Strong currents, difficult conditions and pipes over 40 years' old were among the obstacles encountered by divers during repairs on a small but vital pipeline linking North Sea gas platforms to the Norfolk coast.
A 3.5in mono-ethylene glycol (MEG) line running alongside the main gas pipeline was fractured by trawler lines. Since MEG injected into the piped gas prevents the build-up of hydrates that clog valves at the onshore refinery, restoring the platform’s return MEG supply was an urgent matter if far greater expense was to be avoided.
"A major survey of the 45 mile pipeline was undertaken to find the leak -- that took almost three weeks," explained Hydratight's field technician, Bob Till.
"We were then called in to complete the repair, which involved replacing 61m of damaged pipe and took another three weeks."
This was Hydratight's third repair to these lines: though the pipes are fairly deep, this is a busy shipping and fishing area and the pipeline is easy prey for trawler lines.
Though Hydratight engineers knew the job from an historical perspective, there were still big physical challenges in achieving the repair.
Most important was the unavailability of the same size pipe work. The damaged line was 3.5in diameter, but this is no longer available. Instead 4in pipe was used, with a suitable reducer.
The pipe was also 35m down, in strong tidal currents that meant divers only had a two hour window to work every six hours.
"We had a team of 20 divers, two at a time, working round the clock in shifts for three weeks. One man worked and the other provided cover each dive. In fact, they could work for only 35 minutes at a time because of decompression considerations," Bob explained.
The teams cleaned the pipe back to sound metalwork for the repair. All outer protection coatings were removed to prepare for the Morgrip connectors, and the seabed had to be dredged so there was room to apply the connectors to the old and new ends.
"This was a tricky job, despite the fairly small pipe diameter," said Bob. "Morgrip jobs have been done on pipes deeper than this and up to 30in in diameter, but this one had its own specific problems to overcome. We used ROVs for observation and even to do some of the dredging as the connectors were applied, but the job was a great success -- the connectors will be good for at least another 30 years."
The project demonstrates the importance of contingency planning by oil and gas operators. Following similar repairs in 2003,The client had bought Morgrip connectors from Hydratight to cover future repairs, and these were ready at Hydratight's Walsall HQ for this job. It was thanks to the client's advance planning that this repair could be completed quickly and efficiently.