A Discussion of Long Term Response Versus Mean Maximum Response of the Selected Design Sea State

The overall aim of the design process is to ensure that a structure can withstand with sufficient margin all foreseen load events. Offshore rules and regulations will typically define a set of rules to be followed by the designer. By fulfilling these it is tacitly assumed that the aimed safety level is achieved. The governing regulations and rules and, not the least, their interpretation may vary between various offshore regions. This may be the case both for the load side of the problem and the strength side of the problem. In connection with design of offshore structures, the largest variability and uncertainty is typically associated with the load side of the problem. The purpose of this paper is to discuss the consequences of two rather common definitions regarding the characteristic environmental load to be used for design purposes: i) The characteristic response is taken as the load for which the annual probability of exceeding it is q (here referred to as the q-probability load), ii) The characteristic response is taken as the mean extreme response of a sea state for which the annual probability of exceeding it is q (q-probability sea state). Results are shown for some generic response cases varying from a linear response case being rather insensitive to wave period to a quadratic response problem associated with a critical wave period band.