Elixir Engineering  was awarded to perform Safety Integration and Valve Criticality Analysis for Dhiab Infill Development Project

Project Summary

Dhiab Field is located 35km SW of the Marmul Field, onshore Oman in the South Oman Salt Basin. The field was discovered in 1985 and first oil was produced to surface in 1987. Dhiab structure is essentially a four-way-dip anticline, complicated by significant faulting.

The field is currently produced under water flooding, a mini water flooding experiment started in 2012 to test the response to flank water injection as a mean to increase field recovery, given the overall absence of strong aquifers to support pressure. The 2016 FDP suggested WF 5-spot development as the development mechanism which is implemented in the field since 2017. The main producing intervals in Dhiab are Middle Gharif, Lower Gharif and Al Khlata Formations, with total STOIIP of 16 million Sm3 as per 2016 FDP.

2016 saw the delivery of Dhiab's most recent FDP. Phase II development was covered. The development has a total of 48 2PUD wells. Inverted "9-spot" patterns, which are part of the CR project, and inverted 5-spot patterns with 250 m spacing for 2PUD.

As per June 2021 the cumulative oil production is ~ 1.44 MMm3, with expected developed and undeveloped reserves of ~ 0.54 MMm3 and ~ 0.57 MMm3 respectively. The total STOIIP expected was estimated at ~ 16.96 MMm3, yielding an expected recovery factor of 21% (currently around ~ 15%). The major items covered under this project scope are listed below, Project scope will be executed in 2 phases

Phase 1

  • 3 nos. of 3”x6” MSV, 300# (MSVs will be free issued by PDO).
  • 3 nos. of Coriolis Meters, 3 nos. of Water Cut Meters & 3 nos. of DLQs for Well Testing with one common Provision for Mobile PI Unit.
  • 12” Common header for all the MSVs. Tie-in provision with DBB to be considered for installation of future MSVs (tentatively 3 nos.)
  • 12" CS/PE, 300# 200 m, A/G Jump-Over Pipeline between New and Old Pipeline.
  • 1 No. of Demulsifier Injection Skid at the New MSV facility up-stream of the jump-over line.
  • 1 No. of Full flow RV (1×100%), 1”D2’ at the common Test header.
  • Re-routing of the existing access graded road (500 meters)

Phase 2

  • 14” CS/PE 300#, 20 km, A/G pipeline from Dhiab to Rahab manifold.
  • 1 no. of Bulk Flow Meter (Coriolis Meter) for the gross flow measurement at Rahab end of the new proposed pipeline.
  • Tie-in Provision for Rahab SW on the new loop line along with DBB arrangement.

Elixir Engineering has done the listed safety studies for Dhiab Infill project.

Safety Studies

  • HAC Schedule
  • HAC Layout
  • Escape Route Layout
  • Safety Equipment
  • HFE Verification Report
  • HSE Activity Plan
  • HFE close out report
Hazardous Area Classification (HAC)

The process of Hazardous Area Classification (HAC) involves determining which elements of a facility are dangerous and which are not, as well as creating zones for the hazardous areas. A hazardous region is described as a three-dimensional place where it is reasonable to assume that a flammable atmosphere will exist at frequencies that necessitate particular safety measures for equipment design and construction as well as the management of other possible ignition sources.

 Zone Classification: Zones are created in hazardous regions according to the probability and length of a flammable atmosphere.  

  • Zone 0 : That portion of a dangerous location where combustible air is persistent or prevalent for extended periods of time.
  • Zone 1 : The portion of a dangerous area where, under normal circumstances, flammable atmospheres are likely to occur
  •  Zone 2 : That portion of a dangerous region where the likelihood of a flammable environment occurring during regular operations is low and, in the event that it does, it will only last briefly.

Non-hazardous areas : Areas that do not occupy any of the above.

Source & Grade of release: Any region from which a flammable gas, vapour, or liquid may be discharged into the atmosphere is considered as a source of discharge for the purpose of area segregation. Based on their expected frequency and duration Three release grades are recognised.

  • Continuous grade release : A discharge that happens frequently and at brief intervals, or that is constant or almost continuous.
  • Primary grade release : A release that is probable to happen periodically or occasionally in normal operation i.e. a release that is planned for in operating procedures.
  • Secondary grade release : A release that is unlikely to happen during regular operations and, in any case, will only happen occasionally and briefly

Fluid Categories

A A flammable liquid that would quickly and significantly evaporate upon release. This group consists of:
(a) Any lighter flammable liquid or any liquefied petroleum gas
(b) Any flammable liquid at a temperature high enough to cause more than 40% volume to evaporate upon release when released, with no additional heat input from the environment.
BA combustible liquid that isn't in category A yet is hot enough to boil when released
CA flammable liquid, not in categories A or B, but which can, on release, be at temperature above its flash point, or form a flammable mist or spray.
G(i)A typical methane-rich natural gas.
G(ii)Refinery hydrogen.
Fire & Gas Dispersion Explosion Assessment (FGDEA)

The goal of the FGDEA is to guarantee that the facility layout minimizes the possibility of escalation to the greatest extent that is practically practicable by identifying and evaluating plausible fire and explosion dangers. To evaluate the impact of believable leaks and determine the possibility of escalation, physical effects modeling (PEM) is used in accordance with PDO SP-1258 (Quantitative Risk Assessment Specification). The study evaluates the potential for impact on workers from hazardous and flammable releases as well as the physical impacts of hydrocarbon emissions, as specified by the potential sources of leakage (PSLs). The physical effects modelling carried out as part of the FGDEA will be used to optimize the Dhiab Infill Development Project and to mitigate escalation and achieve an inherently safe plot, as far as practicable, based on PDO SP-1127 & SP-1190 and confirm the suitability of the current layout of the Maintenance drain pit vent pipe based on the requirements in DEP

The objectives of this study is as follows:

  • Identify hazardous inventories handled and processed in the proposed facilities and their operating conditions;
  • Identify all credible hydrocarbon hazardous events (i.e. Jet fire, flash fire, pool fire, flammable gas dispersion, as well as explosion);
  • Assess the consequences of the final outcomes resulting from releases;
  • Assess the toxic impacts with respect to the requirements in SP-1190;
  • Assess the potential impact on adjacent units as well as buildings (if included in project scope), taking into account the location of the potential releases;
  • Provide an indication of potential escalation from the fire and explosion consequences
  • Identify protection / mitigation measures to prevent escalation as appropriate for the phase of development.

The overall study approach is summarised as follows:

  • Develop assumptions;
  • Establish the assessment criteria;
  • Identify potential leak sources and credible hazardous scenarios;
  • Run the Software and calculate the impact radius for accidental ignition and flammable dispersion;
  • Report the results for accidental ignition and flammable dispersion;
  • Analyse the results against assessment criteria;
  • Report the results for each source of leak and respective credible scenarios;
  • Analyse the results against assessment criteria;
  • Conclude if the identified impact is acceptable and if the case recommend additional mitigation's.

Hazard Identification

Event Tree for Process Hazards
Valve Criticality Analysis (VCA)

HFE-VCA's goal is to outline the requirements for applying HFE concepts to valve design and layout, which includes the following:

  • Analyse and then classify the criticality of valves for a specific application.
  • Advice on choosing the right kind of actuator or valve operator.
  • HFE design specifications for valve placement and orientation.

Valve Criticality Rating


Valves are rated by criticality to help ensure that criticality valves are located to provide for rapid and effective identification and operation. The following three categories are recommended. Risk to health and safety—including the possibility of human error—must be maintained to a minimum.

Category-1 (C-1) Critical Valves

Included in the category of valves are those necessary for regular or emergency operations where quick and unhindered access is crucial. The next sections' descriptions of the "preferred" site must be followed in terms of height, reach distances, and visibility.

These valves satisfy any or all of the subsequent requirements:

  1. Valves essential to production.
  2. Valves essential to process safety or asset integrity
  3. Particularly large valves
  4. MOVs that need quick correction and have a high failure rate.
  5. Valves utilized in a service or in operational circumstances where their failure rates are unknown or potentially unstable
  6. Valves where consequence of failure to obtain quick access would be serious (e.g, process shutdown and/or damage to facilities or personnel).
  7. Valves for which more regular routine maintenance, inspections, and/or operations are anticipated than once every six months.

Access Requirement for C-1 Valves

A permanent raised standing platform must be made available for accessibility. If steps are the only feasible means of access to the elevated platform, then access at ground or deck level is permissible.
The identification and state of valves must be easily observable from an approachable operator position, such as on a nearby walkway, access platform, or in the area surrounding equipment meant for human use.

Category-2 (C-2) Non-Critical Valves

Valves are employed in routine maintenance and inspection procedures, but they are not essential for regular or emergency operations. These valves satisfy any or all of the subsequent requirements:

  1. Valves linked with equipment for which urgent intervention is unlikely to be needed.
  2. Valves with a low operating or inspection frequency (i.e., less than once every 6 months).

Access Requirement for C-2 Valves

The "preferred" location, as shown in Figures 2 and 3, for C-2 valves should be the same as for C-1 valves in terms of height, reach, and visibility. C-2 valves may be located within the “acceptable area” as outlined in Figure 3, depending on their size and the force needed to operate them. A vertical fixed ladder and a small standing surface must be provided for access to C-2 valves in cases where ground level access is not justified.If adequate room and access are maintained for workers, tools, components, and equipment in the design, using auxiliary equipment (such as scaffolding, man lifts, or mobile platforms) to obtain access for maintenance reasons may be permitted.The operator may need to temporarily assume an awkward posture or reach areas not meant for human access in order to identify and inspect the state of C-2 valves, as long as doing so does not result in human error or place the operator in danger of harm or exposure to hazards.

Category‐3 (C-3) Non-operational Valves

Typically, valves are non-operating devices that are employed or examined in specific situations only seldom or infrequently (such as hot tap valves, hydro static test vents, high point vents, or low point drain valves situated in pipe racks), and they are not utilized in activities that are crucial to the HSSE.

Access Requirement for C-3 Valves

Although not necessary, constant access to and visibility of C-3 valves is preferred. No specific location requirements are imposed. Auxiliary equipment like as mobile platforms, human lifts, and/or scaffolding that are used to access C-3 valves must be specified and permitted in the design. C-3 valves should not be accessed with portable ladders. Any suggested exemption or exceptions to this will require careful consideration and approval. Height and reach distances to C-3 valves when operated from auxiliary equipment shall confirm to the “preferred” location.

Mounting heights for hand-wheel operated valves with vertical stems
Mounting heights and clearances distances for hand wheel operated valves with vertical stems


  • The hand-wheel centerline is used to measure heights and distances.For gear-operated valves with a hand- The maximum horizontal distance for gear-operated valves with a hand-wheel and a spinner handle is determined by measuring the hand-wheel's edge that is furthest away from the operator.
  • For rising stem valves, the heights must be at the maximum extent of the valve stem.
  • With the exception of reducing the top limit for the "Preferred" choice location by 100mm (4 in) to accommodate male and female populations in regions like West Africa, Southeast Asia, Southern China, parts of Latin America, India, and Japan, these dimensions are appropriate for male and female personnel worldwide, ranging from the fifth to the ninety-fifth percentile.
  • If the valve is less than 455 mm (18 in.), there should be enough space behind the operator, at least 910 mm (36 in. ), in order to facilitate sitting


  • Measuring is done using the hand-wheel centerline for height or distance. For gear-operated valves with a hand-wheel provided with a spinner handle, maximum horizontal distances is measured to the edge of the hand-wheel furthest from the operator.
  • For 5th percentile males, the upper limit should be set at 1755 mm (69 in), and for 5th percentile females, it should be set at 66 in (1675 mm) in regions like Southeast Asia, Southern China, West Africa, and parts of Latin America, India, and Japan. These dimensions are appropriate for personnel worldwide, ranging from the 5th percentile of the female population to the 95th percentile of the male population.
  • For valves located below 455mm (18in), sufficient clearance of at least 910 mm (36in) should be provided behind the operator to accommodate a squatting posture.
Mounting heights for lever operated valves with vertical stems
Mounting heights and clearances distances for lever operated valves with vertical stems
Mounting heights and clearances distances for lever operated valves with horizontal stems

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