Contains the full lesson along with a supporting toolkit, including teachers’ notes.
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.
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.
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.