free surface of the liquid, which illustratively shows the evolution of waves. The stresses at the free surface are not explicitly modeled in the VOF method, which challenge the establishment of the equilibrium marine atmospheric boundary layer (MABL) throughout the computational domain. Given the importance of the generation of the MABL in a CFD simulation of offshore wind turbines, methods for establishing sustainable wind profiles in a simulation based on the VOF method should be systematically investigated. In the present study, a virtual body force method is developed to maintain the horizontally homogeneous MABL in the VOF-based CFD simulation. The proposed method is quantitatively verified through a simulation of the equilibrium MABL with the sheared wind profile.

Fault Diagnostics for Electrically Operated Pitch Systems in Offshore Wind Turbines

This paper investigates the electrically operated pitch systems of offshore wind turbines for online condition monitoring and health assessment. The current signature based fault diagnostics is developed for electrically operated pitch systems using model-based approach. The electrical motor faults are firstly modelled based on modified winding function theory and then, current signature analysis is performed to detect the faults. Further, in order to verify the fault diagnostics capabilities in realistic conditions, the operating profiles are obtained from FAST simulation of offshore wind turbines in various wind conditions. In this way, the applicability of current signature analysis for fault diagnostics in offshore wind turbine pitch systems is demonstrated.

Offshore wind turbine simulation: Multibody drive train. Back-to-back NPC (neutral point clamped) converters. Fractional-order control

This paper is on offshore wind energy conversion systems installed on the deep water and equipped with back-to-back neutral point clamped full-power converter, permanent magnet synchronous generator with an AC link. The model for the drive train is a five-mass model which incorporates the dynamic of the structure and the tower in order to emulate the effect of the moving surface. A three-level converter and a four-level converter are the two options with a fractional-order control strategy considered to equip the conversion system. Simulation studies are carried out to assess the quality of the energy injected into the electric grid. Finally, conclusions are presented.

Verification of aero‐elastic offshore wind turbine design codes under IEA Wind Task XXIII

ABSTRACTThis work presents the results of a benchmark study on aero‐servo‐hydro‐elastic codes for offshore wind turbine dynamic simulation. The codes verified herein account for the coupled dynamic systems including the wind inflow, aerodynamics, elasticity and controls of the turbine, along with the incident waves, sea current, hydrodynamics and foundation dynamics of the support structure.A large set of time series simulation results such as turbine operational characteristics, external conditions, and load and displacement outputs was compared and interpreted. Load cases were defined and run with increasing complexity to trace back differences in simulation results to the underlying error sources. This led to a deeper understanding of the underlying physical systems. In four subsequent phases—dealing with a 5‐MW turbine on a monopile with a fixed foundation, a monopile with a flexible foundation, a tripod and a floating spar buoy—the latest support structure developments in the offshore wind energy industry are covered, and an adaptation of the codes to those developments was initiated.The comparisons, in general, agreed quite well. Differences existed among the predictions were traced back to differences in the model fidelity, aerodynamic implementation, hydrodynamic load discretization and numerical difficulties within the codes. The comparisons resulted in a more thorough understanding of the modeling techniques and better knowledge of when various approximations are not valid. More importantly, the lessons learned from this exercise have been used to further develop and improve the codes of the participants and increase the confidence in the codes’ accuracy and the correctness of the results, hence improving the standard of offshore wind turbine modeling and simulation.One purpose of this paper is to summarize the lessons learned and present results that code developers can compare to. The set of benchmark load cases defined and simulated during the course of this project—the raw data for this paper—is available to the offshore wind turbine simulation community and is already being used for testing newly developed software tools. Despite that no measurements are included, the large number of participants and the—in general—very fine level of agreement indicate high trustworthy results within the physical assumptions of the codes and the simulation cases chosen. Other cases, such as large prebend flexible blades, large wind shear, large yaw error or transient maneuvers, may not show the same level of agreement. These cases were deliberately left out because the focus is on the specific offshore application. Further on, this benchmark study includes participating codes and organizations by name (contrary to several previous benchmark studies) that gives the reader a chance to find results from one particular code of interest.Copyright © 2013 John Wiley & Sons, Ltd.

Towards the Fully-coupled Numerical Modelling of Floating Wind Turbines

The aim of this study is to model the interactions between fluids and solids using fully nonlinear models. Non- linearity is important in the context of floating wind turbines, for example, to model breaking waves impacting on the structure and the effect of the solid's elasticity. The fluid- and solid-dynamics equations are solved using two unstructured finite-element models, which are coupled at every time step. Importantly, the coupling ensures that the action-reaction principle is satisfied at a discrete level, independently of the order of representation of the discrete fields. To the authors’ knowledge, the present algorithm is novel in that it can simultaneously handle: (i) non- matching fluid and solid meshes, (ii) different polynomial orders of the basis functions on each mesh, and (iii) different fluid and solid time steps. First, results are shown for the flow past a fixed actuator-disk immersed in a uniform flow and representing a wind turbine. The present numerical results for the velocity deficit induced by the disk are shown to be in good agreement with the semi-analytical solution, for three values of thrust coefficients. The presence of a non-zero fluid viscosity in the numerical simulation affects wake recovery and fluid entrainment around the disk. Second, the dynamic response of a cylindrical pile is computed when placed at an interface between air and water. The results qualitatively demonstrate that the present models are applicable to the modelling of multiple fluids interacting with a floating solid. This work provides a first-step towards the fully coupled simulation of offshore wind turbines supported by a floating spar.

Advanced Representation of Tubular Joints in Jacket Models for Offshore Wind Turbine Simulation

Jacket substructures for offshore wind turbines show strong potentials in water depths from 25 up to 70m. A review of state-of-practice and enhanced state-of-the-art modeling of offshore wind turbine jackets is conducted regarding detailed joint properties. The state-of-the-art approach takes advantage of an accurate description of the local joint behavior by use of superelements, enabling more accurate load simulations. Studies conducted in the past highlighted both strong benefits as well as shortcomings of this approach, whereas the drawbacks were mainly related to the size of superelements and the application of local wave loading. This work develops a smart sizing for detailed joint models taking into account the loading and location of the jacket joints. A concept of local wave loading is presented as well. Advice on recommendable parameters is given and enables an optimized superelement application for jacket substructures. As an example the potential for fatigue load reduction is shown using the NREL offshore 5-MW baseline wind turbine and the OC4 reference jacket. The predicted fatigue lifetime was increased by about 15%.

타워와 블레이드의 탄성효과를 고려한 부유식 해상풍력발전기의 동적거동해석

부유식 해상풍력발전기의 시뮬레이션을 위해서 본 연구에서는 2㎿ 육상 풍력발전기에 부유구조물인 Tension Leg Platform(TLP) 구조를 추가하였다. 기상청 관측데이터와 해수면으로부터의 높이에 대해 풍속을 정의하는 지수법칙을 이용하여 풍하중을 산출하고 블레이드와 타워에 일정한 높이간격으로 적용하였다. 상대모리슨 방정식을 이용하여 파랑하중을 모델링하였다. 블레이드의 회전속도를 정격속도인 18rpm 으로 고려하고, 풍하중과 파랑하중 작용 시 2㎿ 의 부유식 해상풍력기의 동적거동 해석을 수행하였다. 파랑하중에 대한 해상풍력기의 공진특성을 조사하기 위해 타워와 블레이드의 탄성체 모델을 구성하여 해상풍력기의 고유진동수를 계산하였다. 타워와 블레이드의 탄성효과가 해상풍력기의 거동에 미치는 영향을 분석하기 위해 타워만 탄성체로 구성된 탄성타워모델과 타워와 블레이드가 탄성체로 고려된 탄성타워 블레이드모델을 각각 강체 모델과 비교하였다.

OC3—Benchmark Exercise of Aero-elastic Offshore Wind Turbine Codes

This paper introduces the work content and status of the first international investigation and verification of aero-elastic codes for offshore wind turbines as performed by the "Offshore Code Comparison Collaboration"(OC3) within the "IEA Wind Annex XXIII - Subtask 2". An overview is given on the state-of-the-art of the concerned offshore wind turbine simulation codes. Exemplary results of benchmark simulations from the first phase of the project are presented and discussed while subsequent phases are introduced. Furthermore, the paper discusses areas where differences between the codes have been identified and the sources of those differences, such as the differing theories implemented into the individual codes. Finally, further research and code development needs are presented based on the latest findings from the current state of the project.