A collision-allowed approach for probabilistic assessment of riser interference

Riser interference assessment is especially crucial in deep water, as the compliancy of risers generally increases with length. This paper presents a probabilistic approach for riser interference analysis, taking into account the stochastic nature of the wave forces. The methodology is founded on the collision-allowed principle, in which infrequent collisions are permitted, provided that structural integrity is not compromised. The collision analysis is traditionally separated into the local and global stages. The procedure for local impact analysis is well established in the literature. For the global analysis, one challenge aspect is that very few collisions, if any, can be observed in a simulation of a practical duration. This difficulty is herein addressed by extrapolating the pre-collision response characteristics to predict the statistics of the critical collision parameters such as impact velocity and kissing angle. This task is made complicated by significant nonlinearities in the loads and responses. The results of the global and local analyses are subsequently combined to evaluate the yield and fatigue limit states. The proposed approach is implemented on a top-tensioned riser system, and it is found to yield good estimates of the system reliability, based on comparison with extensive Monte Carlo simulations.