Patent Publication Number: US-6213045-B1

Title: Flotation system and method for off-shore platform and the like

Description:
RELATED APPLICATIONS 
     This present application is a Continuation Patent Application claiming priority benefit of U.S. Ser. No. 09/384,160 filed on Aug. 27, 1999, now abandoned, which is related to, based on, and claims priority from, Applicant&#39;s Provisional Applications S.N. 60/102,564 filed on Sep. 30, 1998, Ser. No. 60/102,393 filed on Sep. 29, 1998, Ser. No. 60/102,367 filed on Sep. 29, 1998, and Ser. No. 60/098,311 filed on Aug. 27, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a flotation assembly, such as an off-shore floating platform or other structure, and also to a method related to the same. More particularly, the present invention relates to such an assembly and method where an auxiliary flotation support section or sections are provided for floating support to the assembly in a particularly effective manner. 
     1. Background of the Invention 
     For many years, the oil and gas industry has used off-shore fixed platforms resting on the sea floor, to drill and extract oil from under the ocean. More recently, the need to explore and produce in deeper waters requires the employment of floating platforms, since a fixed platform could not be designed for operation at those greater depths. 
     In many respects, the considerations and problems relating to the design, construction and operation of floating off-shore platforms are analogous to those of the design, building and operation of ships. Therefore, it was only natural that those who were engaged for the design of such floating platforms had a background in, or at least derived much of their information from, the ship building industry, particularly since there already exists a large body of design and fabrication expertise in that industry which would be directly applicable to the task of designing, constructing and operating floating platforms. 
     Thus, the current method of providing flotation for off-shore platforms is to construct a steel structure, commonly called a “hull”, that displaces an amount of water equal to its own weight plus the pay load. Generally the upper support surface of the hull is located about 40 to 60 feet above the water level, and the hull would normally extend downwardly below the water surface from 100 to 200 feet. The hull must resist the external pressure of the water below the water line, and this is accomplished by using steel plates, reinforced as needed with internal ribs, stiffeners, bulk-heads and bracing. Further, the entire structure must have sufficient structural strength to withstand the external loads imposed on them from the wind, waves, and possibly other sources. 
     2. Related Art 
     A search of the U.S. patent literature has disclosed a number of patents related to flotation support structures of various kinds, some of these being related to off-shore drilling, and these are the following: 
     U.S. Pat. No. 5,435,262 (Grinius et al.) discloses a semi-submersible off-shore platform comprising a hull having a plurality of stabilizer buoys. Each of the buoys is coupled to the hull and is positioned adjacent to the peripheral edge of the hull. There is a system for stabilizing the semi-submersible platform. 
     U.S. Pat. No. 4,422,803 (Wetmore) discloses an off-shore structure which has at least two similar prefabricated concrete modular subassemblies interconnected in a vertical manner to define a horizontal interface between each pair of subassemblies. There are vertically disposed sheer resistant pin means between the subassemblies and the cement at the interface to secure them together. 
     U.S. Pat. No. 2,552,899 (Manes), shows a “floating drilling rig” that is provided with tanks that provide flotation as it is moved to the drilling site. 
     U.S. Pat. No. 3,572,041 (Graaf) disclosed a spar type floating production facility for under water oil and gas wells. There is a spar section  13  which supports what is called a “super structure component  14 ” that is located above the water surface  16 . The upper part of the spar section  13  consists of  6  large diameter, hollow vertical columns  101 - 106  which are interconnected by a large diameter, hollow grid  107  and a small diameter, hollow upper grid  108 . 
     U.S. Pat. No. 4,126,011 (Lamy et al) relates to an off-shore tower structure, and deals primarily with the method of fabrication. It appears that there is a support structure adapted to rest on the bottom of the body of water and there is another member extending upwardly in the body of water. The platform is made up of two pieces which are floatable. The patent describes in detail the method for assembling and positioning the same. 
     U.S. Pat. No. 4,234,270 (Gjerde et al) shows a support element  6  of cells  10  that surround the upper end of the structure and holds the platform above the sea surface. 
     U.S. Pat. No. 4,310,052 (Rivertz) show an upright platform supporting member  7 , that is provided with surrounding elements  6  that appear to serve as floats. 
     U.S. Pat. No. 4,702,648 (Stageboe et al) shows a platform structure in which there is an anchoring base  2 , that rests on the sea floor. The upper, floating section which is also formed of joined cylinders is joined to the lower location by a tension element. The upper section is formed of relatively elongated cylinders, some of which are long enough to act as supports for the platform. 
     U.S. Pat. No. 4,740,109 (Horton) shows a compliant buoyant tower which is shown in FIG. 19, as being kept upright by a group of elongated cylinders  96 , composed of tank like elements  98 . 
     U.S. Pat. No. 4,766,836 (Behar et al) shows an off-shore tower which is formed of a central buoyant element and a series of tank elements secured around the central element. The outer elements are used for storage. 
     U.S. Pat. No. 5,038,702 (Bowes) shows what is called a “semi-submersible platform” which is supported on columns with pontoons extending between an outboard of the columns. This is provided with pitch stabilization and motion phase control devices such that when the platform is in the “drilling mode”, the platform is able to ride with the storm waves. 
     U.S. Pat. No. 5,088,858 (Massoudi) relates primarily to a method of manufacturing and installing what is termed an “artificial island”, such as a column, pile, harbor, and also a drilling platform capable of Withstanding icebergs, etc. There is a structure  1  having a polygonal construction which has a number of anchors  7 . This is surrounded by what is called an “annulus” of pontoons  16 . There is a detailed description of the method of installing this, and this can be summarized by reading claim  1 , beginning on line  17  of column  10 . 
     U.S. Pat. No. 5,375,550 (Innis) shows a floating platform where there is a plurality of cylindrical air filled platform modules, each having an open bottom end and a closed top end cap  18  of a square configuration. The trapped air chambers are interconnected to increase stabilization of the platform, and the caps  18  collectively provide the support platform. 
     U.S. Pat. No. 5,435,262 (Grinius et al.) shows a semi-submersible off-shore platform having stabilizer buoys  108  attached to the upper surface of the hull. 
     U.S. Pat. No. 3,933,108 (Baugh), shows a system for attaching floats to the riser. The purpose is to make the riser sections sufficiently buoyant to substantially reduce the need for tensioners or other similar apparatus for applying tension to the riser column. 
     SUMMARY OF THE INVENTION 
     It is long been recognized that the use of steel in the construction of off-shore platform hulls requires that a large mass of steel be used, thus producing a high weight-to-buoyancy ratio. In other words, many pounds of steel must be used to produce relatively few pounds of buoyancy. Thus, the hull must be sized to float not only the pay load but also to float itself. 
     Further, steel is prone to corrosion from the marine environment. In spite of coatings, and due to the need for cathodic protection, the maintenance costs of steel structures is high, adding significantly to life cycle cost. Accordingly, it is an object of the present invention to provide a flotation assembly which is particularly adapted for off-shore platforms and other such structures, which maintains the benefits of current design and construction of off-shore platform hulls and other such structures and yet which provide a lower weight-to-buoyancy ratio and other related benefits. 
     More specifically, the present invention enables the effective use of other materials, such as fiber reinforced plastic, that are more corrosion resistant in a marine environment. Further, embodiments of the present invention can provide reduced cost of construction and maintenance, thereby reducing the life cycle cost of operations. Further, in certain preferred embodiments the present invention can be engineered/designed/built using existing technology and construction practices and that can be implemented by use of multiple parts of the same size and design. Other objectives and advantages will be more apparent from a consideration of the following text and drawings. 
     In the design and operation of the present invention, the body of water in which the invention operates (i.e. the ocean or other body of water) can be considered as having a water surface, a “wave impact zone” which is adjacent to the water surface, a quiescent zone which at a greater depth, and an intermediate zone. In the wave impact zone, the hull or other flotation structure is subjected to various forces resulting from the ocean waves, the wind, or possibly other forces or impact forces placed on the structure. Depending on the location of the off-shore structure, in general, this wave impact zone could extend as much as 40 to 60 feet above the water level, measured at a time when the water surface is calm, and also as much as 30 to 50 feet below the water surface. However, within the broader scope, the ranges could vary from values of 80, 75, 70, 65, 60, 55, 50, 45, 40, 35, 30, 25, 20, or 15 feet, above and/or below the water, depending on the circumstances. 
     The quiescent zone is at a sufficient depth below the water surface so that even though the water is not totally unaffected by the wave action at the surface, such effect is minimal or nearly non-existent. This of course is dependent in large part on the height and depth of the wave impact zone. In general, for off-shore oil platforms the upper boundary of the quiescent zone would be at least as low as (or lower than) 50 feet below the water surface. However, depending on the circumstances, within the broader range this could vary from 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, or 70 feet or greater. 
     Obviously there is no sharp line of demarcation between the wave impact zone and the quiescent zone, and so for purposes in establishing the positioning and operation of the components of the present invention, there can be considered to be the “intermediate zone” or what could be termed a “transition zone” where the wave impact zone makes the transition to the quiescent zone. This intermediate zone could be considered to be anywhere between 0 to 30 feet, and also at intermediate ranges of depth, for example 5, 10,15, 20, or 25 feet. 
     The flotation assembly of the present invention is adapted to be positioned in a body of water having a water surface. The assembly comprises: 
     a) a load bearing structure which is adapted to be positioned in an operating position at a support location at the body of water, with a load bearing structure having a support portion to support a load and a flotation support region at which a flotation lifting force or forces can be applied; 
     b) a flotation section comprising a plurality of flotation tubes, each of which has an upper end portion, a lower end portion, and a longitudinal axis which is aligned so as to have a substantial vertical alignment component; 
     c) each flotation tube comprising a surrounding side wall defining an elongate pressure chamber and an upper end closure portion having a downwardly facing pressure surface exposed to pressure in the pressure chamber, the tubes being positioned at laterally spaced locations and arranged relative to the load bearing structure in a manner that each upper end closure portion of at least some of the flotation tubes operatively direct flotation forces to the load bearing structure to create an upwardly directed bearing force against the load bearing structure; 
     d) a source of pressurized gaseous fluid to pressurize the flotation tubes to a level where gas pressure within each tube creates a force against the side wall to alleviate at least in part compressive force of surrounding water pressing inwardly against the side walls of the flotation tubes. 
     The flotation tubes are constructed and arranged to withstand forces created by pressure in the tube in hoop tension, with a bearing force exerted by each end closure portion of each tube being created by gaseous pressure in the related chamber against end portions of the tubes. 
     In one arrangement, the lower end portions of at least some of the flotation tubes are exposed to ambient pressure of water at the lower end portion of the tube to place at least a portion of the flotation tube above a location at which the flotation tube is open to water in hoop tension. 
     In another arrangement, the lower end portions of the at least a portion of the flotation tubes are at least partially closed, and there is an extension tube leading downwardly from the lower end portion and having a lower end of the extension tube being opened to ambient water pressure. 
     In another arrangement, at least one of the flotation tubes is closed at each end to isolate the interior of the chamber of such tube from communication with the ambient water pressure. In this arrangement, the tube that is closed at both ends is pressurized to a sufficiently high level so that the tube that is closed at both ends is in hoop tension and also in axial tension to alleviate radially inward compressive loads and axial compression loads. 
     In a preferred form, the source of pressurized gaseous fluid is arranged to pressurize the flotation tubes to a level at least as great as or greater than the water surrounding at least a substantial portion of a substantial number of the flotation tubes to place each of said substantial portion of said tubes in hoop tension along a substantial portion of each flotation tube. 
     In a preferred embodiment, a load support portion of the load bearing structure comprises a load supporting platform having an upwardly facing support surface. In a specific version of this embodiment, the load supporting platform comprises a hull structure which in an operating position of the assembly in the water is positioned at the water surface and at least in part in a wave impact zone at the water surface. The hull structure further comprises a flotation structure which in the operating position is at least in part below the water surface to provide a buoyancy force in the operating position and having sufficient structural integrity to withstand wave action and/or other external forces enclosed thereon when in the operating position. In at least one arrangement, at least some of the tubes are positioned at circumferentially spaced locations around the hull structure, with upper ends of some of the tubes being in load bearing relationship to the structure that is positioned at circumferential locations around the hull structure to transfer buoyancy forces from some of said tubes to the hull structure. 
     In another version the flotation tubes comprise an auxiliary flotation section below the hull structure with at least a substantial portion of the auxiliary support section positioned below the wave impact zone when the assembly is in the operating position. The upper end closure portions of the tubes are an operative bearing engagement with a lower portion of the hull structure. 
     In yet another arrangement, the flotation tubes are arranged in at least two groups of said flotation tubes. There is a first upper group positioned beneath the load bearing structure to be in load bearing relationship therewith, and a second group of flotation tubes being positioned below the first group of flotation tubes, with each group of the flotation tubes exerting a buoyant force to maintain the hull structure in its operating position. In one arrangement, the load bearing structure is hull structure having a flotation section which is in an operating location beneath the water surface, and the upper group of tubes is positioned below the hull structure. 
     In another arrangement of the above embodiment, the upper portion of the tubes of the first group extend above the water surface and engage the load bearing structure which is above the water surface. In one arrangement of this, the tubes of the first group are pressurized to a sufficiently high level so that the tubes of the first group are not subjected to axial compression loads from the load bearing structure. In another arrangement, the lower ends of the tubes of the first group are open to ambient water pressure with the tubes of the first group withstanding loads from the load bearing structure at least in part in compression loading. 
     In another arrangement where there is a first and second group of tubes, at least some of the flotation tubes of the first group and from the second group are in direct load bearing relationship, with some of the second group of flotation tubes being in load bearing relationship against related aligned flotation tubes of the first group. Thus the buoyancy force from at least some of the flotation tubes in the second group is directed to the aligned flotation tubes there above and to the hull structure. 
     In another arrangement where there are the first and second groups of flotation tubes, the flotation tubes from the first group that are in alignment with some of the flotation tubes of the second group have lower end portions thereof open to pressure of ambient water, and the flotation tubes of the first group that are in such alignment are constructed and arranged to bear compression loads between the hull structure and the aligned flotation tubes of the second group. 
     In a preferred form of the present invention, at least some of the flotation tubes are pressurized to a level so that their side walls are in hoop tension, with the force of the hoop tension increasing with the level of the portion of the flotation tubes in the water. The side wall of each of said tubes in hoop tension is constructed so that an upper portion of the flotation tubes in hoop tension have greater resistive strength in hoop tension relative to the lower part of the flotation tube that is in hoop tension. 
     A preferred form of the present invention is that the flotation tubes are in hoop tension are constructed of fiberglass, and the fiberglass in arranged relative to the material strength, and/or quantity of material in a manner to accomplish greater resistance to hoop tension at upper elevations of the fiberglass flotation tubes than at lower levels of said fiberglass said flotation tubes. 
     In one arrangement, the flotation assembly comprises an off-shore platform which comprises a flotation hull that extends above the water surface and also extends above the water surface and also extends below the water surface. The hull in its operating portion is at least 20 feet above the water surface and extends at least 20 feet below the water surface. 
     Also, in a preferred embodiment the assembly comprises a hull structure which is supported at least in part by the flotation force in the water which has a central opening. The load bearing structure is positioned within the central opening of the hull structure and is able to move axially relative to the hull structure. A drill string or other equipment could be supported from this load bearing structure. Thus the flotation tubes, being in load bearing relationship with the load bearing structure, are able to move with the load bearing structure axially, relative to the hull structure. In a specific form of this embodiment, there are additional flotation tubes located beneath the hull structure, and these are in load bearing relationship with support structure that is in turn connected to the load bearing structure and thus transmit buoyant forces to the load bearing structure. 
     In another embodiment, there is a plurality of groups of flotation tubes, vertically aligned with one another and spaced vertically from one another. A load transfer structure extends vertically form the load bearing structure downwardly. The load transfer structure has vertically spaced load transfer sections, each of which engages upper end portions of a related group of the tubes. Thus buoyancy forces of each group of tubes is transmitted into related load transfer sections into the load transfer structure and to the load bearing structure. 
     It is presently contemplated that the satisfactory flotation tube could be constructed in accordance with the teachings of the present invention where the tube would have a 12 foot diameter, and could be as long as 100 feet in length. The side wall could be a conventional construction and could be made 1 inch thickness. The cap also could be made by conventional fiberglass construction and have a thickness at about 1½ inches. Within the broader scope, the diameter and/or the length of the tube and/or thickness of the tube wall could be increased by 10 percent, 25 percent, 50 percent, 75 percent, 100 percent, 150 percent, 200 percent, 300 percent, 400 percent or 500 percent. Also these could be decreased by 10 percent, 20 percent, 30 percent, 40 percent, 50 percent 60 percent, 80 percent and 90 percent. Also, the thickness of the side wall of the tubes could be increased by increments of one-tenth of an inch up to any desired level up to a maximum of five inches, and could be decreased by increments of one-tenth of an inch of thickness of 0.1 inch. 
     The fiberglass strands would commonly be oriented at about 60-75 degrees relative to the longitudinal center axis of the tube, and this range could be broadened to 55 degrees to 80 degrees, 50 degrees to 85 degrees, and 45 degrees to 90 degrees. Or this angle could be down to 40 degrees, 35 degrees, 30 degrees, 25 degrees, 20 degrees, 15 degrees, 10 degrees, 5 degrees or 0 degrees. 
     In the method of the present invention, the auxiliary flotation support is provided to the load bearing structure by positioning the load bearing structure in the body of water. This load bearing structure has a support portion to support a load and a flotation support region at which a flotation lifting force or forces can be applied. The method further provides providing a flotation section, as described above, comprising a plurality of flotation tubes. Also as described above, the flotation tubes are placed in position at laterally spaced locations so as to direct flotation forces thereof to the load bearing structure to proved flotation support. 
     The method further comprises operating a source of pressurized gaseous fluid to pressurize the flotation tubes, as described above, to obtain the desired amount and distribution of the flotation forces. The flotation tubes being constructed and arranged to withstand the forces created by pressure in the tubes in hoop tension. 
     Other features of the present invention will be apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a prior art off-shore platform assembly having a SPAR configuration; 
     FIG. 2 is a side elevational view similar to FIG. 1, but showing the off-shore platform assembly incorporating the present invention; 
     FIG. 3 is an isometric view showing somewhat schematically the structure of the hull section of FIG. 2; 
     FIG. 4 is a transverse sectional view, taken in horizontal section, of the auxiliary flotation support section of the present invention, as shown in FIG. 2, drawn to a larger scale than in FIG. 2; 
     FIG. 5 sectional view taken along the longitudinal center line of the assembly of FIG. 2, and showing the flotation section which comprises the hull section and the auxiliary flotation section. 
     FIG. 6 is a somewhat schematic view illustrating a single flotation tube and showing graphically the water pressure at various depths from the top of the tube to the bottom of the tube, and also showing the pressure differential between the air pressure within the flotation tube and surrounding water pressure; 
     FIGS. 6A-6G show various arrangements and configurations of the flotation tube; 
     FIGS. 7A,  7 B, and  7 C and  7 D show a flotation tube with four different methods of providing the operative engagement with the hull structure; 
     FIG. 8A is a top plan view of a flotation module; 
     FIG. 8B is a top view of the flotation module FIG. 8A, but with the top bearing member not shown; 
     FIG. 9A is a side elevational view of the modular FIG. 8A, but with the surrounding shroud removed; 
     FIG. 9B is a side elevational view as in FIG. 9A, but with the flotation tubes not being shown; 
     FIG. 9C is a side elevational view of the module of FIGS. 9A and 9B, but showing the module with the surrounding shroud; 
     FIG. 9D is also a side elevational view of module of FIGS. 9A-9C, showing the flotation tubes positioned therein, but not showing the frame nor the shroud; 
     FIG. 10 is a side elevational view of the SPAR of the present invention positioned in the body of water in its operating position; 
     FIG. 11 is a schematic view of the pressurizing system of the invention; 
     FIG. 12 is a top plan view of a second embodiment of the resent invention incorporated in a tension leg platform; 
     FIG. 13 is a side elevational view of the assembly of FIG. 12; 
     FIG. 14 is a top plan view of another form of the auxiliary flotation assembly of the present invention; 
     FIG. 15 is a side elevational view of the auxiliary flotation assembly in FIG.  14 . 
     FIG. 16 is a cross-sectional view taken along lines  16 — 16  of FIG. 18 showing a third embodiment of the present invention, incorporated to provide buoyancy to risers associated with a SPAR; 
     FIG. 17 is a sectional view taken along line  17 — 17  of FIG. 18, also showing the third embodiment; 
     FIG. 18 is a longitudinal sectional view taken along line  18 — 18 , also of the third embodiment; 
     FIG. 19 is a longitudinal sectional view similar to FIG. 18, but showing the third embodiment of the present invention as part of the entire SPAR structure; 
     FIG. 20 is a cross-sectional view taken along line  20 — 20 , showing a fourth embodiment of the present invention; 
     FIG. 21 is a longitudinal section view taken along line  21 — 21  of FIG. 20, also showing the fourth embodiment; 
     FIG. 22 is a longitudinal sectional view such as FIG. 21, but showing the entire SPAR assembly of the fourth embodiment; 
     FIG. 23 is a cross-sectional view taken along line  23 — 23  of FIG. 24, showing a fifth embodiment of the present invention; 
     FIG. 24 is a longitudinal sectional view of the fifth embodiment; 
     FIG. 25 is a longitudinal sectional view similar to FIG. 24, but drawn to a larger scale, showing the fifth embodiment; 
     FIG. 26 is a front elevational view, with certain parts removed for purposes of illustration, showing the entire SPAR structure of the fifth embodiment; 
     FIG. 27 is a elevational view similar to FIG. 26, but showing a modified version of the fifth embodiment; 
     FIG. 28 is a longitudinal sectional view of a sixth embodiment; 
     FIG. 29 is a horizontal cross-sectional taken along line  29 — 29  showing a seventh embodiment of the present invention; 
     FIG. 30 is a longitudinal cross-sectional view of the seventh embodiment, taken along line  30 — 30  of FIG. 29; 
     FIG. 31 is a sectional view taken along line  31 — 31  of FIG. 32, showing an eighth embodiment of the present invention incorporated in a tension leg platform; 
     FIG. 32 is an elevational view showing the tension leg platform of the eighth embodiment; 
     FIGS. 33A through 33H and  33 I are schematic drawings drawn in side elevation of various arrangements of flotation tubes, and also illustrating in graphical form the tension loads that are placed on the various configurations. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, there is shown a typical prior art off-shore platform assembly as a SPAR  10 . It can be seen that there is a buoyant hull structure  12  and a cylindrical section  14  of the SPAR connected to the lower part of the hull structure  12  and extended downwardly therefrom. The cylindrical section  14  extends several hundred feet (e.g. 500 or more feet or as great as 700 to 900 feet) downwardly from the hull section  12 . At the lower end of the cylindrical section  14 , there is a ballast portion  18 . 
     A typical construction of the hull structure  12  is that it would have in plan view a circular configuration having a circular center opening. The top surface of the hull structure  12  provides an upper deck or support surface for the equipment, machinery and other structures. The hull structure  12  is made with plate steel and is compartmentalized, along with various bracing, support beams, etc. The structure  12  is made sufficiently rugged and strong to withstand the various forces of the wind, waves and possibly other external forces, and has enough volume so as to create sufficient buoyancy to support not only its own weight, but that of the rest of the SPAR  10  and the various items placed on, or supported from, the SPAR  10 . 
     Typically, a drill string and possibly other items or equipment would extend through the center opening of the hull structure  12 , and the drill string sections and other items would be lowered through the center opening of the hull structure  12  to a lower location and also raised from the lower location upwardly through the sea water and then through the center opening of the hull structure  21 . There are shown several anchor lines  20  which extend from the spar downwardly and outwardly to anchoring locations in the ocean floor. 
     Reference is first made to FIG. 2, which shows a first embodiment of the present invention, incorporated in a SPAR assembly  30 . In this particular embodiment, the SPAR assembly  30  comprises a flotation section  32  and a truss section  34  which is attached to the lower side of the flotation section and to section  32  and extends downwardly therefrom. The truss section  34  may, in and of itself, be of conventional design. Thus, as shown herein the truss section  34  which is arranged to extend downwardly to a lower level, where it is attached to the ballast section  38 . 
     As shown in FIG. 5, the flotation section  32  is in turn made up of a hull structure  40  and also an auxiliary flotation section  42 . The basic construction of the hull structure  40  is, or may be, substantially similar to the prior art hull structure. One such construction is shown somewhat schematically in FIG. 3, where for purposes of illustration the various plates are shown as being somewhat transparent. It can be seen that there are a plurality of outer side plates  44  arranged (as seen in plan view) in a polygonal or circular shape. There are also inside vertical wall plates  46  also arranged generally in the shape of a circle or a regular polygon. Then there is a plurality of radially extending vertical plates  48  interconnected to juncture points of the inner plates  46  and outwardly to juncture points of the outer plates  44 . This provides for the compartmentalized construction. In addition, as is done in the prior art, there would likely be various additional structural members such as reinforcing beams, etc. Also the hull structure  40  is provided with a center through opening  49 . 
     The hull structure  40  in the present invention differs from the prior art in that the overall volume of the hull structure  40  relative to the overall flotation capability of the flotation section  32  is much smaller. However, the overall construction of the flotation section  32  is such that the upper support surface  50  would be at approximately the same distance above the sea water as a comparable prior art flotation platform assembly, and the upper surface  50  would have a comparable area. However the bottom portion  52  of the hull structure  40  would be, in the operating position of the platform assembly  30 , at a lesser depth beneath the water surface. 
     The additional buoyancy of the flotation section  32  is provided by the auxiliary flotation section  42  which is shown in a horizontal sectional view in FIG.  4  and also shown in the vertical cross section in FIG.  5 . The auxiliary flotation section  42  comprises a plurality of vertically oriented flotation tubes  54  which are maintained in their proper operating position by means of a positioning framework  56 . Also, there is provided a circumferential protective flotation shell or shroud  58  which extends in a cylindrical configuration circumferentially around the flotation tubes  54 . The arrangement of the flotation tubes  54  is in a generally annular pattern, generally matching the plan view configuration of the hull structure  40 . Thus, there is a central opening  60  in the flotation section  42  aligned with the opening  49  of the hull structure  40 . 
     In the particular configuration shown in FIG. 4, the flotation tubes  54  are arranged in six groups  62 , with each group having twelve flotation tubes  54 . In this arrangement the tubes  54  are arranged in three rows of  3 ,  4  and  5  tubes  54 . The positioning framework  56  comprises in this configuration  6  vertical columns or posts  64 , each having a tubular configuration and each being a structural load bearing member. At the upper and lower ends of each column  64  there is a positioning structure  65 , only one of which is shown in FIG. 5, with each comprising six horizontally aligned arms  66 , arranged in a hexagon, with each arm  66  extending between two related columns  64 . Then there are  6  radially aligned positioning members  68  extending inwardly from its related post  64  to join to a related pair of inwardly positioned arms  70  which are arranged in a hexagonal configuration around the central opening  60 . It can be seen that each pair of an inner arm  70  and an outer arm  66  forms with their two related radial members  68  a regular trapezoid. Positioned within the arms  70 , there is an interior shroud  72  having a cylindrical configuration. This shroud  72  has a protective function in that it prevents equipment or various objects that might drop downwardly through the opening  60  from coming into contact with the inner most set of tubes  54 . All of the structural components are desirably hollow, and these are sealed to provide buoyancy. 
     Also as seen in FIG. 5, there can be diagonal cross braces such as shown at  74 . As another option to maintain the tubes  54  in each set in proper spaced relationships, instead of or in addition to the frame sections  65 , there can be positioned upper and lower positioning plates (not shown), with each plate being a planer member having circular cut-outs, with each cut-out, accommodating a related tube  54 . 
     Each of the posts  64  can serve as the structural connecting member between the truss section vertical structure members  76  (see FIG.  2 ). As shown in FIG. 4, each column  64  is positioned so that it intersects the outer jacket or shell or shroud  58 , and the shroud  58  could be formed with cutouts in which the columnar members  64  are positioned. 
     To describe now the structure and function of the flotation tubes  54 , reference is initially made to FIG. 6, and then to FIGS. 6A-6G. FIG. 6 shows the tube  54  somewhat schematically, and this tube  54  comprises a cylindrical side wall  78  and an upper end cap  80  that has a rounded configuration which forms an upwardly facing convex surface  82 . The side wall  78  and the end cap  80  form an interior pressure chamber  84  which in the particular arrangement of FIG. 6 has an open lower end at  86 . 
     The tube  54  is pressurized so that water in the chamber  84  is displaced to a lower level. To describe the structure and function of the tube  54 , let us assume that the tube  54  has a length of 100 feet, and that the upper end cap  82  is about 50 feet below the water surface. Let us also assume (for the sake of simplicity) that the water pressure increases 0.5 pounds for each foot of depth. Thus, in the arrangement of FIG. 6, at the level 50 feet below the surface, the water pressure is 25 pounds per square inch, at the 100-foot level 50 pounds per square inch, and at the 150 foot level, 75 pounds per square inch. Thus, if all of the water in the tube chamber  84  is to be displaced, the air pressure in the chamber must be at the 75 psi level. 
     In actual practice, the pressure will be somewhat less so that the level of the water is a moderate distance (e.g. 2-7 feet) upwardly from the bottom end opening  86 . However, assuming that the air pressure is such that the water level is right at the bottom opening  86 , it will be seen that the pressure differential at the opening  86  from the outside water and the air on the inside would be zero. At the 125 foot level, there would be a pressure differential of 12.5 psi (with the pressure within the chamber  84  being 75 psi and the outside water pressure being 62.5 psi. At the 100-foot depth level, this pressure differential will have raised to 25 psi, and at the location of the end cap  82 , the pressure differential will be about 50 psi. 
     If we assume the interior diameter of the tube  54  to be 12 feet, then the cross-sectional area in a section extending horizontally through the tube  54  would be 16,278 square inches. With the pressure differential at the location of the cap  80  being 50 psi, it can be seen that the total force exerted by the air pressure upwardly against the cap  82  is 813,888 pounds. Each tube  54  is positioned so that its end cap  80  presses upwardly in bearing relationship against the hull structure  40 , thus providing the buoyancy force for the hull structure  40 . 
     Let us now examine how these various forces are reacted into the tube  54 . Since pressure acts perpendicular to the surface with which the pressurized medium is in contact, it can be seen that the air pressure within the air chamber  84  reacts laterally outwardly against the tube side wall  78 . Further, since the tube side wall  78  has a circular cross-sectional configuration, the pressure is exerted radially outwardly to place the side wall  78  in hoop tension. If we again assume that the interior diameter of the side wall  78  is  12  feet, and if we make a further assumption that the thickness of the side wall  78  is two inches, and further if we assume that the maximum pressure differential is 50 psi at the 50 foot depth, then the maximum tension load that would be placed upon each square inch of the material forming the side wall  78  would be about 3,600 pounds. Obviously, at greater depths where the pressure differential between the chamber  80  and the immediately adjacent water becomes less, this force would decrease proportionately. 
     It is also readily apparent that in this particular arrangement there are substantially no axial loads imposed on the side wall  78  (i.e. either tension loads or compression loads that would be parallel to the vertical axis  88  of the tube  78 . 
     When one considers the types of loads which are imposed on each of the tubes  54 , it can then be realized that the tubes  54  lend themselves particularly to being made of fiber reinforced plastic (FRP). The fibers in the side wall  78  can be oriented so as to have a substantial circumferential alignment component to better withstand the hoop tension. In theory, if the only loads that were imposed upon the tube  54  were those resulting from the tube  54  being pressurized, the side wall  78  could be made rather thin, and the lower end very thin. However, for practical reasons (manufacturing, shipping, withstanding occasional impacts from whatever source, etc.) the side wall  78  would be made somewhat thicker than what would be required to withstand the hoop tension loads. 
     Reference is now made to FIGS. 6A-6G to illustrate a number of different possible configurations of the tube  54 . The components of these tube configurations shown in FIGS. 6A-6G will be given numerical designation corresponding to those in FIG. 6, but with a letter suffix (either letter a,b,c, etc.), being used to differentiate those shown in FIGS. 6A-6G. 
     It can be seen that the tube  54   a  of FIG. 6A has the very same configuration as the tube in FIG.  6 . Accordingly, there will be no further description of this tube  54   a . In FIG. 6B, the tube  54   b  has the same overall configuration of the tube  54   a , except that a lower end cap  90   b  is added, and also an extension tube  92   b  extends downwardly from the bottom cap  90   b . This tube  92   b  is open at the bottom and leads into the lower end of the interior chamber  84   b.    
     The reason for having this lower end cap  90   b  and the extension tube  92   b  is so that the lower end opening  94   b  is a further distance beneath the water level. Thus, if the pressure in the chamber  84   b  is sufficiently great to push the water level downwardly below the lower end of the main tube side wall  78  as when the structure bobs up and down in a storm, then the air will not escape, but will simply move periodically to a lower level in the tube  92   b.    
     The tube  54   c  of FIG. 6C differs from the tube  54   a  only in that it is formed with a central tubular member  96   c  which extends the entire length of the tube  54   c  and is open at both the top and the bottom. When the tube  54   c  is pressurized, this tubular member  96   c  would feel a compression load directed radially inwardly. The reason for having this open tube  96   c  is that for some reason it may be desirable to have a member extend from the hull structure  40  to a downward location through this tubular member  96   c.    
     FIG. 6D shows a tube  54   d  that has the upper cap  80   d  and also a lower cap  90   b  which is closed. By having the tube  54   d  totally closed, this tube  54   d  can be pressurized to a higher pressure level than would be possible with the tube configurations of  6 A,  6 B and  6 C. In some situations, it may be desirable to pressurize the chamber  84   d  to a higher level to place the side wall  78  in tension along the vertical axis. For example, with the side wall  78  in axial tension loading, the effective structural rigidity of the side wall  78  would be increased. Further, if for some reason loads were placed on the end caps  80   d  and  90   d  to place an axial compression load on the tube  54   d , the side wall  78   d  would be alleviated from such compression load because of the greater axial force exerted by the two end caps  80 D and  90 D. 
     In FIG. 6E, there is shown two tubes  54   e  that are positioned one on top of the other. Each of these tubes  54   e  has an upper end cap  80   e . It will be noted that the upper tube  54   e  has a pair of vent openings  98   e  at its lower end to permit water to flow inwardly and outwardly from the interior chamber  84   e.    
     In the arrangement as shown in FIG. 6E, the end cap  80   e  of the lower flotation tube  54   e  will be exerting an upward force which would in turn be transmitted into the side wall  78   e  of the upper tube  54   e . Thus, in this particular arrangement, the upper side wall  78   e  would need to be constructed so as to take axial compression loads, in addition to withstanding the loads in hoop tension. For this reason the upper tube may desirably be made of steel or other material better adapted to take compression loads. 
     In the configuration of FIG. 6F, there are also two tubes  54   f  positioned one above the other. In one arrangement the two tubes  54   f  function in substantially the same manner as the two tubes  54   e . However, the lower tube  54   f  has the lower cap  90   f  with the extension tube  92   f . Further, it will be noted there is shown in broken line an upper cap  80   f  that is positioned at the upper end of the lower tube  54   f . In that arrangement, the upper tube  54   f  functions in the same manner as the upper tube  54   e  and would withstand axial compression loads from the lower tube  54   f.    
     However, in an alternative configuration, the upper tube  54   f  would have the configuration of the tube  54   d  of FIG. 6D so that the upper tube  54   f  would have a lower end cap  90   f  that would actually curve convexly into the upper end of the lower tube  54   f , with the end cap  80   f  being eliminated. The upper end of the lower tube  54   f  would, in this arrangement, be sealed by its own upper end cap which would extend concavely into its inner chamber  78   f  so as to provide an air seal. In this instance, the lower tube  54   f  would be pressurized to a sufficiently high level to withstand the surrounding water pressure. The upper tube  54   f  would be pressurized to a pressure level at least as great as the lower tube  54   f , or pressurized to a higher level to ensure that the side wall  78   f  would not be subjected to axial compression loads, but only axial tension loads. 
     However if the pressure in the upper tube  54   f  is only slightly higher than the air pressure in the lower tube  54   f , the upper tube  78   f  would carry only moderate axial compression loads. There would, of course, need to be an airtight seal at the joining location of the upper and lower tubes  54   f.    
     FIG. 6G shows substantially the same configuration as in FIG. 6F, except that there is provided an upper extension tube  92   g  which communicates with the lower part of the upper chamber  84   f . Thus, the arrangement in FIG. 6G would function in substantially the same manner as in the arrangement of FIG. 6F, where the upper cap  80 F is provided. Also, as with the arrangement of FIG. 6E, the upper tube  54   g  would be made of steel or some other material to better withstand the axial compression loads. 
     Reference is now made to FIGS. 7A,  7 B,  7 C and  7 D to describe several ways in which the tubes  54  could be placed in bearing engagement against the hull structure  40 . 
     In FIG. 7A, the tube  54  is shown with its upper edge portion  100  fixedly connected (e.g. by bonding, welding, etc.) to a lower surface  102  of the hull structure  40 . In this instance, there is no upper cover cap  80 , and the air pressure within the chamber  84  bears against the hull structure surface portion  102 . 
     In FIG. 7B, there is shown the flotation tube  54  having the cover cap  80 , and the circumferential edge  104  of the cover cap  80  is configured so as to have sufficient strength to be able to carry the bearing load of the tube  54 . In this configuration, there is cylindrical member  106  which extends around the circumference of the cap  80  and the lower edge of the cylindrical bearing member  106  is pressed against the circumferential load bearing portion  104  of the cap  80 . The upper edge  108  of the bearing member  106  is in engagement with the hull structure  40 , and two small arrows are placed at the edge portion  108  to indicate the upward force exerted by the bearing member  106 . Thus, the pressurized air in the tube chamber  84  urges the cap  80  upwardly, and the force created by the pressure against the cap  80  is reacted into its peripheral bearing portion  104  which in turn transmits the load through the cylindrical member  106  to the hull structure  40 . 
     FIG. 7C shows a third arrangement where the tube  54  has the cap  80 , and there is also the cylindrical bearing member  106 . In this instance, however, the cylindrical bearing member is fixably attached to the hull structure  40 , either by bonding, welding or some other means. 
     FIG. 7D shows yet a fourth arrangement where the tube  54  has the cap  80 , but the bearing member  108  is positioned at an upper central location on the cap  80  so as to distribute the force exerted against the cap  80  upwardly at the central location against the hull structure  40 . In this instance, the cap  80  has to be as sufficient structural strength to be able to transmit the force loads from the air pressure on the outer portions of the cap  80  through the structure of the cap  80  to the bearing member  110 . 
     Reference is now made to FIGS. 8A-B and also FIGS. 9A-D which show an arrangement where a number of the flotation tubes  54  can be provided in the form of a flotation module  112 . 
     The flotation module has four components, namely a top bearing member  114 , a positioning frame  116  which is attached at its upper end to the bearing plate  114  and extends downwardly therefrom to encircle the tubes  54 , and a surrounding shroud  118 . 
     In FIG. 8A (a plan view) the module  110  is shown with the shroud  118 , but in FIG. 9A, a side elevational view, the module  110  is shown without the shroud  118 . In this particular configuration, the upper bearing member  114  is in the form of a circular structure that forms a plate. Alternatively, this bearing member  114  could be formed as an upper bearing frame made up of beams, cross members and other reinforcing (i.e. hull structure). 
     The positioning frame  116  is shown in FIG. 9A with the tubes  54  positioned therein, and in FIG. 9B the positioning frame is shown without the tubes  54 . This positioning frame  116  comprises a plurality of vertical tubular posts  120  which are arranged in a symmetrical hexagonal pattern, and these are connected by truss members  122  which are shown somewhat schematically. The tubes  54  are located within the positioning frame  120  in the configuration shown in FIG.  8 B. For ease of illustration the precise positioning devices for the tubes  54  are not shown, but these could be provided with a positioning plate having openings for the tubes  54 , described previously herein. Alternatively, individual spacers could be used to position the tubes  54  or other arrangements. 
     FIG. 9C shows the entire module  110  in a side elevational view, with the shroud  118  enclosing the major part of the positioning frame  116 , but with the vertical posts  120  positioned in the perimeter of the shroud  118 . In this particular configuration, the shroud  118  can be made in six cylindrically curved segments which are attached between related posts  120 , with the six sections collectively forming the cylindrical shroud  118 . 
     FIG. 9D is a sectional view taken along line  9 D— 9 D of FIG.  8 C. 
     The function of this module  112  is substantially the same as described previously herein, in that the flotation tubes  54  are positioned to provide a buoyancy force for the hull structure  40  which is positioned above. Within the broader scope of the present invention the module  112  would be suitable as stand-alone flotation module to provide buoyancy for mooring lines or TLP tendons. 
     This modular form of positioning the flotation tubes  54  can be of value in various ways. For example, in the initial installation of the flotation assembly, it may be more effective to pre-construct the modules  112  and then install the modules at the site at which the flotation assembly is to be established. Also, if it is necessary to make a replacement of one or more tubes, this could quite possibly be accomplished more effectively by simply removing an entire module and substituting this with another module, as opposed to attempting to replace one or two of a set of tubes  54  which are at less accessible locations beneath the hull structure  40 . 
     To describe now the steps in constructing and deploying the flotation assembly of the present invention, let us first assume that the flotation assembly is in the form of the SPAR assembly as shown in FIG.  10 . It is presently contemplated that the preferred procedure in assembling and installing the flotation assembly is to construct it at a convenient location, with the SPAR assembly  30  complete and the flotation tubes  54  already installed therein. Then the entire SPAR assembly would be moved in to the water to assume a horizontal floating position. The flotation hull  40  would provide more than adequate buoyancy force for that portion of the structure and the ballast section  38  would be emptied to provide a flotation force at the opposite end. Additional flotation devices could be employed if needed. 
     If the tubes  54  are of the open bottom configuration, these would not provide flotation during deployment while in the horizontal position. At the location where the entire SPAR structure  30  is to be placed in its operating position, the ballast section  38  is flooded so that this sinks in the water to position the SPAR structure  30  in a vertical position. The mooring lines are attached and then deployed. 
     When the SPAR structure with the flotation tubes already positioned therein are vertically oriented, then the tubes  54  are themselves pressurized which provide the additional buoyany for full operation. Normally, the buoyancy provided by the flotation tubes  54  is not necessary when the SPAR structure  30  is being towed to the operating location, because the top side pay load weight is not at that time on the hull structure  40 . The SPAR itself (with the hull structure  40  and the ballast section  38 ) would likely be enough to float the unloaded SPAR structure  30  in its horizontal position. If not, as indicated above, additional flotation devices could be provided. However before the top side weight is added to the hull structure  40 , the flotation tubes  54  need to be pressurized to provide the additional flotation. 
     Reference is now made to FIG. 10 which shows the SPAR assembly  30  of the present invention in its installed location. For ease of illustration, only two of the flotation tubes  54  have been illustrated, it being understood that the entire array of flotation tubes  54  would be present. Also, the shroud or shell  58  has been omitted. The surface of the water is shown where there is very little wave action. It will be assumed that the upper surface of the hull structure  40  has been loaded with equipment, supplies, etc., these being shown somewhat schematically simply as three boxes  128 . 
     An upper portion  130  of the hull structure  40  rises above the level of the sea water, and a lower portion  132  of the hull structure is positioned below the sea water surface. For example, in a typical installation of the present invention as an off-shore platform, the top surface  50  of the hull structure  40  could be approximately 40 to 60 feet above the sea surface for an average size flotation assembly, this height dimension being indicated by the arrow  133 . 
     The lower part  132  of the hull structure  40  would extend downwardly to about 50 to 100 below the water surface in an average size platform assembly, as indicated at  134 . 
     As indicated previously, there is what can be considered to be a wave impact zone which extends from a level above the water surface downwardly to a depth where the forces imposed by the wave action, wind, and other exterior occurrences would reasonably require that the displacement hull  40  have the rugged structure to properly withstand all of these forces, yet be properly sized for effective operation. This impact zone is indicated by the arrow  136 . In general, this impact zone generally can be considered to be a depth as low as 30 feet below the water line and as high as 40-60 feet above the water line. Obviously, as indicated previously, these ranges could vary greatly, being larger or smaller, depending upon the location of the platform and various other factors. 
     Then there is what may be called a transition zone  138  where the movement of the water and forces of the water exist to some extent but are not considered to be appreciable, this we consider the intermediate or transition zone  138 . Then beneath the intermediate zone would be the quiescent zone  140  where the forces exerted by the action in the water or from other sources is considered to be less significant. 
     As indicated previously, the SPAR assembly  30  can extend downwardly from the water surface by as much as 700 to 900 feet, with the main truss section  34  fixedly attached to the hull structure  40  and extending downwardly to the ballast section  38 . The entire truss section and the ballast section  38  act in the manner of a pendulum to maintain the hull in a horizontal or nearly horizontal position. Also as indicated previously, anchor lines can be used to properly locate the assembly  30 , and there would normally be a drill string or other equipment which extends downwardly through the central opening  49  of the hull structure  40  and also through the central opening  59  of the flotation section  42 . 
     As indicated earlier, the hull structure  40  is commonly made of steel and has sufficient structural strength so that it can withstand the forces that would be created in severe storm conditions. These forces are considered to be greatest in what has been termed the wave impact zone, and this wave impact zone would extend from a location above the water surface and also below. Accordingly, one of the main design considerations in optimizing the present invention is to determine the reasonably predictable location and height dimension of this impact zone and select the dimensions of the flotation hull  40  accordingly. 
     The auxiliary flotation section  42  (see FIG. 5) should be positioned at a sufficient depth so that the flotation tubes  54  are substantially isolated from the forces that are created during severe operating conditions (e.g. in very stormy conditions). The outer shroud or jacket  58  (see FIGS. 4 and 5) is provided to provide protection from less severe loads, such as lesser wave or current action or impact loads that might occur from equipment impacting the flotation section. Also, as indicated previously, there is also provided the inner shroud  72  which protects the flotation tubes  54 . 
     Thus, the outer shroud  58  and the inner shroud  72  are provided to prevent damage to the tubes  54  or possibly the associated positioning framework from loads such as impact loads from equipment that could be dropped or misplaced from crashing into the tubes or other such occurrences. However these are not arranged to resist the more massive loads which could be imparted by very stormy weather conditions. 
     The closed chamber  141  formed by the outer shell  58 , inner shell  72  and hull structure  40  is occupied, in part, by tubes  54 . The remaining space between the tubes can also be pressurized, lowering the water level  141   a  in the chamber  141  to provide additional buoyancy. Alternately, the spaces between tubes  54  and enclosed in chamber  141  can be filed with lightweight concrete or other suitable filler to provide buoyancy and additional structural integrity. 
     Let us now direct our attention to the buoyancy function that the tubes  54  provide. At this point, it would be helpful to look again at the analysis of the functioning of the tube  54  with reference to FIG.  6 . It will be recalled that the side wall  78  of the tube  54  would normally experience only loads in hoop tension. A simplified explanation of the main function of the long cylindrical side wall is as follows. First, it should occupy a sufficient volume so that the water displaced by that volume is sufficiently large to create a high buoyancy force relative to its “foot print” (i.e. the space occupied in a horizontal plane to contribute substantially to the flotation of the hull structure. Second, it should do this in a manner so that it has sufficient structural strength to withstand the pressure of the water bearing against the side wall  78  which would tend to collapse the side wall  78  because of the water pressure. With the arrangement as shown in FIG. 6, the only load which the side wall  54  needs to withstand is those loads created in hoop tension due to the interior chamber  84  being pressurized to the extent necessary to withstand the compressive force of the surrounding water. 
     Now we look to the cap  80 . This cap  80  is actually what could be considered the load bearing member. The upper surface  82  of the cap  80  has water pressure bearing downwardly on it at a depth of 50 feet. This would be approximately 25 pounds per square inch, and if we assume that the tube side wall  78  has a 12-foot diameter, the cross-sectional area would be about 16,277 square inches, with the downward force of the water being 25 pounds per square inch, there would be about 407,000 pounds of force pressing downwardly on the top cap  80 . However, the air pressure in the chamber  84  is three times the level of the water pressure pressing down on the cap  82 , creating a net upward force of in the neighborhood of 800,00 pounds. 
     Another advantage of the present invention is that the buoyancy of the auxiliary flotation section  42  can easily be controlled by the inflating and deflation system which is shown somewhat schematically in FIG.  11 . There is a source of compressed gas, (i.e. air, an inert gas or other gaseous substance)  142  which connects to a plurality of valves  144 , with each of the valves  144  being connected to a related flotation tube  54 . To create more buoyancy, one or more of the valves  144  could be opened to move more compressed gas into the selected flotation tubes  54  to lower the water level in those tubes and thus increase the buoyancy. Alternatively, one or more of the valves  144  can be vented to let gas out of one or more of the flotation tubes  54  to decrease the buoyancy. Further, the pattern of flotation can be controlled by selecting the tube or tubes  54  which are to be filled with more compressed gas or to be vented. Thus, if the hull structure  40  is being loaded unequally at one location as opposed to another, and it is desired to shift the location of the buoyancy forces accordingly, this could be quite easily done with the present invention. 
     Thus, the outer shroud  20  and the inner shroud  72  are provided to prevent damage to the tubes  54  or possibly the associated positioning framework from loads such as moderate wave and current loads or impact loads from equipment that could be dropped or misplaced from crashing into the tubes or other such occurrences. However these are not arranged to resist the more massive loads which could be imparted by very stormy weather conditions. 
     Reference is now made to FIG. 12 where there is shown in plan view a tension leg platform  150 . This tension leg platform  150  comprises four vertically aligned cylindrical legs  152  which are spaced from on another in a square pattern. Interconnecting the four legs  152  are four horizontally extending structures  154 , each having a rectangular cross-sectional configuration, and with these structures  154  formed in a square configuration with the legs  152  being at the corners of the square. 
     The four vertical legs, could be, for example, from 40 to 90 feet in diameter, and the horizontal elongate structures  154  could each be,for example, 200 feet in length. The actual support platform (not shown herein for ease of illustration) is positioned at the upper end portions of the legs  152 . 
     Below each of the legs  152 , there is provided a related flotation assembly  156  which is, or may be, the same or similar to the flotation assemblies which are described earlier in this text and shown in the accompanied drawings. 
     The flotation assemblies may be retrofit for existing platforms, or incorporated into the original design of the platforms. 
     Another embodiment  158  of a flotation assembly is shown in plan view of FIG. 14, and FIG. 15 is a side sectional view taken along line  15 — 15  of FIG.  14 . 
     The flotation tubes  54  are arranged in six groups of seven flotation tubes each, each group being generally designated  160 . In each group  160 , there are 6 tubes  54  arranged in a hexagonal pattern and a center tube  54 . Each group  160  of seven flotation tubes  54  has upper and lower perimeter frames  162  which are (or may be) the same or similar to those described elsewhere in this text, and there is also for each group  160  a surrounding cylindrical shroud  164 . 
     In FIG. 14, there is only shown one of the groups  160  of tubes, and it is to be understood that similar groups  160  of tubes  54  are positioned in the other five areas. For identification, the six areas where the groups  160  are situated are designated generally as  166 . 
     To position the tube groups  160  there is a larger assembly frame  168 . This assembly frame  168  comprises six vertical hollow columns  170 , arranged in a hexagonal pattern. Extending radially inwardly from each column  170  are upper and lower horizontal struts  172 , with each upper and lower pair of struts  172  connecting to a related interior vertical column  174 , these also being arranged in a hexagonal pattern matching that of the outer vertical columns  170 . The six interior vertical columns  174  are each interconnected by upper and lower struts  176  arranged in a hexagonal pattern. 
     It can be seen that each of the receiving areas  166  is defined by an adjacent pair of the outer columns  170 , and adjacent pair of two interior columns  174 , and their related interconnecting struts  172  and  176 . 
     The flotation assembly  158  of FIG. 14 is formed with an open center area  178 . One of the advantages of the arrangement of FIG. 14 and FIG. 15 is that with each group  160  of seven tubes  54  being held in position by the frame  162  and surrounded by the shroud  164 , it can function essentially as a unit which can be inserted into its area  16  in the larger assembly frame  168 . Thus, if replacement or repair is required for one or more of the tubes  54  of any one group  160 , the entire group unit  160  can be removed and replaced by another group  160  while the repairs and replacements are being made. 
     It is believed that the operation of this embodiment of FIGS. 14 and 15 is readily understandable from reviewing other portions of this text, so this will not be repeated in this section of this text. 
     Third Embodiment of the Present Invention 
     The third embodiment of the present invention is designated generally as  200  and is shown in FIGS. 16 through 19. This flotation assembly  200  is in the form of a SPAR and comprises a hull  202 , a SPAR frame  204  and a flotation apparatus  206  which in this embodiment can be considered to be a flotation module. The hull  202  and the SPAR frame  204  are fixedly connected to one another and act as a unit while the flotation module  206  is positioned within the hull  202  and the frame  204 , and is able to move vertically independently of the hull  202  the SPAR frame  204 . 
     In this particular embodiment, the hull  202  can be a conventional hull (or a nearly conventional hull) where the volume of the hull is sufficiently great so that it is able to float both the hull  202  and the SPAR frame  204 . Also, as an alternative, the hull  202  could be replaced with a flotation assembly such as that shown as  32  in FIGS. 2 and 5. 
     To describe now the flotation apparatus  206 , there is first a central section  208  which extends from a lower location upwardly and thence through the center opening  210  of the hull  202 . Then there is a surrounding section  212  which surrounds the lower part of the central section  208 . 
     The central section  208  comprises three groups of flotation tubes  214  (with seven tubes  214  in each group), with these three groups being the following, a lower group  216 , an intermediate group  218  and an uppermost group  220 . The tubes  214  of the uppermost group  220  bear against the lower surface  222  of a central load carrying support structure  224 . This load carrying structure  224  functions as a flotation support region against which the flotation lift forces of the upper group  220  of the tubes  214  bear directly. 
     The upper load carrying support structure  224  can be utilized in either or both of two ways. First, the upper surface  228  of the support structure  224  could support objects placed thereon. Second, in functioning as a support for the risers, at least some of the tubes  214  have a central tubular member which is arranged in a manner similar to the open ended tube  96   c  in the flotation tube  54 C shown in FIG. 6 c . The risers thus can extend upwardly through vertically aligned sets of three tubes  214 . In the following description of the various arrangements of the lower, intermediate, and upper tube groups  216 ,  218 ,  220 , respectively, it is to be understood that in each instance, each vertically aligned set of tubes that are used to have the riser extending therethrough will have the tube configuration of the tube  54 C of FIG. 6 c.    
     In the particular arrangement shown herein, the lower flotation tubes  214  of the lowermost group  216  are substantially the same as shown in FIG. 6 a , where there is the upper cap  80   a  and the lower open end at  86   a . Assuming that the level of the water in the lowermost tubes  214  is near the bottom end of these tubes, the air pressure in the tube chambers  84   a  is slightly below the water pressure at the level of the water at the lowermost ends in the lowermost tubes  214 . 
     The tubes  214  located in the intermediate group  218  can function in either of two modes. First, the upper and lower ends  80  and  90  of each of the intermediate tubes  214  of the group  218  can be closed, and the air pressure in the chamber of the intermediate tubes  214  can be raised to a level equal to (or even slightly greater than) the pressure in the lowermost tubes  214  the group  222 . In like manner, the tubes  214  and the uppermost group  220  also can be closed at both the top and bottom ends and also pressurized to the same pressure level (or even greater than the pressure level in the intermediate lower tubes  214  in the intermediate lower groups  220  and  222 , respectively). In that arrangement, the side walls  78  of each of the tubes in the three groups are all, of course, placed in hoop tension, but in addition have axially aligned tension loads placed thereon. 
     Alternatively, the tubes  214  of the intermediate and upper sets  218  and  220  can be made so as to have openings at their lower ends, as indicated at  98   e  in FIG. 6 e . In that situation, the side wall  78  of each of the tubes  214  in the upper and lower groups  220  and  218  would be made of a material with a high strength to weight ratio for resisting the axial compressive loads, such as steel or some other material. 
     The three tube groups  216 - 220  are enclosed in a surrounding structural cylinder  232  which is constructed to resist axially aligned loads. This structural shell  232  is fixedly connected to the upper support structure  224 . This shell structure  232  is also fixedly connected to the annular section  212  as will be described immediately below. 
     The surrounding section  212  of the flotation assembly  206  comprises an annular horizontally aligned support structure  234  and a structural frame  236  which is connected to the perimeter portion of the support structure  234  and extends downwardly therefrom. More particularly, this support frame  236  comprises a plurality of vertical support tubes  240  interconnected by struts  242  (see FIG.  17 ). Contained within the structure frame  236 , there are six groups  243  of flotation tubes  214 . There are radially inwardly extending struts  244 , and these in turn connect to inner struts  246  which have a hexagonal pattern similar to that of the outer struts  242 . Overall, this arrangement shown in FIG. 17 is rather similar to that shown in FIG. 4, except that in FIG. 17 there are fewer flotation tubes. The SPAR frame  204  is (or may be) the same as the truss section  34  shown in FIG.  2 . 
     In operation of flotation assembly  200  being positioned in a body of water, the flotation tubes  240  in the surrounding flotation section  212  bear against the annular support structure  234  to push it upwardly. The support structure  234  in turn transmits a vertical lifting force into the shell  232  that in turn bears against the upper load carrying structure  224 . 
     In addition, the tubes  214  of the three vertical tube groups  216 ,  218  and  220  bear against the upper load carrying structure  224 . As indicated previously, the flotation module  206  is constrained within the hull  202  and the SPAR frame  204  to limit any lateral movement, but this flotation module  206  is able to move vertically independently of the hull  202  and SPAR frame  204 . 
     Fourth Embodiment 
     This fourth embodiment will now be described with references to FIGS. 20,  21  and  22 . The situation for which this fourth embodiment is particularly adapted is where there is an existing prior art SPAR and the flotation capacity of the existing SPAR is not adequate to meet increased performance demands, in that the additional weight created by equipment, risers, etc. exceed the present flotation capacity of the SPAR. 
     As shown in FIG. 22, the SPAR  260  comprises the hull  262  and the SPAR truss  264  and the ballast section  266 . To provide the additional support, there is added to the hull  262  at an intermediate location a surrounding annular, horizontally aligned flange  267 . Attached to the outer edge of the flange  267  and extending downwardly therefrom is a surrounding cylindrical shroud  268  which provides an annular flotation space  269  between the shroud  268  and the radially inward cylindrical outer surface portion  270  of the hull  262 . 
     Positioned in this flotation space  269  is a plurality of flotation tubes  274 . These flotation tubes  274  can be brought to the location of the SPAR  260  and then lowered to a position a short distance below the shroud  268 . Each of the flotation tubes  274  is at that time filled with water. Then a small amount of air is injected into the upper end of the tube  274  and the tube is caused to rise into the annular space  269  enclosed by the shroud  268 . As the additional flotation tubes  274  are brought into their position within the shroud  268 , these can be positioned by suitable means so that these remain in proper vertical alignment. When the tubes  274  are all in place, then additional pressurized air can be directed into the tubes  274  to lower the water level in the tubes  274  and thus cause the tubes  274  to apply a flotation force against the flange  266  in the manner described above. 
     Fifth Embodiment 
     The fifth embodiment is shown in FIGS. 23 through 27. This fifth embodiment is similar to the first embodiment as shown in FIGS. 2 and 5, except that the conventional hull of the first embodiment has been replaced with a support structure which is positioned at or above the water surface. 
     In this fifth embodiment, there is a SPAR  290  comprising an upper flotation section  292 , a SPAR truss  294 , and a lower ballast section  296 . The flotation assembly  292  comprises an upper annular support structure  298 , an upper group  300  of flotation tubes  302 , a lower group  304  of flotation tubes  306 , and upper and lower shroud sections  308  and  310 , respectively. The support structure  298  and the upper and lower flotation tube groups  300  and  304  define a central through opening  305 . 
     As can be seen in FIG. 23, the arrangement of the upper group  300  flotation tubes  302  is substantially in the same arrangement as shown in FIG. 4 of the first embodiment. Thus, there are vertical columns  311  in a hexagonal configuration, upper and lower positioning frames  312  comprising perimeter struts  314 , radially extending struts  316  and inner struts  318 , with the flotation tubes  302  arranged in six groups in six trapezoidal frame sections. 
     The lower group  304  of flotation tubes  306  has exactly the same arrangement as shown in FIG. 23, and each of the flotation tubes  306  in the lower group  304  is positioned in vertical alignment with, and bears against, an aligned upper flotation tube  302  immediately above. Thus, it can be seen that the lower surface  320  of the support structure  298  forms the flotation support region of the support structure  298  where the upper ends of the upper tubes  302  bear against the lower surface  320 . The support structure  298  is made as a platform with sufficient structural strength to support the various loads which are placed thereon (e.g. the weight of personnel, equipment, the flotation forces, as well as the loads imposed thereon by wave action and other influences). 
     Also, the upper shroud section  308  is made with sufficient structural strength to withstand the various forces to which it is subjected by wave action and other forces. The structural strength of the upper shroud section  308  can be provided in various ways. For example, a structural member can be placed around the periphery of the central opening  305 , at the location of the upper group  300  of tubes  302  at the location of the upper group  300  of tubes  302  with radially aligned struts extending through the region occupied by the upper tubes  302  to form to a perimeter structure. Also, the perimeter structure itself could be made sufficiently strong to withstand such forces. 
     The lower shroud section  310  would normally be located below the impact zone and entirely (or at least in large part) located in the quiescent zone. Accordingly, the lower shroud section  310  could be made in much the same manner as the shroud  58  of the first embodiment. 
     The SPAR truss  294  comprises a plurality of vertical columns  311  which extend downwardly from the support structure and which are interconnected by various cross braces  324  that are both horizontally and diagonally aligned. These vertical tube member  322  extend all the way up to the upper support structure  298  to form with the upper support structure  298  a unitary rigid SPAR structure. In FIG.  27  the SPAR is shown with an outer shroud  319  extending downwardly over the upper portion of the lower flotation tubes  306 . 
     In operation, with the SPAR  290  being positioned in a body of water, as indicated in FIG. 25, the level of the water is at about the mid height of the upper group  300  of the flotation tubes  302 . It will be noted that at the lower end of these flotation tubes  302 , there are vent openings  98   e , as shown for the flotation tubes  54   e  in FIG.  6 E. Therefore, in accordance with the explanation previously with the flotation tube arrangement of FIG. 6E, the upper flotation tubes  302  will function in a manner to provide a flotation force which would be proportional to the air volume of each of the flotation tubes  302  that is above the water level in the flotation tubes up to the level of the surrounding water. In addition, the side wall of each of the tubes  302  will need to be constructed to take compression loads from the support structure  298  and transmit these to the upper ends of the lower group  304  of flotation tubes  306 . The flotation tubes  306  function in the manner of the flotation tubes  54   a  shown in FIG.  6 A. Thus, the side wall of the lower flotation tubes  306  will experience no axial compression loads, but will experience forces in hoop tension. 
     Sixth Embodiment 
     A sixth embodiment of the present invention is shown in FIG.  28 . This sixth embodiment has similarities to the fifth embodiment, and FIG. 28 is drawn somewhat schematically as is FIG. 24 which shows the fifth embodiment. 
     This sixth embodiment of FIG. 28 differs from the fifth embodiment primarily in that the lower set of flotation tubes does not have any direct bearing engagement with the upper set of flotation tubes. Rather, the buoyancy forces of the lower flotation tubes is transmitted into structure which in turn transmits these loads through structure to an upper support structure above the water level. Thus, while both the upper and lower flotation tubes provide a buoyancy force, in terms of transmitting these buoyancy forces into the structure, they operate separately from one another but combine in the sense that they do provide a net buoyancy force. 
     Also, it is to be understood that the sixth embodiment as shown in FIG. 28 could be incorporated in a float assembly, such as a SPAR assembly, as shown in FIGS. 26 and 27. 
     The flotation assembly  330  of this sixth embodiment comprises an upper annular support structure  332  located above the water level, and a lower support structure  334  located below the water level. (the water level being indicated at  335 ). 
     There is an upper group  336  of upper flotation tubes  338  which bear against the lower surface of the upper support structure  332 , and also a lower group  340  of lower flotation tubes  342  that bear against the bottom surface of the lower support structure  334 . 
     There is an interconnecting structure  344  which defines a through central opening  346 . The upper end portion  348  of this interconnecting structure  334  connects to the upper support structure  332 , and an intermediate portion  350  of the interconnecting structure connects to the lower support structure  334 . Thus, the buoyancy forces of the lower set of tubes  342  are reacted from the structure  334  into the interconnecting structure  350 , and then transmitted upwardly through the upper portion  352  of the interconnecting structure  344  and thence into the upper support structure  332 . 
     It will be noted that the lower end portions  352  of the upper flotation tubes  338  are open. Thus, with the upper flotation tubes  338  being pressurized so that the water level in the tubes  338  is only a short or moderate distance above the lower ends  342  of these flotation tubes  338 , the air pressure in the flotation tubes  338  will be substantially equal (or exactly equal) to the water pressure in the open water at the water level  354  in each of the tubes  338 . 
     Therefore, in the arrangement of FIG. 28, both the upper and lower sets of tubes  338  and  342  are not subjected to axial compression loads. Rather, both sets of tubes  338  and  342  are subjected only to hoop tension. The summation of the forces exerted by the pressurized air in the two sets  336  and  340  of tubes  338  and  342  is equal to the total weight of the flotation assembly  330 . 
     Also, there is an upper shroud  355  surrounding the upper tubes  338  and a lower shroud  356  surrounding the lower tubes  342 . As in one or more of the prior embodiments, the space  358  that is enclosed by the upper shroud  355  and the lower space  360  enclosed by the lower shroud  356  can also be filled with pressurized air (or other gas) to provide additional buoyancy. 
     It is to be understood that while the upper shroud  355  has been shown schematically by a single line, this upper shroud  355  can be made as a structural member that forms with the upper structure a unitary structure capable of withstanding impact loads of waves, etc. thus, the tubes  338  are protected from such impact loads and can be designed to withstand primarily the load which result from the tubes performing their flotation functions. This sixth embodiment can be incorporated in a truss structure as is done in the fifth embodiment of FIGS. 23-27. 
     Seventh Embodiment 
     The seventh embodiment of the present invention is shown is FIGS. 29 and 30. This embodiment is similar to the embodiment shown in FIG. 28, except that the lower support structure and lower flotation tubes of FIG. 28 have been deleted. 
     Thus, in this seventh embodiment, there is a flotation assembly  370  comprising an annular support structure  372  which can be similar to the support structure  332  of the sixth embodiment. Also, there is a plurality of flotation tubes  374  arranged in six groups  376 . There is a cylindrical outer structure  378  which surrounds the flotation tubes  374 , the upper circumferential edge portion of which surrounds it is fixedly attached to the upper annular structure  372 . The two structures  372  and  378  form a unitary structure which has sufficient structural strength to withstand the various impacts of waves and other forces. 
     The lower edge portion  380  of the circumferential structure  378  extends downwardly around the tubes  374  to a sufficient depth to protect the tubes  374  from any substantial forces resulting from wave action. Also, the surrounding structure  378  has sufficient strength to protect the tubes  374  from other external forces such as impacts from various items, etc. The bottom ends of the flotation tubes are open. 
     Also, there is an inner cylindrical structure  382 , the upper edge portion  384  of which is connected to the inside surface  386  of the structure  372 . This cylindrical structure  382  extends downwardly, with the lower edge portion  388  thereof reaching to the lower ends  381  of the tubes  374 . 
     In a preferred version, the inner cylindrical structural member  382  is structurally interconnected with the outer cylindrical structure  378  in a manner to enable both the structures  378  and  382  to cooperate with one another to create overall strength. More specifically, this is accomplished by providing a plurality of radially extending plates or frame members  390  that extend radially from the location of the outer structure  378  inwardly to connect to the inner structure  382 . As shown herein, these plates or frame member  390  connect to vertically extending tubular member  392  which are in turn made part of the outer cylindrical structure  378 . If the flotation assembly  370  is to be used as part of a SPAR structure then the tubular members  382  would extend downwardly as in the fifth embodiment to form the main vertical members of the SPAR truss. 
     Extending between the vertical tubular member  392  are circumferential struts  394  which cooperate with the frame member  390  to provide six circumferential areas or regions in which the flotation tubes  376  are positioned. Thus, with regard to the positioning of the flotation tubes, the arrangement in FIG. 29 is quite similar to that shown in FIG.  23 . While the flotation assembly  370  is shown by itself, it is to be understood that this could be incorporated with other structures (such as a SPAR structure, as indicated above). 
     It is believed that the operation of the flotation assembly  370  is evident from the prior description. Accordingly, this will be covered very briefly in this portion of the text. The outer cylindrical structure  378  obviously serves the function of a shroud simply to protect the tubes from impacts either from waves or various objects. 
     In addition, the three structural components, namely the upper annular structure  372 , the outer cylindrical structure  378  and the inner cylindrical structure  382  are interconnected with one another in a manner to provide an overall structure having adequate structural strength to withstand the impacts and waves and other forces to which the flotation assembly  370  might be subjected. It is evident that the radially interconnecting frame members  390  extending between the inner cylindrical structure  382  and the outer cylindrical structure  392  provide reinforcing and also enable the inner and outer structures  382  and  378  to cooperate with one another in load bearing relationship. Further, these structures  378 ,  372 , and  384  could be compartmentalized so that those portions of the structure  378  and  380  that are under water will provide a flotation force. 
     Eighth Embodiment 
     The eighth embodiment of the present invention is shown in FIGS. 31 and 32. This eighth embodiment incorporates the present invention in a tension leg platform, similar to that shown in FIGS. 12 and 13. However, this eighth embodiment differs form the embodiment shown in FIGS. 12 and 13 in that instead of showing a flotation assembly that is provided for the existing tension leg platform the flotation tubes of the present invention are positioned within the structure of the tension leg platform. 
     As shown in FIGS. 31 and 32 there is a tension leg platform  400  which comprises four legs  402 , each having a hexagonal cross-sectional configuration, and four horizontally extending structures  374  interconnecting the legs  402  in a square pattern. 
     As can be seen in FIG. 31, in each of the legs, there is provided a plurality of flotation tubes  406 . It can be seen that the flotation tubes  406  in each leg are arranged in groups and are held in position by a frames  408  which can be similar to the positioning frames described previously herein. 
     Also, in each of the interconnecting structures  404  there is a plurality of flotation tubes  410 . These tubes  410  could also be provided with suitable positioning means to keep these properly aligned and positioned within these structures  404 . 
     The shrouds  412  are best shown in FIG.  32 . These shrouds  412  enclose each of the four legs  402 , and the shrouds  412  can be provided as described previously in this text. Further, these shrouds  412 , in addition to providing the enclosing and protective function, are able to serve as structural members to withstand the various forces imposed on the tension leg platform  400 . Further, the positioning frames  408  can be provided in a manner that these connect to the vertical column members  414  in a manner to provide reinforcing. 
     With regard to the design and functioning of the flotation tubes  406  and  410 , in the simplest form these can be provided in the manner of the basic flotation tubes shown at  54   a  and FIG.  6 A. Alternatively, other options, such as shown in FIGS. 6B-6G would also be acceptable. For example, the flotation tubes  406  for the legs  402  may more advantageously be designed by having sets of upper and lower tubes. Since these various options have been described previously in this text, these will not be discussed further relative to this seventh embodiment. 
     Various Design Aspects of the Present Invention 
     One of the significant benefits of the present invention is that the flotation tubes can meet a wide variety of requirements. Reference is now made to FIGS. 33A to  33 H and  33 J which show rather schematically various arrangement. In FIG. 33A there is shown a flotation tube  500   a  which is substantially the same as flotation tube of  54   a  of FIG.  6 A. It can be seen that the tube  500   a  has an open bottom end  502   a . To the right of the tube  500   a  there is shown a diagram illustrating the hoop tension loads imposed on the tube  500   a . As explained previously, since the air pressure within the chamber  504   a  of the tube  500   a  is substantially the same throughout, and since the water pressure decreases at higher levels, the hoop tension increases along the side wall of the tube  500   a  in an upward direction. 
     There is a structure  508   a  being supported by the tube  500   a . The water level in the tube  500   a  is shown at  510   a , and the water level of the surrounding water is designated  512   a.    
     Obviously, this situation requires that as the hoop tension increases, the strength of the side wall of the tube  500   a  must increase correspondingly, either by adding high strength material or simply more of the same material. Alternatively if the tube is to have the same structural characteristics along its entire length, it must be designed to meet the maximum hoop tension requirements the exist at the top end of the tube. 
     With reference to FIG. 338, there is shown a flotation tube  500   b  which has its length doubled relative to the tube  500   a  of FIG.  33 A. There is shown to the right of the tube  500   b  a diagram illustrating the hoop tension. It can readily be seen that at the very top of the tube  500   b  the hoop tension is approximately twice as great as at the top of the tube  500   a , thus requiring the extra material and/or additional reinforcing to provide additional structural strength. Obviously, there is a practical limit beyond which it is more prudent to select other design options other than simply increasing the length of the tube yet further. 
     In the arrangement of FIG. 33C we have two tubes  500   c  stacked one on top of the other, with the individual tubes  500   c  having the same length as the tubes  500   a , but with the two tubes  500   c  stacked one on top of one another having the same length as the tube  500   b . In this instance, the tubes  500   c  have vent holes  506   c  at the lower end thereof. It can be seen that this is substantially the same arrangement as shown in FIG. 6E or in an alternative design, one or more of the extension tube arrangements such as shown at  96 G could be used. 
     As explained earlier in this text, the lower tube  500   c  could be the same as a tube  500   a , and a graph illustrating the hoop tension in the lower tube  500   c  is located at the right of FIG. 500 c . The upper tube  500   c  would have the same hoop tension pattern as the lower tube  500   c , and this is shown at the right of the upper tube  500   c . However, with the pressure in the upper tube  500   c  being less than that in the lower tube  500   c , as explained previously with reference to FIG. 6C, the upper tube  500   c  would be subjected to axial compression loads. Accordingly, in this particular design option, the material selected for the lower tube  500   c  would like to be in a material which has substantial strength to weight ratio in tension (such as fiberglass or other plastics), while the upper tube  500   c  would be made of a material which will withstand not only the loads in hoop tension, but also the axial compression loads. This may dictate that the upper tube  400   c  be made of a metal (such as steel) or other material having such characteristics. 
     However, as will be apparent skilled in this art, there are other design considerations, such resistance to corrosion, ability to resist shock loads (e.g. impact with some object), ease of installation and/or replacement, maintenance, etc. Thus, some of the tubes may be designed to incorporate a combination of materials where two different materials may be combined for a blend of capabilities, such as providing structural strength with other characteristics, such as resistance to corrosion, etc. Thus there could be a situation where the material providing structural strength would be more susceptible to corrosion, and thus may be incorporated with the surface layer of material more resistant to corrosion, as well as possibly adding structural strength to resist other types of loading, etc. This brings us to the fourth design option of FIG.  33 D. The design option shown in FIG. 33D is similar to that shown in FIG. 33C in that there are upper end lower flotation tubes  500   d , supporting a structure  508 . This arrangement is similar to that shown in FIG. 6F in a situation where the upper tube  500   d  is pressurized sufficiently to a high level so that the surrounding side wall of the upper tube is subjected to higher tension loads, and also to either little or no loading axially, or even axial tension loads to ensure that the axial compression loads will not be imposed upon the upper tube  500   b . As in FIGS. 33A,  33 B and  33 C, the structure, at  508   d  that is being supported is at the level of the water  512   d.    
     The distribution of the tension loads is shown in the graph immediately to the right of the upper and lower tubes  500   d . As illustrated herein, the pressure in the upper flotation tube  400   d  is sufficiently high so that in addition to the hoop tension loads, the upper tube  400   d  is subjected to a tension force axially. 
     In FIGS. 33E,  33 F,  33 G and  33 H there is the situation where the structure being supported is above the level of the water. 
     The arrangement shown in FIG. 33E is analogous to that shown in FIG. 33A, and in like manner, the arrangement shown in  33 F,  33 G and  30 H are analogous to, respectively, the arrangement shown in FIGS. 33B,  33 C and  33 D. 
     In FIG. 33E, the flotation tube  500   e  has an open bottom  502   e , and the tube chamber  40   e  has sufficient cross-sectional area and has a base sufficient length so that when the chamber  504   e  is pressurized to a sufficient level, the water level  510   e  in the tube  500   e  is sufficiently low so that the overall buoyancy force (which would be proportional to the difference in the elevation in the water level  512   e  to the surrounding water level at the water surface  512  so that the structure  506   e  is positioned at the desired level above the water surface  512 ). 
     The force distribution in hoop tension is illustrated in the graph immediately to the right of the tube  500   e , and it can be seen that the hoop tension increases from the lower location of the water level  510   e  upwardly to the water surface  512 . Then the hoop tension for the upper portion of the upper portion  514   e  of the tube  500   e  that is above the water surface  512   e  remains substantially constant. If a load is imposed on the structure  506   d  to increase the overall weight imposed on the flotation tube  500   e , then the tube  500   e  would sink lower into the water. However, this would result in an increased buoyancy force since the volume of the air in the chamber  504   e  that is below the level of the water  512   e  would increase. Further, since this would increase the pressure in the chamber  504   e , the additional pressurized air could be pumped into the chamber  504   e  to maintain the level of the water  510   e  in the tube  500   e  at a low level, relative to the overall length of the tube  500   e  to provide a sufficiently buoyant force. In that instance, the hoop tension in the upper portion of the tube  500   e  would increase to a higher level. 
     Reference is now made to FIG. 33F, where there is a tube  500   f  that is supporting a load or structure  506   f  which has a greater overall weight, either due to the weight of the structure  506   f  itself or due to added equipment or other items carried by the structure  506   f . In this instance, the tube  500   f  has the same cross-sectional area as the tube  400   e  of FIG. 33E, but the length of the tube  500   f  is increased. It can be seen that the water level  510   f  in the tube is much lower to increase the overall volume of air in the tube  500   f  and thus increase the buoyancy force. 
     As shown in FIG. 33F, the structure  508   f  is at a level above the water level  512   f . However, the elevation of the structure  506   f  can be lowered, simply by reducing the amount of air in the tube chamber  504   f . As the air is let out of the chamber  504   f  and the structure  506   f  moves to a lower elevation, the water level  510   f  in the tube  500   f  remains nearly the same, since the overall net buoyancy force has, in this instance, decreased only slightly, due to the fact that the greater portion of the lower end portion of the tube  500   f  is surrounded by water both outside and inside the tube so that there is some flotation force exerted by the volume displaced by the tube portion that is immersed (both inside and outside) by water. 
     It can be seen that from the graph immediately to the right of the tube  500   f  that there is the same pattern of hoop tension distribution as shown in FIG. 33E, but the hoop tension force has increased in the upper portion of the tube  500   f  in comparison with the tube  500   e.    
     FIG. 33G shows an arrangement which is substantially the same as in FIG. 33C, except that the upper and lower tubes  500   g  are positioned at a higher level so that the upper portion of the upper tube  500   g  is above the water line  512   g . On the assumption that the two tubes  500   g  are each at the same length as the tubes  500   c , and also have the same diameter, in order for the upper tube  500   g  to be positioned above the water level  512   g , the weight of the structure  508   g  plus any load placed thereon would be less than that of the structure  508   g  of FIG.  33 C. Also, as shown in the graph immediately to the right of the tubes  500   g  in FIG. 33G, the hoop tension imposed on the lower tube  500   g  is the same as that in the lower tube  500   c  in FIG.  33 C. However, the hoop tension imposed on the upper part of the upper tube  500   g  is lower than that imposed on the upper tube  500   c  of FIG.  33 C. The pressure level in both of the upper and lower tubes  33 G is lower than, respectively, the pressure in the corresponding upper an lower tubes  500   c  shown in FIG.  33 C. 
     FIG. 33H shows the arrangement similar to that shown in FIG. 33D, except that the tubes  500   h  are located in the water so that the structure  508   h  is above the water level  512   h . The upper tube  500   h  in FIG. 33H is, as in FIG. 33D, enclosed and pressurized to a sufficiently high level to remove any axial compression loads and actually impose a moderate axial tension load on the upper tube  400   h.    
     Thus, there is created the pattern of hoop tension as shown to the right of the tubes  400   h . On the assumption that the weight of the structure  508   h  and any loads carried thereon increases, then the structure  508   h  and the tubes  500   h  will move to a lower level and the pressures in both of the tubes  500   h  will be increased accordingly to provide the proper support. 
     Reference is now made to FIG. 33J where there is shown a flotation assembly where there is a group of tubes (three groups in all) arranged one above the other. The underlying design in the assembly of FIG. 33J is similar to that shown in FIG. 28 in that the buoyancy force of each group of tubes are not transmitted from lower tubes directly to upper tubes. Rather these are transmitted through lower and intermediate structural components up through the main structure  508   j  that is being supported. More specifically, in FIG. 33J there is an upper group  518   j  of upper tubes  520   j , an intermediate group  522   j  of tubes  524   j  and the lower group  526   j  of tubes  528   j . At the lower end of the assembly there is a ballast section  530   j.    
     There is an intermediate load transmitting frame generally designated  532   j , and this could be in the form of a SPAR frame as described previously herein. In the schematic drawing of FIG. 33J, this interconnecting structure  532   j  is shown as an elongate central member  534   j  that extends from the bottom wall of the structure  408   j  downwardly to connect to the ballast section  530   j . The upper group  518  of tubes  520  bear directly against the bottom surface of the structure  508   j ; the intermediate group  522   j  of tubes  524   j  bear against an intermediate support structure  536   j  that is fixedly connected to the central interconnecting structural member  534   j . Then the lowermost group  526   j  of tubes  528   j  bear against a lower low-transmitting structure  538   j  that also is fixedly connected to the central structure for  534   j.    
     With regard to the mode of operation of this float assembly in FIG. 33J, each of the tubes  520   j ,  524   j  and  528   j  would have the same force distribution in hoop tension as shown in FIG.  33 A. Thus, the buoyancy forces the groups of tubes are separate from one another in terms of load path, with the load paths of the lower groups  522   j  and  526   j  being directed upwardly through the structure  432   j , and the load path of the uppermost tubes  520   j  being transmitted to the upper structure  508   j.    
     It is evident that the various design options which are shown in FIGS. 30A-30H and  30 J could be employed in a variety of embodiments, such as those shown previously therein. Also, it is evident that while FIGS. 33A-33H show only a single tube  500  or two tubes  500  are shown in each Figure, there would in most instances be a plurality of such tubes, as shown in various embodiment described previously herein.