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An oil production plant is a facility that processes production fluids from oil wells to separate key components and prepare them for export. Typical oil well production fluids are a mixture of oil, gas, and produced water. An oil production plant is distinct from an oil depot, which does not have processing facilities. Oil production plants may be associated with onshore or offshore oil fields.
Many permanent offshore installations have full oil production facilities. Smaller platforms and subsea wells export production fluids to the nearest production facility, which may be on a nearby offshore processing installation or an onshore terminal. The produced oil may sometimes be stabilized (a form of distillation) which reduces vapour pressure and sweetens "sour" crude oil by removing hydrogen sulfide, thereby making the crude oil suitable for storage and transport. Offshore installations deliver oil and gas to onshore terminals which may further process the fluids prior to sale or delivery to oil refineries.
Onshore oil production
The configuration of onshore oil production facilities depends on the size of the oil field. For simple fields comprising a single well or a few wells, an oil storage tank may be sufficient. The tank is emptied periodically by a road tanker and transferred to an oil refinery. For larger production rates a rail tanker transfer facility may be appropriate. For larger fields, a full three-phase processing facility is required. Three-phase separators separate the well fluids into its three constituent phases: oil, gas, and produced water. Oil may be transferred by road or rail tanker or by pipeline to an oil refinery. Gas may be used on the site to run gas engines to produce electricity or can be piped to local users. Excess gas is burned in a ground flare. Produced water may be re-injected into the reservoir. Small fields can use portable integrated packages, like vapor-tight tanks.
Offshore oil processing options
There is a wide variety of options for the processing of produced oil. These range from minimal offshore processing with all produced fluids sent to an onshore facility, to full offshore processing to make products to a specification suitable for sale or use with no further onshore processing. The decision on what facilities to provide depends on a number of factors:
• the optimal size of the offshore installation
• whether an onshore terminal is required or available
• what export routes (pipelines, tankers) are available
• the project constraints in terms of cost, schedule, and resources
• the impact or implications for future projects
The Gulf of Mexico and the North Sea are two mature producing areas that have taken different approaches to the facilities provided.
Plant configuration
A further consideration is the number of separation trains and the number of stages of separation. Trains of process facilities operate in parallel, and stages are operated in a sequential series. The number of trains depends on flow rates, the availability of plants, and the available plot area. Single trains are capable of handling 150,000 to 200,000 barrels of oil per day (23,847 – 31,797 m3/day). Vessel sizes can be up to 14 to 19 feet (4.27 to 5.79 m) in diameter and up to 30 feet (9.14 m) long. Vessels on the Gulf of Mexico deepwater installations are 12 to 14 feet (3.66 to 4.27 m) in diameter and 60 to 70 feet (4.27 to 21.34 m) long.[4]
The number of stages of separation depends on:
• wellhead pressure
• the gas/oil ratio
• the vapor pressure specification of the crude oil export stream
First-stage separators in the Gulf of Mexico typically operate at 1500 to 1800 psi (103.4 to 124.1 bar), they operate as 2-phase liquid and vapour separators with a liquid residence time of 1 to 2 minutes. Produced water is removed in the low-pressure (LP) 3-phase separator. This operates at 150 –250 psi (10.3 – 17.2 bar).[4]
In the North Sea first stage separators generally operate at < 750 psi (< 51.72 bar). These are operated as 3-phase (vapor, oil, and water) separators and are sized to provide 3 – 5 minutes of liquid residence time. Pressures are set to maximize gas separation at as high a pressure as possible. Up to 5 stages of separation are common in the Gulf of Mexico and up to 4 stages on platforms in the North Sea.
Materials of construction
A range of construction materials are used for oil processing plants. Carbon steel is extensively used as it is inexpensive. However, it is unsuitable for corrosive service where a number of corrosion-resistant alloys and other materials are required. The table illustrates typical materials for service on a plant that processes sour fluids.
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