Abstract
The predicted responses of an offshore structure when the wave induced kinematics are computed from different estimation methods can change significantly. The sometimes controversial results have recently motivated the development of a new methodology for wave kinematics prediction. While the methods commonly used by the offshore industry are empirical and semi-empirical modifications of Linear (random) Wave Theory, the new approach (Hybrid Wave Model) satisfies the principles of hydrodynamics and explicitly considers the non-linear effect of the wave-wave interactions on wave elevation, kinematics and evolution. This methodology has been proven to be more accurate and reliable for the estimations of wave kinematics, but its impact on the prediction of the structural response is yet to be investigated. In this study, the performance of the new methodology arid 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. A numerical scheme based on a Finite Element time integration technique (Newmark-beta method) was used for the response evaluation after it had been validated and calibrated by an analytical (linear) solution and measured responses for regular waves. 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.
Ramos Heredia, Rafael Juda (1995). Comparisons on offshore structure responses to random waves using linear and high-order wave theories. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1995 -THESIS -R366.