Design, Fabrication, and Installation of the Neptune Export Lateral PLETs
Authors: Antani Jayesh , Dick William , Balch Donald , van der Liej Thiemen 
Published: October 2008

As development projects in deep water Gulf of Mexico move further away from existing infrastructure, it becomes advantageous to consider subsea tie-in of their export systems with existing deepwater pipeline systems offering spare transport capacity. The Commercial Agreements between Owners of such systems and third parties utilizing these tie-in locations normally mandate that the latter provide a spare connection for Owner's use. This necessitates incorporating large heavy wyes, a spare male hub and spare subsea valve into the Pipeline End Termination (PLET), with consequent increase in size and weight. The resulting functional design has to be optimized to obtain a PLET size and weight that satisfies installation vessel constraints. The subject matter describes the successful design of such large and heavy PLETs for second end installation by the J-technique. Although several PLETs have been installed by this technique, to date, the Neptune PLETs are the largest and heaviest " second-end?? installed in water depths upto 4000 ft, and required modifications to the installation vessel, Allseas' DP pipelay vessel Solitaire.

Introduction

In early 2007, Enbridge Offshore Facilities, L.L.C. (Enbridge) installed 20-inch oil and 12-inch gas export pipelines for the transportation of production from the Neptune TLP in Green Canyon block 613. These export pipeline systems originate as steel catenary risers from the TLP, and terminate at PLETs P3 (gas) and P7 (oil) in Green Canyon Block 650. The PLET piping is jumper-connected to the adjacent gas and oil wye sled assemblies (WSA, W3 and W7) incorporated in the Mad Dog laterals of the Caesar and Cleopatra Pipelines of the Mardi Gras Transportation System.

The Neptune export PLETs incorporate subsea wyes, valves and connector hubs, and provide spare oil and gas connection hubs for tie-in of future laterals, at the same locations in block GC 650. The P3 and P7 PLET design incorporates provisions for the safe passage of pigs specially designed to traverse multi-diameter pipeline systems. This Paper describes the design, fabrication and installation of the Neptune oil and gas export lateral PLETs in 4200 ft depths in Green Canyon Block 650, Gulf of Mexico.

PLET design

The PLET Structure is comprised of a mudmat, main structural frame and an installation yoke. The main structural frame supports the piping and components (wye, valves, hubs), while the mudmat distributes the load to the seabed while minimizing settlement. The yoke is hinged to the structural frame, to minimize torsion related rotation of the PLET pipeline assembly during installation and lay down of the PLET onto the seabed.

To ensure a low center of gravity, the wye is oriented horizontally, and the mudmat is attached to the bottom of the PLET framing. This assists in minimizing pipeline bending moment at the PLET/pipeline interface, and in mitigating rotation tendency of the PLET as it is lowered from the surface to the seabed. The water depth for the gas PLET P3 is 4221 ft, while the oil PLET P7 is in 4267 ft depth.