STAMP-based analysis of deepwater well control safety

The complexity of a deepwater well control system makes defining appropriate safety requirements with traditional safety analysis methods difficult. Hence, there is a need for a complex systems approach for better understanding the development and prevention of accidents during deepwater drilling. Differing from traditional methods based on reliability theory, we use the system-theoretic accident model and process (STAMP) and system-theoretic process analysis (STPA) methods to establish a hierarchical control and feedback loop model of a well control system. In view of the characteristics of complexity and dynamism, the safety analysis is regarded as a system control and feedback problem in this work. Using this model, the systematic hazards and safety related constraints are firstly defined, the safety control structure (SCS) is then established, the inappropriate control actions (ICAs) are identified in the next step, and the key factors that contribute to ICAs are finally determined. Guided by the STAMP/STPA, we construct the process of well control to simulate the consequences of ICAs during deepwater drilling by using the dynamic multiphase simulation software, OLGA. The simulation takes the lack of control actions and the provided control actions of a late shut-in as examples. The STAMP/STPA method proves to be an effective solution for evaluating the safety of deepwater well control from the perspectives of control and constraint. By modeling the process of a well kick, shut-in, and well killing, it was quantitatively indicated that the rational control actions within a certain time period can prevent accidents from occurring and escalating, thereby ensuring the safety of the system. The differences between traditional methods and STAMP/STPA are compared in this paper, and the limitations that need to be solved in the future are pointed out.