Wave Impact Forces on Offshore Structures: Re-Examination and New Interpretations

Abstract

ABSTRACT A new mathematical model is presented for determining the time histories of impact forces on horizontal circular members and flat deck structures of offshore platforms. Comparison with characteristics of measured data allows explanation of various anomalies, as well as further insight into the effects of measurement system dynamics. Extensions of the analysis are described for application to problems of impact on vertical cylinders, as well as overhanging decks or jacket structure legs during platform transport. INTRODUCTION The problem of wave impact forces on offshore platforms has primarily been concentrated (in the past) on the case of horizontal members in the splash zone, where these support members experience impact forces due to contact with the crests of incident waves. A number of publications related to this problem area have appeared in OTC and related publications ([1]-[5]), where both theoretical analyses and experimental results have been presented and discussed. The results have been primarily concerned with vertical impact forces on the cylinder structure, but some consideration was also given to horizontal forces. While most of the experimental data in these papers was that from model tests, data on a large Ocean Test Structure (OTS) in the Gulf of Mexico together with an associated theoretical analysis-correlation study were presented in [5]. Although these results have arrived as some generally accepted value for the peak vertical impact force, there were a number of questions related to the nature of the load time history. Various effects associated with the measurement procedures; the influence of inertial reactions; effects of measurement system dynamics; influence of filters used in data acquisition and analysis processes; etc. have an influence on the measurements that must be considered when determining the nature of the impact forces and their magnitudes. Such aspects were not uniformly considered in all of the prior publications. Another important problem area, of more recent concern, involves impact forces on platform deck structures (particularly for older platform installations). Large impact forces occur due to extreme waves whose crests can inundate the deck structure due to insufficient air gaps. These reduced air gaps occur as a consequence of subsidence of the local seabed region (or settlement of the platform over time), as well as the increased extreme wave heights that were not forecast as well when these earlier platforms were installed. A particular illustrative case is that of the the Ekofisk platform. The Ekofisk platform deck is composed of flat plating, for which type of structure there are large loads possible. Other platforms have their deck composed of a finite number of beams on which grating is places, so that a reduced "hard structure" region is present. For such cases the horizontal deck loads are then usually the dominant loads on the deck structure. The nature of the loads on flat plate decks differs from that on horizontal cylinders, and the theoretical methods used for each type of structural element are describes in the present paper. The approach used in this paper is similar to techniques used in analysis of ship slamming phenomena.