The simulation error caused by input loading variability in offshore wind turbine structural analysis

Abstract

Stochastic representations of turbulent wind and irregular waves are used in time domain simulations of offshore wind turbines. The variability due to finite sampling of this input loading is an important source of simulation error. For the OC4 reference jacket structure with a 5 MW wind turbine, an error of 12–34% for ultimate loads and 6–12% for fatigue loads can occur with a probability of 1%, for simulations with a total simulation length of 60 min and various load cases. In terms of fatigue life, in the worst case, the lifetime of a joint was thereby overestimated by 29%. The size of this error can be critical, i.e., ultimate or fatigue limits can be exceeded, with probability depending on the choice of number of random seeds and simulation length. The analysis is based on a large simulation study with about 30,000 time domain simulations. Probability density functions of response variables are estimated and analyzed in terms of confidence intervals; i.e., how probable it is to obtain results significantly different from the expected value when using a finite number of simulations. This simulation error can be reduced to the same extent, either using several short simulations with different stochastic representations of the wind field or one long simulation with corresponding total length of the wind field. When using several short‐term simulations, it is important that ultimate and fatigue loads are calculated based on the complete, properly combined set of results, in order to prevent a systematic bias in the estimated loads. Copyright © 2014 John Wiley & Sons, Ltd.

Simplified rotor load models and fatigue damage estimates for offshore wind turbines

“…For jackets, this depends to some degree on the turbine and its rating, but it is generally thought that for turbines on jackets the rotor loads tend to be relevant for fatigue. This is supported by the finding that the variability of wind loads has a much stronger effect on wind turbines atop jackets than the variability of wave loads [8]. It is also the case that the complex geometry of jackets makes fatigue estimation more challenging.…”

Development of a comprehensive database of scattering environmental conditions and simulation constraints for offshore wind turbines

“…FASTv8, NREL 5 MW turbine, OC3 monopile, or OC4 jacket) that are not rare in academia (e.g. Zwick andMuskulus, 2015 or Morató et al, 2017), the given values represent a well-founded guidance for simulation setups. Furthermore, these results should sensitise the research community to the problem of initial transients especially in the case of fatigue.…”

“…Commonly, time series are investigated to estimate times of initial transients (Zwick and Muskulus, 2015). Although this is a straightforward approach, here it is considered to not be expedient.…”

“…So far, these values are frequently chosen without profound knowledge. Some approaches to gain a deeper insight into these constraints (Stewart, 2016;Zwick and Muskulus, 2015) concentrate on simulation lengths or specific types of substructures and are not taking realistically scattering environmental conditions into account. In this work, the scattering of the conditions is addressed and different bottom-fixed substructures are analysed.…”

“…This recommendation is based on an analysis which has not been further specified. For a jacket foundation, Zwick and Muskulus (2015) conducted a study investigating lengths of simulations and initial transients and also concluded that 10 min is sufficient, as long as 10 min time series are merged before the rainflow counting is applied. The required time of initial transients is determined by checking the rotor speed to reach a steady state.…”