TDW Offshore completes North Sea low pressure isolation operation

TDW Offshore Services has completed a low pressure isolation operation on an export pipeline riser in the North Sea.

Offshore staff

STAVANGER, Norway -- TDW Offshore Services has completed a low pressure isolation operation on an export pipeline riser in the North Sea.

In 2007, a passing vessel collided with the southeast face of a satellite platform jacket, damaging the 12-in. (30-cm) export riser. Production from the platform was immediately shut-in via the emergency shutdown valves, leaving the pressure in the pipeline at approximately 4 barg.

Before production could resume, the operator needed to repair the riser. Following analysis, engineering, and testing, TDW developed a solution using its range of specialist pipeline pigging, pig tracking, and remote communications technology. It consisted of the following elements:

•A custom-designed TDW pig trap and pigging spread
•A high friction pig train furnished with the SmartTrack remote tracking and pressure-monitoring system
•A SmartTrack subsea remote tracking and pressure-monitoring system
•A SmartTrack topside tracking and monitoring system with radio link to the dive support vessel
•A pipeline isolation ball valve.

In August, TDW used its remote-controlled SmartTrack technology to isolate the damaged riser section from the gas inventory in the export pipeline without venting or flooding the pipeline, or displacing the pipeline inventory. A three-module high friction pig train created an isolation against the gas pressure in the pipeline. The company was then able to verify and record the pipeline inventory gas pressure, and close and isolate the Emergency Shut Down Valve (ESDV) 050. The redundant topside pipework upstream of the ESDV was removed, and a temporary spool and valve were installed. Leakage over the ESDV was monitored closely, with a view to minimizing pressure build-up in the spool.

Using the pig trap and pigging pump, the high friction isolation pig train (HFIPT) was launched and pigged with water to the pre-determined isolation position within as straight spool section of the vertical riser. Using TDW’s remote tracking technology, technicians onboard the dive support vessel (DSV) tracked the position of each pig to verify that the HFIPT was located below the damaged section of riser that was designated for replacement. Communication skids were positioned over the three pigs and connected to the pig monitoring system. By doing so, TDW was able to monitor the downstream pressure of each isolation pig continuously throughout the operation using its “through pipe wall” communications technology that makes it possible to send isolation integrity data by radio link to a DSV.

The existing topside pipework was removed and replaced with new pipework. Divers were deployed from the DSV. The section was successfully cut and removed using a crane onboard the DSV. A mechanical connector was locked onto the existing riser. The new riser was attached to a crane on the platform and lowered down to rope access workers who installed it on the topside pipework closing spool and to the existing riser located above the HFIPT.

Following installation of the new riser section, TDW verified that the ESDV and new valve were operating properly and fully open. After purging the riser and topside
pipework, TDW used its pigging pump to slowly increase the water pressure to begin pigging the HFIPT downstream away from the platform to the launcher. The ESDV and 12-in. (30-cm) valve were then closed, and the pipeline gas inventory pressure was increased to keep the HFIPT moving forward. After all pigs were recovered in the temporary pig trap, the ESDV and new valve were closed. Using the platform crane and ROV, all pigging equipment was removed and the TDW crew demobilized. All offshore operations were carried out in about 10 weeks, the company reports.

By using its remote tracking and pressure monitoring technology, TDW says it repaired the damaged riser while maintaining a continuous flow throughout the operation.

12/03/2009

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