Oil production plant
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 (a form of distillation) which reduces vapour pressure and sweetens "sour" crude oil by removing hydrogen sulphide, thereby making the crude oil suitable for storage and transport.
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)/coalescer is routed to a produced water degasser operating at near atmospheric pressure to removed 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.
Onshore oil terminals generally have fired heaters followed by separators and coalescers to stabilise the crude and remove any produced water and light hydrocarbons not separated offshore. Onshore separators tend to operate at a lower pressure than the offshore separators and so more gas is evolved. The associated gas is generally compressed, dew-pointed and exported via a dedicated pipeline. If gas export is uneconomical then it may be flared. Onshore terminals frequently have large crude oil storage tanks to allow offshore production to continue if the export route becomes unavailable. Export to the oil refinery is either by pipeline or tanker.
Onshore gas terminals may have facilities for removal of liquids from the incoming gas stream. Gas treatment processes may include glycol dehydration, gas sweetening, hydrocarbon dew-point control and gas compression before gas distribution to users.
Utility and support systems
In addition to production and gas and oil treatment systems a range of ancillary, support and utility systems are provided to support production and occupation of an offshore installation. Systems include:
- Glycol regeneration - water-rich glycol is heated and stripped with dry gas to drive off the water
- Amine regeneration - rich amine is heated and stripped with sweet gas to drive off CO2 and H2S
- Fuel gas - used to power gas turbines, and to purge relief, vent and flare systems
- Purge gas - to provide blanket gas for tanks and to purge vessels prior to maintenance
- Inert gas - to provide blanket gas for tanks and to purge vessels prior to maintenance
- Diesel fuel - for firewater pump diesel engines and start-up of generators
- Aviation fuel - helicopter refuelling
- Atmospheric vent - gas disposal for low pressure systems and maintenance purging
- Relief and Flares - safe collection and disposal of excess gas under normal and shutdown conditions
- Pipeline pigging - to clear and monitor pipelines
- Well test facilities - test separator to determine flowrates from individual wells
- Seawater - used for flushing, cooling, washdown
- Firewater - seawater for fire fighting
- Water injection - deaerated seawater injected into the oil reservoir to drive oil towards production wells and maintain reservoir pressure
- Potable water - drinking water bunkered from supply vessels or made onboard by reverse osmosis of seawater
- Cooling medium - to cool gas and oil streams; comprising either seawater (direct) cooling or a fresh water/glycol mixture
- Heating medium - to heat gas and oil streams, HVAC; comprising hot oil or a fresh water/glycol mixture
- Closed (process) drains - to drain process equipment prior to maintenance
- Open drains - drainage from deck areas, hazardous area drains are segregated from non-hazardous area drains, disposed overboard
- Sewage treatment - maceration and overboard disposal of 'black' (toilet) and 'grey' (sink and shower) water
- Plant/service air - for powering air-driven tools, purging vessels
- Instrument air - for operating pneumatic actuated controllers and valves
- Electricity generation - diesel or fuel gas for diesel engine or gas turbine driven electricity generation
- Chemical storage and injection - to aid separation of well fluids and maintain operation of facilities, may include methanol, glycol, corrosion inhibitor, scale inhibitor, oxygen scavenger, H2S scavenger, emulsion breaker, foam breaker, wax inhibitor
- Seal oil and lube oil storage - for compressors, gas turbines and diesel engines
- Hydraulic oil - operation of subsea and subsurface well equipment
- HVAC - for enclosed process plant and accommodation
- Drilling facilities - bulk chemical storage, well drilling equipment
- Magnus Process Flow Diagrams 1983; NW Hutton Process Flow Diagrams 1987; Shearwater Process Flow Diagrams 2005
- Ken Arnold and Maurice Stewart (1998). Surface Production Operations, Volume 1: Design of Oil-Handling Systems and Facilities. Oxford: Elsevier Science & Technology. ISBN 9780750678537.
- "Three phase separation". Retrieved 11 February 2019.
- "Forties pipeline oil specification". Retrieved 10 February 2019.
- "Custody transfer metering". Retrieved 11 February 2019.
- "Oil in Water Grand Banks" (PDF). Retrieved 10 February 2019.
- "Glycol dehydration". Retrieved 11 February 2019.
- "Northern Leg Gas Pipeline specification" (PDF). Retrieved 10 February 2019.
- Gas Processors Suppliers Association (2004). Engineering Data Book. Tulsa, Oklahoma: GPSA. pp. Section 18 Utilities.