This invention relates to a structure floating on a body of water. Three or more spar buoy-type floats support the structure above the water. The structure is connected to anchors in the floor of the body of water by elongated members such as large diameter pipe for example. There are no other anchoring connections in the system. Each spar buoy has a unique structure so that vertical forces and overturning moments on the floating structure are minimized. The spar buoys have a buoyancy means having a volume of two parts. g The buoy of each spar buoy has a volume of two parts. The first part can be defined as resulting from a straight, vertical, prismatic shape which runs the entire vertical length of the buoyancy means. buoy. The volume of this prismatic portion comprises between about 40 and 80 percent of the total displacement. The buoyancy means have a second or second part has an auxiliary volume of displacement which runs considerably less than the vertical length of the prismatic portion. This critical arrangement of buoyancy between these two parts as taught in this invention minimizes mooring forces imposed on the vertical elongated members, such as occur to react forces on the structure due to passing waves.
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11. A floating structure for use in the body of water having an expected maximum wave which comprises:
a deck; buoyancy means rigidly supporting said deck, said buoyancy means including at least three slender, vertical float members, each such float member having two parts, the first part resulting from a straight, vertical, prismatic shape which runs the entire vertical length of the vertical float member, the volume of the prismatic portion comprising between about 40 and about 80 percent of the total displacement of the vertical float member, and an auxiliary portion exterior said prismatic portion and, comprising between about 20 and about 60 percent of the total displacement of the buoyancy means below still water, said auxiliary portion being placed below the trough of the expected maximum wave; anchor means at the bottom of said body of water; an elongated member connecting each said vertical float member and said anchor means, said elongated members being parallel; said structure being free of any anchoring connection with the water bottom other than said parallel elongated members.
1. A floating structure having limited lateral movement for use in a body of water which comprises:
a working deck; buoyancy means for supporting said working deck, said buoyancy means including a plurality of slender vertical float members; anchor means in the floor of the body of water; horizontally spaced-apart, parallel, elongated members interconnecting the said buoyancy means and said anchor means whereby said deck is maintained parallel to and at a substantially constant angle with reference to the horizontal; each said vertical float member of said buoyancy means having prismatic volume resulting from a straight, vertical, prismatic shape which runs the entire vertical length of the buoyance means, the volume of the prismatic portion comprising between about 40 and80 percent of the total displacement of the buoyancy means, and the structure having an auxiliary buoyancy portion having a volume of displacement between about 20 and about 60 percent of the total displacement of the buoyancy means, said auxiliary volume being placed below the trough of an expected maximum wave; said platform and buoyancy means being free of any anchoring connection with the water bottom other than said parallel elongated members.
17. A floating structure for use in a body of water having an expected maximum wave height hmax which comprises:
a deck; buoyancy means rigidly supporting said deck, said buoyancy means providing a total still water displacement b and including at least one slender vertical float member having a still water draft h, comprising two parts, the first part resulting from a straight prismatic shape which runs the entire vertical length of the vertical float member and an auxiliary portion exterior to said prismatic portion having an overall vertical length l, said auxiliary portion being placed below the trough of the expected maximum wave, for which the shape of the slender, vertical float member is defined by a value of r, said parameter r being the ratio of the maximum radius of the auxiliary portion to the radius of the prismatic portion, which value r is Equations (32) through (34); anchor means at the bottom of said body of water; elongated member interconnecting each said slender vertical float member and said anchor means, if there is more than one vertical float member, the said elongated members associated therewith are parallel; said structure being free of any anchoring connection with the water bottom other than said elongated member.
34. A floating structure for use in a body of water having an expected maximum wave height hmax which comprises:
a deck; buoyancy means rigidly supporting said deck, said buoyancy means providing a total still water displacement b and including at least one slender vertical float member having a still water draft h, each said vertical float member comprising two parts, the first part resulting from a straight prismatic shape which runs the entire vertical length of the vertical float member and an auxiliary portion exterior to said prismatic portion having an overall vertical length l, said auxiliary portion being placed below the trough of the expected maximum design wave, for which the shape of the slender, vertical float member is defined by a value of r, said parameter r being the ratio of the maximum radius of the auxiliary portion of the radius of the prismatic portion, which value r is between about [rbt -(0.3441 kips/ft.2 )(h2 /b)] and about [rbt +(34.41 kips/ft.)(h/b)] where rbt is determined from Equations (10), (16) and (17); anchor means at the bottom of said body of water; an elongated member connecting each said vertical float member and said anchor means, said elongated members associated therewith being parallel; said structure being free of any anchoring connection with the water bottom other that said parallel elongated members.
25. A floating structure for use in a body of water and having an expected maximum wave height hmax which comprises:
a deck; buoyancy means rigidly supporting said deck, said buoyancy means providing a total still water displacement b and including at least one slender vertical float member, said vertical float member having a still water draft h, comprising two parts, the first part resulting from a straight prismatic shape which runs the entire vertical length of the vertical float member and an auxiliary portion exterior to said prismatic portion having an overall vertical length l, said auxiliary portion being placed below the trough of the expected maximum wave, for which the shape of the slender, vertical float member is defined by a value of r, said parameter r being the ratio of the maximum radius of the auxiliary portion to the radius of the prismatic portion, which value r is between about [r1 -(0.3341 kips/ft.2)(h2 /b)] and about [r2 +(34.41 kips/ft.)(h/b)] where r1 is determined from equations (20) through (22) and r2 is determined from equations (23) through (25); anchor means at the bottom of said body of water; an elongated member interconnecting each said vertical float member and said anchor means, if there is more than one vertical float member, the said elongated members associated therewith are parallel; said structure being free of any anchoring connection with the water bottom other than said parallel elongated members.
41. A floating structure for use in a body of water which comprises:
a deck; buoyancy means rigidly supporting said deck, said buoyancy means providing a total still water displacement between about 15,000,000 pounds and about 60,000,000 pounds and including at least three slender, vertical float members, each such vertical float member having a still water draft between about 75 and about 150 feet and comprising two parts, the first part resulting from a straight, vertical prismatic shape which runs the entire vertical length of the vertical float member and an auxiliary portion exterior to said prismatic portion, said auxiliary portion being placed below the trough of the maximum design wave, for which the shape of the slender, vertical float member is defined by either (a) values of p and r which when plotted as a point falls into the shaded regions of either FIGS. 15B through 23B or falls into shaded regions obtained by a linear interpolation between the shaded regions of these figures, said interpolation being made on the basis of still water displacement and still water draft, or (b) values of (l/h) and r which when plotted as a point falls into the shaded regions of either FIGS. 15A through 23A, or falls into shaded regions obtained by a linear interpolation between the shaded regions of these figures, said interpolation being made on the basis of still waster displaceent and still water draft; anchor means at the bottom of said body of water; an elongated member interconnecting each said buoyancy means and said anchor means, said elongated members associated therewith being parallel; said structure being free of any anchoring connection with the water bottom other than said parallel elongated members.
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This application is a continuation-in-part application of copending application Ser. No. 754,628, entitled "Vertically Moored Platforms," filed Aug. 28, 1968, Kenneth A. Blenkarn and now abandoned.
1. Field of the Invention
This invention relates to a structure floating on a body of water. More particularly, the invention relates to a floating structure from which drilling or production operations are carried out. In its more specific aspects, the invention concerns a floating structure having buoyancy means placed especially with respect to the trough of a design wave so as to minimize mooring forces imposed on the vertical elongated members which anchor the structure, such as those forces which may be caused by passing waves.
2. Setting of the Invention
In recent years there has been considerable attention attracted to the drilling and production of wells located in water. Wells may be drilled in the ocean floor from either fixed platforms in relatively shallow water or from floating structures or vessels in deeper water. The most common means of anchoring fixed platforms include the driving or otherwise anchoring of long piles in the ocean floor. Such piles extend above the surface of the water with a support or platform attached to the top of the piles. This works fairly well in shallower water, but as the water gets deeper, the problems of design and accompanying costs become prohibitive. In deeper water it is common practice to drill from a floating structure.
In recent years there has been some attention directed toward many different kinds of floating structures, for the most part maintained on station by conventional spread catenary mooring lines, or by propulsion thruster units. One scheme recently receiving attention for mooring is employed in the so-called vertically moored platform. One such platform is described in U.S. Pat. No. 3,154,039, issued Oct. 27, 1964. A key feature of the disclosure in the patent is that the floating platform is connected to an anchor base only by elongated parallel members. The members there are held in tension by excess buoyancy of the platform. This feature offers a remedy for one of the major problems arising in the conduct of drilling, or like operations from a floating structure. This major problem is that ordinary hull-type barges or vessels, in response to ocean waves, may exhibit substantial amounts of vertical heave and angular roll motion. Such motions significantly hinder drilling operations. Motion difficulties are alleviated to a degree by use of the so-called semisubmersible vessels or structures in which flotation buoyancy is provided by long, slender vertical bottles or tanks. This design suffers the inconvenience that, if carried to the logical extreme of having very little waterplane area, the unit would become statically unstable, requiring careful reballasting to offset changes in vertical loads, such as drilling hook load (e.g., when pulling drill pipe, etc.) or changes in weight of supplies. Some of those problems are eliminated or at least reduced in the vertically moored platform. Being subjected to tension, the elongated parallel members of the vertically moored platform are substantially inextensible and therefore restrain the platform to move primarily in the horizontal direction. This virtually eliminates heave and roll motions. In vertically moored structures heretofore considered, exceptionally strong mooring would be required to resist the vertical forces which might be imposed upon a structure by the orbital motion of passing waves. The present invention describes a means to minimize the mooring forces imposed by the structure on the elongated members, such as those caused by passing waves.
Briefly, a preferred embodiment of this invention concerns a floating structure having limited lateral movement for use in a body of water. It is especially designed for an expected maximum wave. This expected wave is usually called the "maximum design wave." The structure includes a working platform supported by a buoyancy means comprising a plurality of slender vertical float members. The float members are rigidly anchored to the ocean floor by a plurality of horizontally spaced-apart, parallel, elongated members. The volume of the buoyancy means can be defined as comprising two parts, the first part resulting from a straight, vertical, prismatic shape which runs the entire vertical length of each vertical float member. The volume of the prismatic portion comprises from about 40 percent to about 80 percent of the total displacement of the buoyancy means below the "still water" line. The ratio of the displacement of the prismatic portion to the total displacement is called the prismatic ratio p. A second volume of displacement surrounds the prismatic portion and comprises the remainder of the total displacement. This second volume is placed below the trough of the design wave. This critical placement of the second or auxiliary volume and the critical size minimizes the critical mooring forces imposed on the vertical elongated members by the structure due to the orbital motion of the passing waves.
Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawings.
FIG. 1 is a view of a floating structure of this invention;
FIG. 2 illustrates a perspective view of a part of one of the vertical floats of FIG. 1;
FIG. 3 is a section taken along the line 3--3 of FIG. 1;
FIG. 4A illustrates relative vertical forces for ratios of the radii and the lengths of the prismatic portion and the auxiliary portion of the vertical buoyancy means for a still water draft of 100 feet;
FIG. 4B is similar to FIG. 4A and illustrates selection of limits of the prismatic ratios for the same still water draft of 100 feet;
FIG. 5A is similar to FIG. 4A except it is for a still water draft of 125 feet;
FIG. 5B is similar to FIG. 4B except it is for a still water draft of 125 feet;
FIGS. 6A, 6B and 6C illustrate the variation in mooring force for three fundamental types of vertically moored platforms which consist respectively of only one slender, vertical float member; a float member completely submerged; and a buoyancy member according to this invention;
FIG. 7A shows the shape of a typical vertical buoyancy means of my invention;
FIG. 7B shows the forces acting on a typical vertically moored platform comprising only one vertical float member of my invention;
FIG. 8 shows an example of overturning moment on a floating structure such as shown in FIG. 1;
FIG. 9 shows an example of variation in mooring force for a given leg due to the overturning moments shown in FIG. 8;
FIG. 10 demonstrates the typical influence on a floating structure according to my invention due to coupling between net vertical forces on individual legs;
FIGS. 11A and 11B illustrate the net variation in mooring force at one leg of a vertically moored platform which comprises vertical float members of a typical configuration according to my invention;
FIGS. 12A and 12B illustrate the net variation in mooring force at one leg of a vertically moored platform which comprises vertical float members made up of prismatic cylinders;
FIGS. 13A and 13B illustrate the net variation in mooring force at one leg of a vertically moored platform in which the float members are made up of deep spheroidal floats only;
FIG. 14 shows the maximum variation in mooring force at a given leg, expressed as a percent of the displacement per leg, for various values of the shape parameters r and (L/H) (the term r, L and H are defined hereinafter) and for particular platform size and design condition, as noted;
FIGS. 15A through 23A illustrate (a) the best combinations of shape parameters r and (L/H) and (b) the range of practical combinations of these parameters for various platform sizes and design conditions, as noted;
FIGS. 15B through 23B illustrate (a) the best combinations of shape parameters p and r and (b) the range of practical combinations of these parameters for various platform sizes and design conditions, as noted.
Referring to the drawings in which identical numbers are employed to identify identical parts, numeral 10 designates, generally, the floating structure or platform. The floating structure 10 includes a deck portion 12 which may have a derrick 14 mounted thereon. The deck 12 is preferably an enclosed space where quarters, workshop area, etc., are located. This is to aid in streamlining the system. Various auxiliary means, including a port for helicopter, etc., may be provided.
The deck 12 is supported by at least three vertical float means, generally designated by the numeral 16. This includes an upper "skinny" portion 18 and a lower "fat" portion 20. There are enough of these vertical support means 16 to provide stability. This would ordinarily be three or more. There are four shown as indicated in FIG. 3. The size and placement of the lower portion 20 of the float will be discussed later.
The platform is anchored by suitable means to the ocean floor. Shown in the drawing is a baseplate 22. Anchor piles 24 extend into the bottom of the ocean for whatever depth is needed to secure the proper anchorage, e.g., 500 feet. These anchor members are secured in place, for example, by cement 26. Connecting anchor members 24 to the working structure or platform are a plurality of elongated member 28 alternately called risers. These elongated members 28 are preferably large-diameter steel pipe, e.g., 20 to 30 inches in diameter. These elongated members 28 could be cables of wire, chain, and the like. However, it is preferred that they be pipe so that operations can be conducted from the floating structure down through them to underground formations. Preferably, it is desired to drill down through these pipes.
The structure shown in FIG. 1 is essentially rigid in the vertical direction, but is relatively free to move in the horizontal direction. Restraint against horizontal movement is only the horizontal component of riser tension, that component being proportional to the angular departure of the riser from true vertical. Under the action of wind, current and other steady forces, the platform will be shifted horizontally until the resultant horizontal restraint equals such applied loads. In response to wave action the platform will oscillate back and forth about the shifted or average position. The platform will, for storm wave situations, generally oscillate horizontally so as to move with the surrounding fluid. The horizontal motion of the platform will basically satisfy the following relation. ##EQU1## in which
X=the single amplitude horizontal motion of the platform.
A'=the horizontal, single amplitude wave motion of water at the elevation of the platform center of buoyancy. See Equation (2).
B=the buoyancy or displacement of the platform.
H'=the "hydrodynamic mass" of water associated with acceleration of the platform. For most configurations H' is essentially equal to buoyancy.
M=the actual weight of the platform.
T=the wave period.
Tn =the natural sway period, calculated from Equation (3). Water motion A' is calculated, for simple wave theories, according to the following equation. ##EQU2## in which
h=wave height, crest to trough.
S=the submergence of the platform center of buoyancy below still water level.
λ=wave length (=5.12T2, by Airy Theory).
Natural sway period of the platform is expressed as (3) Tn2 =L' (H'+M)/B-M
in which
L'=the length of vertical mooring lines or risers, and other symbols are as previously defined.
For most platform configurations of interest, a design wave 100 feet high would cause the platform to move 50 feet either side of the average shifted position. It is generally to be preferred that steady storm shift of the platform be approximately equal to the single amplitude of the wave induced motion. For the case just described, an appropriate design shift would be 50 feet. For water depth requiring vertical risers 1,000 feet long, such a horizontal shift would correspond to a horizontal restraint equal to 1/20 of the tension in the vertical mooring lines or risers. Thus, tension in the risers should generally be between 15 and 25 times the steady horizontal storm loads. Typically required total tensions in the order of 10,000,000 pounds are to be expected. Typically such a tension could be carried by 16 or 20 pipe risers which have 20 inches outside diameter with a wall thickness of 0.625 inches.
It has been found that when pipes such as risers 28 are under tension and subject to angular rotation, the influence of tension is to concentrate the angular rotation at the ends of the pipe. Accordingly, means are provided in risers 28 to permit this angular rotation with the two terminals of the riser pipe 28. This is provided in the form of a ball joint 30 at the upper end and a ball joint 32 at the lower end.
Another means of providing for the excess stresses which would be built up near the ends of pipe 28 if they were not hinged, is to provide a section of special size and wall thicknesses at the end of the pipe to make them sufficiently strong to withstand the imposed stresses. Other suitable means for limiting this concentration of stress are described in the copending patent application of Blenkarn and Dixon, Ser. No. 748,867 filed July 30, 1968As indicated in FIG. 1, means 52 is provided to move said horizontal fins 50 about a horizontal axis. Recognize that acceleration forces are associated with volumes of displaced and therefore accelerated water. The action of the fins is to trap a surrounding "hydrodynamic mass" or volume of water. In this way acceleration forces are increased by opening the fins out to entrap more hydrodynamic mass. On the other hand, when the fins are folded into a vertical position, they do not influence acceleration forces.
Installation of the vertically moored platform might typically be done according to the following steps:
1. Launch, or otherwise remove the baseplate 22 from a transportation barge.
2. Lower the baseplate 22 to the ocean floor by means of guidelines.
3. Using a semisubmersible drilling unit (preferably dynamically positioned) drive marine conductors 24 (e.g. 30 inches in diameter) through the baseplate following subsea drilling practice.
4. Drill out through the marine conductor 24 for 20-inch surface casing.
5. Run 20-inch surface casing string with the lower portion of ball joint 32 at the top of this 20-inch casing string.
6. Cement casing string in the well.
7. Repeat the operation for other conductors and casings installed in the baseplate.
8. Bring platform 10 to the location.
9. Ballast the platform to float at the designed draft.
10. Using, for example, derrick 14, run elongated members 28 in the manner normally employed for installing marine risers, passing the elongated members through vertical conductor pipes down through the platform.
11. Make up ball joint 32 at the bottom of each elongated member 28.
12. Weld off, or otherwise fix, the top of elongated members 28 at the platform deck.
13. Deballast the platform 10 in order to apply the proper tension to the elongated members 28.
While a limited number of embodiments of the present invention have been shown, various modifications can be made thereto without departing from spirit or scope of the invention.
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| Apr 30 1979 | Standard Oil Company (Indiana) | (assignment on the face of the patent) | / |
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