Dynamic Analysis of Long Floating Structures in Waves

Abstract

This paper describes a frequency domain dynamic analysis technique for calculating the response of long floating structures in waves. Due to the large dimensions of the structure, it is necessary to account for the correlation of wave forces along the structure length to accurately calculate the response. The sea surface is therefore modeled as short-crested and is described by a directional wave spectrum. The dynamic analysis technique uses a superposition principle in which the short-crested sea is composed of numerous harmonic wave components with different frequencies and directions of propagation and amplitudes calculated from the directional wave spectrum. Hydrodynamic coefficients are calculated using a 2-dimensional diffraction program. The structure is modeled using beam elements in the ANSYS finite element program and moorings are represented using spring elements. The response of the structure to each wave component is calculated in a series of harmonic analyses. Spectral analysis is used to calculate the variance of the responses in a given storm event. The responses of interest for a given exceedence probability are then determined. The covariance of the responses is calculated and coincident response combinations are produced based on the assumption that the responses have a multi-variate normal distribution. The magnitude of coincident responses is often of interest in structural design. For example, a member subject to bi-axial bending could be designed to have excess resistance if the maximum value of the orthogonal bending moments obtained from a dynamic analysis is used rather than the coincident bending moments.