Independence Trail-Pipeline Design Considerations
Authors: Al Sharif Majid 
Published: October 2008

Design optimization of long trunklines in deep water requires striking a balance between technical and economical pressures while satisfying constrains imposed by project execution plan, manufacturing, installation, pre-commissioning and operation. The paper present approaches and compromises adopted within the context of Independence Trail Pipeline project and lessons learned while building the world deepest offshore pipeline.

Introduction

At a record water depth of 7,912 fsw in the Gulf of Mexico, the 138 mile long Independence Trail Pipeline (ITP) is the deepest pipeline ever to be laid. The ITP pushed the industry published capability limits at the time and provided new basis for its future projections developed to support building larger and deeper trunk lines.

ITP initiates from West Delta Block 68 at a water depth of 118 fsw and terminates at Mississippi Canyon Block 920 at a water depth of 7912 fsw (Figure 1.0). At the deep end a Steel Catenary Riser (SCR) connects the pipeline to the semi floating production facility (Figure 2.0). The requirements of 1.0 Billion Cubic Feet (BCF) per day gas throughput, at a Maximum Allowable Operating Pressure (MAOP) of 3640 psi, and the ability to withstand a maximum external pressure of 3516 psi, necessitated an outside pipeline diameter of 24 inches Double Submerged Arc Welded (DSAW) with wall thickness ranging from 0.95-inch to 1.35-inch and a minimum grade of API-5L X-65 material. The pipeline was constructed in India via Welspun-Gujarat Stahl Rohren's J-C-O expanded pipe methodology and made from low-carbon, low- sulfur, microalloyed steel plate manufactured by Azovstal in the Ukraine and Voest Alpine in Austria with thermo-mechanical process control (TMPC) including accelerated cooling. Tennessee Gas Pipeline Company (TGP) shallow water pipeline network was selected for transportation of the gas from WD68 (Figure 3.0) to shore.

This paper describes an overall framework of the decision making process for selecting project specifications and other issues associated with a typical large capital value deep water trunk lines such as ITP which are characterized by dependent upstream and downstream facilities and must achieve a low level of risk during manufacture, transport and installation while maintaining high level of reliability in operation.

Design Basis

The unique commercial aspect of the overall project (i.e. five producers and infrastructure developers collaborating in the fast track development of ten remote gas fields (Figure 4.0) through a new supply corridor in the Eastern Gulf of Mexico), led to making the definition of what is considered a basic design premise to work from a challenging task especially since, the design basis was part of the negotiation between pipeline owner (i.e. Enterprise Products Partners, LP) and individual producers. In addition to the above; the location of the hub, topside facilities capacity and shallow water pipeline system to be connected to were moving targets that needed to be taken into consideration during basis development in order to:Minimize subsea development cost for each producerMaximize facility production capacity,Minimize maximum required compression pressure,Reduce deliverability risk,Meet existing commercial agreements terms.