During the August 1991 Maintenance Shutdown of the Ekofisk Field, the 36" riser for the Ekofisk-Emden Gas Transmission Pipeline and a 70 m section of the subsea pipeline downstream of the 2/4 Riser platform were replaced in their entirety. The need for replacement was due to subsidence of the seabed due to reservoir compaction which had resulted in riser movement and deflections that would have caused unacceptable level of stress in the riser without implementing the remedial repair program.

The hydroplug sealing device utilized to isolate the pipeline was a three-module modified version of earlier two-module hydroplug designs used in 1981, 1984 and 1987 for various riser and valve repairs on the Ekofisk-Emden Pipeline Transportation System. The redesign of the hydroplug included the addition of a third module used for centralizing the umbilical in the pipeline and modifications to both the slip and seal modules to ensure positive sealing in the 3 mm thinner walled subsea line pipe.

This paper describes the project premises and planning, the required hydroplug modifications, verification testing and the actual performance of the hydroplug deployments necessary to isolate the P/L during the riser replacement and valve repair program. This is the first known field procedure where subsea habitat welding activities have been conducted behind a high pressure gas barrier plug.

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