Madjid Karimirad

Wave- and Wind-Induced Dynamic Response of a Spar-Type Offshore Wind Turbine

This paper addresses coupled wave and wind-induced motions of spar-type 5-MW wind turbines in harsh and operational environmental conditions. Global dynamic motion responses have been analyzed by aero-hydro-servo-elastic time-domain simulations. The aerodynamics is based on an advanced blade element momentum theory. Panel method and Morison formula accounting for the instantaneous position of the structure are applied for hydrodynamics. Hydrodynamic drag and considering geometrical updating introduce nonlinearities. Hydrodynamic nonlinearities were found to cause excitation of the natural frequencies in the low frequency range more than in the wave frequency range. Extrapolation methods are applied to estimate the maximum responses. A previous study showed that the uncertainty of such an extrapolation for the present concept is less than 2%. In this study it is found that the mean values of the dynamic responses are primarily wind induced and the standard deviations of the responses are primarily wave ind...

Extreme Dynamic Structural Response Analysis of Catenary Moored Spar Wind Turbine in Harsh Environmental Conditions

Floating wind turbines can be the most practical and economical way to extract the vast offshore wind energy resources at deep and intermediate water depths. The Norwegian Ministry of Petroleum and Energy is strongly committed to developing offshore wind technology that utilises available renewable energy sources. As the wind is steadier and stronger over the sea than over land, the wind industry recently moved to offshore areas. Analysis of the structural dynamic response of offshore wind turbines subjected to stochastic wave and wind loads is an important aspect of the assessment of their potential for power production and of their structural integrity. Of the concepts that have been proposed for floating wind turbines, spar-types such as the catenary moored spar (CMS) and tension leg spar (TLS) wind turbines seem to be well-suited to the harsh environmental conditions that exist in the North Sea. Hywind and Sway are two examples of such Norwegian concepts; they are based on the CMS and TLS, respectively. Floating wind turbines are sophisticated structures that are subjected to simultaneous wind and wave actions. The coupled nonlinear structural dynamics and motion response equations of these turbines introduce geometrical nonlinearities through the relative motions and velocities. Moreover, the hydrodynamic and aerodynamic loading of this type of structure is nonlinear. A floating wind turbine is a multibody aero-hydro-servo-elastic structural system; for such structures, the coupled nonlinear equations of motion considering nonlinear excitation and damping forces, including all wave- and wind-induced features, should be solved in the time domain. In this thesis, the motion and structural responses for operational and extreme environmental conditions were considered to investigate the performance and the structural integrity of spar-type floating wind turbines. The power production and the effects of aerodynamic and hydrodynamic damping, including wind-induced hydrodynamic and wave-induced aerodynamic damping, were investigated. Negative damping adversely affects the power performance and structural integrity. In this thesis, the controller gains were tuned to remove servo-induced instabilities. The rotor configuration effect on the responses and power production was investigated by comparing the upwind and downwind turbines. To develop robust design tools for offshore wind power, the competencies of the offshore technology and wind technology must be combined. Both the offshore and wind energy industries have begun to extend their existing numerical codes to account for the combined aerodynamic and hydrodynamic effects on the structure. As a result verifications of extended codes by doing experiments and code-to-code comparisons are needed. One of the aspects of the present research was to fill this gap by performing hydrodynamic and hydro-elastic comparison between commercial codes. For both CMS and TLS concepts, the comparisons were carried out prior to using the tools to study the behaviour of the CMS and TLS under wave- and wind-induced loads. Offshore structures encounter a variety of operational and harsh environmental conditions. Limit states such as ultimate, fatigue, accidental collapse and serviceability limit states (ULS, FLS, ALS and SLS) are defined as the design criteria for offshore structures. In performing realistic ultimate limit state analysis, the extreme responses of a floating wind turbine over its life should be estimated. This estimation requires detailed analysis of the extreme response. In the present thesis, extreme value analysis for spar-type wind turbines subjected to simultaneous wave and wind actions was preformed. The structural responses and the effect of modelled forces such as turbulence on these responses were investigated. The joint distribution of the environmental characteristics of the wave and wind was applied through the contour surface method. Stochastic wave and wind analysis showed that, while rigid body modelling was sufficient for obtaining accurate motions, consideration of the elastic behaviour of the tower/support structure was necessary to predict structural responses. The blades structural responses were found to be significantly affected by the turbulent wind. However, the mean and standard deviation of global motion and structural responses were not affected by the turbulence. Thus, to reduce the simulation time in fatigue analysis, a constant wind speed model can be applied. The CMS and TLS wind turbines are inertia-dominated structures, and the hydrodynamic viscous drag did not affect their wave-induced responses, while an increase in viscous drag could effectively reduce the resonant responses of such turbines. Under operational conditions, aerodynamic damping was found to be active in reducing both wave frequency and resonant responses. The results showed that, for a floating wind turbine, extreme response could occur in survival conditions, while for a fixed wind turbine, the extreme response occurs in operational cases related to the rated wind speed. To estimate the extreme value responses, extrapolation methods were used to reduce the sample size in Monte Carlo simulations. The accuracy of methods to estimate the extreme responses as a function of sample size and methods applied was investigated. The normalized responses for both CMS and TLS offshore wind turbines were presented to draw more generalized conclusions.

STC (Spar-Torus Combination): A Combined Spar-Type Floating Wind Turbine and Large Point Absorber Floating Wave Energy Converter — Promising and Challenging

This paper deals with a novel concept by combining a spar-type floating wind turbine (FWT) and a Torus (donutshaped) point absorber-type wave energy converter (WEC) that is referred as the ‘Spar-Torus Combination’ (STC) herein. Concept feasibility study has been carried out by doing numerical simulations. It showed that the STC results in a positive synergy between wind and wave energy generation in terms of both capital investment and power production. As a novel concept, the STC concept is considered a simple compact combination of two technologies that have had high technology readiness level (TRL). It is suitable for deep water deployment and is not sensitive to seabed conditions and wave directions. Therefore, it is interesting to pursue a further development of this concept. The paper presents the technical information about the STC and highlights some challenging areas of the STC that should be carefully looked at to make it a proven concept.Copyright

V-shaped semisubmersible offshore wind turbine subjected to misaligned wave and wind

The dynamic behavior of the V-shaped semisubmersible offshore wind turbine subjected to misaligned wave and wind loads in operational conditions is presented in this paper. During the life time of an offshore wind turbine,wave and wind can be misaligned which may affect the dynamic response and as a result the functionality of the floating wind turbine. Especially for asymmetric floating structures such as the V-shaped semisubmersible, the misalignment of the wave and wind may result in unexpected behavior. In the present study, integrated aero-hydro-servo-elastic analysis for coupled mooring-floater-turbine is carried out in order to investigate possible effects under misaligned wave and wind conditions. For misaligned wave and wind conditions, the wave-induced as well as the wave-wind-induced motions, tension of mooring lines, and functionality of the turbine such as power production, rotational speed, and controller actions like blade-pitch-angle are studied and presented. The results show that the V-shaped semisubmersible offshore wind turbine is not affected in an undesirable way by the misaligned wave and wind loads in operational conditions and can be considered as enough robust in such environmental conditions. Also, the functionality and power production of the current concept is not affected by the misalignment of the wave and wind. The wave-induced responses of the V-shaped floating wind turbine are relatively small compared to wave-wind-induced responses. The dynamic responses of the V-shaped semisubmersible offshore wind turbine in coupled wave-wind-induced analyses are mainly dominated by the wind loads effects.

Comparative Study of Spar-Type Wind Turbines in Deep and Moderate Water Depths

This paper compares the dynamic responses and performance of two spar-type wind turbines, DeepSpar and ShortSpar, in deep and intermediate water depths, respectively. The oil and gas industry has implemented spar platforms in deep water areas. Spar platforms show good hydrodynamic performance due to their deep draft. The same idea is applied to offshore wind turbines to present a reliable concept. Hywind is an example of a successful offshore wind turbine based on the spar concept in deep water. The good performance of spar-type wind turbines motivates us to study the feasibility of using these turbines in moderate water depth. Spar-type 5-MW wind turbines in deep and moderate water depths are compared. The power performance, dynamic motions, tension responses, accelerations, structural shear forces and bending moments are studied. Simo/Riflex/TDHMILL3D is used to perform the coupled wave- and wind-induced analyses. Simo/Riflex, developed by MARINTEK, is a commercial tool for analyzing the coupled wave-induced responses of moored offshore structures. TDHMILL3D, is an external DLL that accounts for spar motions while calculating the aerodynamic thrust at each time step using the turbine characteristics and relative velocities. Different environmental conditions are used to compare the responses. The results show that spar-type wind turbine in the moderate water depth exhibits good performance and that its responses are reasonable compared to those of spar-type wind turbine in deep water. This finding indicates the feasibility of implementing the same rotor-nacelle assembly for both concepts. The total mass (the structural mass plus the ballast) of the ShortSpar is 35% less than that of the DeepSpar, while the statistical characteristics of the power generated are almost the same. The reduced mass of the ShortSpar helps to achieve a more cost-effective solution for floating wind turbines in moderate water depth.Copyright

Extreme Structural Dynamic Response of a Spar Type Wind Turbine

Proper performance of structures requires among other things that its failure probability is sufficiently small. This would imply design for survival in extreme conditions. The failure of a system can occur when the ultimate strength is exceeded (Ultimate Limit State) or fatigue limit (Fatigue Limit State) is passed. The focus in this paper is on the determination of extreme responses for ULS design checks. The present paper deals with coupled wave and wind induced motion and structural response in harsh condition up to 14.4 (m) significant wave height and 49 (m/sec) 10-min average wind speed (at top of tower, 90 m) for a parked floating wind turbine. In survival condition the wind induced resonant responses (mainly platform pitch resonance) are dominant. Due to platform resonant motion responses, the structural responses are close to Gaussian. The dynamic structural responses show that the process is wide banded. The critical structural responses are determined by coupled aero-hydro-elastic time domain simulation. Based on different simulations (20 1-hour, 20 2-hours, 20 3-hours and 20 5-hours) the mean up-crossing rate has been found in order to predict the extreme structural responses. The most probable maximum of the bending moment and the bending moment having up-crossing rate of 10−4 are found to be close in the present research. The minimum total simulation time in order to get accurate results is highly correlated to the needed up-crossing rate. The 1-hour and 2-hours original values cannot provide any information for 10−4 up-crossing rate. Comparison of different simulation periods shows that the 20 1-hour simulations can be used in order to investigate the 3-hours extreme bending moment if the proper extrapolation of up-crossing rate used.Copyright

Effect of Shut-Down Procedures on the Dynamic Responses of a Spar-Type Floating Wind Turbine

The design standards (IEC, DNV and GL) define a minimum set of combinations of external conditions and design situations as load cases. Like other design load conditions, the design situations relating to fault and shut-down events shall be addressed. Emergency shut down occurs in the presence of severe faults to prevent turbine damage. For pitch-regulated turbines, blade pitching to feather provides an effective means of aerodynamic braking. The blades are pitched to feather at the maximum pitch rate. This action exerts huge loading on the turbine and may challenge the structural safety. In this paper a 5-MW spar-type wind turbine is used as a case study. By using the HAWC2 code, the turbine pitch actuator fault and shut-down scenarios are simulated through external Dynamic Link Libraries. The shut-down scenarios are: normal shut down with blade pitching, emergency shut down with blade pitching, and emergency shut down with blade pitching and mechanical brake. Due to the occurrence of fault, the pitch angle of one blade is fixed from a specific occurrence time. The supervisory controller reacts by pitching the remaining two blades to the maximum pitch set. The maximum yaw motion value is observed after the first revolution of the rotor during which the tower-top torsion experiences a change of direction. Negative platform pitch motion as well as tower-bottom bending moment are induced due to the pitching activity of the two blades. The response extremes of the main shaft bending moment and the yaw motion exhibit clear variation with the blade azimuth when emergency shut down is initiated. The tower-bottom bending moment and nacelle acceleration are relatively more affected by the wave loads. For a given blade azimuth, larger response variation is observed under harsher environmental conditions. Under the fault scenario, the effects of different shut-down procedures on the response extremes are investigated. It is found that the response extremes are affected significantly by the rotor speed. Among the three procedures, normal shut down, which is associated with the slowest decaying aerodynamic excitations and the highest rotor speed, usually leads to the largest response extremes near the rated wind speed. The employment of mechanical brake reduces rotor speed, motion responses and structural responses effectively. During shut down, the responses of yaw motion, nacelle fore-aft acceleration, main shaft bending moment, and tower-bottom side-to-side moment may be of concern for the floating wind turbine studied.Copyright

Experimental study on gyroscopic effect of rotating rotor and wind heading angle on floating wind turbine responses

Limited fossil resources, daily increasing rate of demand for energy and the environmental pollution fact have made people revert to renewable sources of energy as a solution. One type of renewable energy is offshore wind energy which has high potential without any sound and visual noises. Recently, a lot of researchers have carried out on the issue of offshore wind turbine. Because of incapability of most of software programs to simulate gyroscopic effect of rotating rotors, in this articles a significant effort has been made to fabricate and test an offshore wind turbine under different rotor rotation velocities and different heading angle of wind so as to obtain the effects of these parameters on structure responses. Study on the response of a wind turbine under environmental loads has had a notable importance due to the fact that structure behavior can strongly affect procedure of modeling and optimizing wind turbine structures. On the other hand, frequency-domain structural response of a wind turbine can also make engineers be informed about of appropriate mooring system for a special environmental condition. Consequently, it has been observed that increasing the rotor rotation velocity leads the peak of spectrums shift to a higher frequency due to the gyroscopic effect appeared as a damping term, and changing heading angle of wind may lead to a change in heave and pitch amplitudes in the time domain response, and heave, sway and surge motion in frequency response.

Effect of the Beam Element Geometric Formulation on the Wind Turbine Performance and Structural Dynamics

In this paper, the original double symmetric cross section beam formulation in RIFLEX used to model the blades is compared against a newly implemented generalised beam formulation, allowing for eccentric mass, shear and elastic centres. The generalised beam formulation is first evaluated against an equivalent ABAQUS beam model (Using the generalised beam formulation implemented in ABAQUS) which consists of DTU 10MW RWT (reference wind turbine) blade in static conditions. A static displacement is applied to the tip, which is close to an operating load. The results appear very similar and ensure that the implementation is correct. The extended beam formulation is afterwards used on the Landbased 10MW turbine from DTU with external controller. This case study aims at evaluating the effect of the newly implemented formulation on realistic, flexible structure. During the study, the blades were discretised using both the old and new formulation, and dynamic simulations were performed. The effect of the beam formulation was evaluated using several wind conditions that are thought to be characteristic of operating conditions. Results show slight difference between two formulations but could be more significant for next generation flexible blades. -------------------------------------------------------------------------* corresponding author. virgile.delhaye@sintef.no **Current affiliation : Queens University Belfast (QUB), United Kingdom (UK) 1 Earlier MARINTEK, SINTEF Ocean from 1st January 2017 through a merger internally in the SINTEF Group INTRODUCTION There is a need to improve the structural predictions of the blade in aerodynamic codes without compromising with codes efficiency. This comes from the increased complexity and size of the new blade designs. In particular, improving the aerodynamic performances of the blade by tailoring the fibre reinforced plastic layers is an important research topic (Kooijman (1996) [1]). In this context, the use of advanced beam model able to better predict blade dynamics and give a more realistic description of the load transfer into the wind turbine becomes crucial for industry. A significant contribution in this field comes from helicopter technology and was developed by Hodges et al. (1999) [2], Yu et al. (2002) [3]. The idea is to reduce a three-dimensional anisotropic elastic problem to a twodimensional cross section analysis and to a one-dimensional beam analysis (e.g. the variational asymptotic methodology by Berdichevsky (1979) [4]). These methodologies have been used to successfully describe the structural behaviour of single blade submitted to static and dynamic loadings (Otero (2010) [5]). The inclusion of such formulation in multibody codes, which are able to handle a whole wind turbine, is less common. An anisotropic beam formulation was recently implemented in the multibody aeroelastic code HAWC2 (Branner et al., 2012 [6], Kim et al., 2013 [7]), in order to simulate full wind turbine structure and to capture the bend-twist effect arising in the nextgen blades as observed by Lobitz et al. (2000) [8] (2003) [9]. In the present paper, a similar step is presented in RIFLEX with the implementation of a generalised cross-section formulation to discretize the blades. Virgile Delhaye* SINTEF Ocean1 Trondheim, Norway Madjid Karimirad** SINTEF Ocean1 Trondheim, Norway Petter Andreas Berthelsen SINTEF Ocean1 Trondheim, Norway Proceedings of the ASME 2017 36th International Conference on Ocean, Offshore and Arctic Engineering OMAE2017 June 25-30, 2017, Trondheim, Norway

Wave Energy Converters

Wave power presents as the movements of water particles close to the ocean surface. The energy intensity depends on the height and frequency of waves. A large amount of wave power in random sea motivates us to think of using ocean wave energy for generating electricity. We use wave energy converters (WECs) to change potential kinematical energy of sea waves to electrical energy. Waves in the ocean have a vast amount of renewable energy. This makes the ocean a renewable source of power in order of terawatts (TW). The global power resource represented by waves that hit all coasts worldwide, has been estimated to be in the order of 1 TW.