PDS method

To prevent and limit risks, manufacturers need to implement safety barriers. There are various types of safety barriers, such as safety instrumented functions (SIF); implemented by safety instrumented systems (SIS), they have enjoyed growing popularity since the publication of standards governing their design, use, monitoring and maintenance over time (IEC 61508 and 61511 standards). The design of a SIS requires an assessment of the integrity level (SIL) of the RIS to demonstrate that the risks are under control. The aim is to demonstrate that the proposed RIS architectures can indeed achieve the required SIL levels. To do this, it is necessary to calculate the probability of failure for each RIS (PFD _avg for systems in stress mode, or PFH for systems in continuous mode). The PDS method, widely used in the offshore industry (and particularly in Norway), is the answer to this need.

This article presents the principles of the PDS method and highlights the main differences with the approach presented in IEC 61508-6. More specifically:

• it traces the origins of the method;

• it presents the formulas for calculating the probability of failure under stress of a RIS (PFD _avg );

• it compares the PFD values _avg obtained by applying the PDS method with those obtained by applying the formulas in IEC 61508-6.