The objective of the project is to carry out Comprehensive study and develop FEED (Front End Engineering Design) work of the proposed facilities with modifications to the original FEED already performed including associated systems and develop the (Engineering, Procurement and Construction) EPC package to enable to further proceed with EPC. QGWP objective is to reuse and recycle treated QG1 and PMP wastewater streams in compliance with environmental regulations set by the MME.
SIL Classification
Key Objectives of the SIL Study
- Ensure Process Safety: Through the SIL study, we assess and verify safety systems to mitigate risks associated with treated wastewater reuse.
- Optimize Environmental Impact: The project aims to enhance sustainable water management practices, reducing Qatar Gas’s overall environmental footprint.
- Increase Reliability of Safety Systems: By identifying and addressing potential failures, our SIL study ensures that safety systems perform effectively under various operational conditions.
- Qualitative Classification Approach
- The classification followed a qualitative approach for assessing initiating event frequency and consequence severity to determine the required SIL level for a particular Safety Instrumented Function (SIF).
- Purpose of SIL Classification Study
- The primary purpose was to assess if sufficient protection layers were present against hazardous scenarios, often caused by process deviations. Reliability of the SIF was analyzed, with additional Independent Protection Layers (IPLs) recommended to meet tolerable risk threshold criteria. [To know more on SIL, click here.]
- Identification of SIFs
- The first step in the SIL Classification was correctly identifying each SIF through a review of P&IDs, Cause & Effect Diagrams, and HAZOP outputs. A SIF includes initiators (sensors for hazardous situations), a logic solver, and final elements (actions for safe process state).
- Defining SIF Design Intent
- After identifying each SIF, its design intent was defined with a unique purpose to prevent specific hazardous events. Multiple design intents for one SIF were assessed independently.
- Primary Final Elements Selection
- Primary final elements were selected to directly prevent hazardous situations, with minimum actions required (e.g., stopping inlet flow or cutting a heat source).
- Multiple Action Success Criteria
- When multiple actions were necessary, success criteria were defined, using an “M out of N” (MooN) format to specify how the SIF should function to meet the design intent.
- Non-Essential Final Elements
- Non-primary elements provided additional protection or supported restart after shutdown but were not essential for minimum process safety and thus excluded from SIL requirements.
- Demand Likelihood and Failure Consequence Assessment
- After defining the SIF, its demand likelihood and failure consequence were assessed to determine the required SIL level. Demand scenarios included process deviations caused by equipment malfunctions or improper field component operations.
- Consequence of Failure on Demand (CoFoD)
- CoFoD represents the potential consequence if the SIF fails to perform its intended action during a demand scenario. In defining CoFoD, no other safeguards were assumed to function, allowing for a worst-case consequence assessment.
- HAZOP as Input for Demand Scenario and CoFoD
- Both the Demand Scenario and CoFoD were derived from the HAZOP report, which was reviewed and validated during a workshop through brainstorming, with additional input added as necessary.
- Likelihood and Consequence Assessment
- After identifying the SIF, a Likelihood assessment was conducted, followed by a Consequence assessment. This assessment evaluated the potential impact on safety, environment, and financial damage, ranking from Class 1 to Class 5 (Class 5 being the most severe).
- SIL Level Determination Using Qatar Gas SIL Matrix Software
- Once the consequence assessment was completed, results were entered into Qatar Gas’s SIL Assessment Matrix-based software to determine the required SIL level, categorized from Tolerable SIL 1 to SIL 3 (Unacceptable Risk).
- Assessment of Independent Protection Layers (IPLs)
- After the initial SIL Classification, an additional assessment identified any Independent Protection Layers (IPLs) that could provide further risk reduction beyond the SIF. If suitable IPLs were identified, credits were applied to lower the required SIL target for the studied SIF.
However, for an IPL to be considered valid, it was subjected to the following criteria:
- Specificity: The IPL must be specifically designed to prevent the hazard in question
- Independence: The IPL must be able to function separately from any other functions, and shall not be subject to common cause failure arising from any demand scenario (e.g. BPCS control failure).
- Dependability: The IPL must be suitable in a way such that it can be relied upon to perform the specified actions required to prevent the occurrence of demand scenario.
- Auditability: The IPL must be provided with suitable provision for regular inspection and testing to ensure its functionality.
Examples of system or devices suitable for use as IPL, that meet the above set of criteria included the:
- BPCS control, when the controlling function is independent from source of failure;
- Process alarms, when there is sufficient time for operator response;
- Mechanical devices, such as PSV, rupture disks.
The final SIL target was assigned based on the initial SIL assessment, with consideration of any IPLs identified which provided additional reduction to the initial SIL level. This final SIL level was converted to reliability requirements, expressed in terms of average probability of failure on demand (PFDavg), and subject to SIL Verification. For corresponding PFDavg target for each SIL target, refer table below.
Relationship between SIL, PFD and RRF values has been established in Table:
The SIL study / assessment was performed using Risk Matrix Method. This was used for evaluating the SIL of a Safety Instrumented Function and there by establishing the SIL of a Safety Instrumented System (SIS), developed from knowledge of the risk factors associated with the Equipment Under Control (EUC) and the EUC control system. Risk Graph Method used the information provided in the P&IDs, HAZOP Report, Cause & Effect diagrams and other documents.
Conclusion
The SIL Classification study for Qatar Gas’s wastewater reuse and recycling project has successfully identified, evaluated, and classified the necessary Safety Instrumented Functions (SIFs) to manage potential risks in line with industry standards. By employing a qualitative approach to assess the likelihood and severity of potential demand scenarios, the study determined appropriate SIL levels to ensure robust protection against hazardous events. The assessment also incorporated Independent Protection Layers (IPLs) where applicable, reducing SIL requirements and enhancing overall system reliability. Through this comprehensive analysis, Qatar Gas can confidently implement effective safety measures that safeguard personnel, protect the environment, and support operational continuity, reinforcing its commitment to safety and sustainable practices.
