Abstract:
Column-stabilized floating offshore platform structures ( 10 ) having spaced apart buoyant main vertical columns ( 11 ) joined at lower ends by horizontal lower truss members ( 13 ) in a pin connection and joined at upper ends by a buoyant deck mount structure ( 14 ), and/or by horizontal truss members, to form a moment connection. A buoyant keel tank ( 15 ) having a central moon pool ( 15 A) can be retracted and extended relative to the main columns between a retracted transport mode and an extended operating mode. Ballast of the columns and keel tank can be adjusted to raise or lower the center of gravity of the structure with respect to its center of buoyancy to stabilize the structure and compensate for variable or fixed loads, deck payloads, environmental conditions, and operational and installation stages. A three-sided deck mount allows on-site float-over deck installation.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority of U.S. Provisional Application Ser. No. 61/029,565, filed Feb. 19, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to offshore floating vessels and platform structures used in the exploration and production of oil and gas products, and more particularly to an offshore floating platform structure having vertical columns connected at a lower end by lateral trusses, a telescoping keel tank supported beneath the columns, and a rectangular ring-like deck mount structure or a three-sided deck mount structure at the top of the columns open on one side to allow on-site float-over deck installation. 
     2. Brief Description of the Prior Art 
     In the following discussion the term “truss”, as used herein, refers to a welded or bolted cross braced open frame structure formed of slender tubular members. A truss bridges between vertical column structures to stabilize a semi-submersible vessel at the water surface when floating with respect to wind, wave, current and other horizontal loads. As used herein, the term “moment connection” means a connection designed to transfer moment as well as axial and shear forces between connecting members. The term “pin connection” means a connection designed to transfer axial and shear forces between connection members, but not moments. 
     Floating vessels and semi-submersible floating vessels, such as floating production platforms, storage and offloading vessels, tension leg platforms (TLPs) and SPAR structures, are commonly used for oil well drilling, oil production and living and working quarters. It is desirable to design floating structures with minimum heave (vertical) oscillations to waves in the ocean environment. 
     Conventional column-stabilized semi-submersible vessel or platforms typically comprise three or more large diameter vertical columns that are spatially separated and connected at their bottom ends by large horizontal pontoons. The columns and pontoons of modern semi-submersibles are usually constructed of shells formed of thin metal plates backed with welded stiffeners, frames, stringers, bulkheads and stiffeners and may have compartments or voids for ballast and storage. The deck structure is above the water with a sufficient distance between the still water level to the bottom of the deck to allow waves to pass across the columns without impacting on the bottom of the deck. The center of gravity of the entire semi-submersible vessel is generally high due to large deck loads above the water. The columns and the pontoons provide stability and the necessary upward buoyancy required to support the structure, the downward payload of the deck and equipment, and live loads. The center of gravity (CG) of a conventional semi-submersible vessel is usually maintained above the center of buoyancy (CB), unlike a SPAR structure. The center of gravity (CG) positioning controls the roll and pitch period of the vessel and also the vessel stability. Due to the shallow draft of the columns, bringing the center of gravity (CG) below the center of buoyancy (CB) demands a large amount of ballast compensation in the pontoons, in addition to the buoyancy required to support the deck. Thus a conventional semi-submersible requires large water displacement to support the deck payload. Other problems with conventional semi-submersible platforms is that they are not well suited for dry tree support because their heave oscillation varies from a small magnitude in calm sea or small wave conditions to large in stormy rough sea or high wave conditions, the added mass and ballast mass of the pontoon is too large to effectively shift the natural period away from the calm wave period, and damping is very poor and predominantly radial in nature. Thus, the conventional semi submersible platforms may be acceptable for dry-tree support in low and moderate sea states but not in extreme sea states. 
     A conventional semi-submersible structure is structurally stable in severe wave environment due to the fact that the conventional boxed shell pontoons, either all around or on two sides, provide the vessel with a strong “moment connection” at the bottom of the columns at their bottom ends. The deck structure is typically simply supported or placed at the top of the columns and is a hinge or “pin connection” at the top of the columns and has no capacity to transfer moment to the columns through the connection. In the conventional semi-submersible, the pontoon predominantly provides the required buoyancy and the columns are separated and sized for column stabilized requirements. The pontoon mass is also large to accommodate a large volume of ballast water required to lower the center of gravity (CG) sufficiently to provide adequate stability in extreme sea conditions. 
     The wave forces are large on the pontoon because the large volume and mass is located at a shallow draft. Thus, the moment connection at the bottom between the columns and the pontoon are subjected to these wave forces and the connection is also subjected to severe storm loadings and fatigue loadings. 
     My previous U.S. Pat. No. 6,671,124, which is hereby incorporated herein by reference, discloses column-stabilized floating structures having a plurality of vertical buoyant caissons bridged together in distantly spaced relation by a plurality of open frame horizontal truss pontoon members and vertical truss columns at a lower end. A work deck is secured to the top ends of the vertical caissons. The buoyancy of the caissons is selectively adjusted by means of ballast control. Water is selectively pumped into or out of keel tanks at the bottom of the truss structure such that the water mass and weight is adjustably tuned to raise or lower the center of gravity of the entire mass of the floating structure relative to its center of buoyancy. 
     My previous U.S. Pat. No. 6,899,492, which is hereby incorporated herein by reference, discloses jacket frame floating structures comprising one or more elongate vertical support columns formed of an open cross-braced jacket formwork of tubular members interconnected together and at least one cylindrical buoyancy capsule disposed in the open framework near an upper end and at least one cylindrical second buoyancy capsule near a lower end in vertically spaced relation. The buoyancy capsule(s) may be a single, or a plurality of upper and lower capsules bundled in circumferentially spaced relation with a central opening therethrough. Alternatively, a keel tank may replace the lower capsule. The buoyancy of the upper buoyancy capsule(s) is adjustably tuned to provide a buoyant force and a sufficient water plane area and moment of inertia required for stability of the floating structure, and the water mass and weight of the lower buoyancy capsule(s) or keel tank(s) is adjustably tuned to raise or lower the center of gravity of the entire mass of the floating structure with respect to its center of buoyancy. 
     My previous U.S. Pat. No. 6,942,427, which is hereby incorporated herein by reference, discloses floating offshore fluid storage caisson platforms having a large diameter vertically oriented buoyant column or caisson, or multiple caissons, defining a storage chamber, and a telescopic keel tank disposed at the bottom end thereof, and may have deck on top of the caisson(s). The structure can be transported horizontally either dry on a transporting vessel or towed with its keel tank in a fully retracted position. At the field of operation, the structure initially floats horizontally. The keel tank is extended and then slowly flooded to move the center of gravity of the structure toward the keel tank and with the heavier tank, the structure tilts upright to assume an operating vertical position with the telescopic keel tank extended downward with respect to the caisson, and thereafter as the storage chamber is filled with fluid, the relative position of the keel tank is adjustably tuned to raise or lower the center of gravity of the entire mass of the structure with respect to its center of buoyancy and maintain the center of gravity of the structure below its center of buoyancy and stabilize the structure vertically at a desired draft. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned problems and is distinguished over the prior art by column-stabilized floating offshore platform structures having buoyant main vertical columns bridged together in spaced apart relation by horizontal lower truss members at a lower end forming a “pin connection” and joined together at upper ends by a buoyant deck mount structure, and/or by horizontal truss members, to form a “moment connection”. A buoyant keel tank having a central moon pool can be retracted and extended relative to the main columns between a retracted transport mode and an extended operating mode. Ballast of the columns and keel tank can be adjusted to raise or lower the center of gravity of the structure with respect to its center of buoyancy to stabilize the structure and compensate for variable loads, fixed loads, deck payloads, environmental conditions, and operational and installation stages. The deck mount structure has box-like generally rectangular sides and may be in the form of a ring or may have a C-frame configuration with three box-like generally rectangular sides to define a wide opening between two laterally adjacent main columns on one side of the platform to allow on-site float-over deck installation of the platform deck from a barge onto the top of the deck mount structure. 
     A buoyant keel tank having a central moon pool is secured at the bottom ends of telescoping columns mounted in the main columns and can be retracted and extended relative to the main columns between a retracted transport mode and an extended operating mode. The weight and buoyancy of the main columns and keel tank is adjustably tuned to raise or lower the center of gravity of the entire mass of the structure with respect to its center of buoyancy according to ballast and variable or fixed loads including deck payloads, to stabilize the structure, and to compensate for different operational, environmental, survival and installation stages of the structure. 
     The present semi-submersible floating offshore platform incorporates technology that has significant differences and advantages over conventional semi-submersible structures in its structural load path, in its hydrodynamic performance, and in its ease of fabrication and transportation. Cost of fabrication is reduced with the simplicity of the hull design and eliminating large pontoon structures. 
     In contrast to conventional semi-submersible structures, the present semi-submersible platforms have a “moment connection” at the top of the columns and hinge or “pin connection” at the bottom of the columns provided by the submerged lateral truss structures connecting the columns on all four sides. The submerged lateral truss structures eliminate the large shell type boxed pontoons. These features significantly reduce the overall wave forces on the platform. 
       FIG. 12  illustrates schematically the forces on a moment connection at the top of a column and a truss pin connection at the bottom of the column, as utilized in the present invention. The “moment connection” of the deck mount structure at the top of the column forms a moment connection that transfers moment “M” as well as axial and shear forces between the connected members. The trusses at the lower end of the column form a pin connection that transfers axial and shear forces “P” between the connected members, but not moments. The force “P” acts in equal and opposite directions and is equal to the moment “M” divided by the distance “D” between the upper and lower horizontal tubular members of the truss. 
     The moment connection at the top of the columns and lateral truss pin connection at the bottom significantly increases structural stability of the present semi-submersible platforms. The trusses are open frame structures formed of small diameter tubular members and are transparent to wave action, thus, wave forces on the platform are reduced significantly. Because the wave forces are predominantly only on the vertical columns and much less on the trusses, the fatigue life of the platform is enhanced. This feature also reduces the mooring loads and force requirements for dynamic positioning thrusters. The lateral trusses also simplify the construction and total fabrication cost of the platform. 
     The moment connection at top of the columns is obtained by the box type framed deck mount structure welded to the top of the columns on three sides, leaving one side open, known as a “C-frame”, or on all four sides, known as a “Ring-frame” structure. The C-frame deck mount structure is designed with a very strong transverse side or back and two lateral sides connected to the transverse side or back and reinforced by triangular corner braces or corner knee braces. The box type deck mount structure is disposed well above the water surface with sufficient clearance to meet maximum wave heights, thus the platform is less subject to wave forces. The C-shaped deck mount structure also allows on-site float-over deck installation of the platform deck from a barge onto the top of the deck mount structure or main columns. 
     The columns are designed to take the vertical loads of the deck and have a large cross sectional area are of sufficient length to provide the required buoyancy for the platform, and are spaced apart to provide stability and adequate water plane area. 
     The keel tank is designed such that the mass and the ballast lower the center of gravity (CG) in the fully extended operating condition. The keel tank is retracted such that the platform is of a compact height during fabrication and transportation. This feature also allows the platform to work at a site for a particular period of time and then easily be relocated to another site by retracting the keel tank to assume a compact height. 
     The telescoping keel tank with the moon pool opening assists in all the phases of operation including fabrication, installation, in-place performances, and riser and/or tendon tensioning. Utilizing the telescoping keel tank for self-installation eliminates the need of an installation vessel onsite. When the risers are tensioned with the help of the keel tank, the deck is freed from the riser load. This is very advantageous in deepwater applications. 
     The platform with the framed deck support structure with a moment connection at the top of the columns above water and open frame truss pin connection at the bottom of the columns submerged also make the platform suitable for using vertical tension moorings with tethers to function as a tension leg platform (TLP) for deepwater applications. The TLP embodiment may be provides with or without the keel tank. With the keel tank, the tethers are connected to the keel tank at four corners and the ballast is adjusted to provide the required tension to the tethers. Without the keel tank, the tension leg tethers are connected to lower end of the four main columns and the ballast of the columns is adjusted to provide tension to the tethers. 
     Elimination of the pontoons and providing the rigid deck mount structure at the top of the columns reduces the mass and overall wave forces on the platform and enables the tension leg (TLP) configuration to be used in ultra deepwater and enables control the natural period of the platform and thereby reduce heave and pitch motions of the platform significantly resulting in smaller dynamic tension in the vertical moorings, such as tendons or tethers. 
     The present semi-submersible floating offshore platforms provide several deck installation options: (1) The deck structure and related equipment may be preinstalled at the top of the hull at the construction site; (2) the deck structure and related equipment may be integrated to the hull at quayside; or (3) in an embodiment having a generally C-shaped deck mount the platform deck may be floated over the hull on a barge and installed on the top of the hull at the operation site. 
     The structural and hydrodynamic response features of the present semi-submersible floating offshore platforms allow them to respond to effectively to heave motions and environmental forces in ultra-deepwater and the platform deck is independent of vertical riser and mooring loads, making them suitable for dry tree support in either low or moderate sea states as well as in extreme sea states. 
     With the present floating offshore platforms, the deck could be swapped out for different applications for different needs and different phases of the operations such as: drilling, production, and riserless well intervention, etc. For example, a drilling facility deck may be installed by the float-over technique, and when drilling is completed, the drilling deck removed with the help of the barge, and then a production deck installed by the float-over method. When well servicing is needed to improve the performance of the well production, then the production deck is replaced with a riserless well-intervention deck. After the well is reinstalled back to full production, the well-intervention deck may be replaced with the production deck. 
     Other features and advantages of the invention will become apparent from time to time throughout the specification and claims as hereinafter related. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the semi-submersible floating offshore drilling and production platform structure in accordance with the present invention as viewed from the top shown without a deck installed on the deck mount structure. 
         FIG. 2  is a perspective view of the semi-submersible floating offshore drilling and production platform structure as viewed from the bottom with a deck installed on the deck mount structure. 
         FIG. 3  is a perspective view of a modification of the semi-submersible floating offshore drilling and production platform structure without a deck mount structure as seen from the top without a deck mount structure. 
         FIG. 4A  is a perspective view showing somewhat schematically, an example of a bottom connector for connecting the lower ends of the knee braces to the keel tank. 
         FIG. 4B  is an elevation view showing somewhat schematically, a self-locking latch mechanism for connecting the upper ends of the knee braces to the columns. 
         FIGS. 5A and 5B  are side elevation views showing, somewhat schematically, the semi-submersible floating offshore drilling and production platform in a transport mode, and in an operating mode, respectively. 
         FIGS. 6A and 6B  are longitudinal cross sections showing, somewhat schematically, an example of a hydraulic locking mechanism at the upper and lower end portions, respectively, of the telescoping cylindrical column located inside the main columns. 
         FIGS. 7A and 7B  are schematic side elevation views illustrating the steps in fabricating, transporting, and self-installing of the semi-submersible floating offshore drilling and production platform to a site of operation. 
         FIGS. 8A through 8D  are schematic perspective views illustrating the steps in installing a deck on the top of the semi-submersible floating offshore drilling and production platform in a float-over deck installation technique. 
         FIGS. 9A and 9B  are schematic side elevation views showing the deck installed and the platform positioned above a well head on the sea floor with a dry tree placed on the platform deck and a pneumatic tensioner supported on the keel tank, and alternatively a riser extending between the well head and dry tree with the riser tensioned by a pneumatic tensioner disposed at the bottom of the deck, respectively. 
         FIG. 10  is a schematic perspective view showing an alternate tension leg embodiment of the platform. 
         FIGS. 11A and 11B  are schematic side elevation views illustrating the steps in fabricating, and transporting the tension leg platform to a site of operation, installing the deck, and tensioning the tendons. 
         FIG. 12  is a schematic illustration showing a moment connection at the top of a column and a truss pin connection at the bottom of the column. 
         FIG. 13  is a schematic side elevation of a plurality of the floating platform structures secured together with a deck secured at the top end to form a very large column-stabilized floating offshore structure capable of use as a floating airport, port, bridge or mobile offshore base. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIGS. 1 and 2 , there is shown, somewhat schematically, a column stabilized semi-submersible floating offshore drilling and production platform structure  10  in accordance with the present invention. The platform structure  10  has four main vertical columns  11 , which may be of generally rectangular or circular cross section, with short horizontal lateral extensions  12  at their lower ends facing in opposed relation, and lateral truss members  13  formed of cross-braced tubular members extending horizontally between the extensions at the lower end of the main columns connecting the adjacent columns. 
     A generally C-shaped or U-shaped deck mount structure  14  is secured to the top ends of the main vertical columns  11  for receiving and supporting a deck D (seen in  FIG. 2 ). The deck mount structure  14  has three box-like generally rectangular sides  14 A,  14 B and  14 C formed of metal plate defining a hollow interior and is open on one side defining a wide opening between two laterally adjacent main vertical columns  11 . The deck mount structure  14  is reinforced at two inside corners by a triangular box-like diagonal brace  14 E to add additional strength and support and reduce deformation at the free ends of the lateral sides of the open area. The deck D may be installed on the deck mount structure  14  by several different alternative methods, as discussed hereinafter. 
     Alternatively, as shown in dashed line in  FIG. 1 , the deck mount structure  14  may also be provided with a fourth box-like generally rectangular side  14 D to form a generally rectangular ring enclosed on all four sides, and provided with a triangular box-like diagonal brace  14 D in each inside corner. 
     The bottom ends of the main vertical columns  11  are enclosed by end plates  11 A. A generally rectangular ring-like keel tank  15  is supported beneath the bottom ends of the vertical columns  11  by smaller cylindrical vertical telescoping columns  16  each passing slidably through a bottom end plate  11 A of a respective main vertical column. The cylindrical vertical telescoping columns  16  are mounted in the main vertical columns  11  to be extended and retracted relative to the main vertical columns. The generally rectangular keel tank is formed of metal plate defining a hollow interior compartment capable of being ballasted and de-ballasted and has a central moon pool opening  15 A. 
     The cylindrical telescoping columns  16  and keel tank  15  may be raised and lowered by conventional raising and lowering mechanisms for extensible and retractable movement at various distances relative to the generally rectangular columns  11  for carrying out various operations, and may be locked at a fully extended or retracted position by a locking mechanism described hereinafter or other conventional locking mechanisms. 
     Conventional ballast control means, pumps and piping systems are provided for selectively pumping water into and out of the main vertical columns  11  and keel tank  15  to partially or fully flood and empty the columns and keel tank and thereby adjust the weight and ballast. Such raising and lowering mechanisms and means for flooding and de-flooding are conventional and well known in general shipboard and submarine ballast design practice, and therefore not shown or described in detail. 
       FIG. 3  shows a modification of the platform  10  wherein the upper ends of the four main vertical columns  11  have short horizontal lateral extensions  12  facing in opposed relation, and lateral truss members  13  formed of cross-braced tubular members extend horizontally between the extensions at the upper end of the main vertical columns connecting the adjacent columns, and leaving one side open defining a wide opening between the upper ends of two laterally adjacent main vertical columns. Thus, the platform  10  has lateral truss members  13  extending horizontally between the lower end of the main vertical columns  11  on all four sides, and lateral truss members  13  extending horizontally between the upper end of the main vertical columns on three sides, and leaving one side open. The three-sided truss configuration is reinforced at two inside corners by a diagonal brace or knee strut to add additional strength and support and reduce deformation at the free ends of the lateral sides of the open area. A diagonal brace  13 A may also be secured diagonally between the top corner of the lower lateral truss members and the lower corner of the upper lateral truss members. 
     This modification may be used with or without the generally C-shaped or U-shaped deck mount structure  14 . If the deck mount structure  14  is not used, lateral truss members  13  may be provided between the upper end of the main vertical columns on all four sides, and a deck may be secured to the top ends of the main vertical columns in a conventional manner to provide smaller deck load and total hull weight. This would also move the center of gravity (CG) of the entire structure downward. Braces may be provided between the top of the upper trusses and the deck or columns if required for structural integrity. 
     Referring additionally to  FIGS. 4A and 4B , a pair of knee braces  17  may be releasably connected between the keel tank  15  and the lower end of each main vertical column  111  on each of the four sides of the platform structure by a crane structure on the deck. The knee braces  17  are rotatably connected at a bottom end to the top surface of the keel tank  15  by a ball joint connection  18 , and are releasably connected at a top end to the lower end of each main vertical column  11  by a receiving and self locking latch mechanism  19 . The knee braces  17  may be provided with universal joints at both ends. However, the joint at the bottom end is not removable whereas the joint at the top of the knee brace connecting it to the column is removable when needed. When the keel tank  15  is in a retracted position, the knee braces  17  are stowed in a horizontal position on the top of the keel tank with one end connected by the ball joint connection  18 , and, when the keel tank is extended, their opposed end is received in the self locking latch mechanism  19  and connected to the lower end of the main vertical column  11  such that each pair of braces extend angularly between the keel tank and the adjacent columns. 
       FIG. 4B  shows, somewhat schematically, an example of a self locking latch mechanism  19 . The latch mechanism  19  has a U-shaped frame  19 A with a spring biased latch member  19 B hingedly mounted in the outer end of each leg of the frame. The latch members  19 B are biased normally outwardly in laterally opposed relation. As shown in dashed line, when the upper end of the knee brace  17  enters the U-shaped frame  19 A, the latch members  19 B are pressed inwardly against the spring pressure, and as the upper end of the brace passes by them, they spring back out to capture the upper end of the knee brace. A pair of stop pins  19 C limit the inward and outward travel of each latch member  19 B. The inner surface of the U-shaped frame  19 A may be provided with resilient pads  19 D for engaging the upper end of the knee brace  17 . The upper end of the knee brace  17  may be removed from the latch mechanism  19  by removing the outward travel limit pin  19 C by mechanical means or retracting it hydraulically. 
     In a preferred embodiment, the knee braces  17  are designed to be neutrally buoyant for ease of crane handling and installation. The knee braces  17  take the axial load without transferring a moment arm load to the columns. Thus, the fatigue life of the knee braces at their connections is enhanced. Eight knee braces, two per side, are used such that over all structural stability of the vessel is achieved. The locking connections at the top of the knee braces are designed to be unlocked such that the braces can be disconnected from the columns and stowed back horizontally on top of the keel tank to allow the keel tank to be de-ballasted and retracted to a compact floating or transportation draft. 
       FIGS. 5A and 5B  are side elevation views showing somewhat schematically, the semi-submersible floating offshore drilling and production platform structure  10  in accordance with the present invention in a transport mode and in an operating mode, respectively. In  FIG. 5A , the knee braces are not shown and the keel tank  16  is shown fully retracted in the transportation mode. In  FIG. 5B , the keel tank  15  is fully extended with the knee braces  17  extending angularly between the keel tank and the adjacent columns  11 . 
     As mentioned above, the cylindrical columns  16  and keel tank  15  may be raised and lowered by conventional raising and lowering mechanisms for extensible and retractable movement at various distances relative to the generally rectangular columns  11  for carrying out various operations, and may be locked at a fully extended or retracted position by a locking mechanism. When the keel tank  15  is telescoped down, it could be locked in-place to resist the heave added mass forces during operation if desired. 
     In some installations, depending upon the severity of the platform motion and the loads, the knee-braces may be eliminated, and several mechanisms may be used to lock the telescoping cylindrical column  16  to the generally rectangular columns  11 . For example, a hydraulically operated locking system may be placed inside the main vertical columns  11  and locked to withstand vertical loads due to wave and inertial loads on the keel-tank. The locking mechanism of such a locking system would be operated by hydraulic pressure and controlled from the top of the deck. 
       FIGS. 6A and 6B  illustrate somewhat schematically, an example, of a hydraulic locking system. In this example, an upper end portion ( FIG. 6A ) and a lower end portion ( FIG. 6B ) of the outside diameter of the telescoping cylindrical column  16  is provided with a reduced diameter portion  16 A with opposed circumferential tapered portions  16 B above and below the reduced diameter. An outer ring  20  having an interior radial shoulder  20 A is secured to the interior of the main vertical column  11  at its upper end and lower end. An expandable split ring  21  having a radial flange  21 A at one end and a tapered interior surface  21 B at opposed ends is disposed between the exterior of the telescoping column  16  and the interior of the outer ring  20 . It should be noted that the radial flange  20 A in the outer ring  20  at the upper end ( FIG. 6A ) and the radial flange  20 A in the outer ring  20  at the lower end ( FIG. 6B ) are disposed in vertically opposed relation. 
     During downward travel of the telescoping column  16 , when it reaches its lowermost extent, the split ring  21  is expanded radially inward such that the upper tapered surface  16 B at the upper end of the telescoping column  16  engages the interior tapered surface  20 B at the top of the split ring and its radial flange  21 A engages the radial shoulder  20 A of the outer ring  20  to prevent further downward movement ( FIG. 6A ) and the column  16  takes the tension load. Similarly, during upward travel of the telescoping column  16 , when it reaches its uppermost extent, the split ring  21  is expanded radially inward such that the lower tapered surface  16 B at the lower end of the telescoping column engages the interior tapered surface  21 B at the bottom of the split ring  21  and its radial flange  21 A engages the radial shoulder  20 A of the outer ring  20  to prevent further upward movement ( FIG. 6B ) and the column  16  takes the compression load. 
     Thus, the telescoping column  16  may be operated to provide a reduced length for compression load and longer length for the tension/pulling load when the waves act on the keel tank  15 , thereby enhancing the structural load carrying efficiency of the telescopic inner column. Once these two locks at the upper and lower ends of the column  16  are engaged by the hydraulic system, then the keel tank  15  is fixed at the desired telescoped location. As discussed above, the knee braces  17  also share the axial loads, and the locking system may be provided as an alternative to the knee braces if they are eliminated, or provided in addition to the knee braces to share loads between the keel tank and the upper hull. 
       FIGS. 7A and 7B  are schematic side elevation views illustrating the steps in fabricating, and transporting the semi-submersible floating offshore drilling and production platform  10  to a site of operation. The platform is fabricated in the shipyard and skidded into the water to float on the retracted keel tank. It is then lifted on to the deck of a transportation barge B for dry transport to the operation site. When the barge reaches the operation site, the barge is flooded so that the semi submersible platform is floating on its retracted keel tank. The platform is allowed to free float in the sea and the barge is moved away from it. At this stage, the keel tank  15  is flooded to fully extend the telescoping cylindrical columns  16  and place the keel tank at the maximum distance beneath the upper main columns  11 . A crane C on the deck is used to mechanically lift the upper end of the knee braces  17  and the upper ends are connected to the rectangular columns to extend between the rectangular columns and the keel tank. The platform is allowed to float with maximum telescoped keel tank extension. Mooring lines M anchored to the sea floor are attached to the four main columns of the platform. Production risers are pulled up and hung from the sides of the keel tank. The keel tank is de-ballasted to adjust the production riser tension loads and obtain the required freeboard of the columns. Conventional riser tensioners may be supported on the keel tank and used for tensioning the risers if needed. Conventional thrusters may be installed on the keel tank for dynamically positioning the platform, or for assisted dynamic positioning. Adequate gas storage is possible for the dynamic positioning system in the keel tank. 
       FIGS. 8A through 8D  are schematic perspective views illustrating the steps in installing a deck on the top of the semi-submersible floating offshore drilling and production platform. The deck D is transported by a barge B to the site of the platform ( FIG. 8A ). The barge approaches from the open side of the C-shaped or U-shaped deck mount structure  14 . The open side is sufficiently wide to provide clearance between the inside walls of the deck mount structure and columns for the barge to move into the open area of the platform with the deck D disposed above the deck mount structure  14  ( FIG. 8B ). The barge is positioned such that the deck D is disposed just over the deck mount structure, and the deck mating is accomplished with conventional equipment on the barge and also ballasting/de-ballasting both the barge and the keel tank of the platform ( FIG. 8C ). After the deck has been mated and secured to the deck mount structure, the barge is moved out from the open side of the platform ( FIG. 8D ). 
       FIG. 9A  is a schematic side elevation view showing the deck installed and the platform positioned by mooring lines M above a well head on the sea floor with a dry tree on the platform deck and a pneumatic riser tensioner supported on the keel tank. Riser tensioners may be may be supported on the outer sides of the keel tank or on the inner sides of the central moon pool  FIG. 9B  shows and alternate arrangement wherein a riser extends between the well head and dry tree with the riser tensioned by a pneumatic tensioner disposed on the deck. 
     The knee braces  17  take the vertical loads, thus, oil storage is feasible in the keel tank and the platform may be utilized in ultra deepwater dry-tree support for oil and gas production and also serve as a floating production storage and off-loading (FPSO) vessel. 
       FIG. 10  is a schematic perspective view showing an alternate tension leg (TLP) embodiment of the platform  10 A. The components shown and described previously are assigned the same numerals of reference, but will not be described again to avoid repetition. In this embodiment, the telescoping keel tank and vertical telescoping columns are eliminated, the bottom ends of main vertical columns  11  are sealed closed by a bottom end plate, and the generally rectangular columns  11  are ballasted and de-ballasted. Conventional pumps, control means, and piping systems are provided for selectively pumping water into and out of the columns  11  to partially or fully flood the columns and thereby adjust the weight and ballast. Such means for flooding and de-flooding a support column are conventional and well known in general shipboard and submarine ballast design practice, and therefore not shown or described in detail. In this embodiment, a cross-braced open tendon support frame  22 , similar in construction to the lateral truss members, are secured to the lower end of the main vertical columns  11  and extend a short distance radially outward therefrom. Each tendon support frame  22  is provided with a conventional tendon top connector for securing the top end of at least one tendon T extending from an anchor on the seabed. Such tendon top connectors are conventional and well known in the art, and therefore not shown or described in detail. 
       FIGS. 11A and 11B  are schematic side elevation views illustrating the steps in fabricating, and transporting the tension leg platform embodiment  10 A to a site of operation. In this embodiment the platform  10 A is fabricated in the shipyard and skidded into the water to float in an inverted position on the box-like deck mount structure  14 . It is then lifted on to the deck of a transportation barge B for dry transport to the operation site. When the barge reaches the operation site, the barge is flooded so that the platform is floating on its deck mount structure  14 . The platform  10 A is allowed to free float in the sea and the barge is moved away from it. A crane C on the deck of the barge is used to invert the platform and may be facilitated by partially flooding the columns  11  on one side while lifting the opposed side such that the deck mount structure is at the top and the columns are partially submerged. The columns are ballasted to place the deck mount structure a distance above the water surface for installation of the deck. 
     The deck D is transported by a barge to the site of the platform. The barge approaches from the open side of the C-shaped or U-shaped deck mount structure  14 . The open side is sufficiently wide to provide clearance between the inside walls of the deck mount structure and columns for the barge to move into the open area of the platform with the deck disposed above the deck mount structure. The barge B is positioned such that the deck D is disposed just over the deck mount structure  14 , and the deck mating is accomplished with conventional equipment on the barge and also ballasting/de-ballasting of both the barge and the columns of the platform. After the deck has been mated and secured to the deck mount structure, the barge is moved out from the open side of the platform. At this stage, the columns are ballasted to achieve a proper draft for connecting the top ends of the tendons T to the top connector in the tendon support frames. The columns are then de-ballasted to adjust and apply tension load on the tendons and obtain the required freeboard of the columns. 
     With the present floating offshore platforms, the deck could be swapped out for different applications for different needs and different phases of the operations such as: drilling, production, and riserless well intervention, etc. For example, a drilling facility deck may be installed by the float-over technique, and when drilling is completed, the drilling deck removed with the help of the barge, and then a production deck installed by the float-over method. When well servicing is needed to improve the performance of the well production, then the production deck is replaced with a riserless well-intervention deck. After the well is reinstalled back to full production, the well-intervention deck may be replaced with the production deck. 
     The present offshore floating platform structures may also be utilized for other offshore floating structure applications. For example,  FIG. 13  shows schematically a plurality of the platform structures connected together by horizontal truss members and a large deck or joined decks connected together to form a very large column-stabilized floating offshore structure capable of use as a floating airport, port, bridge or mobile offshore base. 
     While the present invention has been disclosed in various preferred forms, the specific embodiments thereof as disclosed and illustrated herein are considered as illustrative only of the principles of the invention and are not to be considered in a limiting sense in interpreting the claims. The claims are intended to include all novel and non-obvious combinations and sub-combinations of the various elements, features, functions, and/or properties disclosed herein. Variations in size, materials, shape, form, function and manner of operation, assembly and use, are deemed readily apparent and obvious to one skilled in the art from this disclosure, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed in the following claims defining the present invention.