Abstract:
A method for forming a sealed floating roof for a storage tank is disclosed. A plurality of roof panels are positioned adjacent to one another such that respective depressions of adjacent roof panels disposed adjacent to one another form a walled trough at interconnections of the plurality of roof panels. A self-propelled automatic welder is positioned and guided within the walled trough to make a weld joint at the walled trough, and thus forming the sealed floating roof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/929,191, entitled “WELDED FULL CONTACT FLOATING ROOF”, filed Jun. 27, 2013, now U.S. Pat. No. 8,973,771, which is a continuation of U.S. patent application Ser. No. 12/075,229, entitled “WELDED FULL CONTACT FLOATING ROOF AND METHOD”, filed Mar. 6, 2008, each of which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The invention pertains to sealed floating roofs for storage tanks. 
     Above ground storage tanks are frequently used to store hydrocarbon fluids. Because the stored fluid is volatile, the storage tank is often equipped with a floating roof itself sometimes under a fixed roof. The floating roof floats on the stored fluid and moves up and down with the fluid level. Floating roofs greatly reduce fluid evaporation, preventing loss of the stored fluid and reducing pollution due to hydrocarbon evaporation into the atmosphere. 
     Additionally, it is desirable to provide floating roofs that are full-contact roofs, allowing the roof structure to be in direct contact with the fluid surface. Such roofs do not allow an air gap between the bottom of the roof surface and contained fluid. When air gaps exist, they allow an evaporation zone over the top of the fluid that increases the risk of leakage around the floating roof structure. However, full contact floating roofs must be tightly sealed to prevent direct vapor leakage or evaporation through small gaps in the roof structure. Such leakage or evaporative losses can create unwanted pollution as well as the economic loss of stored product. 
     However, typical floating roofs are large and must be assembled on site. The structure typically comprises a framework of segments that are assembled to form and open lattice, and a plurality of roof panels or sheets that are then attached to the framework to form the upper surface of the roof. Roof panels or sheets are typically attached by bolting them in place, and sometimes sealed with sealants that are generally known in the industry. 
     Such construction can prevent the desired sealing effectiveness of the roof from being achieved. Sealants can degrade over time due to environmental conditions, and may be attacked by contact with the stored fluid or by vapors. Bolted connections are not vapor tight. Effecting repairs can require draining the storage tank, resulting in lost revenue, and may require workers to operate in a hazardous environment. Accordingly it is desirable to provide a seal between the roof panels and the framework that is highly resistant to degradation over time, and that will provide a strong, durable roof over its life expectancy. 
     One possible approach is to weld the edges of the roof panels to the framework, so that every seam between the roof panels and the framework is permanently sealed. However, if welding were to be done by hand, the large size of the typical floating roof would require a large expenditure of man hours, and the work would often have to be done in an extremely uncomfortable environment. 
     Accordingly, it is desirable to provide a floating roof, especially one intended for full contact, with welded construction so that the roof panels are welded to the framework. It is further desirable to accomplish this welding with an automatic welder. 
     SUMMARY 
     The invention comprises a floating roof comprising frame segments and roof panel top and bottom sheets that are shop welded to the frame segments to form fully sealed roof panels. These roof panels are then assembled and welded together in the storage tank to form a fully welded, full contact floating roof. In a preferred embodiment, the frame segments are aluminum extrusions, which allow strong, yet relatively lightweight construction and high resistance to corrosion over time. Individual, fully welded roof panels may be constructed and pre-tested at a factory location, then shipped to the job site for final assembly. It is generally desirable to assemble the roof panels into a staggered rectangular grid. In this manner, standard-sized rectangular roof panels may be used to complete almost the entire roof, with differently shaped panels only required to form the outer, circular circumference of the roof. 
     For example, a typical rectangular panel of a preferred embodiment of the invention would be framed using four lengths of an extruded aluminum frame segment, with top and bottom sheets edge-welded around their entire perimeters to the frame segments, forming a fully sealed roof panel. These roof panels can then be tested at the factory for seal and weld integrity, and modified as desired for a particular installation. For example, sniffers or other test equipment may be inserted into a panel through its top sheet, allowing a customer to operate real-time test equipment once the roof is placed in operation. The completed roof panels may then be shipped to the job site for assembly. 
     Once at the job site, the roof panels may be supported on legs or temporary supports, and frame segments of adjacent panels riveted together, preferably using a self-piercing rivet gun such as Model #ESN50. Those of skill in the art will recognize that, during this assembly process, the roof panels must be supported in a way that insures that they are properly leveled with respect to each other. Once the roof panels have been riveted together to form the overall roof structure, the roof structure is completed by welding the roof panels together. 
     To weld the roof panels together, a self-propelled automatic welder, such as Model #BUGHDT1010 by HMT, Inc., using components manufactured by Bug-O Systems and Lincoln Welding Equipment may be used. However, it is necessary to properly guide the welder so that the track of the weld is correctly positioned along the contact seam between the adjacent roof panels. Accordingly, it is desirable to provide a guide to correctly position the automatic welder. 
     To accomplish this goal, the frame segments are preferably extruded with a formed depression in an upper edge of the frame segment. Due to the nature of the construction, a single form of frame segment may be used, leaving a flat side turned outward from the roof panels. When the roof panels are riveted together, these flat sides form the outer wall of the roof panel, and are riveted to the flat sides of the adjacent roof panels&#39; frame segments, with two such frame segments mechanically coupled “back-to-back.” When correctly positioned, the depressions in the upper edges of two such joined segments will be adjacent, and will form a walled trough in the upper surface of the beam. 
     As those of skill in the art will recognize, many alternatives to such construction may exist. For example, an entire beam could be extruded as a single piece, with a depression formed in its upper surface, without departing from the spirit of the invention. However, such an extrusion would be heavier and harder to position and control during assembly of the roof. Further, such a construction method would essentially require top and bottom sheets of the roof panels to be welded into place at the job site, increasing the complexity of the on-site construction, and making testing of individual cells in the roof much more difficult. 
     With a roof thus assembled, each walled trough between roof panels acts as a directional guide for the automatic welder, and an automatic welder thus controlled will maintain an appropriate path. It is therefore desirable to modify the aforementioned stock automatic welder by attaching a guide wheel to its carriage to insure that it follows the guide track in the upper surface of the framework. In a preferred embodiment, guide wheels are attached to both the front and rear of the automatic welder&#39;s carriage, to insure that one end does not skew during transit. 
     Completion of the floating roof can thus be accomplished by positioning the automatic welder to transit along the walled troughs, using the walls for guidance and forming a continuous welding bead in the trough. Those of skill in the art will recognize that, without departing from the spirit of the invention, guidance of the automatic welder may be accomplished by a variety of alternate methods, such as providing multiple parallel troughs for guide wheels, radio or light frequency remote controls, direct linkage remote controls, or computer driven programmable controls integrated into the welder itself. While functional, such alternatives may increase the complexity of the assembly operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross section of a frame segment extrusion of one embodiment of the present invention; 
         FIG. 1B  is a cross section of two adjacent roof panels of one embodiment of the present invention; 
         FIG. 2A  is a top view of a roof panel of one embodiment of the present invention; 
         FIG. 2B  is a bottom view of a roof panel of one embodiment of the present invention; 
         FIG. 3  is a side schematic view of an automatic welder operating to weld roof panels together in an assembled roof; and 
         FIG. 4  is a schematic view of an assembled full contact floating roof of one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1A , an extruded frame segment  10  of a preferred embodiment of the present invention is seen in cross-section. Referring also to  FIG. 1B , a cross-section of two adjacent assembled roof panels  12 ,  14  employing multiple frame segments  10  is shown. Frame segments  10  of  FIGS. 1A, 1B  comprises an upper lip  16  which supports upper roof sheet  18  and which is placed into sealing contact with upper roof sheet  18  by welding along the perimeter  20  of upper roof sheet  18 . Similarly, frame segment  10  comprises a lower lip  22  that is placed into sealing contact with lower roof sheet  24  by welding along the perimeter  26  of lower roof sheet  24 . When so assembled, frame segments  10  and upper and lower roof sheets  18 ,  24  form a sealed cavity  28  that forms a component of a full contact floating roof. 
     Roof panels  12 ,  14  are preferably constructed and initially tested at a factory before being shipped to a job site for assembly into a full contact floating roof. (Such as depicted in  FIG. 4 ) When placed adjacent to each other for assembly into a roof, roof panels  12 ,  14  are first leveled and properly aligned, then mechanically joined together, as by use of stainless steel rivets  30 . 
     Frame segment  10  additionally preferably comprises a first top depression  32  and a second top depression  34 . When joined in complementary orientation, as shown in  FIG. 1B , the first top depressions  32  of two frame segments  10  combine to form a walled trough  36 . Similarly, the second top depressions  34  combine to form a weld seam  38 . 
     Referring now to  FIGS. 2A and 2B , top and bottom views of assembled roof panels  212  are shown respectively. Frame segments  210  are angle-cut at the corners to provide squared-off corners  215 . Upper roof sheet  218  is fully welded to frame segments  210  along its perimeter  220 , and lower roof sheet  224  is fully welded to frame segments  224  along its perimeter  226 . Frame segments  210  are also welded at corners  214  to provide a completely sealed roof panel. Although roof panels  212  are preferably rectangular in shape, those of skill in the art will recognize that this shape is a matter of engineering preference, and that panels with curved edges will be required to form the perimeter of a circular floating roof as shown in  FIG. 4 . If desired, one or more portals  228  may be opened in the upper roof sheet  218 , to provide access to the interior of the roof panel  212  for the insertion of leak sniffers or other instrumentation. 
     Referring now to  FIG. 3 , a schematic view of an automatic welder  310  for use in completion of the floating roof is shown. The automatic welder  310  comprises a carriage  312  mounted on wheels  314 , allowing it to roll across the upper surface  316  of the floating roof. A weld head  320  is held in position by control arm  318 , allowing weld head  320  to be positioned to form a weld along weld seam  38  of  FIG. 1B . Guide wheels  322 ,  324  are attached to the automatic welder  310 , and are fitted into walled trough  36  of  FIG. 1B , controlling the line of motion of the automatic welder  310 , and allowing the automatic welder  310  to track each weld seam in the floating roof under assembly to completely seal the roof. 
     Those of skill in the art will recognize that other means of guiding the automatic welder, such as (without limitation) radio frequency controls, directly connected steering controls, or alternate guide lines formed in the roof under construction could be used without departing from the spirit of the invention. Additionally, guidance could be accomplished without using multiple guide wheels. 
     Referring now to  FIG. 4 , a schematic top view of a welded, full contact floating roof of the present invention is shown. Floating roof  410  preferably comprises a plurality of rectangular roof panels  412  and perimeter curved roof panels  414 . Those of skill in the art will recognize that panels  412  and  414  are constructed in the same fashion, with curved panels  414  requiring curved frame segments along one side. Further, if the floating roof is of square or rectangular configuration, curved roof panels will be unnecessary. 
     One or more panels, for example  412 , may be provided with man-way access, to allow personnel to access the lower portion of the roof if needed. Each of the seams  418  between adjacent roof panels  412 ,  414  is welded by use of the automatic welder as discussed above, providing a full contact floating roof that is fully sealed against evaporation by welds. 
     Those of skill in the art will also recognize that the floating roof of this invention may be fitted with a sliding edge seal (not shown) around its perimeter as known in the art. Additionally, the roof may be equipped to be held at a particular height, for example, for maintenance operations, by providing it with non-penetrating cable attach points (non-shown) on its upper surface, or by providing non-penetrating leg supports (not shown) on its lower surface. Thus, these attachments can be made without penetrating the roof, preserving its sealing integrity. 
     The above examples are included for demonstration purposes only and not as limitations on the scope of the invention. Other variations in the construction of the invention may be made without departing from the spirit of the invention, and those of skill in the art will recognize that these descriptions are provide by way of example only.