Multi-Hazard Risk Analysis: Case of a Simple Offshore Structure

Keywords: Risk Assessment, Offshore Structures, Modeling, Seismic Risk Assessment, Limit States, Structural Failure, Random Vibration Methods, Linear Models, Seismic Loads, Wind Loads, Wave Loads, and Dynamic Response Analysis.

Abstract: This report develops and illustrates methodology for multi-mode, multi-hazard risk assessment. The case study involves an offshore structure subjected to earthquake shaking and storm-generated wind, wave, and current loads. Two classes of risks are calculated: (1) conditional failure probabilities under short-term loads; and (2) long-term or overall risks, which depend on occurrence patterns of loads of different intensities. This report shows that the relative importance of different loads depends on the limit states and on the load and structural characteristics. For a flexible offshore structure, the storm loads dominate in the case of a displacement limit state, but are unimportant compared to the earthquake load in relation to an acceleration limit state. Parameter uncertainty is shown to always increase the long-term risk; in the case study, the increase is more pronounced for the displacement limit state than for the acceleration limit state. Also, it is shown that the ambient load increases the risk due to earthquake loads alone, especially at the lower resistance levels.