Nonlinear Wave Amplification Around Column-Based Platforms in Steep Waves

The wave diffraction and upwelling around vertical columns such as platform legs and offshore wind turbine foundations is studied. Nonlinear contributions in steep wave conditions are particularly addressed. Data and analyses from a previous experiment with fixed cylinders in deep water regular and random waves are reviewed, and essential findings are highlighted. Measured extreme crests are clearly higher than linear predictions, connected with an increase in basic harmonics as well as with second-order components. Improved results are obtained with fully second-order modeling, while this is still incomplete, and such analyses can be computationally demanding as well as non-robust.

An alternative, simplified empirical nonlinear correction approach based on these findings, previously suggested for engineering design prediction of extreme crests in random waves, is validated against a re-analysis of the fixed column data. In addition, validation is also made against model test data for two floating production semis. The method agrees well with the data, taking into account the simplicity of the method and the complexity of the problem, and is a tool for early design phases. Detailed studies will still require model tests and CFD development.