The Effect of a Non-Linear Wave Force Model on the Reliability of a Jack-Up Platform

The reliability against failure due to excessive stresses in the legs of a jack-up drilling rig is investigated.A 3-D linear Timoshenko beam model of the jack-up is used to determine a non-linear relation between the wave height and the maximum leg stresses. The non-linearities follow from the use of Morison’s formula, Stoke’s fifth order wave theory and integration of wave forces to the actual water surface. Loads due to current, wind and gravity are included, partly through probabilistic models. Uncertainties in structural parameters such as the drag coefficient and the yield stress of the leg material are taken into account.With this model, the probability of failure due to excessive leg stresses is determined using first and second order reliability methods. These results are compared to the safety factors according to established codes of practice for allowable stresses. A similar analysis is given for the safety against overturning. It is found that the established practice for site approval of jack-up platforms for these two failure modes leads to probabilities of failure which differ by orders of magnitude.