Buoyant leg structure alternative to Spar, TLP

When operators consider a floating solution for deepwater, they may need to look at a new alternative to tension leg platforms (TLPs) and Spars. The newly developed Buoy-ant Leg Structure (BLS) offers advantages of its own.

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By Robert Copple, Dave Kalinowski, Dave Schwartz
I.D.E.A.S.

Test scenarios show savings drives solution

When operators consider a floating solution for deepwater, they may need to look at a new alternative to tension leg platforms (TLPs) and Spars. The newly developed Buoy-ant Leg Structure (BLS) offers advantages of its own.

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This is a marginal-field drilling and production unit with four well slots.
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Hydrodynamic and structural analyses, as well as model basin tests, have confirmed that the BLS works well in deep and ultra-deepwater. Studies of various fabrication, assembly, and installation scenarios indicate that the overall cost of the structure, for various payloads in deepwater, is favorable when compared to TLPs and Spars.

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Most operators consider TLPs and Spars for floating development. A number of each have been installed, and they are performing well. The only way to get an operator to consider a new concept like the BLS is to show significant cost advantages.

The BLS is similar to the SPAR, but vertically restrained like a TLP and able to provide lateral support for drilling and production risers like a bottom-founded platform. In its simplest configuration, a BLS consists of a large-diameter cylindrical buoyant unit, which supports a deck at the surface. The lower end of the buoyant unit is attached to a smaller-diameter transition unit. This unit is relatively short and is attached to a long, small-diameter restraining unit that extends to an anchor at the sea floor.

The BLS is restrained in heave and compliant in surge, sway, pitch, roll, and yaw. Numerous studies and model basin tests have confirmed that deck motions are within the normal operating range for drilling and production. Fabrication and installation are relatively uncomplicated. Studies indicate the final installed cost of the BLS is competitive.

The smallest units, at 1,000-ton payload, serve either as wellhead or utility facilities. Larger units, at 6,000-ton payload, serve as marginal-field drilling and/or production units with a limited number of wells. The largest units, at 16,000-ton payload, serve as drilling and production platforms in ultra-deepwater.

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This is a 16-slot drilling and production unit.
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Economically competitive water depths for the smallest BLS units range from about 1,000 ft to about 6,000 ft, while the largest units are best suited for deep and ultra-deepwater up to 10,000 ft.

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Drilling and/or production risers are contained in a center well or in sleeves within the buoyant unit. A limited number of risers can be contained within the transition and restraining units. With a larger number of risers, guide frames outside of the restraining unit can laterally brace the risers. The buoyant unit, therefore, shields the risers from wave and current forces. If all the risers are contained within the restraining unit, they are shielded for their entire length.

Because risers are more flexible than the restraining unit, BLS motions will not cause significant bending or axial stresses in the risers. This means there is no need to decouple the risers from the structure.

The single most critical element in the well system is the riser, be it a production riser, drilling riser, off-take riser, or tubing riser. Riser weight and movement relative to the platform and to other risers are a key problem. The BLS design incorporates surface-completed wells. This gives it an advantage over TLPs because it does not require tensioners, and over Spars because it does not require buoyancy modules. In addition, a tapered stress joint and/or keel wear joint are not necessary.

Well system scenarios

Using the small field development BLS with four well slots, all wells are predrilled from a mobile offshore drilling unit (MODU). Casing strings are landed in a marine housing/template. The BLS is installed with combination drilling and production risers. The risers are tied back and the wells completed with tubing, wellheads, and trees. The rig is of minimal design, capable of workovers and minor redrills.

Wells are batch drilled to include the 13 3/8-in. casing. Casing strings are landed in a marine housing/template. The BLS is installed with combination drilling/production risers. The risers are tied back, and the rig drills and installs the rest of the casing strings. The wells are completed with tubing, wellheads, and trees. This case would require a slightly larger rig than in the first example.

All wells are drilled from the BLS. Conductors are landed in the template. The 22-in. surface string is brought back to the BLS as a drilling riser. The 16-in. liner is hung off down hole. A combination 13 3/8-in. drilling riser and high-pressure blowout preventer (BOP) are installed after the 22-in. combination riser is removed, and the hole is drilled for the 13 3/8-in. casing string. The well is completed with a tubing riser, a 9 5/8-in. production riser, and a 13 3/8-in. workover riser. These operations require a rig with full drilling capacity.

In a large field development BLS with 16 well slots, there are several scenarios. In the first, all wells are drilled from the BLS with the exception of the conductors that are batch set by a MODU. It would also be possible to drive the conductors from the unit that installs the template, which would eliminate the need for a MODU. The 22-in. surface string is brought back to the BLS as a drilling riser. The 16-in. liner is hung off down hole. A combination 13 3/8-in. drilling riser and high-pressure BOP are installed after the 22-in. combination riser is removed, and the hole drilled for the 13 3/8-in. casing string. The well is eventually completed with a tubing riser, a 9 5/8-in. production riser, and a 13 3/8-in. workover riser. A rig with full drilling capacity is required.

Conductors and surface strings, through 22-in., are batch set. The next string has 7,000 ft of penetration and would most likely be a liner, meaning it is not tied back to the surface. The BLS would arrive with the 16-in. combination risers and proceed from there. A 22-in. drilling riser is installed with a moderate-pressure BOP. The hole is drilled for the 13 3/8-in. casing string. The well is then completed with a tubing riser, a 9 5/8-in. production riser, and a 13 3/8-in. workover riser. This requires a rig with full drilling capacity.

BLS evaluation

The following generic casing program has been used in all cases:

  • Conductor - 36-in. Penetration = 300 ft
  • Surface - 22-in. Penetration = 3,000 ft
  • Intermediate - 16-in. Penetration = 7,000 ft
  • Intermediate - 13 5/8 in. - Penetration = 11,000 ft
  • Liner - 11 7/8-in. Penetration = 15,000 ft
  • Liner - 9 5/8-in. Penetration = 17,500 ft

Small field development unit

This scenario, with about four wells, will most likely involve predrilling the wells from a MODU or at least batch-drilling several casing strings. The BLS would arrive on location with a set of the casing strings required for the next hole size to be drilled. These strings would be fixed to the BLS.

In the case of the wells being predrilled, the 13 3/8-in. workover risers would be made up in their guide frames so they are ready to be tied back to the BLS and mud line wellhead. The BLS can accommodate either single or dual production risers. The 9 5/8-in. production risers are then installed, and the wells are completed.

Key features of this scenario include the fact that production/workover risers are already made up and installed with the BLS. Multiple casing production risers can be installed and have minimal impact on BLS design. There is no requirement for riser transition joints or wear joints. There is no requirement for riser tensioners or buoyancy modules.

Surface BOPs

In the case where wells have been batch drilled to include the 16-in. strings, the BLS would arrive on location with 16-in. drilling risers installed. After tying back the 16-in. drilling riser and installing the BOP, the rig would drill the 11,000 ft of hole for the 13 3/8-in. casing string, then run and cement the string. The 13 3/8-in. string would be tied back and landed on a BLS deck with or without disconnect hardware at the mud line. The 16-in. drilling riser can be removed or left in place if the 13 3/8-in. is eventually removed. The rest of the well is drilled with the 13 3/8-in. drilling riser and surface BOP. The BLS can accommodate either single or dual production risers. The 9 5/8-in. production risers are installed, and the wells are completed.

Features of this scenario include the drilling risers being installed with a BLS, and a high degree of versatility when choosing or tying back risers. Multiple casing production risers can be installed with minimal impact on BLS design. Mudline tieback hardware can be eliminated in some cases. There is no requirement for riser-transition joints or wear joints. Also, there is no requirement for riser tensioners or buoyancy modules.

Large field development BLS

This scenario would feature about 16 wells. It will most likely involve major drilling from the BLS. The construction barge used to install the template could preset conductors, or the conductors could be drilled in with the BLS.

The BLS would arrive on location with the 22-in. surface strings in their guide frames. The rig would drill the hole for the 22-in. through the riser and run and cement the string, tying it back to the BLS as a drilling riser. After installing the BOP, the hole for the 16-in. casing is drilled, and the string run and cemented. The 16-in. riser is tied back as a drilling riser and the 22-in. riser removed.

After tying back the 16-in. drilling riser and installing the BOP, the rig would drill the 11,000 ft of hole for the 13 3/8-in. casing string and run and cement the string. The 13 3/8-in. string would be tied back and landed on a BLS deck with or without disconnect hardware at the mudline. The 16-in. drilling riser can be removed or left in place assuming the 13 3/8-in. riser is eventually removed. The rest of the well is drilled with the 13 3/8-in. drilling riser and surface BOP. The BLS can accommodate either single or dual production risers. The 9 5/8-in. production risers are installed, and the wells are completed.

Features of this scenario include the fact that the drilling riser is installed by the BLS. This offers a high degree of versatility when choosing and tying back risers. Multiple casing production risers can be installed with minimal impact on BLS design. Mudline tieback hardware can be eliminated in some cases. There is no requirement for riser-transition joints or wear joints. There is no requirement for riser tensioners or buoyancy modules, and the solution uses surface BOPs.

Production risers

Wells completed from a BLS will require at minimum a tubing riser and a production riser in a single-casing system. The operator may require another production riser as a back up in a dual casing system.

For production risers, the BLS has an advantage over TLPs and Spars in that decoupling and stress/wear joints are not required. In addition, the buoyancy characteristics of the BLS may have an advantage in that if more risers are required, they can be tied back.

The BLS was the subject of a joint industry project initiated with funding provided by BP, ChevronTexaco, Conoco, JNOC, and Shell to further develop the concept. In addition to a preliminary design of an ultra-deepwater BLS, both the small and the large BLS units were tested in the model basin in November 2000 and March 2001.

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