Design and Installation of the 20-in. OD Mississippi Canyon Block 397 Gas Pipeline
Authors: Vogt G.B. , Walker L.D. , Lammert W.F. 
Published: October 2008

ABSTRACT
This paper addresses the problems, solutions and techniques used to design and install Southern Natural Gas (S.N.G.) Company's 20-Inch Mississippi Canyon Block 397 Pipeline. The S.N.G. pipeline will transport natural gas from the Exxon U.S.A. Alabaster Platform located in Mississippi Canyon Block 397, offshore Louisiana to a point on S.N.G. 's Venice to Lake Washington Pipeline near Venice, Louisiana. The 60 mile 20-Inch O.D. pipeline is the deepest large diameter pipeline laid in the Gulf of Mexico to date. Water depth along the route of the pipeline varies from approximately 460 feet at the Alabaster Platform, increasing to a maximum water depth of 1220 feet in the Mississippi Canyon area and decreasing to a negligible water depth at the landfall site southwest of Venice, Louisiana.

INTRODUCTION
The Southern Natural Gas Mississippi Canyon Block 397 pipeline project is an excellent example of a pipeline which encounters an array of conditions which prompted special design considerations and installation techniques. A tremendous team effort was used to study, research, test, design and install this pipeline. The Southern Natural Gas project team assembled for this project consisted of personnel from Guillot-Vogt Associates, Inc. for permitting, design and project management; Hudson Engineering for design and stability analysis; Materials & Welding Technology, Inc. for review of welding and pipe specifications and numerous personnel throughout the S.N.G. organization for project management, materials management, inspection and overall operations. Addressed in this paper is the technology used in areas such as pipe properties, anti-corrosion and weight coatings, span and buckle considerations and the installation equipment utilized.
The results of the design/economic analysis and construction data are presented along with the final design criteria which allowed for reduced installation tension. The buckle arrestor design, special coating considerations and other significant aspects of the pipeline are outlined along with actual field data obtained.

PIPELINE ROUTING
Preliminary pipeline design was performed and the results were used to study and evaluate various pipeline routes between the proposed Exxon Mississippi Canyon Block 397 Platform and an onshore facility near Venice, Louisiana. The proposed platform was installed on a large topographic rise, caused by a diapiric intrusion, known as the Alabaster Knoll. The water depth at the platform location is approximately 460 ', which is near the top of the Alabaster Knoll. The routing of the pipeline down the Alabaster Knoll was critical due to the slope and surface features of the bottom in that area. Within a radius of 10,000' of the knoll in almost every direction the water depth increases to a depth of 1200' to 1500'. Routing considerations included studying the use of existing pipeline systems in the area versus a direct route to an onshore facility. The use of an existing pipeline transmission system to transport the natural gas could not be arranged, therefore a direct route to an onshore facility was pursued.
Once it was determined that the pipeline would be installed directly to an onshore facility, various pipeline routes were analyzed.