Impact Forces on a Jacket Deck in Regular Waves and Irregular Wave Groups .
Authors: Murray J.J. , Winso F.N. , Kaplan P. 
Published: October 2008

This paper describes a series of physical model tests carried out on a model of a jacket structure with a horizontal deck. The deck was represented by a closed rectangular box-shape structure withno support members. The dynamometer measured vertical and horizontal forces as well as moments about the transverse axis. The model was subjected to both regular waves and wave groupsselected from irregular wave spectra. Wave spectra were modelled using a deterministic approach which matched thespectrum shape. The measured waves were scanned to identify groups containing waves with crest heights expected to make contact with the deck. A zero-crossing analysis was run on the measured wave groups to determine their period and crest heights. The model was then subjected to a regular wave having the same period and crest height.

Introduction

The deck of an offshore jacket structure is generally positioned at an elevation above the mean water level that is not expected to be reached by any wave crest throughout the structure's durationof operation. This elevation is determined as part of the design process involving prediction of the highest expected wave crest height. The maximum expected crest height is estimated from probability-based models aimed at predicting the largest wave ina particular return period. This so-celled design wave is specified in terms of height and wave period. The intention is to position the deck at a sufficient elevation to avoid an impact, rather than design for one. However, for various reasons, situations arise where the probability of a wave impact is sufficient to require an estimate of the impact load from certain wave conditions and its subsequent transmission to the remainder of the structure. Therefore, an acute understanding of the various componentsaffecting a wave impact force is necessary.

Offshore structure designers traditionally use the design wave method to establish the ultimate design load. The design wave approach assumes that the largest wave will appear somewhere in the irregular wave spectrum time series. This approach furtherassumes that the impact force resulting from this design wave will represent that which results from the largest wave in the irregular spectrum that contacts the deck. Estimating wave impact loads on decks typically involve both physical and numerical models. In most cases approximations based on linear numerical models with empirical factors derived from thephysical model tests are adequate. The associated errors are assumed to be small in terms of the global effects.

However, ocean waves are not regular in nature, but are considered a random process modelled as the linear superpositionof numerous contiguous regular wave components. The result is a relatively complicated wave surface shape as compared to regular waves. Since the wave impact force is delivered to the deck from water in the region of the wave crest, such parameters as the form of the surface profile at the instant of impact and thedeck's depth of immersion can have a significant effect on the impact force.