FSA stage 4 on long operating oil and gas asset

MAR Consulting partly produced a functional safety assessment, FSA stage 4 of a long operating oil and gas asset in the Norwegian Continental Shelf (NCS). The main responsibilities were to assess the safety requirements specifications (SRS) for the different safety instrumented functions (SIF) required to operate the asset; and how they have been maintained and applied during the operating lifetime of the facility.

Of special importance to an FSA stage 4 is to understand how functional safety has been maintained during the many years of operations. In general, nowadays, there is special focus and emphasis on functional safety during the design and initial commissioning of a platform, further validated by FSAs stage 1-3. Unfortunately, often times this focus evaporates after the platform is handed to operations. Operations can represent decades in production with hazardous substances present, whereas the design and, specifically, commissioning takes only months with limited hazardous substances introduced in the process.

Depending on several factors, the probability of failure on demand (PFD) increases with time, reducing the safety integrity level (SIL), as shown in Fig. 1 below. Some of the factors are:

  • the periodicity of proof tests done on the equipment composing the SIFs;
  • competency and knowledge of personnel performing the proof tests;
  • age of the equipment.

These factors greatly affect the proof test coverage (PTC). A so-called “perfect” PTC of 100% means that the equipment is restored to the original state after the proof test is performed. Opposingly, PTC of 0% is similar to not performing a proof test at all. The illustration below (Fig. 1) shows the effect of PTC on the PFD over time.

PFD vs. Time
Fig. 1: Probability of failure on demand (PFD) vs Time; considering different proof test coverage (PTC) levels.

In summary, operational assessments (such as FSA stage 4, periodic reliability assessments, etc …) are vital to ensure the functional safety levels estimated during design are still maintained. In other words, to validate that assumptions made during the design phase are still valid; and that the risk is sustained within the acceptable levels defined by the stakeholders.

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