This peper wes selected for presentetion by the OTC progrem Commillee following review of informetion conteined in en ebstract submilled by the euthor(s). Contents of the peper, es presented, have not been nsviewed by the Offshore Technology Conference end ere subject to correction by the euthor(s). The meteriel, es presented, does not necesserily reflect eny posnion of the Offshore Technology ConIensnce or ns off'lC8I'8, Electronic reproduction, distribution, or storage of eny part of this peper for commerciel purposes without the wriIIen consent of the Offshora Technology ConIensnce is prohibned. Permission to nsproduce in print is nsstricled to en ebstrect of not mora then 300 words; illustrations mey not be copied. The ebstract must contein conspicuous ecknowledgment of whens end by whom the paper wes presented. AbstractNeptune 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 con-fIrming 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 drilling rig moored alongside the Spar, is discussed.

The Spar is a deep-draft floating steel caisson, which can be designed to drill, produce and store oil. It is very stable and relatively insensitive to deck loads. Drilling and production equipment are supported on the decks above the waterline. Oil may stored in the lower portion of the hull. The centerwell protects drilling and production risers down to 650 feet. The Spar can be moored with conventional or taut catenary mooring lines using embedment or pile anchors. Because of its deep draft and large displacement mooring line dynamic loads are relatively low. Fabrication and installation plans together with cost estimates are included for a 36-well design for Chevron in the Gulf of Mexico. Predicted motions under normal and maximum sea states are shown together with the results of global motion analysis using both frequency and time domain programs. The sensitivity of the design to mooring parameters, draft, diameter and Spar mass properties is discussed. Potential applications are briefly reviewed. INTRODUCTION The concept of a Spar (a slender vertical structure) stable floating ocean platform has been recognized for years. The Flip Ship (Fisher and Spiess, 1963) was built in 1962 as a stable platform for oceanographic measurements. Its favorable motion properties are well documented (Rudnick 1964, 1967, 1971). The potential for use of the Spar in the offshore industry was recognized in the 1970s with the construction of Shell's Brent Spar for oil storage and offloading in the North Sea. Other potential uses and the salient motion properties were discussed by van Santaan and deWerk (1976). Research on the Spar as a storage and/or production vessel was carried out by some oil companies in the mid 1970s, however additional use of the Spar concept did not materialize. It has again recently been proposed as a low cost production facility for remote subsea well sites (Kerckhoff and Pijfers, 1989). This paper describes a full drilling and production Spar designed for a 2,700 foot water depth in the Gulf of Mexico. Oil storage is not a requirement for this site, but could be a major advantage of the Spar for remote areas of the world. The proposed Spar is an order of magnitude larger (in displacement) than the Brent or Flip designs, however, it retains the stability and simplicity intrinsic to the concept. SPAR CONCEPT The Spar is a large, deep draft, cylindrical, floating platform (Figure 1) designed to support drilling and production equipment and to store oil. It is permanently moored with multipoint catenary anchor lines over a multiwell seafloor template. Wells can be drilled and completed with the BOP stack either on the surface or on the seafloor. Risers are held in tension by hydraulic tensioners or by buoyancy modules (Figure 2). The most obvious features of the Spar are its extreme draft, straight sides, large centerwell, and large displacement (Figure 1).

This paper reviews the performance of the Neptune Spar for the fwst two years of its producing history. Equipment operation and required modifications together with causes and extent of downtime are discussed. Facility capacity proved to be limited so a facility expansion was undertaken coinciding with the shutdown scheduled for installing the remaining buoyancy cans. As called for in the original field development plan, additional wells have been drilled under the Spar and tied back with dry trees to the existing production deck manifold. The Spar was moved aside 250 feet to accommodate a MODU drilling the new wells into the existing well pattern. Challenges of designing the MODU mooring to prevent contact between it and the Spar mooring are detailed. A remote subsea well has been drilled and tied back to the Spar with a flexible pipe riser. Installation of the remaining buoyancy cans and completion of the new wells are discussed together with the concerns raised in operating a moored MODU close to the moored Spar. Introduction Oryx Energy Company installed a production Spar at Viosca Knoll Block 826 in September 1996. The location is in the Gulf of Mexico about 90 miles due South of Mobile, Alabama, where the water depth is 1,930 feet. Seven wells were pre-drilled to total vertical depths ranging from 8,000 feet to 13,000 feet with semi-submersible rigs. A packaged 650 hp workover rig was set on the Spar and used to run the production risers and complete the wells. Production started on March 11, 1997, building to 26,000 barrels of oil and 22 million cubic feet of gas per day by September. In February 1998 the Spar was moved aside with its mooring system and a moored semisubmersible drilled three additional wells into the existing seafloor pattern after which the MODU was moved to an adjacent block to drill and complete one more well which was tied back to the Spar. Again using a packaged workover rig set on the Spar, the new wells were tied back and completed, bringing the number of producing platiorm wells to ten. At the end of January, 1999, with two of the new wells completed, production was 27,500 barrels of oil and 30 million cubic feet of gas per day. Background The Neptune Spar produces oil and gas from a four block unit operated by Oryx Energy Company with CNG Producing as an equal partner. To develop the 50–75 million BOE reserves a phased development based on a Spar platform was selected. Seven wells were predrilled with MODUs, before and during construction of the Spar hull and topsides, as Phase I of the drilling effort. The 72 foot diameter steel hull (Figure 1) was built in two sections which were assembled afloat into a single, 705 foot long cylinder with buoyancy compartments at one end and a 32 foot square centerwell throughout the length. A short tank at the other end of the hull provided temporary buoyancy to help float the hull out to the installation site at Viosca Knoll Block 826. The hull was towed to the site and upended by flooding the temporary buoyancy tank after which the six preinstalled mooring lines were connected to the Spar and tensioned up. Each mooring leg (Figure 2) consists of a 180 foot long 84 inch diameter pile, approximately 200 feet of 4-3/4 inch chain, 2,400 feet of 4-3/4 inch spiral strand wire rope and 1050 feet of 4-3/4 inch chain running from 300 feet outside the fairlead up the side of the hull through a chain jack and down into a chain locker.

The Tension Buoyant Tower (TBT) consists of a cylindrical spar type buoyancy module moored to the sea bottom by a group of tendons arranged in a pattern close to the centerline of the buoyancy module. The buoyancy module consists of buoyant hard tanks extending to a depth of 250 feet or more depending on the specific requirements of the design. An upper stem extends below this point to a depth of 500 feet or more. Seven tendons pass through a centerwell in the buoyancy module and extend to the seafloor, passing through a lower stem before template. Local tendon bending stresses are limited by tapered tendon sleeves at the entrance points to the upper and lower stems. The tendons and sleeves are allowed to pivot at these points, eliminating the need for elastomeric flex joints. Coupled analysis of the design was performed using the finite element program COPIPE. The key parameters in the design include: base tension, location of platform CG and CB, sleeve design and location of tendon support points within the stems. The results of analysis showing the sensitivity of the design to these and other parameters will be discussed. Application of the TBT to Gulf of Mexico applications ranging from 1,000 feet to 10,000 feet will be reviewed. INTRODUCTION Deep water Gulf of Mexico development will undoubtedly involve the use of innovative structures for drilling and production. We have already seen the first deep water tension leg well platform at Jolliet, the World's deepest floating drilling and production platform at Green Canyon Block 29 and in 1992 will see another deep water drilling and production platform at Auger. All of these were preceded by the guyed drilling and production tower at Lena. As water depth increases it becomes ever more difficult to provide structures which allow surface drilling and completion. The TBT allows drilling from the structure coupled with traditional surface trees. The TBT (Figure 1), is a compliant offshore structure used for drilling and producing oil and gas in water depths ranging from 1,000 feet to 10,000 feet. Low cost and simplicity are its principal features. The structure relies on buoyancy to keep it upright and on bundled vertical tendons to maintain its position. The lower ends of the tendons are fixed to a base which is pile fastened to the seafloor. Although the TBT can be designed to support any deck load, it is well suited to supporting drilling and production facilities in the range of 5,000 to 25,OOO tons. The drilling and production equipment is similar to that used on conventional fixed or compliant platforms. Conductors are supported by sleeves and/or spacers from the well deck to the mudline. They are pretensioned to level required to prevent buckling under the extreme pitch motions of the TBT, approximately 6 degrees. Wells can be drilled from, and completed on the deck of the TBT as is done on fixed platforms.

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