US3285213A

US3285213A - Floating structures

Info

Publication number: US3285213A
Application number: US409417A
Authority: US
Inventors: Whittle Frank
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-11-08
Filing date: 1964-11-06
Publication date: 1966-11-15
Anticipated expiration: 1983-11-15
Also published as: DE1431323A1; NL6413047A; GB1012370A

Description

Nov. 15, 1966 F. WHITTLE 3,235,213

FLOATING STRUCTURES Filed Nov. 6, 1964 v 5 Sheets-Sheet 1 Nov. 15, 1966 F. WHITTLE 3,235,213

FLOATING STRUCTURES Filed Nov. 6, 1964 5 Sheets-Sheet 2 28 $2M 3r ammx 7ZW Nov. 15, 1966 F. WHITTLE 3,285,213

FLOATING STRUCTURES Filed Nov. 6, 1964 5 Sheets-Sheet I5 32 12 r 3 8 F/G.5. @I'AV 12 FIG 6.

United States Patent 3,285,213 FLOATING STRUCTURES Frank Whittle, Walland Hill, Chagford, Devon, England Filed Nov. 6, 1964, Ser. No. 409,417 Claims priority, application Great Britain, Nov. 8, 1963, 44,160/ 63 21 Claims. (Cl. 114-.5)

This invention relates to buoyant structures or artificial islands such as for example floating platforms utilised in the drilling of off-shore oil wells.

Floating platforms for drilling are already known and existing constructions include (a) converted vessels with more or less conventional hull shapes and (b) decks supported by a plurality of vertical tubular buoyancy chambers of bottle-like form. This latter type has a much lower tendency to roll or pitch in a heavy sea, but is much less stable than the former type, in fact, for any buoyant vessel in which the supporting pressure is the water pressure on the vessel the more stable it is the more the tendency to respond to rolling and/ or pitching couples due to waves.

Floating platforms which are both stable and unresponsive to waves, that is to say steady, have many other applications besides drilling platforms. Of these, some would remain more or less permanently in one position; others may be described as semi-permanent in that they would remain on one location .for long periods; and yet others would need to be capable of being moved after relatively short periods, i.e., to be mobile.

Some permanent applications are as follows: lightships; bright supports, supports for floating oil refinery units; radar bases; radio relay stations; radio beacons and stations for other navigational aids; tanker mooring for loading and unloading; deep water pier heads; pipeline pumping stations; cable relay stations; bathing platforms, etc.

In the semi-permanent category are the following: production platforms for off-shore oilfields; floating power stations for off-shore oilfields etc; weather stations; missile launching bases; anti-submarine bases; fishery bases; mother ships for refuelling etc; floating factories; helicopter or other aircraft seadromes etc.

Of the mobile applications in addition to the drilling platform are the following: pile driving rafts crane barges; deep sea exploration bases; whaling factories etc.

Other applications might be mobile, semi-permanent or permanent according to circumstances, e.g., supports for artificial breakwaters; supports for anti-submarine nets; artificial harbours; floating docks, etc.

In general, artificial islands will be of use for any purpose which requires steadiness and does not call for appreciable mobility other than such as is necessary to tow them into position.

The provision of an off-shore drilling platform suitable for use in shallow waters presents little difliculty since the stability and steadiness of such a structure may be ensured by supporting the platform on legs resting on or piles driven into the sea bed. Off-shore drilling for example for oil is tending however to take place in deeper water, up to and even beyond the hundred fathom line which normally defines the limit of a Continental Shelf, with the result that the engineering difliculties presented by the provision of a stable structure supported from the sea bed in water depths in excess, for example of a hundred feet are very considerably increased, particularly when the drilling site is exposed to extreme weather conditions. Moreover, there are numerous deep water applications other than oil well drilling for which a steady platform is desirable or even essential.

Accordingly it is an object of the present invention, to

3,285,213 Patented Nov. 15, 1966 provide a stable buoyant structure which, when required, can be moored in deep water and the construction of which reduces to a minimum movement resulting from Wave or swell action.

According to the present invention, a buoyant structure comprises a deck supported by a plurality of spaced tubular flotation chambers disposed relative to each other and to the deck such that, in plan, the centre of area and the mean plan position of the centre of gravity of the complete structure substantially coincide, each chamber comprising a hollow cylinder of substantially uniform internal cross-section having its longitudinal axis disposed substantially normal to the plane of the deck, and open at its lower end only; conduit means interconnecting the chambers for maintaining the pressure of air or gas contained within each of the chambers substantially equal and means for ensuring positive stability of the structure when afloat.

The means for ensuring positive stability for the floating platform may include a ballast weight or weights supported beneath the flotation chambers or the deck where by the centre of gravity of the structure is arranged to be below the centre of buoyancy (the vertical position of which will be midway between the external and internal water levels) and preferably at least as low as the internal mean Water level of the flotation chambers.

Means may be provided for mooring the structure to the sea bed, in such a manner, as to compensate for variation in the depth of water due to tidal action and to afford additional stability to the structure.

The structure may include [means for ensuring positive buoyancy in the event of damage to the flotation chambers.

The deck of the structure is preferably vertically displaceable relative to the flotation chambers.

Preferred embodiments of the invention are described hereinafter with reference to the accompanying drawings which are of a diagrammatic nature only and in which:

FIG. 1 is a plan view of a buoyant structure adapted for use as a drilling platform,

FIG. 2 is a side view of the structure shown in FIG. 1,

FIG. 3 is a view similar to that in FIG. 2 but illustrating an alternative means for stabilizing the structure,

FIG. 4 is a side view of a flotation chamber,

FIGS. 5 and 6 are detail views of outrigger arms,

FIG. 7 is a side view of a drilling platform, illustrating a mooring and stabilizing arrangement,

FIG. 7a is a side view of a drilling platform showing an alternative stabilizing arrangement,

FIG. 8 illustrates a drilling platform with the flotation chambers raised for towage, and

FIG. 9 shows the platform of FIG. 8 with the flotation chambers and ballast lowered.

Flotation chambers The basic principle on which the fiotating. platform of the present invention has been devised, may best be explained by consideration of an upright cylindrical chamber of uniform diameter closed at the top but open at the bottom so that when placed in a suitable liquid medium for example water, the chamber is supported by gas or air contained within the chamber, the water rising within the chamber until the pressure of the air or gas contained therein equals the head of water corresponding to the difference between the level of the external and internal water lines. If the centre of gravity of such a vessel is sufliciently low it will have a pendulum stability rather like a parachute.

The chamber is thus supported by the internal air pressure acting against the closed top and since variations in the internal water level merely tend to increase or decrease the air pressure within the chamber a rise or fall of the water within which the chamber is apparently immersed does not produce any moment tending to cause the chamber to roll or pitch. Such a single chamber would, however, rise and fall with change in surface level and would readily capsize unless the centre of gravity were below the mean of the external and internal water levels.

It is to be noted that in contrast to a normal buoyancy chamber--where the supporting pressure are external and tend to collapse the structure inw-ardsin this case the supporting pressure is tending to burst the vessel and, as is well known, vessels subject to bur-sting pressure are far lighter and much simpler in construction than those subject to a collapsing pressure. For example, a cylinder of I20 diameter resting vertically in sea water with a 37' difference between the external and internal water lines would be capable of supporting a load of well over 300 tons and yet the wall thickness need be no more than one-tenth of an inch for a hoop stress in the cylinder wall of less than tons/sq. ins. (It follows that for maximum structural strength a cylindrical vessel of circular section should have a herni-spherical top.) It will be clear that buoyancy chambers of the type described may be extremely light in construction with a weight of the order of 2% of its supporting capacity.

Where a light but stiff construction is desirable the flotation chambers may have a section consisting of 2, 3 or more lobes with the cusps formed by the junctions being connected by tension bracing which may conveniently cooperate with external bracing to transfer the lift of the chamber to other structures. For small structures such as bathing platforms it may be practicable to utilise flexible and inflatable flotation chambers.

By utilizing a deck or hull, supported by a plurality of tubular flotation chambers of the type hereinbefore described and by interconnecting the chambers by ducts or conduits to permit the free passage of air or gas between the chambers such that the pressure is maintained substantially equal at all times, then the uniformity of the air pressure within the chambers is not affected by a rise or fall of the water level in one or more of the chambers due to the action of waves or swell which cannot therefore induce any vertical forces tending to cause roll or pitch.

Preferably, the structure consists of at least three cylindrical flotation chambers, the sizes and spacing being chosen, inter alia, to achieve a plan position of the centre of area closely corresponding with the mean plan position of the centre of gravity. By increasing the spacing between the chambers or increasing the number of chambers or both the tendency of all the chambers to rise or fall simultaneously under wave action may be considerably re- .duced; thus it is desirable that the spacing should be so chosen that it does not correspond with the anticipated average length of the swell i.e. the horizontal distance between consecutive wave crests or troughs. In some circumstances it may be desirable to provide means for varying the spacing to suit varying conditions.

In the foregoing it has been assumed that the buoyancy forces are transmitted by an air cushion, but .of course any other gas could be used. Also the air or other cushion could be separated from the internal water surface by a flexible diaphragm (e.g. a plastic bag) or a film of light fluid such as oil which is immiscible with sea water.

In the embodiment illustrated in FIGURES 1 and 2 the structure comprises four flotation chambers 10 disposed at the respective corners of a square platform 12. The flotation chambers 10 are joined one to another by air pressure equalizing ducts 14 and by bracing members 15 to interconnect the chambers 10 so as to form part of a rigid structure, surrounding the platform 12. Cooperating and vertically extending guide means 117, means 17 mutually slidable releasable locking are provided at each corner of the platform and of the rigid structure respectively to permit of relative movement, between the platform 12 and the assembly of floation chambers. The

means

17, 117 may be, for example, in the form of vertical rods 117 fixed to each chamber 10 and telescopically slidably received in an associated sleeve 17 fixed to the platform 12. Outlets 16 from the chambers 10 to the ducts 14, are located in the upper portion of the chambers 10 at positions above the highest likely internal water level within the chambers.

In the case of a vertical cylinder, pendulum stability will be obtained if the centre of gravity of the system is below the centre of buoyancy, i.e., lower than half the distance between the external and intern-a1 water lines. It can easily be shown that this also applies for any number of vertical cylinders (the centre of gravity must remain fixed in relation to the assembly as a whole i.e., ballast free to swing would not contribute to stability). How ever, generaly speaking, it will be desirable to have the centre of gravity as low as possible and probably at least as low as the internal mean water line, which, because of the equal air presure in all chambers, will be at the same distance below the external means water line in all chambers.

In most cases, it would be possible to arrange that much of the equipment or other load to be carried by the platform 12 can be installed well below deck level e.g., such items as diesel electric generating sets, fresh water storage etc. In many cases, however, substantial ballast may be necessary, particularly where the use of the structure requires that a substantial proportion of the equipment is at a high level above the deck, e.g., a drilling platform where the centre of mass of the derrick and the drill pipe racked in it might be about 50 feet above deck level, the deck itself having a freeboard of 20 feet or more.

There is much scope for the disposition of the ballast, which may, for example, be provided in the form of a number of individual ballast masses, each one being associated with a buoyancy chamber.

In the arrangement shown in FIGURES l and 2 a ballast mass 18 is attached, in rigid relationship to each of the buoyancy chambers 10, to the lower end of a structure comprising an inverted pylon 20 rigidly attached at its upper end to the roof of the buoyancy chamber 10' and extending downward therefrom. In another possible arrangement shown in FIGURE 4 each ballast mass 18 is suspended from three or more cables 22 attached at their upper ends to spaced points around the lower end of the chamber 10. In this latter case, means may be provided for hoisting the ballast mass within the chamber e.g. by the fitting of a remotely controlled winch within the chamber or as depicted in FIGURE 4 by a hoisting cable 24 led through an airtight gland 26 in the chamber 10 to an external winch (not shown).

Alternatively, as shown in FIG. 3 a single ballasting mass 28 may be suspended from three or more cables 30, attached at their upper ends to widely spaced points on the structure of the floating platform or other suitable points. Thus, so long as the cables 30 remain in ten sion, the position of the mass relative to the platform 12 will not vary. This form of suspension allows the ballasting mass to be raised or lowered, or its plan position altered, by appropriate adjustment of the cables 30. In FIG. 3 mooring cables are denoted by reference 31, and are attached to an anchorage on the platform close to the centre of plan 35 of the structure.

In most cases it will almost certainly be desirable to have additional means of. stabilizing the platform. One example is shown in FIGURE 5 wherein the platform 12 is provided with three or more outrigger booms 32 with cables 34 running from the end of each boom for connection to sinker weights resting on the sea bed, the cables 34 being tensioned sufficiently to ensure that there is some degree of tension at all times with means for compensating for changes in sea level due to tides etc. The longer the outrigger booms, the more effective the stabilizing cables would be. In certain cases where the installation is more or less permanent and the sea bed is suitable, the lower end of the stabilizing cables may be attached to piles driven into the sea bed.

To facilitate the recovery and paying out of the stabilizing cables, it is preferably that tension is maintained in the cables by devices indirectly connected to the cables. For example, the effective length of the cable may be varied as shown in FIGURE 6 by arranging that the outrigger booms 32, over the outer end of which the cables 34 are led, are themselves pivotally attached at their inner end as at 36 to the deck 12 of the structure. A tensioning device 38 may be provided between each boom 32 and a fixed point on the platform 12 whereby the booms '32 may be elevated or lowered with respect to the platform, in the manner of a lufiing crane jib. In the example shown in FIG. 5 the booms 32 may be fixed rigidly to the platform and provided at their outer end with an arm 40, arranged as a bell crank lever which on displacement by a tensioning device 42, causes a pulley 44 attached to the arm and carrying the stabilizing cable 34 to be displaced in the vertical plane and thus to vary the effective length of the cable 34.

One form of a tensioning device might be a piston and cylinder arrangement, in which the position of the piston relative to the cylinder is regulated by altering the quantity of fluid trapped between the piston and the upper and lower parts of the cylinder. If this fluid is compressible, air for example, it may be necessary to include also a damping device, such as a dashpot to prevent oscillations in the whole system. Sundry combinations of winches and block and tackle systems (as used in hoisting) may be used as tensioning devices.

Where the depth of the water and nature of the sea bed permits, additional stability may be provided as shown in FIG. 7a by fitting light extensible rigid legs 50, extending downwards to the sea bed from spaced points on the floating platform, the length of the legs 50 being adjusted as necessary to ensure that they remain continuously in contact with the sea bed whilst not bearing any appreciable load. When the sea bed is comprised of soft mud, incapable of supporting load other than by its buoyant I effect, the said legs 50 may be fitted with feet comprised of chambers 52 having buoyancy relative to the mud so that they float thereon, and may also carry ballast masses 54 near their lower ends. If the sea bed is relatively stable the feet 52 can be fixedly anchored thereto in a conventional manner to prevent horizontal movement of the platform 12 but permitting vertical movement thereof due to the telescopic construction of the legs 50. In FIG. 7a the legs 50 are arranged beneath the platform 12. In some cases it may be preferred to attach the legs directly to the buoyancy chambers 10. As shown in dotted lines a girder type leg 150 is adjustably attached to each chamber 10, provision being made for raising and lowering the legs e.g. by a rack and pinion indicated generally at 152, the legs being locked against movement when in the operative position.

Another method of providing additional stability for the platform is shown in FIG. 7 wherein a downwardly extending structure or pylon 56 is rigidly attached to the platform 12 and braced to piles in the sea bed by pairs of cables 60, one of each pair extending from the bottom end of the downwardly extending pylon 56 to an anchorage point on the sea bed (i.e. a pile or heavy weight) and the other from a much higher point of the pylon 56 to theauchorage pointeach pair of cables 60 and the pylon 56 thus being in triangular arrangement. At least three pairs of cables would be necessary-in the form of two tetrahedrons. The pylon 56 may, as shown in FIGURE 7 be slidably mounted with a bell mouthed tube 62, the anchoring cables 60 being attached to the tube 62 so that the tension in the cables 60 can be adjusted by extending or collapsing the pylon 56. It would, ,of course, be necessary to ensure that all. cables are maintained in tension all the time by arranging that there is an upwards pull on the pylon assembly. In the case of a telescopic pylon, the outer part, to which the cables are attached, may form part of an immersed buoyancy system including a chamber 64 of sufficient capacity to ensure permanent tension in the cables 60.

Means for compensating for tilting loads Large movements of deck load, wind loads on super structure etc., will all have a tendency to cause the platform to tilt. If there are stabilizing means in addition to pendulum stability, these additional stabilizing means will be suflicient to deal with small changes in tilting load, e.-g., if Outriggers and cables are used then a movement of deck load or a wind load or both would result in an increase or reduction of tension in the cables and as long as tension is not reduced to zero in any cable, the platform would remain level except for the very small effect due to change of strain in the cables. However, in order to avoid excessive loads on the stabilizing means or to compensate for tilting loads when pendulum stability only is relied on, it may be desirable to provide means for applying a couple in opposition to the tilting couple. There are several ways in which this may be done, namely:

(1) By arranging that certain items of deck load which do not have to be in any particular fixed position can be moved to another position.

(2) By arranging that ballast can be moved in a horizontal plane to adjust the plan position of the centre of gravity.

(3) By providing weights which can be run out or in along Outriggers so that a comparatively small weight can produce a large compensating couple.

(4) By using immersed or nearly immersed additional enclosed flotation chambers, the position and/or volume of which can be adjusted as required (wave forces would have little or no tendency to move immersed flotation chambers vertically as long as they are sufliciently deep to remain immersed at all times).

(5 By having additional floation chambers of the ame nature as the main flotation chambers but capable of being moved in relation to the main flotation chambers so as to adjust the centre of pressure (these compensating chambers would also need to be connected by air passages with the air spaces in the main flotation chambers by adjustable or flexible ducting).

(6) By bodily movement of the platform and its load in relation to the main flotation chambers.

(7) By providing ballast tanks at widely spaced points within or on the structure between which water ballast may be pumped.

A number of other more elaborate methods of applying a compensating couple can be visualized such as water propellers rotating in a horizontal plane, or suitably disposed air propellers.

Means for providing compensating couples will be most needed in applications, such as drilling platforms, where many tons of drill pipe etc., change position in space both horizontally and vertically as drilling proceeds. Also in this application, wind loads on the super-structure are likely to be higher than in many other applications.

Compensating means of one or more of the types described will, of course, be most needed in certain applications where pendulum stability only is relied on, because, unless the centre of gravity of the system is very low, considerable tilting might otherwise occur before the resulting movement of the centre of gravity provides a sufiicient compensating couple.

Where outriggers with vertical cables are used as stabilizing means, a convenient way of sensing the need for applying a compensating couple would be to measure the tension in each of the cables. For this purpose a tension measuring device could "be used similar to that used to indicate the weight on the bit in drilling operations. Alternatively, Where piston and cylinder assemblies are included in the cable system, the -fluid pressure in the cylinders would be a measure of the tension of the cables. It will be obvious that any device used for measuring cable tension could also be used as the actuating means for operating whichever compensating couple device or devices is used, probably through some more or less conventional relay and amplifying mechanism.

Means for preventing pendulum oscillations Where pendulum stability only is relied on, a platform would have a natural frequency of oscillation which might be excited by certain Wave frequencies or frequency of wind gusts. One way of preventing the pendulum effect would be to provide damping devices 70 (FIG. 3) e.g. sea anchors or inverted parachutes, or other sur faces which would resist vertical movement, suspended from outrigger arms 71 by light cables or booms 72.

In locations where ocean currents are negligible, large vertical surfaces fixed well below the surface of the water would have a powerful damping effect. These, however, would be undesirable if there was any appreciable current.

In applications where a very small tilting is acceptable, any of the above described means for applying a compensating couple could also be used to oppose a tendency to oscillate, by being made responsive to a tilt sensing device or devices such as pendulums, spirit levels, gyroscopes etc.

Means for preventing vertical oscillations When pendulum stabilization only is relied on, the platform will have a natural frequency of vertical oscillation and it may be necesary to damp this out by devices. It may readily be seen that inverted parachutes or similar devices suspended from outriggers would serve this purpose, as well as that of damping out pendulum oscillation.

Means for providing reserve buoyancy The nature of the main buoyancy system is such that .if one flotation chamber becomes damaged to the extent of ceasing to become airtight, then all cease to be effective unless there is a compressor 101 connected directly or indirectly to the chambers 10 by, for example, a con- .duit 102 as shown in FIGURES l and 2 for pumping air into the chambers 10 as fast as it escapes (as indicated below, an air compressor is a desirable feature of the apparatus). However, if air does escape, the platform will tend to sink on an even keel and it may be prevented from sinking entirely by arranging that the platform is itself a water-tight vessel of suflicient buoyancy to prevent sinking. Alternatively, the upper parts of each flotation chamber may have a sufficient volume partitioned off to act as reserve buoyancy chambers.

the main flotation chambers in a raised position as shown in FIGURE 8 and when on location to lower the buoyancy chambers and elevate the platform for example from the position shown in FIGURE 9. In such an application it would also be desirable to have that part of the system .which acts as ballast, capable of being raised and lowered so that the whole apparatus can be launched or towed in comparatively shallow water.

Means for raising and lowering the platform The construction of the system is such that the platform can be raised or lowered relative to the sea surface by pumping air into or allowing air to escape from some point in the supporting air system. Since all the air spaces in the flotation chambers are interconnected the air connection for this purpose can be in any one of the flotation chambers or in the interconnecting ducting. It is desirable to provide for this purpose, not only air compression apparatus, but also a reserve of compressed air for use in emergencies.

The use of air to elevate or lower the flotation cham bers makes possible a very convenient way of changing from the towing position, with the platform providing the buoyancy and the flotation chambers elevated, to the operating position with the platform elevated and the flotation chambers lowered. The operating sequence might be as follows:

(1a) Air is allowed to escape from the flotation chambers until the flotation chamber assembly sinks to its operating position, relative to the platform by coming to rest on stops 217 (FIG. 1) attached to the latter.

(2b) The platform is locked to the flotation chambers assembly.

(3c) The vertically movable ballast is lowered to a point sufficient to provide pendulum stability, after elevation of the platform.

(4d) Air is pumped into the flotation chambers until the platform is raised to its operating height.

(5e) Any supplementary stabilizing means are set in position.

(It will be clear that this sequence need not be rigidly adhered to-for example, (3) above may precede 1) and/ or (2).)

Preferably interengaging locking and guide means 17, 117 (FIG. '2) are provided between the flotation chamber system and the platform to ensure that the relative plan position of the chambers and the platform is maintained during raising and lowering operation; such guide means being such as to enable the platform to be locked to the flotation system irrespective of the state of the sea.

Means for mooring the structure and preventing drift In the Continental Shelf waters, i.e., depths up to about 600' a normal anchoring system will be suflicient to hold the platform against any tendency to drift off location due to currents, wind forces and lateral waves forces. It is important however that the moorings should be so connected to the structure that the line of action of the cables should pass through or near the centre of pressure, i.e. the centre of area of the sections of the flotation chambers at the level of their tops, so that tilting couples due to changes in tension in the mooring cables induced by tendency to horizontal movement of the platform are avoided. Where the water is very deep or the sea bed unsuitable for anchoring it may be necessary to provide engine driven screws to neutralize the tendency to drift 1n the manner already proposed for other floating platforms.

For most purposes, it will be very important to ensure that the platform cannot move very far from location, partly to ensure that its position is always known to moving vessels, to which it would otherwise provide a navigational hazard and partly because in most circumstances accurate location will be important for its purposes. This is particularly true of a drilling platform where the tolerance in position is a matter of feet only, in relation to the well being drilled.

In the case of weather observation platforms and the like, it may be suflicient to adjust location by conventional navigational means, but where this is not sufliciently accurate, one possible way of detecting a tendency to move off location would be to provide a vertical cable extending to the sea bed, with means for detecting the change in the angle of the cable relative to the platform that would be caused by drifting. The means of detection could be suitable electrical contacts which could be used to control the drift opposing devices. This means should be satisfactory for use in deep ocean waters. The

cable would, of course, need to be kept in tension at all times.

Means for adjusting to tidal rise and fall In a number of applications it will be desirable to have constant freeboard i.e. the platform as a whole must be capable of rising and falling with changes of sea level due to tides etc. This means that the distance between the platform and the sea bed will vary. Means of allowing for this have already been suggested in the cases where the platform is stabilized by cables or legs extending to the sea bed. On the other hand, in certain applicationsoil well drilling, for example-it may be desirable to maintain the height of the platform above the sea bed constant so that the freeboard varies with rise and fall of sea level. This can be arranged by admitting air to or exhausting air from the flotation chambers in the manner already described. The control of the amount of air in the flotation chambers may be made responsive to the stress in the stabilizing cables where used or to a special cable linking the platform to the sea bed. In this latter case any tendency for the platform to rise bodily would increase the tension in the cable and this could be used to release air from the flotation system, and when there is a reduction of tension in the cable due to a tendency for the platform to descend bodily, this reduction may be made to operate an air compressor or, preferably, a valve to a compressed air reservoir kept continuously charged by a small air compressor. With a compressed air reservoir it might be possible to deal with short term tendencies to rise and fall such as might be caused by the Wave pitch corresponding with the spacing of the buoyancy chambers.

Means for adjusting in azimuth Where appropriate an anchoring system may be used to prevent rotation in azimuth as well as drift, but where the circumstances are such that an anchoring system is not practicable due either to the great depth of water or the nature of the sea bed, it may be necessary to provide special means for preventing rotation in azimuth and/ or for changing the position in azimuth as, for example, when it is desired to drill a large number of Wells from one location. One obvious way of adjusting in azimuth would be to use two or more screw propellers or reaction water jets at some distance from the centre of the platform and mounted in such a manner as to move the platform in azimuth only. The motors driving these screws or the pumps supplying the jets may be switched on or off in response to any conventional azimuth sensing device such as a magnetic, radio, or gyro compass.

In some instances it may be useful to have the deck structure capable of being rotated in relation to the flotation chamber assembly.

Use multiple platforms For bridges across deep water, or for very large artificial islands, it will usually be necessary to use a number of platforms of the type described, independent in respect of their buoyancy system but interconnected by rigid structure 100 (FIGS. 1 and 2). In some circumstances the stabilizing effect of this connecting structure will reduce the need for other stabilizing means; for example a pier in water above a rapidly shelving sea bed may comprise a floating structure of the type described linked to shore by a bracing system which permits vertical motion but is torsionally stiff and restrains lateral horizontal motion. Again, three or more structures of the kind described though individually unstable would be collectively stable if connected together by rigid bracing.

What is claimed is:

1. A buoyant structure comprising a platform, a plurality of spaced tubular flotation chambers disposed relative to each other and to the platform such that, in plan, the centre of area of the structure and the mean plan position of the centre of gravity of the structure substantially coincide, each flotation chamber comprising a tubular element of substantially uniform internal cross-section with its longitudinal axis disposed substantially in a vertical plane, each flotation chamber being generally devoid of surfaces against which vertical components of hydrostatic pressure can act, each flotation chamber having an open lower end portion, means closing an upper end portion of each flotation chamber, conduit means connecting together all of the plurality of flotation chambers, said flotation chambers and conduit means together at all times defining a continuous and uninterrupted air space whereby the pressureof air contained in any one chamber is at all times maintained substantially equal to the pressure of air in the remaining chambers, said flotation chambers being devoid of buoyancy means in the form of closed chambers and the sole means for imparting the total buoyant support for said structure is the air entrapped in said flotation chambers and conduit means, said structure being intrinsically unstable buoyancy-wise, and non-buoyant stabilizing means for insuring positive stability of the structure when afloat whereby the structure is unresponsive to wave motion and does not give rise to stabilizing forces due to buoyancy.

2. The buoyant structure as defined in claim 1 wherein the non-buoyant stabilizing means is a mass suspended fromsaid structure beneath said platform such that the centre of gravity of the structure is below the centre of buoyancy thereof.

3. The buoyant structure as defined in claim 1 wherein means are provided for vertically adjusting said platform relative to said flotation chambers.

4. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is a mass suspended from said structure beneath said platform such that the centre of gravity of the structure is below the centre of buoyancy thereof, and means are provided for vertically adjusting said platform relative to said flotation chambers.

5. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is an individual ballast mass suspended from and below each flotation chamber.

6. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is an individual ballast mass associated with each of said flotation chambers, means for rigidly suspending each ballast mass from its associate chamber, and said suspending means being secured internally of each of associated chamber at a point remote from said open lower end portion.

7. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is a ballast mass associated with eachflotation chamber, and three cables are attached respectively at their upper ends to the open lower end portion of each of said flotation chambers.

8. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is a single ballast mass suspended from at least three cables attached to widely spaced points of said platform.

9. The buoyant stnucture as defined in claim 1 wherein said non-buoyant stabilizing means includes atleast a single ba'llast mass normally disposed in a first position below at least one of said chambers, and means for moving said ballast mass into the interior of said one chamber in a second relative position of said one chamber and said ballast mass.

10. The buoyant structure as defined in claim -1 wherein said non-buoyant stabilizing means includes at least three outrigger booms extending outwardly from widely spaced locations on said platform, a mooring cable led over each boom for connection to an associated mooring on the sea bed, and means for maintaining a predetermined tension in each mooring cable and for compensating for variations in the depth of water.

11. The buoyant structure as defined in claim 1 including means for introducing air into the space defined by the chambers and conduit means at a rate at least equal to the rate at which air escapes the space to insure posi- 1 1 tive buoyancy of the structure in the event of damage to the conduit means or chambers.

12. The buoyant structure as defined in claim 1 including a pylon extending downwardly fromsaid platform and rigidly attached thereto, at least three parts of cables in the configuration of at least two tetrahedrons for connecting said pylon to moorings on the sea bed, one of each pair of cables extending from the lower end of the pylon to an associated mooring on the sea bed, and the other of each pair of cables extending from a point on said pylon spaced upwardly from said lower end to said associated mooring.

13. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is a mass suspended from said structure beneath said platform such that the centre of gravity of the structure is below the centre of buoyancy there-of, means are provided for vertically adjusting said plat-form relative to said flotation chambers, stabilizing means including at least one outrigger boom extending outwardly from said platform, a mooring cable associated with said boom for connection to an associated mooring on the sea bed, and means for maintaining a predetermined tension in said mooring cable and for compensating for variation in the depth of water.

14. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is a mass suspended from said structure beneath said platform such that the centre of gravity of the structure is below the centre of buoyancy thereof, means are provided for vertically adjusting said platform relative to said flotation chambers, and means for compensating for tilting movements of the platform resulting other than from atmospheric conditions.

15. The buoyant structure as defined in claim 1 wherein said non-buoyant stabilizing means is a mass suspended from said structure beneath said platform such that the centre of gravity of the structure is below the centre of buoyancy thereof, damping mean-s suspended from said structure, and said damping means including surfaces disposed in a generally horizontal plane thereby offering resistance to hydrostatic forces acting in a generally vertical plane.

16. The buoyant structure as defined in claim 3 wherein said vertically adjusting means includes cooperative vertically extending sliding guide means and locking means disposed between said platform and said plurality of flotation chambers for permitting relative vertical displacement therebetween and enabling the platform to be locked to the flotation chambers in anyone of a plurality of different relative vertical positions.

17. The buoyant structure as defined in claim 12 wherein said pylon comprises an inner and an outer element capable of relative telescopic movement, said cables being attached to said outer element, and said outer element being supported by an immersible buoyancy chamber 1'2 aflording positive buoyancy to the outer element sufiicient .to maintain continuous tension in the cables.

18. A buoyant structure comprising a platform, a plurality of spaced tubular flotation chambers disposed relative to each other and to the platform, each flotation chamber comprising a tubular element with its longitudinal axis disposed substantially in a vertical plane, each flotation chamber having an open lower end portion, means closing an upper end portion of each flotation chamber, conduit means connecting together all of the plurality of flotation chambers, ballast means associated with at least one of said flotation chambers, and means for hoisting said ballast means from a first position beneath said one flotation chamber to a second position in the interior of said one flotation chamber.

19. The buoyant structure as defined in claim 18 wherein said hoisting means includes a cable passing through an opening of said closing means, and means are provided for eflecting an airtight seal between the cable and the closing means.

20. The buoyant structure as defined in claim 18 including stabilizing means in the 'form of at least one outrigger boom extending outwardly from said platform, a mooring cable led over said boom for connection to an associated mooring on the sea bed, means for movably securing the boom to said platform, and compensating means disposed between said boom and the platform for maintaining a predetermined tension in said mooring cable thereby compensating for variations in the depth of water.

21. The buoyant structure as defined in claim 18 including stabilizing mean-s in the form of at least a single outrigger boom extending outwardly from said platform, said boom including a movable terminal end portion, a mooring cable led over said terminal end portion for connection to an associated mooring on the sea bed, and compensating means disposed between said movable terminal end portion and the platform [for maintaining a predetermined tension in each mooring cable thereby compensating for variations in the depth of water.

References Cited by the Examiner UNITED STATES PATENTS 1,511,153 10/1924 Armstrong 114-43.5 1,749,958 3/ 1930 Randell 1'1443.5 X 2,399,611 5/ 1946 Armstrong 114- 435 2,399,656 5/ 1946 Arm-strong 114-0.5 2,777,669 1/ 1957 Willis et al. 114-05 2,881,591 4/'l959 Reeve G l-46.5 2,889,795 6/ 1959 Parks 1140.5

FERG'US S. MIDDLETON, Primary Examiner.

MILT-ON BUCHLER, Examiner.

T. M. BLIX, Assistant Examiner.

Claims

1. A BUOYANT STRUCTURE COMPRISING A PLATFORM, A PLURALITY OF SPACED TUBULAR FLOTATION CHAMBERS DISPOSED RELATIVE TO EACH OTHER AND TO THE PLATFORM SUCH THAT, IN PLAN, THE CENTRE OF AREA OF THE STRUCTURE AND THE MEAN PLAN POSITION OF THE CENTRE OF GRAVITY OF THE STRUCTURE SUBSTANTIALLY COINCIDE, EACH FLOTATION CHAMBER COMPRISING A TUBULAR ELEMENT OF SUBSTANTIALLY UNIFORM INTERNAL CROSS-SECTION WITH ITS LONGITUDINAL AXIS DISPOSED SUBSTANTIALLY IN A VERTICAL PLANE, EACH FLOTATION CHAMBER BEING GENERALLY DEVOID OF SURFACES AGAINST WHICH VERTICAL COMPONENETS OF HYDROSTATIC PRESSURE CAN ACT, EACH FLOTATION CHAMBER HAVING AN OPEN LOWER END PORTION, MEANS CLOSING AN UPPER END PORTION OF EACH FLOTATION CHAMBER, CONDUIT MEANS CONNECTING TOGETHER ALL OF THE PLURALITY OF FLOTATION CHAMBERS, SAID FLOTATION CHAMBER AND CONDUIT MEANS TOGETHER AT ALL TIMES DEFINING A CONTINUOUS AND UNINTERRUPTED AIR SPACE WHEREBY THE PRESSURE OF AIR CONTAINED IN ANY ONE CHAMBER IS AT ALL TIMES MAINTAINED SUBSTANTIALLY EQUAL TO THE PRESSURE OF AIR IN THE REMAINING CHAMBERS, SAID FLOTATION CHAMBERS BEING DEVOID OF BUOYANCY MEANS IN THE FORM OF CLOSED CHAMBERS AND THE SOLE MEANS FOR IMPARTING THE TOTAL BUOYANT SUPPORT FOR SAID STRUCTURE IS THE AIR ENTRAPPED IN SAID FLOTATION CHAMBERS AND CONDUIT MEANS, SAID STRUCTURE BEING INSTRINSICALLY UNSTABLE BUOYANCY-WISE, AND NON-BUOYANT STABILIZING MEANS FOR INSURING POSITIVE STABILITY OF THE STRUCTURE WHEN AFLOAT WHEREBY THE STRUCTURE IS UNRESPONSIVE TO WAVE MOTION AND DOES NOT GIVE RISE TO STABILIZING FORCES DUE TO BUOYANCY.