Igor Prislin (PhD student) and Jun Yang (MS student)

Predictions of Short-Crested Irregular Ocean Waves

A nonlinear hybrid wave model is developed for the relation among short-crested ocean wave kinematics, pressures and elevation. The model is based on sound hydrodynamics principles and employs both conventional perturbation and phase modulation methods to describe wave-wave interaction in a short-crested ocean wave field. The understanding of nonlinear effects on wave elevation, pressures and kinematics may lead to more accurate prediction of wave loads impact on offshore structures and analysis of ocean wave measurements.

This research may strengthen our capability of modeling and measuring ocean waves and therefore provide more reliable and accurate estimation of wave loads on offshore structures.

An extended Maximum Likelihood Method (EMLM) is employed to obtain a directional wave amplitude spectrum based on three or more fixed-point measurements. The initial phase of each free-wave components is then computed through fitting with the measurement. Nonlinear interactions between two free-wave components are computed using a conventional perturbation method for close wave frequencies and a phase modulation method for disparate frequency scales. The effects of nonlinear interaction are then subtracted from wave measurements. The procedure of decomposition is iterative to reach required accuracy. Based on the decomposition, wave properties other than the measured ones and nearby the measurements can be predicted. The predictions are being examined by two sets of field measurements (Texaco Harvest data and FULWACK data) and laboratory measurements (OTRC wave basin).

Conventional perturbation method was studied by Longuet-Higgins (1963). His work has been extended to study directional waves up to second order with randomly selected phases (e.g. Tayfun 1986).

The phase modulation method for modeling wave interaction in a short-crested wave field is proposed and developed through this research. Also to our knowledge, the hybrid wave model is the first serious attempt to deterministically decompose a nonlinear and directional wave field based on field and laboratory measurements. The model also allows deterministic prediction of various wave properties nearby the measurements.

October 1994 to September 1997

A deterministic wave model is developed for analyzing measurements of directional seas.
A combination of using conventional and modulation phase methods is developed for study the effects of nonlinear wave interaction on wave elevation, kinematics and pressures up to second order of wave steepness.
Based on the proposed model, a corresponding numerical scheme is being developed (with the support of a JIP) for decomposing a directional irregular wave field of given wave pressure or elevation, and/or velocities. It will provide accurate predictions of wave properties.