Operational failure model for semi-submersible mobile units in harsh environments

The Semi-submersible Mobile Unit (SMU) plays a vital role in the development of offshore oil and gas fields. Extreme weather conditions such as high winds, waves, and icy conditions impose an extraordinary load on the platform. This paper presents a detailed operational failure model considering the wind and wave combined loading of extreme weather conditions. The model is developed in a probabilistic framework using the Bayesian network (BN) to assess the operational risk quantitatively. The BN represents conditional dependencies of the weather effects and the operational characteristics of the SMU. The proposed model is tested and validated using a well-known accident - the Ocean Ranger. The model predicts a very high probability of operational failure (capsizing) in the prevailing weather conditions, which is confirmed by the fateful event on Feb 15, 1982. The proposed model is a useful and reliable tool to develop and monitor the operational failure envelope of the SMU in given environmental conditions. This work helps to promote safe and reliable offshore development.