Energy Absorbtion During Ship Impact on Offshore Steel Structures

Abstract Increasing attention is being paid to ship collision resistance of offshore structures. This paper deals in general with ship impact of any type of structure. However, the examples and the energy absorption capabilities of structural members cover only structures built up by unstiffened tubular members. In order to place the energy absorption capabilities of separate members subjected to well defined loadings in a broader perspective, an outline of the philosophy underlying the DnV requirements to strength verification both during accident and in "as damaged condition" is presented. It should be kept in mind that the structures should be checked for accidental loads in order to promote sound, ductile behavior. III defined types of loadings which are not normally considered but which still may occur, shall not lead to excessive damages or loss of lives and structures. Due to the uncertainties with respect to magnitude and duration of accidental loads in general and ship impact loads in specific, a very sophisticated approach to determine the exact strength and energy absorption capability of structural elements and whole structures may be considered exaggerated. The primary goal for the investigations and verifications should be to try to identify the ductile or brittle behavior of a given structure when subjected to given assumed loading. Following this philosophy, the relative energy absorption capabilities of different deformation modes become as important as the exact quantification of each energy absorptions capability. However, in order to make requirements of practical use, even qualitative requirements must be quantified, and to this end specific sizes of ships are defined hitting structures with prescribed velocities. Tests and refined calculations are necessary in order to establish energy absorption capabilities of separate members. However, once these capabilities are known, the design effort can in many cases be performed using simple hand calculations' rather than the computer. It is hoped that the information contained pertaining to energy absorption capabilities of various deformation modes may serve as a tool in the strength verification of structures subjected to ship impact - or any other impact. It is recognized however that each impact case even in seemingly identical structures are very often "one of a kind" cases since minor changes in the relative strength and ductility of separate members may change the response of the structure significantly. Introduction This paper is subdivided into the following main parts:–Outline of common requirements pertaining to structures impacted by ships–Energy absorption capabilities of commonly used structural elements. (Based on tests and calculations). All results, i.e. the energy absorption capabilities of various deformation modes have been compared to the simplest possible analytical expressions in order to establish energy absorption calculation methods which can readily be utilized in hand calculations. In a number of cases, the absorbed energy has been compared to the simple plastic moment times the discredited rotations.