Abstract:
A leg assembly for a compliant offshore tower provides a structure allowing foundation drive piles to be driven and attached entirely externally to the space frame of the tower, and simultaneously providing restraints on bending forces within the drive pile—flex leg connection structure, allowing for ease of construction and assembly and greater strength and life-expectancy for the structure.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/093,070, which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to flex-leg assemblies for offshore platforms, specifically the flex-leg assemblies useful in providing vertical support and lateral stability for deepwater compliant tower platforms. 
       BACKGROUND OF THE INVENTION 
       [0003]    Traditional bottom-founded platforms using fixed or rigid structures are effective to support topsides facilities and risers in relatively shallow water depths, but they become economically ineffective in greater water depths. At water depths of  1000  feet or more, the tonnages required and the physical dimensions of the structure increase significantly, and more severe demands are imposed on construction equipment used to assemble the structure onshore and offshore. These factors increase the costs of deep-water structures significantly. 
         [0004]    Most deepwater developments have been accomplished using a class of structures which use compliancy to minimize the impact of periodic environmental loadings. A number of compliant structure concepts have been used extensively for deepwater developments, including tension leg platforms, spars, and floating production systems. These concepts are floating or buoyant structures that can perform effectively in water depths up to several thousand feet. 
         [0005]    Another compliant alternative is the compliant tower. Rather than floating, compliant towers are bottom-founded and more stable. Compliant response of the structure produces inertial loadings that are beneficial, rather than damaging. This beneficial effect results in structures which are much less demanding in terms of the tonnages required and the capabilities required of construction equipment and procedures. Compliant towers are thus a cost-efficient alternative. 
         [0006]    Compliant response requires that the natural periods of the structure are controlled to avoid resonance with periodic environmental forces. Ideally, the sway period is controlled so that the natural period in sway is approximately twice the period of peak waves in the design environment. Various approaches have been utilized effectively to control the flexibility of the tower and produce compliant responses. These approaches include extending the foundation piling to an attachment point well above the mudline, and applying a restoring couple at the attachment point to resist lateral sway motion. Additional measures include varying the width of the tower frames, and enhancing the strength of key members of the structure. Use of such design measures has supported the introduction of a class of towers known as “slim” towers that are smaller, require less steel, and are more easily constructed in the fabrication phase onshore and in the assembly phase offshore. 
         [0007]    One type of compliant tower is the compliant piled tower, such as that disclosed in U.S. Pat. No. 4,696,603 to Danaczko, et al. For such a tower, drive piles are driven into the seabed, but are extended well above the mudline to provide a restoring couple for stability against platform sway. A space frame, with its base resting on the ocean bottom, is extended to a level fifteen to thirty meters above the waterline. A drilling and production deck is placed on top of the space frame. A number of flex piles are driven at preselected positions located by guides placed around the periphery of the space frame. Each of the flex piles is extended along the space frame legs to a preselected elevation below the waterline, where it is attached to the space frame leg. The stiffness, number, and location of the flex piles are used to control the period of the sway response of the compliant platform. 
         [0008]    Those experienced in the design and construction of offshore deepwater structures understand the limitations existing with the compliant piled tower concept. Vertical extension of the drive piles requires that the flex piles be installed offshore in sections, with a number of sections required above the section that is driven into the sea floor. Assembly and connection of these sections offshore requires increased cost and risk due to the large number of connections that must be safely and reliably completed in a deepwater offshore environment. Because the piles are long and slender, the pile driving process is inefficient due to lower energy transfer compared to driving shorter, thicker piles. 
         [0009]    A second type of compliant tower uses long tubular members, or flex legs, which extend from the lowest levels of the space frame, essentially parallel to the space frame legs, to a preselected connection well above the base of the space frame. The flex legs are assembled to the space frame prior to moving offshore for final installation. The flex legs are located around the periphery of the space frame and are permanently connected to the drive piles during the offshore installation phase. Drive piles must be driven before the space frame is lowered into place. The space frame and twelve or more pre-attached flex legs must be simultaneously stabbed over the drive piles. To insure correct alignment of the drive piles for this step, a drive pile template must be used for foundation pile installation. Further, the offshore subsea stab-over of the space frame onto the drive piles reduces allowed tolerances for dimensions of the space frame in both fabrication and installation phases. The requirements for the drive pile template and reduced fabrication and installation tolerances increase the cost, complexity, and time required to install a flex leg tower. 
         [0010]    Other factors contribute to increasing the required cost of a flex leg tower. Determining the most effective base widths for the space frames is critical to the designer in controlling the natural periods for sway. The base width sets the spacing for the flex piles and the drive piles. It is desirable to reduce base width to reduce cost. However, if the base width is decreased, tower flexibility is increased with accompanying increases in the sway periods and reductions in sway response. The resultant reduced spacing of the drive piles also decreases the leverage of the piles and mitigates the advantages provided by reduced tower base dimensions. 
         [0011]    The construction phase introduces other limitations, challenges, and costs for a flex leg configuration. The drive pile template must be transported offshore and placed on four pre-driven leveling piles in the seafloor. These leveling piles are used to ensure that the space frame will be level. Four dedicated hydraulic leveling jacks are attached to the pile driving template to provide final leveling. The design of the tower requires that the verticality of the tower be controlled to ensure that the moments caused by deviations from vertical are within acceptable limits. The leveling jacks rest directly on the leveling piles and provide lifting forces to the tower to place the assembled tower in the desired vertical position. The leveling jacks are unrecoverable, and are thus an additional cost. 
         [0012]    The foundation pile template may be either a separate template which is removed following the pile installation, or a section of the rigid tower frame which becomes a permanent part of the space frame. If the template is to become a part of the permanent structure, the drive piles must be lengthened by twenty to twenty-five meters to allow the connection between the foundation pile and flex leg to be made. 
         [0013]    It has been proposed in earlier designs that the drive piles be offset laterally relative to the flex legs to simplify pile installation. U.S. Pat. No. 4,696,604 to Finn, et al. discloses a symmetrical arrangement of drive piles and flex piles around the space frame legs. This arrangement was intended to balance the moments caused by the offsets of the drive piles and flex piles. However, this arrangement required that at least one foundation pile for each space frame leg would be located interior to the space frame, complicating drive pile installation. Additionally, ring frames would be required to control the relative lateral movements of the flex piles caused by the moments due to axially eccentric flex legs and drive piles. 
         [0014]    It is thus desirable to develop a flex leg assembly for a compliant flex leg tower that effectively balances the load induced moments resulting from offset foundation pile configurations. It is further desirable to provide such an assembly that locates piles exterior to the space frame, thus allowing efficient installation of the drive piles with reduced risks. It is additionally desirable to provide such an assembly that improves control of the tower sway response, reduces materials required, eliminates the need for a pile installation template, simplifies leveling of the structure, and allows fit-for-purpose designs of the flex legs and the drive piles. 
       SUMMARY OF THE INVENTION 
       [0015]    The invention is directed to a leg assembly for use in providing the vertical support and lateral stability for an offshore platform comprising a space frame and load-bearing support legs. A flex leg is rigidly attached to a support leg at a point approximately one-half the height of the structure. The flex leg extends downward and essentially parallel to the support leg from this rigid attachment point. 
         [0016]    At the sea floor, a drive pile is driven through a drive pile sleeve into the sea floor, then attached to the drive pile sleeve, preferably by grouting. The flex leg is connected vertically to the drive pile sleeve to pass vertical loads from the flex leg to the drive pile. Lateral support for the flex leg in area where it is connected to the drive pile is provided by a lateral support frame, which prevents lateral relative movement between the flex leg, drive pile sleeve, drive pile, and support leg. The lateral support frame is connected to the support leg, the flex leg, and drive pile sleeve at multiple, vertically separate, points, or alternatively a plurality of vertically separated lateral support frames may be utilized. The support leg is rigidly attached to the lateral support frame. The flex leg, drive pile sleeve, and drive pile can move vertically relative to the lateral support frame. 
         [0017]    Because the flex leg member and the drive pile are not concentric, load induced moments could cause wear or other damage to the flex leg member and the drive pile if their relative horizontal motion is not constrained. By restraining the flex leg member and drive pile with a lateral support frame, load induced moments are restrained from developing in the flex leg or drive pile. 
         [0018]    In a preferred embodiment, the leg assembly comprises a plurality of flex legs in a symmetric array relative to the support leg, and a plurality of drive piles. The lateral load induced loads at each flex leg and drive pile array are balanced against opposing members within the group and to opposing arrays at opposite support legs in the space frame. The lateral support frame balances net lateral loads from the flex leg, support leg, and drive pile group to opposing groups in the structure. 
         [0019]    A number of advantages are derived when using this leg assembly. The connection of the flex legs to the drive piles with the lateral support frame allows all drive piles to be positioned exterior to the space frame. The result is a greatly simplified installation of the drive piles, as they are all accessible with no interior framing clashes, and installable using conventional driving procedures. A pile driving template is not required when using this configuration as drive pile sleeves, pre-assembled to the lower sections of the flex legs, serve to locate the piles during installation. Load induced bending forces from the flex leg to drive pile interfaces are minimized, which is important in controlling wear in the flex leg and drive pile sleeve. The number of flex legs can be minimized as there is no requirement for matching numbers of flex legs and drive piles as the balancing forces are provided by the lateral support frames, nor is there a requirement for symmetry of the drive pile array. 
         [0020]    Additional advantages arise with the reduced lateral spacing of the flex legs relative to prior designs. This reduced lateral spacing is achievable due to the fact that the flex legs and drive piles are not concentric. It is advantageous to minimize the lateral flex leg spacing and to increase the drive pile lateral spacing. The flexibility of the platform is increased with reduced lateral spacing of the flex legs, which produces reductions in sway response loads. Increases in pile spacing produce greater drive pile leveraging and reduced pile loads. In addition, the conventional pile installation also simplifies leveling of the platform which is important for long, slender structures due to moments which develop as a result of out-of-vertical postures. An active leveling system can be provided by applying jacking forces between the drive piles and the drive pile sleeve. The jacks are fully recoverable for use on future projects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1A  is a elevation view of a prior art compliant tower flex leg attachment structure. 
           [0022]      FIG. 1B  is an elevation view of the prior art compliant tower flex leg attachment structure of  FIG. 1A , showing the response of the structure to environmental forces. 
           [0023]      FIG. 1C  is a graphical view of the forces imposed on the flex legs and drive piles in the prior art configuration of  FIG. 1A . 
           [0024]      FIG. 1D  is a graphical view of the moments imposed on the drive piles in the prior art configuration of  FIG. 1A . 
           [0025]      FIG. 2  is an elevation view of a compliant tower incorporating an embodiment of the leg assembly of the present invention. 
           [0026]      FIG. 3  is a plan view of a lateral support frame, seen as a section of the compliant tower of  FIG. 2  through section line  3 - 3 ′. 
           [0027]      FIG. 4  is an elevation view of the flex leg to drive pile connection of a preferred embodiment of the present invention, including partial sectional views showing the arrangement of the drive pile, the drive pile sleeve, and lateral support frame, viewed from the position of line  4 - 4 ′ of  FIG. 3 . 
           [0028]      FIG. 5  is a plan view of compliant tower framing including the flex leg attachments, seen as a section of the compliant offshore structure of  FIG. 2  through section line  5 - 5 ′. 
           [0029]      FIG. 6  is an alternative embodiment of the view of  FIG. 2 , utilizing equal numbers of flex legs and drive piles. 
           [0030]      FIG. 7A  is a graphical view of the forces imposed on the flex legs and drive piles in the leg assembly of the present invention. 
           [0031]      FIG. 7B  is a graphical view of the moments imposed on the drive piles in the leg assembly of the present invention. 
           [0032]      FIG. 8  is a graphical view of the relationship between sway response and sway natural periods with lateral spacing of the drive piles and flex legs. 
           [0033]      FIG. 9  is a graphical view of drive pile loads produced by variations in the space frame width including similar changes in drive pile spacing, and the case where the drive pile spacing is kept constant while varying the space frame width. 
           [0034]      FIG. 10  is an elevation view of an alternative embodiment of a compliant tower incorporating the leg assembly of the present invention, including buoyancy modules. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Referring to  FIGS. 1A and 1B , a prior art leg assembly  112  is shown. The configuration shown in  FIGS. 1A and 1B  is that of U.S. Pat. No. 4,696,604 to Finn, et al. A space frame  114  comprising support legs  122  rests on the sea floor  118 . Drive piles  136  are engaged by drive pile sleeves  142 . (The upper portions of the compliant tower structure are not shown). As those of skill in the art will recognize, Drive piles  136  are usually connected to drive pile sleeves  142  by grouting, although other well known connection methods, such as swage connections, may be also used. Shear plates  144  provide a connection between drive pile sleeves  142  and flex legs  124 . Flex legs  124  are laterally constrained relative to support legs  122  via upper coupler  146  and lower coupler  147 . Upper coupler  146  and lower coupler  147  are vertically located at levels intermediate plan levels  116  of the space frame  114 . 
         [0036]    Upper coupler  146  and lower coupler  147  are provided to place the flex legs  124  in a symmetric array about support legs  122 , and to balance the large, force-induced moments cause by the eccentricity of the drive piles  136  and flex legs  124 . Upper coupler  146  and lower coupler  147  hang from flex legs  124  and slide vertically on support legs  122 . Forces in flex legs  124  can vary significantly, twisting upper coupler  146  and lower coupler  147  relative to support legs  122  and subjecting upper coupler  146  and lower coupler  147  to binding at support legs  122 . Additionally, upper coupler  146  and lower coupler  147  restrain relative vertical movement of flex legs  124 , reducing the effectiveness of flex legs  124 . 
         [0037]    As reflected in  FIG. 1B , when the compliant tower tilts in response to environmental forces, drive piles  136  are subjected to reaction forces, as will be flex piles  124 . The resultant bending of drive piles  136  and flex piles  124  results in the relative lateral movement of the longitudinal axes of drive piles  136  and flex piles  124 . Shear plates  144  cannot preclude this movement, because (as reflected in  FIGS. 1C and 1D ), reaction force R 136  causes bending force M in the shear plate  144 . Bending force M is thus transmitted to flex leg  124 , causing reaction forces R 146  and R 147  to develop at upper coupler  146  and lower coupler  147 , respectively. 
         [0038]    Referring to  FIG. 1C , unbalanced reaction forces R 146  and R 147  resulting from the interaction of upper coupler  146  and lower coupler  147  with flex leg  124  are shown graphically. The cause of the bending of the flex leg  124  in this manner is a rotational bending force M, which is translated through shear plate  144  from drive pile sleeve  142  and drive pile  136 , which experiences reaction force R 136 .  FIG. 1D  graphically depicts the corresponding bending force  150  in the flex leg  124  as this bending occurs in the flex leg  124 . 
         [0039]    Referring again to  FIG. 1A , the result of such unbalanced forces is large bending forces in the flex legs  124  between upper coupler  146  and lower coupler  147 , and excessive wear in the flex legs  124  where they are attached to the support legs above and below upper coupler  146  and lower coupler  147 . Additionally, these problems require that the flex legs  124  and drive piles  136  of this prior art embodiment must be in a symmetric array around the support legs  122 , forcing a complicated and expensive fabrication and assembly process, as one set of flex leg  124  and drive pile  136  must be located in the structural interior of space frame  114 . 
         [0040]    Those of skill in the art will recognize that the spacing between the flex legs  124  and drive piles  136  is severely limited because the moment in the flex legs  124  is directly proportional to this spacing. As a result, the ability to increase the leverage of the drive piles  136  is very limited. 
         [0041]    Referring now to  FIG. 2 , a compliant tower  10  comprising an embodiment of the leg assembly of the present invention is shown. Compliant tower  10  includes a substantially stiff slender space frame  12  extending from the sea floor  20  to a level approximately fifteen meters above the water surface  22 . The space frame  12  comprises steel tubular members, with essentially vertical support legs  14 . Support legs  14  extend to the sea floor  20 . Compliant tower  10  supports topsides drilling and production facilities  16 . Drilling equipment  19  extends from topsides drilling and production facilities  16  through the sea floor  20 . 
         [0042]    Referring to  FIGS. 2-5 , leg assemblies  23  of the present invention are provided at each of the support legs  14 . In a preferred embodiment, the leg assemblies  23  comprise a drive pile  24 , a drive pile sleeve  26 , a drive pile sleeve receptacle  27 , a lateral support connection  29  (in the preferred embodiment comprising upper lateral support frame  28  and lower lateral support frame  30 ), shear plate  32 , flex leg receptacle  34 , flex leg  36 , and flex leg guides  42 . Flex leg  36  comprises a lower section  38  and an upper section  40 . 
         [0043]    In a preferred embodiment, each leg assembly  23  comprises a plurality of drive piles  24  and of flex legs  36 . Moreover, the leg assembly of the present invention may be constructed with more drive piles  24  than flex legs  36 , or equal numbers of each, or with more flex legs  36  than drive piles  24 . 
         [0044]    Referring to  FIG. 3 , an example of four leg assemblies  23  of the present invention, each connected to one support leg  14  of a space frame  12 , is shown. Those of skill in the art will recognize that conductor guide  18  provides vertical guidance for drilling equipment ( 19  of  FIG. 2 ). Lower lateral support frames  30  tie space frame  12  to each of drive pile sleeve receptacles and flex leg receptacles  34 . Lower lateral support frames  30  are preferably joined to space frame  12  at a horizontal level with the interior cross bracing  13  of space frame  12 . Similarly, upper lateral support frame  28  ( FIG. 2 ) is preferably joined to space frame  12  at a higher horizontal point, again level with the interior cross bracing  13  of space frame  12 . Together, upper lateral support frame  28  and lower lateral support frame  30  comprise lateral support connection  29 . However, those of skill in the art will recognize that, although this embodiment is preferred, a single connection to space frame  12  could be engineered in a manner to support upper and lower attach points to leg assembly  23 , or three or more connections to space frame  12  could be used to support upper and lower attach points to leg assembly  23 . Thus it will be understood that the spirit of the invention is satisfied by providing two, vertically separated, lateral support points in the leg assembly  23 . 
         [0045]    The number of flex legs  36  is preferably fewer than the number of drive piles  24 . Upper and lower lateral support frames  28  and  30  prevent load induced moments from developing in the leg assemblies  23  as a result of the eccentricities between the drive piles  24  and flex legs  36 . Upper and lower lateral support frames  28  and  30  eliminate the need for symmetry of the drive piles  24  to balance the moments and prevent moments in the leg assemblies  23 . Optimization of the design of the leg assembly  23  independent of the drive pile  24  design can be performed as a result of the offsets between the two elements. The close proximity of the flex leg  36  to the space frame  12  improves the flexibility of the compliant tower  10  and reduces the sway global response. The increased spacing of the drive piles  24  increases the leverage of the drive piles  24  and reduces the pile forces, which results in reduced pile penetrations and tonnages. The increased leverage also improves the potential for using fewer drive piles  24  as a result of the reduced pile loads. (The term “drive pile” as used in this application also includes other pile forms including drilled piles or jetted piles.) 
         [0046]    Referring now to  FIG. 4 , a side elevation view of the lower portion  25  of one embodiment of a leg assembly  23  of the present invention is shown. Drive piles  24 , driven into the sea floor  20  are driven through drive pile sleeves  26 . Drive pile sleeves  26  are horizontally connected to drive pile sleeve receptacles  27 ,which in turn are rigidly connected to either upper lateral support frame  28 , or lower lateral support frame  30 . Flex leg  36  is horizontally connected to flex leg receptacles  34 , which in turn are rigidly connected to either upper lateral support frame  28 , or lower lateral support frame  30 . Additional connective security is provided by tying flex leg  36  to adjacent drive pile sleeves  26  via shear plates  32 . Shear plates  32  can alternatively be made in the form of tubular members, or with more flexible materials such as elastomeric materials. 
         [0047]    With this arrangement, the drive piles  24 , flex legs  36 , and support legs ( 14  of  FIG. 2 ) all move laterally in equal amounts because of upper lateral support frame  28  and lower lateral support frame  30 , but drive piles  24  and flex legs  36  are free to move vertically relative to upper lateral support frame  28  and lower lateral support frame  30 . 
         [0048]    As will be understood by those skilled in the art, once the space frame  12  is leveled, drive piles  24  can be rigidly attached to drive pile sleeves  26 , as by the preferred process of grouting. Other methods of forming such a rigid attachment, such as forming a swage connection, may also be used. 
         [0049]    Accordingly, the interconnection of upper lateral support frame  28  and lower lateral support frame  30  to space frame  12  (as shown in  FIG. 3 ) means that drive piles  24 , flex leg  36 , and support leg  14  (as shown in  FIG. 3 ) are constrained from relative lateral motion. Moreover, this construction arrangement no longer requires a symmetric arrangement of drive piles and flex legs around support leg  14 , allowing all drive piles  24  to be positioned to the exterior of space frame  12 . Further, this positioning of drive piles  24  allows drive piles  24  to be driven after the remainder of the structure is positioned on the sea floor  20 , avoiding the pre-positioning problems and expense of the stab-over methods discussed above. 
         [0050]    Leveling piles may be used to provide a level base for a compliant tower to be set upon offshore. Current practice includes the use of four jacking systems at the interface between the compliant tower and leveling piles to provide final leveling capability. These systems are used to ensure that the tower is vertical within desired tolerances. In this practice, the four leveling jack systems are not recoverable and have one time usage, representing additional cost. The leg assembly of the present invention provides accessible leveling surfaces at the tops  60  of the drive piles  24  and tops  50  of the drive pile sleeves  26 . These accessible surfaces act as jack support surfaces for the final leveling of the tower. Using these accessible surfaces, existing jacking system designs can be used that are recoverable and reusable. In some cases, a support (not shown) may be located interior to the top  50  of the drive pile sleeves  26 . This capability greatly affects installation simplicity and required resources. 
         [0051]    Additionally, supporting space frame  12  with leveling piles will reduce sway natural periods and increase sway response, if the leveling piles remain in a supporting connection with the space frame  12 . In the leg assembly of the present invention, leveling piles  48  can be located at the lower end  38  of flex legs  36 , reducing the undesirable impact on natural sway periods and the need to separate the structure from leveling piles  18 , thereby further simplifying the installation process. 
         [0052]    Referring to  FIG. 5 . flex leg guides  42  are positioned further up the space frame  12  (relative to  FIG. 4 ), to insure that flex legs  36  are constrained essentially parallel to support legs  14 . The flex leg guides  42  provide lateral restraining forces to the flex legs  36  without vertical restraint to flex leg assemblies  23 , thereby allowing the flex leg assemblies  23  to move vertically relative to the flex leg guides  42  and support leg  14 . Referring again to  FIG. 2 , flex legs  36  are rigidly attached to space frame  12  at a desired positions  44  above the sea floor  20 . 
         [0053]    Referring to  FIG. 6 , an alternative embodiment to that of  FIG. 3  of the leg assembly  23  of the present invention is shown, utilizing an equal number of flex legs  36  and drive piles  24 . As those of skill in the art will recognize, such combinations are a matter of engineering choice, depending on such matters as known environmental conditions, cost factors, the need to provide additional support for a compliant tower, and desired moment arms for the drive piles  24 . 
         [0054]    Referring now to  FIGS. 7A and 7B , graphical representations of the response of the leg assembly of the present invention are shown. Due to the lateral constraints provided by the lateral tying together of flex leg receptacles  34  and drive pile sleeve receptacles  27 , reaction forces R 27  and R 28  are provided, removing bending forces in flex leg  36  and drive pile  24 . As depicted by graph  60 , the leg assembly of the present invention maintains zero bending force in the flex leg, precluding wear of the assembly components from such bending. 
         [0055]    The flexibility provided by the flex legs  36 , is a function of the cross sectional area, length, number, distance from rotation axis, and material composition of flex legs  36 . This flexibility is a key element in controlling the sway response of compliant tower  10 . The natural sway period must be approximately twice the period of time varying forces such as wind and waves. One alternative in controlling the sway response is to vary the width of space frame  12 , which varies the distance of flex leg  36  from the axis of rotation. A reduction in the width of space frame  12  increases natural sway periods and reduces sway response. 
         [0056]    Referring to  FIG. 8 , these effects are displayed graphically for a compliant tower utilizing the present invention. An increase in natural sway period as the width of the tower is reduced is shown by first plot  52 . Second plot  54  demonstrates the reduction in sway response such as overturning moment as the natural sway period is increased with a reduction in the width of the space frame  12  and corresponding flex leg  36  spacing. In a preferred embodiment, the width of the space frame  12  and spacing of flex legs  36  is reduced, while drive piles  24  are maintained at a greater spacing, thereby preventing the drive pile loads from increasing as the base width of the space frame  12  is reduced. 
         [0057]    Referring to  FIG. 9 , third plot  56  shows how pile loads increase as their spacing is decreased. Fourth plot  58  demonstrates that pile loads remain essentially constant for a case in which the width of space frame  12  is decreased while maintaining the spacing of drive piles  24  constant. This effect is an advantage as drive pile  24  tonnages are reduced, saving cost, and installation is simplified with reduced pile lengths. 
         [0058]    Referring to  FIG. 10 , an alternative embodiment of the compliant tower of  FIG. 2  includes buoyancy modules  46  in the upper levels of space frame  12  to reduce load induced moments produced by the offset drive piles  24  of the present invention. Buoyancy modules  46  serve to offset gravity loading from the topsides weight and reduce corresponding load levels in the support legs  14  and flex legs  36 . Buoyancy modules  46  will also attract greater environmental loading from current and waves. Reductions in sway response will also develop as buoyancy modules  46  add considerably to the mass of the system and increase natural sway periods. The net impact from the preceding factors will produce load reductions in the flex legs  36 , upper and lower lateral support frames  28  and  30 , shear plates assemblies  32 , and drive piles  24 . 
         [0059]    Those of skill in the art will recognize that, if necessary to adapt to varying conditions such as heavy topsides in relatively shallow water conditions, flex legs  36  may comprise telescoping members (not shown) to allow varying flex leg length as conditions require. 
         [0060]    The above descriptions are provided as examples only, and are not intended to limit the scope of the invention as claimed below.