Purpose of Oil and Gas Separation on Offshore Structures
Reservoir fluid is a complex mixture of gas, oil, water, solids and other contaminants that must be separated and processed. Separation is the process whereby reservoir fluid is separated into its component phases of gas, liquid and solid. The separated products are further processed to sellable, reuseable and disposable quality after separation.
Reservoir fluids are usually gathered in an Oil and Gas production facility where separation and processing takes place. If the well location is in a deep offshore location, the cost of laying long distances pipelines to onshore processing facilities might make the project to be uneconomical. When it is uneconomical to lay long distances pipeline to onshore facility, the reservoir fluid is separated into its component parts and stored on offshore platforms like the Floating, Production, Storage and Offloading (FPSO) facilities. Periodically, vessels go to the FPSO to load products and transport such to the customers. A new area being exploited is the floating liquefied natural gas (FLNG) facilities where processing of gas to LNG can take place.
Oil and gas separation is also carried out offshore in order to avoid flow assurance issues associated with transporting reservoir fluid from long distance offshore location to onshore processing facility. Some of these issues are hydrates formation, wax deposition, asphaltenes problems, corrosion etc. All these happen when there is a hydrocarbon-water mixture.
Separating water from oil and gas on an offshore platform will also reduce the cost of waste disposal as separated water can be treated on the platform and reinjected or disposed off immediately. A smaller (less expensive) diameter pipeline size will then be required to transport oil offshore rather than a bigger, more expensive pipeline size that will be required for gross liquid transport.
Method of Oil and Gas Separation
The primary mechanisms in oil-gas separation are gravity settling, centrifugal force and impingement.

Gravity Separation
In gravity separation, gravitational force is used by reducing velocity so the liquid can settle out in the space provided. Primary separation is achieved by the density difference between liquid and gas. The retention period is typically five minutes, allowing gas to bubble out, water to settle at the bottom and oil to be taken out in the middle. They generally have their mist extractor close to the gas outlet to remove any oil or liquid droplets as the gas flows out.

Centrifugal Force
Centrifugal force is used to effect primary separation by changing the direction of flow. Centrifugal elements cause cyclic flow of the incoming fluid at velocities high enough to separate the fluid into an outer layer or cylinder of liquid and an inner cone or cylinder of gas or vapour. Required velocity for centrifugal separation is between 40 to about 300 ft/sec. The centrifugal elements are usually properly sized tangential inlet or internal spiral in the separator. Most centrifugal separation elements are used in vertical separators, although some can be found in horizontal separators.

Impingement
These include all units that use devices such as impingement plates/device or a pack of tower packing on the inlet to accomplish primary separation. The elements eliminates from the gas stream those entrained liquid droplets which have little no gravity difference between them and the gas phase. It provides large impingement surface area which the droplets will hit, coalesce, and collect to form larger droplets that will be heavy enough to fall to the bottom. Impingement devices are usually vane type or knitted wire.
The vane types consist of a labyrinth formed with parallel metal sheets with suitable liquid collection pockets. The surface of the element is usually wet, and liquid particles striking it are absorbed into the liquid film.. When these droplets are large enough they settle under gravitational force (aided by the centrifugal force) into the collection surface. The collected liquid in the vanes goes through drain pipe to the liquid accumulation section
In knitted wire, the process of separation involves the entrained liquid particle striking the metal surface and flowing downstream through capillary space provided by adjacent wires. The liquid collects at these spaces and continue its downward flow. At the lower face of the mist pad, surface tension holds the liquid droplets. When the liquid droplets are large enough, gravity force exceeds both the surface tension and the upward gas velocity force, and the droplets fall.

Challenges associated with bringing the separate phases onshore
For liquid flow through a pipeline, at low temperature (below the cloud point of the crude), wax deposition might occur if the crude oil being transported in the pipeline is waxy. Wax deposition on pipeline walls reduces the internal diameter, causing pressure and production losses as a result of blockage. The removal of wax deposits requires production shutdowns with incurred revenue losses. Pipeline are insulated to ensure crude remain above the wax deposition temperature. Chemical injection (Wax inhibitors) is also used to prevent crude from forming wax in the pipeline.
For gas transportation, the temperature of gas cools down over the journey through the offshore pipeline flowing under the sea causing liquid to drop out of the gas and resulting in unwanted multiphase flow through the pipeline Multiphase flow might lead to severe slugs of liquid travelling in the pipeline. A slugcatcher is usually required at the onshore receiving facility to handle the slugs.

Hydrates are formed in the presence of gas, free liquid water and temperature below the hydrate formation temperature. If the gas is not dehydrated before transporting in pipeline, water may drop out alt low temperature causing formation of hydrates. Hydrate can clog the line causing production loss. The presence of water in the gas pipeline can also cause corrosion of the pipeline. Hydrocarbon and water dewpointing is carried out on the gas stream before export through the pipe to prevent liquid from dropping out of the pipe. This is done by reducing the temperature of the gas stream and collecting any condensed liquid before gas transport. Hydrate inhibitor can lso be added to the gas stream to prevent hydrate formation.

REFERENCES
1) Campbell J. M., et al., Gas Conditioning and Processing, Vol 2: The Equipment Modules, 8th Ed., John M. Campbell and Company, Oklahoma (2004).
2) Havard Devold., Oil and Gas Production Handbook, 3rd Ed., ABB, Oslo, (2013).