Until recently, the cutting of an installed subsea pipeline was a comparatively rare event, usually necessitated by the installation of a tie-in or repair of a damaged section. However, with the possibility of subsea emergency shutdown valves (ESV's) being required on many lines in the near future, many operators are reviewing not only the currently available ESV's but also intervention systems.
The primary element of any intervention system is a plug or other device to isolate a section of pipeline. The functions of the plug are:
To isolate divers and equipment working on a section of pipeline from pressure surges in the remainder of the line.
To dispense with the need to flood the line with water prior to intervention. A gas line can take several weeks to dry out after flooding, which is obviously an undesirable situation.
Some manufacturers have proposed plugs capable of withstanding full line pressure, so that a gas pipeline, for example, would not need to be depressurised prior to intervention. Since most ESV's will require diver assistance for installation, most operators and other interested parties have advocated the use of low pressure plugs, thus reducing risks to divers but requiring pipeline depressurization.