Prioritization Approach for Systems-Theoretic Process Analysis (PA-STPA) : Applied for Subsea Systems

A Considerable amount of scenarios and constraints were produced by the basic systems-theoretic process analysis (STPA) application. It increased the difficulty to allocate resources for problem-solving. While these problems also applied to all other traditional hazard analysis tools, only STPA had not implement prioritization in the process. The purpose of this study is to develop a prioritization approach for STPA. Prioritization approach for systems-theoretic process analysis (PA-STPA) was proposed based on risk-based decision making (RBDM) approach. The proposed approach consisted of five main steps, four main steps from the basic STPA process and one additional step for solution proposal. Additionally, assessment processes were added to each step to quantify the qualitative assessment results. The obtained values were used to rank the results. PA-STPA has been applied to "the subsea gate box (SGB)" system and compared to the result from basic STPA. The study found that PA-STPA had higher analysis efficiency than basic STPA process while having the risk of reducing the safety level of the results by screening out high-risk scenarios. This effect might differ, depending on the unsafe control action (UCA) screening criteria used. The subjectivity of the assessment, formula used for calculation and result safety issue had been identified as weaknesses of PA-STPA. Several countermeasures had been done to resolve these issue. The unsolved problems were discussed and proposed as further works to refine the proposed PA-STPA.