Design And Installation Of Auger Pipelines

Abstract

ABSTRACT Shell's Auger Tension Leg Platform (TLP)in Garden Banks Block 426 has been installed in a record water depth of 2864 FSW(873 m). The two 12-inch oil and gas export pipelines from the TLP to shallow water platforms are also the deepest constructed to date in the Gulf of Mexico, requiring record breaking J-Lay construction technology. This paper describes design of the pipelines, including route selection, determination of the wall thickness, buckle arrestor and J-Lay collar design, and connection to the Auger TLP by means of first time use of a steel catenary riser. Construction topics discussed include a description of the deepwater J-lay system and a comparison of shallow water S-Lay and deepwater J-Lay. PIPELINESYSTEN DESCRIPTION The Auger pipelines were designed and constructed to transport oil and gas from Shell's Auger TLP located in the Deepwater Gulf of Mexico Garden Banks Area (GB426) to existing facilities in shallow water (Figure 1). The oil pipeline is a 12.75" (323.9 mm)outside diameter pipeline, 72 miles (116 km) in length, that runs from the TLP to one of Shell's existing platforms in the Eugene Island area (Block EI 331 'An) in a waterdepth of 250 ft (76 m). The gas pipeline is also 12.75" in diameter and runs from the TLP to Hess's existing platform in the Vermilion South Addition area (BlockVR397 'An) in a waterdepth of 397ft (121 m). The oil pipeline is designed for a 50 MBPD flow rate and the gas pipeline is designed to transport up to 150 14MSCoFfDnatural gas. Both pipelines are rated for ANSI 900service, or 2160psig (14.9 MPa)operating pressure. PIPELINE ROUTE SELECTION Several problems exist in routing deepwater pipelines in the Gulf of Mexico:large distance to existing shallow water pipeline infrastructure,generally very rough terrain in the deepwater region (beyond the 800 ft (244m)waterdepth contour),lack of reliable bottom contour and bottom feature data, andpresence of large areas classified by the Minerals Management Service as environmentally sensitive, which means that crossing of these areas by pipelines is prohibited. All these problems were present for the Auger TLP pipelines and route selection, including multiple surveys, required much more time and effort than at anytime in the past, Especially the oil pipeline routing faced the problem of having to circumvent environmentally sensitive areas and its final route is far from a straight line between the TLP and EI 331 "A" platform. Figure 1, which shows the routing of the deepwater portion of the pipelines, illustrates the impact of some of the problems caused by rough terrain. Because a portion of the pipeline (beyond the 1200- 1500foot waterdepth contour) could not be installed with conventional S-Lay, a choice had to be Dade:install a 20-mile-long section of each pipeline by first-time use of J-Lay along a smooth seabed, orlay as long as possible with S-Lay, and have only two 10-mile-longsections of J-Layed pipeline to install.