Combining contours of significant wave height and peak period with platform response distributions for predicting design response

Abstract

Model tests of the Troll A platform with increased topside weight have been carried out at Marintek during the summer of 2007. During the model tests, first estimates for the q-probability response values were obtained using the 90% value of the 3-h response extreme value distribution for the sea state on the q-probability contour line for significant wave height, H s, and the peak period, T p, which gave largest response (i.e. the worst sea state). A comprehensive analysis program, where model test results and numerical calculations are used, has been carried out in order to establish a reliable set of design loads for the modified platform. This work revealed a need for verification of the applied percentile level for the design loads and was the motivation for this study. The verification process involves the following steps: 1) Executing a full long-term analysis. This requires that a sufficient number of 3-h time domain simulations are carried out for a sufficient number of different sea states. It is important that the numerical calculations reflect the underlying physics with sufficient accuracy. 2) Screening analysis to determine the worst sea state in view of the problem under consideration along the 10 −2-probability contour. 3) Establish the 3-h response extreme value distribution for this sea state. 4) The target percentile level is then obtained by the percentile giving the same result as the long-term analysis. The percentile levels are estimated for one wave direction and a set of response quantities, but the main focus has been put on the overturning moment and the base shear. A general observation is that the percentile levels vary somewhat for the response quantities. It is further found that the “correct” percentile level may depend on whether maxima or minima are considered.