Experiences from Setting Safety Integrity Level (SIL) Targets in the Offshore Petroleum Industry
Authors: Hoff Bjorn , Heyerdahl Richard , Schumacher Robert R. 
Published: October 2008

Performance standards for safety functions, expressed as safety integrity levels (SIL), is gaining worldwide acceptance after issue of standards such as the ISA S84.011, IEC 615082 and the draft IEC 615113. Although the concetp of establishing availability requirements and performance standards is not new in the offshore petroleum industries4, the issue of the IEC standards have sparked a renewed interest. In Norway, the effect of the IEC 61508 and the draft IEC 61511 standards has been that operators on the Norwegian Continental Shelf have started analyzing protective systems on new and existing facilities in order to establish an appropriate SIL rating for such systems.

This paper addresses some experiences gained from applying SIL analysis techniques for installations located on the Norwegian continental shelf. The focus is on some practical observations that may benefit personnel that are involved in projects aiming to carry out a SIL analysis.

It is emphasized that the focus of this paper is not on how to demonstrate that a particular system meets a specified SIL level. Rather, focus is on how to establish revelant SIL targets for such systems. For information on how to verify that systems meet the required SIL targets, reference is made to e.g. the IEC 61508 and 61511 standards.

Introduction

The concept of Safety Integrity Levels was first brought to the general attention of the Norwegian offshore industry with the issue of the IEC 61508 standard for functional safety of electrical/electronic/programmable electronic safety related systems. Companies involved with design and production of such systems rather quickly adopted the requirements and principles in the standard. As a result, vendors initiated processes to qualify their equipment for certain SIL-levels. Third party verification and certification of such equipment and systems is also becoming increasingly wide spread. The result of this is clearly seen in marketing material, with a steadily increasing part of brochures being used to highlight that a particular valve, transmitter or control system can form part of e.g. a " SIL 2" safety instrumented system. Table 1 indicates that this SIL requirement would imply having a system with a failure rate of between 1 failure pr. 100 and 1 failure pr. 1000 demands.

A consequence of adopting the new standards is that it will be required to define a Safety Integrity Level target value for instrumented safety functions on e.g. an offshore process plant. Various methods have been presented for how this can be done, most involving a team of engineers and operators to carry out a semi-quantitative review. A commonly used method is the risk-graph approach as described e.g. in the IEC 61511 standards. This method is also presented in guidelines issued by organizations such as UKOOA6(Fig. 1).

Experiences have shown that the results of a risk graph review may in some cases be conservation (i.e. result in a high SIL rating), presenting challeanges with respect to meeting the established SIL targets.