The Behavior Of Steel Offshore Structures Under Accidental Collisions

Abstract

Abstract In order to design steel offshore structures against impact loads due to accidental collisions caused by ships, it is necessary to evaluate the plastic energy absorption capability of its members. In this paper the local energy absorbed at the point of impact and in its immediate vicinity is first considered. Simple estimates based on upper bound solutions are discussed. Then the overall energy absorption capability of a tubular member is studied when it deforms in bending. The varying degree of axial and rotational restraint provided by the end supports is included in the analysis. A simple method for estimating the extent of damage resulting from a collision caused by a typical supply vessel is then suggested, and a specific example of application is discussed. Introduction The collision between ships and platforms or other installations in offshore oil and gas fields is a type of accident which can have catastrophic effects, and as such is a cause of growing concern. Several studies have dealt with the problem of collisions offshore from the point of view of probability of occurence and degree of severity. For example in [1], major failures recorded in the Gulf of Mexico between 19L,8 and 1977 were analysed, indicating that 797. were related to drilling and production, 7% to storms and 5% to collisions. On the basis of oil spilled, only 1% of volume spilled resulted from collisions. Thus it appears that collisions account for a relatively small percentage of all major failures offshore. However, other studies indicate that the probability of occurrence of such incidents is by no means irrelevant. In [2], for example, values for the probability of occurrence of a collision involving different types of vessels and an offshore platform are given. In the case of supply ships the probability is considerable (p > 10−2), for oil tankers involved in offshore loading the probability is medium (P > 10−3), while for passing ships not involved in offshore operations the probability is very low (P > 10−7). Some studies sponsored by the UK Department of Energy [3] also address the question of the risk of shiRip1atformco11ision, and the general conclusion that can be taken from these various sources is that such risks cannot be ignored. In fact, not only surface ships should be considered as potential causes of accidental impacts, since the possibility of collisions involving submersibles is certainly also real, as discussed in [4]. Several studies have addressed the question of how to minimize the risks and extent of damage associated with collisions offshore, such as methods for improving navigation [5] and the development of special fendering systems [6]. However, practically no effort has been devoted to improving or adapting the structural design of platforms to this type of accidental load.