Honeycomb Buoyant Island Structures

Abstract

Multi-cylinder buoyant island structures, such as may be used for staging and storage related to offshore oil and gas production activities, and methods of use and manufacture.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Applications No. 61/792,862 filed Mar. 15, 2013, and No. 61/701,505 filed Sep. 14, 2012, both of which are incorporated by reference in their entireties.

BACKGROUND

1. Field of the Invention

The present invention relates to buoyant structures, including, but not by way of limitation, buoyant island structures for use in or near offshore oil and gas production operations, especially in environments affected by ice formation.

2. Description of Related Art

Offshore drilling operations present additional challenges as compared with land-based installations because of harsh environmental conditions and lack of proximate support structures. Drilling platforms must be self-sustaining, making them larger than their onshore counterparts. Such platforms may be in remote locations that lack natural staging areas (islands) that can be used for refueling flights and the like.

Traditionally, offshore structures take two characteristic forms, that of bottom-supported units and floating units. Floating units, for example, include drill ships and semisubmersibles. Bottom-supported offshore drilling rigs include the submersible and the jackup. Submersible rigs can be bottle-type submersibles, where large diameter cylinders are welded to smaller horizontal cylinders to form a submersible cube structure. When flooded, the cylinders sink to rest on the bottom.

Jackups, another type of bottom-supported drilling rig, utilize large vertical trusses or, alternatively, columnar legs. A platform outfitted with these legs is generally towed to location, where crew members jack the legs down until they come into firm contact with the ocean seafloor. The platform can then be leveled and then jacked up until it is clear of the water's surface and high waves.

In harsh winter environments, such as those found in the arctic, offshore structures must be able to withstand the tremendous forces of moving pack ice that surrounds them for most of the year. Arctic submersibles come in several forms: the conical drilling unit, the mobile arctic caisson, and the concrete island drilling system. Arctic submersibles often employ a heavy steel or concrete linear wall, which surrounds the equipment below the waterline. Arctic submersibles are extremely heavy and more costly to manufacture, due to the additional reinforced concrete necessary to protect the wide base area of the existing bottle-type or jackup designs.

SUMMARY

The present embodiments include offshore buoyant island structures that can withstand the ice environment of the Arctic and Antarctic regions while reducing the cost and time required to build such offshore rigs.

Buoyant structures, a relatively new entry into the offshore drilling rig space, have the advantage of being less expensive than the fixed platforms described above, as well as providing attractive fabrication and installation schedules. Relying on a cell spar design, buoyant structures may be configured as compliant structures, meaning that they can be configured to move somewhat in response to environmental forces. Single-spar configurations often include a floating platform having a single vertical cylinder supporting a deck. At least a portion of the cylinder can be flooded with water to submerse a portion of the cylinder, which can also be weighted at the bottom with dense material (e.g., more dense than seawater) to lower the center of gravity and improve stability. Truss spar configurations can employ a cylindrical hard tank to provide buoyancy. The cylindrical tank can be attached to a smaller soft tank by a truss, with the soft tank located at a bottom part of the rig and containing a heavy ballast material.

Cell-spar configurations can include a platform with a large (e.g., central) cylinder surrounded by smaller cylindrical tubes, which may be of alternating lengths. The smaller tubes may contain variable-ballast tanks and redundant, independent cells to improve stability. Some of the present embodiments include a cell spar design in a modified buoyant structure to provide a buoyant island structure that is capable of surviving harsh arctic environments, that can be used as a staging area (e.g., for helicopter traffic supplying logistics to the drill and production sites), and that can be manufactured at lower cost with faster fabrication and installation times. In some instances, drilling sites may be as much as 1600 km from the closest populated area, and multiples ones of the present island structures can be located at 300-400 km intervals between a drilling site and a populated area.

This disclosure includes embodiments of methods and apparatuses related to buoyant structures, including, but not by way of limitation, platforms in environments affected by ice formation.

Embodiments of the present offshore buoyant structures comprise: an inner cylinder having a radius; a plurality of outer cylinders coupled around the inner cylinder such that each outer cylinder is not separated from immediately adjacent ones of the outer cylinders by a distance greater than the radius of the inner cylinder; where the support structure is configured to be disposed in a body of water with the inner cylinder and outer cylinders substantially vertical. In some embodiments, each of the outer cylinders is in contact with two adjacent ones of the outer cylinders. In some embodiments, the structure is disposed in a body of water with the inner cylinder and outer cylinders substantially vertical, and the support structure further comprises; a platform coupled to and supported above the cylinders. Some embodiments further comprise: one or more spacers disposed between at least two of the central and outer cylinders. Some embodiments further comprise: a plurality of intermediate vertical cylinders disposed between the inner cylinder and the outer cylinders. Some embodiments further comprise: an outer shell coupled around the outer cylinders. In some embodiments, where the outer shell comprises steel, iron, alloy, and/or reinforced concrete. In some embodiments, portions of the outer shell each extend inward between adjacent ones of the outer vertical cylinders. In some embodiments, at least a portion of the outer perimeter of the outer shell is curved in shape. In some embodiments, the outer shell has a tapered cross-sectional shape. In some embodiments, the outer shell has a triangular cross-sectional shape. Some embodiments further comprise: a heating element coupled to the outer shell. In some embodiments, at least one cylinder includes a variable-buoyancy mechanism. Some embodiments further comprise: at least one suction anchor coupled to the cylinders.

Some embodiments of the present methods comprise: positioning an embodiment of the present buoyant support structures in a body of water such that the cylinders are substantially vertical. Some embodiments further comprise: coupling the support structure to a suction anchor secured to a seafloor under the body of water. Some embodiments further comprise: coupling a platform to and above the cylinders.

Some embodiments of the present methods (of constructing an offshore support structure) comprise: coupling a plurality of outer cylinders around an inner cylinder having a radius such that each outer cylinder is not separated from immediately adjacent ones of the outer cylinders by a distance greater than the radius of the inner cylinder; where the coupled inner cylinder and outer cylinders are configured to be disposed substantially vertically in a body of water to support a platform for production of hydrocarbons. In some embodiments, the outer cylinders are coupled such that each of the outer cylinders is in contact with two adjacent ones of the outer cylinders. Some embodiments further comprise: disposing the support structure in a body of water with the inner cylinder and outer cylinders substantially vertical; and coupling a platform to the inner cylinder and outer cylinders. Some embodiments further comprise: disposing one or more spacers between at least two of the central and outer cylinders. Some embodiments further comprise: coupling a plurality of intermediate vertical cylinders between the inner cylinder and the outer cylinders.

Some embodiments of the present methods further comprise: coupling an outer shell around the outer cylinders. In some embodiments, the outer shell comprises steel, iron, alloy, and/or reinforced concrete. In some embodiments, the outer shell is configured and coupled to the outer cylinders such that portions of the outer shell each extend inward between adjacent ones of the outer vertical cylinders. In some embodiments, at least a portion of the outer perimeter of the outer shell is curved in shape. In some embodiments, the outer shell has a tapered cross-sectional shape. In some embodiments, the outer shell has a triangular cross-sectional shape. Some embodiments further comprise: disposing one or more production apparatuses within an interior region defined by the outer cylinders. Some embodiments further comprise: coupling a heating element to the outer shell. In some embodiments, where at least one cylinder includes a variable-buoyancy mechanism. Some embodiments further comprise: coupling the cylinders to at least one suction anchor.

Any embodiment of any of the present systems, apparatuses, and methods can consist of or consist essentially of—rather than comprise/include/contain/have—any of the described steps, elements, and/or features. Thus, in any of the claims, the term “consisting of” or “consisting essentially of” can be substituted for any of the open-ended linking verbs recited above, in order to change the scope of a given claim from what it would otherwise be using the open-ended linking verb.

Details associated with the embodiments described above and others are presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings illustrate by way of example and not limitation. For the sake of brevity and clarity, every feature of a given structure is not always labeled in every figure in which that structure appears. Identical reference numbers do not necessarily indicate an identical structure. Rather, the same reference number may be used to indicate a similar feature or a feature with similar functionality, as may non-identical reference numbers. The figures are drawn to scale for certain embodiments (unless otherwise noted), meaning the sizes of the depicted elements are accurate relative to each other for at least the depicted embodiment.

FIG. 1 depicts a perspective view of a first embodiment of the present buoyant island structures with a plurality of suction anchors.

FIG. 2 is a top view of a plurality of the structures of FIG. 1.

FIGS. 3A and 3B are top and perspective views, respectively, of a plurality of the structures of FIG. 1 tethered together.

FIGS. 4A-4D depicts a process for installation of the structure of FIG. 1.

FIG. 5 depicts a ballasting step for the installation of the structure of FIG. 1.

FIGS. 6A-6B depict top and side cutaway views, respectively of a second embodiment of the present buoyant island structures with a plurality of suction anchors.

FIGS. 7A-7B depict top and side cutaway views, respectively of a third embodiment of the present buoyant island structures with a plurality of suction anchors.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The term “coupled” is defined as connected, although not necessarily directly, and not necessarily mechanically; two items that are “coupled” may be unitary with each other. The terms “a” and “an” are defined as one or more unless this disclosure explicitly requires otherwise. The term “substantially” is defined as largely but not necessarily wholly what is specified (and includes what is specified; e.g., substantially 90 degrees includes 90 degrees and substantially parallel includes parallel), as understood by a person of ordinary skill in the art. In any disclosed embodiment, the terms “substantially,” “approximately,” and “about” may be substituted with “within [a percentage] of” what is specified, where the percentage includes 0.1, 1, 5, and 10 percent.

The terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”) and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a system or apparatus that “comprises,” “has,” “includes” or “contains” one or more elements possesses those one or more elements, but is not limited to possessing only those elements. Likewise, a method that “comprises,” “has,” “includes” or “contains” one or more steps possesses those one or more steps, but is not limited to possessing only those one or more steps.

Further, a structure (e.g., a component of an apparatus) that is configured in a certain way is configured in at least that way, but it can also be configured in other ways than those specifically described.

The present buoyant structures are configured for use in environments affected by ice formation. The present embodiments include a structure having a honeycomb assembly of a series of cylinders (designed to be vertical, in use), some or all of which can include variable-buoyancy mechanisms for varying the buoyancy of the structure, such as, for example, via one or more discrete chambers that can be filled with air, air and seawater, seawater alone, or a fixed ballast. Such cylinders may comprise steel, iron, and/or other metals or alloys, as are known for hulls of offshore vessels and the like.

Referring now to the drawings, and more particularly to FIG. 1, shown there and designated by the reference numeral 100 is an exemplary embodiment of the present buoyant structures. In the embodiment shown, structure 100 includes a multiple cylinders 110 (structure 100 is configured as a cell spar structure). In the depicted embodiment, buoyant structure 100 is configured to be held in place relative to the seabed or seafloor by at least one Suction Can Foundation (“SCF”) 120. For example, in the embodiment shown, an SCF 120 is disposed at the bottom of each cell.

For example, each SCF 120 can be forced into the seafloor during installation of the structure, and provides both lateral and vertical support while permitting the structure to act as a compliant structure around the center of rotation. The center of rotation refers to an angular motion pivot point at the base of a cell 110 at or near a corresponding SCF 120. Each SCF 120 may be drawn into the seabed by ballasting some of the tanks cells 110 and allowing the weight of structure 100 to force the SCF down into the seafloor. Alternatively, or in conjunction, the pressure within a SCF 120 can be lowered to suck up a portion of the seabed aggregate within the SCF. In the embodiment shown, an SCF is coupled to a lower end of each cell 110; however, in other embodiments, SCFs 120 may be provided at the lower ends of only some of cells 110. In the

Embodiments of the present structures can include any number of cells 110 that achieves the purposes of the present disclosure and permits structure 100 to function as described. For example, FIG. 1 depicts a seven-cell honeycomb configuration, with a plurality of outer or peripheral cells 110 surrounding an inner or central cell 130. The present honeycomb structures can comprise any of various configurations, such as, with more outer cells 110 of any of various diameters (e.g., equal to any one of, or between any two of: 5, 10, 15, 20, or more meters) surrounding an inner cell 130 of any of various diameter (e.g., equal to any one of, or between any two of: 5, 10, 15, 20, or more meters). For example, in other embodiments, such as the one described in more detail below and shown in FIGS. 6A-6B, the present honeycomb structures can include outer or peripheral cells that surround intermediate cells that, in turn, surround an inner or central cell. In some embodiments, such as one shown in FIG. 2, outer cells 110 have identical diameters that are also equal to the diameter of central cell 130. For example, in the embodiment shown, cells 110 can each have a diameter of 10 meters, such that structure 100 has an upper area, with a maximum transverse dimension 140 of 30 meters, that is large enough to support a variety of structures and/or helicopter landing pads to provide staging, storage, and/or other support for remote drilling operations.

In some embodiments, outer cells 110 are configured to maintain a closed pattern, in which each outer cell 110 contacts two adjacent outer cells 110. FIG. 1 is illustrative. Each outer or peripheral cell 110 maintains contact along the longitudinal axis (150) of the structure with two other adjacent outer cells 110. In this embodiments, outer cells 110 each also contacts center cell 130. For these closed-pattern configurations, the diameters of the respective cells can be varied to maintain contact between the cells of each type (central, peripheral) and/or to maintain contact between different types of cells (between central and peripheral cells) for various numbers of outer cells 110 (and/or or intermediate cells). In other embodiments, such as the ones described below with reference to FIGS. 6A-6B and 7A-7B, spacers may be disposed between adjacent cells. Structure 100 (and cells 110 and 130) may be configured for a variety of water depths (e.g., equal to any one of, or between any two of: 5, 10, 25, 50, 100 meters or more) and can have any suitable height to position the top of structure 100 at or above sea level when installed.

In some embodiments, multiple ones of the present structures can be tethered or otherwise coupled together to form an island structure. For example, FIG. 2 illustrates three structures 100 being floated together, and FIGS. 3A and 3B depict nine structures 100 tethered or otherwise coupled together to form a larger buoyant island structure. In some embodiments, gaps between cells 110, 150 of a single structure 100, and or gaps between cells 110 of adjacent structures 100, may be covered (e.g., with plates of steel or the like) to provide a substantially unbroken work surface.

FIGS. 4A-4D depict an example of a method for delivering and installing structures 100. A first (left) one of structures 100 is shown in 4(a) as already secured to a seafloor 160 by SCFs 120. A second (right) one of structures 100 is shown in 4(a) being floated in a horizontal configuration to the installation site. Cells 110 and 150 of structure 100 are generally hollow, and therefore will generally float (at least before ballast is added to the cells) such that a tugboat or other vessel may be used to tow structure 100 to an installation site. Once at or near the installation site, ballast is added to cells 110 and/or 150 (e.g., sub-compartments within the cells) to rotate the bottom end of structure 100 (and SCFs 120) downward toward seafloor 160, as shown in 4(b). Ballast may include seawater (e.g., pumped into the cells) and/or gravel, mud, or other seafloor material (e.g., dredged into the cells). As structure 100 becomes vertical, as in 4(c), it can be moved closer to its position (e.g., next to an adjacent structure 100) and its respective SCF(s) 120 secured to seafloor 160, as in 4(d).

FIG. 5 depicts an example of a method for adding ballast to the cells of the present structures. In the embodiment shown, a ship 200 dredges seafloor material through an intake conduit 204 and deposits the seafloor material (e.g., and seawater) into cells 110 (and/or cells 150) through their respective tops via a delivery conduit 608.

FIGS. 6A-6B depict another embodiment 100a of the present structures that is similar in some respects to structure 100, except that structure 100a comprises a plurality outer or peripheral cells 110a, a central cell 130a, and a plurality of intermediate cells 170. As described above, peripheral cells 110a, central cell 130a, and intermediate cells 170 can each have any of a variety of diameters (e.g., equal to any one of, or between any two of: 5, 10, 15, 20, or more meters). For example, in the embodiment shown, outer cells 110a each has a diameter of 5.8 meters, central cell 130a has a diameter of 16 meters, and intermediate cells 170 each has a diameter of 10 meters. As such, in this embodiment, apparatus 10a has a transverse dimension of nearly 50 meters, resulting in a large work space or staging area. In this embodiment, structure 100a may be delivered to an installation site in pieces and assembled at the installation site, such as is described above for multiple ones of structure 100.

In the embodiment shown, outer cells 110a are configured in a closed configuration in which each of outer cells 110a contacts two adjacent outer cells 110a. The relatively small diameters of outer cells 110a can permit the perimeter defined by outer cells 110a to flex somewhat under pressure from ice, thereby decreasing stability somewhat to increase movement (and thereby discouraging and/or inhibiting ice formation around structure 100a and impingement on structure 100a by adjacent ice sheets 190). This is especially true for embodiment in which only some of outer cells 110a are directly secured to the seafloor by SCFs 120. For example, in the embodiment shown, some of outer cells 110a do not have spacers 180 joining them to intermediate cells 170, such that those outer cells 110a without spacers may be permitted more movement to break up and reduce formation of solid ice sheets in close proximity to structure.

In the embodiment shown, spacers 180 may be disposed between and in contact with adjacent cells. Spacers 180 are configured to bridge gaps between cells of to permit additional flexure between cells. Spacers may be fashioned in various shapes and sizes, including with straight sides, concave sides, convex sides, or as truss segments, and may be welded or otherwise secured to cells to tether cells together. In one embodiment, spacers may be used with multiple cells of different diameter (not shown). A person of skill in the art would understand the frequency with which spacers are required along the longitudinal axis of the cells, according to the engineering stresses imparted on the structure. In some embodiments, spacers may include pipes extending between the cells, and such pipes can be used, for example to carry fluids within the structure (e.g., seawater for ballast, production fluids, and/or the like).

FIGS. 7A-7B depict another embodiment 100b of the present structures that is similar in some respects to structure 100, except that structure 100b comprises a plurality outer or peripheral cells 110b, a central cell 130b, and a plurality of spacers 180 between outer cells 110b and between central cell 130b and outer cells 110b. As described above, peripheral cells 110b and central cell 130b, can each have any of a variety of diameters (e.g., equal to any one of, or between any two of: 5, 10, 15, 20, or more meters). For example, in the embodiment shown, outer cells 110a each has a diameter of 10 meters, and central cell 130a has a diameter of 16 meters. As such, in this embodiment, apparatus 10b has a transverse dimension of nearly 40 meters, resulting in a large work space or staging area. In this embodiment, structure 100b may be delivered to an installation site in pieces and assembled at the installation site, such as is described above for multiple ones of structure 100. In this embodiment, outer cells 110b are not configured in a closed configuration, and each of outer cells 110b does not contact adjacent outer cells 110b. Instead, spacers 180 are disposed between outer cells 110b. In some embodiments, an outer shell 200 having a tapered cross-sectional shape to discourage and/or inhibit ice formation around structure 100b and impingement on structure 100b by adjacent ice sheets 190) is disposed around outer cells 110b. For example, in the embodiment shown, a close-fit outer shell 200 that extends around peripheral cells 110a and has a height 210 selected to extend above and below an expected sea level 220. Height 210 can be, for example, between 3 and 15 meters (e.g., equal to any one of, or between any two of: 3, 5, 7, 10, and 15 meters. Height 210 may be selected for various implementations to meet or exceed an average or expected swell height in a particular body of water, and/or to meet or exceed average or expected ice ridge height in a particular body of water. In this embodiment, shell 200 has a triangular cross-section with a point 220 that is spaced apart from outer cells 110b, as shown.

While shell 200 extends around the entire perimeter of structure 100b, other embodiment can include a shell that extends around only a portion of the perimeter of an individual structure, such as to facilitate joining multiple individual structures with shell portions extending outward from the joined group of structures.

The present structure embodiments with a honeycomb cell design and an outer shell includes several benefits. For example, because the structure is rounded with no sharp edges, moving ice can better flow around it. Moreover, as the pressure of the ice increases, the resistance of the assembled cylinders increases due to the interlocking of the elements of the outer shell. The individual arcuate elements of the outer shell can also function as miniature arches, resolving inward forces into compressive stresses that are laterally distributed through the shell in a lateral direction. In other embodiments, shell 200 can include a series of alternatingly oriented arcs (as opposed to the depicted series of similarly-oriented arcs in the depicted embodiment), such that forces carried laterally through the shell will attempt to thrust outward at the base of each concave arcuate element, but will be restrained by the adjacent and opposingly-oriented convex arcuate elements. In addition, outer shell 200 can be coupled to outer cells 110b, which can assist in restraining the lateral thrust as the arcuate elements transfer inward forces to compressive stresses.

In the harsh environment of the arctic, extreme low temperatures may impact the resilience of the outer shell 200 material. Basic steel, for example, may not possess impact toughness when exposed to low temperatures and may fail in a brittle manner. Thus, materials used for outer shell 200 should generally be selected to have strong low-temperature performance (e.g., higher grade steels, other metals, and/or alloys). In some embodiments, excess power from the platform may also be channeled into outer shell 200 to elevate the temperature of the shell to a range in which the shell performs adequately. In some embodiments, outer shell heaters may increase the temperature of outer shell 200 to a point that is sufficient to reduce ice formation in the immediate vicinity of the structure, thereby further lessening potential stresses from the ice pack.

The above specification and examples provide a description of the structure and use of exemplary embodiments. Although certain embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the scope of this invention. As such, the various illustrative embodiments of the present devices are not intended to be limited to the particular forms disclosed. Rather, they include all modifications and alternatives falling within the scope of the claims, and embodiments other than the one shown may include some or all of the features of the depicted embodiment. For example, components may be combined as a unitary structure. Further, where appropriate, aspects of any of the examples described above may be combined with aspects of any of the other examples described to form further examples having comparable or different properties and addressing the same or different problems. Similarly, it will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments.

The claims are not intended to include, and should not be interpreted to include, means-plus- or step-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” or “step for,” respectively.

Claims

1. An offshore buoyant structure comprising:

an inner cylinder having a radius;

a plurality of outer cylinders coupled around the inner cylinder such that each outer cylinder is not separated from immediately adjacent ones of the outer cylinders by a distance greater than the radius of the inner cylinder;

where the support structure is configured to be disposed in a body of water with the inner cylinder and outer cylinders substantially vertical.

2. The support structure of claim 1, where each of the outer cylinders is in contact with two adjacent ones of the outer cylinders.

3. The support structure of claim 1, where the structure is disposed in a body of water with the inner cylinder and outer cylinders substantially vertical, the support structure further comprising;

a platform coupled to and supported above the cylinders.

4. The support structure of claim 1, further comprising:

one or more spacers disposed between at least two of the central and outer cylinders.

5. The support structure of claim 1, further comprising:

a plurality of intermediate vertical cylinders disposed between the inner cylinder and the outer cylinders.

6. The support structure of claim 1, further comprising:

an outer shell coupled around the outer cylinders.

7. The support structure of claim 6, where the outer shell comprises steel, iron, alloy, and/or reinforced concrete.

8-12. (canceled)

13. The support structure of claim 1, where at least one cylinder includes a variable-buoyancy mechanism.

14. The support structure of claim 1, further comprising:

at least one suction anchor coupled to the cylinders.

15. A method comprising:

positioning a buoyant support structure of claim 1 in a body of water such that the cylinders are substantially vertical.

16. The method of claim 15, further comprising:

coupling the support structure to a suction anchor secured to a seafloor under the body of water.

17. The method of claim 15, further comprising:

coupling a platform to and above the cylinders.

18. A method of constructing an offshore support structure, the method comprising:

coupling a plurality of outer cylinders around an inner cylinder having a radius such that each outer cylinder is not separated from immediately adjacent ones of the outer cylinders by a distance greater than the radius of the inner cylinder;

where the coupled inner cylinder and outer cylinders are configured to be disposed substantially vertically in a body of water to support a platform for production of hydrocarbons.

19. The method of claim 18, where the outer cylinders are coupled such that each of the outer cylinders is in contact with two adjacent ones of the outer cylinders.

20. The method of claim 18, further comprising:

disposed the support structure in a body of water with the inner cylinder and outer cylinders substantially vertical; and

coupling a platform to the inner cylinder and outer cylinders.

21. The method of claim 18, further comprising:

disposing one or more spacers between at least two of the central and outer cylinders.

22. The method of claim 18, further comprising:

coupling a plurality of intermediate vertical cylinders between the inner cylinder and the outer cylinders.

23. The method of claim 18, further comprising:

coupling an outer shell around the outer cylinders.

24-28. (canceled)

29. The method of claim 18, further comprising:

disposing one or more production apparatuses within an interior region defined by the outer cylinders.

30. The method of claim 23, further comprising:

coupling a heating element to the outer shell.

31. The method of claim 18, where at least one cylinder includes a variable-buoyancy mechanism.

32. The method of claim 18, further comprising:

coupling the cylinders to at least one suction anchor.