Reliability analysis of wake-induced collision of flexible risers

Collision between risers is an important design and operational concern, especially in deep water since the probability of collision tends to increase as the riser length increases. Riser collision is due to the joint effects of many processes, i.e. environmental loads, hydrodynamic interference and surface floater motions and the most of them are stochastic processes. This paper provides an approach for estimating the failure probability of riser collision by considering these joint effects and their stochastic nature. Firstly, a procedure for establishing the distribution function of the extreme minimum relative distance between two risers is introduced based on simulation tools and statistical data. Numerical simulation is performed to compute the minimum distance between risers for a given duration. Repeated simulations are applied so that the extreme value distribution can be established. Secondly, reliability analysis is performed by considering the uncertainties of input parameters related to environmental loads and riser system. The collision probability is calculated based on both the First/Second Order Reliability Method and the Monte Carlo simulation techniques.