Fire and gas detection systems, process shutdown systems and emergency shutdown systems are examples of SIS used to prevent abnormal operating conditions from developing into an accident. Such systems are thus installed to reduce the process risk associated with health and safety effects, environmental impacts, loss of property, and business interruption costs. In the PDS method failure of such systems is referred to as “loss of safety”.
 
Addressing safety and reliability in all relevant phases of the safety system life cycle therefore becomes paramount both with respect to safe as well as commercial operation. It must be verified that all safety requirements for the SIS are satisfied, and that the risk reduction actually obtained from the SIS is in line with what is required. Here, the PDS method plays an important role in predicting the risk reduction obtained from the safety instrumented functions (SIF) that are performed by the SIS.

IEC 61508 and IEC 61511 have become the main standards for design, construction, and operation of SIS in the process industry. The Norwegian Oil Industry Association (OLF) has developed a guideline (OLF guideline no. 070) to support the implementation of the two IEC standards. In the regulations from the Norwegian Petroleum Safety Authorities (PSA), specific references are given to the IEC standards and the OLF 070 guideline. IEC 61508 allows using different approaches for quantifying loss of safety. In the OLF guideline, it is recommended to use the PDS method for this purpose.

The PDS method has been applied in numerous projects and in many different contexts. The main application, however, has been related to computer-based safety systems in the offshore and onshore oil and gas industry. The PDS method has e.g. been utilised in:

• A large number of third-party reliability verifications of offshore and onshore safety systems.
• Projects that consider the effects of integrating the process control, process shutdown and emergency shutdown systems.
• Comparative reliability assessments of different control and safety systems.
• A study for specifying emergency shutdown (ESD) system requirements on offshore installations.
• Studies to compare different voting configurations of gas detectors, and to evaluate new detector design.
• Optimisation of the functional testing interval for offshore equipment, considering both safety and maintenance cost.
• A large number of High Integrity Pressure Protection System (HIPPS) reliability studies, for onshore, offshore and subsea applications.
• In a number of SIL verification studies and preparation of Safety Analysis Reports (SAR).
• In reliability analyses of railway signalling systems (i.e. typical high demand systems).