Long term effect of operating loads on large monopile-supported offshore wind turbines in sand

The long-term probability distributions of a spar-type and a semisubmersible-type offshore floating wind turbine response are calculated for surge, heave, and pitch motions along with the side-to-side, fore–aft, and yaw tower base bending moments. The transfer functions for surge, heave, and pitch motions for both spar-type and semisubmersible-type floaters are obtained using the fast code and the results are also compared with the results obtained in an experimental study. The long-term predictions of the most probable maximum values of motion amplitudes are used for design purposes, so as to guarantee the safety of the floating wind turbines against overturning in high waves and wind speed. The long-term distribution is carried out using North Atlantic wave data and the short-term floating wind turbine responses are represented using Rayleigh distributions. The transfer functions are used in the procedure to calculate the variances of the short-term responses. The results obtained for both spar-type and semisubmersible-type offshore floating wind turbine are compared, and the study will be helpful in the assessments of the long-term availability and economic performance of the spar-type and semisubmersible-type offshore floating wind turbine.

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On the Modeling of Nonlinear Waves for Prediction of Long-Term Offshore Wind Turbine Loads

Volume 6: Nick Newman Symposium on Marine Hydrodynamics; Yoshida and Maeda Special Symposium on Ocean Space Utilization; Special Symposium on Offshore Renewable Energy ◽

10.1115/omae2008-57855 ◽

2008 ◽

Author(s):

P. Agarwal ◽

L. Manuel

Keyword(s):

Wind Turbine ◽

Wind Turbines ◽

Return Period ◽

Wave Model ◽

Offshore Wind ◽

Offshore Wind Turbine ◽

Offshore Wind Turbines ◽

Irregular Wave ◽

Inflow Turbulence

In the design of wind turbines—onshore or offshore—the prediction of extreme loads associated with a target return period requires statistical extrapolation from available loads data. The data required for such extrapolation are obtained by stochastic time-domain simulation of the inflow turbulence, the incident waves, and the turbine response. Prediction of accurate loads depends on assumptions made in the simulation models employed. While for the wind, inflow turbulence models are relatively well established, for wave input, the current practice is to model irregular (random) waves using a linear wave theory. Such a wave model does not adequately represent waves in shallow waters where most offshore wind turbines are being sited. As an alternative to this less realistic wave model, the present study investigates the use of irregular nonlinear (second-order) waves for estimating loads on an offshore wind turbine, with a focus on the fore-aft tower bending moment at the mudline. We use a 5MW utility-scale wind turbine model for the simulations. Using, first, simpler linear irregular wave modeling assumptions, we establish long-term loads and identify governing environmental conditions (i.e., the wind speed and wave height) that are associated with the 20-year return period load derived using the inverse first-order reliability method. We present the nonlinear irregular wave model next and incorporate it into an integrated wind-wave-response simulation analysis program for offshore wind turbines. We compute turbine loads for the governing environmental conditions identified with the linear model and also for an extreme environmental state. We show that computed loads are generally larger with the nonlinear wave modeling assumptions; this establishes the importance of using such refined nonlinear wave models in stochastic simulation of the response of offshore wind turbines.

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Numerical Prototyping of Floating Offshore Wind Turbines: Virtual Operation in Real Environmental Conditions

Volume 9: Ocean Renewable Energy ◽

10.1115/omae2020-18174 ◽

2020 ◽

Author(s):

Daniel Milano ◽

Christophe Peyrard ◽

Matteo Capaldo

Keyword(s):

Wind Turbines ◽

Wind Wave ◽

Fatigue Analysis ◽

Offshore Wind ◽

Offshore Wind Turbine ◽

Wave Direction ◽

Offshore Wind Turbines ◽

Floating Offshore Wind Turbines ◽

Wave Conditions

Abstract The numerical fatigue analysis of floating offshore wind turbines (FOWTs) must account for the environmental loading over a typical design life of 25 years, and the stochastic nature of wind and waves is represented by design load cases (DLCs). In this statistical approach, combinations of wind speeds and directions are associated with different sea states, commonly defined via simplified wave spectra (Pierson-Moskowitz, JONSWAP), and their probability of occurrence is identified based on past observations. However, little is known about the difference between discretizing the wind/wave direction bins into (e.g.) 10deg bins rather than 30deg bins, and the impact it has on FOWT analyses. In addition, there is an interest in identifying the parameters that best represent real sea states (significant wave height, peak period) and wind fields (profile, turbulence) in lumped load cases. In this context, the aim of this work is to better understand the uncertainties associated to wind/wave direction bin size and to the use of metocean parameters as opposed to real wind and sea state conditions. A computational model was developed in order to couple offshore wind turbine models with realistic numerical metocean models, referred to as numerical prototype due to the highly realistic wind/wave conditions in which it operates. This method allows the virtual installation of FOWTs anywhere within a considered spatial domain (e.g. the Mediterranean Sea or the North Sea) and their behaviour to be evaluated in measured wind and modelled wave conditions. The work presented in this paper compares the long-term dynamic behaviour of a tension-leg platform (TLP) FOWT design subject to the numerical prototype and to lumped load cases with different direction bin sizes. Different approaches to representing the wind filed are also investigated, and the modelling choices that have the greatest impact on the fidelity of lumped load cases are identified. The fatigue analysis suggests that 30deg direction bins are sufficient to reliably represent long-term wind/wave conditions, while the use of a constant surface roughness length (as suggested by the IEC standards) seems to significantly overestimate the cumulated damage on the tower of the FOWT.

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Experimental and Numerical Studies on the Dynamic and Long-Term Behavior of Offshore Wind Turbines in Clay

Geotechnical Testing Journal ◽

10.1520/gtj20170043 ◽

2018 ◽

Vol 41 (2) ◽

pp. 20170043 ◽

Cited By ~ 1

Author(s):

Swagata Bisoi ◽

Sumanta Haldar

Keyword(s):

Wind Turbines ◽

Offshore Wind ◽

Offshore Wind Turbines ◽

Numerical Studies ◽

Long Term Behavior

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Analysis of the Fatigue Damage on the Offshore Wind Turbines Exposed to Wind and Wave Loads Within the Typhoon Area

23rd International Conference on Offshore Mechanics and Arctic Engineering, Volume 1, Parts A and B ◽

10.1115/omae2004-51347 ◽

2004 ◽

Cited By ~ 1

Author(s):

Atsushi Yamashita ◽

Kinji Sekita

Keyword(s):

Fatigue Damage ◽

Wind Turbines ◽

Offshore Wind ◽

Wave Loads ◽

Wave Load ◽

Offshore Wind Turbines ◽

Simple Addition ◽

Recorded Data ◽

Term Data

For the design of offshore wind turbines exposed to wind and wave loads, the method of combining the wind load and the wave load is significantly important to properly calculate the maximum stresses and deflections of the towers and the foundations1). Similarly, for the analysis of the fatigue damage critical to the structural life, the influences of combined wind and wave loads have not been clearly verified. In this paper fatigue damage at the time of typhoon passing is analyzed using actually recorded data, though intrinsically long-term data more than 10years should be used to properly evaluate the fatigue damage. This paper concludes that the fatigue damage of the tower caused by the wave load is not substantial and, thus, the fatigue damage by the combined wind and wave load is only 2–3% larger than the simple addition of the independent fatigue damages by the wind and the wave loads. The fatigue damage of the tower top, which is required to reduce the diameter in order to minimize the aerodynamic confliction with blades, is larger than that of the tower bottom. The fatigue damage at the foundation by the combined wind and wave load is 25% larger than the simple addition of the wind and wave damages, as the foundation is directly exposed to the wave load. For the foundation, the proper structural section can be designed in order to improve the structural performance against fatigue.

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Model Tests on the Long-Term Dynamic Performance of Offshore Wind Turbines Founded on Monopiles in Sand

Journal of Offshore Mechanics and Arctic Engineering ◽

10.1115/1.4030682 ◽

2015 ◽

Vol 137 (4) ◽

Cited By ~ 18

Author(s):

Zhen Guo ◽

Luqing Yu ◽

Lizhong Wang ◽

S. Bhattacharya ◽

G. Nikitas ◽

...

Keyword(s):

Dynamic Response ◽

Wind Turbines ◽

Model Tests ◽

Offshore Wind ◽

Frequency Change ◽

Test Results ◽

Scaled Model ◽

Structural Dynamic ◽

Offshore Wind Turbines

The dynamic response of the supporting structure is critical for the in-service stability and safety of offshore wind turbines (OWTs). The aim of this paper is to first illustrate the complexity of environmental loads acting on an OWT and reveal the significance of its structural dynamic response for the OWT safety. Second, it is aimed to investigate the long-term performance of the OWT founded on a monopile in dense sand. Therefore, a series of well-scaled model tests have been carried out, in which an innovative balance gear system was proposed and used to apply different types of dynamic loadings on a model OWT. Test results indicated that the natural frequency of the OWT in sand would increase as the number of applied cyclic loading went up, but the increasing rate of the frequency gradually decreases with the strain accumulation of soil around the monopile. This kind of the frequency change of OWT is thought to be dependent on the way how the OWT is cyclically loaded and the shear strain level of soil in the area adjacent to the pile foundation. In this paper, all test results were plotted in a nondimensional manner in order to be scaled up to predict the consequences for prototype OWT in sandy seabed.

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