Prediction of Low-Frequency Offshore Structure Response to Irregular Waves Using Linear and High-Order Wave Theories

Abstract

Abstract The predicted responses of an offshore structure when the wave kinematics are computed from different estimation methods can change significantly. The sometimes controversial results have recently motivated the development of a new methodology for random wave kinematics prediction known as Hybrid Wave Model (HWM). In this paper, the performance of the new methodology and other methods currently used for kinematics prediction was tested. The (surge) response of two offshore structures designed specially for deep-oil production was estimated using three methods (Hybrid Wave Model, Wheeler "Stretching" and Linear Extrapolation) and compared with the corresponding laboratory measurements. The wave forces were computed from the conventional Morison Equation evaluating the ambient wave kinematics from the wave elevation measurements and the response was computed using a numerical scheme based on a Finite Element time integration technique (Newmark-beta method). The comparisons between measured and predicted responses using kinematics calculated from the Hybrid Wave Model showed excellent agreement, specially for the low frequency components, while those using methods based on linear modifications rendered poor underestimations. The low frequency (peak) responses of these deep-water offshore structures were found to be greatly dominated by very low frequency wave excitations, which are mainly due to the wave-wave interactions. This work will show the necessity of high-order methods to evaluate irregular wave kinematics and induced responses.