fig3 generally illustrates a cylindrical hull 10 constructed according to the invention . the hull 10 is formed from a plurality of separate hull sections 11 that will be more fully described below . one receiver 13 is illustrated for a configuration where the hull 10 may be used to receive the upper leg of a space frame or topside as disclosed in u . s . pat . no . 5 , 558 , 467 . although only one receiver is shown , a suitable number of receivers 13 would be provided . also , it is important to note that the hull construction of the invention is suitable for a variety of structures and not only the type of structure disclosed in the above - referenced patent . fig4 illustrates the structural arrangement of a portion of one section 11 of the cylindrical hull 10 according to the invention and a smaller portion of a second section 11 . the outer shell 12 of the hull 10 is provided with a plurality of longitudinal stiffeners 14 , rings 16 and tension - type lateral supports 20 for the rings 16 . the lateral supports 20 are provided only : at the same locations as selected radial girders 30 and are anchored at each end of a hull section ( compartment ) in order to act in tension under all loading conditions . the inner shell 22 of the hull 10 is provided with circular ring stiffeners 24 and optional tension - type lateral supports 26 for rings 24 . these structures are discussed in further detail below . as seen in fig5 , each ring 24 on the inner shell 22 may also be provided with a flange 31 , although those rings on the inner shell that weld to the radial girders will not normally need flanges since their spans are relatively short . fig5 illustrates the rings 24 both on and off of the inner shell 22 to provide a clear understanding of the construction . the flanged rings 24 are preferably rolled t &# 39 ; s , to provide ease of construction , but may also be formed by any other suitable means such as cut flat plate for the webs with a rolled flange welded thereon . as seen in fig6 , a flat circular plate 28 , which will form a watertight compartment division also referred to as a “ flat ”, has a central opening approximately equal to the outer diameter of the inner shell 22 and an outer diameter approximately equal to the inner diameter of the outer shell 12 . as seen in fig7 , a plurality of circular stiffeners arranged concentrically are added to the plate of the flat for stiffening . the curved stiffeners 32 are formed from angle iron or bulb tees and are placed on edge on the circular flat plate 28 with the flanges toward the outer shell plate to facilitate the cold bending operations of the stiffeners themselves . as seen in fig8 , a plurality of radial girders 30 are welded to the flat and to the circular stiffeners to support the stiffeners and further stiffen the flat . the radial girders 30 on the flats are preferably spaced apart in line with every third or fourth longitudinal stiffener 14 on the outer shell following a radial line toward the center of the cylinder . the radial girders 30 are essentially fabricated tees that may be tapered in depth and that are welded to the outer and inner shells 12 and 22 when the shells are erected onto the flats . the drawings and description illustrate the form of the radial girders that are suitable for the major portion of the length of the hull . due to increased hydrostatic pressures at the lower end of a deep draft hull , the radial girders at the lower end may need to be deeper , have haunched ends or be spaced more closely together and the opposite is the case for those girders at the upper end of the cylinder . however , the general principle illustrated in the drawings and described above is still the same . when the curved shell plate sections are set , fig8 a , the radial girders and the plate forming the flat are then welded to the inner diameter of the outer shell 12 at one end and to the outer diameter of the inner shell 22 at the other end . fig9 and 10 illustrate more detailed views of the arrangement of the longitudinal stiffener 14 , rings 16 , tension - type lateral supports 18 and 20 , circular stiffeners 32 and radial girder 30 at the outershell . fig9 illustrates the configuration at the locations of the radial girders on the flats while fig1 illustrates the configuration at the locations between the radial girders . ( it is noted that fig9 and 10 illustrate these portions of the hull structure in the normal upright and installed position . however , it will be seen from the description of the construction sequence below that each section 11 of the hull is constructed upside down compared to its orientation in the floating structure .) it can be seen that each longitudinal stiffener 14 is formed from an angle iron or bulb tee welded on one edge to the inner diameter of the outer shell 12 . at the locations of both the flats and any circumferential splices of the outer shell plate , as seen in fig1 a , the ends of the longitudinal stiffeners 14 are preferably cut at an angle ( normally 45 degrees to meet abs requirements for terminating stiffening that is not attached to another stiffening member ) instead of being square ended . at some locations in the cylinder , these sniped ends of the longitudinal stiffeners may need to be reinforced . as shown in fig1 b , the preferred method of reinforcement is to omit sniping the end of the stiffeners and , instead , cut the end of the stiffener square at a location approximately the stiffener depth back from the flat and add a thicker plate 15 ( in the shape of a sniped end ) to the flat side of the stiffener . except at the locations of the circumferential splices of the outer shell plates , these reinforcing plates can be added to the stiffeners in the shop before the stiffeners are welded to the shell plate . at the locations of the circumferential splices of the outer shell plates , this reinforcement detail is used although the sniped plates are positioned and welded after the shell is spliced instead of welding them on in the shop in order to provide access from both sides to make the circumferential welds in the shell plate . the rings 16 are formed out of webs cut in a curved shape from flat plate and at all locations , except for those rings 16 a that attach to the radial girders , the rings are flanged with flanges 17 rolled to the curvature of the inner edge of the webs and are attached to the webs . all the rings 16 and 16 a are welded on edge to the inner diameter of the outer shell 12 so as to be substantially parallel to the flat circular plate 28 . the rings with flanges 16 are designed to be of equal depth within any one compartment to facilitate the tension - type lateral bracing 20 . the rings without flanges 16 a are welded to the radial girders which provide the lateral support required since the spans of these rings between girders are relatively short . each hull section 11 is constructed upside down compared with its in - place orientation . the circular flat plate 28 is placed on a level work surface as seen in fig6 . a circular hole 29 is provided through the flat plate 28 that will serve to receive the access shaft . the curved stiffeners 32 are placed and welded in position on the flat plate 28 as seen in fig7 . it can be seen that the curved stiffeners 32 are preferably rolled to match the circumference of their position on the flat plate 28 and form concentric circles . this gives the advantage of all the stiffeners 32 crossing all the radial girders 30 in a perpendicular orientation which makes for easier welding of the stiffeners 32 to the radial girders 30 . a further advantage of the curved stiffeners is the equalization of the spans of the flat plate between stiffeners and between the stiffeners and the shells . the curved stiffeners 32 are welded in position on the flat plate 28 by any suitable means including manual and tracking - type semi - automatic welding units . it is preferable that the sections of curved stiffeners 32 be placed such that the joints necessary to form a continuous circular stiffener 32 do not radially overlap . the radial girders 30 , which have cutouts to fit over the curved stiffeners 32 , are placed over the curved stiffeners 32 as seen in fig8 and 8 a and welded in place to both the curved stiffeners 32 and the flat plate 28 . it can be seen that the radial girders 30 are preferably provided with a flange 31 rigidly attached to the edge of the girder for stiffening purposes . a tubular access shaft not shown is then placed in hole 29 and welded to both the flat plate and two of the radial girders to form a water tight seal between the access shaft and the flat and support the weight of the shaft in service . the construction of the outer shell is illustrated in fig1 a - e . the metal 12 a that will form the outer shell 12 is cut into pieces which are connected together preferentially to form a plate the height of a full or partial compartment and a portion of the circumference ( normally ⅛ th to ⅓ rd ). the longitudinal stiffeners 14 are welded in place on this section of the outer shell while the plate is in the flat position . the stiffeners 14 b and 14 c are varied in length and end - prep as required to interface with the intersecting structure . at the end 14 d which will be attached to the flat to form a single compartment , the stiffeners 14 b are stopped short to abut the unflanged ring which is added in the jig . at the end 14 e which will be attached to another section in the final assembly of the full cylinder , the stiffeners are cut short , fig1 c , and the reinforcing sniped - shape section 15 a is pre - positioned and temporarily attached so it is readily available to be re - located and welded as soon as the circumferential splice of the outer shell plate is completed . this section of the outer shell plate 12 a , with the longitudinal stiffeners 14 , is laid in a jig form 34 which has the same curvature as the outer shell . the weight of the stiffened plate forms the plate to the curvature of the outer shell with little or no additional force . rings 16 , which are provided with cutouts to fit over the longitudinal stiffeners 14 , are placed in position and welded to the outer shell section and to the longitudinal stiffeners . the ring 16 a , without a flange 17 , that will abut the radial girders 30 is also placed and welded in position . the lateral supports 20 for the rings 16 ( best seen in fig9 and 11 e ) are placed and welded transverse to the flanges 17 and anchored at each end . these are the tension - type lateral supports noted above . by anchoring these elements at each end , the lateral supports 20 for the rings 16 act in tension in both bracing directions , resulting in a very efficient member size . the lateral supports 18 for the longitudinal stiffeners 14 are placed and welded at the ends of the longitudinal stiffeners 14 and also anchored at both ends so as to similarly act in tension at all times . the lateral supports 18 at the end away from the pre - installed flat are installed in the jig . the lateral supports 18 on the end with the pre - installed flat are added after the outer shell plate sections are erected onto , the flat since these lateral supports are anchored at each end to the webs of the radial girders . other non - structural components such as access ladders are then added . the remaining sections of the outer shell 12 are constructed in the same manner . the sections may be turned over to add exterior ( convex side ) appurtenances such as anodes and strakes prior to assembly of the sections to the flat or these additions may be deferred until the shell is vertically erected to the flat . with the upper cylindrical section of the hull compartmented solely with flats , the compartment at the waterline will not normally require a second watertight outer shell section ( cofferdam section ) typical of the prior art . if the section of the hull that will be at the water line is required to have a cofferdam section for reasons of preference or special operational requirements , the construction of the cofferdam section and the portion of the hull inside the cofferdam section is the same as described above . the construction of the inner shell 22 is illustrated in fig1 and 13 . the metal 22 a that will form the inner shell 22 is cut into sections the length of a portion of the circumference ( typically ⅛ th to ⅓rd ) and preferentially the height ( width ) of a mill plate . the portion of the height and circumference will depend upon the fabricator . as illustrated in fig1 a and b , this section is mechanically rolled to the circumference of the inner shell and laid on a jig form 36 matching the curvature of the inner shell . additional pieces are placed on the jig form 36 and welded together to form the height of the section . as illustrated in fig1 a and b , the mechanically curved rings 24 are placed and welded in position on the section of the inner shell 22 . lateral supports for the rings 24 are generally not required but if they were , tension - type lateral supports 26 similar to those for the outer shell rings would be used . other non - structural components such as access ladders are then added . the remaining sections of the inner shell 22 are constructed in the same manner . the sections may be turned over to add exterior ( concave side ) appurtenances , eg . anodes , prior to assembly to the flat . to assemble one hull compartment , or partial compartment 11 , one inner shell section is stood up with one of its ends adjacent to the flat plate 28 as seen in fig1 a , aligned and plumbed with the flat plate 28 , and welded to the flat plate 28 and the ends of the radial girders 30 . the unflanged ring 24 a sits on the radial girders 30 to establish the correct position and is welded to the webs and flanges of the radial girders 30 . the remaining sections of the inner shell are positioned and welded in place in the same manner to form the inner shell 22 as seen in fig1 b . the sections that form the inner shell 22 are spliced together by welding and the rings 24 are also welded together . the outer shell is erected in the same manner . as seen in fig1 a and b , each section of the outer shell is stood in position and welded to the flat plate 28 as well as to the radial girders 30 . the sections of the outer shell and the rings are spliced together by welding . at the proper time in this sequence , the access shaft 33 , if used , is added . appurtenances are added at any time in the pre - fabrication and erection sequences which the fabricator considers desirable . to join one section 11 of the hull 10 to the next , a temporary erection brace assembly , similar to spokes on a bicycle wheel , not shown , is placed between the inner shell 22 and the outer shell 12 at the opposite end from the flat plate 28 . as seen in fig1 and 17 , the constructed hull section 11 is set on skidways 38 with temporary shoes 40 and upending shoes 42 . the hull section 11 is rotated either on the upending shoes or in the air so that the longitudinal axis of the hull section 11 is in a horizontal position and placed adjacent to a previously constructed hull section 11 that is also in a horizontal position . the end of the hull section 11 with the flat is placed next to the end of the adjacent hull section 11 where the temporary brace assembly is located . the two sections are moved together and then the outer shell , inner shell and access shaft shell plates are welded together . it can be seen from this explanation and fig3 that the hull sections 11 are assembled upside down but connected to form a hard tank so the side of the flat plate 28 with the radial girders 30 faces the bottom of the hull when in the installed position . the radial bulkheads are eliminated . this eliminates rigid supports of the inner and outer shells at four places which allows both types of shell plate and their curved stiffening to act as full rings which greatly increases their structural efficiency compared to the prior art where the shell plate and girder stiffening acted primarily in bending . the smooth lines of the full rings compared to the many intersections of girders and struts in the prior art results in the elimination of nearly all “ hot spot stress ” areas in the main framing which results in the elimination of insert plates and other means of remediating the stress concentrations common in the prior art . additionally , the area of stiffened plate eliminated with the radial bulkheads is approximately 10 % more than the area of stiffened plate added for additional flats to complete the compartmentation , which contributes to the lower steel weights of the invention as well as its greatly enhanced constructability , compared to the prior art for the same sized hull . the longitudinal stiffeners on the outer shell are not made structurally continuous across the flats . this configuration eliminates the practices of either 1 ) penetrating the flats with the stiffeners and sealing up the resulting holes or 2 ) adding brackets on each side of the flat at each stiffener . on a typical spar hull , this configuration eliminates over 800 such penetrations and their seal - plates or over 1 , 600 brackets . eliminating these penetrations or brackets allows the stiffener welding to be largely completed in the shop / pre - assembly stages while , in the prior art , the penetration and bracket work must necessarily be deferred until later in the fabrication sequence when the sub - assembly sections are connected at the erection site . this configuration further simplifies the construction by eliminating the need for the longitudinal stiffeners on either side of a flat to align with each other as was the case in the prior art . termination of the longitudinal stiffeners on the outer shell short of the flats causes the ends to cantilever from the last ring . this ring is spaced from the flat the proper distance to balance the bending moment in the stiffener at the cantilever with the maximum moments at other spans of the stiffeners resulting in the most efficient stiffener sizing . to maximize the cantilever moment capacity of the stiffeners , the free ends are laterally braced using the tension - type members . these slender members run transverse to the ends of the stiffeners , are attached to each stiffener and are anchored at each end so they act in tension under loadings from either axial direction . this approach eliminates bracket - type lateral bracing and results in significant savings in fabrication material and labor . terminating the stiffeners short also requires they be cut back ( sniped ) at 45 degrees to meet the requirements of the american bureau of shipping . there are several ways to reinforce the sniped ends of the stiffeners , including : lapping a thicker snipe - shaped piece onto the end of the stiffener , replacing the sloping section of stiffener web with a thicker piece in line with the web , adding a stiffener perpendicular to the web and following the incline or adding a section of flange along the incline . the advantage of this invention is that any of the methods of reinforcement can be done in the shop using automated equipment in advance of assembling the stiffeners to the shell plate . an exception to this is : at the locations of the circumferential splices where the sniped shape reinforcements are pre - made and only positioned when the stiffeners are being added then permanently put in place after the circumferential welds on the shell plate are complete . similar to the lateral bracing for the ends of the longitudinal stiffeners , the invention laterally braces the curved rings on the outer shell ( and the inner shell rings if required ) using slender members transverse to these girders and anchored to the longitudinal stiffeners at either end of a compartment so they always act in tension under loadings from either axial direction . to be most effective , this type of bracing requires the rings in any one compartment between flats be of the same depth . this is easily accomplished in the design by setting a ring depth and varying : the ring spacing , the flange widths or thicknesses of the rings to accommodate the differing hydrostatic loadings at different locations along a compartment &# 39 ; s depth . this type of lateral bracing virtually eliminates the need for the so - called “ tripping brackets ” used extensively in the prior art . since there are over 3 , 000 tripping brackets ( often measuring several feet in each direction ) in a typical spar hull , the lateral bracing approach in the present invention greatly reduces the material and labor required to complete the hull fabrication . this same lateral bracing technique is applicable to bracing the repetitive stiffening in any other parts of the hull . the circular centerwell is always in hoop tension which allows the shell to be stiffened with only ring stiffeners and eliminates the need for longitudinal stiffeners which greatly simplifies the stiffening of this shell plate . the concentric circular centerwell also provides a constant distance between the two shell plates . this allows a highly repetitive and uniform stiffening pattern for the flats using radial girders . combined with the radial girders are circular stiffeners that create perpendicular intersections of all the girders with the stiffeners , which is very desirable for welding as well as creating uniform spans of the flat plate between stiffeners themselves and between stiffeners and the shell plates , thus eliminating variations and non - uniformities which beget the special stiffening , inserts , and other remediations common in the prior art . all of the configuration details embodied in the present invention contribute to reducing the number of pieces that need to be cut , fitted , and welded together to form the completed cylinder . eliminating the tripping brackets and eliminating the filling of holes in the flats were mentioned above , but the configuration of the rings themselves compared to the girders in the prior art , the elimination of longitudinal stiffeners on the centerwell shell and the uniformity of the stiffening patterns of the flats all contribute to a significant reduction in the number of pieces and fewer pieces translate into lower costs . with the flats closer together , the compartments can be economically assembled with the flats parallel to the ground and the curved shell plates erected vertically so the effects of gravity do not affect the curvature of the shell plates as would be the case if the sections of shell plate were combined into full cylinders in the horizontal position . vertical erection of each compartment locates a higher percentage of the welding close to the ground and facilitates crane access to the interior throughout the erection of a complete compartment . while specific embodiments and / or details of the invention have been shown and described above to illustrate the application of the principles of the invention , it is to be understood that this invention may be embodied as more fully described in the claims , or as otherwise known by those skilled in the art ( including any and all equivalents ), without departing from such principles .