A recently completed subsea hot tap undertaken by Stolt Comex Seaway Ltd on behalf of client Wintershall Noordzee BV has firmly established an important new technique for pipeline intervention and modification, which will significantly reduce the cost of developing marginal oil and gas fields. Although hot tapping has been used routinely on land pipelines, it has, until now, been uneconomic for high capacity subsea pipelines. This is due to the difficulty of achieving the required weld quality for the retrofitting of tees under hyperbaric conditions without shutting down production. This problem has been overcome by the application of a unique induction heating system to provide continuous preheating of the weld zone under extreme line flow and pressure conditions. The system, together with a number of other special techniques, was employed in October 1994 for the subsea installation of a 36in by 16in tee on the Den Helder gas trunkline, which will receive gas export from Unocal BV's Halfweg development in 1995.
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