Though the fundamental principles of plant layout such as safety, operability, maintainability, constructability, compliance to process requirements and provision for future expansion apply to offshore plant layout as well, there are profound differences in how these principles are applied to offshore plant facilities. Offshore facilities layout are governed by many other factors such as helicopter approach, pipeline approach, jack-up rigs, drilling rigs, supply boat operations, installation barge and maintenance barge access, vessel mooring requirements etc. which are not applicable to onshore plant layout.
Offshore modules invariably tend to be modularized as against onshore plant layouts which are by and large stick built. The modules which are installed on the jacket are called the topsides. In offshore industry, weight control of the topside module is very important as it governs the barge or vessel which will be deployed to install the jacket. Additionally, the overall height of the topsides is also required to comply with the installation barge requirements. Depending on the lift capacity of the barge the topside module may be a single lift or split into more than one module.
Primary Considerations for Topsides Layout
The key criteria for topsides layout are summarized in the figure below. The ultimate goal of any layout development is to minimize the risk in achieving the projective objectives and minimizing the risk to the facilities throughout its operating life. The design development process should incorporate the hazard evaluation and risk mangagement process to achieve this objective. The layout development requires involvement of all the relevant engineering disciplines and concerned stakeholders including safety, integrity, operations, maintenance and construction divisions.
Orientation of Offshore Facility
Final orientation of an offshore facility is largely governed by the prevailing wind direction, sea current characteristics, pipeline approach to the facility, service vessels, jack-up rigs, construction barges and drilling rig limitations. The orientation of the facility should permit natural ventilation and dispersion of flammable vapours away from the low hazard areas. Low hazard areas such as Living Quarters should be located upwind or crosswind of the high hazard facilities such as process areas so that any hydrocarbon leaks from the process facilities are directed away from the living quarters due to the prevailing wind direction. Similarly any hydrocarbon pools due to leaks or spillage should be directed away from the safe areas and the facilities by prevailing water current direction.
Approach of supply vessels and maintenance barges should be against the prevailing wind direction and sea current. Similarly, the approach and takeoff orientations of helicopters are aligned as close as possible into the prevailing winds and govern the orientation of Helideck. To minimise the potential of vessel impact on the facility, the boat fender should be located such that the vessel approach to the platform is against or across the prevailing wind direction and sea current. The approach of vessels and helicopters to the platform have better control when done against or perpendicular to the wind or current direction. It is important to verify the details of supply boats that will be used to ensure that the mast of the supply boat does not collide with platform structure. The risers should not be located in the boat fender area. The riser fender should preferably be located on the face opposite or at right angles to the boat fender. Since risers are a source of significant amount of hydrocarbons, they should be located such that prevailing wind direction or water current will disperse the hydrocarbon gases and oil spills respectively away from the process facilities. The lowest deck level should be located such that it clears the design wave (usually the 100 year wave crest) height plus a minimum air gap of 1.5m.
Very often, the various requirements conflict with each other and the selected layout is the best compromise which poses a minimum risk to the facilities. Typical sketch of a wellhead platform is shown below.
Safety
Safety is one of the key drivers of offshore plant layout. Since offshore facilities tend to be confined, the effect of fire and explosion in one unit can have a significant impact on the overall safety of the facility. Due to limitations in the footprint, offshore facilities cannot always afford the luxury of maintaining separation distances which are possible in onshore facilities. Often the layout should include the necessary mitigations in place to limit escalation and counter the consequences of fire and explosion hazard. Examples of such a mitigation are the provision of fire walls or blast walls to protect buildings in low hazard area. The inherently safer design approach eliminates reliance on complex safety systems, operating and emergency procedures and instead relies on implemenation of design to reduce the residual risk to the plant facilities.
Safety can be achieved by
- minimizing hydrocarbon and hazardous material inventory
- minimizing loss of containment
- minimizing escalation in the event of hydrocarbon leak, fire or explosion
- minimizing the probability of ignition
- providing safe means of escape and emergency evacuation
- providing active and passive fire protection
Minimizing Hydrocarbon and Hazardous Material Inventory
Process design should limit the hydrocarbon inventory in the facility. The storage of hazardous material on the platform should be limited to the extent possible for safe operation. Piping lengths should be optimized during design. Pipelines are a source of significant amount of hydrocarbon inventory. Hence riser ESDVs should preferably be located at the lowest level of the platform to isolate the pipeline inventory from topsides.
Limiting Escalation
One of the approaches to limiting escalation is segregation of high and low hazard areas and locating the low hazard areas upstream of the high hazard areas with respect to the prevailing wind direction and sea current. High hazard areas comprise of facilities which contain hydrocarbon and are a potential source of fire and explosion hazard. On the other hand, low hazard areas comprise of facility which have limited sources of fire and explosion hazard. Emergency utility systems such as fire water pumps should be located in low hazard areas and should be separated from high hazard areas to ensure their availability for an emergency event. Figure below identifies some of the high hazard and low hazard areas in an offshore facility.
Minimizing Ignition Potential
By separating the source of ignition from the hydrocarbon it is possible to minimize the ignition potential. Accordingly, adequate separation shall be maintained between systems containing flammable hydrocarbons and potential ignition sources. Ignition sources such as generators, flare tips should be installed away from hazardous areas.
Flares are located downwind or crosswind of the safe areas and process facilities. When located downwind, an ignited flare can be a potential source of ignition for hydrocarbon leaks from the upwind process facilities. This risk can be eliminated by locating the flare crosswind of the safe areas and process facilities.
Hazardous Area Drawings
Hazardous areas are those where there exists a risk to the facilities due to probability of fire or explosion due to presence of flammable gases or vapors and source of ignition. The probability of explosion is governed by the hazard triangle comprising of fuel, oxygen and ignition source as its sides. Hazardous area classification drawings are prepared to define the hazardous areas according to the probability of occurrence of an explosive gas / air mixture in order to:
- Ensure that sources of ignition are segregated from sources of flammable gas.
- To establish a basis for the correct selection of instruments and electrical equipment to be used in these areas.
The three classifications as per API RP 505 are:
Zone 0: Explosive mixture is present continuously (continuous hazard).
Zone 1: Explosive mixture is present during normal operation (intermittent hazard).
Zone 2: Explosive mixture is not present during normal plant operation (low hazard - exists only during abnormal plant operation).
Minimizing Loss of Containment
Loss of containment can be minimized by reducing the possiblity of mechanical damage to equipment and pipelines that contain hydrocarbons or other fluids that can cause harm to personnel and environment. Layout shall ensure that there is no possiblity of damage to equipments or pipelines while handling items from the barge to the platform, or during mechanical handling of items within the platform up to the laydown areas. A dropped object protection study should be carried out to identify all possible scenarios and ensure that the necessary mitigations are in place to protect the equipment and pipelines from physical damage. An example of mitigation for pipeline protection from dropped objects is the use of concrete mattresses.
Safe means of Escape and Evacuation
Plant layout should ensure safe means of escape and evacuation of personnel in the event of an emergency. An Escape, Evacuation and Rescue Assessment (EERA) study should be carried out on every offshore facility to ensure that efficient escape, evacuation and rescue of plant personnel is possible in the event of an emergency. On every deck (platform) level, at least two escape routes shall be provided which lead to the stairways. Width of primary escape routes leading to the stairways may vary between 1200-1500 mm depending on platform specifics. Clear headroom of 2200mm is required throughout the escape routes. Width of secondary escape routes from the working areas near the equipment to the primary escape route may vary between 800-1000mm depending on project specification. Stairways should be located as far as practically possible (preferably on diagonally opposite ends of the platform) and should be located on the exterior of deck perimeter. The helideck is considered as a primary means of evacuation and the lifeboat is considered as the secondary means.
During design development, it shall be ensured that instrument boxes, valve actuators including handles and spindles do not protrude into the escape routes. Designers often fail to recognize that offshore safety equipments are located on the periphery of the deck. These safety equipments protrude into the escape routes located on the periphery and compromise the minimum required escape route width. This is often discovered at the construction stage and stands out as one of the major punch list items. Recognising this aspect at an early stage will ensure compliance to the safety requirements.
Muster Areas and Temporary Refuge
All offshore facilities should have provision for a muster area and/or temporary refuge where personnel can gather in the event of a fire or explosion or hydrocarbon release for further instructions until the emergency situation is bought under control. If there is an escalation of the emergency situation, the further instruction to plant personnel could be the evacuation and abondonment of the facilities by helicopter or life-boat/life-raft. Hence it is important that the muster area/temporary refuge is located in a safe area and the escape routes from the muster area/temporary refuge up to the evacuation areas are safe for personnel movement and further evacuation. As against muster areas which are in an open space, the temporary refuge is an enclosed space. The prefix temporary implies that it has a limited endurance period. The temporary refuge should be designed to provide protection for the defined endurance period against fires, explosions and hydrocarbon releases. Minimum endurance period is specified as one hour but more time may be specified depending on the time required to evacuate the facilities.
Operability
The layout of the offshore facility should ensure safe and unhindered access for the following operations:
- Access to frequently operated manual valves.
- Access to take field mounted instrument readings.
- Access to emergency shutdown valves and emergency isolation valves.
- Access to pigging operations, sampling operations, process vents and drains.
- Access to rotating machinery such as pumps, compressors and air coolers which require routine monitoring and inspection.
This may require platforms to be permanently installed for operational and inspection access.
Plating or Grating
Use of grating in offshore facility will promote natural ventilation and minimize the accumulation of vapours and potential overpressure in the event of a explosion or blast. Use of grating also prevents accumulation of hydrocarbon liquid pools and hence minimizes the probability of pool fires. However use of grating makes the equipments located on the higher elevation more susceptible to damage caused by fire at lower level. Use of plating will serve to contain the spills on a particular deck level and can also provide protection against flame impingement to equipments located above due to fire at lower level.
Maintainability
The spacing between the equipments shall allow maintenance of valves, equipment and machinery to be carried out efficiently. Since layout on an offshore facility tend to be congested, maintenance and removal of equipment can be difficult or impossible if the necessary provisions are not made during each design stage. Provision of appropriate mechanical handling equipments, maintenance routes and laydown areas shall be made in the layout. Mechanical handling of maintainable items of equipment involves primarily two operations: the lifting (raising and lowering) operation and the horizontal movement of the item from the individual laydown area to the main laydown area at that level. Individual laydown areas at each level should connect through a maintenance route to the main laydown area. Monorail beams, trolley hoists and pad eyes shall be provided for safe removal of equipment and placing them on trolleys or roller skates. Horizontal movement of items through use of monorails is preferred over transporting them by trolleys or roller skates. Mechanical Handling Study should address every aspect of maintenance requirements in sufficient detail. Mechanical handing requirements may often dictate the spacing between two deck levels. Hence it is important to consider the mechanical handling aspect in sufficient details while finalizing the layouts. The subject of mechanical handling on offshore platforms is exhaustive and will be dealt with in a separate article.
Laydown Areas
Laydown areas should not overlap the escape routes. Laydown area should be structurally designed for the heaviest item of equipment that will be placed in the area. Cranes shall be located such that they are be able to transfer the materials from the supply boat to the laydown area and vice versa from laydown area to the boat. The main laydown areas on each level of the platform should be located on the periphery and staggered to allow the crane unhindered access for handling of material at each level. To prevent blind lifts, the staggering should be preferably achieved in such a manner that the laydown area is in the direct line of sight of the crane operator. Mechanical handling routes from the individual laydown areas should terminate into the main laydown area on that deck level. Since the laydown area will be subject to the lifting operation, consideration should be given to designing the laydown areas for impact loads.
Living Quarters
Living Quarters (LQ) on an offshore facility include the accomodation for personnel, dining and recreation faciities and administration offices. A living quarter is considered as a low hazard area and should be located as far as practically possible, upwind or crosswind of the process units or hydrocarbon facilities including wellhead areas, gas compression units, flares, pig trap areas, pipelines etc. The location should ensure that any gas leaks or hydrocarbon spills are carried away from the living quarter due to the prevailing winds or sea currents. The location should also ensure minimum impact due to fire or explosion in hydrocarbon containing units. Consideration should be given to locating the living quarter on a separate platform and connected by a bridge to the process areas. Machinery with heavy noise or vibration should not be located on the living quarters. In many facilities the helideck is located on the living quarter platform or close to the living quarter. This provides the advantage of efficient evacuation of personnel in the event of an emergency.
Living Quarter should be under positive pressure and its air intake should be from a safe location. Risers or ESD valves and large inventory of hydrocarbons should not be located below the living quarters.
Helideck
The location of helideck shall be such that it provides unobstructed and safe access for helicopter approach, landing and take-off against the prevailing wind direction. The helideck should comply with local aviation regulations. It is recommended that the location and layout of helideck be reviewed and approved by the local aviation regulation authorities at an early stage of the project.
Helideck design requirements are discussed in detail in the following article : Offshore - Helideck Design Guidelines
Pigging Facilities
Pigging facilities should be preferably grouped together in one area with the end closures facing the sea. This avoids any accidental projectile from the pig trap hitting and damaging the process faciities. This allows the mechanical handling facilities to be optimized for all the pig traps.
Piping Requirements
API RP 14E provides guidance on design and installation of offshore production platform piping systems. However it does not cover plant layout related requirements. This section provides some guidance on piping requirements which may have an impact on the overall facility layout and should be given due consideration during the layout development phase of the project. The important considerations are:
- Space for expansion loops depending on the stress analysis findings. The provision for expansion loops on connecting bridges to handle the differential movement between the two platforms should be made at an early stage as the placement of loops impacts the structural design of the bridge.
- Lines with no pocket requirements such as Flare lines to the flare tower and drain headers to the drain drum should be studied at an early stage as the routing of these lines may impact the spacing between the deck levels.
- Piping layout should take into consideration the minimum headroom of 2200mm or as stipulated in the project specification.
- Orifice meter runs should be studied at an early stage of the layout due to the minimum upstream and downstream straight length requirements.
- Piping containing hazardous fluids should not be routed through the utility areas and other non-hazardous areas.
- Adequate provision for future piping should be made in the layout. The future piping volumes should be preferably shown in the plant layout or 3D Model so that they are not inadvertently occupied by other piping as layout development progresses.
- Constructability of future equipments and piping should be addressed during design including constraints and proposed mitigation.
- Crevice corrosion is a major concern on offshore platforms and many Company standards tend to use shoe supports for lines two inches and above. The required support design should be taken into consideration so that any additional piping elevation requirements do not compromise the required minimum headroom during design development.
- Ensure that piping is not routed over pumps and other rotating equipments which may obstruct mechanical handing or maintenance requirements. Also ensure that piping layout allows sufficient room for operational access.
Studies and Assessments
- Plot Plan Review
- Quantitative Risk Assessment
- HAZID Analysis
- Fire and Blast Analysis
- Heat Radiation Analysis
- Constructability Study
- Mechanical Handling Study
- Escape, Evacuation and Rescue Assessment (EERA) Study
- Dropped Object Protection Study
Reference Industry Codes and Standards
- CAP 437: Standards for offshore helicopter landing areas.
- ICAO Annex 14: Aerodromes - Volume I - Aerodromes Design and Operations. ICAO stands for International Civil Aviation Organization.
- API RP 2A: Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms.
- API SPEC 2C: Specification for Offshore Pedestal-mounted Cranes.
- API RP 14E: Recommended Practice for Design and Installation of Offshore Production Platform Piping Systems.
- APIR RP 14J: Recommended Practice for Design and Hazard Analysis for Offshore Production Facilities.
- SHELL DEP 80.00.10.12: Layout of Offshore Facilities.
- BP GN 44-100: BP Group Guidance on Offshore Facilities Layout.