US3654886A

US3654886A - Tethered platform flotation

Info

Publication number: US3654886A
Application number: US49417A
Authority: US
Inventors: Daniel Silverman
Original assignee: BP America Production Co
Current assignee: BP America Production Co
Priority date: 1970-06-24
Filing date: 1970-06-24
Publication date: 1972-04-11
Anticipated expiration: 1989-04-11

Abstract

This invention relates to a structure floating on a body of water. The structure comprises a working deck with buoyancy means supporting the deck. The buoyancy means comprises one or more slender vertical floats which have a unique structure having two parts. The first part comprises a straight, vertical, prismatic volume which runs the entire vertical length of the vertical floats. The volume of the prismatic portion comprises between about 40 and 80 percent of the total displacement. There is a second or auxiliary volume of displacement which is submerged below the trough up the maximum wave to be expected. The relative buoyancy of the two parts can be adjusted so as to minimize the vertical forces on the structure due to passing waves. This invention is concerned with additional means to vary the flotation of the prismatic part and to control the water plane area of the floats so as to reach smaller minimum variations of vertical forces on the structure.

Description

United States Patent Silverman [151 3,654,886 [451 Apr. 11, 1972 [54] TETHERED PLATFORM FLOTATION [72] Inventor:

[73] Assignee: Amoco Production Company, Tulsa, Okla. 22 Filed: June 24, 1970 [21] App]. No.: 49,417

Daniel Silverman, Tulsa, Okla.

Primary Examiner-Trygve M. Blix Attorney-Paul F. Hawley and John D. Gassett ABSTRACT This invention relates to a structure floating on a body of water. The structure comprises a working deck with buoyancy means supporting the deck. The buoyancy means comprises one or more slender vertical floats which have a unique structure having two parts. The first part comprises a straight, vertical, prismatic volume which runs the entire vertical length of the vertical floats. The volume of the prismatic portion comprises between about 40 and 80 percent of the total displacement. There is a second or auxiliary volume of displacement which is submerged below the trough up the maximum wave to be expected. The relative buoyancy of the two parts can be adjusted so as to minimize the vertical forces on the structure due to passing waves. This invention is concerned with additional means to vary the flotation of the prismatic part and to control the water plane area of the floats so as to reach smaller minimum variations of vertical forces on the structure.

18 Claims, 10 Drawing Figures PATENTEDAPR 11 I972 3, 654,886

SHEET 1 OF 5 INVENTOR. FIG. I DANIEL SILVERMAN TTORNEY PATENTEDAPR 1 1 I972 SHEET 2 OF 5 A B C 2 2 2 e e F F F E V I E M L E E E W A S E W E IIQI O W S F 2 W M O O I I C H I 2, O F E E S T T I 0 FS .HVH OF F W 0% E W IO M I m V T m I A E M E S 2 v 0 II. T O 5 FW 0% T.- 4 F I allE II ll 58 2 W W m 5 O R I M E O O O O O O O O O O O O. O O O O O A EmE t womom E0:

mw .52 mo zoi LEADING CREST FOLLOWING CREST CREST FOLLOWING TROUGH CREST INVENTOR. DANIEL SILVERMAN TORNEY PATENTEDAPR 1 1 I972 SHEET 3 [IF 5 M Q- MAM;

A N M FIG. 6

N R A m N E VV mu 8 ID E N A D XM A/r bvey PAIENTEIIAPR 1 1 I972 SHEET l [IF 5 92 SERVO AMPLIFIER MANMEML ACCELEROMETER 86 TENSION MEASURING MEANS FIG.7

INVENTOR. DANIEL SILVERMAN TTORNEY PATENTEDAPR 1 1 I972 3. 654, 8 86 SHEET 5 OF 5 I no H0 H2 H2 I8 38 /e/ j 46 46 M [Li /36 F o n4 L0 TO YANCHOR T Tb ANCHOR INVENTOR. DANIEL SILVERMAN A TORNE Y CROSS REFERENCE TO RELATED APPLICATIONS This application is related to application Ser. No. 754,628 entitled Vertically Moored Platforms filed Aug. 28, 1968, now abandoned, and to a continuation-in-part application of that application, Ser. No. 17,485, entitled Vertically Moored Platforms, filed Mar. 9, 1970, in the name of Kenneth A. Blenkarn.

BACKGROUND OF THE INVENTION 1. Field ofthe 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 general, the structure may be restrained by vertical tension members or tethers, or conventional catenary anchor cables, or hydraulic or hydromechanical thrustor positioning means to have a minimum of horizontal motion. This invention is particularly applicable to a floating structure having buoyancy means of a specific type adapted to minimize vertical heave forces and heave motion of the structure due to 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 US 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 semi-submersible 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 water plane 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. Means are described to minimize the mooring forces imposed by the structure on the elongated members, such as those caused by passing waves.

BRIEF DESCRIPTION OF THE INVENTION This invention is preferably applied to a floating structure having limited lateral movement for use in a body of water,

5 which 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 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.

As will be shown later, this type of structure can be designed to compensate the flotation and orbital (or inertial) effects so that they essentially neutralize each other, leaving a zero resultant vertical wave effect on the structure. Since wave motion may vary over wide ranges in period and height, the best that can be done in the design of a structure is to maintain a minimum vertical force on the structure for a range of wave height and wave period. This is all taught in the above mentioned applications, Ser. Nos. 754,628 and 17,485.

This invention is concerned with means which are made part of the flotation system of the structure, by means of which the flotation system can be adjusted to have a different prismatic ratio p so as to balance the buoyancy and inertial effects of the waves on the structure.

This invention is applicable to many types of floating structures, and can be applied to any structure, whether it has been designed to minimize the buoyancy and inertial effects of the waves or not. However, this invention is most applicable to, and is most effective with, those structures in which a partial balance is reached between the buoyancy and inertial forces of the waves, and will be described in terms of such structures. More particularly, for convenience, it will be described primarily in terms of a vertically moored or tethered structure, and reference will be made to the above mentioned applications for structural detail.

BRIEF DESCRIPTION OF THE DRAWINGS Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the appended drawings.

FIG. 1 is a floating structure typical of the type on which this invention can be used.

FIGS. 2A, 2B and 2C illustrate the variation in mooring force for three fundamental types of vertically moored plat forms, which consist respectively of only one slender, vertical float member; a float member completely submerged, and a member combining elements of both types.

FIG. 3 illustrates one embodiment of this invention utilizing a single control buoyancy pipe.

FIGS. 4 and 5 illustrate other embodiments utilizing a plurality of control buoyancy pipes.

FIG. 6 illustrates the detail of a preferred embodiment.

FIG. 7 illustrates a complete control system utilizing the embodiment of FIG. 6.

FIG. 8 illustrates an embodiment utilizing control buoyancy pipes on a floating structure anchored by conventional laterally set anchors and accompanying lines.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings in which identical numbers are employed to identify identical parts and particularly to FIG. 1. Numeral l 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.

The platform is anchored by suitable means to the ocean floor. Shown in the drawing is a base plate 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 members 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.

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.

T, the natural sway period, calculated from Equation (3). Water motion A is calculated, for simple wave theories, according to the following equation.

A =5 -21rs/x in which h wave height, crest to trough. S the submergence of the platform center of buoyancy below still water level. )t= wave length 5.121", by Airy Theory) Natural sway period of the platform is expressed as T,, =L (H+M)/BM (3) 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 by 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 one-twentieth 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.

The vertical members 16 of the structure are connected by cross bracing 34. This cross bracing is preferably all located below the still water line indicated by line 36. As mentioned earlier, this structure will be subjected to various wave forces. In Naval engineering, when designing floating structures, or other marine structures for that matter, it is quite common to select what is known as a maximum design wave. The maximum design wave will have a crest 38 and a trough 40.

There are concepts disclosed herein which teach the means by which the mooring forces are minimized when using structures as exemplified by the embodiment of FIG. 1. A particularly desirable shape for the vertically positioned elongated floats is illustrated in FIG. 1. With reference to such a shape, the following applies. The volume of buoyancy or displacement can be conceived as being made up of two parts. The first part results from a straight, vertical, prismatic shape which has the diameter of upper portion 18 and runs the entire vertical length of the structure. The volume of this prismatic portion of the structure comprises between about 40 percent and about percent of the total displacement. The second or auxiliary volume of displacement is that part which is the annulus volume between the prismatic volume and the outer wall of enlarged portion 20. This auxiliary volume is placed below trough 40 of the maximum design wave.

The auxiliary volume should be placed in a smooth and streamlined fashion, as indicated above, as an annular space around the basic prismatic volume. The size of the auxiliary volume in the annulus portion of the bottles should be reduced to the extent of displacement provided by the bracing 34 within the structure which is below the trough of the design wave. The auxiliary volume in the annular space should be streamlined and flared into the basic prismatic volume to the extent practical. While I have discussed a prismatic volume and an auxiliary volume, it is to be understood that these two volumes can be continuous and that it is not necessary that they be separated into physical compartments.

If a vertically moored platform is to be used, it is usually necessary that variations of vertical mooring forces, which arise in reaction to forces imposed on the structure by wave action, be minimized within the range of wave lengths of importance. Wave lengths of importance vary from one wave area to another but many are typically in the range of from about 500 feet to 2,000 feet. Wave action on the structure results in (a) a net vertical force on the structure, (b) a net couple on the structure due to vertical forces on individual bottles, and (c) a net overturning moment on the structure due to horizontal wave forces. All of these forces contribute to the variation in mooring force.

The structure shown minimizes the variation in mooring force, for the range of wave lengths of importance, by permitting offsetting contribution from each of the contributing factors: net vertical force, net couple of vertical forces and net overturning moment. If this structure is not designed to obtain proper distribution of forces, one of these forces might be overpowering. For example, if vertical forces on individual bottles are eliminated or minimized, thereby eliminating or minimizing the net vertical forces and the net couple due to vertical force on the structure, the variation in mooring force is due entirely to overturning moment and can be undesirably large, especially for the longer wave lengths. On the other hand, if the buoyancy arrangement is such that a small amount of net vertical force is admissible for all wave lengths, there is a phenomenon associated with this force, the net coupling of vertical forces, which causes a net reduction in overturning moments at the larger wave lengths. Therefore, a careful selection of buoyancy distribution can result in a minimization of mooring force variations over the entire range of important wave lengths.

The vertical forces on the structure are dominated by forces which fall into two categories: namely, (a) variable buoyancy forces and (b) vertical water acceleration forces or inertial forces. While there are other contributions to the net vertical forces, they are of lesser importance. All of these forces on the structure were calculated by elementary, commonly understood means. However, the dominant two forces were combined for the calculations into one net force, heave, which is discussed below. The two categories of dominant vertical forces act in opposite direction to one another and one of the concepts of this discussion is to carefully adjust the magnitudes of these forces to obtain the desired net vertical force. This is possible with my design for certain ratios (L/H) of the length (L) of the enlarged portion to the total design draft (H) and for certain ratios (r) of the radius R of the enlarged portion to the radius R of the prismatic portion for a selected draft where L, H, R and R, are defined in FIG. 1. As shown above, the prismatic ratio p is defined as the ratio of the displacement of the prismatic portion to the total displacement.

I shall first consider the net vertical forces on the structure. These various net vertical forces can be calculated by using the following equation.

F net change in vertical force, positive upwards.

A('r total displacement below the instantaneous water level.

M0) design displacement, or total displacement below design still water level.

k wave decay factor, i.e., 21r/)\ where )t wave length (Airy Theory).

p water mass density.

3 gravitational acceleration A( y) cross-section area (varies with depth of y) y) hydrodynamic mass coefficient and varies with depth, i.e., (y) (mass of cylinder added fluid mass)/mass of cylinder H design draft y a vertical coordinate measured position upwards from the base of the buoyancy means.

1 a vertical coordinate measured position upwards from the design still water level to the instantaneous water surface yH). In Equation (4) terms [A(17) A(O)] give the force due to variable buoyancy, and the remaining term gives the force due to vertical water acceleration.

Consider first two very elementary types of vertically moored structures as shown in FIGS/2A and 2B. FIG. 2A shows a buoy consisting only of one cylinder. This buoy is moored by one or more vertical tethers such that it is not free to move vertically, but it can move horizontally or rotate. The buoy does not have an annular, or auxiliary portion; all displacement is from the prismatic portion. Therefore, the prismatic ratio p which is defined as the ratio of the displacement of the prismatic portion to the total displacement, equals one (p l). The three curves in FIG. 2A show the variation of net vertical force on the cylinder due to passage of a single wave from three different wave trains. The three wave trains have periods of 10-, 14- and -seconds; the corresponding wave lengths are 512-, 1004- and 2048-feet, respectively. In this example and all subsequent examples it is assumed that the wave height corresponding to each wave length equals either one-tenth of the wave length of the maximum design wave height, whichever is smaller. In this and most of the subsequent examples, except where noted, the maximum design wave height is IOO-feet. Therefore, the corresponding wave heights for the curves in FIG. 2A are 5 l .2-, and lOO-feet, respectively. The variation of net vertical forces is expressed as a percent of total displacement. For example, a 20-second wave causes a reduction in net vertical force of about 32 percent of the displacement when the wave trough is aligned at the axis of the cylindrical buoy. When the crest is aligned with the axis of the cylindrical buoy there is an increase in net vertical force of about 22 percent of the displacement. By virtue of the decrease in net vertical force at the trough and the increase at the crest, this example demonstrates that forces due to variable buoyancy are dominating for the high prismatic ratio. As an explanation of terminology, the term leading crest is that part of the wave half way between the trough and the next crest. The term following crest is that part of the wave train at a point one-half way between the crest and the next trough.

FIG. 2B shows similar curves for another fundamental configuration of a vertically moored structure. In this case the entire displacement is contributed by a spherical cavity at the bottom of the buoy and the portion of the structure projecting upwards from the sphere has an extremely small cross-section. Consequently, the prismatic portion contributes essentially nothing to the total displacement, and the annular, or auxiliary, portion contributes the entire displacement. The prismatic ratio equals 0 (p O). the curves of FIG. 2B show that the maximum variation in net vertical force is about 30 percent of the displacement in the long period, 20-second wave. However, in this example the vertically moored structure experiences an increase in net vertical force when the wave trough is aligned with the buoy, rather than a decrease as with the cylindrical buoy in FIG. 6A. This example demonstrates that for a low prismatic ratio the forces due to vertical water acceleration are dominating.

This study shows that for a low prismatic ratio, forces due to vertical water acceleration are dominating while for a high prismatic ratio, forces due to variation in buoyancy are dominating. Moreover, for an intermediate value of the prismatic ratio p, there exists a balance between variable buoyancy forces and vertical water acceleration forces such that the variation in net vertical force for the wave lengths of interest are substantially smaller than in the two fundamental cases examined above.

Consider for example a vertically moored structure as shown in FIG. 2C. In this case the parameters describing the physical properties of the bottle are r 1.853, (L/H) 0.5 and p 0.545. In addition, the maximum design wave height, h is 100 feet and the draft, H, is feet, which is the same as in the two preceding examples. The curves of FIG. 2C show the variation of net vertical force when the bottle is subjected to the same three waves. In this case the maximum variation in force is about 7 percent and it occurs under the influence of both the 10- and 20-second waves, although it is an increase for the lO-second wave and a decrease for the 20-second wave. FIG. 2C shows that for a bottle with this specific distribution of displacement, vertical water acceleration or inertial forces dominate for short period waves while variable buoyancy forces dominate for longer period waves. Furthermore, there is a wave (about a 16-second wave) for which there is virtually no variation in net vertical force because there is a perfect balance between the variable buoyancy and vertical water acceleration forces.

If the prismatic ratio had been slightly greater, buoyancy forces would have dominated as in FIG. 2A, consequently the maximum variation due to the 20-second wave (a decrease in net vertical force) would have been greater than 7 percent. If the prismatic ratio had been smaller, as in FIG. 28, vertical water acceleration forces would have dominated and consequently the maximum variation due to the lO-second wave (an increase in net vertical force) would have been greater than 7 percent. Therefore, for the range of waves of interest, to 20-second period waves, a best balance between the two influencing vertical forces is obtained for the combination ofparameters in FIG. 2C, r= 1.853, L/H= 0.5 andp=0.545.

The basic configuration of the bottle is described by any two of the three parameters. The most fundamental set is r and L/H where p is a function of these two parameters. On the other hand, it is convenient to express the design of the buoyancy members in terms of p and r. It is also recognized that the most practical selection of r and L/H may not always provide the minimum net vertical force and therefore some means are required to change the parameters in operation to subject the structure to the minimum vertical net variation in force, which brings us to the need for this present invention.

The essence of this invention lies in apparatus which can be made to automatically control the ratio r so as to maintain a balance, or, at least, a smaller minimum heave force, than is possible by a fixed value of r, for a range of wave heights and periods.

One embodiment of this invention is illustrated in FIG. 3. Here, I show the bottle 16 (as in FIG. 1) with a tubular pipe 46 fastened along the side of the prismatic portion 18. This pipe 46 is long enough so that its top 47 extends above the level of the maximum wave crest 38, and its bottom end is below the trough 40 of the maximum wave. The top can be open as shown, or closed, with slightly different effects. The bottom end of pipe 46 is provided with a closure means or valve 48 operable by rod means 50, although other types of valves can be used equally well.

When the valve 48 is open, water will rise inside pipe 46 to the same level as outside. Thus, there is substantially no flotation effect due to pipe 46 (other than the negligible displacement of the walls of the metal pipe). When the valve 48 is closed, then the flotation due to the entire exterior volume of the pipe becomes effective.

As shown in FIG. 4 a plurality of pipes may be used for control of flotation. These are shown as 56, 57 and 58 fastened to the outside of the prism 18 by means such as welding, as indicated. These can be operated singly or together to provide a plurality of control flotation values (which can be expressed as values of Ar in the design parameter r). If the flotations are designed in binary ratios of say 1, 2, 4, 8, etc., a number of flotation values equal to 2" can be obtained, where n number of pipes 56, etc. Thus for four pipes of proper ratios of diameter, different values of control flotation can be obtained by proper choice of valve closings.

If the

pipes

56, 57 and 58 are placed on the outside, they add to the total flotation. However, they can equally well be placed inside the wall 18 where they reduce the total flotation as shown by pipe 56'. Also, I show in FIG. 5 how two pipes can be mounted one inside the other to provide three different values of flotation dependent on which, if any, of the

volumes

70, 68 are open to the sea.

I have shown in FIG. 2 how, with assumed parameters, an optimum balance in vertical forces can be made for the range of periods from 10 to seconds and from 52 to IOO-feet wave amplitude. I have shown how for this design, the vertical forces are balanced for a 16-second l00-foot wave, while for all others there is an unbalanced force remaining. I show also in FIG. 2C how the longer period waves dominate in the buoyancy effect, while for shorter period waves the inertial forces predominate. It is thus clear that if we had means such as shown in FIG. 4 with a plurality of pipes, we could design the structure such that at l6-second periods the prism 18 would have less buoyancy and the additionally required buoyancy would be provided by part of the pipes 46. Then if the wave period went to longer values we would reduce the flotation to compensate and if the period went to shorter values, we would add flotation to compensate. Theoretically if we had enough control buoyancy available we could maintain a substantially complete balance between the two opposing sets of vertical forces for all values of wave period in the chosen range.

In FIG. 6 I show a preferred embodiment of my invention in the control flotation element 74. This comprises the pipe 75 with a special type of valve in which an annular flow space 76 is provided between an outer tapered tube 75 and an inner tapered plug 80 with surface 78. The plug 80 is supported by rod 82. As the rod 82 is moved up or down the constriction at the valve is increased or decreased. The hydraulic resistance of the annular orifice serves to reduce the rate of flow of water into the pipe 75 as the wave crest approaches, and reduce the rate of water flow out of the tube as the wave trough approaches. Thus the water inside the tube will rise to level, say, 84 when the crest reaches the pipe, (to level 38) and fall to 86 when the trough reaches the pipe (to level 40'). The flotation effect is proportional to the difference in levels, 38-84 on the crest and 86-40 on the trough. Since under resistive restraint of the orifice 76 the level difference can be made anything desired from zero to a maximum of 38-36, etc., this embodiment provides in one pipe, the flexibility of a great number of smaller pipes, such as in FIG. 4.

In FIG. 7 I show a system by means of which the position of the rod 82 can be controlled by a servo responsive to the tension in the tether or other sensor output, to control the flotation so as to bring the fluctuation in tension to a minimum. 1 show the flotation unit 16 with the flotation control element 74 attached to its side. The plug 80 is operated by rod 82 which carries a rack 96 in contact with a pinion driven by motor 94 which is mounted on a bracket 97 attached to pipe 74. In the tether 86 connected to the element 18 is placed a tension sensor 88. This can be of the strain gauge type or other types, all of which are represented by commodities available on the market and well known in the art. The output of the tension sensor 88 goes by switch 89 to line 90 to a servo amplifier 92 which sends control signals over line 93 to the drive motor 94. The servo amplifier has an additional input from a sensor sensitive to the level of the sea, via line 102. If the phases of the signals are the same, that is, if the tension increases when the water level increases, then the structure is buoyancy controlled, and the flotation element 74 must be adjusted for lower flotation, that is, the valve 80 must be opened more. On the other hand, if the phases of the two sensors are opposite, then the structure is inertial controlled, and the flotation must be increased, that is, the valve 80 must be closed further.

No detail is deemed necessary for the servo amplifier since this art is well known, and systems are available for purchase and text books are available.

In the event that the structure 16 is not vertically tethered, then it will be necessary to have another sensor to replace the tension sensor 88. One that might be suitable would be an accelerometer (or velocity or displacement sensor). This is indicated as 98 which, through switch 99, can be connected to the servo amplifier 92 in place of sensor 88. Here again the art of acceleration sensors is well known and they are available on the market so further description is not deemed necessary.

Most of the description has been restricted to an embodiment comprising a vertically tethered platform, which is restrained to have essentially no vertical motion. However, because the wave effects can cause very large forces in the tether members, it becomes important to control the flotation to bring the variation in vertical forces to a minimum for any particular wave condition.

There is another large class of vessels similar to the abovedescribed tethered platform. These are the semi-submersible drilling vessels. A generalized drawing of such a vessel is shown in FIG. 8. Here a vessel much like that of FIG. 1 is shown. This drilling vessel is generally anchored by means of a number of catenary cables to anchors in the sea floor. The anchor cables 116 go around sheaves 114 and 112 to winches on the deck 120. While these cables restrict the horizontal motion, the vessel can move vertically through a fairly wide range due to the long catenaries.

This vertical motion, or heave, of the drilling vessel is undesirable for a number of reasons.

I. It places great stresses on the anchor cables as the vessel is moved up and down by the waves.

2. Because the drilling tools in the hole are of substantially constant length it is very difficult to maintain a desired value of pressure of the bit on the rock due to the vertical motion of the deck, and expensive devices are required to compensate for this heave in order to be able to drill.

3. In logging, a precise measurement of depth of the sonde in the well is required. This is difficult to determine, without providing expensive devices to compensate for heave of the drilling vessel.

4. In drilling, a riser pipe is required, which is fastened to the well head at the sea floor, and must extend up to the vessel. Normally large and expensive slip joints and seals are required to retain the drilling mud in the riser pipe as the vessel heaves.

For all of these reasons this invention is also applicable to, and would be valuable if applied to semi-submersible drilling vessels and similar structures.

While a limited number of embodiments of this invention have been shown, various modifications can be made thereto, all of which are felt to be part of this invention, the scope of which is to be determined only by the scope of the appended claims.

lclaim:

l. A floating structure for use in a body of water which comprises:

a deck;

buoyancy means for supporting said deck, said buoyancy means including at least one slender vertical float member;

said at least one vertical float member of said buoyancy means having a volume defined in two parts, the first part resulting from a straight, vertical, prismatic shape which runs the entire vertical length of the buoyancy means, the volume of the prismatic portion comprising a predetermined portion X, of the total displacement of the buoyancy means, and an auxiliary portion having a volume of displacement comprising (l-X) of the total displacement, said auxiliary volume being placed below the trough of an expected maximum wave; and

control buoyancy means comprising at least one vertical tubular means associated with said at least one float member, said tubular means of such length that its top is above the crest of, and its bottom end is below the trough of, said expected maximum wave, said tubular means having adjustable closure means below said trough of said maximum wave.

2. A floating structure asin claim- 1 in which said tubular means is closed at the top.

3. A floating structure as in claim 1 in which said tubular means is open at the top.

4. A floating structure as in claim 1 in which said tubular means is exterior to the float member with which it is as sociated.

6. A floating structure as in claim 1 in which said tubular means is positioned within the outer contour of the float .member with which it is associated.

6. A floating structure as in claim 1 in which said control buoyancy means comprises a plurality of tubular means.

7. A floating structure as in claim 1 in which said adjustable closure means is adapted to provide an adjustable resistance to flow of water into and out of said vertical tubular means.

8. A floating structure as in claim 7 including servo means to control the resistance to flow of said closure means in response to the heave of said structure.

9. A floating structure as in claim 1 in which said predetermined portion X comprises between 40 and percent of the total displacement.

10. A floating structure as in claim 1 in which said predetermined portion X comprises between 45 and 65 percent of the total displacement.

11. A floating structure as in claim 1 in which said structure is tethered by at least one substantially vertical tension member.

12. A floating structure as in claim 11 and including servo means to control the buoyancy of said control buoyancy means in response to the tension in said tension member.

13. A floa ing structure as in claim 1 in which said structure is restricted in its range of horizontal motion by means other than vertical tethers.

14. A floating structure as in claim 13 in which said horizontal motion of said structure is restricted by catenary anchor means.

15. A floating structure as in claim 13 and including servo means to control the buoyancy of said control buoyancy means in response to at least one parameter of the vertical motion of said structure.

16. A floating structure as in claim 15 in which said parameter is the vertical acceleration of said structure.

17. A floating structure as in claim 1 in which said structure is anchored to the floor of the body of water only by parallel elongated members.

18. A floating structure as in claim 1 in which said vertical tubular means has its upper end open.

Claims

1. A floating structure for use in a body of water which comprises: a deck; buoyancy means for supporting said deck, said buoyancy means including at least one slender vertical float member; said at least one vertical float member of said buoyancy means having a volume defined in two parts, the first part resulting from a straight, vertical, prismatic shape which runs the entire vertical length of the buoyancy means, the volume of the prismatic portion comprising a predetermined portion X, of the total displacement of the buoyancy means, and an auxiliary portion having a volume of displacement comprising (1-X) of the total displacement, said auxiliary volume being placed below the trough of an expected maximum wave; and control buoyancy means comprising at least one vertical tubular means associated with said at least one float member, said tubular means of such length that its top is above the crest of, and its bottom end is below the trough of, said expected maximum wave, said tubular means having adjustable closure means below said trough of said maximum wave.

2. A floating structure as in claim 1 in which said tubular means is closed at the top.

4. A floating structure as in claim 1 in which said tubular means is exterior to the float member with which it is associated.

6. A floating structure as in claim 1 in which said tubular means is positioned within the outer contour of the float member with which it is associated.

9. A floating structure as in claim 1 in which said predetermined portion X comprises between 40 and 80 percent of the total displacement.

13. A floating structure as in claim 1 in which said structure is restricted in its range of horizontal motion by means other than vertical tethers.