Cost Savings Associated with Improved VIM Prediction Accuracy

Fatigue damage to mooring lines and riser system components due to current driven Vortex Induced Motions (VIM) governs the design of deepwater floating platforms, especially in the Gulf of Mexico (GoM). Fatigue estimations based on scaled model testing were traditionally used for VIM predictions of deepwater platforms. The excessive reliance on scaled model testing was partly due to the unavailability of reliable numerical tools for quantifying the fatigue that is induced. Practical difficulties in VIM estimation using scaled model testing induces assumptions such as ignoring the effects of external damping, Reynolds number scaling, non-linear stiffness, and current profile.

It has been observed both experimentally and numerically that the external damping of the mooring and riser system reduces the VIM amplitudes and these studies have been presented in the RPSEA5404 project reports [1]. A comparison study has been undertaken to quantify the savings in mooring and riser system cost of a deepwater platform due to the consideration of effects of external damping on the platform VIM performance. The damping levels due to mooring and riser system have been established from a numerical model, based on the methodology established in the RPSEA5404 project [11]. The VIM amplitude reductions due to external damping established from the RPSEA5404 project have been used in the study. Mooring and riser configurations that satisfy the fatigue life requirements corresponding to different damping levels have been established for the cost comparison purposes.