Linking HAZOP and SIL for Well Intervention & Workover Systems

The ORS approach to linking HAZOP and SIL

Hazard and Operability (HAZOP) is a hazard analysis method where you identify hazards and operability issues through a multi-discipline workshop by use of parameters and guidewords. Based on the output, actions for risk mitigation are recommended where applicable. A standard method for application of process or operability-related parameters and guidewords may not cover all aspects for equipment (rather than process) driven operations, such as for well intervention or workover. Hence, you should apply an approach utilizing a combination of accidental scenarios based on historical data and process/equipment/operability-related parameters/guidewords. ORS proposes a simplified approach for linking a customized HAZOP with LOPA, SIL classification, and verification of workover/well intervention systems.

Step 1: Identification of risk

Firstly, apply a HAZOP methodology customized for well intervention/workover systems. Focus on:

• Barrier status and shutdown sequences for various operational modes – Identification of the minimum list of main components needed to prevent applicable accidental scenarios

• Semi-quantitative approach to demand rate (Combine experience and specific well data)

• Interfaces to auxiliary systems due to use of fail-as-is (rather than fail-safe) safety functions

• Consideration of contingencies as Independent Protection Layers (IPL) based on the available time between incident occurrence/detection and realisation of hazard

Step 2: PFD Requirements for Well Intervention Safety Functions

• Based on information from the customized HAZOP, perform a Layers of Protection Analysis (LOPA) for the Safety Function(s) in question

• The LOPA is based on the Safety Functions in the various operational modes. For each operational mode, evaluate the demand rate, barrier status, and contingencies (from the HAZOP)

• Quantify risk reduction from contingencies and applicable barriers that can be justified as IPLs, with a special focus on operator intervention

• Operator intervention is assessed by including TESEO tables for operator responses to various hazardous events

• Utilize available data to estimate the required PFD level for each operational mode

Step 3: Validation of SIFs

By combining detailed knowledge of the system and the identified minimum list of safeguards from the HAZOP, a Reliability Block Diagram (RBD) is developed for each SIF. In this case, you should give special attention to the following:

• Definition of operating envelope for SIF (Limitations for SIF applicability)

• Common components between protection layers, including the SIF in question

• Components in auxiliary systems needed to achieve safe state, e.g., hydraulic system, close assist system, etc.

• Contingencies not already listed as IPLs may provide redundancy

Based on the RBDs, you can perform a Fault Tree (FT) analysis to demonstrate SIL compliance regarding PFD requirements. In this case, you should carefully evaluate common cause failures (CCF). A separate assessment of hierarchical and software requirements should be performed as part of the SIL validation process.

Step 4: Monitoring of SIF Performance during Operation

Test and maintenance procedures for well intervention systems ensure typically loop testing of the SIFs, i.e., end-to-end tests. Normally, you perform function tests prior to operations, excluding the cutting capability. You need to reflect on this in the SIL verification process. Since SIL verification is dependent on the actual oil/gas well, you should also update the SIF performance monitoring accordingly during operations to validate the design assumptions during operations. This is because the demand rate for the SIF may vary depending on the location for the well intervention or workover operation. There should be a sufficient margin between the required and achieved PFD to prevent SIF re-design or excessive testing on high-demand wells.

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Lastly, do you have any comments that you would like to share? Or perhaps you would like more information about any of the steps mentioned above? Feel free to contact us.