UNDERWATER TIE-IN OF 24 INCH OFFSHORE PIPELINE

Abstract

A 47 mile 24 inch underwater pipeline was required to link the Esso/BHP Halibut field to onshore production facilities. To hasten the completion of this line. Esso elected to use two pipe laying barges: one starting from the shore and one from the platform, and to make an underwater tie-in where the two lay barges met. To complete the tie-in, Esso engaged Ocean Systems Australasia Pty Ltd to make a sea floor welded connection in their underwater welding habitat, using the TIG (Tungsten Inert Gas) hyperbaric welding process and the saturation diving technique. The support vessel for this work was the semi-submersible derrick barge Choctaw. The tie-in was 38 miles offshore in the open waters of the Bass Strait in 218 feet of water. The pipe wall thickness at the tie-in was three-quarter inch.

The Halibut 24 inch pipeline underwater tie-in was successfully completed in a total elapsed time of 383 hours (15 days 23 hours) compared with the estimated total time of 348 hours. Three major factors contributed to the success of the operation; these were:

The ability of the semi-submersible derrick barge Choctaw to stay on location during the bad weather when swells of up to 20 feet were experienced.

The efficient organisation and implementation of the work by Ocean Systems.

The close alignment of the two sections of the pipeline on the seabed by the pipelaying contractor, Ingram Contractors Australia Pty Ltd.