Design and verification of a SIL-rated shutdown function for a subsea power grid

Abstract

The offshore oil and gas industry is moving towards a direction where more and more process equipment are being placed subsea. Accordingly, various electrical equipment, including pumps, separators, gas compressors, water injectors, etc., are required to drive the machinery loads subsea. The ultra-deepwater and long step-out situation is extremely harsh for the electrical equipment. It is common now to find that a Hazard and Risk Assessment study has identified a requirement to stop a particular electric motor driven machine to prevent or mitigate a hazard, e.g. overpressure. This function could be a part of an independent protection layer or a safety instrumented function. A process shut down function (PSD) constitutes such an independent protection layer. PSD system handles abnormal situations and shuts the plant down if the dedicated process control system is unable to maintain control within predefined limits.

This master thesis was performed in collaboration with Siemens AS, which are currently developing a subsea power distribution system called the Siemens Subsea Power Grid. The Subsea Power Grid includes the shutdown of a large electric motor as part of a safety instrumented function. The associated safety instrumented system faces demanding reliability targets that are expressed in terms of safety integrity level (SIL). Documenting that the SIL requirements have been met for new installations is often a challenging task.

The main objective of this master thesis has been to introduce a process shutdown system integrated in the Siemens Subsea Power Grid and to verify that the proposed PSD function reaches the required safety integrity level. This is achieved through the following scope of work: 1. Define and describe a case with a Siemens Subsea Power Grid driving a subsea process that requires a SIL 2-rated PSD function 2. Describe a solution on how PSD functionality can be implemented in the Siemens Subsea Power Grid 3. Describe the reliability-related requirements to a SIL 2-rated PSD function 4. Investigate common quantitative analysis methods to calculate probability of failure on demand 5. Discuss availability of relevant reliability data for the SPG case 6. Perform necessary analyses and calculations to verify a SIL 2-rated PSD function 7. Analyze the impact on the reliability and availability of the Siemens Subsea Power Grid after the introduction of a SIL 2-rated PSD function.

Reliability block diagram and fault tree analysis were selected as the methods to calculate probabilities of the PSD system failure if the demand to function occurs. These methods are mature and well proven and gave similar results. The analysis verified that the proposed PSD system fulfills the stated requirements and can be a part of a SIL-2 safety loop.

Results may serve as a guideline for the Subsea Power Grid system designers in planning technology qualification programs.