Spar history
SPARs have been used for decades as marker buoys and for gathering oceanographic data. The first significant spar for our purposes is Flip, a structure owned by the U.S. Navy and operated by the Scripps Institution of Oceanography in California (Fisher 1963). Flip was put into service in 1965 and was used primarily for ocean acoustic measurements. Operating draft is about 270 feet. Its diameter is 12 ft at the waterline tapering to 20 ft over most of the hull. its heave natural period is 29 s. Although Flip occasionally has been tethered to the sea floor, it it more commonly allowed to drift in the ocean currents.
In the early 1960s, Nippon Telegraph installed a spar off the coast of Japan to carry a microwave relay station. This spar was 445 ft long with a stepped hull ranging in diameter from 10 to 20 ft. The topside structure is a cylinder, 50 ft in diameter by 33 ft high, with equipment, accommodations and a heliport on top. A four-point catenary mooring of 3 in.-chain connected to a 175 ton clump weights keeps the spar in place. Operating draft is 330 ft. In the mid-seventies Shell installed an oil storage and offloading spar at Brent Field, in the North Sea. The hull is 95 ft in diameter, necks down to 55 ft at the water plane, and the operating draft is 357 ft. this spar was designed to store 3000,000 barrels of produced crude and to transfer it to bow loading tankers, the oil storage system used a water displacement principle which allowed the tanks to be designed for ambient pressures. The mooring system consists of six lines, each made up of a 1000 ton concrete gravity anchor, 2600 ft. of 3.5 in. wire rope, and 935 ft. of 4 in. chain. The top part of the superstructure rotates to allow the tanker to weathervane about the spar. Accommodations, power plant, other equipment and pumps are in the superstructure and a heliport is fitted.
Agip installed a flare spar off West Africa in 1992 designed to flare 100 million cubic feet of gas per day. The spar is 233 ft long, with a diameter of 7.5 ft which tapers to 5.5 ft through the water plane. With a draft of 170 ft, it is held in place with a four leg chain catenary mooring. In 1993, Shell installed a loading spar at Draugen. The spar topsides include a rigid boom to which the flexible loading hose is attached, the hull diameter is 28 ft and the operating draft about 250 ft.
The world’s first production spar was the Neptune Spar installed in 1996 by Oryx Energy Company (now Kerr McGee) and CNG (Vardeman et al, 1997). Design production rates were 25 mbod and 30 mmcfd respectively. The maximum topsides weight with the workover rig is 5500 tons. Wells would be predrilled with a semi-submersible and completed with a platform workover rig placed temporarily on the spar. The Neptune spar has a hull 705 ft long with a 32 ft centrewell and a diameter of 72 ft. The six-point mooring system consisted of driven pile anchors, 4.75 in. spiral strand wire rope and chain for the section leading up to the fairleads and onto the hull.
There are 14 spars in production or under construction. The spar, along with the Tension Leg Platform, is the only floating platform which up to now has been used for dry trees. The reason for this is that these are the only platforms with small enough heave and pitch motions to allow the risers to be safely and economically supported by the floater.