Advances in Deepwater Pipeline Route Selection - A Gulf of Mexico Case Study
Authors: Tootill N.P. , Vandenbossche M.P. , Morrison M.L. 
Published: October 2008

The selection of pipeline routes and the location of the associated tie-in structures require detailed knowledge of the seabed conditions and shallow geology. The areas along the routes and regionally around the routes must be accurately surveyed. This is especially important for the complex geology and seabed topography found in the deepwater Gulf of Mexico.

The Mardi Gras project used an Autonomous Underwater Vehicle (AUV) to provide high quality survey data. This data allowed the project to take pipeline routing decisions with a very high degree of confidence. This has ultimately led to the successful installation of the Mardi Gras Pipelines.

Introduction

BP and its partners are developing several deepwater prospects in 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 by means of the Mardi Gras Transportation System. The deepwater segment of Mardi Gras consists of oil and gas pipelines and steel catenary risers with diameters ranging from 16-inch to 28-inch.

The process of selecting a pipeline route is a complex one. The factors involved can be grouped into two main categories: primary and secondary. Primary factors include the location of the host (start) and destination (end) of the pipeline, and installation parameters that may affect route geometry. Secondary factors include bathymetry, seafloor character, sub seabed geology, geohazards, bio-environmental issues and existing infrastructure. The final factor, which is distinct from these two groups, is cost. Shorter pipelines cost less. Therefore the challenge for any pipeline route selection team is to find the shortest route while conforming to the requirements set out by the primary and secondary factors.

Some examples of geohazards that may be encountered are: steep slopes, unstable slopes, hard seabed sediments, bedforms and environmentally sensitive areas, the most well known of which are chemosynthetic communities. Man made hazards include existing offshore installations, pipelines or cables, shipwrecks, military dumping grounds, and maritime regulations.

Mardi Gras was confronted with all of these in selecting the deepwater pipeline routes. Over 330 miles (531 kilometers) of pipeline (not including the portion of Okeanos laid by Shell Pipeline Co. LP.) were planned in water depths ranging from 400 feet (122 meters) to 7300 feet (2225 meters).

Figure 1 Regional Map(Available in full paper)

The goal of the pipeline survey was to ensure that the pipelines followed the optimum routes based on the best survey data available. This included using the first commercial use AUV system in conjunction with C&C Technologies Inc.

The methodology employed can be summarized as follows.Define pipeline route requirements and the data requirements necessary to select a route.Commence desktop study using company and public domain information.Assess regional data for selection of corridor for high resolution survey.Initial AUV survey of pipeline route corridor.Assessment of survey results, modeling of any areas of concern, and acceptance of general route.Additional survey, at ultra high resolution, of specific areas of concern or interest.Geotechnical evaluation of the route including sampling and slope stability analysis.Selective visual inspection of contacts with ROV system.Final pipeline route selection.