As a result of designing two high pressure piggable isolation plugs (26 in. and 30 in.) for a riser replacement project in the Indian Ocean, Pipeline Engineering (Catterick Bridge, UK) is developing a range of isolation tools for pipeline rehabilitation work. Ideal applications for the tool are valve changeout, renewal of fittings, pipeline diversion, replacement of corroded sections, and riser replacements.
To deploy the tool, it is first loaded into the pig launcher. After closing the door, pressure is applied behind the tool. The plug is then pigged down the line using a medium such as inhibited seawater, water/glycol mix, nitrogen, or the pipeline product. Sometimes, it is necessary to insert a slug of water or glycol/water mix in front of the tool to balance the travel speed/pressure. Then the plug is positioned at the point of isolation using an acoustic pinger for approximate positioning, or a Tracerco (radioactive device for more accurate positioning).
Once the tool is in position, a hydraulic system activates the polyurethane seals and steel vice jaws, locking the plug in position and sealing the line. Three methods of hydraulic control are possible:
- Fully tethered, by means of hydraulic umbilical hoses passing from hydraulic pumps, gauges and recording equipment outside the pipeline through a stuffing box on the closure door down the pipe to the plug.
- Control umbilical, tethered using a high strength telemetry cable from a central control console through the stuffing box down to on-board hydraulics.
- Fully remote, where the on-board hydraulics are controlled by means of through-the-wall communications, usually using very low frequency radio telemetry.
Once the work has been carried out, the grippers and seal are released and the plug is pigged back up to the trap and removed. Plugs normally have a 100% built-in redundancy so that if one module fails totally, the remaining module is capable of holding the full line test pressure.
As the plug will often be left for a period of time in the pipeline, a double block and bleed configuration is normally called for. This means that two plugs are used in tandem and the central annulus between the plugs is constantly monitored to check the integrity of the seal and to monitor for any by-pass. As a further safety measure, plugs can also be designed to be self-actuating when a differential pressure is applied.
On older pipelines, large differences in internal diameter are quite normal. Therefore, in order to reach the isolation position, the plug will require as much internal clearance as possible. Using a patented technique, Pipeline Engineering has developed a tool with a clearance of 40 mm radially, providing up to 80 mm clearance on the pipeline diameter. Similarly, older lines can have tight bends restricting the tools travel down the line. By providing sufficient clearance, the plug can also transverse these bends.
Independent inspectors such as DNV would normally approve and witness the design, stress calculations, and testing of such tools.
