Patent Publication Number: US-6666624-B2

Title: Floating, modular deepwater platform and method of deployment

Description:
BACKGROUND OF INVENTION 
     This invention relates generally to platforms from which offshore operations, such as petroleum drilling and production, can be carried out and methods for installing or deploying these offshore platforms. The invention is particularly concerned with (1) methods for deploying, normally in relatively deepwater, floating platforms comprising two or more separately fabricated modules or structures and (2) the resulting platforms whose low heave, pitch and roll motions enable them to support surface wellhead equipment. 
     As hydrocarbon reserves decline, the search for oil and gas has moved offshore into increasingly deeper waters where economic considerations and physical limitations frequently militate against the use of platforms supported on the ocean or sea floor. Thus, most offshore drilling and production in deep water is conducted from floating platforms that support the drill rig, derrick, and associated drilling and production equipment. The three types of floating platforms that see the most use in deepwater are semisubmersible platforms, tension leg platforms (TLPs), and spars. 
     Semisubmersible floating platforms typically consist of a flotation hull usually comprising four or more large diameter vertical columns supported on two or more horizontal pontoons. The columns extend upward from the pontoons and support a platform deck. The flotation hull, when deballasted, allows the platform to be floated to the drill site where the hull is ballasted with seawater to submerge it such that the deck remains above the water surface. The platform is held in position by moorings lines anchored to the sea floor. Partially submerging the hull beneath the water surface reduces the effect of environmental forces, such as wind and waves, and results in a relatively stable work deck. Although the semisubmersible platform is stable for most drilling operations, it still exhibits a relatively large heave response to the environment that makes the use of surface wellheads (wellheads located above the water surface) undesirable because of the complexity and cost of riser tensioners and other clearance systems required to permit relative movement between the riser pipes and platform. Instead, the wellheads are typically located on the seafloor, and relatively complex and costly subsea equipment is used to produce hydrocarbons. However, the cost of accessing the wellheads for servicing and workovers becomes more difficult and costly as the water depth increases, thereby making the use of conventional semisubmersibles in deep water somewhat undesirable. 
     Tension leg platforms (TLPs) are also used to produce hydrocarbons in deep water. These platforms are moored to the ocean floor using semirigid or axially stiff (not axially flexible), substantially vertical tethers or tendons (usually a series of interconnected tubulars). The TLP platform is comprised of a deck and hull similar in configuration and construction to the semisubmersible platform. The hull provides excess buoyancy to support the deck and to tension the tethers and production risers. The deck supports drilling and production operations. The use of axially stiff tethers tensioned by the excess buoyancy of the hull to moor the platform tends to substantially eliminate heave, roll and pitch motions, thereby permitting the use of surface wellheads and all the benefits that accompany their use. However, heave restraining the entire platform, including the drilling rig, crew quarters and equipment, requires a substantial amount of additional buoyancy and tether steel, which in turn substantially increases the cost of the TLP. 
     Another type of floating structure used in offshore drilling and production operations is a spar. This type of structure is typically an elongated, vertically disposed, cylindrical hull that is buoyant at the top and ballasted at its base. The hull is anchored to the sea floor by flexible taut or catenary mooring lines. Although the upper portion of a spar&#39;s hull is buoyant, it is normally not ballastable. Substantially all the ballast is located in the lower portion of the hull and causes the spar to have a very deep draft, which tends to reduce heave, pitch and roll motions. The main problem with the use of spar platforms is the difficulty in deploying them in deep water. The elongated hull must be towed to the desired offshore location on its side and then upended in the water so it can be vertically oriented. After it is upended, its deck and associated equipment must be placed on the top of the hull. Both of these operations require the use of a large floating crane and other equipment at the offshore location, thus making the installation a complex and expensive endeavor. 
     It is clear from the above discussion that the three types of platforms commonly used in deepwater exploration and production have significant disadvantages. Thus, there exists a need for other platform designs that result in structures that not only possess low heave, pitch and roll motions but are also relatively inexpensive and simple to build and easy to deploy in relatively deep offshore waters. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, it has now been found that a floating platform or other apparatus can be more easily constructed and deployed in a body of water if it is comprised of two or more buoyant and ballastable structures or hulls that are separately fabricated in conventional shipyards or other fabrication facilities and then individually floated to the desired location in the body of water. Here, the lower structure is anchored to the floor of the body of water, usually using flexible and substantially non-vertical mooring lines, and then ballasted down until completely submerged. After the lower buoyant and ballastable structure has been anchored and completely submerged in the body of water, an upper structure is floated over the lower structure, and both structures are selectively ballasted and/or deballasted until the top of the lower structure mates with the bottom of the upper structure under the water surface to form a floating apparatus that can support a deck or platform from which offshore operations are conducted. The lower structure remains completely submerged in the water without touching the bottom of the body of water. 
     The resulting floating apparatus comprises an uppermost buoyant and ballastable structure partially submerged in the water without contacting the floor of the body of water and a lower buoyant and ballastable structure, which typically has a height greater than about 50% of the height of the uppermost structure, that is completely submerged in the water without contacting the floor of the body of water. The bottom of the uppermost structure is fixedly mated to the top of the lower structure, and the lower structure is anchored to the floor of the body of water, usually with flexible and non-vertical mooring lines. The uppermost structure typically supports a deck from which drilling, production, and workover operations are carried out. The relatively deep draft, usually greater than about 150 feet, of the combined structures coupled with the moorings used on the lower structure results in substantially reduced heave, pitch and roll motions and thereby makes it feasible to employ surface wellheads. Normally, it is not necessary for obtaining the desired heave, pitch and roll responses of the combined structures to anchor the uppermost structure to the floor of the body of water with moorings of any kind. However, the uppermost structure may contain winches or other devices for tensioning the mooring lines used to anchor the lower structure. 
     In some instances, especially when it is desired to provide oil and/or gas storage capabilities to the invention, more than two separate buoyant and ballastable structures can be utilized in the floating apparatus. Such a system can be constructed by fabricating the additional structure or structures in the shipyard and floating them to the desired offshore location where they are selectively ballasted and/or deballasted as described above such that the top of one structure mates with the bottom of another. The resulting apparatus will then comprise two or more structures completely submerged in the water with the uppermost structure only partially submerged and supporting a deck from which offshore operations can be conducted. Typically, only the lowermost structure will be anchored to the floor of the body of water, usually with flexible, non-vertical mooring lines. In this apparatus, the lowermost structure(s) can be designed to store oil and/or gas, and all of the structures combined may have a draft of as much as about 150 to 400 feet. 
     The apparatus and method of the invention have significant advantages over conventional offshore platforms and installation methods. The individual modules or structures comprising the apparatus of the invention can be made in simple shapes (e.g., square and rectangle boxes) and in relatively small sizes (e.g., heights usually less than about 150 feet) that allow the structures to be fabricated in conventional shipyards with conventional equipment. The uppermost structure can be built in the shipyard with the deck and associated drilling, production, and/or workover equipment preinstalled so that vertical lifting devices are not needed offshore to fit the platform and its equipment to the supporting structure. Furthermore, since the individual modules or structures are buoyant and ballastable, they can be towed to the desired offshore site without using barges and can be fixed together without the need for heavy lift equipment. Finally, the heave, pitch and roll resistance of the combined modules or structures allows the use of surface completions and wellheads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 in the drawings is a side elevation view of an embodiment of the apparatus of the invention containing two buoyant and ballastable modules or hulls attached to one another such that one is on top the other; 
     FIG. 2 is a plan view of the apparatus of the invention shown in FIG. 1 taken along the line  2 — 2 ; 
     FIG. 3 is a plan view of the apparatus of the invention shown in FIG. 1 taken along the line  3 — 3 ; 
     FIG. 4 is a side elevation view showing the upper and lower buoyant and ballastable modules or hulls of FIG. 1 floating separately in a body of water at a preselected offshore location before they are aligned, ballasted and mated to form the apparatus of the invention shown in FIG. 1; 
     FIG. 5 is a side elevation view showing the upper and lower buoyant and ballastable modules or hulls of FIG. 1 after the lower buoyant and ballastable module has been anchored or moored to the floor of the body of water and the upper module aligned thereover but before the upper and lower modules have been mated to form the apparatus of the invention shown in FIG. 1; and 
     FIG. 6 is a side elevation view of another embodiment of the apparatus of the invention containing three buoyant and ballastable modules or hulls attached to one another such that the upper module is on top of the middle module and the middle module is on top of the lower module. 
    
    
     All identical reference numerals in the figures of the drawings refer to the same or similar elements or features. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-3 in the drawings illustrate one embodiment of the apparatus of the invention, an offshore platform  10  for use in conducting drilling, production and/or workover operations in relatively deep water, e.g. water having a depth of between about 1,500 and 13,000 feet. It will be understood, however, that the apparatus of the invention is also suitable for other uses that require low motion support offshore in a body of water having a depth as low as 400 to 800 feet, but typically above 1,000 feet, such as supporting an industrial plant, storing supplies, accommodating personnel, landing aircraft and the like. 
     The platform  10  comprises deck  12  supported by a floating modular structure  14  that is comprised of upper hull structure  16  and lower hull structure  18 . The bottom of upper hull  16  is attached to and fixedly mated with the top of lower hull  18  by hull securing devices  20 . These securing devices may be any type of mechanical connector conventionally used to join large tubulars either above or below water. Examples of such connectors include self-locking pipe connectors, marine riser connectors, and hydraulic type connectors. In lieu of or in addition to mechanical connectors, the two hulls can be fixedly joined by permanent welds between the bottom of upper hull  16  and the top of lower hull  18 , or by net compression supplied by buoyancy control between the two adjoining hulls as will be described in more detail hereinafter. The modular structure  14  floats in body of water  21 , which, for example, may be an ocean, sea, bay or lake. 
     Lower hull  18  (see FIGS. 1 and 3) is comprised of four vertical lower hull columns  22 , four lower hull bottom pontoons  24  and, in some cases, four lower hull top pontoons  25 . The hull also contains a lower hull central column or well bay structure  26  that is connected to columns  22  by lower hull diagonal tubulars  28  and lower hull gusset plates  30 . The well bay serves to shield any risers supported by platform  10  from the wave action of the water  21  or other environmental forces. If, however, it is desirable to design lower hull  18  to be transparent to the horizontal movement of water  21 , the central column or well bay  26  can be eliminated from the lower hull or replaced by a conventional open truss framework of tubulars. 
     Lower hull  18  is anchored to the floor  32  of body of water  21  by mooring lines  34  and piles or other anchoring devices  36  to prevent large horizontal movements of modular structure  14 . Normally, sets of two, three or four mooring lines are attached to each of the four lower hull columns  22 . The mooring lines  34  may be taut, as shown in FIG. 1, or catenary and usually comprise a combination of steel chain and wire or synthetic rope as shown in FIG.  1 . These mooring lines are flexible and usually oriented in a substantially non-vertical position, usually from about 20 degrees to about 55 degrees from the vertical position, depending on the depth of body of water  21 . These characteristics distinguish them from the tendons used to anchor TLPs, which tendons are typically a series of interconnected semirigid tubulars oriented in a substantially vertical position. The mooring lines  34  are attached to the lower hull  18  using fairlead and chain stopper assemblies  38 . Although flexible mooring lines are normally used to anchor the lower hull to the floor  32  of body of water  21 , tendons can be used if desired. 
     The upper hull  16  (see FIGS. 1 and 2) is comprised of four vertical upper hull columns  40  and, in some cases, four upper hull pontoons  42 . The upper hull also contains an upper hull central column or well bay  44 , that is connected to columns  40  by upper hull diagonal tubulars  46  and upper hull gusset plates  48 . The upper hull well bay  44  serves to shield any risers supported by the platform  10  from the wave action of the water  21  or other environmental forces and, if its bottom is closed or plugged, can also be used for increased buoyancy of the upper hull during deployment of modular structure  14  as described in more detail hereinafter. If increased buoyancy is not needed, upper hull well bay  44  can be perforated—instead of continuous as shown in FIG.  1 —to decrease its resistance to water but still protect the risers from wave action. 
     The combination of upper hull  16  stacked on top of and fixedly attached to lower hull  18  forms floating modular structure  14 , which in turn supports deck  12 . In the embodiment of the invention shown in FIG. 1, deck  12  is used to support conventional oil and gas drilling and production equipment, such as derrick  50 , crew quarters  52  and heliport  54 . As pointed out above, however, deck  12  can be used to support other operations besides oil and gas drilling, production and workover. Any operation requiring support in an offshore environment can be carried out on deck  12 . 
     As shown in FIG. 1, the height of upper hull  16  is less than the height of lower hull  18 . Although this is the usual case, the heights of the two hulls may be the same or the height of the upper hull may be greater than that of the lower hull. Normally, the height of each individual hull ranges from about 80 to about 150 feet, preferably between about 100 and about 125 feet. The height of upper hull  16  is usually kept under about 125 feet to facilitate its fabrication in dry dock and the attachment of deck  12 . Such heights make it possible to build the individual hulls in conventional size shipyards or other fabrication facilities without the need for employing extra large construction equipment, such as oversized cranes and dry docks. 
     Although upper and lower hulls  16  and  18  are depicted, respectively, in FIGS. 2 and 3 as being in the shape of a square box, i.e., each hull having the same length as width, it will be understood that the width and length of the same hull and each individual hull can be different. The width of each hull typically ranges between about 90 and about 280 feet, usually from about 120 to about 250 feet. If the length is greater than the width, i.e., the hull is rectangular in shape, the length will usually exceed the width by between about 10 and 20 percent. 
     It will be understood that, although upper hull  16  and lower hull  18  typically each have a quadrilateral shape and contain four structural columns, each hull may have other shapes and contain a different number of structural columns. For example, each hull can take the shape of a triangle and contain three columns, a cylinder, which is itself a single column, or a hexagon and contain six columns. Furthermore, the shape and number of structural columns possessed by one hull may be different from the shape and number of structural columns possessed by another. For example, the upper hull could be triangular in shape and contain three structural columns while the lower hull is cylindrical or vice versa. Likewise, although upper hull columns  40  and lower hull columns  22  are depicted in FIGS. 1-3 as having a square cross sectional shape that is uniform in the vertical direction, they may have different cross sectional shapes, such as triangular, rectangular or circular, and the cross sectional shape may vary in the vertical direction. 
     Each hull  16  and  18  is designed to be both buoyant and ballastable and therefore contains ballast compartments or tanks, not shown in the drawings. These ballast compartments are usually located in lower hull bottom pontoons  24 , in upper hull pontoons  42  if present, in lower hull columns  22  and in upper hull columns  40 , thereby giving each hull adjustable ballast capability. Obviously, each hull contains conventional equipment associated with the ballast compartments, such as ballast pumps, manifolds, valves and piping, which allow ballast, typically seawater, to be pumped in or out of the ballast compartments to adjust the position of each hull in the water  21 . 
     Since it is the buoyancy of modular structure  14  that supports deck  12  and its payload of associated equipment, the size of the columns and pontoons will typically depend on the size of the payload. Normally, the width and length of the lower hull columns  22  and the upper hull columns  40  range between about 20 and 60 feet, while the height of the columns usually is between about 70 and 120 feet. The width of lower hull bottom pontoons  24 , lower hull top pontoons  25 , and upper hull pontoons  42  is typically the same as the width of columns  22  and  40  while the length varies from about 50 to about 230 feet. The pitch and roll motions of modular structure  14  can be decreased by increasing the length of the lower hull bottom pontoons  24  and upper hull pontoons  42  and thereby increasing the distance between the lower hull columns  22  and upper hull columns  40 , respectively. Typically, the height of lower hull bottom pontoons  24  is greater than that of lower hull top pontoons  25  and upper hull pontoons  42  and ranges between about 20 and 60 feet. However, it should be understood that it may not be necessary to utilize pontoons  25  and/or  42  in the modular structure  14  as is discussed in more detail below, and they may be eliminated altogether. 
     The upper and lower hulls  16  and  18  are usually individually ballasted so that modular structure  14  floats in body of water  21  such that the bottom of deck  12  is between about 20 and 60 feet above the water surface  56  and the modular structure  14  has a draft between about 100 and 300 feet, usually greater than about 150 feet and less than about 250 feet. A draft of this depth reduces the heave response of platform  10  to such a level that surface well completions can be utilized. Thus, platform  10  is shown in FIG. 1 to contain a riser  58  extending upward from water floor  32  through the bottom of deck  12  and terminating at wellhead  60 . Typically, platform  10  is designed to support between about 4 and 30 risers and associated surface wellheads. 
     As shown in FIG. 2, upper hull  16  contains guide frame  62  comprised of guidance sleeves  64 . Each riser supported by platform  10  passes through a guidance sleeve in the guide frame, which sleeve supplies lateral support to its riser. Each sleeve is usually between about 2 to 3 feet high and is typically formed by welding the small end of a cone-shaped sleeve onto each end of a short cylinder. Although not shown in FIGS. 1-3, the upper hull  16  usually contains 3 or 4 guidance frames spaced equally apart, while lower hull  18  typically contains 2 or 3 evenly spaced frames. 
     The use of a modular floating structure comprised of separate hulls disposed beneath one another as a platform for conducting offshore oil and gas operations has a number of advantages. It allows the individual hulls or modules to be separately fabricated, usually of steel, in small enough sizes and shapes that they can be made in conventional size shipyards with conventional equipment, thereby reducing manufacturing costs. Moreover, by pre-installing the deck and its associated equipment on the top of one of the modules during fabrication in dry dock, the need to employ cranes and other expensive equipment for an offshore installation is eliminated. Also, since the individually fabricated modules or hulls are buoyant and ballastable, they can be floated rather than barged to the desired offshore location. Finally, combining the modules offshore into a much larger structure results in a draft sufficiently deep that heave motions are reduced to a level that allows the use of surface wellheads, further decreasing the cost of producing oil and/or gas offshore. 
     FIGS. 4 and 5 illustrate one embodiment of the method of the invention. After upper and lower hulls  16  and  18  have been fabricated in the same or separate shipyards and the deck  12  with its associated equipment  50 ,  52 , and  54  has been installed on top of hull  16  in the shipyard, the two hulls are individually floated out of the shipyard and separately towed by boat in a low-draft position to the desired assembly or deployment site in body of water  21 . FIG. 4 shows the two hulls in their low-draft positions α and γ at the desired offshore assembly location after the towboats have departed. During the towing process, upper hull columns  40 , upper hull pontoons  42 , and upper hull well bay  44  provide the buoyancy required to float upper hull  16  (with deck  12  attached) in its low-draft position α to the desired offshore location. If the weight of deck  12  and its associated equipment is sufficiently low, it may be feasible to design the hull  16  without pontoons  42  and the buoyancy they provide. If the pontoons are not included in the hull, the well bay can be tied to upper hull columns  40  with a conventional open truss structure of tubulars not shown in the drawing. 
     The buoyancy required for floating lower hull  18  is provided by lower hull columns  22 , lower hull bottom pontoons  24 , and lower hull top pontoons  25 . If the added buoyancy that pontoons  25  provide is not needed, they can be eliminated and replaced with a conventional open truss structure. Such an open structure has the advantage of being transparent to the horizontal movement of water  21  and therefore tends to minimize drag response induced by wave energy and water current. 
     Once the upper and lower hulls arrive at the desired offshore location, deployment of platform  10  is begun, as shown in FIG.  5 . Normally, the first step in deployment is to ballast down the lower hull  18  until its top is near the water surface  56 , but far enough above the surface so that workers can stand and work on the top of the hull without being endangered by water and environmental forces. Next, the lower hull  18  is attached to mooring lines  34 . Prior to floating the hulls to the desired offshore location, one end of each mooring line is attached to a pile or other anchoring device  36  sunk into the floor  32  of body of water  21 . The other end of each mooring line is attached to the end of a lighter weight messenger line, and the mooring line is left lying on the floor  32  of the body of water. The other end of each messenger line is attached to a buoy or buoyant can, not shown in FIG. 5, floating at the water surface  56 . The messenger lines are then used to attach the mooring lines to the hull by pulling them into the fairleads  38  using winches or other equipment not shown in the figure. Stoppers above the fairleads hold the mooring lines in place. During the attachment process the hull  18  is pulled down further into the water and the mooring lines are overtensioned by the buoyant forces on the hull. 
     After the mooring lines have been attached to lower hull  18  and overtensioned, the hull is ballasted down further, usually by pumping water  21  into ballast compartments located in lower hull columns  22  and lower hull bottom pontoons  24 , until the lower hull is completely submerged in body of water  21  as shown in FIG.  5  and the tension on the mooring lines is decreased to the desired value. 
     Upper hull  16 , which carries deck  12 , is floated over and aligned with completely submerged lower hull  18  as shown in FIG.  5 . The upper hull  16  is then ballasted down by pumping water  21  into ballast compartments located in upper hull columns  40  and upper hull pontoons  42 , and the bottom used to prevent water from entering upper well bay  44 , thereby providing extra buoyancy during the towing of the upper hull, is removed. Enough ballast is added so that the bottom surfaces of the upper hull columns  40  contact and mate with the respective upper surfaces of the lower hull columns  22 , usually such that there are no vertical gaps between the columns. In order to obtain proper mating between the surfaces, it may be necessary to selectively and separately ballast and deballast each hull. 
     Once the upper hull  16  and lower hull  18  are mated, they are normally attached to each other and held together with mechanical locking devices  20 . It is possible, however, to weld the contact surfaces together from the inside of the hulls after they have been mated and thereby dispense with permanent locking devices. Alternatively, the hulls can be held together by buoyancy control to keep them in net compression at all times. If after the two hulls are mated there is slack in the mooring lines, it is taken up, usually by the use of winches mounted on upper hull  16 , and the lower hull  18  is slightly deballasted to raise the combined hulls enough to induce the desired tension forces in the mooring lines. The mating of the two hulls completes the installation or deployment process and results in the formation of the apparatus of the invention as shown in FIG.  1 . 
     The resultant platform is now ready for offshore operations including the installation of risers with surface wellheads, such as riser  58  shown in FIG.  1 . Normally, the upper hull is supported entirely by the bottom hull, which is held floating in place by mooring lines  34 . The draft of the combined hulls is sufficiently deep to significantly reduce heave, pitch and roll motions while the mooring lines control lateral motion. It is normally not necessary to use other types of anchoring devices, such as substantially vertical and axially stiff tendons and risers, on the lower hull. Moreover, the upper hull is typically devoid of mooring lines and tendons. There is no need to directly anchor the upper hull to the floor of the body of water. Its attachment to the lower hull is sufficient to provide it with the required stability. 
     As can be seen in FIG. 5 the bottom of upper hull  16  is a mirror image of the top of lower hull  18  because both hulls are designed similarly. However, this may not always be the case. For example, upper hull  16  may not contain upper hull pontoons  42  and upper hull well bay  44  may not extend all the way to the bottom of the hull. In fact, it is possible that well bay  44  will extend downward from deck  12  to only about half the height of hull  16 . Also, lower hull  18  may not contain lower hull top pontoons  25  or lower hull well bay  26 . Regardless of whether the bottom of upper hull  16  and top of lower hull  18  are mirror images of each other, the area encompassed by the periphery of the bottom surface of the upper hull, e.g., the area encompassed by the outer boundaries of FIG. 2, is normally substantially the same as the area encompassed by the top surface of the lower hull, e.g., the area encompassed by the outer boundaries of FIG.  3 . However, in some cases the area encompassed by the periphery of the bottom surface of the upper hull may be either larger or smaller than the area encompassed by the top surface of the lower hull. 
     Another embodiment of the apparatus of the invention is shown in FIG.  6 . This embodiment is similar to the one shown in FIG. 1, but contains a third module or hull  64  attached to and disposed below lower hull  18 . This hull is usually the same shape as the upper and lower hulls, but instead of containing columns it normally is a box-like structure with the center open to accommodate risers running downward from deck  12  to the floor  32  of the body of water  21 . The hull  64  contains multiple compartments, not shown in FIG. 6, that are designed to store oil and/or gas produced through the risers. The draft of this three-module embodiment of the apparatus of the invention is obviously greater than that of the embodiment shown in FIG.  1  and normally ranges from about 200 to about 400 feet. 
     The embodiment of the apparatus of the invention shown in FIG. 6 is deployed or installed using the method described for the installation of the embodiment shown in FIG.  1  and described with respect to FIGS. 4 and 5. Hull  64  is separately built in a shipyard and towed in a low-draft position, as is the upper and lower hulls  16  and  18 , respectively, to the desired offshore location. Then hull  64 , instead of hull  18 , is attached to mooring lines  34  as described previously. After hull  64  has been ballasted down and completely submerged in water  21 , the lower hull  18  is positioned and aligned over the hull  64  and then ballasted and/or deballasted until its bottom surface mates with and attaches to the top surface of hull  64 . The combination of the two hulls is ballasted down such that the combined structure is fully submerged in water  21 . Finally, upper hull  16  is floated over and aligned with the top of lower hull  18  and then mated with the lower hull as described previously with respect to FIG.  5 . 
     The embodiments of the apparatus of the invention shown in FIGS. 1 and 6 are comprised, respectively, of two and three buoyant and ballastable modules or hulls. It will be understood that the apparatus of the invention is not restricted to the use of two or three hulls. Any number of buoyant and ballastable hulls can be used to construct the apparatus of the invention in accordance with the methods of the invention described above. Moreover, the individual hulls can be designed similarly or differently depending on the offshore location and desired use of the apparatus of the invention. In addition, although the embodiment of the invention shown in FIG. 6 contains three modules and is designed to store oil and/or gas in the bottom module  64 , it will be understood that oil and/or gas can be stored in an embodiment of the apparatus of the invention containing only two buoyant and ballastable modules. For example, the embodiment of the invention shown in FIG. 6 can be modified by not employing lower hull  18 , thereby forming another embodiment of the invention in which the bottom of upper hull  16  is directly attached to the top of hull  64 . 
     Although this invention has been described by reference to several embodiments and to the figures in the drawing, it is evident that many alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace within the invention all such alternatives, modifications and variations that fall within the spirit and scope of the appended claims.