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.