SPAR MOORING

Spar description

General

The basic form of the spar platform, the classic spar, is a deep-draft, caisson-type, floating structure with a fully compartmented upper section that is buoyant and with 2 lower sections that are flooded. The lowest compartments in the upper buoyant section are configured for variable seawater ballast to maintain draft and trim under varying topside loading conditions. The hull uses standard ship-type plate and stiffener construction and contains an open centerwell (moonpool). The applicable water depth range is considered to be from 1,500 ft. to 10,000 ft., although shallower and deeper depthsare achievable.                                               

Stationkeeping is provided by lateral, catenary anchor lines which are attached to the hull near its center of pitch for low dynamic loadings. Because of the very favorable interaction of the hull and mooring system, a spar uses a taut catenary system of chain and wire, terminating at the sea bed in piled anchors, installed by driving or suction techniques. This system has much shorter scopes than conventional full catenary arrangements and is correspondingly more economical.
                                                                                                                        Rigid Steel production risers are located within the centerwell where the protected water allows each riser to be supported in tension by its own buoyancy module. This riser arrangement permits the use of surface (dry) trees at the deck level. The very favorable motions of a spar platform permit the use of steel catenary risers for both export pipelines and import flowlines from remote well sites.
                                                                                                                           Using the normally flooded center section, a spar can be configured for oil storage at a low marginal cost. Since the size of the hull is usually proportional to the topside payload and the corresponding production throughput, the hull can normally store an 8- to 10-day supply of oil without increasing the diameter or draft for this purpose. This aspect of the spar's design makes it suitable for shuttle tanker turn around, even from rather remote locations.

Finally, a spar platform is relatively insensitive to water depth and very insensitive to seabottom topography and geology, so it is a very good candidate for relocation several times during its 20- to 40-year design life.
                                                                                                                           The result is a system that, compared to other systems for very deep water, has been shown to cost less to build and install, to have greater flexibility, and to have more favorable motions when subjected to the offshore environment.

Characteristics

-Applicable in 1,500 ft. to 10,000 ft. water depths
-Cost relatively insensitive to water depth, most competitive in very deep water
-Hull cost is relatively insensitive to deck payload
-Platform motions are mostly lateral (minimal heave)
-Uses standard shipyard and offshore deck construction
-Hull is initially transported by towing in a horizontal, self-floating position
-Wells can be pre-drilled or drilled from the platform
-Production risers are steel pipe with surface trees
-Hull can be configured for significant liquids storage if this is advantageous
-Particularly applicable in remote locations which lack infrastructure
-Economical to relocate to other sites, in both deeper and shallower water.

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