Offshore platform is used for exploration of oil and gas from the seabed and processing. Central processing platform is used for processing the well head fluid which is coming through the riser from the wellhead platforms. Main functions for the central processing platform is to separate the oil, gas and produced water from the 3-phase well head fluid. Along with CPP, Well head platforms also used for extraction of oil from wellhead.
Typical offshore process platform have following process
1. Separation
2. Gas compression and dehydration
3. Produced water conditioning
4. Sea water processing and injection system
Separation
Separation is main process in the central processing platforms, in which oil, gas and water is separated from the well fluid. Well fluid from various well platforms / subsea manifold reaches the process complex via subsea pipelines and risers and is further processed in more than one train. Each train will normally consist of a production manifold, well fluid heater, Inlet Separator, crude oil manifold, crude oil heater, Surge Tanks and MOL Pumps. Well fluid is received in the production manifold. Demulsified chemical is dozed in production manifold to promote breaking up of water–oil emulsion
Then it is heated in well fluid heater with hot oil flowing in shell side and well fluid in tube side. This heating enables better separation of oil and water in Inlet separator.
The well fluid from well fluid heater reaches Inlet separator. Inlet separator is a vessel in which 3 phase separation of well fluid into oil, gas and water occurs.
Separation is by gravity mainly assisted by chemical and heat. Residence time in the vessel is an important criterion for better separation. Separated gas is routed to gas compression and dehydration Module. Compressed gas is sent to the lift gas network for lifting the producers and excess gas is exported via pipelines to shore. Separated oil flows to the oil manifold.
Separated water flows to produced water conditioning unit. From oil manifold crude oil flows to crude oil heater in which crude oil is heated with hot oil. This enhances the separation of oil & water in surge tanks second stage separation. Demulsified chemical dozed in oil
manifold further promotes the breaking of water oil emulsion.
Surge Tank :-Surge tank is maintained at a lower pressure to stabilize crude i.e. to remove maximum of associated gas from the crude oil. Oil from surge tanks can be either pumped directly with pumps or can be diverted to third stage separators (Surge tank–3). Separated crude oil is pumped with crude transfer pump / MOL pumps to export trunk lines. Separated gas is diverted to gas compression module after boosting the pressure LP booster compressor. Separated water is diverted to produce water conditioning Unit.
Gas Compression and Gas Dehydration
Gas Compression:-Gas from separators, surge tanks and export gases if any from other process platforms are compressed to about 90–100 kg/cm2 pressure as per the field gas lift requirement. Normally gas turbine driven centrifugal compressors, process gas compressors are used. Gases compressed in process gas compressors are dehydrated to prevent formation of gas hydrates. Gas hydrates are formed at low temperatures when moisture is present in hydrocarbon gases. These gas hydrates are ice like substance which prevent the smooth flow or block the flow of gases in gas flow lines. Gas hydrates can be formed in adjustable choke valves, PCV’s & GLV’s in GI Lines where throttling of gases give rise to low temperatures. This can affect production phenomenally from gas lift wells.
Gas Dehydration:- Compressed gas is dehydrated in a glycol contactor with Tri-Ethylene Glycol (TEG) as an absorbent for moisture from compressed gas. Glycol contactor is bubble cap tray column with many bubble cap trays. TEG flows counter current with compressed gas from the top of the column. TEG coming in contact with compressed gas in the bubble cap trays selectively absorbs the moisture from the gas and dehydrates it. The dehydrated gas is sent to feed gas lift wells in priority basis and remaining to export gas line. The TEG rich in moisture is sent for re–concentration, converted to lean glycol and recycled back to contactor for dehydration.
Produced Water Conditioning
The water produced along with oil and gas from the wells is to be treated to within acceptable levels of quality in terms of oil ppm before it is discharged in to the sea. Produced water conditioning unit normally consists of flash vessel, CPI separators, IGF (induced Gas floatation Unit) and sump caisson.
- Flash Vessel: – flash vessel receives water from both Inlet separators and surge tanks. It is maintained at 0.8 kg pressure. In flash vessel most of the dissolved gases in the produced water flashes out which is routed to LP flare header. associated oil from the produced water in the flash vessel is routed to closed drain header and is collected in the sump caisson.
- CPI separators:- Water from the flash vessel flows to many CPI separators in parallel. Oil from CPI separator is collected in a tank from which it is pumped to oil manifold. Gas goes to LP Flare header.
- IGF unit:- Water from CPI separator flows to IGF unit. IGF is a tank in which gas bubbles are aerated with motor driven agitators. These bubbles float the oil droplets to surface .This collected oil is pumped to CPI separators.
- Sump Caisson:-Water from IGF is routed to sump caisson which is a vessel with bottom end open through which water continuously drains into the sea. Oil floating in the surface of the sump caisson is collected in the blow caisson and lifted up and flown into the skimmer with gas injection.
Water injection system and Sea water processing
- Water Injection:- Water Injection is done to maintain reservoir pressure as well as water flooding. To prevent damage to the reservoir the quality of water injected is strictly complied with. Also, the health of the pipelines carrying the injection water to the wells and well platforms is taken care of by dozing chemicals to prevent corrosion and generation of h2s by srb colonies. The major components of water injection systems are sea water lift pumps, coarse filters, fine filters, deoxygenating towers, booster’s pumps, main injection pumps, chemical dosing system, chemical dozing system flocculent, scale inhibitor, corrosion inhibitor, chlorination etc.
- Sea Water Lifting and Filtering:- Water from sea is lifted with seawater lift pumps and fed to Coarse Filters and fine filters for filtering. Coarse filters the particles are filtered to 20 microns. Fine filters the particles are filtered up to 2 microns. Poly electrolyte and coagulants are added in sea water lift pump discharge to promote coagulation of suspended particles.
- De–Oxygynation and Pumping:- The filtered water flows to deoxygenating towers for removal of oxygen. De-oxygenation prevents formation of aerobic bacterial colonies (sulphur reducing bacteria) in the WI flow lines. Vacuum pumps and oxygen scavenger chemical dozed facilitates oxygen removal in the towers. Booster pumps take suction from de-oxygenation towers and feed Main Injection Pumps. Scale inhibitors, bactericide and corrosion inhibitor chemicals are dozed in the discharge of booster pumps. MIP’s discharge the treated water to Water Injection subsea pipelines to wells and well platforms for injecting in to water injection Wells
Sea water processing and injection system:
- Power Generation
- Water Maker/Utilities/Sewage Treatment
- Emergency diesel generators
- Communication systems
- Air compressors
- Fire water pumps
- Fire detection & Suppression system
- Nitrogen Air receiver
- Life boats
- Pedestal Cranes
Some central processing platforms have living Quarters, well services/drilling modules. Living quarter are depends upon the clients choice if they want to add the living quarters module in processing complex or they want to install another living quarter platforms separately.
Installations of central processing platform
Central processing platforms are mostly 6 to 8 leg jacket installations platforms. They are typical four decks in process platforms like sub-cellar deck, cellar deck, mezzanine deck, top deck. May be the name of the decks varies as per clients requirements. These decks are fabricated as per the process and stacked one into another in onshore. Then these central processing platforms are installed in float over process by big barges.
Float over concept for installation of processing platforms is reducing the amount of offshore work. But this is totally depends upon the operating clients. Some client’s wants module wise offshore installation and some clients float over installation because installations drive the cost of the project too much.
Offshore platform types
Fixed platforms are anchored to the seabed with piles or other devices. They are the most common type of offshore platform and can be used for both drilling and production.
- Steel jacket platforms are the most common type of fixed platform. They are made of steel and are anchored to the seabed with piles. Steel jacket platforms are relatively inexpensive and can be easily modified to accommodate different drilling and production requirements.
- Concrete caisson platforms are made of concrete and are also anchored to the seabed with piles. Concrete caisson platforms are more expensive than steel jacket platforms, but they are also more durable. They are
often used in areas where the seabed is soft or unstable.
Floating platforms are not anchored to the seabed and are used in deeper water. They are more expensive than fixed platforms, but they are also more flexible and can be moved to different locations.
- Semi-submersible platforms are the most common type of floating platform. They have a hull that floats on the surface of the water and legs that reach down to the seabed. Semi-submersible platforms are stable in
deep water and can be used in a variety of weather conditions.
- Tension-leg platforms are also a type of floating platform. They have a hull that floats on the surface of the water and legs that are tethered to the seabed with cables. This design provides very good stability and allows tension-leg platforms to be used in very deep water.
- Spar platforms are a type of floating platform that is shaped like a cylinder. They have a hull that floats on the surface of the water and a spar that extends down to the seabed. Spar platforms are used in water depths up to about 3,000 meters.
Drillships are mobile platforms that are used for drilling in deep water. They are typically anchored to the seabed, but they can also be moved to different locations. Drillships are the most expensive type of offshore platform, but they are also the most versatile.
Jackup rigs are mobile platforms that can be moved from one location to another. They are used for drilling in shallow water. Jackup rigs are less expensive than drillships, but they are not as versatile.
Floating production, storage, and offloading (FPSO) units are used to produce, store, and offload oil and gas. They are typically anchored to the seabed. FPSO units are a cost-effective way to produce oil and gas in deep water.
The type of offshore platform that is used depends on a number of factors, including the water depth, the type of drilling operation, and the cost.