Not applicable.
1. Field of the Invention
The disclosure relates generally to floating offshore structures. More particularly, the disclosure relates to buoyant semi-submersible offshore platforms for offshore drilling and production. Still more particular, the disclosure relates to the geometry of the hull and pontoons of semi-submersible offshore platforms.
2. Background of the Technology
Most conventional semi-submersible offshore platforms comprises a hull that has sufficient buoyancy to support a work platform above the water surface, as well as rigid and/or flexible piping or risers extending from the work platform to the seafloor, where one or more drilling or well sites are located. The hull typically comprises a plurality of horizontal pontoons that support a plurality of vertically upstanding columns, which in turn support the work platform above the surface of the water. In general, the size of the pontoons and the number of columns are governed by the size and weight of the work platform and associated payload to be supported. The draft of an offshore structure generally refers to the vertical distance between the waterline and the bottom of the structure.
Conventional shallow draft semi-submersible offshore platforms are used primarily in offshore locations where water depth exceeds about 300 feet (91 meters). A typical shallow draft semi-submersible platform has a draft between 60 ft and 100 ft. (18.3 m and 30.5 m), and incorporates a conventional catenary chain-link spread-mooring arrangement to maintain its position over the well site. The motions of these types of semi-submersible platforms are usually relatively large, and accordingly, they require the use of “catenary” risers (either flexible or rigid) extending from the seafloor to the work platform, and the heavy wellhead equipment is typically installed on the sea-floor, rather than on the work platform. The risers have a catenary shape to absorb the large heave (vertical motions) and horizontal motions of the structure. Due to their large motions, conventional semi-submersible platforms usually do not support high-pressure, top-tensioned risers.
Increasing the draft of a semi-submersible offshore platform can improve its stability and reduce its range of movement. Doing so involves lengthening the columns and locating the pontoons at a greater depth below the surface of the water, where wave excitation forces are generally lower. As a result, a deep draft semi-submersible offshore platform (i.e., having a draft of at least about 150 feet (about 45 m)) usually has significantly smaller vertical and rotational motions than a conventional shallow draft semi-submersible platform, thereby enabling the deep draft platform to support top-tensioned drilling and production risers without the need for disconnecting the risers during severe storms. In addition, the surface area of the upper and lower surfaces of the pontoons can be increased, resulting in the vessel having a greater added mass, and hence, increased resistance to movement through the water and heave natural period. With increased heave natural period, the peak wave energy can be avoided.
In both conventional and deep draft types of semi-submersible offshore platforms, the hull is divided into several closed compartments, each compartment having a buoyancy that can be adjusted for purposes of flotation and trim. Typically, a pumping system pumps ballast water into and out of the compartments to adjust their buoyancy. The compartments are typically defined by horizontal and/or vertical bulkheads in the pontoons and columns. Normally, the compartments of the pontoon and the lower compartments of the columns are filled with water ballast when the platform is in its operational configuration, and the upper compartments of the columns provide buoyancy for the platform.
The location of final assembly of a semi-submersible offshore platform may involve integration of the hull (i.e., the pontoons and columns) and work platform (topside) at the shipyard (quayside), offshore at its operation site, or nearshore (integration site). For integration at the shipyard, the work platform is lifted and mounted to the hull with heavy lifting equipment (e.g., heavy lift crane), and then the fully assembled semi-submersible platform is transported to the operation site using a heavy lift or tow vessel. This approach may not be possible for deep draft semisubmersible platforms that have relatively long columns. For integration at the operation site, the hull is transported offshore to its operation site, either by towing it at a shallow draft, or by floating it aboard a heavy lift vessel. When the hull is at the operation site, it is ballasted down by pumping sea water into the pontoons and columns, and the work platform is then either lifted onto the tops of the columns by heavy lift cranes carried aboard a heavy lift barge, or by floating the work platform over the top of the partially submerged hull using a deck barge. In either case, the procedure is typically effected far offshore (e.g., 100 miles, or 161 km), is performed in open seas, and is strongly dependant on weather conditions and the availability of a heavy lift barge, making it both risky and expensive. For nearshore integration, the work platform is lifted and mounted to the hull with heavy lift cranes or heavy lift barge in the water close to the shore, and then the assembled platform is transported to the operation site. As compared to assembly at the operation site, nearshore assembly is generally less expensive and less risky. However, as the water is generally shallower proximal the shore, nearshore integration may not be possible for some deep draft semi-submersible structures due to the length of the columns—due to water depth, the hull may not be capable of being ballasted down far enough to allow mounting of the work platform to the hull with a heavy lifting crane or heavy lift barge.
During drilling or production operations, it is generally desirable to minimize the motion of the offshore platform to maintain the position of the platform over the well site and to reduce the likelihood of damage to the risers. One component of offshore platform motion is heave, which is the vertical linear displacement of the offshore platform in response to wave motion. For use in conjunction with top tensioned risers or dry tree solutions, the floating structure preferably has heave characteristics such that the strokes (relative motion between the hull and the buoyancy can or risers) and the tension of the risers are within acceptable limits. Further, for use in conjunction with steel centenary risers or wet tree solutions, the floating structure preferably has heave characteristics such that the riser fatigue and strength requirements are within acceptable limits.
For most semi-submersible floating structures, heave is governed by the draft of the structure and the geometry of the hull. As previously described, in general, the deeper the draft of the structure, the less heave. However, increasing the draft of the hull may inhibit the ability to employ quayside topside integration. Further, increasing the draft of the hull usually results in increased hull weight, as well as increased materials and manufacturing costs.
Accordingly, there remains a need in the art for a semi-submersible offshore platforms with acceptable heave characteristics in lower draft applications, and which can be manufactured more cost effectively.
These and other needs in the art are addressed in one embodiment by a semi-submersible offshore structure. In an embodiment, the structure comprises an equipment deck disposed above the surface of the water. In addition, the structure comprises a buoyant hull coupled to the equipment deck and extending below the surface of the water. The hull comprises a first vertical column and a second vertical column, each column having an upper end proximal the deck and a lower end disposed subsea. In addition, the hull comprises a first elongate horizontal pontoon having a longitudinal axis, a first end, and a second end opposite the first end. The pontoon includes a first node disposed at the first end of the pontoon and positioned below the lower end of the first column, a second node disposed at the second end of the pontoon and positioned below the lower end of the second column, and an intermediate section extending axially from the first node to the second node. Further, the first node has a width Wi measured perpendicular to the longitudinal axis in bottom view, the second node has a width W2 measured perpendicular to the longitudinal axis in bottom view, and the intermediate section has a width W3 measured perpendicular to the longitudinal axis in bottom view. Moreover, the width W3 varies moving axially from the first node to the second node.
These and other needs in the art are addressed in another embodiment by a semi-submersible offshore structure. In an embodiment, the structure comprises a work platform disposed above the surface of the water. In addition, the structure comprises a first vertical column and a second vertical column, each column extending from an upper end at the work platform to a lower end disposed subsea. Further, the structure comprises an elongate horizontal pontoon coupled to the lower end of the first column and the lower end of the second column. The pontoon has a longitudinal axis, a first end, and a second end opposite the first end. The pontoon includes a first node positioned below the lower end of the first column, a second node positioned below the lower end of the second column, and an intermediate section extending axially from the first node to the second node. Still further, the first node has a lower surface area A1, the second node has a lower surface area A2, and the intermediate section has a lower surface area A3. Moreover, the ratio of area A3 to the sum of the area A1 and the area A2 is between 0.45 and 0.60.
Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior structures, systems, and methods. 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.
For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:
The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate 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 and connections. Further, the terms “axial” and “axially” generally mean along or parallel to a central or longitudinal axis (e.g., the drillstring axis), while the terms “radial” and “radially” generally mean perpendicular to the central or longitudinal axis. For instance, an axial distance refers to a distance measured along or parallel to the central or longitudinal axis, and a radial distance refers to a distance measured perpendicularly from the central or longitudinal axis.
Referring now to
Referring now to
Referring now to
As previously described, the four straight, elongated pontoons 21 are connected end-to-end to form a closed loop hull 15. In particular, each end 21a, b of each pontoon 21 intersects with one end 21a, b of another pontoon 21 to form corners 28. For example, as best shown in
Referring still to
Moving axially from first end 21a to second end 21b, first node 24 extends axially from first end 21a to a bulkhead 31 generally coincident with a vertical plane P24 perpendicular to axis 22 at the start of opening 24; intermediate section 25 extends axially from first node 24, bulkhead 31, and plane P24 to second node 26 and a bulkhead 32 generally coincident with a vertical plane P26 perpendicular to axis 22 at the end of opening 24. Thus, intermediate section 25 is the portion of each pontoon 21 that extends along opening 23, whereas nodes 24, 26 are the portions of each pontoon 21 that underlie columns 50 and intersect an adjacent pontoon 21. Due to the intersection of two pontoons 21 at each corner 28 and each node 24, 26, it should be appreciated that first node 24 of one pontoon 21 is coincident (and overlaps) with second node 26 of a different pontoon 21 in bottom view. Intermediate section 25 is the only portion of each pontoon 21 that does not intersect or overlap with another pontoon 21 in bottom view (
Referring still to
Referring again to
As best shown in
Referring now to
Referring now to
Referring now to
As previously described, the four straight, elongated pontoons 121 are connected end-to-end to form a closed loop hull 115. In particular, each end 121a, b of each pontoon 121 intersects with one end 121a, b of another pontoon 121 to form corners 128. For example, as best shown in
In this embodiment, pontoons 121 each have a rectangular cross-section taken perpendicular to its longitudinal axis 122. However, in general, the pontoons (e.g., pontoons 121) of offshore structures in accordance with the principles described herein may any suitable cross-section including, without limitation, circular, oval, triangular, etc.
Referring still to
In bottom view, the lower surface of each node 124 has a surface area A124, the lower surface of each node 128 has a surface area A128, the lower surface of each intermediate section 126 has a surface area A126. It should be appreciated that each node 124 is coincident with one node 128, and thus, the lower surface area A124 of each node 124 is the same as the lower surface area A128 of each node 128. Further, in this embodiment, lower surface area A124, A128 of each node 124, 128 is the same, and lower surface area A126 of each intermediate section 126 is the same.
Referring still to
As best shown in
Referring again to
Each column 150 has a width W150 measured perpendicular to axis 155 in side view (
As previously described, the heave characteristics of an offshore floating structure (e.g., platform 10, platform 100) are influenced by the draft of the structure and the geometry of the structure. Regarding geometry, a critical factor affecting heave is the shape of the lower pontoons (e.g., pontoons 21), and in particular, the shape of the lower surface of the pontoons, which are subject to the vertical forces imposed by waves. The shape of the lower surface of a pontoon may be characterized by a “pontoon lower surface area ratio” defined as the ratio of the lower surface area of the pontoon excluding the nodes to the total lower surface area of the nodes of the pontoon as follows:
where:
In the embodiment of platform 100 previously described, the sum of the lower surface areas of nodes 124, 128 of one pontoon is lower surface area A124 plus lower surface area A128, and the total lower surface area of the remainder of each pontoon 121 is lower surface area A126. Thus, the pontoon lower surface area ratio for platform 100 previously described is:
For pontoon 21, as well as most conventional pontoons for semi-submersible offshore structures, the pontoon lower surface area ratio is typically between 0.75 to 1.0. However, for embodiments of “dog bone” shaped pontoons in accordance with the principles described herein (e.g., pontoons 121), the pontoon lower surface area ratio is preferably between 0.45 and 0.6. In particular, each pontoon 121 previously described has a pontoon lower surface area ratio of about 0.54.
The shape of the lower surface of each pontoon may also be characterized by a “minimum pontoon-to-column width ratio” defined as the ratio of the minimum width of the pontoon in bottom view measured perpendicular to the pontoons central or longitudinal axis to the width of a column supported by the pontoon at the intersection of the column and the pontoon (i.e., width of column footprint) in bottom view measured perpendicular to the pontoons central or longitudinal axis as follows:
In the conventional pontoon design employed in offshore platform 10 previously described, width W50 of each column 50 is uniform along its entire length, and thus, the width of each column 50 at its intersection with pontoon 21 as measured perpendicular to axis 22 of pontoon 21 is width W50. Further, width W21 of each pontoon 21 is constant or uniform along its entire length, and thus, the minimum width of each pontoon 21 is width W21. Thus, the pontoon-to-column width ratio for conventional pontoon 21 previously described is:
In the embodiment of platform 100 previously described, width W150 of each column 150 is uniform along its entire length, and thus, the width of each column 150 at its intersection with pontoon 121 as measured perpendicular to axis 122 of pontoon 121 is width W150. Further, width W121 of each pontoon 121 is at a minimum along middle portion 126b, and thus, the minimum width of each pontoon 121 is width W126b. Thus, the pontoon-to-column width ratio for “dog bone” shaped pontoon 121 previously described is:
For pontoon 21, as well as most conventional pontoons for semi-submersible offshore structures, the pontoon-to-column width ratio is typically between 1.15 and 1.25. However, for embodiments of pontoon 121 of platform 100, the pontoon-to-column width ratio is preferably less than 1.0, and more preferably between 0.65 and 0.75. In particular, each pontoon 121 previously described has a pontoon-to-column width ratio of about 0.7.
As compared to pontoons employed in conventional semi-submersible offshore structures (e.g., pontoons 21 employed in platform 10), embodiments described herein including “dog bone” shaped pontoons (e.g., platform 100 including pontoons 121) offer the potential for a hull with reduced weight and reduced material requirements. Further, without being limited by this or any particular theory, by reducing the vertical area or surface area of the lower surface of the hull, it is believed that embodiments described herein offer the potential for reduced heave as compared to conventional offshore platforms, particularly in shallower draft applications (e.g., ˜120 foot draft applications). By reducing draft without a substantial increase in heave as compared to a conventional designs, embodiments described herein also offer the potential increase the ease of quayside topside integration.
Without being limited by this or any particular theory, the preferred ranges for the pontoon lower surface area ratio and the pontoon-to-column width ratio offer the potential for a pontoon that experiences reduced heave, while providing sufficient strength and rigidity. For example, if the pontoon lower surface area ratio gets sufficiently small, implying the lower surface area of the pontoon outside the nodes is relatively small, the pontoon may not have sufficient strength and rigidity when subjected to subsea loads and torques. Likewise, if the pontoon-to-column width ratio gets sufficiently small, implying the minimum width of the pontoon is relatively small, the pontoon may not have sufficient strength and rigidity when subjected to subsea loads and torques.
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 invention. 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.
To further illustrate various illustrative embodiments of the present invention, the following example is provided.
To investigate the impact of the “dog bone” pontoon on heave motion, the motion response of a semi-submersible offshore structure having the shape and geometry of the embodiment of platform 100 previously described and shown in
S
R(ω)=[RAO(ω)]2*S(ω)
where:
SR(ω) is the heave response spectrum, S(ω) is the wave spectrum, and w is the wave frequency
This application claims benefit of U.S. provisional application Ser. No. 61/104,545 filed Oct. 10, 2008, and entitled “Dog Bone Multi Column Floater,” which is hereby incorporated herein by reference in its entirety.
Number | Date | Country | |
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61104545 | Oct 2008 | US |