Pipeline End-Sled Design for the Green Canyon 209 Lateral Pipeline
Authors: Istre Michael R. 
Published: October 2008

This paper presents a review of Discovery Producer Services' GC 209 Lateral Pipeline Project (formerly referred to as the Allegheny Lateral). The primary focus of this paper is on the design, fabrication, testing, and installation of the lateral's pipeline end-sleds (PLES) and 12-inch OD rigid interconnecting jumper located in Green Canyon 209. The technical challenges addressed by this paper are:PLES site selection and surveyDesign and layout of equipment - diverless connectors, valves, and piggable wyeExpansion and flexibility - design and testing of a rigid jumper connectionInstallation issues in 3,000 ft water depths including contingency leveling of a PLES to make a rigid jumper connection

The issues and solutions presented in this paper should assist in future design, development and expansion of a deepwater pipeline transmission system. The successful use of large diameter, high-pressure diverless connections will assist in the establishment of a future transmission infrastructure beyond the continental shelf.

Introduction

The GC 209 Lateral is a 27 mile 16-inch diameter, natural gas pipeline extending from a subsea tie-in on Discovery Gas Transmission's 30-inch mainline in Ewing Bank 873. British-Borneo's Allegheny mini-TLP in Green Canyon 254 was connected to the GC 209 Lateral with a 3 mile, 12-inch diameter gas export pipeline. The 16-inch OD portion of the pipeline is designed to transport 350 MMSCFD of natural gas and condensate to the Discovery system. Two pipeline endsled (PLES) units and a rigid interconnecting jumper were successfully installed in 3,000 ft water depth and the entire system was commissioned in the summer of 1999, on schedule and under budget. The extensive design and contingency planning involved in this aspect of the project allowed for an efficient installation. The PLES designs also incorporated features that make the dual diameter system fully piggable during operation for maximum flow efficiency.

Layout and Site Selection

Particular attention was given to the site selection for the pipeline interconnection. While serving as the transition point from the 12-inch OD pipeline to the 16-inch OD pipeline, this is also the location of the future tie-in to the 16-inch pipeline. A combination of area bathymetric surveys, flexibility analyses, system expansion, and installation considerations were used in the selection and confirmation of an appropriate site.

Route Selection.

The pipeline route was controlled by four predominant factors:restricted departure angle from the mini-TLP,significant bottom features,avoidance of chemosyntheic communities, andblock lease holder objections.

Based on these factors, a preliminary route was chosen and hazard survey performed. The hazard survey included the collection of geophysical and bathymetric data, location of magnetic anomalies, and drop core samples.

The orientation of the TLP required a northwesterly departure angle. The tie-in point to the existing Discovery system was slightly northeast of the TLP location. Three wells and the departing oil export pipeline also needed to be avoided. Previous mapping of the sea bottom, confirmed by the hazard survey, indicated several significant bottom features to avoid. Most notable were three seamounts and an escarpment near the TLP site. Figure 1 shows the route and bathymetry in the area near the Allegheny field.