Multi‐hazard reliability assessment of offshore wind turbines

Abstract

A probabilistic framework is developed to assess the structural reliability of offshore wind turbines. Probabilistic models are developed to predict the deformation, shear force and bending moment demands on the support structure of wind turbines. The proposed probabilistic models are developed starting from a commonly accepted deterministic model and by adding correction terms and model errors to capture respectively, the inherent bias and the uncertainty in developed models. A Bayesian approach is then used to assess the model parameters incorporating the information from virtual experiment data. The database of virtual experiments is generated using detailed three-dimensional finite element analyses of a suite of typical offshore wind turbines. The finite element analyses properly account for the nonlinear soil-structure interaction. Separate probabilistic demand models are developed for three operational/load conditions including: (1) operating under day-to-day wind and wave loading; (2) operating throughout earthquake in presence of day-to-day loads; and (3) parked under extreme wind speeds and earthquake ground motions. The proposed approach gives special attention to the treatment of both aleatory and epistemic uncertainties in predicting the demands on the support structure of wind turbines. The developed demand models are then used to assess the reliability of the support structure of wind turbines based on the proposed damage states for typical wind turbines and their corresponding performance levels. A multi-hazard fragility surface of a given wind turbine support structure as well as the seismic and wind hazards at a specific site location are incorporated into a probabilistic framework to estimate the annual probability of failure of the support iii structure. Finally, a framework is proposed to investigate the performance of offshore wind turbines operating under day-to-day loads based on their availability for power production. To this end, probabilistic models are proposed to predict the mean and standard deviation of drift response of the tower. The results are used in a random vibration based framework to assess the fragility as the probability of exceeding certain drift thresholds given specific levels of wind speed.

Structural Reliability Analysis of Wind Turbines: A Review

“…The fragilities were expressed as a function of wind speed or wave height. In a follow-up work [111], a probabilistic framework was proposed to evaluate the reliability of OWTs subjected to multiple hazards including extreme wind and seismic loads.…”

Some Results on the Vulnerability Assessment of HAWTs Subjected to Wind and Seismic Actions

“…Some specific research has already been developed for onshore HAWT, considering the variability of ground motion intensity [30,31] and wind speed [32] separately, and for offshore HAWT under the combination of extreme wind and wave conditions [33]. Fragility is defined as the conditional probability of a damage measure (DM) attaining or exceeding a damage limit state for a given …”

Wind turbines and seismic hazard: a state-of-the-art review

“…Nonetheless, the contribution of wind loading to the wind turbine's fragility was found insignificant compared with the one derived from the seismic excitation, and this is valid even for low‐intensity earthquake events. The fragility analysis, conducted by Mardfekri & Gardoni, highlighted also the governance of the bending over the shear failure mode, respectively, as normally anticipated for slender structural systems like the wind turbines. Moreover, coupling the fragilities with the annual probability density functions for seismic and wind hazards related to the selected sites (Mexican Gulf and Californian coast), the obtained probability of complete damage was found higher than the nominal annual failure probability of 10 −4 , prescribed by DNV‐OS‐J101 as the target safety level for wind turbines' support structures and foundations.…”