ABSTRACT:

This paper describes the design and installation of three offshore pipelines and a power cable to platform Osprey in Cook Inlet, Alaska. Although hardly classified as a Mediterranean area, the methodology of this unusual installation project is applicable to other areas of the world. The absence of offshore pipe laying equipment necessitated an innovative approach. The project included, (1) drilling four cased borings from a 65 m [210 ft] high bluff, (2) pulling three heavy wall pipelines 3.2 km [2 miles] through the borings to the platform, and, (3) pulling three pipeline risers through short radius pull-tubes into the platform. The fourth borehole was used for a power cable to the offshore platform. The installation involved three pipelines, a 219 mm [8.625-inch] oil line with a wall thickness of 19 mm [0.750 inch], a 219 mm [8.625-inch] high-pressure water injection line with a wall thickness of 22.2 mm [0.875 inch] and a 168 mm [6.625-inch] gas line with a wall thickness of 19 mm [0.750 inch]. The heavy wall pipeline pipe was required both for pressure considerations and ocean floor stability. The pipelines were coated with three-layer polypropylene for abrasion resistance and reduction of friction. A custom designed chain pulling device were used to pull the pipelines through the short radius pull-tubes to the deck of the platform. Measured pulling loads as high as 1197 kN (269 kips) agreed with theoretical analysis but were significantly higher than empirical calculations calibrated from longer radius pull-tube installations. The as-laid pipelines were surveyed for spans using side-scan sonar and multi-beam fathometer instruments. Spans were corrected using flexible concrete mattresses.

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