Abstract:
A variable-diameter tank construction system is provided. The tank construction system uses sets of wedges (shims) and bevel-faced spacers (washers) in conjunction with the field connections of the vertical edges of adjacent modular tank wall panels, so as to create an angular deviation between the tangent lines of the watt panels immediately adjacent to and on either side of the vertical joint between the connected panels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/834,875, filed Mar. 15, 2013, which application is related to U.S. Provisional Patent Application Ser. Nos. 61/658,225, filed Jun. 11, 2012, and 61/749,435, filed Jan. 7, 2013, the disclosures of which are incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates in general to modular storage tanks, and in particular to open-top modular storage tanks used in the petroleum industry. More particularly, the disclosure relates to systems and methods for constructing circular storage tanks of different diameters. 
     BACKGROUND 
     It is increasingly common in the oil and gas industry to use hydraulic fracturing (colloquially known as “fraccing”or “fracking”) to aid in the recovery of petroleum fluids such as crude oil and natural gas from subsurface formations. Hydraulic fracturing is a process involving the injection of a “fraccing fluid” under pressure into spaces such as cracks and fissures within a subsurface petroleum-beating formation, such that the fluid pressure forces the cracks and fissures to become larger, and/or induces new fractures in the formation materials, resulting in more and/or larger flow paths through which petroleum fluids can flow out of the formation and into a well drilled into the formation. Fraccing fluids typically carry particulate materials called “proppants” that are intended to stay inside the enlarged or newly-created subterranean fissures after the fraccing fluid has been drained out of the formation and hydraulic pressure has been relieved. 
     There are various different types and formulations of fraccing fluids, but regardless of the type of fraccing fluid being used, one thing that is common to all fraccing operations is the need for temporary storage of very large volumes of fraccing fluid, both to provide a reservoir of frac fluid fir injection into subsurface formations, and to store frac fluid circulated out of the well after completion of fraccing operations. Storage tanks having volumes of 250,000 to 1,500,000 U.S. gallons or more are commonly required for this purpose. For practical and environmental reasons, such tanks are typically of modular design so that their components can be shipped by truck to remote well sites, where they can be erected on site and eventually disassembled and shipped off site after they are no longer needed. The costs of shipping storage tank components to and from remote well sites and the costs of erecting and disassembling the tanks on site can be considerable. Accordingly, modular storage tank systems that minimize these costs are highly desirable. 
     Open-top storage tanks most commonly are circular, as this is the most stable and efficient structural configuration for a liquid storage tank. Modular circular tanks typically comprise multiple horizontally-curved steel wall panels having a radius corresponding to the radius (or half-diameter) of the finished tank. The vertical side edges of the curved wall panels abut and are fastened to the vertical edges of adjacent wall panels by suitable structural connection means, such that when all of the wall panels have been erected and interconnected, they form a circular tank having a particular height, diameter, and liquid storage capacity. External braces are typically installed at intervals around the perimeter of the tank to stabilize the panel assembly, and a suitable liquid-tight liner is installed inside the tank, covering a prepared ground surface inside the tank perimeter and extending up and typically over the tank wall. The tank is then ready to receive a fraccing fluid or other liquid that needs to be stored. 
     Field connection of adjacent modular tank wall panels is commonly facilitated by fabricating the panels with continuous steel end plates or structural angles (a.k.a. angle irons) along their vertical side edges, such that the face of each end plate (or the face of one leg of each angle iron) lies in a plane coincident with a radius of the assembled tank, and perpendicular to the tangent tine of the immediately adjacent region of the curved panel. Therefore, the faces of the end plates on adjacent panel edges will come into mating contact upon erection, such that the panels can be securely connected using structural bolts extending through bolt holes provided in the panels&#39; vertical end plates or angle irons. 
     These field connections between adjacent tank wall panels must carry tensile forces induced by hoop stresses in the watts of the completed tank due to hydrostatic forces exerted by the liquid stored in the tank. The magnitude of the hoop stresses in the tank wall is proportionate to the density of the stored liquid, and it increases linearly with the depth of liquid in the tank. Accordingly, the tensile force that needs to be transferred across the vertical joints between adjacent tank. wall panels, per unit of vertical distance, will increase linearly toward the bottom of the tank. The most efficient and economical structural design will therefore result in the bolt spacing in the panel end plates (or angle irons) being increasingly closer toward the bottom of the tank wall. 
     Alternatively, the bolt hole spacing could conceivably be kept constant by using different sizes of structural bolts at different locations. This alternative would require stocks of different sizes of bolts and would give rise to the risk that bolts that are too small might inadvertently be used in lower regions of the tank, potentially leading to catastrophic failure of the panel connections. However, it could be workable subject to appropriate quality control and field inspection during tank construction. 
     Modular circular storage tanks as described above are typically designed and fabricated with tank wall panels intended to be tank-diameter-specific. In other words, for a given finished tank diameter, the wail panels which have a radius of curvature corresponding to one-half the tank diameter. Accordingly, in order to accommodate different tank volume requirements, it is necessary to provide multiple sets of tank wall panels having different radii of curvature. This increases the overall cost of maintaining a stock of modular tank assemblies sufficient to meet anticipated requirements. 
     In addition, modular tanks with tank-diameter-specific wall panels can increase tank assembly transportation costs, such as when a tank of one size is used on one drilling site, and when the drilling rig is later moved to another well site (which might be comparatively close by) at which the frac fluid storage requirements are significantly less than or greater than at the first site. In that scenario, if the storage tank. used at the first site has a capacity greater than required at the second site, it could be moved to and used at the second site to save transportation costs (as compared to transporting the tank away from the first site and shipping a smaller tank to the second site); however, that option is disadvantageous in that the larger tank is being inefficiently used, an unnecessarily large area on the well site needs to be prepared to erect the tank, and the tank erection and disassembly costs will be greater than if a smaller tank had been used. 
     In the alternative scenario where the tank storage capacity at the second site is greater than the requirement at the first site, there will be no alternative but to ship out the tank used at the first site and transport a larger tank to the second site. 
     For these reasons, there is a need for systems and methods for constructing modular storage tanks in which the modular wall panels can be used to construct tanks of different diameters. The present disclosure is directed to that need. 
     BRIEF SUMMARY 
     The present disclosure teaches a variable-diameter tank construction system using sets of wedges (shims) and bevel-faced spacers (washers) for use in conjunction with the field connections of the vertical edges of adjacent modular tank wall panels, so as to create an angular deviation between the tangent lines of the wall panels immediately adjacent to and on either side of the vertical joint between the connected panels. This allows the use of tank wall panels having a given radius of curvature to be used to construct substantially circular and structurally sound storage tanks having diameters either greater than or less than twice the panels&#39; radius of curvature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments in accordance with the present disclosure will now be described with reference to the accompanying Figures, in which numerical references denote like parts, and in which: 
         FIG. 1  is a perspective view of a plurality of prior art curved storage tank wall panels assembled to form the perimeter wall of a circular open-top storage tank. 
         FIG. 2  is a side view of a typical storage tank wall panel as in  FIG. 1 . 
         FIG. 3  is atop view of the tank wall panel in  FIG. 2 . 
         FIG. 4A  is a plan cross-section through an angle iron welded to a vertical side edge of the tank wall panel in  FIG. 3 , with bolt holes to facilitate connection of the wail panel to an adjacent wall panel. 
         FIG. 4B  is a plan cross-section through the bolted connection between the vertical side edges angles of adjacent tank wall panels as in  FIG. 4A , in accordance with a prior art tank construction method. 
         FIG. 5  is a side view of a set of wall panel adaptor wedges in accordance with the present disclosure, for adapting prior art curved storage tank wail panels for use in constructing a storage tank having a diameter less than double the wall panels&#39; radius of curvature. 
         FIG. 5A  is an isometric view of the set of waft panel adaptor wedges shown in  FIG. 5 . 
         FIG. 6  is a cross-section through an exemplary wall panel adaptor wedge as in  FIGS. 5 and 5A . 
         FIG. 7  is a side view of an exemplary set of spacer bars for use in conjunction with wall panel adaptors as in  FIGS. 5 and 5A . 
         FIG. 8  is a cross-section through left-hand and right-hand variants of the spacer bars in  FIG. 7 . 
         FIG. 9  is a plan cross-section through the bolted connection of the vertical side edges angles of adjacent tank watt panels using adaptor wedges as in  FIGS. 5 and 5A  in conjunction with left-hand and right-hand spacer bars as in  FIGS. 7 and 8 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates a plurality of prior art curved tank wall panels  10  assembled to form an open-topped circular storage tank  100 .  FIG. 2  is an elevation view of an exemplary curved tank wall panel  10  comprising a horizontally-curved tank wall plate  12  reinforced by a plurality externally-mounted, horizontally-curved structural stiffeners  14 , and with secondary vertical stiffeners  16  extending between vertically adjacent horizontal stiffeners  14 . The spacing of horizontal stiffeners  14  becomes smaller toward the bottom of wall panel  10 , thus reducing the vertical span of wall plate  12  in order to keep flexural stresses in wall plate  12  within safe limits as hydrostatic pressures exerted against wall plate  12  increase toward the bottom of wall panel  10 . An angle iron side edge stiffener  20  is provided along each vertical side edge of wall panel  10 . 
       FIG. 3  is atop view of tank wall panel  10 , illustrating the horizontal curvature of wall plate  12 . and horizontal stiffeners  14 . Although the dimensions of wall panel  10  and the tank ultimately constructed from wall panels  10  are variable to suit project-specific requirements, the particular wall panel  10  shown in  FIGS. 2 . and  3  is designed for purposes of a 12-foot-high storage tank having a nominal storage capacity of 750,000 U.S. gallons. As indicated in  FIG. 3 , this wall panel  10  has a chord length (i.e., the straight-line distance between vertical side edges) of approximately 26.7 feet. The dimensions of other variants of will panel  10  would, of course, be different to suit different tank storage capacities and other design criteria. 
       FIG. 4A  is an enlarged sectional detail of the welded connection of a side edge stiffener  20  to wall plate  12  and to intervening horizontal stiffeners  14  (the ends of which typically will be coped to fit around stiffener  20 ). Side edge stiffener  20  has a radially-aligned end face  22  (corresponding to one leg of the angle iron forming stiffener  20  in the illustrated embodiment) with suitably spaced bolt holes  23  having centerlines CL 23 .  FIG. 4B  illustrates side edge stiffeners  20  of two adjacent wall panels  10 , connected by means of bolts  25  passing through bolt holes  23  in stiffeners  20 , with the end faces  22  of the two stiffeners  20  in mating contact. 
       FIGS. 5 ,  5 A, and  6  illustrate an exemplary set of elongate wall panel adaptor wedges  30  in accordance with the present disclosure, for installation between the end faces  22  of the side edge stiffener  20  of adjacent tank wall panels  10  (as will be explained in greater detail later herein). As best understood with reference to  FIG. 6 , each adaptor wedge  30  comprises a primary plate element (or leg)  32  having an orthogonal face  32 A and a beveled face  32 B, with orthogonal face  32 A and beveled face  32 B enclosing a wedge offset angle A 30 . In the illustrated embodiment, wedge offset angle A 30  is 6 degrees, but this angle may vary from one embodiment to another. Primary leg  32  of adaptor wedge  30  has bolt holes  34  corresponding to bolt holes  23  in side edge stiffeners  20 . The centerline CL 34  of bolt holes  34  is, preferably, perpendicular to orthogonal face  32 A, as shown in  FIG. 6 . 
     Adaptor wedges  30  could be provided in single lengths corresponding to side edge stiffeners  20  (which are 12 feet long in the illustrated embodiment). As illustrated in  FIGS. 5 and 5A , however, adaptor wedges  30  can be conveniently provided in sets of smaller lengths for ease of handling and installation. Because the spacing of bolt holes  23  typically varies along the length of side edge stiffeners  20 , when adaptor wedges  30  are provided in smaller lengths, the spacings of bolt holes  34  in the wedges respective primary legs  32  will vary within each set of wedges  30 . This is reflected in  FIGS. 5  and  5 A, in which the illustrated wedges are designated by reference numbers  30 A,  30 B,  30 C, and  30 D. In wedges  30 A and  30 B (which are identical in the illustrated set of wedges), the spacing of bolt holes  34  is greater than in wedges  30 C and  30 D. The spacing of bolt holes  34  in wedge  30 C is greater than in wedge  30 D, and, in fact, the spacing of bolt holes  34  in wedge  30 D reduces toward one end. 
     Accordingly (and having regard to the explanation set out previously herein regarding the variable fastener spacing for connections between abutting side edge stiffeners  20 ), each wedge in each set of wedges would typically be intended for installation at a different vertical location along the length of a field connection between two abutting side edge stiffener  20 . More specifically, considering the exemplary wedge set shown in  FIGS. 5 and 5A , wedges  30 A and  30 B (having wider bolt hole spacings) would be used in upper regions of a side edge stiffener connection, wedge  30 D would be used in a lower region of the connection, and wedge  30 C would be used in an intermediate region. Since the spacing of the bolt holes  34  in each wedge will always match the spacing of particular bolt holes  23  in stiffeners  20 , it will be virtually if not completely impossible to install the wedges incorrectly. 
     Due to the offset angle A 30  between orthogonal face  32 A and beveled face  32 B of primary leg  32  of each adaptor wedge  30 , the installation of wedges  30  in each vertical field joint between tank wall panels  10  will create a corresponding angular offset between the tangent lines of adjacent curved wall panels  10 . This will result in a reduced effective tank radius as measured at the field connections of assembled tank wall panels  10 , and the “included angle” of each wall panel  10  will be increased. Accordingly, offset angle A 30  for a given set of adaptor wedges  30  can he selected such that for particular tank wall panels  10  having a particular radius of curvature R, requiring a quantity “X” of wall panels  10  to create a storage tank having a diameter D equal to 2R, a quantity of “X” minus 1 wall panels  10  (or, in the more general case, “X” minus “n” wall panels  10 ) could be used to construct a storage tank having a diameter less than 2R. 
     As illustrated in  FIGS. 5A and 6 , each adaptor wedge  30  is preferably of generally L-shaped configuration, with a secondary plate element (or leg)  36  oriented perpendicular to orthogonal face  32 A. Secondary leg  36  is preferably located an appropriate distance from centerline CL 34  of bolt holes  34  in adaptor wedge  30  such that secondary leg  36  serves as a stop member abuttable against the toe of one side edge stiffener  20  in a field connection between two adjacent stiffeners  20 , so as to align the centerlines CL 34  of bolt holes  34  in adaptor wedges  30  with the centerlines CL 23  of bolt holes  23  in the side edge stiffeners  20 . Accordingly, accurate field positioning of adaptor wedges  30  will entail only vertical adjustment of the positions of adaptor wedges  30  relative to side edge stiffeners  20 . 
     In theory at least, bolts  25  could be simply inserted through the bolt holes in abutting side edge stiffeners  20  and an adaptor wedge  30  installed between the stiffeners  20 , If that is done, however, bolts  25  and their corresponding nuts will not seat properly against the side edge stiffeners  20  due to the angular offset between the stiffeners resulting from the installation of adaptor wedge  30 . Tightening bolts  25  in this scenario would induce undesirable flexural stresses in the bolts, and while this could theoretically be tolerated by selecting bolts strong enough to withstand such flexural stresses in addition to intended axial tension stresses, this is not ideal or desirable. It is much more desirable and preferable for bolts  25  to be stressed in axial tension only, as would be the case when bolts  25  are tightened with end faces  22  of side edge stiffeners  20  in mating contact, as seen in the arrangement illustrated in  FIG. 4B . 
     Accordingly, adaptor wedges  30  as taught in the present disclosure are preferably used in conjunction with elongate spacer bars  40  as illustrated in  FIGS. 7 and 8 . Spacer bars  40  have bolt holes  42  spaced to match the spacing of bolt holes  23  in side edge stiffeners  20  and bolt holes  34  in adaptor wedges  30 . As shown in  FIGS. 7 and 8 , each spacer bar  40  has a first (or inner) surface  41  and a second (or outer) surface  44 . Outer surface  44  is machined or otherwise formed to provide beveled surfaces  43  surrounding bolt holes  42 , beveled surfaces  43  being angularly offset from the plane of inner surface  41  by an offset angle A 40 , with offset angle A 40  being equal to one-half of the offset angle A 30  of the associated adaptor wedges  30 . When a spacer bar  40  is positioned against the outer face of each side edge stiffener  20  in a wall panel connection including adaptor wedges  30 , the beveled surfaces  43  of the spacer bars  40  on each side of the connection will be parallel to each other. Therefore, when bolts  25  are installed in this connection assembly, their bolt heads and nuts will bear uniformly against the beveled surfaces  43  of the corresponding spacer bars  40 , such that when bolts  25  are fully torqued they will, for all practical purposes, be under axial tension only. 
     Similar to the case of adaptor wedges  30 , the spacings of bolt holes  42  in spacer bars  40  will vary in accordance with the variable spacing of bolt holes  23  in side edge stiffeners  20 . A single elongate spacer bar  40  could be provided on each side of each tank wall panel connection, with bolt holes  42  spaced to match all bolt holes  23  in side edge stiffeners  20 . Alternatively, however, a set of shorter spacer bars (indicated by reference numbers  40 A,  40 B,  40 C,  40 D, and  40 E in  FIG. 7 ) may be provided for convenient handling and installation instead of full-length spacer bars. If shorter spacer bars ( 40 A,  40 B, etc.) are provided, they can be interchangeable for use on either side of a tank wall panel connection if they have symmetrical bolt hole patterns (as in the exemplary spacer bars shown in  FIG. 7 ). However, if full-length spacer bars  40  are provided, or if shorter spacers bars with non-symmetrical bolt hole patterns are provided, these will have to be provided in left-hand and right-hand variants, as indicated by reference number  40 (L) and  40 (R) in  FIGS. 8 and 9 . 
     In an unillustrated alternative embodiment, individual bevel washers could be used at each bolt location instead of elongate spacer bars  40 . 
     As previously described, the connection detail shown in  FIG. 9 , using adaptor wedges  30 , allows storage tank wall panels  10  having a given radius of curvature to be used to construct a storage tank having a nominal diameter less than twice the panels&#39; radius of curvature. In another unillustrated alternative embodiment, adaptor wedges generally similar to those previously described (with or without secondary legs) could be installed between adjacent side edge stiffeners  20  in an orientation reversed from the orientation of adaptor wedges  30  in the connection detail in  FIG. 9 , in order to allow storage tank wall panels  10  having a given radius of curvature to be used to construct a storage tank having a nominal diameter equal to greater than twice the panels&#39; radius of curvature. In other words, the thicker portion of the wedges in this variant embodiment would be oriented toward the inside of the tank, such that the wedges spread the radially inner edges of the adjacent side edge stiffeners  20  apart from each other (instead of abutting each other as in  FIG. 9 ), and with the offset angle A 30  between the end faces  22  of the stiffeners being divergent toward the inside of the tank instead of the opposite case illustrated in  FIG. 9 . Therefore, by way of non-limiting example, if a total of 12 tank wall panels  10  having a radius of curvature R would be required to construct a storage tank having a uniform diameter equal to 2R, the storage volume of the tank could be increased by approximately 17 percent by using “reverse” adaptor wedges having a suitable bevel angle to allow the inclusion of one additional wall panel  10 . 
     It will be readily appreciated by those skilled in the art that various modifications to embodiments in accordance with the present disclosure may be devised without departing from the scope of the present teachings, including modifications using equivalent structures or materials hereafter conceived or developed. It is to be especially understood that the scope of the present disclosure is not intended to be limited to described or illustrated embodiments, and that the substitution of a variant of a described or claimed element or feature, without any substantial resultant change in functionality, will not constitute a departure from the scope of the disclosure. 
     In this patent document, any form of the word “comprise” is to be understood in its non-limiting sense to mean that any item following such word is included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one such element is present, unless the context clearly requires that there be one and only one such element. 
     As used herein, relational terms such as “perpendicular,” “vertical,” and “coincident” are not intended to denote or require mathematical or geometric precision. Accordingly, such terms are to be understood in a general rather than precise sense (e.g., “generally perpendicular” or “substantially perpendicular”) unless the context clearly requires otherwise. 
     Wherever used in this document, the terms “typical” and “typically” are to be understood in the sense of representative or common usage or practice, and are not to be understood as implying invariability or essentiality.