Floating Deep Draft Semi-Submersible Offshore Platforms and Methods for Assembling and Deploying Same

Abstract

A semi-submersible platform for offshore operations includes a buoyant hull including an adjustably buoyant base and a plurality of adjustably buoyant columns extending vertically from the base. In addition, the platform includes a deck moveably coupled to the columns. The deck is configured to move vertically up and down relative to the hull.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

This disclosure relates generally to floating offshore structures. More particularly, this disclosure relates to buoyant semi-submersible offshore platforms for offshore drilling and production operations. Still more particular, the disclosure relates to buoyant semi-submersible offshore platforms having increased draft and associated stability, while being able to be constructed dockside.

The draft of a floating offshore structure refers to the vertical distance between the waterline (sea surface) and the bottom of the structure when the structure is deployed in water (e.g., in an offshore operating location). In general, increasing the draft of a floating offshore structure improves its stability and reduces its range of movement (vertical and rotational). Enhanced dynamic stability provides several potential advantages including increased comfort of on-board personnel, easier operation of topsides equipment, and reduced stresses applied to structures supported by the platform such as drilling risers, production risers, etc. Consequently, low motion floating offshore platforms for conducting drilling and/or production operations (i.e., floating offshore platforms that experience heave less than 10-15 ft.) allow the use of top tensioned risers and dry production trees disposed on the platform, both of which offer operational advantages that may lead to enhanced hydrocarbon recovery.

Tension Leg Platforms (TLPs) and Spar platforms are two types of floating structures platforms for conducting offshore drilling and/or production operations. TLPs and Spars both experience motions sufficiently small to support top tensioned risers and dry tree production trees. The TLP's vertical motions are constrained by stiff tendons that extend from the structure to the sea bed, and the Spar's vertical motions are diminished as a result of its relatively deep draft (in the order of 400 to 500 ft.) extending down to the low dynamic pressure zone.

Semi-submersible offshore platforms are another type of floating structure for conducting offshore drilling and/or production operations. Conventional semi-submersible platforms include a hull with sufficient buoyancy to support a work platform above the surface of the water. The hull is typically made of a plurality of horizontal pontoons that support a plurality of vertically upstanding columns, which in turn support the work platform. The hull is divided into several closed compartments, each compartment having buoyancy that can be adjusted for purposes of flotation and trim. Typically, a pumping system pumps ballast water into and out of the compartments, as desired, to adjust buoyancy.

Conventional shallow draft semi-submersible platforms typically have a draft less than about 100 ft. (about 30.5 m). The motions of shallow draft semi-submersible platforms are usually relatively large, and thus, they are generally not suitable for top tensioned risers and dry production trees. Consequently, shallow draft semi-submersible platforms usually require the use of “catenary” risers and wet production trees disposed at the sea floor. The draft of a floating semi-submersible platform can be increased to reduce motions and improve stability by lengthening the hull columns and locating the hull pontoons at a greater depth below the surface of the water. As a result, deep draft semi-submersible platforms having drafts greater than 150 feet (about 45.0 m) usually experience significantly smaller motions than conventional shallow draft semi-submersible platforms, thereby enabling the deep draft semi-submersible platforms to support top-tensioned risers and dry production trees. Such smaller motions also reduce the likelihood of inducing damage to risers and mooring systems.

BRIEF SUMMARY OF THE DISCLOSURE

Embodiments described herein include a semi-submersible platform for offshore operations. In an embodiment, the platform comprises a buoyant hull including an adjustably buoyant base and a plurality of adjustably buoyant columns extending vertically from the base. In addition, the platform comprises a deck moveably coupled to the columns. The deck is configured to move vertically up and down relative to the hull.

Embodiments described herein also include a method for assembling a deep draft semi-submersible offshore platform. In an embodiment, the method comprises (a) floating a buoyant hull in water at a dockside location. The buoyant hull has a draft less than 40 ft. at the dockside location. In addition, the method comprises (b) moveably coupling a deck to the hull during (a) to form the semi-submersible offshore platform. The deck is configured to move vertically relative to the hull.

Embodiments described herein also include a method for deploying a deep draft semi-submersible offshore platform to an offshore operating site. In an embodiment, the method comprises (a) floating the semi-submersible platform to the operating site. The platform comprises a buoyant hull and a non-buoyant deck moveably coupled to the hull. The hull is disposed at a draft D. In addition, the method comprises (b) ballasting the hull at the operating site to increase the draft D of the hull at the operating site. Further, the method comprises (c) raising the deck vertically relative to the hull at the operating site.

Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention in order that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:

FIG. 1 is a schematic side view of a conventional semi-submersible platform being assembled at a dockside location;

FIG. 2 is a schematic side view of the conventional semi-submersible platform of FIG. 1 deployed and anchored at an offshore operating site;

FIG. 3 is a schematic side view of an embodiment of a deep draft semi-submersible platform in accordance with the principles described herein deployed and anchored at an offshore operating site;

FIG. 4 is a top schematic view of the deep draft semi-submersible platform of FIG. 3;

FIG. 5 is an enlarged partial schematic side view of one of the jack-up assemblies of FIG. 4;

FIG. 6 is a schematic side view of the deep draft semi-submersible platform of FIG. 3 at a dockside location illustrating the vertical range of movement of the deck relative to the hull;

FIGS. 7A-7J are schematic side views of the deep draft semi-submersible platform of FIG. 3 being assembled at a dockside location, deployed to an offshore operating site, and installed at the offshore operating site;

FIGS. 8A-8C are enlarged partial schematic side views of an embodiment of a jacking system for raising and lowering the deck of FIG. 3 relative to the columns of FIG. 3;

FIG. 9 is an enlarged partial schematic side view of an embodiment of a jacking system for raising and lowering the deck of FIG. 3 relative to the columns of FIG. 3;

FIG. 10A is a schematic top view of the deep draft semi-submersible platform of FIG. 3 including a plurality of column support structures; and

FIG. 10B is a schematic top view of the deep draft semi-submersible platform of FIG. 3 including a plurality of column support structures and deck extensions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. Any reference to up or down in the description and the claims will be made for purposes of clarity, with “up”, “upper”, “upwardly” or “upstream” meaning toward the surface of the borehole and with “down”, “lower”, “downwardly” or “downstream” meaning toward the terminal end of the borehole, regardless of the borehole orientation. Any reference to up or down in the description and the claims will be made for purpose of clarification, with “up”, “upper”, “upwardly” or “upstream” meaning toward the surface of the borehole and with “down”, “lower”, “downwardly” or “downstream” meaning toward the terminal end of the borehole, regardless of the borehole orientation.

As previously described, Tension Leg Platforms (TLPs) and Spar platforms are two types of floating offshore structures that have sufficient dynamic stability to enable the use of top tensioned risers and dry production trees. However, as water depth increases, the cost of tendons can render TLPs cost prohibitive. In deep water, Spars may offer a better option than a TLP from a cost perspective, however, Spars pose installation challenges. In particular, due to the deep draft of a Spar, the hull is typically towed to the offshore operating site in a horizontal orientation (i.e., on its side), and then up-righted to a vertical orientation at the operating site under very controlled conditions. Next, the Spar topside is installed atop the hull using a heavy lift vessel. Depending on the weight of the topsides modules, multiple lifts may be required. These stages of Spar construction add significant costs to the delivery and installation.

Deep draft semi-submersible platforms having a draft greater than about 150 ft. (about 45.0 m) are another type of floating offshore platform capable of employing top-tensioned risers and a dry production tree. However, similar to Spars, deep draft semi-submersible platforms pose installation challenges. More specifically, semi-submersible offshore platforms are built by constructing the hull, and then mounting the deck to the top of the vertical columns of the hull. The location of final assembly may involve integration (i.e., installation of the deck onto the hull) at the shipyard (dockside or quayside), nearshore, or at its offshore operating site, which is typically far offshore (e.g., over 100 miles, or 161 km).

For integration of the semi-submersible platform at the shipyard, the deck, also referred to as the topside, is lifted and positioned atop the columns of the hull with heavy lifting equipment (e.g., heavy lift crane), and then the fully assembled semi-submersible platform is floated out to the operating site using a heavy lift or tow vessel. Dockside water depths are typically on the order of 30-40 ft., and thus, for dockside integration, the hull is deballasted such that its draft is always less than the 30-40 ft. water depth. For example, referring now to FIG. 1, an exemplary floating semi-submersible platform 10 is shown being assembled in water 11 at a dockside location 12. As previously described, dockside water depths are typically on the order of 30-40 ft., and thus, water 11 at dockside location 12 has a depth D_w12 of about 30-40 ft. Platform 10 includes a buoyant hull 20 and a deck or topside 30 disposed thereon. Hull 20 includes a plurality of adjustably buoyant horizontal pontoons 21 and a plurality of adjustably buoyant vertical columns 25 extending upward from pontoons 21. Hull 20 has a draft D₂₀ that is limited by depth D_w12 of water 11 at dockside location 12. In other words, at dockside location 12, the draft D₂₀ is less than the water depth D_w12. A heavy lift crane 40 positioned on land adjacent water 11 at the dockside location 12 lifts deck 30 upwards and positions it atop columns 25 to assemble platform 10. As shown in FIG. 2, once assembled, platform 10 is floated from the dockside location 12 to the offshore operating site 14, and then hull 20 is ballasted to increase the draft D₂₀ to the desired operating draft. In general, the greater the draft D₂₀ and the lower the deck 30, the greater the dynamic stability of platform 10. However, at dockside location 12, the draft D₂₀ is limited by the depth D_w12 at dockside location 12 (i.e., the draft D₂₀ must be less than the depth D_w12 at the dockside location 12).

For offshore integration (i.e., integration at the offshore operating site 14), hull 20 and deck 30 are separately transported offshore to the operating site 14, either by towing at a shallow draft or by transport aboard heavy lift vessels. Then, at the operating site 14, the hull 20 is ballasted down by pumping sea water into the pontoons 21 and columns 25, and then deck 30 is either lifted onto the tops of columns 25 by heavy lift cranes carried aboard a heavy lift barge or by floating deck 30 over the top of the partially submerged hull 20 using a deck barge. In either case, the procedure is typically performed far offshore in open seas, and is strongly dependent on weather conditions and the availability of a heavy lift barge.

For nearshore integration, hull 20 and deck 30 are separately transported to a nearshore assembly site (closer to shore than the offshore operating site 14), either by towing at a shallow draft or by floating aboard heavy lift vessels. Then, at the assembly site, hull 20 is ballasted down by pumping sea water into the pontoons 21 and columns 25, and then deck 30 is either lifted onto the tops of columns 25 by heavy lift cranes carried aboard a heavy lift barge or by floating platform 30 over the top of the partially submerged hull 20 using a deck barge. The assembled platform 10 is then floated from the nearshore assembly site to the offshore operating site 14.

As compared to offshore integration, nearshore integration is generally less expensive and less risky. However, dockside integration is generally less expensive and less risky than both nearshore and offshore integration. In addition, dockside integration provides the added advantage that it can generally be performed in a shorter timeframe than both nearshore and offshore integration. Unfortunately, dockside integration is usually not possible for deep draft semisubmersible platforms, which have relatively tall/deep columns. In particular, referring again to FIG. 1, the draft D₂₀ of hull 20 is limited by depth D_w12 of water 11 at dockside site 12, which is typically 30-40 ft. deep. Hull 20 has an overall height H₂₀, and thus, hull 20 and columns 25 extend to a height H₂₅ above the waterline 13 (i.e., surface of the water 11) that is equal to the difference between height H₂₅ and draft D₂₀. Thus, for a given draft D₂₀ (e.g., draft D₂₀ limited by water depth D_w12), increasing height H₂₀ of hull 20 to facilitate deep draft offshore operations, results in an increased height H₂₅. However, a sufficiently large height H₂₅ may exceed the maximum operating limits of the heavy lift crane 40 (i.e., crane 40 may not be able to lift the heavy deck 30 above the columns 25 and/or reach horizontally far enough to mount the deck 30). In FIG. 1, crane 40 is shown operating at its maximum lifting height. In other words, in FIG. 1, crane 40 cannot lift deck 30 to a height greater than height H₂₅. Thus, the maximum lifting height and reach of crane 40 limits the maximum height H₂₅, which in turn limits the maximum total height H₂₀ of hull 20 that can be integrated with deck 30 at dockside location 12, which has a limited water depth D_w12. Moreover, beyond a height H₂₅ of about 120 ft. for a draft D₂₀ of about 30-40 ft., the assembled platform 10 (including the heavy deck 30 mounted atop columns 25) becomes dynamically unstable and prone to capsize at the dockside location 12 as the center of gravity of the platform 10 is spaced sufficiently above the center of buoyancy of platform 10. Consequently, simply increasing the height H₂₀ of hull 20 to enable a deep draft D₂₀ at the operating site 14 (e.g., draft D₂₀ at the operating site 14 greater than 150 ft.) may not be an option if integration at dockside location 12 having a limited water depth D_w12 of about 30-40 ft. However, embodiments described herein offer the potential for deep draft semi-submersible platforms that are suitable for top tensioned risers and dry production trees and can be integrated dockside with dynamic stability via installation of the deck at a lower elevation and subsequent raising of the deck after transport to the operation site.

Referring now to FIG. 3, an embodiment of a deep draft semi-submersible platform 100 in accordance with the principles described herein is shown. In FIG. 3, platform 100 is shown deployed in a deep draft operational configuration at an offshore operating site 14 in water 11. Platform 100 is anchored at the offshore operating site 14 with a mooring system 180 that limits the movements of platform 100 and maintains the position of platform 100 at the operating site 14. In this embodiment, mooring system 180 includes a plurality of catenary mooring lines 181 having upper ends coupled to hull 120 and lower ends anchored to the sea floor. However, in general, any suitable mooring system can be employed to limit the movements of platform 100 and maintain its position at the operating site 14.

In this embodiment, platform 100 includes an adjustably buoyant hull 120, a deck support frame or sub-structure 150 moveably coupled to hull 120, and a deck or topside 160 seated on deck sub-structure 150. The equipment used in oil and gas drilling or production operations, such as a derrick, draw works, pumps, scrubbers, precipitators and the like are disposed on and supported by deck 160. As will be described in more detail below, deck sub-structure 150 can be controllably and adjustably moved vertically up and down relative to hull 120, thereby enabling deck 160, mounted to deck sub-structure 150, to move vertically up and down relative to hull 120.

Referring now to FIGS. 3 and 4, hull 120 includes adjustably buoyant horizontal base 130 and a plurality of adjustably buoyant columns 140 extending vertically upward from base 130. In this embodiment, base 130 of hull 120 includes a plurality of straight, elongated horizontal pontoons 131 connected end-to-end to form a closed loop base 130 with a central opening through which risers may pass up to deck 160. In this embodiment, four pontoons 131 are connected end-to-end to form a generally base 130 having four corners or nodes 133, each node 133 formed at the intersection of two pontoons 131. Each pontoon 131 has the same horizontal length measured between nodes 133, and thus, base 130 has a square perimeter. Although this embodiment of base 130 has a square geometry and includes four equal length pontoons 131, in other embodiments, the number of pontoons (e.g., more or less than four pontoons 131) may differ and/or the geometry of the base (e.g., base 130) may not be square (e.g., triangular, etc.). For instance, a deep draft semi-submersible platform including a hull with a triangular base and three vertical columns may be particularly suited for use in drilling operations. It should be appreciated that additional structural members not shown may be included inside or outside the pontoons (e.g., pontoons 131) to guide risers, stiffen the pontoons connections, increase the added mass effect, etc.

Each pontoon 131 includes ballast tanks that can be selectively ballasted and deballasted, as desired, to adjust the buoyancy of base 130 and hull 120. In particular, each pontoon 131 includes a plurality of horizontally spaced bulkheads, which divide or partition the pontoon 131 into a plurality of horizontally arranged distinct compartments that can be independently and selectively ballasted and deballasted.

Each node 133 of base 130 underlies and supports one column 140, which extends vertically upward therefrom. Thus, in this embodiment, hull 120 includes four columns 140, one column 140 extending vertically from each node 133. Similar to pontoons 131, each column 140 includes ballast tanks that can be selectively ballasted and deballasted to adjust the buoyancy of the column 140 and hull 120. In particular, each column 140 includes a plurality of vertically spaced bulkheads, which divide or partition the column 140 into a plurality of vertically stacked compartments that can be independently and selectively ballasted and deballasted as desired.

As best shown in FIG. 3, each column 140 extends vertically between a first or upper end 140a distal base 130 and a second or lower end 140b secured to one node 133 of base 130, and further, each column 140 has the same height H₁₄₀ measured vertically between ends 140a, 140b. Each pair of adjacent columns 140 is spaced apart a horizontal distance D₁₄₀. In this embodiment, each distance D₁₄₀ is the same. Strakes 162 are provided on the outer surface of each column 140 below sub-structure 150 and deck 160. As will be described in more detail below, strakes 162 are positioned below the lowermost position of sub-structure 150 and deck 160 so as not to interfere with the vertical movement of sub-structure 150 and deck 160 relative to columns 140.

Hull 120 has an overall height H₁₂₀ measured vertically from the bottom of hull 120 to upper ends 140a of columns 140 and a draft D₁₂₀ measured vertically from the waterline 13 to the bottom of hull 140. Thus, the portion of hull 120 (and columns 125) disposed above the waterline 13 has a height H_120′ measured vertically from waterline 13 to upper ends 140a equal to the difference between overall height H₁₂₀ and draft D₁₂₀. As previously described, platform 100 is a deep draft semi-submersible, and thus, at operating site 14, draft D₁₂₀ is greater than 150 ft., and in embodiments described herein, is preferably 150 ft. to 200 ft. Accordingly, height H₁₂₀ of hull 120 is greater than 150 ft. However, as will be described in more detail below, unlike most conventional deep draft semi-submersible platforms, platform 100 can be constructed at dockside location 12 with a draft D₁₂₀ limited to the water depth D_w12 of 30-40 ft. Although the embodiment of platform 100 shown and described herein is a deep draft semi-submersible, in general, embodiments described herein can be sized and configured for any desired draft including, without limitation, shallow draft, deep draft, etc.

Referring now to FIGS. 3-5, deck sub-structure 150 is positioned inside and extends between all four columns 140 (FIG. 3), and is moveably coupled to each column 140 with a jack-up assembly 170 (FIGS. 4 and 5). Deck 160 is positioned within and between columns 140 atop deck sub-structure 150. Thus, deck 160 is moveably coupled to hull 120 via deck sub-structure 150 and jack-up assemblies 170. In this embodiment, deck sub-structure 150 is a structural frame (e.g., truss assembly) that is installed in place. More specifically, deck sub-structure 150 is installed within hull 120 (between columns 140), and moveably coupled to columns 140 at dockside location 12 prior to installation of deck 160 on hull 120.

As best shown in FIGS. 4 and 5, one jack-up assembly 170 couples each column 140 to deck sub-structure 150. In general, each jack-up assembly 170 can comprise any system that simultaneously couples deck sub-structure 150 to each column 140 and allows sub-structure 150 to be controllably moved up and down relative to the corresponding column 140. Examples of suitable systems include those systems known in the art for moveably coupling the legs of a jack-up rig to the deck. In this embodiment, each jack-up assembly 170 includes an elongate toothed rack 171 and a powered pinion 172 that mates and engages the toothed rack 171. The toothed rack 171 is vertically oriented (i.e., oriented parallel to columns 140) and fixably attached to the outer surface of the corresponding column 140 generally facing deck sub-structure 150. The pinion 172 is rotatably coupled to the corner of deck sub-structure 150 immediately opposite the corresponding toothed rack 171. Pinion 172 includes teeth that mate with and engage the teeth of the corresponding rack 171, and further, pinion 172 is free to rotate relative to deck sub-structure 150 and column 140. Thus, rotation of each pinion 172 in one direction causes deck sub-structure 150 to move vertically in a first direction (e.g., upward) and rotation of each pinion 172 in the opposite direction causes deck sub-structure 150 to move vertically in a second direction (e.g., downward) opposite the first direction. Rotation of gears 171 is controlled and powered by a drive mechanism such as a motor (e.g., an electric motor, a hydraulic motor, etc.).

Referring to FIGS. 5 and 6, each rack 171 has an upper end disposed at or proximal upper end 140a of the corresponding column 140 and a lower end disposed a height H₁₇₁ measured vertically from the bottom of hull 120. Thus, jack-up assemblies 170 can move deck sub-structure 150 and deck 160 relative to hull 120 between an uppermost position 160a at upper ends 140a of columns 140 and a lowermost position 160b (shown in phantom) located at height H₁₇₁ from the bottom of hull 120. The uppermost position 160a of deck 160 is spaced from the lowermost position 160b of deck 160 a vertical distance D₁₆₀, which represents the maximum vertical range of movement of deck 160.

Height H₁₇₁ can be varied as necessary, but is preferably set to enable mounting of deck 160 on sub-structure 150 at dockside location 12 with heavy lifting equipment (e.g., crane 40) while ensuring stability of platform 100 at dockside location 12 with a draft D₁₂₀ of about 30-40 ft. (i.e., a draft D₁₂₀ less than the depth D_w12 of water 11). Assuming stability of platform 100, height H₁₇₁ depends on the maximum lift height H₄₀ of crane 40 relative to bottom of hull 120 and the vertical height H₁₆₀ of deck 160 itself as follows:

heightH₁₇₁=heightH₄₀−heightH₁₆₀.

In embodiments described herein, the vertical distance D₁₆₀ is preferably less than or equal to the anticipated maximum change in draft D₁₂₀ of hull 120. In embodiments described herein, the minimum anticipated draft D₁₂₀ of hull 120 is the draft D₁₂₀ at the dockside location 12, which is set to provide nominal clearance from the seafloor at the dockside location 12 as shown in FIG. 6 (typically, 30-40 ft. for most dockside locations 12) and the maximum anticipated draft D₁₂₀ of hull 120 is the draft D₁₂₀ at the operating site 14, which is set to accommodate the environmental conditions at the operating site 14 as shown in FIGS. 3 and 7J (preferably greater than 150 ft. for deep draft applications). For most offshore operations, the maximum change in draft D₁₂₀ of hull 120 is less than 120 ft., more likely between 25 ft. and 75 ft., and nominally about 50 ft. Thus, vertical distance D₁₆₀ is preferably less than 120 ft., more preferably between 25 ft. and 75 ft., and even more preferably about 50 ft. With a vertical range of motion D₁₆₀ of deck 160 between 25 ft. and 75 ft. from upper ends 140a of columns 140 for typical offshore applications and an overall height H₁₂₀ of hull 120 greater than 150 ft., lowermost position 160b of deck 160 is above the vertical mid-point of hull 120. Thus, at least the lower half of hull 120 does not need to accommodate deck 160 or the movement of deck 160. Consequently, as best shown in FIG. 6, stakes 162 are positioned along the outer surfaces of columns 140 below lowermost position 160b of deck 160 so as not to interfere with deck 160, sub-structure 150, or jack-up assemblies 170.

Although only one jack-up assembly 170 is shown between sub-structure 150 and each column 140, in general, one or more jack-up assemblies (e.g., jack-up assemblies 170) can be provided between the deck sub-structure (e.g., sub-structure 150) and each column (e.g., each column 150). In addition, it should be appreciated that other jacking systems other than assembly 170 previously described can be used to couple the deck (e.g., deck 160) and sub-structure (e.g., sub-structure 150) to the columns (e.g., columns 140) and raise and lower the deck. Alternative systems for moveably coupling the deck (e.g., deck 160) and the sub-structure (e.g., sub-structure 150) to the columns (e.g., columns 140) are shown in FIGS. 8A-8C and 9.

Referring briefly to FIGS. 8A-8C, a hydraulic jacking system 170′ for moveably coupling sub-structure 150 to one column 140 is shown. In general, system 170′ can be used in place of system 170 previously described. More specifically, one system 170′ is provided between each column 140 and sub-structure 150, which supports deck 160 (not shown). Each jacking system 170′ includes a plurality of uniformly axially spaced support members 171′ extending from the corresponding column 140 and an extendable hydraulic ram 172′ coupled to sub-structure 150 opposite the vertical column of support members 171′. Each ram 172′ has an upper end 172a′ and a lower end 172b′ that can be hydraulically moved vertically away from each other and toward each other. Each end 172a′, 172b′ comprises a foot 173′ sized to engage a mating support member 171′. Thus, sub-structure 150 (and deck 160 disposed thereon) can be vertically raised along columns 140 in a step-wise manner by releasably coupling foot 173′ disposed at lower end 172b′ to one member 171′ (FIG. 8A), extending hydraulic ram 172′ and releasably coupling foot 173′ disposed at upper end 172a′ to one member 171′ (FIG. 8B), and then disengaging foot 173′ disposed at lower end 172b′ from members 171′ and contracting hydraulic ram 172′ to lift foot 173′. By repeating the process shown in FIGS. 8A-8C, sub-structure 150 (and deck 160 thereon) can be controllably raised upward along columns 140; and by reversing the process shown in FIGS. 8A-8C, sub-structure 150 (and deck 160 thereon) can be controllably lowered along columns 140.

Referring briefly to FIG. 9, a strand jack system 170″ for moveably coupling substructure 150 to one column 140 is shown. In general, system 170″ can be used in place of system 170 previously described. More specifically, one system 170″ is provided between each column 140 and sub-structure 150, which supports deck 160 (not shown). Strand jack system 170″ includes a winch 171″ mounted to the upper end 140a of the corresponding column 140 and a cable 172″ extending from winch 171″ to sub-structure 150. The winch 171″ on columns 140 are operated in unison to raise and lower sub-structure 150 (and deck 160 thereon).

In embodiments described herein, a locking mechanism is preferably provided between each column 140 and sub-structure 150 to releasably lock sub-structure 150 and deck 160 relative to the corresponding column 140 at the desired vertical position. In general, any suitable type of locking mechanism known in the art can be used.

Referring again to FIGS. 3 and 4, deck 160 is seated directly on sub-structure 150, which supports deck 160 and moves deck 160 vertically up and down along columns 140. In embodiments described herein, deck 160 is not buoyant and is not adjustably buoyant. In addition, as best shown in FIG. 4, in this embodiment, deck 160 is sized such that it can be passed horizontally between each pair of adjacent columns 140 during integration. Specifically, deck 160 has a horizontal width W₁₆₀ that is less than the minimum horizontal distance D₁₄₀ between each pair of adjacent columns 140. In this embodiment, deck 160 is square, and thus, each side of deck 160 has the same horizontal width W₁₆₀ that is less than the minimum horizontal distance D₁₄₀ between each pair of adjacent columns 140.

Horizontal forces (e.g., caused by motions and various other sources) may be applied to deck 160 and sub-structure 150. Such forces are transferred to columns 140 and may cause columns 140 to deflect outward (spread apart). Accordingly, sub-structure 150 and/or deck 160 preferably includes rigid column support structures extending around each column 140 to restrict and/or prevent columns 140 from deflecting outward. For example, FIG. 10A illustrates an embodiment of a column support structure 190 disposed about each column 140. In this embodiment, column support structures 190 extend completely around columns 140 and are rigidly secured to deck 160 or sub-structure 150. As another example, in FIG. 10B, column support structures 190 are also disposed about each column 140 and are rigidly secured to deck 160 or sub-structure 150. However, in the embodiment shown in FIG. 10B, a deck extension 161 (schematically illustrated with cross-hatching) extends from each side of deck 160 between a pair of adjacent columns 140. Deck extensions 161 are fixably coupled to the support structures 190 disposed about the corresponding columns 140. In this embodiment, deck extensions 161 are attached to deck 160 and support structures 190 after deck 160 is positioned atop sub-structure 150. This enables crane 40 to lift deck 160 and pass deck 160 between columns 140. Crane 40 can be used to lift and position extensions 161, so that they can be attached to deck 160. It should be appreciated that deck extensions 161 effectively increase the operating area of deck 160 for equipment and personnel.

Support structures 190 are rigid and fixably secured to deck 160, and thus, restrict and/or prevent upper ends 140a of columns from deflecting outward (or inward) relative to each other. In general, support structures 190 can be installed with sub-structure 150 or installed after mounting deck 160 to sub-structure 150 as described in more detail below. In addition, inclusion of support structures 190 increases the interfacing area with each column 140, and thus, enables the use of a plurality of jack-up assemblies 170 for each column 140. Namely, as shown in FIGS. 10A and 10B, four uniformly circumferentially-spaced jack-up assemblies 170 are provided between each column 140 and the corresponding support structure 190. Column support structures 190 provide structural support, as well as foundations for mooring or riser systems.

Referring now to FIGS. 7A-7J, an embodiment of a method for assembling platform 100 at dockside location 12 and subsequently deploying platform 100 is schematically illustrated. In particular, FIGS. 7A-7D illustrate the construction of deck substructure 150 and integration of deck 160 to form platform 100 at dockside location 12; FIGS. 7E-7H illustrate the deployment of platform 100 from dockside location 12 to operating site 14; and FIGS. 71 and 7J illustrate the installation of platform 100 at operating site in a deep draft configuration. In general, hull 120 can be constructed in any suitable means known in the art. For example, hull 120 can be constructed at dockside location 12, constructed at another location and transported to dockside location 12, or constructed in parts at one or more locations, which are transported to dockside location 12 for assembly.

Referring first to FIGS. 7A-7D, jack-up assemblies 170 (previously described and shown in FIG. 4), sub-structure 150, and deck 160 are mated to hull 120. In this embodiment, sub-structure 150 is constructed in place and mounted to pinions 172, which engage toothed racks 171 secured to columns 140 (as previously described and shown in FIGS. 4 and 5). To ease installation, sub-structure 150 is installed in place at vertical height H₁₇₁ as shown in FIG. 7C, which correlates to the lowermost position 160b. Next, as shown in FIGS. 7D-7F, deck 160 is lifted by crane 40, passed between two columns 140, positioned directly over sub-structure 150, and then lowered onto sub-structure 150, thereby completing assembly of platform 100 at dockside location 12. It should be appreciated that crane 40 only needs to lift deck 160 to lowermost position 160b, which is less than the maximum lifting height of crane 40, less than the height H_120′ of hull 120 above waterline 13, and less than the height above the waterline 13 at which deck 160 can be positioned while ensuring stability of platform 100 at dockside location 12 with a maximum draft D₁₂₀ that is less than the depth D_w12 of water 11 at dockside location 12 (typically about 30-40 ft.). In some embodiments, with a draft D₁₂₀ of about 30-40 ft., the height above the waterline 13 at which deck 160 can be positioned while ensuring stability is about 120 ft. Due to the weight of deck 160, the draft D₁₂₀ of hull 120 may increase, however, hull 120 can be controllably deballasted to ensure draft D₁₂₀ is less than the depth D_w12 of water 11 at dockside location 12.

Referring now to FIGS. 7E and 7F, the assembled platform 100 is floated from dockside location 12 to a nearshore location 15. Deck 160 is maintained at the lowermost position 160b, to maintain a relatively low center of gravity for platform 100, thereby ensuring dynamic stability of platform 100 as it is floated to nearshore location 15. Water 11 has a depth D_w15 at nearshore location 15 that is substantially greater than the water depth D_w12 at dockside location 12. Therefore, as desired (e.g., to further enhance the dynamic stability of platform 100 prior to being floated from nearshore location 15 to operating site 14), hull 120 is ballasted at nearshore location 15 to increase the draft D₁₂₀ as shown in FIG. 7F. Deck 160 can be raised as hull 120 is ballasted to ensure the desired height of deck 160 above the waterline 13. However, the draft D₁₂₀ of hull 120 is preferably not increased to the full deep draft D₁₂₀ of hull 120 at nearshore location 15 in order to minimize drag and loads experienced by platform 100 as it is floated from nearshore location 15 to operating site 14. Deck 160 can be raised with jack-up assemblies 170 and sub-structure 150 as necessary to ensure deck 160 is at the desired height above the waterline 13 as draft D₁₂₀ is increased. As shown in FIGS. 7G and 7H, with draft D₁₂₀ increased at nearshore location 15 to enhance stability, platform 100 is floated from nearshore location 15 to operating site 14.

Referring now to FIG. 71, once platform 100 is floated to the operating site 14, hull 120 is ballasted to increase the draft D₁₂₀ of hull 120 to the desired deep draft D₁₂₀. The water 11 at offshore location 14 has a depth D_w14 that is substantially greater than the water depth D_w15 at nearshore location 15 and provides no limitation on the draft D₁₂₀ of hull 120. In embodiments where platform 100 is configured for a deep draft, hull 120 is ballasted to a draft D₁₂₀ of greater than 150 ft. at the operating site. In addition, deck 160 is raised with jack-up assemblies 170 and sub-structure 150 to the desired height above waterline 13 as hull 120 is ballasted. Next, mooring system 180 is installed as shown in FIG. 7J, thereby completing the deployment and installation of platform 100.

In the manner described, embodiments of semi-submersible platforms described herein (e.g., platform 100), including deep draft semi-submersible platforms, can be fully assembled and constructed at a dockside location (e.g., dockside location 12) with a limited water depth (e.g., depth D_w12). Such embodiments allow for construction at dockside locations having water depths that limit installation heights of decks (e.g., 30-40 ft. water depths) with dynamic stability while enabling a deep draft installation at the operating site (e.g., draft D₁₂₀ greater than 150 ft. at the operating site 14). The deep draft capability of embodiments described herein offers the potential for reduced vertical and rotational motions of the deployed platform (e.g., less than 10-15 ft. heave) which make it suitable for use with top tensioned risers and dry production trees. It should be appreciated that the use of top tensioned risers in drilling and production systems allows direct access into subsea reservoirs from the platform deck and offer significant economic advantage to oil and gas production. Also, due the low motion characteristics of the deep draft semi-submersible platforms (e.g., platform 100), motion-induced damage to hanging appendages such as moorings and risers is significantly reduced, thereby expanding the range of applications of the semi-submersible platforms in general and allowing lower cost riser systems, not normally deployed on a conventional semisubmersible, to be extensively used on the deep draft semisubmersible.

While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.

Claims

1. A semi-submersible platform for offshore operations, the platform comprising: a buoyant hull including an adjustably buoyant base and a plurality of adjustably buoyant columns extending vertically from the base;a deck moveably coupled to the columns of the hull, wherein the deck is configured to move vertically up and down relative to the hull.

2. The platform of claim 1, further comprising a plurality of jack-up assemblies moveably coupling the deck to the columns, wherein each jack-up assembly moveably couples the deck to one of the columns

3. The platform of claim 2, further comprising a deck sub-structure moveably coupled to the columns, wherein the deck is seated on the deck-substructure and each jack-up assembly moveably couples the deck sub-structure to one of the columns.

4. The platform of claim 3, wherein each jack-up assembly comprises: a toothed rack fixably secured to the corresponding column; anda powered pinion gear coupled to the deck sub-structure, wherein the powered pinion gear mates and engages the corresponding toothed rack.

5. The platform of claim 4, wherein each toothed rack is oriented parallel to the corresponding column and has an upper end proximal an upper end of the corresponding column and a lower end disposed between the upper end and a lower end of the corresponding column.

6. The platform of claim 3, wherein each jack-up assembly comprises a hydraulic jacking system or a strand jack system.

7. The platform of claim 1, wherein the hull has a vertical height Hh measured from a lower end of the hull to an upper end of the hull, wherein the vertical height Hh is greater than 150 ft.

8. The platform of claim 7, wherein the deck is configured to move relative to the hull between an uppermost position and a lowermost position; wherein the deck is disposed at a vertical height Hu measured from the lower end of the hull at the uppermost position;wherein the deck is disposed at a vertical Hl measured from the lower end of the hull at the lowermost position;wherein the vertical height Hu is greater than 150 ft. and the vertical height Hl is less than 150 ft.

9. The platform of claim 8, wherein the vertical height Hl is less than 120 ft.

10. The platform of claim 8, wherein the uppermost position of the deck is vertically spaced from the lowermost position of deck by a vertical distance Dd greater than 25 ft.

11. The platform of claim 1, wherein the deck is not buoyant and is not adjustably buoyant.

12. The platform of claim 1, wherein the deck is configured to move relative to the hull between an uppermost position and a lowermost position; wherein a portion of each column extending below the lowermost position of the deck has an outer surface comprising strakes.

13. A method comprising: (a) floating a buoyant hull in water at a dockside location, wherein the buoyant hull has a draft less than 40 ft. at the dockside location;(b) moveably coupling a deck to the hull during (a) to form the semi-submersible offshore platform, wherein the deck is configured to move vertically relative to the hull.

14. The method of claim 13, further comprising: (c) floating the semi-submersible platform from the dockside location to an operating site in the water after (b);(d) adjusting the draft of the semi-submersible hull to maintain stability during (c) or during installation at the operating site; and(e) raising the deck vertically relative to the hull at the operating site.

15. The method of claim 14, wherein (e) is performed simultaneous with (d).

16. The method of claim 14, wherein (c) comprises: (c1) floating the semi-submersible platform from the dockside location to a second location;(c2) increasing the draft of the hull at the second location; and(c3) floating the semi-submersible platform from the second location to the operating site after (c2).

17. The method of claim 14, wherein (d) comprises increasing the draft of the hull to a predetermined depth of at least 150 ft.

18. The method of claim 17, wherein (e) comprises raising the deck to a predetermined height above the waterline.

19. The method of claim 18, wherein (e) comprises raising the deck between 25 ft. and 75 ft. relative to the hull.

20. The method of 13, wherein the hull includes an adjustably buoyant base and a plurality of columns extending vertically from the base; wherein (b) comprises: (b1) moving the deck horizontally between a pair of adjacent columns;(b2) raising the deck with a lifting crane at the dockside location; and(b3) coupling the deck to the hull at a height less than a maximum lifting height of the lifting crane.

21. The method of claim 20, wherein the maximum lifting height of the lifting crane is less than 200 ft.

22. The method of claim 13, wherein the deck is non-buoyant.

23. A method comprising: (a) floating the semi-submersible platform to the operating site, wherein the platform comprises a buoyant hull and a non-buoyant deck moveably coupled to the hull, wherein the hull is disposed at a draft D during (a);(b) ballasting the hull at the operating site after (a) to increase the draft D of the hull at the operating site; and(c) raising the deck vertically relative to the hull at the operating site.

24. The method of claim 23, wherein (c) is performed simultaneous with (b).

25. The method of claim 23, wherein (b) comprises increasing the draft of the hull to a predetermined depth; and wherein (c) comprises raising the deck to a predetermined height to set an airgap between the deck and the waterline.

26. The method of claim 23, wherein (c) comprises jacking the deck upward relative to the hull with a plurality of jack-up assemblies that moveably couple the deck to a plurality of vertical columns of the hull.