FPSO-EXPORT METERING, GAS DEHYDRATION, SAMPLING ANALYSIS & EXPORT PIG OR SPHERE LAUNCHER

OBJECTIVE:

After successful completion of this article session the participant will have improved knowledge of the

• Gas dehydration
• Gas export metering
• Export gas compressor
• Gas sampling analysis
• Gas export pig or sphere launcher
• Gas export pipe line

GENERAL:

The gas export and metering facilities are used to raise the pressure of the export specification gas, measure the quality and quantity of the gas and finally export the gas.

A typical system contains

• Export gas compressor
• Gas export metering package
• Gas export pig or sphere launcher
• A gas export pipeline

EXPORT COMPRESSION:

ROUTINE OPERATIONS:

• Monitor correct operation of all instrument control loops.
• Check that correct levels of pressure and temperatures are maintained.
• Make regular checks of all ancillary equipment, such as lube oil and seal gas systems.

NORMAL SHUTDOWN:

Manually initiated shutdown of the gas export system usually:

• Unloads the compressor pressure
• Stops the driver
• Isolates the export system
• Fails safe

EMERGENCY SHUTDOWN:

Shut down is initiated automatically and depressurization takes place whenever following occurs,

• Compressor seal failure
• Cooling failure
• Fire and gas initiation
• Manual initiation

EXPORT METERING:

THE EXPORT GAS FISCAL METER:

• It is designed to comply with fiscal metering requirements
• Provides accurate measurement of the exported product
• The meters are usually Orifice plate type meters

METERING STREAMS:

Gas metering is organised in number of streams

• There are normally 3 identical gas metering in streams
• Each stream typically will handle 50% of the total maximum gas export flow
• One stream will be on standby or being maintained

The objective of the metering stream is to allow,

• Calibration and meter correction factors
• Maintenance
• Accurate metering of the export gas

GAS ANALYSIS:

The gas must be analysed to find its composition and energy content for the client when being exported. Gas analysis instruments are normally provided at the inlet header of the streams.

• Gas chromatography to define the composition of the gas
• H2S analyzers to find the presence of H2S
• Hygrometers (Draw point analyzers)
• Such analysis is usually carried out by the laboratory technicians

GAS LINE PIGGING OPERATIONS AND LAUNCHER:

The reasons, methods and theories behind pigging operations were covered in the oil export section in detail. Some of the facts to remember for gas pigging are as below:

• The extra hazard caused by the compressibility of the gas.
• The launcher / receiver must be depressurized fully and vented before opening to atmosphere.
• Some door mechanism can be the greatest source of injuries.

PIPELINE PROTECTION:

• The topsides of the gas export line are often protected by a set of three individuals High-High pressure switches.
• The system called the High Integrity Presssure Pretection System (HIPPs) operations on a two out of three voting system and causes ESD shutdown.
• The voting arrangement means that if one switch fails, the pipeline is not shutdown. Two switches out of the three must 'fail' before the pipeline will close in. (This prevents unnecessary shut downs due to instrument failure.)
• An export gas pipeline is only ever 'blow down' (depressurized) for emergency or maintenance reasons.
• If a pipeline section must be depressurized, increased methanol injection should be used prior to the well or the lining being shut in. This will help start-up and prevent hydrates.
• Blow down rates are dictated by the low temperatures caused by gas expansion.
• The export pipeline metallurgy is the ultimate limiting factor for depressurization rate. (If the temperature fails too rapidly causing cooling beyond the capacity of the pipeline metal, it may fracture.)

THANK YOU!