The Effectiveness Assessment And Safety Integrity Level Determination For Fire And Gas System On Offshore Platform

Fire,toxic gas leakage and combustible gas explosion are the main problems in petrochemical industry.Fire and Gas System(FGS)is used to detect and mitigate the effects for Loss of Containment(LOC)of hazardous materials,it belongs to Safety Instrumented System and set in suppression protection layer.Traditionally,FGS design is based on engineering experience,causing resource waste by overestimation the risk or exceeding current FGS protection capability because of low risk assessment.To solve these problems,based on performance design conception is widely applied to FGS.The new conception can improve detector coverage and FGS availability in complex leak scenario assessment,especially in high risk exposure scenarios such as offshore installations.Combining engineering practice with performance-based method will be more suitable for FGS design.The paper used the offshore platform with high-risk processing as research object based on the criterion of ISA 84.00.07.The aim is to analysis the influence of basic properties and physical properties to process equipment.According to instrumented system characteristics to discovery the causal logic of FGS.Dividing process installation into zone and setting hazard level for equipment in order to detect LOC events and distinguish different alarms.Using Major Hazard Analysis(MHA)identify LOC leak scenarios and its initial events.FGS performance determination includes several aspects.First,LOC leakage frequency is calculated for process device.Second,gas dispersion modeling is conducted for a typical inflammable gas leakage scenario on offshore platform and the outcome of gas dispersion size and gas cloud boundary concentration will as the scenario database for the detector coverage analysis.The detector coverage is calculated by Effigy software.FGS fault model can be established based on alarm interlocking logic by Fault Tree Analysis(FTA)and safety availability is the results of its logic algorithm.Finally,mitigation effectiveness is quantified using Event Tree Analysis(ETA)through analyzing the interaction between mitigation systems with personnel.In summary,the detector coverage,safety availability,and mitigation effectiveness are the three keys of FGS effectiveness assessment.Risk reduction target of FGS is based on conventional Risk Tolerable Criteria(RTC).The paper proposed a calculation formula for FGS risk reduction objective and compared it with scenario risk.Summarizing the key technical elements in FGS effectiveness assessment and using ETA describes different scenario consequence type which caused by the key technical elements.The FGS effectiveness can be quantified as Potential Life Loss(PLL).Due to the lack of technologies for Safety Integrity Level(SIL)determination of FGS,the paper invent a method to make up the drawback by using modified Layer of Protection Analysis(LOPA)to determine the SIL of FGS that based on the risk distribution principle of LOPA.FGS scenario risk quantification distribution is realized by comparing the proposed FGS risk reduction target.The SIL which determined by LOPA will be verified with FGS that in service to determine if it is up to standard.The FGS risk reduction target makes up for the deficiency of mitigation system in using conventional RTC.Using improved LOPA technique to analyze FGS safety availability can achieve optimal allocation of risk control facilities on offshore platform.