Objectives/Scope: Subsea high-integrity pressure protection systems (HIPPS) have become a standard feature in numerous subsea fields around the globe. Subsea HIPPS' reduced pipeline costs are proven to shorten delivery times for pipelines by decreasing their pressure rating in green field projects or by including high-pressure wells into brownfield projects without modifying rating in existing pipelines.

New frontiers for HIPPS are created by high-pressure (7.5 to 20 kpi) and high-temperature (100° to 200°C) applications, as current pressure transmitters, valves and actuators reach their limits. Furthermore, each HIPPS challenges engineering to achieve a compromise between functional safety and reliability - especially in demanding HPHT fields. These opposing characteristics within the protection systems cannot be treated separately but have to be evaluated as two closely related aspects of engineering.

This paper covers the balancing of these two engineering areas by using the numerical evaluation of probability of failure on demand (PFD) and failure rate (FR) figures. The evaluation is based on safety integrity level 3 (SIL 3) low-demand HIPPS, for which relevant subcomponents and system failure behaviours were simulated. Different HIPPS architectures were applied based on various process sensors, logic solver and final element quantities and their arrangements. The objective of this paper is to investigate the potential improvement of HIPPS design with low PFD and high mean time to failure (MTTF).

Methods, Procedures, Process: Reliability methodologies are based on reliability block diagrams in accordance to IEC 61508 and ISO 12489. Current state of the art safety equipment generic databases for subsea equipment are used to reflect the sub-system's section of HIPPS loops with regards to their functional safety and reliability capabilities.

Results, Observations, Conclusions: This paper indicates optimal and suboptimal architectural configurations and their tradeoff regarding functional safety and reliability.

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