Mardi Gras Deepwater Pipeline Design Overview
Authors: Karlsen Jan S. , McShane Brian M. , Rich Sharon K. , Vandenbossche Michael P. 
Published: October 2008

This paper presents the key deepwater pipeline design issues and the associated project decisions that determined the configuration for the Mardi Gras Transportation System. Points of special interest include wall thickness optimization based on pipeline collapse testing, management of pipeline spans in deep water, lateral buckling of large diameter pipelines, deepwater pipeline crossings, and traversing the Sigsbee escarpment.

Introduction

BP and its partners are developing several deepwater prospects in the Southern Green Canyon and Mississippi Canyon areas of the Gulf of Mexico. These include the Holstein, Mad Dog, Thunder Horse and Atlantis floating production facilities in water depths ranging from 4500 feet to 7300 feet. Oil and gas will be exported via the deepwater segment of the Mardi Gras Transportation System, consisting of pipelines and steel catenary risers with diameters ranging from 16-inch to 28-inch.

A schematic overview of the Southern Green Canyon system is presented in Figure 1. The Caesar Oil Pipeline will consist of 24-inch lateral pipeline segments and a 28-inch trunkline, totaling approximately 115 miles from the Holstein- Mad Dog-Atlantis areas to Ship Shoal 332. The Cleopatra Gas Gathering System parallels the Caesar Oil Pipeline and consists of 16-inch lateral pipeline segments and a 20-inch trunkline.

A schematic overview of the Mississippi Canyon system is presented in Figure 2. The 20-inch Thunder Horse Gas Lateral extends 24 miles from the Thunder Horse facility to a sub-sea tie-in on the Na Kika segment of the Okeanos Gas Gathering System. The Proteus Oil Pipeline consists of a 70 mile long 24-inch / 28-inch pipeline from the Thunder Horse facility to a Booster Platform in South Pass Block 89. There, the Proteus Pipeline will tie into the Endymion Oil Pipeline System.

Wye sled assemblies (WSAs), Pipeline End Terminations (PLETs) and pipeline jumpers will be employed at key deepwater connection points to tie the network of pipelines together and facilitate the later addition of third party production into the system.

Through the process of creating the Mardi Gras system from concept to construction, knowledge has been gained that enhances the industry's capability. Taking into account pipeline diameter, water depth, operating conditions, installation vessels utilized and the system configuration, Mardi Gras has faced several deepwater pipeline aspects that are unique to the industry. This paper focuses on the design of the deepwater pipelines that will be the deepest 28-inch, 24- inch, 20-inch and 16-inch pipes ever installed at 5350 feet, 7300 feet, 6000 feet and 7300 feet respectively.

Pipeline Design - Key Project Decisions

At the start of the project, Mardi Gras recognized that there were several challenges that had to be overcome in order to deliver an optimized, large diameter export pipeline system in deep water. A summary of decisions linked to the pipeline design is listed below.Pipeline collapse testing to verify the design collapse capacity and to investigate strength recovery and increased pipe collapse resistance due to thermal aging during the pipe coating processClassifying the steel catenary risers (SCRs) as part of the pipelines and thus reducing the pressure test requirement for the gas systems