A longtime proponent of spar systems has disclosed more details about the first U.S. oil and gas production project based on the concept.
Roger S. Glanville, Deep Oil Technology Inc., Irvine, Calif., said construction has begun in Finland on the 705 ft by 72 ft hull of the Neptune production spar. It is to be deployed in third quarter 1996.
Rauma Offshore Contracting Oy, Pori, Finland, last February began building the hull under a contract from Neptune field partners Oryx Energy Co., Dallas, and CNG Producing Co., New Orleans. CNG is a unit of Consolidated Natural Gas Co., Pittsburgh.
Rauma by mid-April had cut more than 1,000 tons of steel and had begun assembling the hull, Glanville told a Houston meeting of the Southwestern Finnish American Chamber of Commerce.
Under a separate contract, J. Ray McDermott SA, New Orleans, in August is to begin constructing the Neptune spar's topsides (OGJ, Feb. 20, p. 100).
McDermott also is responsible for transporting and installing major spar components. These include the topsides, hull, mooring system, production system, and oil and gas pipelines.
Glanville said production spars can be effective development options for deepwater oil and gas fields of many sizes.
Designed to produce 25,000 b/d of oil and 30 MMcfd of gas, the $100 million Neptune spar is expected to recover 50-75 million bbl of oil equivalent during the life of the field. Industry sources estimate total project costs could amount to as much as $300 million, including wells, pipelines, the production system, and other items.
NEPTUNE HULL
More than simply marking the first U.S. use of a new deepwater production option, deployment of Neptune spar is to occur on a fast track schedule more common in shallow water fields.
Glanville said fast track construction of Neptune spar's 14,000 ton hull is possible in part because the structure is less complicated to fabricate than alternative hull designs. Given allowances for the hull's variable wall thickness, he said, steel workers can rapidly cut the similar sections of steel plating used as the hull wall.
Neptune spar's hull is double walled 20 ft above and 20 ft below the water line.
The spar hull's frame consists of horizontal ring frames, vertical stringers inside the hull, and middeck supports every 40-50 ft. Such supports divide the hull into compartments to guard against catastrophic failures caused by losing all buoyancy at once. Deep Oil has calculated that the hull could lose as much as 40 ft of buoyancy without risking damage to wellhead or riser assemblies.
ln addition, the vessel is to be outfitted with variable ballast tanks that can be filled or emptied to offset temporary loads. Variable ballast capacity will be almost 5,000 tons, so the Neptune spar will show little response to changes in deck load.
With a draft of 650 ft, Neptune spar's buoyancy is centered nearer the top of the structure than on most vessels. Unlike a semisubmersible production unit with buoyancy divided among four legs, an operator needn't worry constantly about keeping a spar in balance, Glanville said.
"The spar's buoyancy is all in one section in the middle, and the center of gravity always is below the center of buoyancy," he said.
SPAR INSTALLATION
Plans call for Rauma to build the spar hull in two sections to be towed to the Gulf of Mexico and joined in a protected area, such as Mobile Bay off Alabama.
Next, the mated hull is to be towed to Viosca Knoll Block 826 where it is to be upended using the vessel's internal valve system. Meantime, a derrick barge is to retrieve lines of the taut mooring system-to be preinstalled by McDermott-and connect them to the spar hull.
Finally, McDermott's Derrick Barge 50 is to set the three tiered deck atop the upended, anchored hull. The 100 ft by 130 ft, 3,400 ton deck is small enough to install in one piece.
The topsides are to set atop and be connected to the hull by four legs, much the same as if they were affixed to a four pile jacket for a fixed platform. Each leg is to split at the hull and continue for 15-20 ft to help transfer the topsides load.
Neptune production operations are to center on the spar's bottom deck, where equipment is to include surface trees, oil and gas metering, compression, transfer pumps, separators, and water cleanup. The spar was designed with 16 well slots arranged around the edges of a box, each equipped with a riser guide frame extending to the bottom of the hull.
The structure's mezzanine deck is to house an accommodations module, production processing equipment, a generator package, and the unit's flare boom. The top deck is reserved for a large workover rig package to be installed as needed during the life of the field.
Oryx and CNG plan to drill six wells in the field before installing their spar but will not drill from the spar after it is installed. Instead, the combine aims to operate the six predrilled wells for an extended time to monitor reservoir performance, then drill as many as nine more wells during 2 years until the production system is running at full capacity.
The later wells are to be drilled by a semisubmersible rig, requiring operators to move the spar about 250 ft to one side to make room for the rig. Wells on the seabed are to be set 20 ft apart in two parallel rows 40 ft apart and oriented to allow the structure to be moved in the least stressful direction.
In addition, Oryx and CNG have made allowances to tie back as many as six nearby subsea wells to the spar, including four from the northwest and two from the southeast.
PRODUCTION RISERS
Glanville stressed that the spar's well system components are almost 100% off the shelf items. The only prototype component included is an internal tieback connector mounted between the wellheads and stress joints on the unit's production risers, he said.
Otherwise, each production riser is to consist of a standard subsea wellhead to be provided by FMC Corp. and installed on the seabed.
That will be followed by the prototype internal tieback connector; a 45 ft long, tapered titanium stress joint; and a 9 5/8 in., 53.5 lb/ft production riser. The riser will extend through a keel joint and riser constraint ring into and through a 28 in. OD-27 in. ID stem that terminates at the surface tree.
Each tree will be connected through a choke to the production manifold by a 60 ft length of 3 in. ID flexible flow line.
Unlike a tension leg platform, Neptune spar's hull will not support the weight of production risers. Instead, the weight of each riser is carried by two dedicated buoyancy cans. Also, because they extend downward through the center well in the middle of the hull, the spar's risers are protected from wave and current activity for the first 650 ft below the water line.
Glanville credits the spar's deep draft for most of its steadiness at sea. Because wave and current motions decay as they move deeper in the water column, forces hitting the keel of the hull are quite small, he said.
While a spar, like all vessels, tends to pitch about its center of gravity in response to wave action, Glanville said the vessel's natural period in heave is about 30 sec. That's very long compared with waves in the gulf that typically have heave periods of 6-14 sec.
In addition, second order forces have little effect on spars, he said.
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