US3369511A

US3369511A - Marine floating structure

Info

Publication number: US3369511A
Application number: US610197A
Authority: US
Inventors: German John Gordon
Original assignee: Individual
Current assignee: Individual
Priority date: 1964-05-04
Filing date: 1967-01-18
Publication date: 1968-02-20
Anticipated expiration: 1985-02-20
Also published as: GB1102561A

Description

Feb. 20, 1968 .1. G. GERMAN 3,369,511

MARINE FLOATING STRUCTURE I Original Filed April 30, 1965 2 Sheet sSheet l mummy INVENTOR JOHN GORDON GERMAN ATTORNEY Feb. 20, 1968 J. G. GERMAN 3,369,511

MARINE FLOATING STRUCTURE Original Filed April 50, 1965 2 Sheets-Sheet Q -Tm\x g for I o 3; 1000 g "E t E E" Tun I D E for 7\ l000 o 0 .5 .o .o 8 2 ID 20 Period P 0 penod of periurbonons FIG. 3 FIG. 4.

' INVENTOR JOHN GORDON GERMAN BY Ml 11M, Mmf

- ATTORNEY ite rates Patent @flice 3,369,511 Patented Feb. 20, 1968 ABSCT F THE DISCLQE'aURE A perforated motion damping shell surrounds the buoyant chambers of a floating drilling platform to enhance the stability of the platform.

This application is a continuation of Ser. No. 452,108 filed Apr. 30, 1965, now abandoned.

This invention relates to off-shore drilling, and particularly concerns improved floating support platforms and their moorings for drilling apparatus used in sinking boreholes into the sea bottom.

Under the provisions of 35 U.S.C. 119, this application is entitled to the benefits of application for British Patent 18,374, filed May 4, 1964.

Heretofore drilling platforms for the drilling and completion of offshore wells have been generally designed as bottom-supported fixed or moveable (jack-up) structures, consisting of a number of columns or legs standing on the sea bed and supporting an elevated platform. Moveable, or jack-up structures, have proven costly to build because each leg requires to be adjustable to stand on uneven bottom areas, for which purpose very large hydraulic jacking machinery of considerable complexity must be provided. The stable platform performance of floating platform structures, which is necessary, has hitherto been either impossible to achieve to a satisfactory degree, or it was not thought to be possible to design a usefully stable platform combining sufficient static stability and negligible tendency to pitch, roll or bob in the water under all probable sea conditions, in combination with other desirable features such as (1) centre position drill derrick, (2) capability of entering and working in shallow water, (3) low cost, (4) capability of being used as a bottom supported unit in very shallow water.

I have devised a floating moored platform which inherently has good statical stability under ordinary shifting of masses on the platform, such as occur by movement of machinery and materials used in Well drilling, and which moreover has excellent self-stabilization properties to minimize rolling or bobbing under extreme sea agitation. A limited heaving and rolling motion with respect to a drill stem may readily be accommodated by the usual marine rotary table drives. On the other hand, roll in excess of about three degrees from the horizontal is not tolerable due to the stresses placed on the drill stem.

With the aforesaid objectives in mind, I have provided a marine floating structure whose form and disposition of parts yields a natural roll period well above the range of periods of the great majority of perturbations propagated in the sea, and which moreover is highly stable statically and possesses a high degree of damping to achieve rapid steadying and to prevent the development of excessive bobbing (heaving) motion. In structures constructed according to the invention, a horizontal platform which may be of any desired shape is buoyantly supported above the sea on a system of spaced vertical column assemblies each of which includes a tubular vertical column having its lower end secured to a buoyant chamber submerged in the water, said column assemblies forming the corners of a regular polygon. The platform may be moored by cables secured to anchoring means in the sea bottom lying sufficiently distant laterally of the platform so that the cables extend from said means nearly horizontally.

In preferred platform assemblies incorporating my invention, the polygon has an even number of sides, and in an optimum arrangement four column assemblies are provided, including four tubular surface-piercing buoyant columns situated at the corners of a square, the columns having their lower ends or feet greatly enlarged laterally as wholly submerged buoyant thick disc chambers, the top surfaces of which lie below the mean sea surface at a depth of at least one-half the maximum wave height for which the platform is designed, while the platform rides above the mean sea surface a clear distance which is also not less than one-half the maximum wave height.

The column assemblies are further characterized by a central vertical tubular buoyant column extending between the platform and the buoyant disc chamber, and by a vertical enclosing outer shell coextensive with the central column, the shell being extensively perforated over its entire surface and having essentially the same outside diameter as that of the disc chamber.

In structures designed according to the invention, the static metacentric height may have a value anywhere between about 4 and 14 feet, and preferably will be between about 6 and 10 feet. A buoyancy change in draft of about 65 tons per foot is appropriate in a typical structure wherein the diagonal distance between opposite corners of the polygon is between and feet. However, this is a variable quantity depending on the size and lifting capacity requirement of each individual platform design and may be of a different magnitude, depending on requirements.

The structure is moored by long anchor cables extending singly or in fanning groups radially outwardly from the column assemblies to anchors in the sea bed, the length of a cable depending on the sea depth and being preferably five or more times the diagonal dimension of the platform, and preferably 10 or more times such dimension, so that the slope of the chain leading down from the platform will be small, under 35 degrees, and preferably below 30 degrees. In ocean depths deep enough to render mooring systems impracticable, multi-direction controllable propulsion systems will be used to maintain position. These systems will be activated by electronic controls and guidance systems, part of which will be located on the sea bed.

The invention will be described particularly in and by the following description, which is to be read in conjunction with the accompanying figures of the drawings, wherein:

FIGURE 1 is a vertical elevational view of a preferred form of floating platform according to the invention;

FIGURE 2 is a plan view of the platform of FIG- URE 1;

FIGURE 3 and FIGURE 4 are graphs relating respectively to natural roll and bobbing (heave) periods of the structure with respect to a spectrum of perturbation periods;

FIGURE 5 shows a column assembly form useful in a structure having controlled variable ballasting;

FIGURE 6 shows an alternative buoyant column assembly form for deep water sites, having additionally a planar damping baflle; and,

FIGURE 7 shows a plan view of alternative mooring line connections and layout.

In a floating off-shore platform generally designated 10 in the drawings, FIGURES 1 and 2, an extended horizontal platform 11 comprises an upper flat surface 12 and a lower planar bottom 13, containing at least one and preferably two levels for accommodating living quarters, storages for materials, working areas, and ancillary spaces. The lower surface 13 may be anywhere from about 12 to about 30 feet or more above the mean sea surface 14, depending on anticipated sea conditions for which a particular platform is designed. The superstructure and upper and

lower floors

12 and 13, and any intermediate levels, are perforated or apertured centrally at 15, to pass drill pipe and tools there-through, in platforms intended for well drilling. In other applications similar access to the sea may be provided as for instance to provide hatches for raising craft from the sea surface and raising stores and supplies from the decks and holes of supply vessels.

The corners of the platform are extended outwardly over respectively associated buoyant column assemblies 16, which include a central vertical buoyant column 17. The cross sectional area of such buoyant column must be considerable in order to provide adequate static stability under shfits of equipment, and to allow large loads to be lifted, as when withdrawing a drill stem of great length. In a typical design the column 17 may be a cylinder having a diameter of substantially to feet. The column assemblies are not connected by horizontal cross bracing. Each column assembly 16 terminates at its lower end in a submerged foot portion 20. The latter is comprised of a thick hollow disc chamber coaxial with the column 17, and disposed with its upper planar disc face 21 generally as far below the mean sea water, surface 14 as the platform floor 13 lies above such surface. The span between floor 13 and disc 21 should exceed slightly the height of the largest probable waves at the location, and may range between substantially 25 and feet. The diameter of the foot portion 20 is preferably made larger than, that of tubular chamber, 17, and may be as much as 40 to feet.

The thickness of the foot 20 will in general be so proportioned that its contribution of lifting force exceeds the buoyant force exerted by the column 17 of the assembly 16. Such foot portions will henceforth be referred to as the primary buoyant chambers, in order to distinguish them from the surface-piercing tubular buoyant columns 17. In conjunction with the buoyant columns 17, the primary buoyant chambers 20 are designed to carry the load due to the column assembly and platform structures, all equipment and personnel supported thereon, and the weight of the mooring cables 22 to be described later.

The majority of waves propagating in the sea at a drilling site will have crest-to-crest lengths of a few hundred feet so that the platform will span a significant fraction of the wave length, particularly for wave trains of from about five to seven second periods, as may predominate at a given coastal area. It will be seen that the roll-producing couple is greatest when the diagonal distance, or the distance between columns on the same side, is a half wave length. I make the free roll period of the platform to be well in excess of the period of encounter of wave perturbation in order that the structure should have a minimum roll response to the stimulus of wave perturbation. Ahigh degree of damping is also provided to ensure that the structure will be useful and more particularly so that the maximum heave amplitude about any mean Water-plane will substantially not exceed :5 feet.

In order to provide adequate damping, I provide each assembly 16 with an extensively perforated, substantially cylindrical wall or shell 19 which encloses the vertical column 17 and, as shown by FIGURE 1, may conveniently have a diameter substantially the same as that of foot portion 20. I have found that the outer perforated shell, although having a transverse dimension and cross section considerably larger than those of the central tubular column 17 forms a bafiie which surprisingly acts to markedly reduce the disturbing effects produced by sea waves having a wide spectrum of periods and ampliroll axis is well removed from the range of perturbating,

periods encountered, I, therefore, so design the platform that its natural rolling period is very much longer than the periods of any recurring wave trains, preferably above about 20 seconds. A good choice for the natural roll period, I have found, is .25 seconds, which lies well above the periods of swells of appreciable height propagated over long distances toward exposed coast lines, as shown in FIGURES 3 and 4.

As illustrated in the figures accompanying this disclo-- sure, extended mooring cables 22 are provided which are tensioned to hang as catenary curves, ahgned with the central aperture 15 and extending horizontally to anchors 23 in the sea bottom. In such arrangement the length of each cable is preferably such that the cable is as nearly horizontal as practical, and preferably the portion 24 leading down from the fairleads 25 should incline at about 30 to 35 degrees from the horizontal. A bridle such as 18 may be connected with a point on each column assembly well below the surface. It should be understood that alternative anchoring arrangements may be provided within the scope of the present invention.

In a four-column platform as shown, the buoyancy provision should allow for the vertical component of tension in the mooring cables, while adequate structural provision is made for the horizontal component under any conditions of wind and sea. The relatively large horizontal component further serves to oppose the heeling couples due to wind or Waves, providing orrestoring force and a static equilibrium reference. In addition, the

considerable projected area of the cables provides frictional coupling with the sea tending to oppose their displacement Whether by rolling or bobbing motion of the platform.

In deep Water locations, particularly where adequate static stability may *be assured through the use of variable ballasting equipment, the tubular columns 17 may be relatively narrow in section as shown in FIGURE 5, or may i even comprise mere conduits for protection of conductors and fluid lines serving variable ballasting equipment (not shown) located within the foot portions 20. In the embodiment of FIGURE 5, the primary buoyant chambers 20 are enlarged to provide the bulk of the lifting force and serve as roll-damping and heave damping baflies. A group of vertical slender struts 26 suitably braced together connect between the platform and the foot portion to provide a column assembly ofadequate rigidity, and very low cross-section to the waves. In such structures having columns 17 of reduced cross-section, the assemblies 16 may be set a greater distance away from the centre of the platform to improve the stability characteristics of the structure. The buoyant columns 17 would be designed to supply the buoyancy necessary to compensate for load variations due to changes in quantities of fuel, mud and other dead weight items carried by the platform in order to maintain the platform at a constant level.

The foot portions 20 of the column assemblies 16 need not be enlarged where adequate depth exists, and a surface-piercing column 17 of moderate section, possibly for example less than about ten feet in diameter, may be vextended in depth to provide sufficient lifting force. In such an embodiment, which is illustrated by FIG. 6, in addition to the perforated wall or. shell 19, oneor more planar bafiles 27 are located below the mean sea sur-- face a distance at least equal to half the maximum wave height, and preferably a greater distance than this, so that each planar bame lies in water where the orbit diameters of water particles in Wave motion are relatively small.

As shown in FIGURE 7, the mooring lines may be multiplied so that in the aggregate they spread out in all directions with respect to the central aperture 15, to provide a more uniform roll-damping contribution about any horizontal axis.

For use in shallow water where it may be necessary for the platform to rest on bottom, hydraulically operated levelling feet will be fitted at the bottom of the primary buoyant chambers. These will have a downward extension of several feet to take up small differences in the level of the sea bottom Where the platform may be used as a bottom-supported unit. Positive weight will be maintained at all times on the bottom bearing units by introducing ballast water into the buoyant chambers.

Other general advantages to be noted for the proposed platform design are:

(1) Centre drilling position of drilling derrick.

(2) Good towing characteristics-an advantage when changing drilling sites.

(3) Fully enclosedprotection for drilling crew and personnel when working under particularly hot, humid or cold or stormy conditions.

(4) Centre positioning adjustment may be done automatically through the mooring system in conjunction with automatic position sensing equipment and power adjustment of the mooring cable windlasses.

(5) Buoyant columns compartmented for ballasting and to provide additional safety to crew.

I claim: 1. A marine floating structure comprising (a) a horizontal platform supported above the mean sea surface at a height not less than one half the maximum anticipated wave height by (b) a plurality of buoyant vertical column assemblies spaced beneath and supporting said platform, each of said assemblies including (i) a vertical column extending from said platform to a point substantially beneath the sea surface, said column being formed to include a submerged, water-tight primary buoyant chamher coaxial with the vertical axis of the said column of diameter not less than the diameter of the column, and (ii) a substantially cylindrical motion-damping shell surrounding said column and extending both above and below the sea surface, said shell being extensively perforated over its vertical surface said column assemblies being located at the corners of a polygon and so spaced that the free roll period of the platform is substantially greater than the peri- 0d of encounter of wave perturbation, and

(c) means for maintaining the position of the structure relative to the sea floor.

2. A marine floating structure as defined in claim 1 in which said shell extends from a height above the mean sea surface of one half the anticipated maximum wave height to a depth below the mean sea surface of one half the anticipated maximum wave height.

3. A marine floating structure as defined in claim 1 in which there is no horizontal cross-bracing between said column assemblies.

4. A marine floating structure as defined in claim 1 in which said vertical column terminates at its end remote from said platform in a submerged primary buoyant chamber of diameter greater than said column.

5. A marine floating structure as defined in claim 1 in which each of said column assemblies additionally includes one or more planar bafiles located surrounding and extending from said column at a depth below the mean sea surface at least equal to half the anticipated maximum wave height.

6. A marine floating structure as defined in claim 1 in which said means for maintaining the position of the structure relative to the sea floor comprises one or more cables attached to the platform above each of said column assemblies and anchored to the sea floor.

References Cited UNITED STATES PATENTS 12/1965 Kobus ll40.5 5/1966 Blenkarn 6l46