A second order frequency-domain model implemented by a boundary element method was developed for wave elevation around structures. The diffraction of monochromatic waves by a square array of truncated cylinders and a simplified TLP was studied with this model, and the wave run up and the free surface elevation around the structures were presented. The computation results show that the near-trapping phenomena occurs inside the structures, which leads to increased wave height. Special concerns were paid on the examination of the crucial frequencies and maximum wave height of the near-trapping phenomenon.
An arrangement of four truncated cylinders centered at the corners of a square is a simple model of a typical TLP platform. The prediction of the run up on the cylinders is of great interest for the offshore industry, e.g. to determine the height of the platform deck above sea level. Evans and Porter (1997) indicated that wave interaction with four-cylinder structures can result in a considerable enhancement of the local free surface for particular incident frequencies over a very narrow range and the phenomenon is regarded as the near-trapping phenomenon. If these large free surface elevations were to occur in practice, then this would have serious implications for the design of large arrays of offshore structures. It is therefore important to understand when these effects occur and how they might be affected by factors, such as structure form and nonlinearity. Although some interesting interaction affects that arise from nonlinearity have been observed, nonlinear effects are difficult to analyze with complex geometries. In present study, the wave interaction with a square array of truncated cylinders and a TLP platform is investigated in frequency domain based on a Stokes expansion approach. The velocity potential is obtained by a boundary-integral equation method.
