Gas compression: Gas from a pure natural gas wellhead might have sufficient pressure to feed directly into a pipeline transport system. Gas from separators has generally lost so much pressure that it must be re compressed to be transported. Turbine compressors gain their energy by using up a small proportion of the natural gas that they compress.
The turbine itself serves to operate a centrifugal compressor, which contains a type of fan that compresses and pumps the natural gas through the pipeline. Some compressor stations are operated by using an electric motor to turn the same type of centrifugal compressor.
This type of compression does not require the use of any of the natural gas from the pipe, however, it does require a reliable source of electricity nearby.
The compression includes a large section of associated equipment such as scrubbers (removing liquid droplets) and heat exchangers, lube oil treatment etc. Whatever the source of the natural gas, once separated from crude oil (if present) it commonly exists in mixtures with other hydrocarbons; principally ethane, propane, butane, and pentanes.
In addition, raw natural gas contains water vapor, hydrogen sulfide (H2S), carbon dioxide, helium, nitrogen, and other compounds.
Natural gas processing consists of separating all of the various hydrocarbons and fluids from the pure natural gas, to produce what is known as ‘pipeline quality’ dry natural gas. Major transportation pipelines usually impose restrictions on the make up of the natural gas that is allowed into the pipeline.
That means that before the natural gas can be transported it must be purified. Associated hydrocarbons, known as ‘natural gas liquids’ (NGL) ar used as raw materials for oil refineries or petrochemical plants, and as sources of energy.
Gas Compression main function
Gas from Separators, Surge Tanks and export gases if any from other process platforms are compressed to about 90-100 kg/cm2 pressure as per the field gas lift requirement.
Normally Gas turbine driven Centrifugal compressors (PGC’s‐Process Gas Compressors) are used.
Gases compressed in PGC’s are dehydrated to prevent formation of GAS HYDRATES. Gas hydrates are formed at low temperatures when moisture is present in Hydrocarbon gases. These gas hydrates are ice like substance which prevent the smooth flow or block the flow of gases in gas flow lines.
Gas Hydrates can be formed in Adjustable Choke Valves ,PCV’s & GLV’s in GI Lines where Throttling of gases give rise to low temperatures.(Joule Thomson effect). This can affect production phenomenally from Gas lift wells
Metering, storage and export
Most plants do not allow local gas storage, but oil is often stored before loading on a vessel, such as a shuttle tanker taking the oil to a larger tanker terminal, or direct to crude carrier. Offshore production facilities without a direct pipeline connection generally rely on crude storage in the base or hull, to allow a shuttle tanker to offload about once a week.
A larger production complex generally has an associated tank farm terminal allowing the storage of different grades of crude to take up changes in demand, delays in transport etc.
Metering stations allow operators to monitor and manage the natural gas and oil exported from the production installation. These metering stations employ specialized meters to measure the natural gas or oil as it flows through the pipeline, without impeding its movement.
This metered volume represents a transfer of ownership from a producer to a customer (or another division within the company) and is therefore called Custody Transfer Metering. It forms the basis for invoicing sold product and also for production taxes and revenue sharing between partners and accuracy requirements are often set by governmental authorities.
Typically the metering installation consists of a number of meter runs so that one meter will not have to handle the full capacity range, and associated prover loops so that the meter accuracy can be tested and calibrated at regular intervals.
Pipelines can measure anywhere from 6 to 48 inches in diameter. In order to ensure the efficient and safe operation of the pipelines, operators routinely inspect their pipelines for corrosion and defects. This is done through the use of sophisticated pieces of equipment known as pigs. Pigs are intelligent robotic devices that are propelled down pipelines to evaluate the interior of the pipe.
Pigs can test pipe thickness, and roundness, check for signs of corrosion, detect minute leaks, and any other defect along the interior of the pipeline that may either impede the flow of gas, or pose a potential safety risk for the operation of the pipeline. Sending a pig down a pipeline is fittingly known as ‘pigging’ the pipeline. The export facility must contain equipment to safely insert and retrieve pigs form the pipeline as well as depressurization, referred to as pig launchers and pig receivers.
Loading on tankers involve loading systems, ranging from tanker jetties to sophisticated single point mooring and loading systems that allow the tanker to dock and load product even in bad weather.