Failure probability of a jack-up under environmental loading in the central North Sea

Abstract

The annual probability of failure has been calculated for a jack-up subjected to extreme storm loading in the central North Sea. Statistics of extreme loads were generated using models that account for the combined behaviour of winds, waves and current in severe storms. The fluid loading is calculated using well validated theories for the kinematics of extreme random waves and currents and hydrodynamic models of jack-up legs. Non-linear push-over analysis is used to evaluate the ultimate strength and the overturning resistance of the jack-up unit. In the non-linear analysis, windward spudcan sliding is included by means of a force constraint method. The likelihood of structural failure or overturning is the probability that the environmental load will exceed the overturning or structural resistance of the rig. Physical uncertainty in the extreme environmental load dominates the problem and is fully analysed. The effects of physical uncertainty in resistance of the structure are discussed. The results indicate a failure rate of order 10 -5 per year.