Deepwater Gulf of Mexico 2004: America's Expanding ... - OCS BBS

A predominant workhorse of the GOM is the spar. A spar is a vessel with a circular cross-section that sits
vertically in the water and is supported by buoyancy chambers (hard tanks) at the top, a flooded midsection
structure hanging from the hard tanks, and a stabilizing keel section at the bottom. Some unique
features of a spar include
• favorable motion characteristics compared with other floating systems,
• stability (the center of buoyancy is above the center of gravity),
• cost insensitivity to water depth, and
• water-depth capability up to 10,000 ft (3,048 m) and beyond.
A spar is held in place by a catenary mooring system, providing lateral stability. Currently, there are
three competing versions of spars used in the GOM: classic spar, truss spar, and cell spar (figure 43).
The first generation of spar design is the classic spar. It is made up of one cylindrical hull that extends to
the bottom of the structure and surrounds a center opening. This opening allows the wellhead to be on the
platform and permits both drilling and production operations. Approximately 90 percent of the classic
spar’s hull is underwater. The first classic spar was installed in 1996 in 1,935 ft (590 m) of water in the
Neptune field. Other examples of a classic spar are Genesis and Hoover.
The second generation of spar design is the truss spar. In this design, a truss structure (similar to the
space frames used in conventional fixed platforms) replaces the lower portion of the cylindrical hull used
in the classic spar. The truss section is lighter than the equivalent cylindrical section of the classic design,
providing the following advantages:
• construction costs are lower than a classic spar of similar size,
• width of the center opening can be increased to accommodate additional wells, and
• topside equipment can be expanded to handle additional production.
In 2001, the first truss spar was installed over the Nansen field in 3,680 ft (1,122 m) of water. Other
examples of the truss spar are Boomvang, Horn Mountain, and Devil’s Tower. Once installed, Devil’s
Tower will be the deepest spar, operating at a water depth of 5,610 ft (1,710m).
The third generation of spar design is the cell spar. The cell spar’s hull is made up of several identically
sized cylinders surrounding a center cylinder. The main advantages of the cell spar design are reduced
fabrication and transportation costs. The tank of a classic or truss spar requires specialized shipyard
fabrication (large-diameter, steel-plate rolling machines are required). To date, all classic and truss spars
have been constructed in European and Far East shipyards and require transport to the GOM. In contrast,
each cylinder of the cell spar, being of a smaller diameter, can be fabricated using rolling machines that
are readily available in most U.S. shipyards. Once fabricated, the cylinders are then lined up and welded
together. This entire process can be done in the United States, increasing the number of contractors
available for bidding purposes and reducing transportation costs. The main disadvantage is that the cell
spar has no center opening for surface wellheads so only subsea well production is possible. The first cell
spar will be installed in the Red Hawk field in 5,300 ft (1,615 m) of water in late 2004.
Figure 44 shows the different types of production systems installed each year. Data values can be found
in Appendix F. At least eight deepwater production facilities (primarily truss spars) are under
construction or pending installation at this time.
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