Value of information from fatigue monitoring of the monopile foundation of an offshore wind turbine

Value of information (VoI) allows quantifying benefits of adopting structural health monitoring (SHM) even before implementation by altering the prior risk of structural failure using the probabilities of damage detection from SHM data. The VoI is the reduction in the expected failure cost when SHM data becomes available. In this study, an optimized time of SHM deployment for fatigue crack growth in an offshore wind turbine (OWT) monopile foundation is achieved by maximising VoI. The prior probabilities of failure are obtained from stochastic analysis of dynamic responses to random wind and waves characteristic of North Sea environment. Short-term stress time histories are statistically processed to determine the probability distribution functions of their extrema and ranges and statistical extrapolation techniques are then implemented to analyse the long-term probability distributions of axial stresses. The long-term stress ranges and numbers of cycles are determined by the rainflow count and used in fatigue reliability analysis. The probability of detecting a fatigue crack by SHM is modelled and used for calculating the OTW failure risk after repair is conducted based on SHM outcome. The optimal deployment time of the fatigue crack monitoring system for the analysed 5-MW OWT is determined as 21 years past erection.