Offshore platforms are used to explore, extract, and process oil and gas retrieved from the seabed. One vital part of these structures is the central processing platform (CPP), where the well head fluid is pumped from the wellhead platforms through the riser for processing. The most important function of the CPP is to separate the oil, gas, and water from this three-phase well head fluid, to produce the final product of the platform.
Most central processing platforms feature the following components:
1. Separation Unit
2. Gas compression and dehydration
3. Produced water conditioning
4. Sea water processing and injection system
As mentioned above, the main function of central processing platforms is to separate oil, water, and gas from the combined well fluid. This fluid comes from various different well platforms, and is delivered to the CPP via a network of subsea pipelines and risers. In addition, the fluid will be additional processed in multiple trains, which typically contain a production manifold, well fluid heater, inlet separator, crude oil manifold and heater, surge tanks, and MOL pumps. The well fluid is initially received in the production manifold, where a demulsified chemical is added to promote the breaking up of water-oil emulsion.
Next, the mixture is heated in the well fluid heater. Hot oil flows through the shell side, while well fluid flows through the tube side. By heating the well fluid in this manner, the oil and water will be easier to separate in the next component, the inlet separator. This is a vessel where the three-phase separation of well fluid into oil, water, and gas takes place.
The actual separation itself is mainly done by gravity, but this is also assisted by special chemicals, as well as heat. The length of time that the well fluid sits in the vessel is an important factor in how well the components will separate. Once they have done so, the gas is routed to a gas compression and dehydration module, and then any excess gas is sent to the shore through pipelines. Separated water flows into a water conditioning unit, to remove any additional chemicals. The separated oil will flow into the oil manifold. From here, crude oil flows into a crude oil heater, and heated with hot oil. This will ensure that any additional water is better removed in the surge tanks. In addition, demulsified chemicals are added to further promote the breaking up the water-oil emulsion.
The surge tank is kept at a low pressure, so that it can remove as much gas from the crude oil as possible. Oil from the surge tanks can then be pumped directly, or if necessary, diverted into a third-stage surge tank for any additional separation. The separated crude oil will then be pumped by a special crude transfer pump into export trunk lines. The separated gas has its pressure boosted by a pressure LP booster compressor, and then on to a gas compression module.
Gas Compression and Gas Dehydration
Gas Compression:- The gas gathered from separators, surge tanks, and any export gases from other processing platforms is compressed to around 90-100kg/cm2, depending on the field gas lift requirement. Usually, turbine-driven centrifugal compressors will be used for this. The gas is then dehydrated to prevent the formation of gas hydrates- these can form at low temperatures when moisture is present in hydrocarbon gases, and are an ice-like substance which will block the flow of gases within gas flow lines. These gas hydrates may form within adjustable choke valves, PCVs and GLVs in GI lines, since the throttling of gases in these areas can lead to low temperatures. Gas hydrates can have a hugely detrimental impact on production from gas lift wells.
Gas Dehydration:- Once the gas has been compressed, it is then dehydrated inside a glycol contactor, using Tri-Ethylene Glycol (TEG) to absorb any moisture from the gas. The glycol contactor is essential a tray column of many bubble cap trays. As the TEG comes into contact with the compressed gas in the bubble cap trays, it absorbs the moisture from the gas, thereby dehydrating it. This dehydrated gas is next sent to feed gas lift wells, and any additional product is sent to an export gas line. The TEG, which is rich in moisture, is then sent off to be concentrated again, converted back to lean glycol, and then recycled in the contactor.
Produced Water Conditioning
The water which is produced from wells along with oil and gas needs to be suitably purified before it can be discharged back into the sea. To do this, a dedicated produced water conditioning unit is used. This will be formed of a flash vessel, CPI separators, an induced gas floatation unit (IGF), and sump caisson.
Flash Vessel: – The flash vessel takes in water from both inlet separators and surge tanks. Here, it is kept at 0.8 kg of pressure. Within the flash vessel, the majority of the dissolved gases are “flashed out”, and then routed to the Lowe Pressure (LP) flare header. Any oil within the produced water is routed to a closed drain header, and collected in the sump caisson.
Corrugated Plate Interceptor (CPI) separators:– Once the water has gone through the flash vessel, it then flows to multiple CPI separators in parallel. Oil from the CPI separator will be filtered into a tank, from which it is then pumped into the oil manifold. Again, any gas will go to the LP flare header.
Induced Gas Flotation (IGF) unit:– From the CPI separators, the water then flows into the IGF unit. This is essentially a large tank, where gas bubbles are aerated by motor-driven agitators. These bubbles collect oil droplets, and float them to the surface, where they can be pumped back to the CPI separators.
Sump Caisson:-Finally, water from the IGF is then routed to the sump caisson. This is a vessel with an opening through which water is continuously drained back into the sea. Any oil which is floating on the surface of the sump caisson is collected into a blow caisson, and then lifted into a skimmer via gas injection.
Water injection system and Sea water processing
Water Injection:- Water injection is performed to maintain the reservoir pressure, and prevent flooding. To stop the reservoir from being damaged, the injected water is kept at a strict high quality. Furthermore, the condition of the pipelines which carry the injection water to the wells and well platforms is maintained with dozing chemicals, to prevent any corrosion or generation of H2s by SRB colonies.
Water injection systems are made up of the following core components: 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, and a chlorinator.
Sea Water Lifting and Filtering:- Seawater is taken in via seawater lift pumps, and then fed into both coarse filters and fine filters for processing. In coarse filters, the particles are filtered to a level of 20 microns, whereas in fine filters, that filtration takes place at a level of up to 2 microns. Polyelectrolyte and coagulants are then added to the sea water lift pump discharge, to aid with the coagulation of suspended particles.
Deoxygenation and Pumping:- Once the water has been properly filtered, it then flows into deoxygenating towers to remove any excess oxygen. This is done to prevent the formation or aerobic bacterial colonies, which affect Sulphur levels, within the WI flow lines. The oxygen removal is performed through the use of vacuum pimps and oxygen scavenger chemicals. Then, booster pumps take the suction from the deoxygenation towers, and feed it into the main injector pumps. Scale inhibitors, bactericide, and corrosion-inhibiting chemicals are dozed into the discharge of the booster pumps. MIPs then discharge the treated water via water injection subsea pipelines to wells and well platforms.
Processing complexes will also contain the following components:
- 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
In addition, it is possible that central processing platforms will also contain accommodation for workers, as well as drilling modules or well services. However, these components are completely optional, and dependent on the client’s decision.
Installations of central processing platforms
The majority of central processing platforms take the form of six-leg or eight-leg jacket installations. There will usually be four decks: the sub-cellar deck, the cellar deck, the mezzanine deck, and the top deck. The names of these decks may vary, depending on the equipment which is installed in them. Each deck is fabricated according to the individual client’s needs, before being stacked together onshore. Finally, the central processing platform will be installed in a float-over process at sea by large barges.
The float-over concept is intended to cut down on the amount of work that needs to be done offshore. However, whether or not this approach is possible will depend on the needs and budget of the particular client.
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