Science & Technology in Action

2nd Edition

Gas Extraction in the Corrib Field

Shell

This lesson describes the engineering and technology required to bring gas from offshore wells to an onshore reception depot. The electrical and hydraulic control systems are outlined.
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Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

The organic matter that formed the oil and gas reservoirs off the west coast of Ireland was laid down about 300 million years ago – around the time of early dinosaurs. In a long process involving heat and pressure, oil and gas were formed; these were of relatively low density and rose through the surrounding rock, gravel and water. Some of this hydrocarbon material was trapped by impermeable rock layers forming the reservoirs that are tapped today. 

Developments in the oil industry 
Few subsea oil wells at depths of more than 200 m could be exploited economically in the 1960s. Since then developments in extraction technology and dramatic increases in the price of petroleum have changed all that; today tapping of deposits at depths of more than 2000 m is almost routine. 

Extraction infrastructure
When an oil or gas deposit is confirmed the well is lined with steel tubing (termed casing) and then capped with a set of connectors and valves called a ‘wellhead’ or ‘tree’. Further work is suspended until neighbouring deposits have been similarly capped. Decisions on the method of extraction depend mainly on the type of deposit (oil or gas) and the distance to land. What follows relates to a typical offshore gas deposit such as the Corrib Gas Field which lies in 350 m of water 83 km offshore and about 3000 m below the seabed. 

Pipelines
Wells within a few kilometres of one another are connected by small diameter (15 or 20 cm in diameter) pipes called flowlines to a central manifold (a hub) and a pipeline is laid from the manifold to an onshore reception and processing facility – the terminal. 

Pipelines are typically fabricated in units of 12 metres. These are shipped to the pipelay vessel and are welded together as required, to form a continuous pipe. As this is done the pipeline is lowered to the seabed and attached to the manifold. 

 

Quiz questions

  1. Gas and oil are trapped by porous rock layers.
  2. Oil and gas can be extracted form depths of more than a kilometer below the seabed.
  3. It is possible to drill gas wells 20 km deep.
  4. The gas deposits in the Corrib Field are 100 m below the seabed.
  5. Subsea oil wells at depths of more than 200 m have been exploited since the 1920s.
  6. The Corrib extraction infrastructure was put in place by teams of divers.
  7. Methanol is used as a solvent for natural gas.
  8. ROVs are used to carry divers to the subsea gas wells.
  9. Flow control valves are operated directly by electricity.
  10. Flowlines are connected to a central subsea manifold.