Design of Offshore Structures and Ships for Damage Tolerance

Abstract

Accident experiences for offshore structures and ships are briefly described. It is argued that an Accidental Collapse Limit State is necessary to complement other safety measures to achieve an acceptable risk level. The basis for the acceptance criteria and their implementation in the codified probabilistic design criteria in NORSOK are outlined, and the need for establishing such criteria for ships is emphasized. The NORSOK requirements are quantitative, i.e. the ALS check is specified as a survival check of a damaged structural system. The damage may be due to accidental actions such as fires, explosions, ship impacts or fabrication defects corresponding to an annual exceedance probability of 10−4. Survival of the damaged structure under relevant characteristic payloads and environmental actions with an annual exceedance probability of 10−2, should be demonstrated. Risk analysis methodology to establish relevant accidental conditions is discussed. In these analyses possible risk reduction by use of sprinkler/inert gas system or fire walls for fires and fenders for collisions, should be accounted for. Methods for predicting accidental damage and survival of the damaged structure are briefly outlined. While in general nonlinear finite element methods need to be applied, simplified methods, e.g. based on plastic mechanisms, are developed and calibrated using more refined methods, to limit the computational effort required. Finally, the trend towards establishing more prescriptive ALS requirements is also observed.