Dredging, coating removal and cutting of damaged pipeline


Allseas UK, tasked with developing the pipelines connecting the Total Laggan-Tormore gas fields 125km north-west of the Shetland Islands, encountered a damaged flowline due to a dragged vessel anchor.

Underwater Cutting Solutions (UCS) were enlisted to cut out and replace the damaged pipework in Flowline 2 (FL2). FL2 was comprised of 18 inch (457mm) OD x 25mm WT carbon steel pipe grade SAWL 450 FDU, with a 3LPP coating and 50mm thick outer Concrete Weight Coat (CWC).

The solution

In order to prove that the equipment was suited to the required work scope, a coating removal and cutting trial was performed at UCS headquarters. It was found that the CRT was able to clean a strip of concrete coating the full circumference of the pipe in less than 40 minutes, using two linear passes 0.5m long.

The initial phase of the project involved the use of an 8” UCS Dredge System with a bespoke-designed high pressure Barracuda head to clear a channel under the damaged pipework. The Coating Removal Tool (CRT) effectively and rapidly removed concrete coating and 3LPP undercoating to a depth of 1.2m.

Finally, a 42” UCS Diamond Wire Saw completed six cuts, requiring only 44 minutes on average per cutting operation. The remnants were safely recovered to the deck of the Allseas vessel.

Results

Completed on schedule and with no lost time incidents, the dredging, coating removal and cutting operations were successfully delivered with minimal downtime.