Analyzing the risks in subsea wellhead recovery based on fuzzy fault tree

In the process of subsea wellhead system recovery, the operation risks are great, and risk events tend to happen easily, such as recovery tool failure and operation failure, which lead to the failure of subsea wellhead recovery. In this paper, the basic events leading to the failed recovery of subsea wellhead system were sorted out based on the operation process and expert’s field experience of subsea wellhead system recovery, and the fault tree with subsea wellhead system recovery failure as the top event was established based on basic events and logical relations. The expert evaluation mechanism was introducing to carry out linguistic evaluation on the occurrence probability of basic event. The expert’s linguistic evaluation on basic event was converted into the occurrence probability of basic event by using the fuzzy mathematics algorithm. And the high risk factors in the process of subsea wellhead system recovery were identified based on the sensitivity analysis of basic event probability. It is indicated that the damage failure probability of the cutting blade for surface casing and conductor pipe recovery is the highest, and cutter knife freezing, cutting blade damage, inaccurate cutting position and human operation error have greater influences on the failure of subsea wellhead system recovery, so it is necessary to put forward the corresponding prevention and modification measures. The research results can provide an important reference for the safe recovery of subsea wellhead system.