The extreme force on an offshore structure and its variability

For the design or re-assessment of an offshore structure it is essential to have an estimate of the expected value and the associated uncertainty of the extreme force on the structure in a given oceanographic environment. These force characteristics are usually estimated for a large (e.g. 100 yr) crest occurring at the structure. The subject of this paper focuses on how the variability of the design wave force should be interpreted. Basically, there are two possible interpretations of the variability of the design wave force: (1) the uncertainty of the wave force peak that is associated with the largest wave height occurring at a particular point; (2) the uncertainty of the largest wave force occurring in a particular sea state, both within a given duration. For structural reliability purposes, the specific coupling between a wave crest at a particular location and the associated peak force on the structure is not required; only the statistics of the extreme force on a structure in a given extreme environment (sea state) is of interest. In this paper it will be demonstrated through numerical experiments that the Coefficient of Variation (CoV) of the global extreme wave force in a given sea state and period is much smaller than the CoV of the force associated with the extreme wave height occurring at a given location for the same period. For the numerical experiments a recently developed technique based on constrained random time domain simulations has been adopted.