Typical Construction Process for Gas Pipelines

The sequence of pipeline construction on land begins with fencing off the working area (approximately 40m wide) and stripping of topsoil within this area. This topsoil is not removed, but is kept to one side; it is not mixed with subsoil and it is replaced as the topmost layer during reinstatement.

Once the topsoil has been cleared, individual lengths of pipe (typically 12m long) are laid out and then welded together. In the case of the Corrib Gas pipeline, welds will be inspected before the pipe is buried in the ground; any defects will be repaired or cut out and re-welded. All transmission pipelines (upstream and downstream) must be pressure tested before being commissioned for service. The assembled pipeline will also be pressure tested using water at a pressure in excess of 500 bar.

The pipeline will be laid in a trench with a minimum depth of cover of 1.2m (approximately 4ft). Once the trench has been filled in and the soil reinstated there will be no evidence that the pipeline is there at all. For this reason markers will be installed to help locate the pipeline (eg at field boundaries, road crossings, changes of direction). All affected landowners will be made aware of the exact location of the pipeline.

Alternative Techniques

Specialised techniques are sometimes used or needed to construct sections of pipelines where ground conditions are difficult or where there are challenging constraints e.g. river crossings, busy roads, bog, etc.

Trenchless techniques (e.g. directional drilling) have also been used successfully in Ireland to cross the River Boyne and Upper Shannon with gas transmission pipelines.

Trenchless techniques can offer advantages of reduced impact during construction stage, but success is highly dependent on ground conditions which must be investigated in advance. To confirm the feasibility of specialised construction methods in the estuary areas, geophysical and geotechnical surveys will be carried out.