REALISTIC DESIGN WAVES FOR WAVE-IN-DECK PROBLEMS

Due to subsidence, unrealistic environmental assumptions in design or other factors, many fixed offshore installations around the world are prone to large impact forces on the super-structure. Too often, unexpected damages are found, and more accurate analyses reveal the need for reinforcement to comply with the relevant rules and regulations. The common analytical practice is to compute impact forces using 5th order Stokes waves in a CFD solver, with the crest height set to the relevant return period. Using this approach implicitly introduces a series of unrealistic assumptions, and cannot necessarily be expected to produce the force of the desired return period. In the present work, we propose a framework to better estimate the true waves producing the 100-year wave-in-deck forces in a typical North Sea environment. This is done by a long-term analysis of wave-in deck force, hierarchically using several screening filters and solvers of different complexity. The main facilitator is an efficient High-Order Spectral Method, which at the final level is used as initial and boundary conditions in CFD. The resulting waves are short crested and may break-producing very high crest velocities that are normally not accounted for in standard engineering practice.