A comparison of various approaches for predicting extreme wave induced response for design of offshore structures

Abstract

Offshore structures are exposed to environmental loads such as waves, wind, currents etc. and it is important to understand how the structures behave under different conditions. In this thesis the main focus has been on determining the long term extreme response. A case study has been performed on a semi-submersible located in the North Sea.

Because of the randomness in the ocean environment and the corresponding response, statistical methods are required to estimate extreme motions. Two different approaches have been used to estimate the long term extreme response; the all sea state approach and the environmental contour line method. The all sea state approach utilizes the long term variability of the environmental conditions and the variability of response for a given sea state. All sea states are taken into consideration. Normally a full long term analysis is performed. However, if a complicated non-linear problem is under consideration, a full long term analysis is quite time consuming and a simplified method is preferred. One such method is the environmental contour line method that utilizes the long term variability of the environmental conditions to predict the extreme sea states. To establish the extreme response, a short term analysis is performed near the sea states in proximity of the “worst” sea state.

The results obtained with the full long term analysis have been evaluated by using Monte Carlo simulations based the available hindcast data.