offshore central processing platform cpp : – The basic function of the offshore process equipment on as gas producing offshore installation is to remove water prior to transportation of the gas to the onshore reception facility Below mention facilities are provided in process offshore platform.
Major Equipment offshore central processing platform
Wellheads
The gas process commences at the Christmas tree, This is the main isolation valves assembly though which wet gas at full reservoir pressure 2000-3000psig must pass before entering the production flow lines. From the flow line the gas enters a slug catcher where the first phase of liquid removal takes place.
Slug Catcher
The slug catcher is design facilities the removal of large slugs of water and sand prior to the gas entering the production header. In common with most gas process equipment the slug catcher relies on very basic principles fluid dynamics of its operation.
The incoming gas is forced to make a sharp change of direction within the vessel and in so doing, the heavy liquid phase are thrown out into the base of the vessel. The liquids are disposed of via the produced water system.
Meter Run And Test Separator
Having removed any large slugs of liquid or sand could prove injurious to subsequent items of process equipment the gas may flow through a metering orifice bore prior to entry into the production header. Individual metering of each well permit output to be regulated and provides an indication as to the performance of the well.
When more specific information is required the well can be routed through a test separator which can measure the exact quantities of gas water and condensate produced. Regular conditions of the reservoir provide early indications of any problems likely to effect gas production and permits corrective action to be taken.
Production Header
The production header is essentially a manifold which receives the gas from all the slug catchers and redistributes it into a number of process trains. Some installations do not require slug catcher so the production header receives gas direst from Christmas tree flow lines. The next step in the liquid removal process takes place in the production separator.
However before the gas enters the production header it must be reduced in pressure to 98 bar due to the impracticality of construction plant and equipment capable at full reservoir pressure. Pressure reduction and the regulation of gas flow is effected by a control valve or choke valve which is automatic in operation. This valve is normally fitted the slug catcher and production header but in the absence of slug catcher it will be installed on the Christmas tree outlet or wing valve.
Production Separator
The production separator is facilities gas and liquids separator in a similar manner to the slug catcher. However the liquids thrown out as the gas changes direction settle and separate in the base of the vessel.
The water enters the produced water system whilst any condensate present is discharged into the coalescer. The gas continues its journey by entering a heat exchanger.
Heat Exchanger
The purpose of the heat exchanger is to produce gas at a temperature at which the water absorption process within the glycol contactor will be at its most efficient. The type of heat exchanger used will depends upon on reservoir conditions. Some reservoir produce gas at temperature as high as 60 Deg and under these conditions the gas will require cooling before it enters the contactor.
This is normally carried out within a thermostatically controlled fan assisted tube type cooler. Cooling the gas assist in the removal of liquids which condense out as the temperature falls. Once the gas is at the correct temperature it can be passed into the glycol contactor for final drying.
Glycol Contactor
This vessel may function as combined scrubber and contactor, or the scrubber may exits as a separate vessel through which the gas passed prior to entering the conductor. At this stage in the process the gas should be relatively dry. The scrubber will entrain any remaining water droplets within a series of fine metal mesh screens before the gas enters the conductors for final mopping up operations. The contactor consisted of a vertical pressure fitted with a number of horizontal trays over which triethylene glycol (TEG) cascade.
As the gas flow upwards though the vessel it contacts the TEG which acts like liquid blotting paper absorbing any remaining moisture. The gas then passed though a mist extractor designed to remove any entrained glycol before entering the discharge header. The TEG leaves the base of the vessel for circulations though the glycol regeneration plant.
Discharge Header
The discharge header is essentially a manifold which receives gas from the various process trains and redirects it into exports line.
Exports Line
The exports lines usually contains a pig launcher/receivers, a metering box, a condensate re-injection point and emergency shut down valve ( ESDV)
Pig Launcher/Pig receiver
Over periods of time liquids scale and debris will accumulate in the horizontal section of the sub sea pipe line and eventually gas flow will be affected. To counter this problem large poly propylene spheres or pigs are periodically launched under gas pressure to sweep the line by Pig Launcher/Pig receiver. The operation is known as pigging. The line and the debris will be pushed ahead of the pig into a slug catcher at receiving installations or the reception facilities on the beach.
Now technology has changed intelligent pigs are used over traditional pigs. These sophisticated intelligent pigs contain a magnetic eddy currents measuring device energized by an intrinsically safe power and are employed to carrying out an ultrasonic inspection of the subsea pipeline. The pig is propelled by gas pressure and can quickly and efficiently detect any abnormalities in the condition of the pipe line
Condensate re-injection point
condensate is pumped into the export gas for delivery to onshore where it will again be separated for sale as separate commodity to the gas.
Metering Box
Gas leaving the installation enters a subsea pipeline which is invariably shared with other installations in the field. A metering box enables gas production to be calculated to ensure that installation is credited with its due rewards.
Emergency Shut Down Valve (ESDV)
The emergency shut down valve consists of fail safe fire proof valve designed to isolate the platform from the subsea distribution system. This ensures that should an accident occur, the platform will not be fed with gas from rigs. These valves were installed because this is very situation arose on the pipe
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