Application of System-Theoretic Process Analysis to the Isolation of Subsea Wells: Opportunities and Challenges of Applying STPA to Subsea Operations

Abstract

Systems-Theoretic Process Analysis (STPA) is a recently developed hazard identification technique that is based on control and systems theory. Previous studies on STPA emphasize two major strengths of the method: (1) STPA provides a systematic top-down approach that enables early identification of system flaws, and (2) STPA covers a wider scope of hazards compared to traditional methods. Despite these advantages, there are only a limited number of studies that have applied the method to subsea systems. It is therefore of interest to investigate how STPA can be used to formulate new or verify existing requirements to safety-critical systems for subsea facilities. One example is the isolation of subsea wells initiated by the platform emergency shutdown (ESD) system. The purpose of this paper is to apply STPA to this function, and to discuss opportunities, challenges and possible implications of the results obtained from the analysis. The paper starts with a thorough literature study and includes an analysis of the insights and recommendations made from other industry sectors and application areas. This review is followed by the STPA analysis of the proposed system, with focus on the identification of the unsafe control actions and safety constraints for subsea well isolation. It is investigated how STPA is able to address specific design philosophies and subsea operating conditions, like fail-safe function of subsea ESD valves, long distance between top-side control system and subsea valves, and dynamic behavior of the control structure. The paper concludes with discussions and suggestions on how the STPA procedure may be improved for application to subsea systems.

A comparison of hazard analysis methods capability for safety requirements generation

“…Next, we utilized a recommendation by Kim et al 32 when modeling the system. They propose to include the power supply as part of the control action and include it for UCA identification.…”

Combining system-theoretic process analysis and availability assessment: A subsea case study

“…7 Of the mentioned methods, STPA is the approach attracting the most recent attention due to its suitability to analyze complex and software-intensive systems. Some of the advantages and examples of applications of STPA are discussed in the studies of Mahajan et al, 8 Kim et al 9 and Sulaman et al 10…”

“…STPA has been applied in different applications such as automotive, 11 healthcare, 12 aerospace, 13 maritime 14 and subsea. 9,15 As a hazard identification method, STPA can be naturally embedded in safety and security analysis 16,17 by guiding the associated controls and mitigating measures depending on different applications. 12,18,19 So far, the commonality and acceptance of STPA are limited to the academic studies and not yet adapted as best practices in, for example, international standards on safety assessments.…”

“…7 Of the mentioned methods, STPA is the approach attracting the most recent attention due to its suitability to analyze complex and software-intensive systems. Some of the advantages and examples of applications of STPA are discussed in the studies of Mahajan et al, 8 Kim et al 9 and Sulaman et al 10 STPA is based on a rather new accident causation model named systems theoretic accident model and processes (STAMP), which is built on a theoretical basis provided by system theory and control theory. 2 STPA identifies hazards in a systematic way by examining the potential deviations on the defined feedback control loop.…”