Patent Document

FIELD OF THE DISCLOSURE 
     The present disclosure relates in general to storage tanks for liquids, and in particular to open-top modular storage tanks used in the petroleum industry and other industries that require large-scale and inexpensive liquid storage such as for fire-fighting water (field and municipal), potable water storage (remote and municipal), water for fish farming (fresh and sea), mining, etc. More particularly, the disclosure relates to systems and methods for constructing a circular storage tank with secondary containment means to contain liquid escaping the storage tank in the event of leakage. 
     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 hydrocarbon fluids such as crude oil and natural gas from subsurface formations. Hydraulic fracturing is a process involving the injection of a “fraccing fluid” (or “frac fluid”) under pressure into spaces such as cracks and fissures within a subsurface petroleum-bearing 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 hydrocarbon 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 common to all fraccing operations is the need for temporary storage of very large volumes of fraccing fluid at the well site, both to provide a reservoir of frac fluid for 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 2,500,000 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. Open-top liquid 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 of the finished tank. The vertical side edges of each curved wall panel abut and are fastened to the vertical side 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. A suitable liquid-tight liner is then 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. 
     Environmental regulations require storage tanks for many different types of liquids to be provided with secondary containment means to protect against environmental contamination in the event of a tank leak. For example, petroleum storage tanks are commonly constructed within a containment reservoir formed by earthen berms lined with synthetic liners or engineered clay liners installed or constructed below the ground surface. Such secondary containment means may be practical for “tank farm” installations where the primary liquid storage tanks are essentially permanent. However, they are not a practical or acceptable option on well sites requiring tanks for temporary storage of large volumes of liquid (such as fraccing fluid) and where such temporary tanks must be demountable so as to cause little or no permanent environmental disturbance in the area where the temporary tanks were constructed. 
     One known way of providing secondary containment is to build a primary storage tank within a secondary tank structure, such that if the primary tank should develop a leak, the secondary tank will provide a second line of defence against liquid leakage into the surrounding environment. The present disclosure teaches an innovative process for constructing a primary open-top storage tank within a secondary containment tank without significant disturbance to the environment. 
     BRIEF SUMMARY 
     In general terms, the present disclosure teaches a dual-tank liquid storage system and methods for constructing such a liquid storage system, which comprises a lined primary storage tank disposed within a larger lined secondary tank. Construction of this dual-tank system typically involves the following steps: 
     An engineering base course is prepared over the ground area where the system is to be built, with the base course preferably being covered with a geotextile protective layer. 
     A flexible, impermeable liner for the secondary tank is then rolled out over the base course. 
     A flexible, impermeable liner for the primary tank is laid out over the secondary tank liner, with an outer portion being rolled up such that the partially rolled-up primary tank liner fits within the intended perimeter of the primary tank. 
     The perimeter wall of the primary tank is then site-assembled from a plurality of arcuate wall panel sections resting on the secondary tank liner, which is preferably protected by a liner protection material positioned over the secondary tank liner at the primary tank wall panel locations. 
     The rolled-up outer portion of the primary tank liner is unrolled and deployed up and over the fully-assembled wall of the primary tank. 
     An outer portion of the secondary tank liner is then rolled up so as to be temporarily disposed adjacent to the perimeter wall of the completed secondary tank. 
     The perimeter wall of the secondary tank is then site-assembled from a plurality of arcuate wall panel sections resting on the base course. 
     The rolled-up outer portion of the secondary tank liner is then unrolled and deployed up and over the fully-assembled wall of the secondary tank. 
     In the preferred embodiments illustrated and described herein, the primary and secondary tanks are generally circular in shape. In such embodiments, the primary and secondary tank wall panels are of arcuate configuration (in variant embodiments generally circular tanks could be constructed using wall panels that are substantially flat rather than arcuate). However, the methods taught herein can also be applied to the construction of dual-tank liquid storage tank system wherein either or both of the primary and secondary tanks may be of a non-circular configuration. 
    
    
     
       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 ground area that has been prepared for construction of a dual-tank liquid storage system in accordance with one embodiment of a method taught by the present disclosure. 
         FIG. 1A  is a cross-sectional edge detail of the prepared ground area in  FIG. 1 , illustrating a structural base course engineered to support liquid storage tanks. 
         FIG. 2  is a perspective view of a storage tank site as in  FIG. 1  during the initial stages of construction of a dual-tank liquid storage system in accordance with one embodiment of a method taught herein, showing an outer tank liner laid out over the base course, a circular liner protection strip laid over the outer tank liner, and a partially deployed inner tank liner laid out over the outer tank liner. 
         FIG. 3  is a perspective view of a storage tank assembly under construction as in  FIG. 2 , shown after erection of inner tank wall panels over a circular liner protection strip. 
         FIG. 4  is a perspective view of a storage tank assembly under construction as in  FIG. 3 , illustrating the application of liner protection means over vertical joints between adjacent inner tank wall panels and at the interior circumferential juncture between the erected wall panel assembly and the outer tank liner. 
         FIG. 5  is a perspective view of a storage tank assembly under construction as in  FIG. 4 , shown after the inner tank liner has been deployed over and secured to the inner tank wall assembly. 
         FIG. 6  is a perspective view of a storage tank assembly under construction as in  FIG. 5 , illustrating the outer tank wall panels being erected over the base course. 
         FIG. 7  is a perspective view of a storage tank assembly under construction as in  FIG. 6 , shown after the outer tank liner has been deployed over and secured to the outer tank wall assembly. 
         FIG. 8  is a perspective view of a representative pair of modular tank wall panels of the tank assembly illustrated in  FIGS. 3-7 . 
         FIG. 9  is a perspective view of the vertical joint between an abutting pair of tank wall panels as in  FIG. 8 , illustrating an exemplary and non-limiting example of a structural connection between the vertical edges of the abutting wall panels. 
         FIG. 10  is a vertical cross-section through a completed storage tank assembly as in  FIG. 7 , shown with the inner tank filled with liquid. 
     
    
    
     DESCRIPTION 
       FIG. 1  illustrates a prepared ground surface  10  on top of which an engineered base course  20  has been constructed to provide stable ground support for a liquid storage tank assembly comprising a circular primary tank located generally concentrically inside a circular secondary tank in accordance with the present disclosure. Base course  20  will typically be a multi-layered soil structure, and as shown by way of non-limiting example in  FIG. 1A  may comprise a layer of compacted granular material  22  placed over ground surface  10 , then finished with a sand layer  24  which can be levelled and compacted to provide a smooth and dense surface. 
     In one embodiment, granular layer  22  may comprise at least a two inches of ½-inch (12.7 mm) crush compacted to at least 100% of Standard Proctor maximum dry density. However, the appropriate design and construction of base course  20  for a given installation will preferably be determined having regard to the geotechnical properties of the subsoil where the tanks are to be built. Base course  20  will preferably extend radially outward beyond perimeter of the secondary tank by a distance selected to geotechnical requirements and to provide adequate working space during tank construction. For example, for an embodiment of the dual-tank system including a 1.25 million USG (≈4.7 million liters) primary tank having a diameter of 135 feet (≈41 meters) and disposed within a secondary tank having a diameter of 148 feet (≈45 meters), base course  20  preferably will cover a circular area having a diameter of approximately 200 feet (≈61 meters). 
     Base course  20  is preferably covered with a base course protection layer  15 , which in one embodiment may be a double layer of a suitable geotextile. 
     As illustrated in  FIG. 2 , the next step in the tank construction process is to provide a flexible, impermeable secondary tank liner  30 , sized and configured to cover the circular base area of the secondary (i.e., outer) tank and to extend upward and over the inside face of perimeter wall of the secondary tank. Secondary tank liner  30  is laid out over base course  20  (and base course protection layer  15 ) so as to cover a generally circular area within the intended circular perimeter of the secondary tank. 
     A suitable liner protection strip  52  is preferably provided over secondary tank liner  30  along a circular path corresponding to the intended perimeter of the primary tank, to provide a surface upon which the walls of the primary tank can be constructed without causing localized damage to the underlying portion of secondary tank liner  30 . Liner protection strip  52  may be provided in any form suitable for this purpose, such as (by way of non-limiting example) a double ring of geocomposite or geotextile material. In a preferred embodiment, liner protection strip  52  comprises a double layer of geotextile having a felt layer on the top and bottom. 
     Either before or after liner protection means  52  has been placed, a flexible, impermeable primary tank liner  40 , sized and configured to cover the circular base area of the primary tank and to extend upward and over the inside face of the perimeter wall of the primary tank, is laid out over secondary tank liner  30  so as to cover a generally circular area within the intended circular perimeter of the primary tank, but the outer portion  40 A of primary tank liner  40  that will ultimately be extended up and over the primary tank wall is rolled up like a tarpaulin such that the rolled-up wall portion  40 A is temporarily positioned a convenient distance radially inward from the intended perimeter of the primary tank. For example, for a primary tank having a diameter of 135 feet (≈41 meters), primary tank liner  40  will preferably be rolled out such that the diameter of rolled-up wall portion  40 A is approximately 90 feet (≈27 meters), in order to provide ample working clearance from the area where the primary tank is to be erected. 
       FIG. 3  illustrates a plurality of curved modular tank wall panels  62  that have been erected over liner protection strip  52  to form the walls of a primary tank  60 . Persons skilled in the art will understand that the erection of primary tank wall panels  62  may and typically will entail the use of temporary bracing (not shown) to stabilize panels  62 . Temporary bracing may be of any suitable type, and may be provided exterior and/or interior to wall panels  62 . Suitable protective means should be provided to protect secondary tank liner  30  from damage that might otherwise be caused by the installation of temporary bracing. In an alternative (and unillustrated) embodiment of the present tank system construction process, an outer portion of secondary tank liner  30  may be partially rolled up, with the rolled-up portion is positioned fairly close to the perimeter of primary tank  60 , such that exterior temporary bracing can bear directly onto base course protection layer  15  over base course  20  without impinging on secondary tank liner  30 . After all wall panels  62  have been erected to form an inherently stable primary tank  60 , all temporary bracing will be removed. 
       FIG. 4  illustrates the placement of liner protection strips  54  over vertical joints between adjacent primary tank wall panels  62  to protect against localized physical damage to wall portion  40 A of primary tank liner portion  40  when it is extended up and over wall panels  62 , such as liner damage that might occur as a result of movement across vertical joints between adjacent wall panels or panel misalignments due to fabrication and/or erection tolerances. For similar purposes, a continuous liner protection strip  56  is preferably placed along the interior perimeter of primary tank  60  where it rests upon secondary tank liner  30  over base course  20 . It should be understood, however, that liner protection strips  54  and  56  are not essential, and the practical need for same will typically be determined on a case-by-case basis subject to an assessment of the likelihood and potential significance of joint movements and/or tolerance issues. 
       FIG. 5  illustrates a completed primary tank  60 , with wall portion  40 A of primary tank liner  40  deployed to cover the inner surfaces of primary tank wall panels  62  and with an outer edge portion  40 B of primary tank liner  40  extending over the top of wall panels  62  and secured thereto by suitable removable clamp means  45 . 
       FIG. 6  illustrates a plurality of curved modular tank wall panels  72  being erected on top of base course protection layer  15  over base course  20  to form a secondary tank  70 . As shown in  FIG. 6 , the outer portion  30 A of secondary tank liner  30  that will ultimately cover the inner surfaces of the secondary tank wall assembly has been rolled up so that it is inside the perimeter of secondary tank  70 , and preferably as close as possible to primary tank  60  to maximize the working room for erecting secondary tank  70 . 
     Although not illustrated, temporary bracing will typically be used during the erection of secondary tank wall panels  72 , generally as described above with respect to the erection of primary tank wall panels  62 . In cases where the radial distance between primary tank  60  and secondary tank  70  is not large (as in the illustrated embodiment), typically only exterior bracing will be used during the erection of secondary tank wall panels  72 . In alternative embodiments of the tank construction process, secondary tank wall panels  72  could also be temporarily braced against the completed primary tank structure  60 . 
     After all secondary tank wall panels  72  have been erected so as to form an inherently stable secondary tank  70 , all temporary bracing can be removed. Liner protection strips (not shown) may be placed over vertical joints between adjacent secondary tank wall panels  72  and along the interior perimeter of secondary tank  70  where it rests upon base course  20 , generally as described previously with respect to primary tank  60 . 
       FIG. 7  illustrates a completed secondary tank  70 , with wall portion  30 A of secondary tank liner  30  deployed to cover the inner surfaces of secondary tank wall panels  72  and with an outer edge portion  30 B of secondary tank liner  30  extending over the top of wall panels  72  and secured thereto by suitable removable clamp means  35 . The tank assembly is now ready to receive ancillary equipment and appurtenances (e.g., tank inlet and outlet piping; tank level gauges; catwalks and access platforms). 
       FIGS. 8 and 9  illustrate non-limiting examples of curved modular tank wall panels  62  (or  72 ), each comprising a horizontally-curved tank wall plate  63  ( 73 ) reinforced by a plurality externally-mounted, horizontally-curved structural stiffeners  64  ( 74 ), and with secondary vertical stiffeners  66  ( 76 ) extending between vertically-adjacent horizontal stiffeners  64  ( 74 ). The spacing of horizontal stiffeners  64  ( 74 ) preferably becomes smaller toward the bottom of wall panel  62  ( 72 ), thus reducing the vertical span of wall plate  63  ( 73 ) in order to minimizing wall plate thickness requirements while keeping flexural stresses in wall plate  63  ( 73 ) within safe limits as hydrostatic pressures exerted against wall plate  63  ( 73 ) increase toward the bottom of wall panel  62  ( 72 ). 
     An edge stiffener  65  ( 75 ) is provided along each vertical side edge of wall panel  62  ( 72 ). In the illustrated embodiment, and as shown in detail in  FIG. 9 , edge stiffeners  65  ( 75 ) are provided with bolt holes for receiving bolts  68  which will be installed in the field to structurally connect adjacent wall panels  62  ( 72 ). However, the illustrated panel connection detail is by way of non-limiting example only, and persons skilled in the art will appreciate that wall panels  62  ( 72 ) can be structurally interconnected in various different ways, and that the selected structural connection details have no material bearing on the disclosed tank construction systems and concepts. 
       FIG. 10  illustrates the completed dual-tank system in operation, with primary tank  60  filled with liquid. In the event of a leak developing in primary tank liner  40 , any escaping liquid will be retained by secondary tank liner  30  within secondary tank  70 . 
     When the tank system is no longer needed on site, it is a simple matter to disassemble tanks  60  and  70  and to remove their respective liners  40  and  30  and all related components, by essentially reversing the various steps described above. The site can then be landscaped as desired to restore the site to a substantially natural and environmentally undisturbed condition. 
     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, relative or relational terms such as but not limited to “vertical” are not intended to denote or require mathematical or geometric precision. Accordingly, such terms are to be understood in a general sense rather than a precise sense (e.g., “substantially vertical”), 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.

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