Induction welding of branch pipe in the North Sea
Recovering the subsea work skid that stored the induction heating equipment on the sea bed. The actual welding was carried out by welder divers in a subsea habitat structure.

Induction pre-heating dives deep

The Tampen Link project in the North Sea underlines the amazing versatility of induction heating. In this case, EFD Induction expertise is helping weld a branch pipe onto a 20’’ main high-pressure gas pipe located 145 m below the surface of the North Sea… without any interruption in the flow of gas!

As oil production peaks in the North Sea, energy companies are keen to exploit existing pipelines to maximize output, and to export the remaining — and vast — gas resources. A key part of this strategy is the Tampen Link project. Put simply, the project involves linking Statoil’s Statfjord Field gas output to Shell UK’s existing pipeline for export to the UK.

But how is it possible to open up a gas pipeline and connect a new branch without shutting down the flow of gas? The answer is ‘hot tap’ welding, a method involving a drilling (or ‘tapping’) machine that, together with a full-ported valve and a pressure-containing fitting, cuts open the pipe while it is still in operation. However, the actual welding on of the new branch presents its own technical challenges. As the existing pipe is in use, the gas moving at high pressure results in a massive cooling effect. This makes it extremely difficult to preheat the outer surface of the pipe to the temperature required for successful welding.
The traditional way to pre-heat ‘live’ pipes for welding is to use heating mats. But in the case of the Tampen Link, welder-divers working in a sealed subsea habitat 145 m below the surface do the welding. Heating mats would have resulted in unbearable working conditions for them. Induction pre-heating, it seemed, was the only viable option. Once induction heating was selected, EFD Induction began working closely with all the stakeholders involved in order to perfect a technically feasible and safe solution. Key partners in the project included Technip, Perry Slingsby Systems Ltd., and of course Statoil. A crucial task for EFD Induction was to design, make and test customized induction coils to pre-heat the pipes prior to welding. Then there was the issue of minimizing the manual operations to be performed by the divers. Finally, EFD Induction cooperated with its partners to develop equipment capable of delivering heat at low voltage but very high frequency. The solution has been fully tested at the National Hyperbaric Centre in Aberdeen, Scotland, and the project is scheduled to get under way this summer.