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. 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 plant 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 stabilised which reduces vapour pressure and sweetens "sour" crude oil by removing hydrogen sulphide, 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 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.
''See for example: Wytch Farm''

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 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. These are summarised in the following table:

Export options

The export options for oil and gas and the deployment around the world are as follows:

Stabilised crude oil
*Reid Vapor Pressure RVP < 11 psi
*Basic Sediment & Water BS&W < 1% by volume)
*Gulf of Mexico, Canada East Coast, West Africa, Indonesia.
Unstabilised wet crude
*True vapour pressure 150 psia
*BS&W 2%
*North Sea, Azerbaijan, Gulf of Suez, Trinidad.

Gas disposal may take one or more of the following routes:
In the Central and Northern North Sea gas is delivered to St Fergus or Teesside terminals by a small number of large diameter gas pipelines. These operate at 1600 – 2500 psig in the dense phase i.e. above the critical pressure. Operation in the dense phase provides a number of advantages:

it is more efficient to transport gas at high pressure over long distances
it eliminates the problem of multiphase flow
it eliminates the need to treat gas by hydrocarbon dewpointing

These advantages are offset by the additional compression required and thicker walled, more expensive, pipelines are necessary.

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 flowrates, the availability of plant, and the available plot area. Single trains are capable of handling 150,000 to 200,000 barrels of oil per day. Vessel sizes can be up to diameter and up to long. Vessels on Gulf of Mexico deepwater installations are diameter and long.
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, 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 3-phase separator. This operates at 150 –250 psi.
In the North Sea first stage separators generally operate at < 750 psi. These are operated as 3-phase separators and are sized to provide 3 – 5 minutes of liquid residence time. Pressures are set to maximise gas separation at as a 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.
The throughput, number of trains, separation stages and first stage separator pressure for a range of historic offshore installations is shown in the table.

Installation	Location	°API Gravity	Oil production, BOPD	Gas production, m³/day	Inlet separator pressure, bar	No. of trains	No. of stages
Brent C	North Sea	38	150,000	8,500,000	9.6	3	4
Claymore A	North Sea	30	180,000		5.9	1	3
Cormorant A	North Sea	35	60,000	900,000	30	1	3
Statfjord A	North Sea	38	300,000	8,500,000	69	2	4
Murchison	North Sea	36	164,000	1,300,000	45	1	3
Buchan	North Sea	33.5	70,000	600,000	10	1	3
Magnus	North Sea	39	140,000	2,500,000	28	2	2
Brae A	North Sea	33/35, 41/45, 33	100,000 + 12,000 NGL	4,250,000	248	2	3
Namorado I	South America		40,000	640,000		2	3
Namorado II	South America		60,000	420,000
Cherne I	South America		50,000	120,000		1
Bayan A	South China Sea		60,000		27	2	2
Brae B	North Sea	33/35, 41/45, 33	75,000	11,300,000	103	1	4
Ship Shoal	Gulf of Mexico		12,000	4,250	90	1	3
Forties Echo	North Sea	37	67,000	300,000
Eider	North Sea	34	53,000		14 – 29.3	1	1
Seillean	North Sea	36	15,000	6,000,000	17	1	2
East Brae	North Sea	33/35, 41/45, 33	120,000	17,000,000	102	1	3
Gannet	North Sea	38/43	56,000	4,000,000	18 – 70	4	1
Nelson	North Sea	40	145,000		9	1
Tiffany	North Sea	33.6	105,000	3,250,000	17	1	2

Materials of construction

A range of materials of construction are used for oil processing plant. 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.

System or plant	Material of construction
Flowlines and manifolds	Carbon steel
Flowlines and manifolds	Alloy 825
Flowlines and manifolds	Duplex
Separators	Carbon steel
Separators	Carbon steel clad with Alloy 825
Condensate separators	Carbon steel
Condensate separators	Carbon steel clad with 316 stainless steel
Condensate separators	316 stainless steel
Oil pumping and export	Carbon steel
Gas compression	Carbon steel
Gas compression piping	316 stainless steel
Gas compression vessels	Carbon steel
Dehydrated gas	Carbon steel
Gas export	Carbon steel
Flare and vent	Carbon steel
Flare	316 stainless steel
Gas injection	Carbon steel
Seawater	GRP
Seawater	Cunifer
Firewater	Galvanised steel
Seawater for heat exchangers	Titanium
Injection water	Carbon steel
Drains	Carbon steel
Fresh water	Copper
Air	Carbon steel
Cooling and heating media	Carbon steel
Fuel gas	Carbon steel