Tallest structure installed in Gulf

Guntis Moritis
Production Editor

Axial tubes in the tower section help govern the dynamic characteristics in the structure (Fig. 2 [14,079 bytes]).
Ring guides allow the axial tubes to have a different motion than the structure (Fig. 3 [13,437 bytes]).
An articulation point 500 ft above the seafloor also helps govern the dynamic characteristics in the structure (Fig. 4 [8,109 bytes]).
The tower section of the compliant structure is 1,320 ft tall, weighs 20,200 tons, and is 90 x 90 ft (Fig. 5 [10,864 bytes]).
The oil and gas pipelines will be clamped to the same leg of the structure, about 400-ft below the sea level. Shown here are the clamp holders and connecting flanges (Fig. 6 [13,282 bytes]).

The Baldpate deepwater production platform, when installed, will be the tallest freestanding structure in the world.

The $300 million Gulf of Mexico development project is in Garden Banks Block 260, in 1,650 ft of water, 120 miles off Louisiana.

Amerada Hess Corp. (AHC) is project operator with Oryx Energy Co. being a 50% partner.

Baldpate facilities include a compliant tower structure, and process and utility equipment capable of handling 60,000 bo/d, 200 MMscfd, and 75,000 bw/d. Gas and oil pipelines connect the platform to the pipeline infrastructure in the Gulf.

The production platform is the first freestanding (nonguyed) offshore tower. It measures 1,902 ft from the seafloor to the tip of the 260-ft flare booms (Fig. 1 [52,224 bytes]).

AHC explains that the term compliant refers to the structure's response "natural period" of 33 sec compared to 3-4 sec for typical offshore platforms. Axial tubes and an articulation point give the structure its compliance.

Concurrently, the Penn State field (GB 216) is being developed by the GB 260 unit partners. This development involves two subsea wells tied back via flow lines to the GB 260 compliant tower. AHC forecasts production from each of these two wells to be 3,000-5,000 bo/d.

Baldpate field

The field was discovered in November 1991 with well GB 260 No. 1. The well found about 180 ft of pay.

The GB 260 unit includes four blocks. AHC acquired Blocks 259 and 260 in July 1984, and GB 215 was acquired in August 1987. The U.S. Minerals Management Service (MMS) approved the GB 215 S/2, 259, and 260 Unit in February 1991. Block GB 216 was added to the unit in February 1994.

The platform will be set over the temporarily abandoned delineation well GB 260 No. 3. Six additional wells, next to GB 260 No. 3, have been predrilled from a nine-slot template. AHC plans to tieback and complete the wells from the platform with the Ensco 23 drilling rig.

The topsides have three main levels and weigh about 2,400 tons without equipment. The integrated topsides will be installed on the platform in a single lift of about 3,950 short tons.

Reservoir

AHC describes the primary Baldpate pay sands, the "Big sand" and the "Twin sand," as Pliocene-aged basin floor fans truncated along the southwest flank of a shallow tabular salt feature. These sands are sheet-type sands that exhibit excellent lateral continuity and consistency. Average porosity is 29% and permeability is 500 md.

The hydrocarbon column has a 1,600-ft height with the deepest horizon below 17,000 ft subsea.

AHC expects these sands to have a widely uniform drainage pattern, enabling the reserves to be recovered with three wells in each main sand.

The two main Baldpate reservoirs are high-pressure reservoirs, approaching 13,600 psi. Both fluid systems are above saturation pressures, with compositional gradients across the hydrocarbon column.

AHC projects production from the wells will average up to 8,000 bo/d, with 3,000 scf/bbl GORs.

With well GB 259 No. 3, AHC plans to drain a second reservoir, Baldpate North, a somewhat shallower accumulation.

AHC expects total output from the seven wells to exceed 50,000 bo/d and 150,000 MMscfd during the first part of 1999. From both Baldpate and Baldpate North, AHC forecasts an ultimate recovery of 104 million bbl of oil equivalent, of which 60% is projected to be oil.

Drilling and completions

Because of high pressures in the main Baldpate reservoirs, AHC expects surface producing and shut-in pressures to be well above 10,000 psi. Consequently, the project requires 15,000 psig working pressure wellheads.

To achieve the high well flow rates, AHC plans to complete the wells with 4-in. tubing. All seven wells will have a single string, and all zones will be gravel packed with a frac-pack technique.

In each completion, AHC will incorporate a downhole continuous pressure and temperature monitor with surface readout capabilities.

AHC describes the Baldpate 33-34° API gravity oil as having a fairly high paraffin content and cloud point that averages 115° F. Therefore, it plans to take measures to mitigate potential paraffin deposition.

AHC will displace some of the tubing/casing annular fluids with nitrogen for insulation, and will install the capability of downhole continuous injection of paraffin inhibition chemicals.

Methanol injection capabilities will also be installed to combat the potential for downhole hydrate formation.

Structure design

The compliant structure has a natural response period significantly different than the 15-sec periods of high energy, significant waves associated with Gulf of Mexico hurricanes.

AHC says it is critical for a Gulf of Mexico structure to have primary natural periods (such as in bending and torsion) much different than the hurricane wave periods; otherwise, dynamic force amplification, or "resonance," could occur.

All structures have a "natural response period," which is the time for a structure to complete one cycle of response to a force, and return to its original position before impact.

AHC describes most offshore structures as having low natural periods, typically less than 3 sec, and up to 6 sec in deeper water. It says, in 1,650 ft water, it is not practical to keep the period under 6 sec because of the large amount of required steel tonnage to give such a structure sufficient stiffness.

A compliant tower stays away from the problematic hurricane wave periods with a primary bending natural period much higher, about 3 sec for Baldpate.

AHC says this structure is called "compliant" because it responds more "softly" to wave forces, even deamplifying those forces by converting wave energy to structure inertia.

To govern the dynamic characteristics in the structure, the design includes "axial tubes" (Fig. 2 [14,079 bytes] and Fig. 3 [13,437 bytes] ) and an articulation point (Fig. 4 [8,109 bytes]) 500 ft above the seafloor. The tower section has two axial tubes at each of the four legs.

AHC says the articulation point keeps the tower structure from being too stiff, and the axial tube lengths, diameters, and wall thickness result in a primary bending natural period of about 33 sec.

AHC describes that in the Baldpate tower, the axial tubes will do essentially all of the work to keep the upper, articulated part of the structure upright and enable it to accommodate operating loads and withstand typical and extreme weather events.

Once in place and operational, AHC expects the tower to move laterally somewhat more than conventional platforms. It may be displaced as much as 10 ft during more severe winter storms.

During an extreme event, such as a 100-year recurrence hurricane or loop current, AHC expects tower displacement at the water line to reach 25 ft, although about 15 ft will be a fixed offset, induced by the storm winds and waves before the extreme wave. The remaining 10 ft would result from peak winds and waves of the extreme event itself.

In any case, AHC expects the lateral movements to be relatively subtle, with accelerations less than 0.05 of the acceleration of gravity. This is much less than an elevator.

Even during an extreme event, AHC says, over its overall height of more than 1,400 ft above the articulation point, the tower will articulate to just over 1° from vertical.

McDermott Engineering designed the Baldpate compliant tower.

Tower structure

The tower structure was built in two pieces. J Ray McDermott fabricated the base at its Louisiana yard. The base is 351-ft tall, and weighs 8,700 tons. It measures 140 3 140 ft at the bottom and 90 3 90 ft at the top. The largest members have a 144-in. OD and are 35/8-in. thick.

Aker Gulf Marine fabricated the tower section (Fig. 5 [10,864 bytes]) at its Corpus Christi facility. The tower section is 1,320 ft tall, weighs 20,200 tons, and is 90 3 90 ft. The largest members have a 128-in. OD and are 33/4-in. thick. The tower has large pins at the bottom of each of the four legs. These pins will mate with receptacles built into the top base section.

AHC says the structure included stronger steel, 60 ksi, instead of the more common 30 ksi steel.

Aker Gulf Marine also assembled the topsides. These have a 3,950-ton lift weight. In place, the operating loads will peak at 9,800 tons.

The facilities are designed to discharge produced water overboard. AHC indicates the design employs waste heat from the generators and cooling from seawater circulation systems, as well as incorporation of industry-proven process and facility system technologies.

The facility systems are mostly electrically powered, with power delivered by three Solar Taurus 60 turbine-driven generator sets each capable of generating up to 4,750 kw. Two of these units can power the platform at peak output, with the third providing 50% standby.

The facilities include quarters for 28 men.

A total of 37,300 tons of structural steel were used in constructing the tower, piles, and deck.

Pipelines

Allseas in February 1998 installed the 13 mile, 12-in. gas pipeline (0.5-0.625-in. wall API Grade B) and the 17 mile, 16-in. oil pipeline (0.625-0.75 in. wall API Grade B). Both pipelines have a fusion bond epoxy coating and anodes for corrosion protection.

The gas line is owned by the GB 260 partners and ties in subsea to the 30-in. Garden Banks gathering pipeline at GB 85.

Poseidon Oil Pipeline Co. owns the oil line, which ties into the 20 to 24-in. Poseidon pipeline system at a Mobil platform in South Marsh Island Block 205.

Both the oil and gas pipelines are designed to connect to the compliant tower with steel catenary risers (Fig. 6 [13,282 bytes]).

The steel catenary risers are comprised of a section of the heavier wall pipe suspended from a clamp attached to the tower at about 400 ft beneath the waterline.

The pipelines will touch the seafloor about 650 ft from the base of the structure.

The steel catenary risers are coated with neoprene to eliminate potential for external corrosion, and outfitted with helical strakes to suppress vortex-induced vibrations that could be caused by subsurface currents.

Both pipelines have a working pressure equivalent to ANSI 900 (2,200 psi).

A few weeks after the topsides are in place, Cal Dive, with the dynamically positioned diving support vessel "CSO Constructor," will lift, in sequence, the ends of both pipelines, and hang them in their planned catenary configuration.

Installation

Heerema's crane vessel Balder installed the tower base in April 1998. The process included setting two docking piles on four leveling piles, installed in October 1996.

Twelve 84-in. piles, three at each corner, anchor the base to the seafloor. Each 530-in. pile penetrates about 430-ft into the seafloor. The 440-ton piles were installed in one piece.

The tower section will set on (and be grouted to) the base.

AHC expects the structure and facility installation to be completed in mid-May. This includes offshore hook-up, pipeline interconnect, and rig-related activities. Initial production from the first well is planned to begin in August 1998.