4 - Kuwait Oil Company

The risk reduction factor fromeach independent layer can becombined to obtain the total riskreduction factor. Risk reductionfactor (RRF) = 1/PFD Avg.Safety Integrity Level (SIL)Safety Integrity Level (SIL) is astatistical representation of safetyavailability of a SIS at the timeof process demand. When thehazards identification and riskassessment phase concludes thata SIS is required, the level ofrisk reduction afforded by theSIS and the target SIL has to beassigned. The effectiveness of aSIS as an independent protectivelayer is described in terms of theprobability it will fail to performits required function when it iscalled upon to do so.The level of risk reduction varieswith respect to the amount ofrisk that has to be reduced andtolerable risk that has to beachieved. Once the level of riskreduction is determined, it isexpressed as safety integrity level(SIL). An adequate safety relatedsystem (SRS) can then be selectedby choosing a system that fallsunder the appropriate safetyintegrity level.The necessity of SIF is termed asdemand mode and is classifiedas Low demand mode and Highdemand or Continuous mode. Ifthe demand frequency is less thanone per year and test frequencyis greater than two times demandfrequency, then it is called lowdemand mode. If the demandfrequency is greater than oneper year and test frequency isless than two times demandfrequency, then it is consideredhigh demand mode.Target failure measures for SIF at low demand mode of operation is given in Table 1:Safety IntegrityLevelPFDavg.Risk Reduction Factor4≥ 10-5 to