Neptune Project: Spar History and Design Considerations

Abstract

Abstract Neptune marks the first use of this type of structure to support production operations. The paper will discuss existing spar type structures and the development of the Spar design for the Neptune project. Design considerations for hull sizing and mooring system are discussed. Motion analyses and confirming model tests are presented. The well system design is introduced, which allows for the use of surface type trees providing economical well bore intervention capability. The topsides facility layouts are described. The Spar's capability of future drilling operations, using a semisubmersible drillingrig moored alongside the Spar, is discussed. History of Spars 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 andoperated by the Scripps Institution of Oceanography in California. Flip was put into service in 1965 and is used primarily for ocean acoustic measurements. Flip is 350 feet long. Its hull is 20 feet in diameter over the bottom half and tapers down to a 12 foot diameter where it passes through the water plane. A shipshape bow is fitted tothe upper end and houses the accommodations, power plant and other equipment, and the controls. The vessel is towed out to the desired location and upended using its ballast control system. Upending takes approximately 30 minutes, the first 29 being consumed in going from horizontal to 10 degrees of tilt. Once the vessel is upright the instrument booms are extended and the testprogram started. Operating draft is about 270 feet. Although Flip on occasion has been tethered to the sea floor, it is more commonly allowed to drift in the ocean currents. When the program is complete the ballasting operationis reversed and Flip is towed back to port. In the early sixties Nippon Telegraph installed a Spar off the coast of Japan to carry a microwave relay station. This Spar is445 feet long with a stepped hull ranging in diameter from 10 feet to 20 feet. The topside structure is a cylinder, 50 feet in diameter by 33 feet high, with equipment, accommodations and a heliport on top. A four point catenary mooring of 3 inch chain connected to 175 ton clump weights keeps the Spar in place. Operating draft is 330 feet. In the mid seventies Shell installed an oil storage and oftloading Spar at Brent Field, in the North Sea, which is the largest diameter Spar built to date. The hull is 95 feet in diameter, necks down to 55 feet at the water plane, and the operating draft is 357 feet. This Spar was designed to store300,000 barrels of produced crude and to transfer it to bow loading tankers. The mooring system consists of 6 lines, each made up of a 1000 ton concrete gravity anchor, 2600 ft. of 3.5" wire rope, and 935 ft. of 4" chain.