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CROSS-REFERENCE TO RELATED APPLICATIONS 
   None. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO A MICROFICHE APPENDIX 
   Not applicable. 
   FIELD OF THE INVENTION 
   The present disclosure is directed generally to the substructure of an offshore platform that supports drilling and production operations, and methods of assembling such a substructure in the ocean. More particularly, the present invention relates to various embodiments of modular concrete substructures that may be assembled at an offshore location to support the topsides of an offshore platform, and then optionally disassembled when the platform is no longer operational. 
   BACKGROUND 
   Offshore platforms support hydrocarbon drilling and production operations in the ocean. Regardless of the platform type, steel is the industry standard material used to construct both the substructure resting on the ocean floor and the topsides supported by the substructure and extending above the waterline to house personnel and equipment. For countries with limited capacity to fabricate steel, the requisite quantity of steel for the massive offshore platform substructures may be unavailable locally, and obtaining steel from other sources may be economically infeasible. In addition, conventional offshore platform substructures, which are custom designed and constructed in accordance with specific design criteria, such as water depth, wave and tide conditions, and ocean floor characteristics, for example, require long project lead times. Moreover, the heavy equipment necessary to install such steel substructures may not be accessible in remote countries. Therefore, a need exists for a readily available, versatile, easy to install, and economical alternative material to steel for offshore platform construction. 
   SUMMARY 
   In one aspect, the present disclosure is directed to a concrete section of an offshore platform substructure comprising a concrete body with a central opening and at least one guidepost hole extending through a height of the concrete body, wherein a width of the concrete body is greater than the height. The concrete section may further comprise one or more of the following features: at least one alignment nub on a surface of the concrete body, at least one alignment groove on a surface of the concrete body, at least one grout hole extending through the height of the concrete body, at least one window extending through at least a portion of the width of the concrete body. In various embodiments, the concrete section may be ring-shaped or polygonal-shaped. The concrete section may be formed from high-strength concrete. 
   In another aspect, the present disclosure is directed to an offshore platform substructure comprising a base portion resting on the ocean floor, and a plurality of concrete support sections stacked one on top of another on the base portion. The offshore platform substructure may further comprise a guidepost extending through the base portion and the plurality of concrete support sections into the ocean floor, and in an embodiment, the guidepost is grouted into position. The offshore platform substructure may further comprise a tightening cable extending into the base portion and through the plurality of concrete support sections, and in an embodiment, the tightening cable is grouted into position. The offshore platform substructure may further comprise a plurality of alignment nubs engaging a corresponding plurality of alignment grooves between adjacent concrete support sections within the plurality of concrete support sections. In an embodiment, the base portion comprises a concrete base section of substantially the same form as a concrete support section. The base portion may further comprise a concrete foundation poured into place between the concrete base section and the ocean floor. The offshore platform substructure may further comprise a window that allows ocean water to pass through the substructure. In an embodiment, the substructure tapers from a wider width at the base portion to a narrower width at an upper end of the plurality of concrete support sections. In various embodiments, each of the plurality of concrete support sections is ring-shaped with at least one central opening therethrough to receive drilling or production risers, or each of the plurality of concrete support sections is polygonal-shaped with at least one central opening therethrough to receive drilling or production risers. 
   In yet another aspect, a method of assembling an offshore platform with a concrete substructure comprises locating a guidepost in the ocean floor at a well site, towing a plurality of concrete sections to the well site, sequentially engaging each of the plurality of concrete sections with the guidepost, and sequentially sinking each of the plurality of concrete sections, thereby forming a stack of concrete sections on the ocean floor. The method may further comprise aligning each of the plurality of concrete sections, and locking each of the plurality of concrete sections together to prevent relative lateral movement. In various embodiments, the method further comprises applying a weight to the stack of concrete sections to mimic a weight of an offshore platform topsides, jetting in a lowermost concrete section in the stack of concrete sections into the ocean floor, and/or pouring a cement foundation between a lowermost concrete section in the stack of concrete sections and the ocean floor. The method may further comprise drilling an additional guidepost through the stack of concrete sections and into the ocean floor, extending a cable through the stack of concrete sections and applying a tension load to the cable, compressing the stack of concrete sections and grouting the cable into place after compressing the stack of concrete sections. In an embodiment, the method further comprises grouting between each of the plurality of concrete sections. The method may further comprise installing a topsides onto the stack of concrete sections. In an embodiment, installing the topsides comprises floating the topsides over the stack of concrete sections, lowering the topsides to the stack of concrete sections, and jacking up the topsides above a waterline. In another embodiment, installing the topsides comprises lifting the topsides onto the stack of concrete sections. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of the modular concrete substructures and methods of constructing same, reference will now be made to the accompanying drawings, wherein: 
       FIG. 1  schematically depicts a representative installed offshore platform comprising one embodiment of a modular concrete substructure supporting topsides; 
       FIG. 2  is an enlarged cross-sectional side view of a plurality of representative modular concrete support sections resting on a concrete base section; 
       FIG. 3  is an enlarged cross-sectional top view of one of the modular concrete support sections depicted in  FIG. 2 ; and 
       FIG. 4  through  FIG. 8  depict a typical assembly sequence for a modular concrete substructure wherein the topsides may be installed by floating over the substructure and then jacking the topsides up from the substructure on legs. 
   

   NOTATION AND NOMENCLATURE 
   Certain terms are used throughout the following description and claims to refer to particular assembly components. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. 
   As used herein, the term “substructure” generally refers to the supporting base of an offshore platform that rests on the ocean floor and supports the topsides of the offshore platform. The substructure extends from the ocean floor to approximately just below or just above the waterline. 
   As used herein, the term “topsides” generally refers to the deck and other equipment of an offshore platform that is supported by the substructure of the offshore platform. By way of example only, representative topsides may include small, lightweight structures, such as field warehouse facilities; large complex production facilities; or specialty facilities, such as LNG storage tanks. 
   As used herein, the term “high strength concrete” generally refers to a concrete with a compressive strength greater than 6000 pounds per square inch as defined by the American Concrete Institute, wherein compressive strength refers to the maximum resistance of a concrete sample to applied pressure. 
   DETAILED DESCRIPTION 
   Various embodiments of a modular concrete substructure for a fixed offshore platform and methods of assembling a modular concrete substructure will now be described with reference to the accompanying drawings, wherein like reference numerals are used for like features throughout the several views. There are shown in the drawings, and herein will be described in detail, specific embodiments of the modular concrete substructure and assembly methods with the understanding that this disclosure is representative only and is not intended to limit the invention to those embodiments illustrated and described herein. The embodiments of the modular concrete substructure and methods of assembly and/or installation disclosed herein may be used in any fixed offshore platform where it is desired to support topsides. It is to be fully recognized that the different teachings of the embodiments disclosed herein may be employed separately or in any suitable combination to produce desired results. 
     FIG. 1  depicts one representative fixed offshore platform  100  resting at a desired location on the ocean floor  110 , such as at a hydrocarbon well site, for example. The platform  100  comprises a modular concrete substructure  120  that, in this embodiment, extends from the ocean floor  110  to a height above the water level  130 , but in other embodiments, the substructure  120  may extend from the ocean floor  110  to a height below the water level  130 . The modular concrete substructure  120  supports topsides  140 , which may house personnel and equipment needed to drill and/or produce oil and natural gas from the well site. The modular concrete substructure  120  comprises a plurality of pre-fabricated concrete support sections  150  supported by a pre-fabricated concrete base section  160  and a poured concrete foundation  210 . In an embodiment, high strength concrete may be used to form the concrete base section  160 , the concrete support sections  150 , the concrete foundation  210 , or any combination thereof. One or more guideposts  225  may extend through the modular concrete substructure  120  into the ocean floor  110  to strengthen and stabilize the modular concrete substructure  120  and resist the forces of ocean currents. The concrete base section  160 , the concrete support sections  150 , the concrete foundation  210 , may all have a similar shape. In various embodiments, the concrete base section  160 , the concrete support sections  150 , and the concrete foundation  210  may be generally ring-shaped, namely circular when viewed from the top, or polygonal-shaped, such as square or rectangular, for example, when viewed from the top, and with an opening therethrough of sufficient dimension to permit the passage of one or more drilling and/or production risers. One skilled in the art will readily appreciate that the shape of the concrete base section  160 , the concrete support sections  150 , and the concrete foundation  210  may vary, and the concrete foundation  210  may even be irregular depending upon the quality or other characteristics of the firm bottom  220  of the ocean floor  110 . In the embodiment shown in  FIG. 1 , the width (or diameter) of the concrete base section  160 , the concrete support sections  150 , and the concrete foundation  210  is greater than their respective heights. 
   In an embodiment, the concrete base section  160  and the concrete support sections  150  all have approximately identical dimensions. In another embodiment, as shown in  FIG. 1 , the width or diameter of the concrete support sections  150  used in the substructure  120  may vary from bottom to top, with the larger diameter support sections  150  being utilized in deeper water near the base section  160  and transitioning to smaller diameter support sections  150  as the water depth decreases approaching the water line  130 . The use of concrete support sections  150  with varying diameters in this manner may result in a substructure  120  having a tapered shape, namely wider at the base adjacent the base section  160  and narrower at the top adjacent the topsides  140 . 
   Referring now to  FIG. 2 , for illustrative purposes only, an enlarged cross-sectional side view is provided of two specific supports  151  and  152 . In particular,  FIG. 2  depicts an individual concrete support section  151  supported by a base section  160  below and supporting a second concrete support section  152  above.  FIG. 3  depicts a cross-sectional top view of the concrete support section  151 , taken along section line  3 - 3  of  FIG. 2 . As shown in  FIG. 2 , in some embodiments, the base section  160  may be supported by a concrete foundation  210  poured between the base section  160  and the firm bottom  220  of the ocean floor  110  as will be described more fully herein. 
   Still referring to  FIG. 2 , as depicted in phantom lines, the concrete support section  151  may comprise windows  290 , which allow ocean water to pass through the substructure  120  to reduce stress in the substructure  120  due to loading caused by ocean currents. The base section  160  may comprise one or more alignment nubs  250  extending upwardly from its top surface to engage one or more corresponding alignment grooves  270  in the bottom surface of the concrete support section  151 . Similarly, the concrete support section  151  may comprise one or more alignment nubs  260  extending upwardly from its top surface to engage one or more corresponding alignment grooves  280  in the adjacent concrete support section  152 . As depicted, the alignment nubs  250  of the base section  160  extend into the similarly shaped grooves  270  located in the concrete support section  151  to prevent lateral movement of the concrete support section  151  with respect to the base section  160 , and vice versa. Similarly, the alignment nubs  260  of the concrete support section  151  extend into similarly shaped grooves  280  in the adjacent concrete support section  152  to prevent lateral movement of the concrete support sections  151 ,  152  with respect to one another.  FIG. 2  and  FIG. 3  depict alignment nubs  250 ,  260  and their respective alignment grooves  270 ,  280  as being rectangular in shape and having a particular size, number and arrangement. However, one skilled in the art will readily appreciate that the shape, size, number and arrangement of these components  250 ,  260 ,  270 ,  280  may vary. 
   One or more guideposts  225  may extend through corresponding guide conductor holes  226  in the concrete support sections  151 ,  152  and base section  160  into the firm bottom  220  of the ocean floor  130  for some distance, such as several hundred feet, for example, and then grout  235  may be installed around the guideposts  225  to provide additional stability for the substructure  120 .  FIG. 2  and  FIG. 3  illustrate one possible arrangement for the guideposts  225 ; however, one skilled in the art will readily appreciate that the number of guideposts  225  and their arrangement may vary. In an embodiment, only one of the multiple guideposts  225  is pre-installed in the ocean floor  110  before the base section  160  and concrete support sections  150  are installed at the well site. The remaining guideposts  225 , if any, are installed by drilling them through the concrete support sections  150  and the base section  160  into the ocean floor  110  after the complete modular concrete substructure  120  has been assembled at the well site, as will be discussed in more detail herein. 
   Cables  245  may also be inserted through grout holes  246  extending through the height of the concrete support sections  151 ,  152  and into the base section  160 . When the cables  245  are tightened, the concrete support sections  150  compress, and then grout may be injected into the grout holes  246 , thereby causing the entire substructure  120  to act as a single unit rather than a plurality of individual concrete support sections  150  stacked on a base section  160 .  FIG. 2  and  FIG. 3  illustrate one possible arrangement for the cables  245 ; however, one skilled in the art will also readily appreciate that the number of cables  245  and their arrangement may vary. 
   The concrete foundation  210  shown in  FIG. 1 , which may be constructed by pouring concrete between the base section  160  and the firm bottom  220  of the ocean floor  110 , provides substantially uniform support of the base section  160 . Such a uniform surface is important because the base section  160  will support a great deal of weight, namely, the weight of the concrete support sections  150  and the topsides  140 . Without uniform support provided by the concrete foundation  210  in contact with the firm bottom  220 , areas of the base section  160  would be more heavily loaded than other areas. Such a non-uniform load acting on the base section  160  may cause it to crack and possibly fail. 
   Although a uniform surface is needed to support the base section  160 , a concrete foundation  210  is not always required. At some well sites, the ocean floor  110  does not have a firm bottom  220 . Instead, the ocean floor  110  may consist of mud or sand, for example. In those situations, the base section  160  may be seated directly on the mud or sand bottom. Because the mud or sand is soft, it conforms around the base section  160 , thereby providing a uniform surface on which the base section  160  rests. 
   Whether the ocean floor  110  is mud, sand, or something harder, the base section  160  will be designed and constructed from material to withstand the loads placed on it without cracking or failing. The base section  160  and the concrete support sections  150  also have an opening  310  therethrough, as shown in  FIG. 3 , to allow passage of drilling or production risers  320  which will extend from the topsides  110  to the well below the substructure  120 . Although the opening  310  depicted is circular, one skilled in the art will readily appreciate that the shape of the opening  310  may vary to accommodate the drilling and/or productions risers  320 . For example, the opening  310  may be square or rectangular in shape. In addition, one skilled in the art will readily appreciate that multiple openings  310  may also be used to accommodate various configurations of drilling and/or production risers  320 . 
     FIG. 4  through  FIG. 8  schematically depict a sequence of assembly operations for installation of the modular concrete substructure  120  illustrated in  FIGS. 1-3 . Once installed, the substructure  120  may be used to support the topsides  140 , thus forming a fixed offshore platform  100  for use in drilling and/or producing oil and natural gas. For example, to assemble a production substructure  120 , when drilling operations are completed at a well site, a guidepost  225  may be drilled at a desired location into the ocean floor  110  to a depth that depends upon the geotechnical characteristics of the seabed, and then left in place after the drilling rig departs the well site. Typically, the guidepost  225  is vertically driven into the ocean floor  110  to the point of refusal. This guidepost  225  may extend to just below the water surface  130 . Referring first to  FIG. 4 , a guidepost  225  is shown inserted deep into the ocean floor  110  at a well site. A quick-connect  410  may be attached to the upper end of the guidepost  225  to permit additional piping to be connected to the guidepost  225  later, if so desired. 
   The substructure  120  may be assembled around the guidepost  225 , first by installing the base section  160 , and then sequentially installing each of the plurality of concrete support sections  150  until the substructure  120  reaches the desired height. This method of assembly allows the substructure  120  to be used in both shallow water and deepwater installation sites, and further allows for variability of penetration for soft ocean floor  110  conditions. In an embodiment, each of the base section  160  and concrete support sections  150  may be manufactured in a dry dock and then individually towed out to the well site using only a boat  450  and a simple floatation device  420 , such as an underwater salvage lifting bag or a parachute type lifting bag available from J.W. Automarine of Fakenham, Norfolk, for example. Referring again to  FIG. 4 , the base section  160  may be towed to the well site on a floatation device  420  using a tug boat or other type of boat  450 . After the base section  160  reaches the guidepost  225 , divers may slowly deflate the floatation device  420  and manipulate the base section  160  onto the guidepost  225  such that the pre-installed guide conductor hole  226  in the base section  160  slides down over the guidepost  225 . This is possible because the guidepost  225  does not extend all the way to the water surface  130 , allowing the base section  160  to be floated over the guidepost  225  and lowered down onto it. 
     FIG. 5  depicts the base section  160  installed on the guidepost  225  and seated firmly on the ocean floor  110 . Next, a concrete support section  151  is towed out on a floatation device  420  and pulled by a boat  450  to the well site. Upon arrival at the well site, divers may slowly deflate the floatation device  420  and manipulate the concrete support section  151  onto the guidepost  225  such that the pre-installed guide conductor hole  226  in the concrete support section  151  slides down over the guidepost  225 . This is possible because the guidepost  225  does not extend all the way to the water surface  130 , allowing the concrete support section  151  to be floated over the guidepost  225  and lowered down onto it. When the concrete support section  151  lands on top of base section  160 , divers may manipulate the concrete support section  151  until the alignment grooves  270  slide over and engage the alignment nubs  250  located on top of the base section  160 . Once these grooves  270  engage the nubs  250 , the base section  160  and the concrete support section  151  are locked together such that lateral movement of one relative to the other is prevented, similar to the way toy interlocking building block pieces lock together, such as LEGO® brand building blocks, for example. 
     FIG. 6  depicts the base section  160  and a single concrete support section  151  installed at the well site and a second concrete support section  152  being pulled to the well site on a floatation device  420  by a boat  450 . Divers may install the second concrete support section  152  on top of the first concrete support section  151  already installed, again by slowly deflating the floatation device  420  and lowering the second concrete support section  152  onto the pre-installed guidepost  225 . When the second concrete support section  152  lands on top of the first concrete support section  151 , divers may manipulate the second concrete support section  152  until the alignment grooves  280  slide over and engage the alignment nubs  260  located on top of the first concrete support section  151 . Once these grooves  280  engage the nubs  260 , the two concrete support sections  151 ,  152  are locked together such that lateral movement of one relative to the other is prevented. This installation procedure may be repeated, stacking additional concrete support sections  150  adjacent to ones already positioned, until the entire modular concrete substructure  120  has been installed to a desired size and height at the well site, as depicted in  FIG. 7 . 
   Once the entire substructure  120  has been positioned at the well site following the procedure described above, weight in the form of water bags may be applied to the top of the substructure  120  to mimic the weight of the topsides  140  to be installed in order to verify that the substructure  120  will not sink or settle further into the ocean floor  110 . After the substructure  120  has settled, and depending on the consistency of the ocean floor  110 , the base section  160  may then be grouted in to prevent lateral movement of the base section  160  relative to the ocean floor  110 . If the ocean floor  110  is not a hard surface, but a soft surface consisting of mud, sand or other similar material, a concrete foundation  210  need not be constructed between the base section  160  and the ocean floor  110 . Instead, divers may jet in the base section  160  by blowing the mud or sand away from the perimeter of the base section  160  to allow the base section  160  to set into the ocean floor  110  as shown in  FIG. 7 . If the ocean floor  110  consists of a firm bottom  220 , a concrete foundation  210  as shown in  FIG. 1  and  FIG. 2  may be required. To construct such a foundation  210 , divers may place sand bags on the ocean floor  110  in a circular pattern surrounding the base section  160 . Cement is then poured into the dyke created by the sand bags until it fills up the dyke. Because cement is heavier than water, cement displaces water in the dyke as the cement fills up the dyke. Once the cement sets, the concrete foundation  210  prevents lateral movement of the base section  160  relative to the ocean floor  110 . 
   Next, additional guideposts  225  as shown in  FIG. 2  and  FIG. 3  may be installed to provide additional stability for the substructure  120 . A barge, or other type of boat, is positioned over the substructure  120 . According to a method known as the “casing drilling method,” a casing string with a drill bit attached to one end is lowered down to the substructure  120 . Drillers equipped with power tongs then use the casing string with attached drill bit to drill a guide conductor hole  226  in the substructure  120 . After the guide conductor hole  226  is completed, the casing string with attached drill bit is left in place to form the guidepost  225 . This procedure is repeated until all remaining guideposts  225  are installed. Grout may then be injected into the guide conductors  226  and allowed to set. 
   After the guideposts  225  have been installed, cables  245  may be inserted into the grout holes  246  and run down through the concrete support sections  150  into the base section  160 . A tension load may then be applied to the cables  245  to compress the base section  160  and concrete support sections  150 . Grout may also be injected into the grout holes  246  and allowed to set, thus fixing the cables  245  in position. Additionally, grout may be injected between the base section  160  and between the adjacent concrete support section  151  and/or between each of the concrete support sections  150  to provide an additional means of cementing these individual components together. To provide a flowpath for the grout, grooves may be fabricated in the upper surfaces of the base section  160  and the upper and lower surfaces of the concrete support sections  150  around the alignment nubs  250 ,  260  and alignment grooves  270 ,  280 . Compressing the base section  160  and concrete support sections  150  by tightening the cables  245  and injecting grout into the grout holes  246  to fix the cables  245  in place, as well as grouting between the base section  160  and concrete support sections  150  forms a single, sturdy substructure  120 , rather than an individual base section  160  and a collection of individual concrete support sections  150 , each stacked one on top of the other. 
   In some mild environments, the massive size and weight of the substructure  120 , with applied weight from the topsides  140 , may provide enough stability that neither the cables  245  nor the grout is necessary. However, in harsher environments, the weather and ocean currents may be such that using the cables  245  to compress the substructure  120  may be required, but the grouting may not be. In still harsher environments, it may be necessary to use the cables  245  to compress the substructure  120  and also to inject grout into the grout holes  246  and between the base section  160  and the concrete support sections  150  to form a stout substructure  120 . One skilled in the art will readily appreciate that weather and ocean currents at the well site will dictate whether or not the cables  245  will be used and the substructure  120  grouted. Also, the ease with which the substructure  120  may be later disassembled and removed may also be a consideration in determining whether to use the cables  245  and/or grout the substructure  120 . In the absence of cables  245  and grout, the disassembly and removal of the substructure  120  from the well site may be relatively easy. 
   Referring again to  FIG. 7 , the topsides  140  may be installed on top of the completed substructure  120  by a variety of methods. In one embodiment, the topsides  140  may be floated on a floatation device  429  and pulled to the well site by boat  450 . Upon arrival at the well site, the topsides  140  may be floated over the substructure  120  and slowly lowered onto the substructure  120  by deflating the floatation device  429 . Turning now to  FIG. 8 , the topsides  140  may then jack itself up on legs  430  so that the topsides  140  rises above the substructure  120  and the water line  130 . To install the topsides  140  using this float-over method requires that the top surface of the substructure  120  be located sufficiently below the water line  130  to allow the topsides  140  to float over the substructure  120 .  FIG. 8  depicts a topsides  140  supported by a modular concrete substructure  120  and jacked up on legs  430  above the substructure  120  and the water line  130 . 
   In another embodiment, a heavy lift system, such as a derrick barge or the Versatruss heavy lift system employed by Versatruss Americas of Belle Chasse, La., for example, may transport the topsides  140  to the well site and lift the topsides  140  onto the modular concrete substructure  120 . In this scenario, it is desirable to extend the substructure  120  above the water line  130  and into the splash zone, as depicted in  FIG. 1 . Under these circumstances, because the topsides  140  is positioned above the water line  130 , it is not necessary to jack the topsides  140  up on legs, as discussed above. Once the topsides  140  have been positioned onto the modular concrete substructure  120  by either the float over method or the lifting method, the topsides  140  may be connected to the substructure  120  via bolts, rods, ring plates, or other means according to standard procedures familiar to those of ordinary skill in the art. 
   The foregoing descriptions of specific embodiments of modular concrete substructures and methods of assembly or installation to support a topsides, thus forming a fixed offshore platform, have been presented for purposes of illustration and description and are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many other modifications and variations of these embodiments are possible. In particular, the size of the concrete support sections and/or base section may vary depending upon the load they are intended to support, their methods of construction, and the ease with which these components may be transported and installed. Furthermore, the material composition of the concrete used to fabricate these components may vary depending on the material strength required for a specific application and the availability of different types of concrete. The formation of the substructure may be a function of the area of the well site, the water depth, and the size and weight of the topsides to be supported. The assembly and installation methods may also vary depending on the availability of necessary equipment. For example, if a heavy lift barge is unavailable to install the topsides, the float-over method of installing the topsides, as described with respect to  FIG. 7  and  FIG. 8 , may be utilized instead. 
   While various embodiments of modular concrete substructures and methods of assembly or installation have been shown and described herein, modifications may be made by one skilled in the art without departing from the spirit and the teachings of the disclosure. The embodiments described are representative only, and are not intended to be limiting. Many variations, combinations, and modifications of the applications disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is defined by the claims which follow, that scope including all equivalents of the subject matter of the claims.

Summary:
A concrete section of an offshore platform substructure comprises a concrete body with a central opening and at least one guidepost hole extending through a height of the concrete body, wherein a width of the concrete body is greater than the height. An offshore platform substructure comprises a base portion resting on the ocean floor, and a plurality of concrete support sections stacked one on top of another on the base portion. A method of assembling an offshore platform with a concrete substructure comprises locating a guidepost in the ocean floor at a well site, towing a plurality of concrete sections to the well site, sequentially engaging each of the plurality of concrete sections with the guidepost, and sequentially sinking each of the plurality of concrete sections, thereby forming a stack of concrete sections on the ocean floor.