Safety Instrumented Systems: Reconciling the Traditional Prescriptive Approach in API RP 14C With More Recent Risk-Based Approach Methods in IEC 61511
Authors: Currie Mervyn R. 
Published: December 2007

Abstract
Safety instrumented systems for offshore production platforms have traditionally been specified using API RP 14C. Its prescriptive approach does not sit well alongside the more recent risk-based approach in international standard IEC 61511. This later standard requires the safety integrity of these protective systems to be commensurate with the risks they are protecting against, a concept that is not addressed in API RP 14C. A combination of both approaches is proposed to provide assured safety performance, reduced spurious trips, and minimum cost of testing during their operational life.

Introduction
Safety instrumented systems (SIS) prevent hazards on offshore installations and onshore processing facilities initiated by excursions of operating parameters such as pressure and level outside operating limits. They act by detecting the excursions and automatically taking actions to keep within design limits. Such actions typically involve shutting off inflow and may relieve pressure to the flare system. Traditional offshore practice is defined in API RP 14C1 and this has served the industry well over many decades. The practice requires a systematic assessment of process components to determine their safety requirements in accordance with specified rules.
More recent approaches to facilities design use risk-based considerations in conjunction with traditional prescriptive methods. In 2000 the International Electrotechnical Commission (IEC) published IEC 615082. This is a generic standard concerning use of electrical, electronic and programmable electronic systems in safety applications. International standard IEC 615113, published in 2003, applies the principles of IEC 61508 to applications of safety instrumented systems in the process industries.
The approaches in API RP 14C and IEC 61511 to the specification of safety instrumented systems on offshore installations are fundamentally inconsistent. Both approaches will be examined. A means of combining the best of both is offered as a way forward for the offshore oil and gas industry.

API RP 14C
This API recommended practice 14C advocates a structured safety analysis process where a range of undesirable events is addressed for each process component of the offshore installation. Undesirable events include overpressure, leak, liquid overflow, and gas blowby among others. For each type of undesirable event the cause, effect, and detectable abnormal condition are defined. Means of primary and secondary protection are specified in accordance with the rules in Appendix B along with the most effective location of safety devices. Analysis of these events and protective measures is carried out for each process component such as wellhead and flow line, pressure vessel, pump, compressor, etc and is documented using safety analysis checklists (SAC) and safety analysis tables (SAT). The rationale for including or excluding each safety device is documented. The functional requirements relating sensing devices to final elements are documented on the safety analysis function evaluation (SAFE) chart. The SAFE chart includes a summary of primary and secondary protection devices and cross-references to the SAC.
Appendix C specifies how manually initiated emergency shutdown and fire and gas detection facilities should be specified. Appendix D addresses testing and reporting requirements. It recommends monthly test intervals for high and low pressure and level sensors unless they are electronic transmitters connected to programmable electronic systems when quarterly is recommended. However alternative test intervals may be specified, up to a maximum of one year, based on statistical analysis of test results and a monitoring program "to establish acceptance criteria" all subject to "the alternative test interval should ensure device reliability equal to or greater than the reliability demonstrated by the recommended/required test frequency". The author is not aware of any cases where these provisions for alternative test intervals are in use and justified under these API RP 14C requirements. There may be cases where provision of primary and secondary protection to API RP 14C does not provide sufficient protection judged on a quantified risk basis.

IEC 61511 Requirements
IEC 61511, like IEC 61508, advocates a lifecycle approach to the specification and use of safety instrumented systems (SIS).