Uncertainty quantification in subsea lifting operations

Subsea lifting operations are dangerous and expensive. One typical problem is the amplification of dynamic forces on the lifting cable at deep water due to resonance of the cable-equipment system. So, it is necessary to guarantee that the cable is always tensioned to prevent slack conditions that lead to snap loads, and at the same time, the cable must be below its structural limit. Several models have been presented to analyze this phenomenon, but they did not consider uncertainties in the determination of the hydrodynamic coefficients (Ca and Cd), which affect considerably the response of the system. Therefore, the objective of this study is to evaluate the influence of the variability of these coefficients via a statistical description of the problem, using Markov Chain Monte Carlo, accept-reject method, maximum likehood and Monte Carlo simulation in an integrated way. The variability on the structural resistance of the cable is also considered and a reliability study is presented. The stochastic analysis is compared with the deterministic one, and it is concluded that there is a probability of failure and slacking that could be neglected if the deterministic approach is used, which makes the stochastic analysis a more realistic diagnosis of the problem.