Gases Burnt off in Offshore Platforms
The products obtained from offshore platforms are mainly natural gas and crude oil. Before removing the crude oil from the well, they will remove pockets of natural gas located on the sea bed. The obtained natural gas is then processed and transported for industrial purposes and power generating stations.
Natural gas may occur in:
- Undergrounds wells are mainly gas bearing
- Condensate reservoirs (pentanes and heavier)
- Large oil fields (associated gas)
In the case of oil wells, natural gas may present either in solution with the crude oil or as a gas cap above it.
Natural gas comes from underground deposits and contains smaller quantities of heavier hydrocarbons (collectively known as natural gas liquids, or NGL). This is in addition to varying amounts of water, carbon dioxide, nitrogen, and other non-hydrocarbon substances.
Natural Gas Liquids (NGL)
Associated gas found in combination with crude oil comprises mainly methane and NGL. NGL is mainly made up of ethane, which is LPG gasoline. NGL has an atmospheric boiling point in the range of -80o C at atmospheric pressure or -45o C at a vapor pressure of 5 bars.
Liquefaction of Gases (Atmospheric Boiling Point):
Methane (LNG) is stored at -162o C
Propane is stored at -45o C
Butane is stored at -0.5o C
During the process of separating the LNG from the raw feed, gas obtained from the crude oil bed cap is burned in the flare stake (i.e., NGLs are burnt). The raw feed gas is first stripped of condensates. This is followed by removal of acid gases like carbon dioxide and hydrogen sulphide. Carbon dioxide must be removed as it freezes at temperatures above the atmospheric boiling point of LNG, and toxic compounds like hydrogen sulphide are removed as they cause atmospheric pollution when burnt in fuel. Acid gas removal saturates the gas stream with water vapor, and this is then removed by the dehydration unit.
The gas then passes to a fractionating unit where NGL is removed and further split into propane and butane. Finally, the main gas flow is now mostly methane, which is liquefied into the end product, liquefied natural gas (LNG).
Normally in the offshore platform, the fractionating column of pure dry LNG is then passed into the liquefaction plant situated in the platform. Once the LNG is liquefied by using the compressors, it is then stored in tanks at refrigerated condition -162o C- well below its atmospheric boiling point and maintained in it.
The amount of NGL obtained from the LNG fractionating units is considerably negligible, and it is not profitable to install low-duty compressors in the offshore platforms in order to liquefy the NGL like propane and butane, which have a high atmospheric boiling point when compared to LNG.
And if at all we install low-duty compressors for liquefying NGL, before liquefaction we have to carry out a separation process. The NGL will contain propane and butane that has to be separated first. So in order to separate the two gases having the different atmospheric boiling points, we need to install a fractionating unit to separate the two gases. So it is better to burn the NGL in the flare stack rather than liquefying it for storage.