Not applicable.
1. Field of the Invention
The invention relates generally to floating offshore structures. More particularly, the invention relates to unconditionally stable buoyant semi-submersible platforms and tension leg platforms for offshore drilling and production.
2. Background of the Technology
Conventional semi-submersible offshore platforms and tension leg platforms include 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. Whether a semi-submersible or tension leg platform, the hull typically includes 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. For example, in
Wind and wave excitation forces at and below the sea surface continuously seek to move offshore structures. Translational movement of semi-submersible platforms at the sea surface is typically limited by mooring lines extending from the platform to the sea floor, and translational movement of tension leg platforms at the sea surface is typically limited by tendons that extend from the platform to the sea floor and are placed in tension. Mooring lines allow for some vertical movement of the semi-submersible structures (e.g., heave) relative to the sea floor, while tendons restrict and/or prevent vertical movement of tension leg platforms relative to the sea floor. Wind and wave excitation forces may also cause offshore structures (e.g., semi-submersible or tension leg platforms) to tilt or list to one side. For example, in
For semi-submersible platforms, the geometry and arrangement of the columns operate to resist excessive heeling and restore the platform back to its upright, neutral position. However, with extreme wind and/or wave excitation forces, heeling angles can be quite large. At a sufficiently large heeling angle sea water is allowed to flow directly from the sea into one or more openings in the top of the columns of the platform. The smallest heeling angle at which the opening in the upper end of one or more columns is positioned at the sea surface is often referred to as the “downflooding” angle, and is designated as angle β in
Accordingly, there remains a need in the art for offshore platforms that are unconditionally stable and able to resist capsizing. Such offshore platforms would be particularly well-received if they were unconditionally stable, regardless of the geometry and arrangement of the columns and integrity of tendons.
These and other needs in the art are addressed in one embodiment by a platform for offshore drilling and/or production operations. In an embodiment, the platform comprises an equipment deck configured to be disposed above the surface of the water. In addition, the platform comprises a buoyant hull coupled to the equipment deck and configured to extend below the surface of the water. The hull comprises a first column having a central axis, an upper end coupled to the deck, a lower end distal the deck, and a plurality of axially stacked cells between the upper end and the lower end, each cell defining an inner chamber within the cell and an exterior region outside the cell. The plurality of cells includes a first cell extending from the upper end of the first sub-column and a second cell axially positioned below the first cell. The first cell is water-tight. The second cell includes a gas port configured to supply a buoyancy control gas to the inner chamber of the second cell.
These and other needs in the art are addressed in another embodiment by a platform for offshore drilling and/or production operations. In an embodiment, the platform comprises an equipment deck configured to be disposed above the surface of the water. In addition, the platform comprises a buoyant hull coupled to the equipment deck and configured to extend below the surface of the water. The hull comprises a first column and a second column, each column having an upper end coupled to the deck and a lower end distal the deck. The hull also comprises a first elongate pontoon extending between the first column and the second column. The first column comprises a plurality of elongate parallel sub-columns including a first sub-column having a central axis, an upper end at the upper end of the first column, a lower end at the lower end of the first column, and a plurality of vertically stacked cells between the upper end and the lower end, each cell defining an inner chamber within the cell and an exterior region outside the cell. The plurality of cells includes a first cell extending axially from the upper end of the first sub-column and a second cell axially positioned between the first cell and the lower end of the first sub-column. The second cell includes a gas port configured to supply a buoyancy control gas to the inner chamber of the second cell. Moreover, the hull comprises a port configured to allow the water to freely pass into and out of the inner chamber of the second cell.
These and other needs in the art are addressed in another embodiment by a platform for offshore drilling and/or production operations. In an embodiment, the platform comprise an equipment deck configured to be disposed above the surface of the water. In addition, the platform comprises a buoyant hull coupled to the equipment deck and configured to extend below the surface of the water. The buoyant hull is configured to generate a decreasing value of a righting moment followed by an increasing value of the righting moment.
Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, 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, 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.
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
Referring still to
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.
Each pontoon 121 has a width W121 measured perpendicularly to its axis 122 in bottom view. Unlike conventional pontoons that typically have a constant or uniform width along their entire length, in this embodiment, width W121 of each pontoon 121 varies along its length L121-first node 124 has a constant or uniform width W124 and second node 128 has a constant or uniform width W128, however, in intermediate section 126, width W121 varies. In particular, each intermediate section 126 may be divided into a first transition portion 126a having a width W126a, a second transition portion 126c having a width W126c, and a middle portion 126b extending axially between transition portions 126a, b and having a width W126b. Width W126a decreases in first transition portion 126a moving axially from first node 124 to middle portion 126b, width W126c decreases in second transition portion 126c moving axially from first node 124 to middle portion 126b, and width W126b is constant or uniform in middle portion 126b. In this embodiment, width W124 and width W128 are the same, however, width W126b is less than both width W124 and width W128. Further, width W126a, W126c transitions from width W124, W128, respectively, to width W126b. Thus, width W121 of each pontoon 121 is a maximum in nodes 124, 128 (i.e., width W124 and width W128 each represent the maximum width of each pontoon 121), and a minimum in middle portion 126b of intermediate section 126 (i.e., width W126b represents the minimum width of each pontoon 121). Accordingly, each pontoon 121 may generally be described as having a “dog bone” shape in bottom view.
As best shown in
Without being limited by this or any particular theory, the heave characteristics of an offshore floating structure (e.g., 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 121), 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 general, the lower the pontoon lower surface area ratio, the lower the heave. For most conventional pontoons for 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 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:
In general, the lower the pontoon-to-column width ratio, the lower the heave. For most conventional pontoons for 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 design, embodiments described herein also offer the potential to 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.
Referring again to
In this embodiment, each column 150 is a “multi-column” comprising a plurality of parallel, elongated sub-columns 170, each sub-column 170 extending from end 150a at deck 160 to end 150b at base 120. Each elongated, vertical sub-column 170 is oriented parallel to axis 155 and has a radius r170. Further, in this embodiment, each sub-column 170 is equidistant from axis 155 of its respective column 150. In this embodiment, each column 150 is made from four sub-columns 170 equally spaced from axis 155, thereby defining generally square columns 150 with widths W150 equal to about four times sub-column radius r170. The gap in the middle of the four sub-columns 170 defining each column 150 provides a space for storing mooring ropes and/or chains.
Referring now to
In this embodiment, sub-column 170 includes three cells 174—an upper cell 174 extending axially from upper end 170a, a lower cell 174 extending axially from lower end 170b, and an intermediate cell 174 extending axially between the upper and lower cells 174. For purposes of clarity and further explanation, upper cell 174 is also designated as 174U, intermediate cell is also designated as 174I, and lower cell is also designated as 174L. As best shown in
Each cell 174 has an upper end 174a, a lower end 174b opposite upper end 174a, and defines an inner region or chamber 176i within the cell 174 and an outer or exterior region 176o outside the cell 174. In this embodiment, each cell 174 axially above lower cell 174L (i.e., upper cell 174U and intermediate cell 174I) is sealed and water-tight. Consequently, chamber 176i of upper cell 174U is isolated and sealed off from exterior region 176o and the chambers 176i of each adjacent cell 174 (e.g., chamber 176i of intermediate cell 174I), and chamber 176i of intermediate cell 174I is isolated and sealed off from exterior region 176o and the chambers 176i of each adjacent cell 174 (e.g., chambers 176i of upper cell 174U and lower cell 174L). Specifically, upper end cap 172, bulkhead 173, the portion of tubular 171 defining upper cell 174U, and the connections therebetween are water-tight, each being completely free of holes and ports; and bulkheads 173, the portion of tubular 171 defining intermediate cell 174I, and the connections therebetween are water-tight, each being completely free of holes and ports. In other words, chambers 176i of cells 174U, 174I are not in fluid communication with the surrounding environment, each other, or any other chambers 176i. Thus, as used herein, the terms “sealed” and “water-tight” are used to describe a chamber or cell that is completely closed off and not in fluid communication with the surrounding environment, any adjacent chambers or cells, or any inlet or outlet conduits (e.g., ventilation pipes, etc.) during offshore operations. A chamber or cell may have an access panel that allows periodic access to the inside of the chamber or shell for inspection and/or maintenance, yet still be “sealed” and “water-tight” during offshore operations by closing such access panel. Contrary to conventional offshore platform columns that include openings at their upper ends (e.g., platform 10), embodiments of sub-columns 170 described herein do not include any openings (e.g., access trunks, hawse pipe, ventilation pipes, etc.) in their upper ends 170a.
In this embodiment, each chamber 176i disposed axially above the lowermost cell 174 (i.e., lower cell 174L) is completely filled with a gas 106, which contributes to the net buoyancy of sub-column 170, its corresponding column 150, and hull 115. Thus, chambers 176i of upper cell 174U and intermediate cell 174I are filled with gas 106. In general, gas 106 may comprise any suitable gas or gas mixture, but preferably comprises an inert, relatively low cost gas such as air. Since chambers 176i of upper cell 174U and intermediate cell 174I are sealed during offshore operations, the volume of gas 106 within each cell 174U, 174I is constant during offshore operations. Although each chamber 176i above lower cell 174L is completely filled with gas 106 in this embodiment, in other embodiments, solid ballast, liquid ballast (e.g., sea water), or combinations thereof may be included in one or more of the chambers (e.g., chambers 176i) disposed above the lowermost cell to achieve the desired buoyancy of sub-column (e.g., sub-column 170).
Referring still to
Water port 179 is essentially a through hole or opening in lower cell 174L that allows fluid communication between internal chamber 176i of lower cell 174L and the surrounding environment. As previously described, when platform 100 is deployed for offshore operations, lower cell 174L is submerged in the water 101, and thus, port 179 allows water 101 to move into and out of internal chamber 176i of lower cell 174L. It should be appreciated that flow through port 179 is not controlled by a valve or other flow control device. Thus, port 179 permits the free flow of water into and out of chamber 176i of lower cell 174L. Although port 179 has been described as a “water” port, it should be appreciated that gas such as air is also free to flow into or out of chamber 176i of lower cell 174L through port 179. For example, if chamber 176i of cell 174L is completely filled with air, some of that air is free to flow out of chamber 176i via port 179.
A buoyancy control gas 107 may be controllably supplied to chamber 176i of lower cell 174L via port 178, and buoyancy control gas 107 within chamber 176i of lower cell 174L may be controllably exhausted chamber 176i of lower cell 174L via port 178. For example, a buoyancy control gas (e.g., compressed air) may be pumped through port 178 into chamber 176i of lower cell 174L, and buoyancy control gas within chamber 176i of lower cell 174L may be vented through port 178. Thus, port 178 functions as both a buoyancy control gas inlet and outlet. The flow of the buoyancy control gas 107 out of and into chamber 176i of lower cell 174L through port 178 is controlled by a valve 178a. Although the buoyancy control gas 107 may comprise any suitable gas, in embodiments described herein, buoyancy control gas 107 is air.
As previously described, in this embodiment, buoyancy control gas 107 can be supplied to and removed from chamber 176i of lower cell 174L through a single port 178. However, in other embodiments, separate ports may be used to supply gas (e.g., gas 107) to the chamber (e.g., chamber 176i) and vent gas from the chamber. For example, a gas inlet may be coupled to the chamber to supply gas to the chamber, and a separate and distinct gas outlet may be coupled to the chamber to vent gas from the chamber. Such an inlet and outlet each preferably comprise a valve for controlling the flow of gas therethrough.
Since buoyancy control gas 107 (e.g., air) is less dense than water 101, any buoyancy control gas 107 in chamber 176i of lower cell 174L will naturally rise to the upper portion of chamber 176i above any water 101 in chamber 176i. Accordingly, positioning port 178 at or proximal the upper end 174a of lower cell 174L allows direct access to any gas 107 therein. Since water 101 in chamber 176i of lower cell 174L will be disposed below any gas 107 therein, positioning port 179 proximal lower end 174b allows ingress and egress of water 101, while limiting and/or preventing the loss of any gas 107 through port 179. In general, gas 107 will only exit chamber 176i of lower cell 174L through port 179 when chamber 176i is filled with gas 107 from upper end 174a to port 179.
During deployment and operation of platform 100, the buoyancy of lower cells 174L, and hence the buoyancy of corresponding sub-columns 170 and columns 150, and hull 115 may be varied by controlling the volume of gas 107 and water 101 within chamber 176i of each lower cell 174L. A control system (not shown) automatically controls valve 178a, thereby allowing gas 107 to be pumped into or allowed to escape from chamber 176i, based on a variety of factors including, without limitation, the desired buoyancy of hull 115, the heeling angle of platform 100, variations in weight (e.g., top side weight, riser weight, etc.), and the desired draft of hull 115.
Without being limited by this or any particular theory, the flow of water 101 through port 179 will depend on the depth of lower cell 174L and associated hydrostatic pressure of water 101 at that depth, and the pressure of buoyancy control gas 107 in chamber 176i (if any). If the pressure of gas 107 is less than the pressure of water 101 in chamber 176i of lower cell 174L, then the gas 107 will be compressed and additional water 101 will flow into chamber 176i through port 179. However, if the pressure of gas 107 in chamber 176i of lower cell 174L is greater than the pressure of water 101 in chamber 176i of lower cell 174L, then the gas 107 will expand and push water 101 out of chamber 176i through port 179. Thus, gas 107 within chamber 176i of lower cell 174L will compress and expand based on any pressure differential between the air 107 and water 101 in chamber 176i. During deployment and operation of platform 100, gas 107 may be pumped through inlet 178 and associated valve 178a into chamber 176i to increase the pressure and volume of gas in lower cell 174L and decrease the volume of water 101 in chamber 176i, thereby increasing the buoyancy of corresponding sub-column 170 and column 150, and hull 115. Conversely, gas 107 may be exhausted from chamber 176i into the surrounding water 101 via outlet 177 and associated valve 177a to decrease the pressure and volume of gas 107 in chamber 176i and increase the volume of water 101 in chamber 176i, thereby decreasing buoyancy of corresponding sub-column 170, corresponding column 150, and hull 115.
As previously described, cells 174U, 174I are filled with gas 106 and sealed from the surrounding environment, however, the volume of gas 107 in lower cell 174L can be controlled and adjusted. In this embodiment, cells 174U, 174I are sized and configured such that platform 100 is net buoyant even if lower cells 174L are completely filled with water 101. Further, since cells 174U, 174I are sealed and water-tight (i.e., have no unsealed access trunks, hawse pipe for storing mooring chains, etc.), platform 100 has no downflooding angle (i.e., there is no heeling angle at which locations at sub-columns 170 will flood with water).
Referring now to
Cells 174U, 174I of each sub-column 170 of semi-submersible platform 200 are filled with gas 106 (e.g., air), are sealed and water-tight, and thus, have a contain a constant volume of gas 106. However, the volume of gas 107 in each lower cell 174L can be controlled and adjusted as previously described. Similar to platform 100 described above, in this embodiment, cells 174U, 174I are sized and configured such that platform 200 is net buoyant even if lower cells 174L are completely filled with water 101. Further, since cells 174U, 174I are sealed and water-tight (i.e., have no unsealed access trunks, hawse pipe for storing mooring chains, etc.), platform 200 has no downflooding angle (i.e., there is no heeling angle at which locations at sub-columns 170 will flood with water).
As previously described and shown in
As previously described embodiments of platforms 100, 200 comprise columns (e.g., columns 150) made of a plurality of sub-columns (e.g., sub-columns 170). However, in other embodiments, each column of the hull may not be a multi-column formed from a plurality of sub-columns. For example, each column of the hull (e.g., each column 150 of hull 115) may comprise a single column such as that shown in
In the manner described, embodiments of tension leg platform 100 and semi-submersible platform 200 are net buoyant (even with gas-water adjustable cells 174L completely flooded), and further, have no downflooding angle. As will now be described, embodiments of tension leg platform 100 and semi-submersible platform 200 are “unconditionally stable.” In other words, platforms 100, 200 will not capsize regard less of the heeling angle and integrity of tendons 112 or mooring lines of mooring system 212, respectively. In particular, embodiments of platforms 100, 200 are configured such that they will return back to their vertical upright position (i.e., with deck 160 upright) when a moment induces a non-zero heeling angle. Thus, as used herein, the phrase “unconditionally stable” refers to an offshore structure (e.g., platform) that will not capsize regardless of the heeling angle and integrity of the tendons (e.g., tendons 112 associated with TLP 100) or mooring lines (e.g., mooring lines of mooring system 212).
The stability of an offshore platform may be described in terms a “righting moment” that acts on the platform in response to a heeling moment that induces a heeling angle α. In particular, the righting moment is the moment or torque that seeks to restore the platform to its vertical, upright position (i.e., no heeling angle α) after induction of a heeling angle. Conversely, the “heeling moment” is the moment or torque that seeks to tilt the platform away from its vertical, upright position, thereby inducing a heeling angle α. For example, wind acting on an offshore platform generates a heeling moment that begins to tip the platform from vertical and induce a heeling angle α. Without being limited by this or any particular theory, the heeling moment acting on an offshore platform is a function of several external forces such as wind and wave action applied to the platform, while the righting moment is a function of several platform structural characteristics such as draft, buoyancy, weight, column center-to-center spacing, etc.
Calculating the heeling moments and righting moments acting on an offshore structure such as a platform, and graphing same over a range of heeling angles α to assess platform stability are well known in the art of naval architecture. Specifically, various navel architecture standards (e.g., the ABS Guide for Building and Classing Mobile Offshore Units 2008—Part 3, Chapter 3, Section 1) require generation and publication of stability graphs for various offshore structures exposed to a standard, constant 100 knot wind in intact conditions (i.e., no damage to hull compartments) and a standard, constant 50 knot wind (constant) for damaged conditions (i.e., damage to one hull compartment). Referring now to
Referring now to
In general, the stability of an offshore platform such as platforms 100, 200 is a function of various platform structural parameters. As is known in the art, such parameters include:
Embodiments of platforms 100, 200 described herein are configured, via adjustment of one or more of the parameters listed above, to exhibit unconditional stability as illustrated in
where Z=6.
Referring now to
In block 1202, initial values are set for each parameter to be applied for determining platform stability. For example, initial values for GM, FB, ∇, Draft, D2, and CC may be set. The initial values of the parameters may be set based on variety of design considerations including, without limitation, material cost, construction site, transport limitations, parameter values of existing platforms, desired performance characteristics, anticipated offshore environment, etc. For example, Draft may be initialized to 125 feet, center spacing to 150 feet, etc. Additionally, a range value that limits the extent of variation of the parameter, and an increment value specifying the amount the parameter is changed at each step are set for each parameter. The values of the range and increment may be determined based on considerations similar to those used to determine the initial value (e.g., a known range of acceptable values for each parameter).
In block 1204, a first of the parameters is selected for manipulation. A platform incorporating the current parameter values is evaluated for stability in block 1206. More specifically, the righting moment of the platform is evaluated over all heeling angles. Techniques for configuring a platform in accordance with the current parameter values and generating the values of the righting moment of the platform (e.g., values corresponding to the graph of
In block 1208, if the righting moment for the platform being evaluated is greater than zero for all non-zero heeling angles, and/or an increase in value (a positive slope) of the righting moment follows a decrease in value (a negative slope) of the righting moment, then the platform being evaluated is deemed to be unconditionally stable. The method may terminate after identifying a parameter set corresponding to an unconditionally stable platform, or may continue to identify additional parameter sets for other unconditionally stable platforms.
If, in block 1208, the righting moment is not greater than zero for all non-zero heeling angles, and/or if the righting moment does not increase in value following a decrease in value, then, in block 1210, the value of the parameter is checked against the parameter range. If all parameter values within the specified range have not been evaluated, then the value of the parameter is incremented in accordance with the corresponding increment value and a platform in accordance with the current parameter values is evaluated in block 1206.
If, in block 1210, it is determined that all values of the parameter within the specified range have been evaluated, then in block 1214, a next parameter is selected for manipulation, the selected parameter is incremented in block 1212, a platform evaluation continues in block 1206.
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.
This application claims benefit of U.S. provisional patent application Ser. No. 61/324,514 filed Apr. 15, 2010, and entitled “Multi Column Tension Leg Platform,” which is hereby incorporated herein by reference in its entirety.
Number | Date | Country | |
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61324514 | Apr 2010 | US |