Loading uncertainties in extreme waves

Abstract

Uncertainties associated with extreme base shear forces and extreme overturning moments are estimated for an idealized eight-legged jacket platform. The structure is assumed to respond statically, and wave forces due to the drag term in the Morison's equation are considered. Cumulative distribution functions of the extreme response are estimated based on first-order reliability methods (FORM). The FORM analysis includes uncertainties from wave heights, bias due to hindcast wave data, marine growth and wave force calculations (i.e. method of response analysis, wave force model, wave kinematic theory, wave directionality, roughness of cylinders and shielding). The uncertainties associated with the extreme response are estimated for two significantly different wave climates, i.e. the Gulf of Mexico and the North Sea. Uncertainties related to 100 years extreme base shear forces are estimated to have a coefficient of variation (COV) of 95% for the Gulf of Mexico, while the corresponding value for the North Sea is 65%. The COV for the North Sea is found to comply well with analyses performed by SINTEF (Norway). Uncertainties related to 20 years extreme base shear forces are estimated to have a COV between 57% to 98% for the Gulf of Mexico. This COV is significantly higher than the uncertainty for which the new API LRFD code (COV of 37%) is established. Some of this difference is explained due to various assumptions made in the study compared to the API study. However, even if this is accounted for there will still be significant differences in the COVs.