Subsea pipeline welding performed at record depth

It is the first time that hot-tap welding, a way of connecting pipes, has been done at such depths and is the culmination of around 10 years’ research by Cranfield University and industrial partner Statoil.

The work could have a major impact on the offshore gas and oil industries, as Neil Woodward, an engineer for Statoil who is based at Cranfield, explained to The Engineer.

‘Say there’s a pipeline already on the sea bed, it might have been there 20, 30 years and, for whatever reason, it’s more economical to tap into that pipeline and divert a bit of flow somewhere to make the oilfield more profitable, rather than lay a new pipe.

‘It’s called hot tapping because the product is still flowing in the pipe — you don’t close the pipe and that is a big economic driver, because if you have to shut down a pipe you’re potentially talking about millions of pounds a day.’

Subsea pipes are found at various depths and, for those up to 180msw, divers may be used for maintenance and repair. But below these depths, mechanical couplings and other remote-welding techniques are necessary.

To investigate the feasibility of operating at lower depths Cranfield University (with EPSRC funding) installed the world’s highest-pressure dry hyperbaric welding chamber at the University in 1997, able to simulate up to 2,500msw water depths.

In this first phase, the chamber was used for the detailed theoretical and practical research on welding techniques at high pressures and, in 2004, the results demonstrated that welding at these previously unreached deep-sea depths was indeed possible.

Since 2004, qualification work has been performed by Statoil to determine the practicalities of achieving these depths in the field. This culminated in the first successful deep-sea trials conducted in Norway earlier this year.

‘We’ve spent years in the laboratory trying the examine effects of depth, moisture, temperature — all sorts of parameters — to make sure that when we go offshore we can anticipate all the things that may happen. It’s not as easy to do any remedial operations as you can with divers — it’s fully robotic basically.’