In the upstream oil industry, a gas–oil separation plant (GOSP) is temporary or permanent facilities that separate wellhead fluids into constituent vapor (gas) and liquid (oil and produced water) components. Temporary plant Temporary gas–oil separation facilities are associated with newly drilled or newly sidetracked wells where the production potential of the well is being assessed. The plant, comprising a test separator vessel, is connected to the wellhead after the choke valve. The separator allows the fluids to separate by gravity into its component phases: solids such as sand (the densest phase) settle to the bottom of the separator, then produced water and oil which are drawn separately from the base of the separator, and vapor or gas (the lightest phase) separates to the top of the separator vessel from where it is withdrawn. Each of the three fluid phases is metered to determine the relative flow-rates of the components and production potential of the well. In temporary facilities the vapor is generally flared; produced water is disposed of overboard after treatment to reduce its oil content to statutory levels; and the crude oil phase may be diverted to tote tanks for removal and treatment onshore. Alternatively, if the temporary GOSP plant is associated with a permanent production facility, the oil phase may be treated in the installation's permanent gas–oil separation plant. Permanent plant Permanent gas–oil separation plant is associated with permanent offshore production facilities. For a full description of such a plant, see Oil production plant. A gas–oil–and–water separator is called a 3-phase separator. The gas and oil or condensate are pumped through designated pipelines, while the sand and other solids are washed from the separator and disposed of overboard. Reasons for processing Multi-phase production Water need not be separated, and a single liquid (oil and water) phase produced together with a separate gas phase. Chemicals are added so that the crude and water emulsify. This process is then reversed at the storage and processing facility by adding demulsifiers that make the water separate out, and is drawn from the bottom of the tank. After storage, the crude oil can be sold to refineries, which produce fuels, chemicals, and energy products. Pressure The well fluids at the wellhead are at high pressure. Production pressures of greater than are not uncommon, but typically are lower than this. The high pressure is reduced at the choke valve to typically 7 to 30