Choice of Demand Mode for Subsea Safety Systems
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In the real industry project, which new technology and systems have blossomed and have been codified, many problems and challenges regarding the application of international safety systems standard IEC 61508 in low demand and high demand have been encountered. For instance, automatic train protection system (ATP) is argued to be both low demand system and continuous mode system. A low demand blow-out preventer (BOP) system will operate in high demand mode to withstand the well pressure for hours and weeks when it is activated to full closure. In the real case, both reliability assessments for low and high demand mode could be requested due to the vague and ambiguous statement of concepts and definitions in the IEC standard. Current existing researches put main emphasis on low demand systems, but addresses on high/continuous demand systems are few.This master?s thesis which is written in co-operation with Aker Solutions aims at discussing those problems confronted in IEC 61508 and summarizing the existing academic work, mathematical models as well as relevant industry guideline. By assuming the system will operate in both low demand and high/continuous demand mode, Subsea High Integrity Pressure Protection System as a case example is used to illustrate the problems in the case study. Both probability of failure on demand (PFD) and probability of dangerous failure per hour (PFH) are calculated by PDS method as the low and high/continuous demand mode reliability assessment. The results from the case study in terms of SIL are found out to be inconsistent. This problem is discussed and traced back to the general quantitative SIL allocation method in IEC 61508-5. The thesis therefore attempts to develop a general consistent SIL range by carrying out SIL calibriations with Matlab. New correction factor is calculated and a new proposed SIL table is given. The decisions on following IEC 61508 SIL table is further argued. Both pros and cons of different solutions are compared.