Subsea isolation techniques advance

Feb. 1, 2010
Two recent subsea pipeline isolations illustrate the advances made in isolation technology and methods.

Two recent subsea pipeline isolations illustrate the advances made in isolation technology and methods. One isolation was for repair of the line itself, the other for replacement of motor-operated ball valves. This article examines each.


TDW Offshore Services AS in December 2009 completed a pipeline pressure isolation operation in Malaysia with Amserve Engineering on behalf of Malaysia LNG Sdn Bhd, a subsidiary of Petronas, the national oil company of Malaysia.

The operation occurred at the Petronas LNG complex in Bintulu, Sarawak. Natural gas moves from platforms in the South China Sea through four major trunklines to the onshore processing plant. Trunkline 4 (36-in. OD) required pipeline pressure isolation to replace six 22-ton motor-operated ball valves.

Codes generally don't require a cut out for dents less than 5% WT, only a leak repair-reinforcement clamp. Therefore no isolation is required. For dents between 5% and 10% WT, a train of two SmartPlugs with a slug of glycol between them launched into position on either side of the dent and activated in place allows immediate removal of the damaged section.

For dents larger than 10% WT, through which the SmartPlug cannot pass, an upstream SmartPlug moves from the pig launcher to the damage, while the downstream Smartplug is installed in situ via a coiled-tubing injection head through a hot tap and subsea launcher in a post-installed wye-piece.1

For the Malaysian operation TDW carried out pipeline pressure isolation on the receiving end of Trunkline 4, isolating the section by launching its SmartPlug tool at the receiver located at the gas terminal. The 36-in. diameter custom tool travelled about 750 m past the receiver and the beach valve where the tool was set, safely isolating the line while MLNG replaced the six existing MOVs with new ones.

TDW engineered, assembled, and tested the tools at its global headquarters in Stavanger, transporting them to Bintulu via Labuan. Persafe Engineering Sdn. Bhd. transported the new MOVs, with a collective weight of 66 tons, on three chartered MASkargo Boeing 747-400S jets from Germany to Kuala Lumpur before their arrival in Bintulu.

TDW completed the operation in 4 weeks, 6 days ahead of schedule, resulting in considerable savings for MLNG.

SmartPlug allows the operator to monitor pipeline pressure on both sides of the seal and, if appropriate, command the device's battery power into hibernation mode to preserve battery life during extended isolation applications. The longest a SmartPlug has been in position and successfully awakened exceeds 400 days.1

North Sea

The work in Bintulu followed completion of a low-pressure isolation on an export pipeline riser in the North Sea. In 2007, a vessel collided with the southeast face of a satellite platform jacket, damaging the 12-in. OD export riser and a large portion of the connected topside pipeworks (Fig. 1). Any internal isolation would have to be pigged through a dent, a 1.5-D bend, and a narrow 45° traverse.2 Production from the platform was shut in via emergency shutdown valves, leaving line pressure at about 4 barg.

In 2007 a vessel collided with the southeast face of satellite platform jacket, damaging the 12-in. OD export riser (highlighted in red). The impact damaged the riser and a large portion of the connected topside pipeworks (Fig. 1).

The operator sought to cut and remove the damaged riser section and replace it with a new one.

TDW isolated the damaged section of the pipeline riser from the export pipeline gas inventory—allowing replacement of the damaged riser section and associated topside pipework—with multiple high-friction pigs sealing off the damaged riser section and pipework. The solution consisted of the following elements:

• Custom-designed pig trap and pigging spread.

• High-friction pig train furnished with the SmartTrack remote tracking and pressure-monitoring system.

• SmartTrack subsea remote tracking and pressure-monitoring system.

• SmartTrack topside tracking and monitoring system with radio link to the dive support vessel.

• Pipeline isolation ball valve.

The customized pigs outfitted with SmartTrack transponders allow the accurate pig tracking (±2 in.), positioning, identification, and continual pressure monitoring deemed necessary with divers in the water. The SmartTrack system, when mounted in the pig body, provides two-way through-pipe-wall communication. Each pig in the train also has a unique identifier, further minimizing risk in the event the train moves.2

Factory acceptance tests confirmed pressure capabilities before executing the job. A test rig designed and built at TDW facilities in Stavanger had a pig launcher-receiver, a 1.5-D bend, and a 45° traverse to simulate the environment on the satellite platform. Tests of various configurations mirrored the expected scenario as much as possible. Pressure tests included pigging pressure tests, isolation pressure tests, and flip pressure tests.2

TDW isolated the damaged riser section from the gas inventory in the export pipeline in August 2009 without venting or flooding the pipeline, or displacing the pipeline inventory. A three-module high-friction pig train created isolation against the gas pressure in the pipeline.

TDW first verified and recorded the pipeline inventory gas pressure, closing and isolating Emergency Shutdown Valve 050. The company then removed redundant topside pipework upstream of the ESDV and installed a temporary spool and 12-in. diameter valve. ESDV leakage was monitored closely, with a desire to minimize pressure buildup in the spool.

The topside pipeworks shown here (left photo) are not traditionally pigged, requiring pigging for the low-pressure isolation through nonstandard construction pipework fittings (Fig. 2). The nonstandard construction pipework fittings (middle photo) consisted of two 1.5-D bends and a narrow 45° traverse (Fig. 3). The geometry of the damaged section (right photo) of riser was unknown, requiring emphasis on maintaining the section's integrity while running the pig train through the riser (Fig. 4).

A pig trap and pigging pump launched the high-friction isolation pig train and pigged with water to the predetermined isolation position within a straight spool section of the vertical riser (Figs. 2-4). Technicians aboard the dive support vessel tracked the position of each pig, verifying the HFIPT was below the damaged section of riser designated for replacement. Communication skids positioned over the three pigs and connected to the pig's monitoring system allowed the downstream pressure of each isolation pig to be continuously monitored throughout the operation.

Existing topside pipework was removed and replaced with new pipework. Divers deployed from the DSV successfully cut and removed the riser section with a crane aboard the DSV. A mechanical connector 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 to the topside pipework's closing spool and the existing riser above the HFIPT.

Following installation of the new riser section, TDW verified proper operation of the ESDV and new 12-in. valve. After purging the riser and topside pipework, TDW slowly increased water pressure to begin pigging the HFIPT downstream from the platform to the launcher. It then closed the ESDV and 12-in. valve, increasing pipeline-gas inventory pressure to keep the HFIPT moving forward.

TDW recovered all pigs in the temporary pig trap and closed the ESDV and new valve. The platform crane and remotely operated vehicle removed all pigging equipment and the crew demobilized.

Operations took about 10 weeks, May 20-Aug. 2, 2009, during which a continuous flow was maintained.


1. Lim, G.K., and Major, J., "The Challenges of Emergency Pipeline Repairs—Introducing New Rapid Pipeline Repair Methods," 5th Asian Pipelines Conference & Exhibition, Kota Kinabalu, Sabah, Malaysia, Oct. 28-29, 2009.

2. Haldorsen, A.H., "Innovation Low-Pressure Pipeline Isolation," Pipeline Pigging Products and Services Association seminar, Aberdeen, Nov. 18, 2009.

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