Fault tree analysis applied to SIL (SAFETY INTEGRITY LEVEL)RATE SAFETY INSTRUMENTED SYSTEMS)
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This report is the documentation of an M.Sc. thesis carried out for the Technical Safetyand Reliability (TS & R) Group of FMC Technologies and the Department of Production and Quality Engineering, Norwegian University of Science and Technology(NTNU). At FMC Technologies, the TS & R group has guidelines for a number of their analytical tools, but they do not have any guideline for Fault Tree Analysis (FTA).Owing to their increasing need to conduct an FTA, it is therefore pertinent to develop an“easy to follow’’ guideline for FTA. FTA is a suitable tool mainly for analyzing static systems. It is also used partly for analyzing dynamic systems. FTA has been a useful tool for analyzing systems with two possible states, i.e. functioning or failed. This thesis focuses on specific application of FTA in FMC Technologies’ scope of supply. It is a guideline detailing the step-by-step procedure of conducting an FTA. The guideline gives an approach that supports Safety Instrumented Systems (SISs) operating in; low demand/on demand mode and high demand/continuous demand mode. The main objectives of this thesis are as follows: · Develop a step-by-step approach to identify, model and estimate the dangerousundetected contribution from independent and dependent failures, between anycombinations of FTA elements. · Establish a Local Work Instruction (LWI) for FTA with the same format as theGlobal Work Instruction (GWI) 114 FMECA Procedure. It will support both on demandand continuous mode. · Establish a master document that will be used for reporting FTA. · Establish a standalone user manual for an FMC selected tool, adapted forapplication of the LWI FTA. · Apply an FMC SIS delivery, as a pilot. As a follow-up to the objective of this thesis, it is therefore important to identify critical steps that are important in a successful FTA. One of the most important aspects of an FTA is the quantitative and qualitative aspects. The quantitative aspect is mainly aimed at calculating the probability that an undesirable event occurs. This is characterized by the description of the type of system being analyzed, whether low demand system or continuous system. The qualitative aspect takes into account the ranking of all the contributors to a particular undesirable event. This ranking helps to identify design weakness during the design process. It also helps the maintenance department develop a good maintenance strategy for a particular system, since the most critical contributors to an undesirable event can be identified using FTA. It helps the analyst to identify dependencies between events. An uncommon approach was used for the FTA in this thesis. This approach uses RELEX (i.e. FMC selected tool), to perform FTA by pre-processing the ingoing parameters of RELEX. An Excel template and standalone software (java application)were established for the purpose of pre-processing the ingoing parameters of RELEX. Furthermore, the following standards were reviewed in this thesis: IEC 61508, IEC61511, IEC 61025, OLF GL 070 and PDS Handbook. IEC 61508 is a generic standard;covering the functional safety of electrical, electronic and programmable electronic safety-related systems. IEC 61511 is a functional safety standard applied to SIS for the process industry sector. OLF GL 070 is a guideline for the application of the IEC 61508and IEC 61511. The PDS Handbook presents methods used to quantify the safety unavailability and loss of production for SIS. IEC 61025 is a standard mainly applied to FTA. Another important tool used for this thesis is the National Aeronautics and Space Administration (NASA) Fault Tree Handbook. This guideline was developed by NASA and is solely applied to FTA. It presents a detailed methodology for conducting anytype of FTA.