Offshore Facilities: Platforms, Structures in Oil and Gas Piping Systems
An offshore installation involved in production of Oil or Gas is termed as Platform. Each of the offshore platform or structure is chosen primarily due to water depth considerations, and secondarily due to the intended service and quantity of deck equipment necessary to perform its service.
- Fixed steel structures
- Tension leg platforms
- Semi-submersible vessels
- Floating Production systems
Fixed steel structures
The fixed tower structures are the most common offshore structure. These structures are commonly used in water depths less than 150 meters. However this can be used in water depths up to about 300 meters. Within this category there are 4-leg, 6-leg, and 8-leg towers.
Tension leg offshore Platforms
The Tension Leg Platforms are used in water depths greater than 300 meters. They consist of a floating deck structure anchored to pile heads on the sea floor by means of long pipes, which are always kept in tension
As fixed structures are not practical for water depth greater than around 500 meters, so the offshore drilling operation is required to be carried out using a floating vessel.
A specialized marine vessel with good stability and seakeeping characteristics is known as semi-submersible vessel. It can be moved in different locations and anchored to the seabed using mooring system.
In offshore, the specific role for which semi-submersible vessel designs used are: offshore drilling rigs, safety vessels, oil production platforms and heavy lift cranes.
Floating Production Systems
A floating production system is in-effect a floating Oil rig. It contains all the equipment associated with a fixed installation and is used in conjunction with sub sea well heads to explore moderate to deep-water oil fields. FPS is particularly suitable for the development of oil reserves where the installation of a fixed structure would be either impractical or economically not viable.
Piping systems can be broadly classified into two basic categories of Hydrocarbon Piping and Utility Piping
Gas or Oil
The main objective of an offshore installation is to separate Hydrocarbon products into liquid or gaseous forms and to remove any impurities.
Condensate is a clear highly volatile liquid produced as a bi-product of the gas production process. The condensate system is primarily concerned about the removal of water prior to the transportation of condenaste to the onshore facility through subsea pipeline.
Vent systems are normally associated with gas production installations and both high-pressure (HP) and low-pressure (LP) systems are installed
HP vent systems are primarily used to depressurise the High-Pressure vessels during Emergency Shut Down (ESD) or to relieve excess vessel pressure via Pressure Safety Valve (PSV). This system consists of a knock – out vessel and a straight pipe to a safe location.
LP vent systems are same as HP vent system. This is designed to be used for operating at Lower pressure. This provides a source for the release of gases from PSV’s and atmospheric vents.
The flare system has the same function as that of a Vent system. The difference is that the gas is ignited as it leaves the end of the flare boom. This is seen in Oil production platforms where the gas to be vented out is considerable and would constitute hazard if released into the atmosphere through an open vent.
Corrosion inhibitor provides protection to the carbon steel pipes. The inhibitor is injected into Well heads, process piping and subsea pipelines.
Firewater piping system can be Deluge systems or Sprinkler system. Water used for this system may be seawater or treated water. In case of this proper material selection shall be done to make the pipes and fittings to be compatible to the water used.
The deluge system is used to protect the equipment which process/handles hydrocarbon. Deluge piping system remains dry until the deluge valves are opened. Deluge system is intended to cool the plant to reduce further escalation of fire. The deluge system is extended to cover the main structural steel work to avoid weakening if exposed to fire.