An investigation into the influence of analysis methods upon fatigue damage predictions for jack-up rigs.

The influence of wave force linearisation methods on fatigue damage predictions for jack-up rigs is investigated, using Sesam software as the basis for comparison. The possible risk of significantly underpredicting fatigue damage is highlighted. Dynamic spectral fatigue analysis methodology is applied to a representative jack-up rig in 30 and 70 m water depth in the North Sea. Transfer functions are computed using two approaches: regular waves of constant steepness with capping of wave height at low frequencies; stochastic linearisation based on a single sea state per wave scatter diagram. It is concluded that stochastic linearisation based on a single sea state per scatter diagram is insufficient to provide an adequate characterisation of nonlinear drag forces for the dynamic spectral fatigue analysis of jack-up rigs. Superharmonic resonance effects, due to nonlinearities associated with variable submergence and drag forces, are found to be significant when computing transfer functions of structural dynamic response. [ABSTRACT FROM AUTHOR]