SIL assessment of safety instrumented systems in oil and gas stations based on STPA-Bow-tie

The growing imperative for safe and intelligent operation of long-distance oil and gas pipeline systems has led to increased deployment of interlock systems at processing stations, but their reliability and compliance are difficult to guarantee. To achieve process hazard analysis (PHA) and safety integrity level (SIL) assessment in complex hazardous scenarios of oil and gas stations, this paper presents a safety instrumented system (SIS) SIL assessment method based on a combination of system theory, process analysis and the Bow-tie model (STPA-Bow-tie). First, the boundary of the station process system is determined, and unsafe control actions (UCAs) and their key causes are identified based on the hierarchical control structure model of the oil and gas stations. Then, the element mapping relationship among STPA, layers of protection analysis (LOPA), and Bow-tie is proposed, and the Bow-tie model based on Simulink is constructed to realize LOPA, so as to quantify the required risk reduction factor (RRF) and determine the target SIL level of the safety instrument function (SIF). Finally, a Markov model verifies the safety integrity of the interlock circuit. Taking the SIL assessment of an SIS system in a certain oil transfer station as an example, the proposed method demonstrates equivalent SIL determination accuracy to the traditional HAZOP-LOPA method while providing 24 % higher analytical precision and superior visualization of multi-cause/consequence coupling in complex hazard scenarios.