Customized Isolation Operation Solves North Sea Riser Repair

TDW Offshore Services AS has completed an innovative low-pressure isolation operation on an export pipeline riser in the North Sea. The work results from a 2007 incident when 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 4 barg.

Before production could resume, the operator needed to repair the riser. The objective was to cut and remove the damaged riser section, replacing it with a new one. TDW Offshore Services was retained to formulate a low-pressure isolation solution to isolate the damaged section of the pipeline riser from the export pipeline gas inventory. By doing so, the damaged riser section and associated topside pipework production system could be replaced safely.

TDW developed a solution using its range of pipeline pigging, pig tracking and remote communications technology. The approach involved using multiple high-friction pigs 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.

Last 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 first step was to verify and record the pipeline inventory gas pressure, 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.

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 a straight spool section of the vertical riser. Using TDW’s remote-tracking technology, technicians on 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. TDW could monitor the downstream pressure of each isolation pig continuously throughout the operation using its innovative “through pipe wall” communications technology that makes it possible to send isolation-integrity data by radio link to a dive-support vessel.

The existing topside pipework was removed and replaced with new pipework. Divers were deployed from the DSV. The section was 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 to rope-access workers who installed it on the topside pipework closing spool and to the existing riser located above the HFIPT.