SIL Study for reuse and recycle treated QG1 and PMP wastewater streams of Qatar Gas

Waste water streams - qatar gas

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

The classification had followed a Qualitative approach for assessing initiating event frequency and consequence severity to determine the required SIL level for a particular SIF.

The primary purpose of the SIL Classification study was to determine if there were sufficient layers of protection against a hazardous scenario, typically arising from process deviations. As part of the study, the reliability of the SIF was determined, or alternatively further IPL(s) might be recommended in order to meet the tolerable risk threshold criteria. To know more on SIL click Here.

The first step in the SIL Classification was the correct identification of the SIF. These were identified through a review of all P&ID and Cause & Effect Diagram and as an output of the HAZOP. A SIF consists of one or more initiators, the logic solver and one or more final elements. The initiators are the sensors that detect hazardous situation. The final elements are the actions that will be taken by the SIS to bring the process to a safe state on detection of the hazardous situation by the sensors.

After a SIF was identified, its design intent was also defined. Each SIF was given a unique design intent to prevent one hazardous event. In the event that more than one design intent was identified for one SIF, these had been segregated and assessed independently.

Based on the design intent of each SIF, the primary final elements were defined. Primary final elements were selected in such a way that they provide direct prevention of the hazardous situation (e.g. stopping inlet flow to prevent high level, cutting heat source to prevent high temperature) with minimum number of actions required.

Occasionally, multiple actions were required to prevent the hazardous situation, and success criteria needed to be defined as necessary to indicate the way the SIF should function to achieve the desired design intent. This took form of “M out of N” (MooN) actions required to achieve successful operation of the SIF.

Final elements that were not identified as primary elements, have been considered as non-essential for achieving the minimum process safety required, but provide added protection and / or to facilitate restart after equipment or unit shutdown. These actions were not included as part of the SIF and had not been subjected to any SIL requirement but retained in the IPS without additional reliability requirements.

Once the SIF was properly defined, its demand Likelihood and consequence of failure were identified in order to facilitate assessment of required SIL level. Demand scenarios were situations which resulted in a demand on the SIF. These were typically process deviation caused by malfunction of equipment or improper operation of field components and systems.

The Consequence of Failure on Demand (CoFoD) was the potential consequence arising from the demand scenario in the event of the SIF failing to perform its desired action. It was noted that in defining the consequence, no other safeguards were considered to function in order to enable assessment of the worst-case consequence. For both Demand Scenario and CoFoD, the HAZOP report was referred to as input. These were reviewed and validated at the workshop as a brainstorming activity, with additional input added as necessary.

When the SIF was identified, the Likelihood assessment had taken place. Once the Likelihood assessment was conducted, the consequence assessment was done. The consequence assessment considers the magnitude of potential consequence on safety, environment and financial damage, ranking from Class 1 to Class 5 (Class 5 being the most severe). Once the consequence assessment was conducted, the results were fed into the Qatar gas SIL Assessment Matrix based Software to determine the required SIL level, ranging from Tolerable SIL 1 to SIL 3 (Unacceptable Risk).

After the initial SIL Classification had been done, an additional assessment was performed to identify any Independent Protection Layers (IPLs) which provided additional risk reduction apart from the SIF being studied. If any suitable IPL were identified then, credits were given to reduce the required SIL target for the SIF being studied.

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:

SIL assessment &  verification for feed.Table showing the risk reduction factor metrixes

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.