Development of Accidental Collapse Limit State Criteria for Offshore Structures

Accident experiences for offshore structures are briefly described. It is argued that Accidental Collapse Limit State (ALS) checks are necessary to complement other safety measures to achieve an acceptable risk level. The ALS procedure for offshore structures in Norway is outlined, with a focus on risk acceptance criteria. This approach is 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 or fabrication defects corresponding to an annual exceedance probability of 10-4. Survival of the damaged structure under relevant actions with an annual exceedance probability of 10-2, should be demonstrated. Risk analysis methodology to establish relevant damages is discussed. In these analyses possible risk reduction actions should be accounted for. Methods for predicting accidental damage and survival of the damaged structure is highlighted. Finally, the trend towards establishing more prescriptive ALS requirements is also observed. 1