TDW's Customized Isolation Solution Smoothes Path for Safe Repair of Damaged Riser in North Sea

TDW Offshore Services AS announced Dec. 3 that it has successfully completed an innovative 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-inch 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.

A First: Monitoring Pressure During a Low Pressure Isolation
Before production could resume, the operator needed to repair the riser. The objective was to cut and remove the damaged riser section, and replace it with a new one. To facilitate the repair process, TDW Offshore Services was retained to formulate a low pressure isolation solution in order to isolate the damaged section of the pipeline riser from the export pipeline gas inventory. The goal was to replace the damaged riser section and associated topside pipework production system safely and efficiently.

Following intensive analysis, engineering and testing, TDW developed a solution using its range of specialist pipeline pigging, pig tracking and remote communications technology. The unique approach involved using multiple high friction pigs in order to seal off and replace the damaged riser section and pipework. The solution 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.

During August 2009, TDW used its remote-controlled SmartTrack technology to successfully 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 first step was to verify and record the pipeline inventory gas pressure, and close and isolate the Emergency Shut Down Valve (ESDV) 050. The redundant topside pipework located upstream of the ESDV was removed, and a temporary spool and 12-inch valve were installed upstream of the ESDV. Leakage over the ESDV was monitored closely, with a view to minimizing pressure build-up in the spool.

“Through Pipe Wall” Communications Technology Plays Key Role
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.