On an offshore production facility, gas doesn’t leave the separators ready for export or reinjection. It has to be cleaned, cooled, and compressed in a controlled sequence known as the gas compression process. Without this step, gas would arrive too hot, too wet, or at the wrong pressure for the next stage of its journey.
The gas train, a series of treatment and compression stages, is designed to take gas from different separator outlets, prepare it for compression, and then step it up to the required pressure, whether for pipeline export, storage, or reinjection back into the reservoir.
Cooling the Gas | Heat Exchangers
Gas straight from the separator carries heat. Compressing it adds even more. High temperatures aren’t just inefficient; they also increase stress on the equipment.
Cooling is handled by heat exchangers. Two designs dominate offshore work:
- Plate heat exchangers: Compact units with thin, stacked plates where gas and coolant pass on alternate sides in opposite directions.
- Shell-and-tube exchangers: Bundles of tubes running through a water-filled shell, a workhorse design well-suited to large flow rates.
Whenever possible, the facility makes use of waste heat for example, warming oil in the crude train before dumping excess heat to seawater systems. Seawater is effective but unforgiving, so corrosion-resistant alloys like titanium are used in these exchangers.
Removing Liquids | Scrubbers and Reboilers
Even after cooling, gas carries fine droplets of water and light hydrocarbons. If those reach a compressor, they can erode blades and shorten service life.
A suction scrubber strips out the liquids. On many facilities, this includes triethylene glycol (TEG) dehydration:
- Gas rises through several stages where it meets downward-flowing glycol.
- The glycol absorbs water as it works its way down.
- At the bottom, the “rich” glycol is sent to a reboiler, where heat drives off the absorbed liquids.
- After regeneration, the glycol is cooled and recirculated to the scrubber to start the process over again.
Depending on the design, reboilers may be powered by recovered hydrocarbons or by a mix of process heat and electric elements.
Compression | Matching Equipment to the Job
The gas compression process is almost never completed in just one stage. Pressure levels Pressures are built up in stages, using the compressor type best suited to each role:
- Reciprocating compressors: Piston-driven, good for high pressures at low volumes.
- Screw compressors: Two meshing screws handle medium pressures with a steady output.
- Axial compressors: Rows of rotating blades move large volumes at moderate pressure increases.
- Centrifugal compressors: Radial impellers for high flow rates and moderate-to-high pressures, a common choice for main gas trains.
Dividing compression into multiple stages helps distribute the workload, boost efficiency, and simplify upkeep. In major facilities, parallel trains are often used to provide backup capacity.
Guarding Against Surge | Anti-Surge Control
A compressor needs a steady minimum flow. Drop below that point and you risk a rapid reversal of flow that can damage the machine in seconds.
To safeguard compressors, anti-surge systems redirect part of the discharge gas to the suction side. Before re-entry, this gas passes through cooling and scrubbing to remove heat and impurities. Automated controls track performance in real time and fine-tune the recirculation valve, ensuring the compressor stays safely away from the surge point, with rapid valve action if operating conditions near unsafe levels.
The Support Systems That Keep It Running
Several auxiliary systems make the gas compression process reliable over the long haul:
- Load management to share work evenly between compressors.
- Vibration monitoring to catch mechanical issues early.
- Speed control via turbine governors or variable speed drives.
- Lubrication and seal oil systems to keep bearings running smoothly and prevent gas leaks at the shaft seals. Oil in these systems is filtered, degassed, and kept in good condition to avoid contamination.
Why It Matters
The gas compression process is not just about achieving a pressure target — it ensures that natural gas is delivered at the right temperature, moisture content, and flow rate, while also protecting valuable equipment and maximizing energy efficiency. In offshore production, this difference translates directly into steady, reliable output and the avoidance of costly unplanned downtime.
To see how compression builds upon earlier stages of hydrocarbon handling, explore our “Blog on Oil and Gas Separation Process in Upstream“, where we explain how fluids are separated before processing.
Looking ahead, compression is also the crucial link to transportation. Once gas is compressed, it must be moved efficiently to markets via pipelines, LNG carriers, or storage systems. We’ll explore this in detail in our upcoming “Blog on How Oil and Gas Are Transported‘.
Frequently Asked Questions
It is the systematic procedure of cooling, removing impurities, and increasing the pressure of natural gas after it has been separated from the production stream, preparing it for transportation, further processing, or reinjection into the reservoir.
Compressing gas generates heat, and hot gas is harder to compress. Cooling it first reduces energy use, improves efficiency, and protects the compressor.
A gas scrubber removes liquids and fine droplets from the gas stream before compression. This prevents blade erosion and other mechanical damage inside the compressor.
No single compressor type can efficiently handle the entire pressure increase needed offshore. Using multiple stages improves performance, reduces wear, and makes maintenance easier.
Surge happens when flow through the compressor drops below a safe limit, causing gas to reverse direction suddenly. This can cause severe mechanical stress and must be avoided.
