Summary
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 the areas 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. These data allowed the project to make
decisions with a high degree of confidence regarding pipeline routing,
ultimately leading to 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 4,500 to
7,300 ft; see Fig. 1. 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 to 28 in.
The process of selecting a pipeline route is a complex one. The factors
involved can be grouped into three main categories: primary, secondary, and
cost. 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,
subseabed geology, geohazards, bioenvironmental 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 up by the primary and secondary factors.
Examples of geohazards that may be encountered include: steep slopes,
unstable slopes, hard seabed sediments, bedforms, and environmentally sensitive
areas. The most well-known environmentally sensitive areas are the
biological assemblages of tubeworms, clams, mussels, bacterial mats, and a
variety of associated organisms collectively known as chemosynthetic
communities. These occur in many locations in the deepwater Gulf of
Mexico. Manmade hazards include existing offshore installations, pipelines
or cables, shipwrecks, and military dumping grounds. Additionally,
maritime regulations must be observed at all times.
Mardi Gras was confronted with all of these when selecting the
deepwater-pipeline routes. More than 330 miles (531 kilometers) of
pipeline (not including the portion of Okeanos laid by Shell Pipeline Co. LP)
was planned in water depths ranging from 400 ft (122 m) to 7,300 ft (2225
m).
The goal of the pipeline survey was to ensure that the pipelines followed
the optimum routes on the basis of best survey data available. This
included using the first commercial-use AUV system in conjunction with C&C
Technologies Inc.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
30 November 2004
- Revised manuscript received:
11 April 2005
- Manuscript approved:
15 April 2005
- Version of record:
20 March 2006
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