Debottlenecking to increase production from Neptune spar

Nov. 24, 1997
During this month, Oryx Energy Co. plans to remove a number of bottlenecks on its Neptune spar floating production facility, increasing oil throughput capacity by several thousand barrels of oil per day. Oryx currently produces about 26,000 bo/d, 22 MMcfd, and 400 bw/d from seven flowing and gas lift wells, with the highest recorded day rate being 26,500 bo/d. The oil rates are above the 25,000 bo/d and 30,000 MMscfd name-plate capacity of the facility. Oil produced is a light, sweet crude with
Guntis Moritis
Production Editor
During this month, Oryx Energy Co. plans to remove a number of bottlenecks on its Neptune spar floating production facility, increasing oil throughput capacity by several thousand barrels of oil per day.

Oryx currently produces about 26,000 bo/d, 22 MMcfd, and 400 bw/d from seven flowing and gas lift wells, with the highest recorded day rate being 26,500 bo/d. The oil rates are above the 25,000 bo/d and 30,000 MMscfd name-plate capacity of the facility. Oil produced is a light, sweet crude with an API gravity that ranges from 25 to 35°. Production started in mid-March 1997.

Oryx indicates that the debottlenecking will include the compressors, separators, general flow system, and some work on the gas lift system.

Oryx is a 50-50 partner with CNG Producing Co. in the deepwater, 1,930 ft, floating production facility in Block Viosca Knoll 856, about 135 miles southeast of New Orleans. The development encompasses highly faulted sandstone reservoirs around a salt dome structure in four blocks, about 6 miles by 6 miles, in the Gulf of Mexico flex trend (Fig. 1 [125,627 bytes]). Water depths in the blocks range from 1,175 to 3,225 ft and potential pay intervals lie between 6,800 and 14,500 ft.

Current production is from the east side of the salt dome. Because the facility floats (Fig. 2 [170,572 bytes]), Oryx says it has the option of towing the spar to the west side after depleting the east side. Another of its options is to produce the west side with subsea wells tied back with risers to the outside of the spar. Oryx already has plans to tieback some subsea gas and oil wells to the spar.

The oil and gas pipeline risers also are on the outside of the spar.

Oryx estimates total recoverable reserves around the salt dome are 50-75 million bbl of oil equivalent.

Spar facility

Neptune is the world's first spar-based offshore oil and gas production facility (Fig. 2). It was installed on the site in September 1996. The spar's basic dimensions are a 72-ft outside diameter hull, a 65-ft draft, a 55-ft freeboard at the cellar deck, and a 32 by 32-ft center well with room for 16 well heads.

Oryx says a spar facility was selected because it cost less, provides direct access to wellheads, is stable, allows phased development, and can be moved to another location.

A breakdown of the cost of the $172 million development is as follows:

  • Spar hull and mooring -$53 million
  • Topside and installation-$47 million
  • Production risers-$14 million
  • Drilling and completion-$42 million
  • Engineering, project management, and permitting-$16 million
Oryx expects normal day to day movement of the spar to be slight. Estimates are a 1.2 in. or less heave, 1.4 ft or less sway, 1.2 ft or less surge, and 1% of gravity or less maximum top deck acceleration.

Estimates under a 10-year major winter storm are 2.4-in. heave, 1.7-ft sway, 2.1-ft surge, and 2.3% of gravity maximum top deck acceleration. These increase in a 100 year survival event hurricane to 52-in. heave, 5.2-ft sway, 37-ft surge, and 29.9% of gravity maximum top deck acceleration.

The natural periods are 28-sec heave, 161-sec sway, and 161-sec surge.

The spar has the following characteristics:

  • Facility and operating loads-3,400 tons
  • Deck structural steel-1,600 tons
  • Hull structural weight -12,000 tons
  • Hull outfitting and marine systems-895 tons
  • Variable ballast capacity-9,000 tons
A six-leg taut system moors the spar to 84-in. diameter piles.

The wellheads move independently of the spar and therefore flexible lines connect the wells to the production facility (Fig. 3 [14,983 bytes]).

During 1997, the spar survived Hurricane Danny. Oryx says the hurricane passed over the spar but instrumentation was not in place to measure the intensity of the event. At that time the platform was shut in and all personnel had been evacuated to shore.

Oryx plans to install instruments to measure storm effects during the latter part of 1997.

Additional drilling

Oryx plans to drill additional wells in the Neptune development during the first part of 1998. The wells will not be completed or come on stream until the second part of the year.

Because the spar does not have the capacity to house a drilling rig, a semisubmersible rig will be moved in and the spar will be moved out of the way (Fig. 4 [35,320 bytes]). The spar mooring and riser design allows it to move about 250 ft to the side.

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