The chapter on facilities layout was mainly applicable to land facilities. The Layout of Off-Shore facilities, although following the same principles, differs in a number of ways as described below.
Safety considerations remain the main driver: high risk units are segregated from manned areas, such as living quarters and control rooms, and vital units, such as electric power generation. This is usually achieved by locating the utility units between the two.
Provision of redundant escape ways throughout the facilities, to ensure safe escape of personnel in an emergency, becomes more demanding on the layout, due to the confined environment. Making provision the safe evacuation of the personnel is purely Off-Shore specific. It will entail selection of a suitable muster area, in a safe location, and provision of evacuation vessels, such as a life boats. As personnel will remain exposed to the hazard until they have evacuated the facilities, protection of the muster area against fire and explosion must be provided, e.g., by means of blast/fire wall etc.
For what regards the position of the various equipment, the same principles are applied as in the case of land facilities, such as locating high risks units away from each other. Similarly, critical equipment such as electrical generators, fire water pumps, control and technical rooms, turbine and HVAC air inlet, flame heaters are located away and upwind of potential sources of leak (compressors etc.).
The main difference in terms of Equipment layout between Off-Shore and On-Shore facilities is that the fixed minimum separation distances that were applied On-Shore between equipment as per codes, e.g. 30 feet between two compressors, are not applied Off-Shore.
Instead, the distances between equipment of Off-Shore facilities are minimized, to the space strictly required for access/escape ways and for maintenance.
One may wonder how, in such case, is escalation of an incident from on equipment to the other avoided, as it were, in On-Shore plants, by means of separation distance, with which overpressure created by an explosion, or radiation intensity from a fire, rapidly decreases.
A different, more refined, approach is required and applied Off-Shore. For what concerns explosion, first, the most likely explosion scenarios are identified and the blast pressures calculated. Critical equipment and their supports are then designed to resist such blast pressures. Blast studies might also result into the prescription of a blast wall.
The blast calculations entail modelling the facility. Such model does not only extend to equipment, structure but also the level of confinement of the environment, including that created the pipe-work, cable trays etc… Indeed, the latter determines whether a gas cloud resulting from a leak would safely dilute or could accumulate and reach the explosive limit. Reducing congestion is an important factor of safety of Off-Shore facilities. It will allow open air ventilation and dissipation of flammable or toxic vapours.
Escalation of fires in its turn, is prevented by extensive use of Passive Fire Protection coating, applied to the main structure, to critical equipment and their supports.
A special attention is also given to vital facilities, such as shutdown valves that serve to isolate the platform from the connected pipelines. In an emergency, such as a major leak on the facilities leading to a fire, it is essential to isolate the latter from the connected pipelines to prevent fuelling the fire with the inventory of contained in those pipelines. Emergency isolation valves are located as far as possible from hazards, such as on the lowest deck, are of a fire proof design, provided suitable fire protection etc.
Due to the congested deck environment, and uneasy access to equipment, proper review of how its maintenance, in particular that of requiring the dismantling of large and heavy parts (rotating machinery) must be done as part of the Handling Study.
Free space must be provided overhead of equipment, for hoisting the parts, and for their travel up to the nearby trolley/crane pick-up area.
Equipment subject to frequent maintenance, such as for example heat exchangers in severe service (sea water etc.), will be located at a location easily accessible by the crane e.g. upper deck.
The crane itself will be located at a strategic location within reach of both lay down areas and supply boats ferrying dismantled and spare parts.
Other factors come into play into Off-Shore equipment layout, such as relative process elevations between equipment, such as a compressor and its suction scrubber, location of main equipment on the primary structure members, method (lift/slotting) and timing of installation of large equipment in the main structure etc.
