US3299846A - Stable floating support columns - Google Patents

Description

Jan. 24, 1967 JARLAN 3,299,846

STABLE FLOATING SUPPORT COLUMNS Filed Jan. 18, 1965 4 Sheets-Sheet 1 I INVENTOR Gerard Eugene JARLAN PATENT AGENT Jan. 24, 1967 G. E. JARLAN 3,299,846

STABLE FLOATING SUPPORT COLUMNS Filed Jan. 18, 1965 4 sheets sheet .2

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I OI-- N I I I FQI 1 I I I I I p I g I I EU I I I I U1 V g I O I E I E I I I I I E I 1 PI- I P r I LL! 3 I I m II IL I Q I I I Q" I I l l I I I I I I l I n: 9 IH 3 r INVENTOR Gerard Eugene JARLAN RQJMM PATIENT AGENT Jan. 24, 1967 G. E..JARLAN 3,299,346

STABLE FLOATING SUPPORT COLUMNS v I Filed Jan. 18, 1965 4 Sheets-Sheet :5

INVENTOR Gerqrd Eugene JARLAN PATENT AGENT Jan. 24, 1967 G. E. JARLAN 3,299,846

STABLE FLOATING SUPPORT COLUMNS Filed Jan. 18, 1965 4 SheetsSheet 4 INVEN'I'OR Gerard Eugene JARLAN 1e. 9. VM

PATENT AGENT United States Patent O 3,299,846 STABLE FLOATING SUPPORT COLUMNS Gerard Eugene Jarlan, Ottawa, Ontario, Canada, as-

signor to Canadian Patents and Development Limited, Ottawa, Ontario, Canada, a corporation of Canada Filed Jan. 18, 1965, Ser. No. 426,032 13 Claims. (Cl. 114-.5)

This invention relates to marine floating platforms and particularly concerns cup-like floating structures of improved stability formed as upright cylindrical columns having perforated shell walls and perforated bottoms acting as wave energy absorbers, characterised by low wave scattering and decreased reflection from such structures when impinged by deep water waves.

A floating platform in the sea intended to support a superstructure carrying a weight of equipment and living quarters for such purposes as offshore drilling, satellite observation and monitoring, processing plants, and landing stages, requires to be as nearly immovable despite the motions of sea and air as can be achieved. The stability required for offshore drilling platforms restricts the heave displacement to a few feet and the roll to less than 5 degrees of arc, to prevent damage to the drill string, casing, and machinery. Prior art moored floating vessels and platform structures of any form have so far proven unsatisfactory due to their low stability even when exposed to moderate waves, and because of the destructive motion induced by larger waves.

Heretofore bottom-supported platforms have been favored as drilling and observation stations in the sea, but the high cost of such fixed structures in depths over a hundred feet becomes prohibitive, and their safety is questionable, as has been shown in the destruction of certain tower forms under combined wind and wave forces.

I have devised a highly stable cup-shaped floating structure which essentially consists in an upright shell-walled tube of diameter greater than its height and larger than sixty feet which provides the necessary buoyancy either solely by the displacement of its wall when the latter is a hollow annulus, or in conjunction with a central buoyant tank spaced from the wall, having the shell wall which is about three feet thick extensively perforated over its entire surface to achieve a perforation ratio of about 0.4 by a large multiplicity of transverse jet-guiding channels in the form of pipes or ducts having a diameter of about three to four feet, and having the lower end of the tube closed by a perforated shell disc bottom. When the structure is a right cylinder having a diameter preferably about 1.6 times the total height, it may be constructed with a favorable ratio of moment of inertia (I) about a centroidal axis parallel to the base, with respect to displacement volume (V), and with suitable metacentric height, so that its heaving and rolling responses will be small as compared with a structure of the same dimensions lacking the jet-guiding perforations.

Structures according to the invention may be made as broad as desired, and when built in sizes above about 100 feet diameter may be braced interiorly by a plurality of radial walls or septa which are likewise extensively perforated by transverse channels. Such structures may be designed for supporting massive superstructures of many thousands of tons weight, and can be ganged to carry causeways, landing fields, and other extensive civil engineering works. Where the superstructure is lofty, novel gust-deflecting screens are provided operating in conjunction with the exposed perforations over the freeboard portions of the structure.

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FACTORS AFFECTING STABILITY OF BODIES IN THE SEA Very large amplitude heaving'and rolling motions of a prior art solid-walled buoy floating in a sea excited to wave motion are caused by the combined lateral and vertical forces exerted on the buoy by the water. When the crest of a large wave reaches the buoy it'produces a displacement force which is proportional to the wave amplitude at the buoy. If the wave is even partially reflected by the buoy its amplitude at the buoy is increased, hence a very large floating vessel having an extended vertical surface normal to the direction of wave run will produce nearly doubled wave height. The extreme motion of such vessel would render it useless for most purposes. It will therefore be apparent that a floating structure ideally should cause the smallest possible reflection in order to keep the vertical (heaving) force and the lateral (rollproducing) force each as small as possible.

According to the theorem of Bernoulli, the surfaces of a body immersed in water are subjected to pressures which vary in accordance with the velocities of water masses adjacent the body, and also in accordance with the rate of change of these velocities. An explicit analysis of these forces for a body which is itself in motion under impingement by trains of waves of mixed periods and of various amplitudes is so diflicult that scale model studies in wave tanks are essential to reveal the effects of waves of given periods and amplitudes. A number of general observations may however usefully be made in analysing the factors that lead to achieving minimum heave and roll responses.

The natural heaving frequency of a floating body may be established by pushing the structure down slowly a small distance in calm water and then releasing it, and measuring the period of the resultant vertical oscillation. The degree of damping may also be inferred from the rate at which the amplitude of the oscillation diminishes. For low heaving, the natural frequency response should be well below the lowest disturbing frequency, or in other words the natural period of oscillation should be considerably longer than the period of the longest wavelength wave of large amplitude to which the body may be exposed. In the open sea at a given location the maximum wave height may be found, for example, to be 65 feet and the period of this wave to be 17 seconds; if the natural period of oscillation of the body is longer than 17 seconds, and preferably twice the wave period or longer, natural resonance cannot occur. The motion of. the body will be small, and will moreover appreciably lag the wave motion in time phase.

In considering the response of a floating body to forced oscillation, that is, to a periodic exciting force such as the force due to the varying height of water at the body caused by a wave, the magnitude of the disturbing function will depend on the actual displacement of the body at any moment with respect to its displacement under quiet sea conditions and the restoring force will be determined by the change of draft of the body at any moment. Since movement of the body with respect to sea water is opposed by friction, the form and area of the wetted surface must be such that an adequate damping factor operates. The larger the mass of the body, the smaller will be its heaving or rolling response, and a small heave amplitude results particularlywhen the apparent or virtual mass of water which partakes' of motion cophasally with that of the structure is .very large in comparison'with the actual displacement' of the structure. ,This' 'r'nay'be readily perceived by consideririg that while the-idistufbiiig function depends on the difference of displacement from resting displacement, the virtual mass may include a much larger water volume contained by or in proximity to the structures so as to be subjected to essentially the same accelerations as the structure itself. Therefore in evaluating acceleration forces and natural period of oscillation for a floating structure not only the mass of the structure itself must be considered but a certain imaginary water mass M, or hydrodynamic mass must be added to the actual mass to produce the total virtual mass. It will be apparent that by forming the body as a chamber having large surface-to-vol-ume ratio the virtual mass may be large in relation to the displacement for motions in certain directions only, i.e., at right angles to the larger cross-sectional areas. I have found that when a cylindrical cup-like body comprises an extensively perforated shell wall and a perforated shell bottom, the perforations being jet-guiding transverse channels, its hydrodynamic mass is decreased below that of a solid-walled tank of the same dimensions, but the decrease is very small in proportion to the reduction of Wave forces achieved. Consequently a high order of stability is possible for floating structures of such perforated form when their volume is sufliciently large so that the natural oscillation period is above 25 seconds, specifically comprising structures at least sixty feet in diameter. The latte-r show moderately good stability to medium-sized waves, while structures having a diameter of 200 feet or more and height 130 feet or greater are remarkably stable under very large waves.

It is necessary to stress that there exist non-linear phenomena in the definition of the virtual rnass and of the damping terms, which make very difficul-t the expression mathematically of the pitching, rolling and yawing of a floating body.

DISTURBING FORCES ACTING ON BODIES FLOATING There are essentially two sets of forces which contribute to the motions and accelerations of a floating structure exposed to waves:

It is to be noted that for waves of the usual range of period encountered in the open sea, namely from 5 to 20 seconds, these sets of forces will usually be out of phase.

As a consequence of the essentially complete freedom of motion of a floating body subjected to the aforesaid sets of forces, its motion comprises horizontal and vertical components of displacement which are out of phase, and includes a rolling motion which comprises rotation about a transverse horizontal axis in the structure. As will be shown hereinafter, the lateral forces to which a reflecting floating tank are exposed are directed relatively near the mean sea level and well above the centers of gravity and of buoyancy, and also usually well above the metacenter. Unless the magnitude of the resultant of the lateral forces is decreased and its point of application is lowered to reduce the roll-producing moment, the roll angle will be large. It may therefore be readily seen that stability can be directly correlated with reduction of reflection and wave scattering.

Roll-producing forces-Hydrostatic 7 pressure The most significant component of lateral force exerts a normal hydrostatic pressure against the forward vertical surface of the structure and is maximum when the horizontal dimension of the structure measured in the direction of Wave run is a half wavelength, so' that as the forward side lies at the wave crest its rearward side is in the wave trough. It is computed from the relation:

where:

P is the pressure at any depth z below mean sea; h is halfthe wave height, including reflection;

p is the density of water (mass per unit volume); g is the acceleration due to gravity, and

L is the deep water wavelength.

The normal horizontalpressure is highest at a depth equal to the peak wave height below high-water level, and diminishes exponentially below that level, becoming smaller than about 0.1 times its peak at a depth under 0.4 wavelength. When the structure is a cylinder, the pressure component which is aligned with the direction of wave run diminishes to either side of a vertical line in the most forward part of the body, and the height of the wave also decreases along its intersection with the cylinder surface, so that at any level the horizontally applied thrust may be taken as approximately equal to the peak frontal pressure effective over an area of about 40% of the diametral transverse plane.

Roll-producing forces-Ram pressure The second lateral thrust component is associated with the piling up or ramming effect against the structure due to the momentum of moving sea Water, and its peak value may be found from the relation:

i -fl (2) where P is the ram pressure per unit area at any depth z.

The ram pressure will usually not exceed about a quarter of the pressure head P and is computed over the area of the body as projected on a vertical diametral transverse plane.

Roll-producing f0rces.Drag force where P is the drag force at any depth z; C is the drag coefficient for thebody.

The magnitude of this thrust component is about onetwelfth of the value of P and is dependent on the ge0metry and surface form of the body. This force dimin ishes exponentially with depth and is small below about wavelength depth for large wave iarnplitiude, and at any level is computed over the area of the body as projected on a vertical diametral transverse plane.

In relations (2) and (3) wherein h is squared, the resultant lateral force components will obviously be considerably increased even for small reflection.

Roll-producing f0rces.Wind thrust In areas where large waves will impinge the structure, a free'board height of at least thirty-five feet or more, up to about fifty feet, will be necessary. The lateral force exerted by Winds gusting at velocities up to knots, as can be expected in hurricanes, can produce a lateral thrust and roll-producing moment comparable With the thrust and moment due to the sea, particularly if the superstructure carries a derrick or towers. It will be seen that the minimumizationof wlind roll becomes a prob= lem inseparable from the general stability design prob:

"body, and the energy of the flow dissipates lem. As will be made evident hereinafter, novel gustdefiectin-g skirt structures are provided about the uppei periphery of the body, to develop upward jet flow of air, which then deflects the wind as an upward wave of very large dimensions, protecting the superstructure from the .blast. Moreover, as will be shown, the perforations of the freeboard portion absorb a portion of the wave energy of the wind and also part of the wind kinetic energy, minimizing the drag markedly.

H cave-producing forces The heaving force P exerted upon the structure arises from the combined upward ram and drag forces due to the vertical motion of the sea underneath it. The heaving force has its largest magnitude when the body has a span of a half wavelength and the crest of the 'wave is at the forward side of the body and the rearward side lies in the wave trough. The resultant force is centered on the bottom when the wave is in the position described, for a cylindrical body, and for other phases of a single period wave the center of action shifts forwardly and rearwardly from the geometric axis. Therefore the heaving force P also contributes a moment about the center of gravity, G, tending to produce roll.

Since the force is proportional to the plane area of the bottom and to the wave height as a function of depth, it will be apparent that a solid structure which causes a large reflection must experience a correspondingly larger heaving thrust.

When the structure is formed as a shell-walled cylinder which is hollow and the wall is buoyant, for example having a diameter of 200 feet and a wall thickness up to four feet, the plane area of the annulus is only a few percent of the total end area of the cylinder, so that the structure will be favorably proportioned for minimum heaving force and will have a large moment of inertia about any horizontal diametral axis. When a perforated shell disc bottom closes the structure the heaving force is far less than that which would be exerted on a solid disc of area equal to the unapertured area of the perforated shell bottom, and the virtual mass of the structure'is greatly increased, while the presence of the perfor-ated bottom provides a large damping factor.

WAVE ENERGY ABSORPTION IN PERFORATED SHELL BODIES The roll-producing and heaving moments acting on a body become much smaller when the shell Wall and shell bottom are extensively perforated over their entire areas by channels of circular cross-section having diameters of three to four feet and spaced uniformly apart so that the sum of channel cross-sectional areas comprises from 30% to 60% of the cylinder and disc surfaces, and preferably about 40% thereof, the channels having jet-guiding form. When the water level at the outside surface of the body is higher than the level inside the Wall, a hydraulic head acts to propel water along all the horizontal channels, causing a very large translation of water which no longer par-takes of characteristic wavernotion. Instead of the orbital particle motion which the seawater had outside the wall, once it has been transported through the wall it is characterised by coherent jet flow issuing from the orifices normally of the wall and persisting for upwards of twenty or more diameters inwardly from the wall. It is to be noted that while the advancing Wave is made up of particle motions which range from upwardly, through horizontally forward-1y at the crest, to downwardly, and while any forward velocity component is added to the horizontal jet Vfi'lOCdtY resulting from hydraulic head, no vertical component is transmitted through the wall. Similarly, no horizontal component of velocity is transmitted through the shell bottom. Consequently no wave propagation processes can exist within the itself in turbulence and ultimately as heat.

jet-guiding channel desigr'i When the jet-guiding channels have lengths and diameters each between three and four feet, they prove to be most effective to convert the potential energy of a wave incident on the wall into kinetic energy of jet flow in the channels. The hydraulic head acting to produce jet flow may be as small as a few inches for optimally designed channels, and flow rates under a head of only a few feet, when the initial horizontal velocity component of particle orbital motion for a large amplitude wave is superimposed, can exceed thirty feet per second. It the channel length is made appreciably shorter than about 0.8 times its diameter the jet-guiding action and head loss through friction becomes less effective, and if the channel length is below about 0.6 diameter it virtually ceases to guide the flow or form a normal jet and allows substantial wave oscillatory energy to be transmitted through the well. On the other hand when the channel length is increased beyond one diameter, say to four and a half feet for a three foot diameter channel, the friction loss increases substantially and adversely affects the jet velocity, making the mass transport of water less efficient and thereby increasing the wave scattering from the wall. In addition the drag force on the structure is increased.

The stability of the novel buoyant structure of my invention under wave attack is principally due to its inherent capacity for absorbing incident wave energy and transforming it to a form by which minimum force is exerted on the structure without reflecting or scattering a substantial amount of wave energy. The apertured wall acts as a sort of barrier, slightly impeding the wave, and thereby developing a hydraulic head across the ends of the channels. This hydraulic head generates a large volume jet flow through the wall along the axes of the channels, as described, which flow has kinetic energy equal to the greater part of the original wave energy. The injected jets penetrate into the turbulent, aerated water within the space bounded by the shell wall, and adjacent streams impinge each other at small angles. For optimum energy dissipation the jets should flow along flow paths unobstructed by solid structures, for distances of about forty feet or more. The flow within the space will be substantially enough to produce an increase in water level as the jets diffuse within the space and die out, and this head will give rise to outward jets issuing from the rearward sector of the shell wall into the wave trough as the relatively more rapidly travelling sea wave passes to the rearward side of the body. A relatively smaller forward flow through the forward sector also occurs in response to a drop in wave height below the level within the body, contributing to the internal turbulence of the oncoming wave. In a similar manner, reciprocatory flow through portions of the disc base have a quieting effect on the wave motion in depth below the structure.

By virtue of the action described, the magnitude of all of the lateral component forces and of the heaving force is reduced to a fraction of the corresponding thrusts exerted on a solid-walled tank or buoy of the same diam eter. The reduction in total wave force is achieved not only because of the reduced wave height obtained by absence of significantly large reflection effects at the frontal exterior surface and at the inside surface of the rearward wall sector, but also since there is no wave motion within the body, there can be no wave reflection MODEL STUDIES In verification of the foregoing observations, model te'sts were' earried'out using perforated thin tubular walls formed as simple circularly apertured sheet screens, the

openings of which were short axially as to be ineffective as jet-guiding channels, and showed that such screens effectively transmit incident waves. The amplitude of the transmitted waves was found to be only slightly decreased, the screen acting merely as a resistive barrier which did not alter the wave motion to any large degree. Such thin screen wall moreover resulted in significant wave reflection from the frontal surfaces of the forward and the rearward sectors, in proportion to their unapertured area. Any solid-walled structure within a space bounded by the screen, such as a buoyant tank, would be subjected to nearly the full force of the wave, and also would experience the added thrust due to wave scattering from the body surfaces.

I have found from corresponding model tests carried out using perforated, thick shell-walled buoyant cup bodies apertured to retain more than half of their exterior surface area intact, having circular openings formed as transverse channels of length equal to their diameter and scaled to represent three feet, that the impinging waves imposed a smaller thrust than the thrust exerted on the aforesaid thin screen-walled body of identical aperturing ratio and dimensions. The thrust contributions due to hydrostatic pressure head and ram pressure exerted on a correctly designed perforated thick-walled body have been estimated to be only slightly greater for a given height of incident wave, than the thrust accounted for by the fluid friction of jet flow along the channels. By reason of the large flow capacity through the wall, any reflected wave amplitude automatically increases the energy conversion as higher velocity jets, tending to reduce the incident wave.

In model studies of the action of jets of seawater streaming radially inwardly and outwardly of the shell wall and persisting for distances of ten or more channel diameters, it was observed that a volume of water surrounding the model was affected, wherein the internal turbulence of the oncoming wave was increased. In this zone of turbulence the wave potential energy was somewhat lessened as the wave propagated therethrough. The highly turbulent state of this water mass and the well organized issue of water jets horizontally outwardly can be regarded as the reason for the very low observed drag on the structures, this hypothesis being consistent with the observed drop in drag on cylindrical bodies for flow around them at very large Reynolds numbers.

The invention is more specifically disclosed as toits construction and realization, by and in the following description of its preferred embodiments, as illustrated by the accompanying figures of the drawing, wherein:

FIGURE 1 is an elevation view partly in section showing a floating drilling platform constructed according to the present invention;

FIGURE 2 is an enlarged horizontal section through the wall of the body of FIGURE 1 taken on line AA therein, showing compartmenting and wall bracing details;

FIGURE 3 is an enlarged vertical radial section along line 3-3 of FIGURE 2 through the wall, the view being at right angles to the section of FIGURE 2;

FIGURES 4, 5 and 6 are elevation views of a floating body similar to that of FIGURE 1 but in reduced scale, showing the heaving and roll-producing forces acting on the body;

FIGURES 7, 8 and 9 are elevation views of a prior art floating tank of the same proportions and size as the bodies of FIGURES 4-6, showing for comparison purposes the heaving and roll-producing forces acting on the body;

FIGURE 10 shows one form of mooring arrangement for the platform support of FIGURE 1;

FIGURE 11 is an elevation view in vertical diametral section showing a platform support having a large central compartmented buoyant chamber;

FIGURE 12 is a plan view in horizontal section taken on line B-B of FIGURE 11 illustrating bracing septa and variable buoyancy arrangements;

FIGURE 13 is a partly cut away view of a superstructure as in FIGURE 11;

FIGURE 14 is a side elevation view of a freely floating structure showing integral propulsion means;

FIGURE 15 is a partly cut away view in plan of the structure of FIGURE 14; and,

FIGURE 16 is -a vertical section in enlarged scale taken on line C-C of FIGURE 12 showing gust-deflecting means.

Referring to the drawing, one form of floating support structure according to the invention comprises a buoyant cup-like body generally designated 10, having a wall 11, a deck 12 carried above the highest reach of waves such as a wave 13, and being closed over its lower end by a perforated bottom 14. The deck 12 supports a derrick 15, representative of massive loads intended to be supported by the structure in applications other than the well-drilling operation shown herein. A super-structure 16 is carried by the deck, comprising living quarters and storage for machinery and supplies. Mooring chains 17 are shown attached at spaced intervals about the lower margin of wall 11, depending as catenaries from eyes 18, and extending radially outwardly to suitably large weighted masses (not shown) anchoring the outward ends of the chains.

The illustrated structure has a diameter greater than the total height, the 'latter being comprised of a freeboard portion 19 somewhat greater than the maximum amplitude h of wave 13 including reflection component, and including the upper half of an intermittently wetted wall portion 20, and also including a totally immersed wall portion 21. When the structure has a diameter of about 210 feet, the height may be about feet including a freeboard of 40 feet.

A central tube 22 coaxial with the geometric axis of the wall 11 is coextensive with the wall and secured in deck 12 and bottom 14, being of sufficient diameter to contain a casing 23 and drill string (not shown).

As may be the better understood from FIGURES 2 and 3, the Wall 11 is extensively perforated over its whole area by a large number of spaced circular holes 24, whose aggregate area comprises between 0.3 and 0.6 times the wall surface area, and preferably is close to 0.4 times the area. Each hole extends transversely through the wall, whose thickness is from three to four feet, and forms a cylindrical passage 25 serving as a jet-guiding channel for movement of seawater therealong. Where the wall 11 comprises spaced concentric cylindrical plate walls 26 and 27, the channels comprise steel tubes 28 having their ends welded into apertures in the plate walls. The space 29 between the plate'wal'ls comprising that portion of the annulus excluding the volume occupied by the tubes 28, provides the buoyancy required to support the structure and its loads, and may be pressurized as 'by compressed air. Variable freeboard height is provided by pumping water into selected sub-spaces 30 of space 2% separated by vertical end walls 31 by means of pipes 32 leading to pumping equipment (not shown) on the deck. The vertical walls 31 may comprise channels secured integrally along their length between plate walls 26, 27, thereby stiffening the structure to resist deformation by the forces exerted on any sector of wall 11 by the waves.

Further bracing comprises a vertically spaced series of annular decks 33, each of which is disposed in a diametral plane. The radial breadth of the decks may be several times the length of the tubes 28, and may for example be twelve to twenty feet in very large structures. The outer margins of the decks are secured to the inner ,plate wall 27 by peripheral angle members 34, the inner margins of which are stiffened by a peripheral composite channel and angle combination, 135, 136.

The shell bottom 14 preferably is pressurized and airfilled and ballasting is provided in the basal portion of wall 11, so that its mass contribution will add more effectively to the moment of inertia of mass. It is nevertheless feasible to also make the bottom compartmented and to hold ballast, the latter comprising a dense aggregate filling (not shown) for the purpose of lowering the center of gravity of the structure and ensuring a positive metacentric height.

Referring to FIGURES 4 to 9 inclusive, these diagrams show wave force data obtained with scale model tests of a perforated shell-walled cup-shaped buoyant structure 10 (in FIGURES 4-6) and of a buoyant tank 10' (FIG- URES 7-9), each structure 10 or 10' having identical outer dimensions and masses and representing a full-size structure of diameter 200 feet. The models were moored from beneath to float in deep water 36 in a testing basin (not shown), and prototype waves 13 were generated and caused to impinge the models. The wave states diagrammed show a single period wave of length L such that the models spanned a half wavelength, the wave propagating from left to right, and with the crest of the wave incident on the left side of the structure. Orbital directions throughout the wave mass are shown by hollow arrows 35.

. The magnitudes of P P and P as diagrammed by the curved dashed outlines of vector groups 37, 38, 39, and 40, 41, 42, relate to the hydrostatic pressure, ram pressure, and drag forces acting respectively on the structure according to the invention and upon the prior art tank form. In addition the respective magnitudes of P are diagra-mmed by curved dashed outlines of vector groups 43 and 44. Throughout the measurements, it was apparent that the novel structure rested quietly even under large prototype waves, whereas the tank form 10' had a large motion under prototype waves of height 16 feet, and under larger waves, of thirty-four prototype feet, its excursions were so violent the test had to be stopped.

The improvement in stability of the perforated shell model to waves of periods 8-12 seconds was shown by its heave response which was about one-sixth of the heave response of the tank 10. For a wave period of 12 seconds and wave height of 16 prototype feet the heave response was found to be of the order of 1.8 feet with an out-of-phase surge amplitude of about 1.5 feet and roll angle under degrees. At all wave periods, the tests conclusively demonstrated that the heave response of the shell model was never larger than about 22% of the response of the tank. The natural heave frequency was equivalent to a full scale heave period above 60 seconds, and the natural rolling frequency was equivalent to a full scale period of about 26 seconds.

Referring to FIGURE 10, a mooring arrangement for moderate water depths, egg. from about 100 to 1000 feet, comprises opposed pairs of massive reinforced concrete anchor blocks 45 of greater lateral extent than height, resting on seabed 46 below the platform. Depending chains 17 secure-d by their one ends to eyes 18 fast in the bottom margin of wall 11 connect loosely with loops 47 providing allowance for tidal depth changes. A sim'lar array of opposed pairs of catenary chains 17 angular y spaced with respect to the depending chains are similarly secured by their far ends (not shown) to anchor blocks. A strengthened well casing 48 extending from beneath the sea to the upper surface of the platform provides sufiicient deflection to accommodate lateral (surge) displacement. The chains should be adequately strong, for example each should sustain several hundred tons loading.

A monolithic floating perforated shell-walled structure illustrated in FIGURE 11 and FIGURE 12 is constructed of reinforced concrete. A coaxial cylindrical pressure vessel 49 coextensive with the structure has a diameter such that the annular space 50 between the inside of the shell wall 11 and the exterior surface of solid wall 51 is at least forty feet, and preferably is considerably more than fifty feet, to provide room for diffusion of jets flowing inwardly of the channels 25. The tank has a closing bottom wall 52, which may be coplanar with the perforated annulus 14' or which may alternatively depend a considerable distance below the bottom margin of wall 11 as necessitated by buoyancy requirements. coaxial narrow access tube 22 is provided in those applications requiring tools and materials to be lowered into the sea, extending beyond the vessel bottom 52.

The lower portion of the space 53 within the vessel 49 is partitioned radially by members 54 to form compartments in several levels, and occupying the bottom third or half of the vessel. Each compartment in each level is provided with communicating pipelines 32 for admission of pressurized air from a source (not shown) in the superstructure 16. It will be readily understood that the admission of air to expel ballasting water in selected compartments is a standard technique for control of floating height of a structure, and various methods of monitoring the displacement to signal change of draft may be employed, including fully automated depth control systems which per se form no part of the present invention.

When vessel 4% has a diameter such that its bottom area is about one-eighth of the plan area of the whole structure, a very large buoyancy is provided, while the incerase in heave responsive bottom area is relatively small, and the increased load-supporting capacity improves the virtual mass considerably.

As shown in plan view, FIGURE 12, the monolithic structure illustrates a floating observatory station, and includes a plurality of uniformly spaced upright radial wallshereinafter designated septa 56, which are coextensive with wall 11 and are integrally joined therewith and also with the deck 12, the annulus 14, and wall 51. The number employed will depend on the size of the structure, and may comprise from three upwards. Each septum preferably has a wall thickness of two feet and may be perforated by transverse cylindrical channels of any diameter from about two to four feet and preferably with an aperturing ratio of from 0.3 to 0.6. The primary function of the septa is to brace the structure and to transmit the thrust loads exerted on sectors of wall 11 to the rest of the structure including the deck and bottom. Because no wave propagation within the space 50 exists, the channels need not be truly jet-guiding as required for wall 11 or bottom 14', and therefore they may be axially short.

As shown in FIGURE 13 the deck 12 for floating structures of larger sizes, for example having diameters upward of feet to several hundred feet, comprises a system of primary girders, including thickened beam portions 57 radiating inwardly and carried into the upper margins of the septa, and chordal members 58 having their one ends supported on the upper margin of wall 11 midway between the beams 57. Floor-supporting members 59 in turn are carried between the primary girders and the wall margin.

In depths of water where it would not be practical to use chains secured to anchors on the seabed, navigable and dirigible floating structures as depicted by FIG- URE 14 and FIGURE 15 are provided, having opposed propulsion devices such as propellor 60 and balanced rudder 61 carried on the outboard ends of radially extending tubular housings 62 fixed in the wall 11. The housings are preferably mounted as low as possible to minimize wave disturbance at the propulsion units. The latter require to be capable of developing a thrust sulficient to hold the structure in position regardless of the state of the sea and wind, and hence to be able to move the structure in a calm sea at a speed up to five knots. The propulsion and directional controls and power are carried by way of ducts 63, to a conning position 64 in the superstructure. High-powered prime movers 65 and energy converters 66 are used to supply propulsive power and to operate the station.

Referring to FIGURE 16, a gust-deflecting peripheral flange 67 which may be from fifteen to fifty feet in slope length extends peripherally upwardly and outwardly from the margin of wall 11, presenting an apertured conic surface 68 to the wind from any direction. A large plurality of ducts 69 having their lower ends seated in the apertures extend inwardly upwardly from face 68, being curved and changing direction between their nearly horizontal entry and discharge ends by at least 70 degrees, so that the flow of air entering the ducts issues as a nearly vertical stream. The ducts are closely packed so that their total cross-sectional area is nearly equal to the projected area of flange 67 on a vertical plane. The duct lengths are preferably greater than their cross-sectional width, the latter dimension being at least a foot and preferably two to three feet. The duct cross-section may be of any shape, and for minimum head loss should be round or square with rounded corners.

The ducted flange is subjected to only a small drag force due to the wind as compared with the drag force which would be exerted on the superstructure 16 exposed to the wind in the absence of the flange. The incident wind, over a sector which may exceed 90, is deflected efficiently upwardly with little loss of kinetic energy. The reaction force on the flange therefore has a small drag component, T which is directed horizontally, and a larger negative lift component, T directed downwardly. Component T therefore provides a turning moment opposing the roll-producing wind force T (see FIG- URE 1) on the freeboard area. The upward stream of air in turn deflects the main air stream approaching the structure, causing a large standing wave of air shielding the higher parts of the superstructure for a height which is several times the vertical height of flange 67.

Because the exposed wall 11 is apertured, the drag force of wind on the structure is considerably less than the force that would be exerted on a bluff wall, and the turbulence in the air created adjacent the wall by reason of the extreme irregularities of wind velocity just above the sea further decreases the wind drag on the wall surfaces.

I claim:

1. A floating support of span at least sixty feet comprising an upright cup-like body of transverse dimension greater than its height but less than twice the height, said body having a shell wall about three to four feet thick, said wall being hollow and extensively perforated over its surface by a large plurality of transverse jetguiding channels, and a transversely perforated disc bottom closing the lower end of said body.

2. A floating support of span at least sixty feet comprising an upright cup-like cylindrical body having a diameter of substantially 1.6 times the height, said body being hollow and having a wall thickness of three to four feet and a closing bottom of the same thickness, said wall and said bottom being extensively apertured over their entire area by a large multiplicity of transverse jetguiding channels of diameter three to four feet, said channels having a total cross-sectional area comprising from 0.3 to 0.6 times the surface area of said wall and said bottom.

3. A floating support as set forth in claim 2 wherein said wall and said bottom are buoyant and said wall has at least its bottom portion radially compartmented, and arranged for variation of its buoyancy.

4. A floating support as set forth in claim 3 wherein said total cross-sectional area is about 0.4 times the Surface area of any of said wall and said bottom,

5. A floating support as set forth in claim 2 wherein said wall carries mooring cables secured to its lower mar- 6. A floating support as set forth in claim 2 wherein said wall carries laterally projecting opposed tubular housings mounted adjacent the lower end of said wall, said housings supporting propulsive and steering devices.

7. A floating support as set forth in claim 2 wherein said wall supports a deck above high water, and a tubular member extends through said deck and said bottom apertures to provide access to the sea below said support from said deck.

8. A buoyant marine structure for supporting a load above the waves comprising a hollow cylindrical column defining a central space and having a freeboard extending above high water and having a diameter larger than about sixty feet, said column having a shell wall braced to resist deformation under wave attack and having a radial thickness between three and four feet, said wall being extensively apertured by a large plurality of transverse jet-guiding channels of diameter between three and four feet over its surface to provide an aperturing ratio of from about 0.3 to about 0.6, and providing flow passages connecting said space with the sea, and a perforate disc closing the lower end of said column, the height of said column being about five-eights its diameter.

9. A marine structure as set forth in claim 8 wherein said bracing means comprise a plurality of upright radial septa extending inwardly from said shell wall and joined together at their ends, said septa being coextensive with said column.

10. A marine structure as set forth in claim 8 wherein a cylindrical pressure vessel is disposed coaxially of said structure having its wall spaced from said shell wall by a distance of at least forty feet, and said pressure vessel and said shell wall are connected by a plurality of peripherally spaced upright septa coextensive with said column.

11. A marine structure as set forth in claim 8 wherein said bracing comprises annular rings and vertical girders, said shell being comprised of inner and outer spaced cylindrical walls joined by transverse tubes opening through said walls and terminating in said walls, and said girders are disposed between said walls, said shell having closed upper and lower ends.

12. A marine structure as set forth in claim 10 wherein said pressure vessel has at least its lower portion subdivided to form gas-tight compartments, said compartments being provided with pressurizing and flooding lines for variable buoyancy.

13. A marine structure as set forth in claim 1 wherein said body supports a deck above high water and a peripheral outwardly rising flange, said flange being apertured and carrying a large plurality of ducts opening outwardly with their lower ends secured in said apertures, the upper ends of said ducts being nearly vertical.

References Cited by the Examiner UNITED STATES PATENTS 2,475,888 7/ 1949 Hackett 61-46 2,476,309 7/ 1949 Lang. 2,488,542 11/1949 Houghtaling 114-43.5 3,191,388 6/1965 Ludwig 1140.5 X 3,191,570 6/1965 Henderson 114144 3,224,401 12/1965 Kobus 114-405 MILTON BUCHLER, Primary Examiner.

T. M. BLIX, Assistant Examiner.

Claims (1)

1. A FLOATING SUPPORT OF SPAN AT LEAST SIXTY FEET COMPRISING AN UPRIGHT CUP-LIKE BODY OF TRANSVERSE DIMENSION GREATER THAN ITS HEIGHT BUT LESS THAN TWICE THE HEIGHT, SAID BODY HAVING A SHELL WALL ABOUT THREE TO FOUR FEET THICK, SAID WALL BEING HOLLOW AND EXTENSIVELY PERFORATED OVER ITS SURFACE BY A LARGE PLURALITY OF TRANSVERSE JETGUIDING CHANNELS, AND A TRANSVERSELY PERFORATED DISC BOTTOM CLOSING THE LOWER END OF SAID BODY.

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