page_title: Oil production plant
section_title: Offshore processing
hierarchical_section_title: Oil production plant / Offshore processing
caption_attribution_description: I am the author and I drew this diagram myself using Microsoft's Paint program. This image depicts a parallel plate oil-water separator used in industrial wastewater treatment. - mbeychok 19:20, 24 August 2007 (UTC)
context_page_description: An oil production plant is a facility which processes production fluids from oil wells in order to separate out key components and prepare them for export. This is distinct from an oil depot, which does not have processing facilities.
  Typical oil well production fluids are a mixture of oil, gas and produced water. 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 stabilised which reduces vapour pressure and sweetens "sour" crude oil by removing hydrogen sulphide, thereby making the crude oil suitable for storage and transport.
context_section_description: The production plant can be considered to begin after the production wing valve on the oil well Christmas tree. The reservoir fluids from each well are piped through a flowline to a choke valve, which regulates the rate of flow and reduces the pressure of the fluids. The flowlines from each well are gathered together at an inlet manifold and routed into a (first stage) separator, which separates the three fluid phases. Produced water, the densest phase, settles out at the bottom of the separator, oil floats on the top of the produced water phase, and gas occupies the upper part of the separator.    
  Oil from the first stage separator may be cooled or heated in a heat exchanger to aid further separation. Oil is then routed either to a second stage separator, operating at a lower pressure than the first stage to further separate oil/gas/water, or to a coalescer to further remove water. Several stages of separation, operating at successively lower pressures, aim to reduce the amount of dissolved gas and hence reduces the flash point of the oil to meet the export oil specification. For higher oil flowrates parallel trains of separators may be necessary to handle the flow and to provide a turn-down capability.    
  From the final stage of separation, or from the coaleser, oil may be cooled to meet export specifications and is metered to accurately measure the flowrate and then pumped via a pipeline to the onshore terminal. Some installations such as concrete gravity-base structures and FPSOs have integral oil storage tanks which are continuously filled with oil and periodically discharged into oil tankers.      
  Produced water from the separator(s) or coalescer is routed to a produced water degasser operating at near atmospheric pressure to remove dissolved gas from the water. Produced water is then routed to a hydrocyclone to remove entrained oil and solids and then either re-injected into the reservoir or dumped overboard. For overboard disposal the water should have an oil content of less than about 30 parts per million (ppm) oil-in-water. In the early days of the offshore industry parallel plate separator units were used to clean produced water prior to overboard disposal. Hydrocyclones, which are more compact, were introduced in the 1980s.
   
  The associated gas from the top of the separator(s) is also known as flash gas or wet gas as it is saturated with water and liquid alkanes. The gas is typically routed through scrubbers, compressors and coolers to raise the pressure of the gas and to remove liquids. Gas may be dried by counter-current contact with triethylene glycol in a glycol dehydration tower. The dry gas may be exported, used for gas lift, flared, used as fuel for the installation's power generators, or after further compression re-injected into the reservoir.
  Dry gas may be further treated to meet export gas specifications. Excess carbon dioxide (CO₂) can be removed by treatment in an amine gas treating process, whereby CO₂ is preferentially dissolved in a counter-current flow of amine in a contact tower. Hydrogen sulphide can also be removed using amine or by passing the gas through beds of zinc oxide absorbent. The export hydrocarbon dew-point specification (typically 100 barg at 5°C) may be met by chilling the gas to remove the higher alkanes (butane, pentanes, etc). This may be done by a refrigeration system, or passing the gas through a Joule-Thomson valve, or through a turbo-expander to condense out and separate liquids. Export gas is metered to accurately measure the flowrate before being sent to the onshore terminal via gas pipeline. 
  Condensed liquids from the gas treatment plant may be stabilised in a stabiliser (distillation) column to produce a gas product which can be co-mingled with the gas streams and natural gas liquids (NGL) which can be co-mingled with the oil export stream.
caption: Parallel plate separator
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