What is a spar and how has this technology been used in offshore production?

A Spar Platform consists of a large-diameter single vertical cylinder supporting a deck as shown in Figure 1. It has a typical fixed platform topside (surface deck with drilling and production equipment), three types of risers (production, drilling, and export), and a hull moored using a taut catenary system of 6 to 20 lines anchored into the sea floor. Figure 2 shows exterior and interior schematics of the original, or "classic spar".

The spar concept is based on the use of a deep draft floating system that minimizes heave motion under large sea conditions. The structure will move laterally under the restraint of mooring system and will rotate (tilt), but its vertical motion will be relatively small. The early spars were made up of a very large diameter outer cylinder to provide both the buoyancy needed to support the production and drilling facilities and the mass needed to minimize heave motions. A smaller diameter concentric cylinder protects the production risers and their tensioning system. Figure 3 shows a spar under tow to its offshore installation site, giving a good view of the underwater portion (unpainted) and size in relation to the large towing vessels.

Subsequent evolutions of the design led to replacing the middle of the cylinder with a braced truss system to reduce weight. These are referred to as "truss spars" and are compared to classic spars in Figure 4. All spars utilize vertical risers that support dry trees on the topsides. This is in contrast to floating production systems (FPSs) that utilize wet trees on the seafloor. The dry tree provides easy access to the wells for drilling or workover operations.

Spars are currently available in water depths up to 5,000 feet, although existing technology can extend this to about 10,000 feet. Oryx Energy installed the first Spar production platform called Neptune in 1,930 feet of water in the Gulf of Mexico in 1996. It was a 770-foot-long, 70-foot diameter cylindrical structure anchored vertically to the sea floor. The deepest water depth spar in the world is currently Shell's Perdido spar in the Gulf of Mexico in 7,800 feet of water. The largest spar is Shell's Holstein spar in the Gulf of Mexico with a cylinder diameter of approximately 149 feet, although the Aasta Hansteen spar, which is under construction for the Norwegian sector of the North Sea, will become the largest at 184 feet in diameter when installed. See Figure 5 for a summary of all installed and under construction spars in the world.

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