Patent Publication Number: US-2017349419-A1

Title: Gantry tower crane for inspecting cargo hold

Description:
TECHNICAL FIELD 
     The present invention relates to a gantry tower crane for inspecting a cargo hold, and more particularly, to a gantry tower crane for inspecting a cargo hold, capable of inspecting or maintaining and repairing an inside of the cargo hold of a large ship, such as a container ship, an LNG carrier, and a floating liquefied natural gas (FLNG). 
     BACKGROUND ART 
     In general, natural gas is transported in a gaseous state via land or sea gas piping, or stored in a LNG carrier in the form of liquefied natural gas (hereinafter referred to as “LNG”) so as to be transported to a remote source of demand. 
     A ship for carrying LNG is broadly classified into a moss type ship and a membrane type ship, in which the moss type LNG carrier has a structure that more than half of a cargo hold having a generally spherical shape protrudes to the outside of the carrier, and the membrane type LNG carrier has a structure that a membrane type cargo hold is provided inside the carrier or only a part of the membrane type cargo hold protrudes to the outside of the carrier. 
     In the case of the membrane type LNG carrier, since most or all of the cargo holds are provided inside the carrier and basically have a rectangular structure, more LNG can be transported compared to the moss type LNG carrier, and influence of wind can be minimized, so that the sailing performance is also considered excellent. 
     Since the membrane type cargo hold has no support or structure inside the cargo hold due to structural characteristics of the membrane type cargo, scaffolds (mold) are installed by connecting each material one by one to inspect the gas leakage and the membrane damage in the cargo hold, or to perform the maintenance and repair work, and the installation time for such scaffolds is excessively required. 
     In addition, after all the scaffolds (molds) are removed as the cargo hold is constructed, if the maintenance and repair work or the inspection work for the membrane is required, a foothold (scaffold or mold) has to be installed and dismantled again, resulting in significant process disruptions. 
     In the case of typical LNG carriers, the LNG carriers are repaired at a repair shipyard for the maintenance and repair or the inspection of the cargo hold and other tools and materials every five-year cycle. At this time, it is possible to perform the inspection work and the maintenance and repair work by installing the mold in the cargo hold due to the time margin. 
     However, in the case of an ocean platform that has to stay at sea for a long time, such as floating liquefied natural gas (FLNG), since it is not possible to move the ocean platform to the repair shipyard, it is necessary to perform inspection, maintenance, and repair on the inside of the cargo hold in a state of floating on the sea. Thus, it takes a long time for the inspection, maintenance, and repair work. 
     In addition, if the inspection, maintenance, and repair work for the cargo hold is stopped, it causes a great economic loss, so that a method of rapidly performing inspection, maintenance and repair work is required other than the conventional method of installing a scaffold (mold). 
     As described above, since the scaffolds are installed manually when inspecting and repairing the inside of large ships and warehouses, it takes much installation work time. In addition, since there is no separate safety device, there is always a risk of safety accidents caused by mistake such as falling during work, and since it takes much time to dismantle the scaffold after completing the inspection and maintenance work, the manufacturing and inspection costs are raised. 
     In addition, since there is no support inside the cargo hold of the membrane type LNG carriers, in order to inspect the gas leakage, the membrane damage, and the insulation box damage in the cargo hold, or to perform the maintenance and repair work, a gas dome, which is the only passage to the inside of the cargo hold, is used to insert materials, and each material is connected one by one to install the mold. Therefore, the installation and dismantling time is excessively required, and much workforce is necessary. 
     In particular, if the maintenance and repair work for the inside of the cargo hold is inevitably necessary after dismantling the mold, it requires much time and workforce to install the mold, which may lead to serious process disruptions. 
     In addition, although a method of installing a multi-joint robot in the gas dome has been proposed, in order to cover the whole area of the cargo hold, the total length of the multi-joint robot has to be 30 m or more, which is large, and the robot has to be manufactured robust enough to allow an operator to safely ride on the end of the robot. 
     In the case of applying such a multi-joint robot system to an ocean plant such as FLNG, since it is difficult for a long structure or structures having large diameters to enter due to constraints of a space between a top side and a main deck, installation is difficult in reality. 
     For example, Patent Document 1 discloses a ‘movable scaffold structure for working in a cargo hold of a liquefied natural gas carrier’. 
     The movable scaffold structure for working in the cargo hold of the liquefied natural gas carrier according to Patent Document 1 includes: a base body part for supporting a scaffold structure installed in the cargo hold; a fixed leg part installed on a bottom surface of the base body part; a rotation leg part rotatably installed about a pivot point portion with respect to the fixed leg part; a support leg part having an auxiliary leg portion which is adjustable in length by using a telescopic connection portion with respect to the rotation leg part; and a rolling contact part installed on the support leg part to realize movement of the base body part. 
     The pivot point portion includes: a first mounting bracket coupled to one end of the fixed leg part; a second mounting bracket coupled to one end of the rotation leg part; a hinge pin coupled to share a hinge point of the first mounting bracket and the second mounting bracket; a plurality of coupling holes formed in the first mounting bracket and the second mounting bracket over a same radius around the hinge pin; and a fastening pin. 
     The telescopic connection portion includes an adjustment nut screw-coupled to a front end of the rotation leg part to restrain movement of the auxiliary leg portion inserted and fitted into the rotation leg part. In addition, the rolling contact part includes a wheel support horizontally installed at a lower front end of the auxiliary leg portion, and a plurality of rotation wheels installed in a longitudinal direction with respect to the wheel support. 
     Patent Document 2 discloses a ‘movable mold system of an LNG cargo hold’. 
     The movable mold system of the LNG cargo hold according to Patent Document 2 is a mold system provided in the LNG cargo hold and includes: a plurality of guide rails arranged along a longitudinal direction at a side portion of the cargo hold; a frame vertically installed to move along the guide rail; and a plurality of molds detachably coupled to the frame while being vertically spaced apart from each other. 
     DISCLOSURE 
     Technical Problem 
     Although the movable scaffold structure for working in the cargo hold of the liquefied natural gas carrier according to the related art may reduce the number of work processes required for installation and dismantling of the scaffold structure by minimizing an installation range of a multi-layer scaffold structure in the cargo hold, there is a problem that a large-sized cargo hold requires a large number of movable scaffold structure. 
     In addition, in the movable mold system of the LNG cargo hold according to the related art, there is inconvenience that the guide rail has to be fixed in the longitudinal direction along an inner wall surface of the cargo hold, it is inconvenient because the mold has to be installed at regular height intervals, and there is inconvenience due to the installation and dismantling of these movable mold frames. 
     To solve the problems described above, an object of the present invention is to provide a gantry tower crane for inspecting a cargo hold, capable of rapidly installing or dismantling the tower crane for checking, maintaining, repairing, and inspecting an inside of the cargo hold. 
     Another object of the present invention is to provide a gantry tower crane for inspecting a cargo hold, capable of ensuring safety of an operator when checking, maintaining, repairing, and inspecting the cargo hold. 
     Another object of the present invention is to provide a gantry tower crane for inspecting a cargo hold, capable of freely moving or rotating in vertical and horizontal directions such that the entire inside of the cargo hold may be inspected, maintained, and repaired. 
     Another object of the present invention is to provide a gantry tower crane for inspecting a cargo hold, capable of rapidly performing inspection, maintenance, and repair work of the cargo hold. 
     Another object of the present invention is to provide a gantry tower crane for inspecting a cargo hold, capable of installing or dismantling the gantry tower crane through a gas dome formed in an upper portion of the cargo hold. 
     Another object of the present invention is to provide a gantry tower crane for inspecting a cargo hold, which allows the operator to easily access an inner wall surface and a ceiling portion of the cargo hold. 
     Technical Solution 
     To achieve the objects described above, according to the present invention, there is provided a gantry tower crane for inspecting a cargo hold, the gantry tower crane including: two running rails spaced apart from each other in a longitudinal direction of the cargo hold; two prefabricated towers movably mounted along the running rails, respectively; and a basket movable up and down along the prefabricated tower. 
     An overpass connected between the two prefabricated towers widthwise along the cargo hold may be further included. 
     The running rail may include: a rail body having a predetermined length and a predetermined width; a rack mounted on an upper surface of the rail body to allow the prefabricated tower to move; and a guide rail mounted in a same direction as the rack at a position spaced apart from the rack by a predetermined distance. 
     The running rail may include a plurality of moving wheels mounted on a bottom surface of the rail body so as to be movable along a floor surface of the cargo hold. 
     The rail body may include: a first coupling part coupled with another rail body adjacent to one side of the rail body; and a second coupling part coupled with another rail body adjacent to an opposite side of the rail body, wherein a plurality of rail bodies are coupled to each other by the coupling of the first and second coupling parts. 
     The first coupling part and the second coupling part are rotatably hinge-coupled to each other. 
     The prefabricated tower may include: a mobile platform movably mounted along the running rail; and a plurality of tower units vertically coupled to the mobile platform. 
     The mobile platform may include: a platform body having a predetermined size; a plurality of posts vertically fixed to an upper portion of the platform body such that the posts are coupled with the tower unit; and a plurality of pinion gears mounted on a bottom surface of the platform body such that the pinion gears are engaged with the rack so as to be movable along the running rail. 
     The mobile platform may include: a plurality of holes formed in the fixed post at predetermined intervals such that the mobile platform is rapidly coupled with the tower unit; and a pin fitted into the hole. 
     The mobile platform may include a guide roller guided by the guide rail by making close contact with a side surface of the guide rail such that the prefabricated tower is safely moved. 
     The guide roller may include: a guide member making close contact with the side surface of the guide rail; a movable member coupled to the guide member and movably mounted on a side surface of the platform body; and an adjustment bolt fastened to the movable member to change a position of the movable member. 
     The tower unit may include: a lower truss unit coupled to the mobile platform; at least one middle truss unit coupled to an upper portion of the lower truss unit; and an upper truss unit coupled to an uppermost end of the middle truss unit and provided with a passage for an operator. 
     The lower truss unit may include: a plurality of vertical frames having a predetermined length and configured in a rectangular shape; cross frames fixed between the vertical frames such that the cross frames cross the vertical frames at different directions; a plurality of fixing holes formed in a lower portion of the vertical frame such that the lower truss unit is fixedly fitted to the mobile platform; and a coupling hole formed on an upper portion of the vertical frame such that the lower truss unit is fitted to the middle truss unit. 
     The middle truss unit may include: a plurality of vertical frames having a predetermined length and configured in a rectangular shape; cross frames fixed between the vertical frames such that the cross frames cross the vertical frames at different directions; and a horizontal frame fixed to an end of the cross frame. 
     The middle truss unit may include a rotation bar rotatably and elastically mounted on the horizontal frame. 
     The prefabricated tower may include a fixing unit making close contact with an upper surface of the cargo hold to stably mount the tower unit. 
     The fixing unit may include: a fixing plate provided at a same size as the tower unit; a plurality of cylinders mounted on an upper surface of the fixing plate; and a winch mounted on the upper surface of the fixing plate. 
     The fixing unit may include a rotation plate rotatably mounted on an upper surface of the cylinder. 
     The rotation plate may include: a first plate fixed to the upper surface of the cylinder; a second plate rotatably provided on an upper surface of the first plate and coupled to a flat bearing; a third plate mounted on an upper surface of the second plate and formed therein with a groove to which the flat bearing is fitted; and a fourth plate mounted on an upper surface of the third plate and formed therein with a groove to which a tongue of the cargo hold is fitted. 
     The basket is rotatable in a horizontal direction and has a variable length with respect to a wall surface of the cargo hold. 
     The basket may include: an up-down movement frame mounted on an outer surface of the prefabricated tower so as to be movable up and down; a boom support rotatably mounted on one surface of the up-down movement frame and having a variable length; and a work table rotatably mounted at an end of the boom support. 
     The up-down movement frame may include: a first frame provided with a wheel so as to be movable along one surface of the prefabricated tower; a second frame coupled to the first frame and provided with a wheel so as to be movable along an opposite surface of the prefabricated tower; and a support frame protruding such that the boom support is rotatably mounted on one side of the second frame. 
     The boom support may include: a rotation boom support rotatably coupled to one side of the up-down movement frame; a plurality of movable boom supports movable in the rotation boom support in a telescopic manner; and a support bar rotatably mounted on one side of the up-down movement frame and the rotation boom support so as to stably support the rotation boom support. 
     The work table may include: a rotation railing rotatable about one side of the work table so as to communicate with the prefabricated tower; and an auxiliary railing rotated together with the rotation railing, provided on both sides of the rotation railing, and having a predetermined height. 
     The prefabricated tower may include: a mobile platform movably mounted along the running rail; and a plurality of tower units vertically coupled to the mobile platform. 
     The prefabricated tower may include a fixing unit making close contact with a ceiling of the cargo hold so as to stably mount the tower unit. 
     The overpass may include: a foothold serving as a path for an operator; a foothold support having a truss structure to stably support the foothold; and an auxiliary foothold rotatable toward an outside such that a work space extends on one side of the foothold. 
     The foothold may include: a foothold body rotatable about a hinge; and a rotation link provided at predetermined intervals on both sides of the foothold body. 
     The foothold is hinge-coupled so as to be folded or unfolded. 
     The foothold support may include: a first frame and a second frame provided at left and right sides of the foothold support at predetermined angles; two rotation rods rotatably coupled to one ends of the first and second frames in a longitudinal direction; and a connection rod fixed between the two rotation rods. 
     Each of the first and second frames may include: a first ring provided at upper ends of the first and second frames such that the railing frame is rotatably coupled; a second ring provided at a lower portion of the first ring such that the rotation link is rotatably coupled; and a third ring to which the rotation rod and the connection rod are rotatably coupled. 
     The auxiliary foothold may include: a railing frame rotatably coupled to upper sides of the first and second frames; and two railings detachably coupled to the railing frame. 
     The two railings may include: a first railing frame having a predetermined length; and a second railing frame movably coupled to an outer surface of the first railing frame. 
     One of the two railings may further include a work foothold provided to form the work space of the operator. 
     A chain connected between the two railings such that the two railings are stably fixed to each other may be included. 
     Advantageous Effects 
     As described above, according to the gantry tower crane for inspecting the cargo hold of the present invention, the protective cover is provided on the floor surface of the cargo hold, so that the damage or breakage of the cargo hold can be prevented, the mobile platform, the tower unit, and the fixing unit are descended on an installation rail while being assembled, so that the installation is performed easily and rapidly, and baskets are installed in respective prefabricated towers, so that a wide work area (space) for the operator can be obtained. 
     According to the gantry tower crane for inspecting the cargo hold of the present invention, as the basket installed in the prefabricated towers is installed to have a variable length and to be rotatable, it is possible to inspect both wall surfaces and a ceiling of the cargo hold, and to inspect the wall surfaces while moving along an overpass. In addition, it is possible to obtain an effect of being able to perform inspection more safely and precisely in a state where an operator comes closer to the wall surfaces by extending an auxiliary foothold of the overpass. 
     According to a method for installing a gantry tower crane for use in cargo hold inspection, the tower units are descended by a unit descending device while assembling the tower units, so that the tower crane can be rapidly installed in a short time, components necessary for installing the tower crane can be stably inserted (lowered) through the gas dome, the time required to install two prefabricated towers can be reduced by installing the tower crane by the installation rail and then installing the running rail, and all zones of the cargo hold can be inspected as baskets and overpasses are installed. 
     According to a method for installing a gantry tower crane for use in cargo hold inspection, the installation state of the tower crane can be more stably maintained by connecting the two prefabricated towers with each other, and the two prefabricated towers can be moved, which allows to freely move the work area according to the inspection of the cargo hold. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing a state before installing a running rail of a gantry tower crane for inspecting a cargo hold according to a preferred embodiment of the present invention. 
         FIG. 2  is a perspective view showing a state that the running rail of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is installed. 
         FIG. 3  is a perspective view showing a protective cover of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 4  is a perspective view showing a state that a mobile platform is installed on a rail of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 5  is an exploded perspective view showing the rail of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 6  is an exploded perspective view showing a prefabricated tower of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 7  is a bottom exploded view showing the mobile platform of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 8  is an exploded perspective view showing a state that a part of the prefabricated tower of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is assembled. 
         FIG. 9  is an exploded perspective view showing a fixing unit of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 10  is an exploded perspective view showing a tower unit of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 11  is a perspective view showing a basket of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 12  is a bottom perspective view showing the basket of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 13  is a perspective view showing an overpass of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 14  is a bottom perspective view showing the overpass of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 15  is a schematic sectional view showing a state that the overpass of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is folded. 
         FIG. 16  is a perspective view showing a state that a unit descending device of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is installed in a gas dome. 
         FIG. 17  is a partially cut-away perspective view showing an installation state of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
         FIG. 18  is a process diagram for explaining a method for installing a gantry tower crane for use in cargo hold inspection step by step according to the preferred embodiment of the present invention. 
         FIG. 19  is a process diagram including a protective cover installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
         FIG. 20  is a process diagram for explaining the protective cover installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
         FIG. 21  is a process diagram for explaining an installation rail installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
         FIG. 22  is a process diagram for explaining a mobile platform installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
         FIG. 23  is a process diagram for explaining a prefabricated tower installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
         FIG. 24  is a process diagram for explaining a basket installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
         FIG. 25  is a process diagram for explaining an overpass installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
         FIG. 26  is a process diagram for explaining the overpass installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
     
    
    
     BEST MODE 
     Mode for Invention 
     Hereinafter, a gantry tower crane for inspecting a cargo hold according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a perspective view showing a state before installing a running rail of a gantry tower crane for inspecting a cargo hold according to a preferred embodiment of the present invention, and  FIG. 2  is a perspective view showing a state that the running rail of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is installed. 
     According to the preferred embodiment of the present invention, the gantry tower crane for inspecting the cargo hold may include: two running rails  150  spaced apart from each other in a longitudinal direction of the cargo hold; two prefabricated towers  200  having mobile platforms  210 , respectively, so as to be movable along the running rails  150 ; and a basket  300  movable up and down along the prefabricated tower  200 , having a variable length with respect to a wall surface of the cargo hold  10  (see  FIG. 16 ), and rotatable in a horizontal direction. 
     In addition, the gantry tower crane for inspecting the cargo hold of the present invention may further include an overpass  400  connected between the prefabricated towers  200  and serving as a path for an operator. 
     The gantry tower crane according to an embodiment of the present invention is provided with an installation rail  130  for installing the tower crane inside the cargo hold  10  (see  FIG. 17 ) and a running rail  150  for guiding movement of the tower crane. 
     In addition, two running rails  150  are installed in the longitudinal direction of the cargo hold  10  to perform inspection along a wall surface of the cargo hold  10 , and one prefabricated tower  200  may be installed on the two running rails  150 , respectively. 
     In addition, the prefabricated tower  200  may be provided with the basket  300  that is moved up and down along the prefabricated tower  200  and freely rotated in the horizontal direction, and the overpass  400  allowing the operator to safely move (pass) is installed between the two prefabricated towers  200 , so that the entire area of the cargo hold  10  may be inspected, the gantry tower crane required for inspecting the cargo hold  10  may be rapidly installed and dismantled, and the safety of operators is ensured. 
     Meanwhile, a rail  100  recited in the present invention may include the installation rail  130  and the running rail  150  as necessary. Although the two prefabricated towers  200  provided in one pair may be described as a first prefabricated tower and a second prefabricated tower for convenience of explanation, it should be understood that this description may be used in some cases to separate the two prefabricated towers as needed, and may refer to one or both of the two prefabricated towers as necessary. 
     In other words, in the present invention, two running rails  150  provided in one pair may be classified into a first running rail and a second running rail so as to be separated as necessary, and the two prefabricated towers  200  provided in one pair may be classified into a first prefabricated tower and a second prefabricated tower so as to be separated as necessary. 
     In addition, two baskets  300  provided in one pair may be classified into a first basket and a second basket so as to be separated as necessary, and elements separated into first and second elements will not be described using separate reference numerals. 
     In addition, the ‘inspection’ recited in the present invention should be understood as collectively referring to checking, maintenance, and repair of the cargo hold  10 . 
       FIG. 3  is a perspective view showing a protective cover of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention, and  FIG. 4  is a perspective view showing a state that a mobile platform is installed on a rail of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 1 to 4 , the installation rail  130  for installing the prefabricated tower  200  and the running rail  150  for moving the prefabricated tower  200  are installed in the cargo hold  10 . 
     The installation rail  130  and the running rail  150  may be installed on a floor surface of the cargo hold  10 , and a protective cover  110  for preventing the floor surface of the cargo holds  10  from being damaged according to the installation of the rail  100  may be installed. 
     The protective cover  110  is formed of felt, rubber, fabric or the like, so that the rail  100  may not make direct contact with the floor surface of the cargo hold  10 . The cargo hold  10  is formed of invar, which is excellent in corrosion resistance and extremely low in expansion coefficient, so that fine cracks, chips, or scratches may not occur. 
     In other words, the protective cover  110  may be formed of a material having resilience or elasticity to prevent the cargo hold  10  from being damaged by the load or impact applied during the installation of the rail  100  or during the movement of the prefabricated tower  200  along the rail  100 . 
     The protective cover  110  may include a first protective cover  111  installed in the widthwise direction of the cargo hold  10  and a second protective cover  112  installed in the longitudinal direction of the cargo hold  10  at both ends of the first protective cover  111 . 
     The first protective cover  111  may have a predetermined width and a predetermined length to prevent interference or obstruction of a tongue (not shown) protruding from the floor surface of the cargo hold  10 . 
     One side of the first protective cover  111  may have a protruding portion  113  having a predetermined length so as to cover a top surface of the tongue, and the other side may have a coupling portion  114  so as to put on the protruding portion  113 . 
     A groove  115  formed by the protruding portion  113  and the coupling portion  114  is located in the tongue protruding from the floor surface of the cargo hold  10  to prevent the tongue from being damaged or broken. 
     The second protective cover  112  may be provided in the form of a scroll so as to be installed in the longitudinal direction of the cargo hold  10 , and may be formed at a lower end thereof with a groove portion  115  in the longitudinal direction so as to be fitted to the tongue. 
       FIG. 5  is an exploded perspective view showing the rail of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 4 and 5 , the rail  100  may include the installation rail  130  temporarily installed to install the prefabricated tower  200 , and the running rail  150  allowing the prefabricated tower  200  to be movable in the longitudinal direction of the cargo hold  10 . 
     The installation rail  130  may be arranged in the widthwise direction of the cargo hold  10  at a vertically lower portion of the gas dome formed on an upper surface of the cargo hold  10 . 
     In addition, the installation rail  130  may include: a rail body  131  having a rectangular shape which has a predetermined length and a predetermined width; a rack  132  mounted on an upper surface of the rail body  131  to allow the prefabricated tower  200  to move; a guide rail  133  mounted in a same direction as the rack  132  at a position spaced apart from the rack  132  by a predetermined distance; and a plurality of moving wheels  134  mounted on a bottom surface of the rail body  131  so as to be movable along a floor surface of the cargo hold  10 . 
     A plurality of installation rails  130  may be provided in the widthwise direction of the cargo hold  10 , and rail bodies  131  may have a first coupling portion  135  and a second coupling portion  136  so that the installation rails  130  may be coupled to each other. 
     The first coupling portion  135  may be provided on one side of the rail body  131  so as to be coupled with another adjacent rail body  131 , and a second coupling portion  136  may be provided on the other side of the rail body  131  so as to be coupled with another adjacent rail body  131 . 
     The first coupling portion  135  and the second coupling portion  136  may be coupled to each other in the form of a male and female coupling, and the first coupling portion  135  and the second coupling portion  136  may be installed or detachably coupled with each other by a pin (not shown) and the like. 
     In addition, the moving wheel  134  is used to install and dismantle the installation rail  130 , and allows the installation rail  130  to be freely moved and installed in a state that the installation rail  130  is lowered on the cargo hold  10  before the prefabricated tower  200  is installed. 
     In other words, the moving wheel  134  may include a damping wheel which may restrain the movement of the moving wheel  134  according to the load of the rail body  131  or the weight (load) of the prefabricated tower  200 . 
     Such a damping wheel may freely move by the elastic force of a spring (not shown) when no load or weight is applied, and the elastic force of the spring is restrained when a load or weight is applied, so that the movement may be restrained as a brake (not shown) performs braking. 
     Such a damping wheel or gas spring may be any typical damping wheel or gas spring. 
     In addition, the installation rail  130  may include an auxiliary installation rail  140  to move the prefabricated tower  200  more safely. 
     The auxiliary installation rail  140  may have the same length as that of the rail body  131 , and may be provided at both ends thereof with a wheel (not shown) to facilitate the movement. 
     The auxiliary installation rail  140  may be installed at a position spaced apart from both sides of the rail body  131  by a predetermined distance, and the auxiliary installation rail  140  further supports the prefabricated tower  200  through an outrigger  236  when the prefabricated tower  200  moves on the installation rail  130 , so that the prefabricated tower  200 , which moves vertically, may be stably moved. 
     A groove (not shown) that may be fitted around the tongue (not shown) may be formed in the auxiliary installation rail  140 , and the tongue is pressed by fitting a fixing pin (not shown) at a position adjacent to the groove, so that the auxiliary installation rail  140  may be firmly fixed to the tongue. 
     In addition, the running rail  150  may include: a rail body  151  having a predetermined length and a predetermined width; a rack  152  mounted on an upper surface of the rail body  151  to allow the prefabricated tower  200  to move; a guide rail  153  mounted in a same direction as the rack  152  at a position spaced apart from the rack  152  by a predetermined distance; and a plurality of moving wheels  154  mounted on a bottom surface of the rail body  151  so as to be movable along a floor surface of the cargo hold  10 . 
     The rail body  151  of the running rail  150  may have a first coupling portion  155  and a second coupling portion  156  at both ends of the rail body  151 , respectively. The first coupling portion  155  and the second coupling portion  156  may be rotatably hinge-coupled to each other. 
     The running rail  150  as described above may be lowered together with the prefabricated tower  200  while being coupled to both side surfaces of the prefabricated tower  200 , separated from the prefabricated tower  200 , and spread and installed in the longitudinal direction of the cargo hold  10 . 
     In addition, the rail  100  may include a rotation rail  170  which may rotate the prefabricated tower  200 . 
     The rotation rail  170  has the same configuration as the installation rail  130  and the running rail  150 , and has both ends formed in an arc shape, so that the rotation rail  170  may be rotated together with the prefabricated tower and may be coupled to other rails  130  and  150 . 
     In other words, the installation rail body  131  and the running rail body  151  provided adjacent to the rotation rail  170  may be formed in the same arc shape as the arc of the rotation rail  170  so as to be coupled by the rotation of the rotary rail  170 . 
       FIG. 6  is an exploded perspective view showing a prefabricated tower of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention, and  FIG. 7  is a bottom exploded view showing the mobile platform of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
       FIG. 8  is an exploded perspective view showing a state that a part of the prefabricated tower of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is assembled,  FIG. 9  is an exploded perspective view showing a fixing unit of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention, and  FIG. 10  is an exploded perspective view showing a tower unit of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 6 to 10 , the prefabricated tower  200  is movable in the longitudinal direction of the cargo hold  10 , and the assembling of the basket  330  to the prefabricated tower  200  is completed in a state that the installation rail  130  is installed. Next, the prefabricated tower  200  is moved along the running rail  150  after changing the direction by the rotation rail  170 . 
     In other words, the prefabricated tower  200  may change the direction thereof from the installation rail  130  to the running rail  150  by the rotation rail  170 . 
     The prefabricated tower  200  may include: a mobile platform  210  movably mounted along the running rail  150 ; a plurality of tower units  230  vertically coupled to the mobile platform  210 ; and a fixing unit  250  making close contact with an upper surface of the cargo hold  10  to stably mount the tower unit  230 . 
     The prefabricated towers  200  are moved along the running rails  150 , and may be installed on a pair traveling rails  150 , respectively. This allows inspecting the cargo hold  10  on both sides of the cargo hold  10  by using two prefabricated towers  200 , and inspecting all front and rear wall surfaces of the cargo hold  10  by installing the overpass  400  between the two prefabricated towers  200 . 
     The mobile platform  210  for moving the tower unit  230  along the running rail  150  may include: a platform body  211  having a predetermined size; a plurality of posts  212  vertically fixed to an upper portion of the platform body  211  such that the posts  212  are coupled with the tower unit  230 ; a plurality of pinion gears  214  mounted on a bottom surface of the platform body  211  such that the pinion gears  214  are movable along the running rail  150 ; and a guide roller  215  guided by the running rail  150  by making close contact with a side surface of the running rail  150  such that the prefabricated tower  200  is safely moved. 
     In addition, the mobile platform  210  may include a plurality of holes  213  formed in the post at predetermined intervals such that the mobile platform  210  is rapidly coupled with the tower unit  230 , and a pin (not shown) fitted into the hole. 
     The mobile platform  210  has a platform body  211  having a predetermined size, and a plurality of posts  212  may be fixed to the platform body  211  to fix the tower unit  230 . 
     The post  212  may have a plurality of holes  213  for firmly fixing the tower unit  230 , and the post  212  may be provided with a fixing pin (not shown) which is fixedly fitted to the hole  213  and the tower unit  230 . 
     The mobile platform  210  is movable along the running rail  150 , and a pinion gear  214  that engages with the rack  132  of the installation rail  130  and the rack  152  of the running rail  150  may be rotatably mounted on a bottom surface of the mobile platform  210 . 
     In addition, the guide roller  215  may be installed to allow the stable movement along the rails  130  and  150 , and guide rollers  215  may be installed on both side surfaces of the mobile platform  210 . In other words, the guide roller  215  is moved while making close contact with the guide rails  133  and  153  of the rails  130  and  150 , thereby moving the prefabricated tower  200  more stably. 
     A plurality of guide rollers  215  are installed on both side surfaces of the mobile platform  210 , and grooves having a predetermined size may be formed in both side surfaces of the platform body  211 . 
     The guide member  216 , which makes close contact with the guide rails  133  and  153 , is substantially shaped as reversed ‘T’, and a movable member  217  for moving a position of the guide member  216  may be coupled to an upper portion of the guide member  216 . The movable body  217  is fitted to the groove of the platform body  211 , and an adjustment bolt  218  for moving the guide member  216  may be fastened to the movable body  217 . 
     The tower unit  230  may include: a lower truss unit  231  coupled to the mobile platform  210 ; a plurality of middle truss units  237  coupled to an upper portion of the lower truss unit  231 ; and an upper truss unit  242  coupled to an uppermost end of the middle truss unit and serving as a path for an operator. 
     The tower unit  230  is vertically constructed from the floor surface to the ceiling of the cargo hold  10 , and includes a lower truss unit  231  fixed to the mobile platform  210 , a plurality of middle truss units  237  having the same shape and structure as the lower truss unit  231 , and an upper truss unit  242  fixed to the uppermost end of the tower unit  230 . The tower unit  230  may further include a fixing unit  250  fixed to an upper surface of the upper truss unit  242  so as to allow the prefabricated tower  200  to make close contact with the ceiling of the cargo hold  10  and to stably maintain the contact state. 
     The tower unit  230  may refer to one in which the lower truss unit  231 , the middle truss unit  237 , and the upper truss unit  242  are integrated, and may refer to one that further includes the fixing unit  250 . In other words, the tower unit  230  may refer to the prefabricated tower  200  except for the mobile platform  210 . 
     The truss units  231 ,  237 , and  242  may have a rectangular parallelepiped shape having a predetermined height, a plurality of vertical frames  232  having a predetermined length are arranged in a rectangular shape in the lower truss unit  231 , and cross frames  233  crossing each other are fixed between the vertical frames  232 . 
     In addition, the vertical frame  232  of the lower truss unit  231  may have a hole  234  corresponding to the post  212  and the hole  213  of the platform body  211 , and the post  212  and the vertical frame  232  may be firmly fixed by the fixing pin (not shown) passing through the holes  213  and  234  in a state that the post  212  and the vertical frame  232  are fitted to each other. 
     In addition, a coupling hole  235  may be formed at an upper end of the vertical frame  232 , so that the middle truss unit  237  may be coupled to the vertical frame  232 . 
     An outrigger  236  (see  FIG. 4 ) for maintaining the prefabricated tower  200  in a stable state by moving along the auxiliary installation rail  140  when the prefabricated tower  200  is moved is rotatably installed in the lower truss unit  231 . 
     The outrigger  236  may be installed at an angle of approximately 45° from the lower truss unit  231  so as to stably support the prefabricated tower  200 . The support angle of the outrigger  236  may be varied as necessary. 
     As shown in  FIGS. 4 and 6 , the outrigger  236  has a variable length in a telescopic manner so as to support the prefabricated tower  200  on both sides of the prefabricated tower  200 , so that fall down, rollover, and the like may be prevented when the prefabricated tower  200  moves along the installation rail  130 . 
     One end of the outrigger  236  may be rotatably coupled to an upper end of the lower truss unit  231 , and the other end of the outrigger  236  may be movably coupled to the auxiliary installation rail  140 . 
     In addition, the middle truss unit  237  may include: a plurality of vertical frames  238  having a predetermined length and configured in a rectangular shape; cross frames  239  fixed between the vertical frames  238  such that the cross frames  239  cross the vertical frames  238  at different directions; a horizontal frame  240  fixed to an end of the cross frame  239 ; and a rotation bar  241  rotatably and elastically mounted on the horizontal frame  240 . 
     The vertical frame  238  is provided at a lower portion thereof with a protrusion portion that is fitted to the vertical frame of other middle truss unit located under the middle truss unit  237 , and formed at an upper portion thereof with a groove portion into which the protrusion portion of the vertical frame of another middle truss unit  237  located over the middle truss unit  237  is fitted. 
     In addition, the horizontal frame  240  may be fixed to the vertical frame  238  of the middle truss unit  237  between the cross frames  239 , and the horizontal frame  240  may be provided with the rotation bar  241  for temporarily hooking the running rail  150 . 
     Accordingly, the truss units  231  and  237  may be inserted into the cargo hold  10  to assemble the tower unit  230  of the prefabricated tower  200  while simultaneously inserting the running rail  150  together, so that the time required for installing the gantry tower crane for inspecting the cargo hold can be remarkably shortened. 
     The rotation bar  241  has a hook shape, and may be elastically installed so as to be rotatable by a torsion spring (not shown) and the like installed therein. 
     This allows the running rail  150  to be temporarily hooked to an outer surface of the middle truss unit  237 , and the rotation bar  241  is rotated as the running rail  150  is lifted up to a predetermined height so that the running rail  150  may be separated from the middle truss unit  237 . 
     The upper truss unit  242  has the same structure and shape as the truss units  231  and  237 , and the upper truss unit  242  may be provided with a passage (see  FIG. 9 ) through which the operator may pass. 
     When the rotation railing  351  is unfolded in a state that the basket  300 , which is to be described as follows, is installed on the upper truss unit  242 , a passage may be formed in a partial space of the upper truss unit  242  so that it is possible to pass through the overpass  400  installed between the prefabricated towers  200 . 
     In addition, the fixing unit  250  may be provided at the upper end of the prefabricated tower  200  so as to stably fix the prefabricated tower  200  by making close contact with the ceiling of the cargo hold  10 . The fixing unit  250  may press the prefabricated tower  200  against the ceiling of the cargo hold  10  to stably fix the prefabricated tower  200 . 
     The fixing unit  250  may include: a fixing plate  251  provided at a same size as the tower unit  230 ; a plurality of cylinders  252  mounted on an upper surface of the fixing plate  251 ; a winch  253  mounted on the upper surface of the fixing plate  251 ; and a rotation plate  254  rotatably mounted on an upper surface of the cylinder  252 . 
     The rotation plate  254  may include: a first plate  255  fixed to the upper surface of the cylinder  252 ; a second plate  256  rotatably coupled with a flat bearing  256   a  on an upper surface of the first plate  255 ; a third plate  257  mounted on an upper surface of the second plate  256  and formed therein with a groove (not shown) to which the flat bearing  256   a  is fitted; and a fourth plate  258  mounted on an upper surface of the third plate  257  and formed therein with a tongue insertion portion  258   a  to which a tongue of the cargo hold is fitted. 
     The upper truss unit  242  is provided at the upper surface thereof with the fixing unit  250 , and the upper truss unit  242  is provided at the upper surface thereof with the fixing plate  251  having a predetermined thickness so as to stably fix the fixing unit  250 . 
     A plurality of cylinders  252  may be provided on the upper surface of the fixing plate  251  so as to be pressed against the ceiling of the cargo hold  10 , and the fixing plate  251  may be provided at the upper surface thereof with a winch  253  for moving up and down the basket  300 . 
     Two winches  253  may be provided so that the basket  300  may be stably moved up and down, and a pulley for guiding the movement of a cable (not shown) of the winch  253  may be provided. 
     In addition, the rotation plate  254  in which a plurality of plates stacked may be provided on the upper surface of the cylinder  252 , and when the prefabricated tower  200  changes the direction thereof from the installation rail  130  to the running rail  150  through the rotation rail  170 , the rotation plate  254  is pressed against the ceiling of the cargo hold  10 , which allows the prefabricated tower  200  to rotate in a stable posture. 
     In other words, since the prefabricated tower  200  is vertically installed from the floor surface to the ceiling of the cargo hold  10 , the prefabricated tower  200  may be moved in a very unstable state when the mobile platform  210  is moved. 
     Therefore, in order to allow the prefabricated tower  200  to rotate in a more stable posture, the rotation plate  254  of the fixing unit  250  installed on the upper surface of the upper truss unit  242  may be pressed against the ceiling of the cargo hold  10 . 
     Accordingly, the prefabricated tower  200  is rotated while maintaining a more stable posture in a state that the rotation plate  254  is pressed against the ceiling. 
     In addition, when the inside of the cargo hold  10  is inspected, the fixing unit  250  installed on the upper surface of the upper truss unit  242  maintains the rotation plate  254  being pressed against the ceiling of the cargo hold  10 , so that the safety of the operator and the stable behavior of the basket  300  can be ensured. 
     The rotation plate  254  of the fixing unit  250  may be provided with the first plate  255  on the upper surface of the cylinder  252 , the second plate  256  provided with the flat bearing  256   a  may be mounted on the upper surface of the first plate  255 , and the third plate  257  formed in the bottom surface thereof with a groove so as to be coupled to the flat bearing  256   a  may be rotatably coupled to the upper surface of the second plate  256 . 
     In addition, the fourth plate  258  having a predetermined thickness may be provided on the upper surface of the third plate  257 , and the tongue insertion portion  258   a  in which the tongue protruding from the ceiling is fitted may be formed in the upper surface of the fourth plate  258 . 
     The tongue insertion portion  258   a  may be substantially shaped as a cross, so that the rotation plate  254  may be coupled to the tongue regardless of the direction in which the prefabricated tower  200  is installed. 
       FIG. 11  is a perspective view showing a basket of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention, and  FIG. 12  is a bottom perspective view showing the basket of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 11 and 12 , the basket  300  of the gantry tower crane for inspecting the cargo hold according to the embodiment of the present invention is horizontally moved and rotated toward the wall surface of the cargo hold  10 , so that the operator can inspect the entire area of the cargo hold  10 . 
     To this end, the basket may include: an up-down movement frame  310  mounted on an outer surface of the prefabricated tower  200  so as to be movable up and down; a boom support  330  rotatably mounted on one surface of the up-down movement frame  310  and having a variable length; and a work table  350  rotatably mounted at an end of the boom support  330 . 
     The up-down movement frame  310  may include: a first frame  311  provided with a wheel  313  so as to be movable along one surface of the prefabricated tower  200 ; a second frame  312  coupled to one side of the first frame  311  and provided with a wheel  313  so as to be movable along the other surface of the prefabricated tower  200 ; and a support frame  314  protruding such that the boom support  330  is rotatably mounted on one side of the second frame  312 . 
     The up-down movement frame  310  is moved up and down by the winch  253  installed on the upper portion of the prefabricated tower  200 , and the up-down movement frame  310  may include a first frame  311  coupled to one side of the prefabricated tower  200 , and a second frame  312  coupled to the other side of the first frame  311 . 
     A plurality of wheels  313  that rotate along the vertical frames  232  and  238  of the prefabricated tower  200  may be installed in an inner surface of the up-down movement frame  310 , and the first frame  311  and the second frame  312 , which configure the up-down movement frame  310 , may be fixed to each other with a bolt or a pin in a state of making close contact with the outer surface of the prefabricated tower  200 . 
     In addition, the support frame  314  may be fixed to the up-down movement frame  310  so as to install the boom support  330 , and the support frame  314  may include a first support frame  314   a  on which the boom support  330  is installed and a second support frame  314   b  on which an auxiliary frame is installed. 
     The boom support may include: a rotation boom support  331  rotatably coupled to one side of the up-down movement frame  310 ; a plurality of movable boom supports  332  movable in the rotation boom support  331  in a telescopic manner; and a support bar  333  rotatably mounted on one side of the up-down movement frame  310  and the rotation boom support  331  so as to stably support the rotation boom support  332 . 
     In addition, the work table  350  may include: a rotation railing  351  rotatable about one side of the work table  350  so as to communicate with the prefabricated tower  200 ; and an auxiliary railing  352  rotated together with the rotation railing  351 , provided on both sides of the rotation railing  351 , and having a predetermined height. 
     The boom support  330  has a variable length so that the operator may move toward the wall surface of the cargo hold  10 , and may be coupled to horizontally rotate about the support frame  314  of the up-down movement frame  310 . 
     One side of the rotation boom support  331  may be rotatably coupled to the up-down movement frame  310 , and the other side of the rotation boom support  331  may be coupled to a plurality of movable boom supports  332  such that the length is varied in a telescopic manner. 
     In other words, the rotation boom support  331  horizontally rotates the work table  350  in a desired direction about the up-down movement frame  310 , and the movable boom support  332  moves the work table  350  to a desired position (distance) so that the operator may be moved to the desired position. 
     In addition, the support bar  333  may be installed on a bottom surface of the rotation boom support  331  and the support frame  314  of the up-down movement frame  310  so as to support the rotation boom support  331  more stably. 
     In addition, the work table  350  on which the operator can ride may be coupled to the end of the boom support  330 , the work table  350  may be provided with a railing along the periphery of the support plate, and the rotation railing  351  communicating with the passage of the upper truss unit  341  may be installed on one surface of the railing of the work table  350 . 
     In other words, the rotation railing  351  allows one side of the railing to be opened and closed as necessary, and the rotation railing  351  may be provided with auxiliary railings  352  serving as railings on both left and right sides of the rotation railing  351  when the rotation railing  351  is unfolded. 
       FIG. 13  is a perspective view showing an overpass of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention,  FIG. 14  is a bottom perspective view showing the overpass of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention, and  FIG. 15  is a schematic sectional view showing a state that the overpass of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is folded. 
     As shown in  FIGS. 1, 2 and 13 to 15 , the overpass  400  may be installed between a pair of prefabricated towers  200  and may be used as a passage through which the operator can pass or a work space. In other words, the overpass  400  may be selectively used as a work passage or the work space of the operator as necessary, and the overpass  400  may be used as the work space as the auxiliary foothold  450  is deployed. 
     The overpass  400  may include: a foothold  410  serving as a path or a work space for an operator; a foothold support  430  having a truss structure to stably support the foothold  410 ; and an auxiliary foothold  450  rotatable toward an outside such that the work space extends on one side of the foothold  410 . 
     The foothold  410  may include: a foothold body  412  rotatable about a hinge  411 ; and a rotation link  413  provided at predetermined intervals on both sides of the foothold body  412 . 
     Each of the first and second frames  431  and  432  of the foothold support  430  may include: a first ring  431   a  and  432   a  provided at upper ends of the first and second frames  431  and  432  such that the railing frame  451  is rotatably coupled; a second ring  431   b  and  432   b  provided at a lower portion of the first ring  431   a  and  432   a  such that the rotation link  413  is rotatably coupled; and a third ring  431   c  and  432   c  to which the rotation rod  433  and the connection rod  434  are rotatably coupled. 
     The auxiliary foothold  450  may include: a railing frame  451  rotatably coupled to upper sides of the first and second frames  431  and  432 ; and two railings  452  detachably coupled to the railing frame  451 . 
     The railing  452  may include: a first railing frame  452   a  having a predetermined length; and a second railing frame  452   b  movably coupled to an outer surface of the first railing frame  452   a.    
     One of the two railings may further include a work foothold  453  provided to form the work space of the operator. 
     A chain  454  connected between the railings  452  such that the two railings are stably fixed to each other may be further included. 
     As shown in  FIGS. 1 to 15 , the overpass  400  may be installed between the two prefabricated towers  200 . This overpass  400  allows passing from a first prefabricated tower to a second prefabricated tower, and securing the work space in which the operator may work by unfolding one of the two railings  452  to the outside of the foothold  410  as necessary. 
     The foothold  410  of the overpass  400  may have a predetermined width and a predetermined length, and the foothold  410  may have a structure that may be fitted to each other. 
     In addition, the foothold  410  may have a folding structure that the foothold  410  may be folded so as to be inserted through the gas dome of the cargo hold  10 . In other words, the foothold  410  may have a foldable structure so as to be inserted into the cargo hold  10  through the gas dome (not shown). 
     The foothold  410  may be rotatable about the hinge  411  such that the foothold may be folded or unfolded, and the foothold body  412  having a predetermined length and a predetermined width may be coupled to both sides of the hinge  411 . 
     In addition, the rotation link  413  rotatably coupled to the foothold support  430  while supporting the foothold body  412  may be fixed to both side surfaces of the foothold body  412 . The rotation link  413  may be rotatably coupled to the second rings  431   b  and  432   b  of the foothold support  430  so as to fold the foothold body  412  as necessary while supporting the foothold body  412 . 
     The foothold support  430  may include: a first frame  431  and a second frame  432  provided at left and right sides of the foothold support  430  at predetermined angles; two rotation rods  433  rotatably coupled to one ends of the first and second frames  431  and  432  in a longitudinal direction; and a connection rod  434  fixed between the two rotation rods  433 . 
     The foothold support  430  may have a truss structure that is coupled to a bottom surface of the foothold  410  to firmly support the foothold  410 , and may have a rotation structure that the foothold  410  may be folded as necessary. 
     The foothold support  430  may include the first frame  431  and the second frame  432 , each of which having a truss structure. The first frame  431  and the second frame  432  may have the same shape. 
     As shown in  FIG. 15 , the first frame  431  and the second frame  432  have a predetermined height, and the first rings  431   a  and  432   a  may be provided at uppermost ends of the first and second frames  431  and  432 , respectively. 
     In addition, the second rings  431   b  and  432   b  may be provided at lower portions of the first rings  431   a  and  432   a  so as to be rotatably coupled to the rotation link  413  of the foothold  410 . In other words, the rotation link  413  of the foothold  410  may be rotated about the second rings  431   b  and  432   b.    
     In addition, the first and second frames  431  and  432  may be coupled so as to be rotatable in a direction symmetrical to each other. In other words, as shown in  FIG. 15 b   , the third rings  431   c  and  432   c  may be provided at lower ends of the first and second frames  431  and  432 , respectively. 
     In addition, rotation rods  433  may be rotatably coupled to the third rings  431   c  and  432   c  in the longitudinal direction of the foothold  410 , and the connection rod  434  passing through the third rings  431   c  and  432   c  may be provided between two rotation rods  433 . 
     Each of the first frame  431  and the second frame  432  may be rotatably coupled about the rotation rod  433 , and the foothold body  412  may be folded or unfolded by the rotation of the first frame  431  and the second frame  432 . 
     The first rings  431   a  and  432   a  of the first frame  431  and the second frame  432  may be rotatably coupled with a railing frame  451  formed as a rod having a predetermined length. In other words, the railing frame  451  may be rotatably fitted to the inside of the first rings  431   a  and  432   a.    
     The railing frame  451  may be coupled to the first frame  431  and the second frame  432 , and the railing frame  451  may be fixed with the railing  452  having a predetermined height. 
     The railing  452  may be separated or coupled to the railing frame  451  by fitting. The railing  452  may have a structure which may vary the length of the railing  452  to secure a work space as necessary. 
     The railing  452  may include a first railing frame  452   a  having a predetermined length and a second railing frame  452   b  movably coupled to an outer surface of the first railing frame  452   a.    
     The first railing  452   a  may be formed as a circular pipe having a predetermined length, and the second railing  452   b  may be movably coupled along the outer surface of the first railing  452   a  to vary the length of the railing  452  as necessary. 
     In addition, the railing  452  may further include a work foothold  453  to use the work foothold as a foothold for working. The foothold  453  may have a structure that may be unfolded or folded according to a variable length, such as the railing  452 , or a structure in which the length is varied in a telescopic manner. 
     In addition, two railings  452  may be provided with the chain  454  to stably maintain a state that the railing  452  is unfolded so as to be used as a work foothold. The chain  454  may include a rope or a cable as well as an iron chain. 
     Hereinafter, the coupling relation of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention will be described in detail. 
       FIG. 16  is a perspective view showing a state that a unit descending device of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention is installed in a gas dome, and  FIG. 17  is a partially cut-away perspective view showing an installation state of the gantry tower crane for inspecting the cargo hold according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 16 and 17 , the gantry tower crane for inspecting the cargo hold according to an embodiment of the present invention may be provided with a unit descending device  500  which may insert the rail  100 , the prefabricated tower  200 , the basket  300 , and the overpass  400 , which are separated from each other, through the gas dome (not shown) formed in the cargo hold  10  and into the cargo hold  10 . 
     The unit descending device  500  may include a fixing frame  530  having a cylindrical body  510  coupled to an inner surface of the gas dome (not shown) and cylinders  520  that alternately move up and down on the outer surface of the cylindrical body  510 . 
     The cylinders  520  may be installed in directions symmetrical to each other with respect to the cylindrical body  510 , and the cylinders  520  may include a first cylinder  521  and a second cylinder  522  so as to be moved up and down alternately. 
     The unit descending device  500  is installed in the gas dome and inserts the rail  100 , the prefabricated tower  200 , the basket  300 , and the overpass  400  into the cargo hold  10 , and unit descending device  500  sequentially assembles the prefabricated tower  200 . 
     In other words, the unit descending device  500  descends the lower truss unit  231  and a plurality of middle truss units  237  while sequentially assembling (coupling) the lower truss unit  231  and a plurality of middle truss units  237 . 
     The prefabricated tower  200  mounts the lower truss unit  231  on the first cylinder  521  of the unit descending device  500 , and a first middle truss unit  237  is coupled to the upper surface of the lower truss unit  231 . 
     Accordingly, the first middle truss unit  237  is lowered by the first cylinder  521 , and the lowered first middle truss unit  237  is lowered after the first middle truss unit  236  is coupled in a state of being mounted on the second cylinder  522 . 
     In other words, the middle truss unit  237  is lowered by the reciprocation of the first cylinder  521  and the second cylinder  522 . At this time, the descending height of the first cylinder  521  and the second cylinder  522  is shorter than the height of the middle truss unit  237 , so that the first cylinder  521  and the second cylinder  522  lowers the first middle truss unit  237  to the upper surface of the gas dome by the reciprocation. 
     The second middle truss unit  237  is coupled to the upper surface of the lowered first middle truss unit  237 , and a second middle truss unit  237  is descended to the upper surface of the gas dome by the reciprocation of the first cylinder  521  and the second cylinder  522 , and then coupled with a third middle truss unit  237 . 
     The middle truss units  237  are sequentially coupled to each other, the lower truss unit  231 , the middle truss unit  237 , and the upper truss unit  242 , which configure the tower unit  230 , are coupled to each other, and the fixing unit  250  is sequentially coupled and descended. 
     The tower units  230  including the lower truss unit  231 , the middle truss unit  237 , and the upper truss unit  242  may be firmly fixed by the coupling of the fixing pin (not shown) and the like. 
     In addition, the running rail  150  may be lowered in a state that the running rail  150  is coupled on both sides of the tower unit  230 . 
       FIG. 18  is a process diagram for explaining a method for installing a gantry tower crane for use in cargo hold inspection step by step according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 1 to 18 , a method for installing a gantry tower crane for use in cargo hold inspection according to an embodiment of the present invention may include: (a) a step S 100  of installing an installation rail on a floor surface of a cargo hold in a widthwise direction; (b) a step S 200  of movably installing a pair of mobile platforms, which are inserted into an interior of the cargo hold, on the installation rail; (c) a step S 300  of assembling a pair of prefabricated towers by coupling tower units to each of the mobile platforms; (d) a step S 400  of installing a running rail on both ends of the installation rail in a longitudinal direction of the cargo hold; and (e) a step S 500  of installing a basket on each of the prefabricated towers such that the basket is movable up and down along the prefabricated tower. 
     Hereinafter, the method for installing the gantry tower crane for use in the cargo hold inspection according to the present invention will be described with reference to  FIGS. 18 to 26 , and the method will be described with reference to  FIGS. 1 to 17 . 
       FIG. 18  is a process diagram for explaining a method for installing a gantry tower crane for use in cargo hold inspection step by step according to the preferred embodiment of the present invention. 
     As shown in  FIG. 18 , in the method for installing the gantry tower crane according to an embodiment of the present invention, the installation rail  130  may be installed on the floor surface of the cargo hold  10  in the widthwise direction in step S 100 . 
     The installation rail  130  may be inserted (or lowered) through the gas dome formed in the ceiling of the cargo hold  10 . 
     The gantry tower crane according to the present invention may be installed such that the gantry tower crane is inserted through the gas dome formed in the ceiling of the cargo hold  10 . The gantry tower crane, which is disassembled after the completion of the inspection of the cargo hold  10 , is discharged to the outside of the cargo hold  10  through the gas dome. Therefore, it should be understood that all of the gantry tower crane described below are inserted or discharged through the gas dome. 
     In step S 200 , the mobile platform  210  may be movably installed on the installation rail  130  installed on the floor surface of the cargo hold  10 . 
     The mobile platform  210  may be mounted with the tower unit  230  formed with a plurality of middle truss units  237  so as to form a height ranging from the floor surface to the ceiling of the cargo hold  10 , and the fixing unit  250  may be provided on the upper surface of the tower unit  230 . In step S 300 , the mobile platform  210  may move the prefabricated tower  200  including the tower unit  230  and the fixing unit  250  as necessary. 
     The mobile platform  210  may move the prefabricated tower  200  from the installation rail  130  to the running rail  150 , as well as along the running rail  150 . 
     As the prefabricated tower  200  is installed, the mobile platform  210  moves to both ends of the installation rail  130 . In step S 400 , the moved mobile platform  210  may be provided with the running rail  150  that is lowered in a state of being installed in the middle truss unit  237 . 
     As shown in  FIG. 10 , the running rail  150  may be lowered in a state of being hooked by the rotation bar  241  of the middle truss unit  237 . The running rail  150  is lifted by a predetermined height at the upper portion of the gas dome by using a crane (not shown) and the like. 
     The running rail  150  may be installed along the floor surface of the cargo hold  10  in the lengthwise direction of the cargo hold  10  as the running rail  150  is separated and descended from the rotation bar  241  of the middle truss unit  237 . 
     The prefabricated tower  200  moved to the both ends of the installation rail  130  moves to a position of the gas dome again, so that the basket  300  may be mounted on the prefabricated tower  200 . 
     In step S 500 , the basket  300  may be installed on the first prefabricated tower disposed on the installation rail  130 , and on the second prefabricated tower. 
     The basket  300  has a work space having a predetermined size, so that the operator may inspect the cargo hold  10 . 
       FIG. 19  is a process diagram including a protective cover installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
     As shown in  FIG. 19 , in step S 50 , the protective cover  110  for preventing the damage or breakage of the cargo hold  10  may be installed before the installation rail  130  is installed in the cargo hold  10 . 
     In addition, in step S 70 , the unit descending device  500  capable of lowering the tower unit  230  and the fixing unit  250  while assembling the tower unit  230  and the fixing unit  250  may be installed in the gas dome (not shown) of the cargo hold  10 . 
     The step of installing the protective cover  110  and the step of installing the unit descending device  500  may be performed before the installation rail  130  is installed. 
     As shown in  FIGS. 19 and 20 , the protective cover installation step S 50  may include: a step S 51  of installing a first protective cover  111  in the widthwise direction of the cargo hold  10 ; and a step S 55  of installing a second protective cover  112  in the longitudinal direction of the cargo hold  10  at both ends of the first protective cover  111 . 
     The first protective cover  111  may have a plate shape having a predetermined size, a protruding portion  113  may be provided on one side of the first protective cover  111 , and a coupling portion  114  corresponding to the protruding portion  113  may be formed on the other side of the first protective cover  111 . 
     The protruding portion  113  and the coupling portion  114  of the first protective cover  111  are coupled to each other in the form of surrounding the outer surface of the tongue that protrudes from the floor surface of the cargo hold  10  so as to protect the tongue together with the floor surface. 
     In addition, the second protective cover  112  may be provided in the form of a scroll having a predetermined length, and the first protective cover  111  and the second protective cover  112  may be inserted (or lowered) into the cargo hold  10  by using the crane (not shown) provided on the upper side of the cargo hold  10 . 
     In addition, the protective cover  110  is installed on the floor of the installation rail  130  and the running rail  150  to prevent the damage or breakage of the floor surface of the cargo hold  10 , and may be formed of a material having elasticity such as felt, fabric, rubber, or a synthetic resin. 
     In addition, the protective cover  110  prevents the cargo hold  10  from being damaged or broken by preventing the prefabricated tower  200 , the basket  300 , the overpass  400 , and the like from making direct contact with the cargo hold  10  when inserting (lowering) the prefabricated tower  200 , the basket  300 , the overpass  400 , and the like, and prevents the damage or breakage of the cargo hold  10  due to falling of a tool and the like during the work of the operator. 
     As shown in  FIG. 19 , in the unit descending device installation step, the unit descending device  500  is moved to the upper surface of the gas dome of the cargo hold  10 . In order to protect the inner surface of the gas dome through the descent of the unit descending device  500 , the cylindrical body  510  of the unit descending device  500  is coupled to the inner surface of the gas dome, while the unit descending device  500  may be seated on the upper surface of the cargo hold  10 . 
     The unit descending device  500  may insert (or lower) the rail  100 , the prefabricated tower  200 , the basket  300 , and the overpass  400 , which are separated from each other, into the cargo hold  10 . 
     The unit descending device  500  may be provided on both side surfaces thereof with a first cylinder  521  and a second cylinder  522 , which form one pair, and the unit descending device  500  may be provided at a center thereof with the cylindrical body  510 , which is fitted to the inner surface of the gas dome to prevent the damage or breakage of the gas dome. 
     The unit descending device  500  may be moved to the position of the gas dome by the crane and the like, and the cylindrical body  510  may be coupled to the inner surface of the gas dome as the unit descending device  500  is lowered. 
     In addition, the unit descending device  500  may be seated in a stable state on the upper surface of the gas dome simultaneously with the coupling of the cylindrical body  510 . In other words, as the unit descending device  500  is moved by the crane (not shown) and lowered from the upper side of the gas dome, the unit descending device  500  may be seated on the upper surface of the cargo hold  10  together with the coupling of the cylindrical body  510 . 
     The installation rail  130  may be installed using the unit descending device  500  or the crane (not shown). 
       FIG. 21  is a process diagram for explaining an installation rail installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 18, 19, and 21 , in the installation step S 100  of the installation rail  130 , the installation rail  130  may be installed on the floor surface of the cargo hold  10  so that the prefabricated tower  200  vertically installed in the cargo hold  10  may be moved in a straight-line direction. 
     The installation step of the installation rail  130  may include: a step S 110  of lowering the installation rail  130  inside the cargo hold  10 ; a step S 120  of arranging installation rails  130  such that an installation rail  130  which is lowered beforehand and installation rails  130  which are sequentially lowered are disposed in a straight line, and sequentially coupling the installation rails  130  with each other; and a step S 130  of separating the auxiliary installation rail  140  coupled to both side surfaces of the installation rail  130 . 
     The auxiliary installation rail  140  may be firmly fixed so as not to move in the tongue of the cargo hold  10 . 
     In step S 110 , the installation rail  130  may be lowered into the cargo hold  10  by the crane. 
     In step S 120 , a plurality of installation rails  130  may be sequentially inserted while being arranged in a row in the widthwise direction of the cargo hold  10 , and coupled to each other such that adjacent rail bodies  131  are arranged in a straight line. 
     In the installation rails  130  arranged in a row, the first coupling portion  135  of the rail body  131  and the second coupling portion  136  of other rail body  131  disposed adjacent to the first coupling portion  135  of the rail body  131  are coupled to each other. At this time, the first coupling portion  135  and the second coupling portion  136  may be fixed by a fixing device such as the fixing pin (not shown). 
     In addition, in step S 130 , the auxiliary installation rail  140  may be separated and installed on both sides of the installation rail  130  in a state in which the installation rails  130  are completely installed. 
     The moving wheel  134  is installed on the bottom surface of the rail body  131  so as to be movable, and the installation rail  130  may be switched into a stopped state so that the rail body  131  is not moved by the self-weight of the rail body  130  as the rail bodies  131  are coupled. 
     In other words, as the rail bodies  131  are coupled, the total weight of the rail body  131  increases, so that the moving wheel  134  is restrain from moving due to the weight of the rail body  131 . 
     In addition the auxiliary installation rail  140  separated from the rail body  131  is provided for more secure movement of the prefabricated tower  200 , and the auxiliary installation rail  140  may be fixed to the tongue (not shown) of the cargo hold  10  by a plurality of fixing pins (not shown) and the like in a state that the outrigger  236  coupled to the lower truss unit  231  is moved to a position where it can be reached. 
       FIG. 22  is a process diagram for explaining a mobile platform installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 1, 2, 17, 18, 19, and 22 , in step S 200 , the first prefabricated tower and the second prefabricated tower, which are two prefabricated towers  200 , may be movably mounted on the installation rail  130  of the cargo hold  10 , and the mobile platform  210  capable of moving the prefabricated tower  200  may be inserted and installed. 
     The mobile platform  210  inserted into the cargo hold  10  may be movably mounted on the installation rail  130 . 
     The mobile platform installation step may include: a step S 210  of seating the mobile platform  210 , which is inserted by the crane (not shown) on the installation rail  130 ; a step S 220  of engaging a pinion gear  214 , which is mounted on the mobile platform  210  during the seating of the mobile platform  210 , with a rack  132  mounted on the installation rail  130 ; and a step S 230  of adjusting guide rollers  215  mounted on both side surfaces of the mobile platform  210  such that the guide rollers  215  make close contact with the running rail  130 . 
     In order to allow the prefabricated tower  200  to move along the installation rail  130 , the mobile platform  210  is inserted into the cargo hold  10  and the tower unit  230  is assembled and descended, so that the mobile platform  310  and the tower unit  230  are coupled to each other. 
     The mobile platform  210  may be inserted and lowered inside the cargo hold  10  by using the crane. At this time, two mobile platforms  210  may be inserted. 
     In step S 210 , the inserted mobile platform  210  is seated on the installation rail  130 . 
     In step S 220 , the mobile platform  210  seated on the installation rail  130  may be coupled to the installation rail  130 , such that the pinion gear  214  mounted on the platform body  211  and the rack  132  provided on the installation rail  130  are engaged with each other. 
     In addition, in step S 230 , a position of the guide roller  215  may be adjusted by the installation rail  130  and the mobile platform  210  to move the mobile platform  210  in a more stable posture when the mobile platform  210  moves. 
     As shown in  FIG. 7 , the adjustment bolt  218  installed on the side surface of the mobile platform  210  is rotated, and the guide member  216  makes close contact with the side surface of the installation rail  130  with the movement of the movable body  217  by the rotation of the adjustment bolt  218 . 
     The adjustment of the guide roller  215  may be controlled by the adjustment bolt  218  protruding from the side surface of the mobile platform  210 . In other words, the operator rotates the adjustment bolt  218  protruding from the side surface of the mobile platform  210 . 
     Accordingly, the movable body  217  fastened to the adjustment bolt  218  moves to the side surface of the mobile platform  210 . As the adjustment bolt  218  is fastened to the movable body  217 , the guide member  216  coupled to the inside the movable body  217  is moved. 
     The moved guide member  216  protrudes outward of the mobile platform  210  or moves inward of the mobile platform  210  according to the rotation direction of the adjustment bolt  218 . As the guide member  216  protrudes to the outside of the mobile platform  210 , the guide member  216  makes close contact with the guide rail  133  of the installation rail  130 . 
     As described above, as the guide member  216  makes close contact with the guide rail  133  of the installation rail  130 , the mobile platform  210  may move in a more stable state. 
       FIG. 23  is a process diagram for explaining a prefabricated tower installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 18, 19, and 23 , the mobile platform  210  may be provided with the prefabricated tower  200  by assembling the tower unit  230  with the fixing unit  250 . 
     In step S 300 , the prefabricated tower  200  may be assembled by coupling the tower unit  230  and the mobile platform  210  that are lowered while being assembled by the unit descending device  500 . 
     The prefabricated tower installation step may include: a step S 310  of seating a lower truss unit  231  on the unit descending device  500 ; a step S 320  of coupling a middle truss unit  237  to an upper surface of the lower truss unit  231  seated on the unit descending device  500 ; a step S 330  of repeatedly assembling and lowering another middle truss unit  237  on an upper surface of the middle truss unit  237 ; a step S 335  of coupling an upper truss unit  242  and the fixing unit  250  to an upper end of the middle truss unit  237 ; a step S 340  of lowering the tower unit  230  that has been assembled by the unit descending device  500 ; and a step S 350  of fixing the lower truss unit  231  of the tower unit  230  to a post  212  of the mobile platform  210 . 
     When moving the prefabricated tower  200 , the outrigger  236  provided on the tower unit  230  may be movably coupled to the auxiliary installation rail  140  spaced apart from the installation rail  130  by a predetermined distance. 
     In step S 310 , the lower truss unit  231  is seated on the unit descending device  500 , and the lower truss unit  231  maintains a seated state by the first cylinder  521  or the second cylinder  522  of the unit descending device  500 . 
     In step S 320 , the middle truss unit  237  is coupled to the upper portion of the lower truss unit  231 . In other words, the middle truss unit  237  may be fitted to the coupling hole  235  in the upper portion of the lower truss unit  231 , and then fixed by fitting the fixing pin (not shown). 
     In step S 330 , a plurality of middle truss units  237  are provided, the middle truss units  237  are assembled and fixed to each other, and when the middle truss unit  237  is coupled, the unit descending device  500  descends the tower unit  230  while the first cylinder  521  and the second cylinder  522  are alternately moved up and down. 
     In step S 335 , the upper truss unit  242  and the fixing unit  250  are firmly coupled to the upper end of the middle truss unit  237 . The tower unit  230  including the lower truss unit  231 , the middle truss unit  537 , the upper truss unit  242 , and the fixing unit  250  is sequentially lowered while being assembled by the unit descending device  500 . 
     In step S 350 , the tower unit  230  lowered by the unit descending device  500  is fixed to the mobile platform  210  mounted on the installation rail  130 . 
     The fixing hole  234  formed in the lower truss unit  231  of the tower unit  230  is aligned with the hole  213  of the mobile platform  210 , and then fixed with the fixing pin and the like. 
     In addition, the prefabricated tower  200  in which the mobile platform  210 , the tower unit  230  and the fixing unit  250  are integrated unfolds the outrigger  236  so as to be stably moved when moving along the installation rail  130 . 
     The outrigger  236  is coupled to the auxiliary installation rail  140  spaced apart from the installation rail  130  by a predetermined distance to move the prefabricated  200  more safely. 
     In step S 400 , the prefabricated tower  200  in which the mobile platform  210 , the tower unit  230  and the fixing unit  250  are integrated may be provided at both sides thereof with the running rails  150  as the running rails  150  coupled to the tower unit  230  are lowered. 
     In other words, the running rail  150  is lowered simultaneously with the tower unit  230  by the crane and the like in a state of being hooked by the rotation bar  241  of the tower unit  230 . As the running rail  150  is lifted by the crane and the like, the rotation bar  241  is rotated inwards of the middle truss unit  237  by the elastic force of the spring (not shown), so that the running rail  150  and the tower unit  230  are separated from each other. 
     Since the running rail  150  is provided with the moving wheels  154 , the running rail  150  is installed along the floor surface of the cargo hold  10  at both ends of the installation rail  130  in the longitudinal direction of the cargo hold  10 . 
     At this time, as for the two prefabricated towers  200 , the running rail  150  is installed after the tower unit  230  and the fixing unit  250  are installed on the mobile platform  210  in the first prefabricated tower. 
     In addition, during the installation of the running rail  150  of the first prefabricated tower, the unit descending device  500  assembles and lowers the tower unit  230 , the upper truss unit  242 , and the fixing unit  250  so as to install the second prefabricated tower. 
     In other words, steps S 310  to S 350  are repeated, and accordingly, the work time required for assembling and installing the second prefabricated tower  200  can be remarkably reduced. 
       FIG. 24  is a process diagram for explaining a basket installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
     As shown in  FIG. 24 , in step S 500 , the basket  300  having a space allowing the operator to ride and work may be installed between the prefabricated towers  200 . 
     The basket installation step may include: a step S 510  of moving the prefabricated tower  200  to a position of the gas dome of the cargo hold  10  so as to install the basket  300  on the prefabricated tower  200 ; a step S 520  of lowering a first frame  311  of an up-down movement frame  310  such that the first frame  311  is movable on an outer surface of the prefabricated tower  200 ; a step S 530  of lowering a second frame  312  of the up-down movement frame  310  such that the second frame  312  is movable on the outer surface of the prefabricated tower  200 ; a step S 540  of installing the first frame  311  and the second frame  312  to allow the first frame  311  and the second frame  312  to move up and down on an outside of the prefabricated tower  200 ; and a step S 550  of moving the prefabricated tower  200  installed with the basket  300  to the running rail  150 . 
     In step S 510 , the prefabricated tower  200  is moved to the position of the gas dome by the mobile platform  210 . 
     This is performed to install the basket  300  on which the operator may ride in the first and second prefabricated towers, which are two prefabricated towers  200 . 
     In addition, after the basket  300  is installed on the first prefabricated tower, the first prefabricated tower is moved to the running rail  150 , and the second prefabricated tower is moved to the position of the gas dome to install the basket  300 . 
     The basket  300  is lowered into the cargo hold  10  through the gas dome by integrating the boom support  330  and the work table  350  in the second frame  312 , and the first frame  3111  corresponding to the second frame  312  is lowered to integrally assemble the frames, thereby installing the basket  300 . 
     In step S 20 , the first frame  311  is lowered through the gas dome while the prefabricated tower  200  is moved to the position of the gas dome. 
     Meanwhile, the lowering of the first frame  311  and the second frame  312  may be achieved by firstly lowering the first frame  311  by the crane and temporarily hooking the first frame  311  on the winch  253  installed on the prefabricated tower  200 . 
     In addition, in step S 530 , the boom support  330  and the work table  350  are integrally assembled to the second frame  312  and lowered by the crane through the gas dome. 
     In step S 540 , the lowered first and second frames  311  and  312  are integrally assembled with fixing devices such as a pin or a bolt in a state of making contact with each other, thereby installing the basket  300  on the prefabricated tower  200 . 
     Since the basket  300  is provided with a plurality of wheels  313  on the first frame  311  and the second frame  312 , the basket  300  may be freely moved up and down by the winch  253 . 
     The first prefabricated tower in which the basket  300  is assembled is moved along the running rail  150 , and at this time, the prefabricated tower  200  may move in a more stable state by coupling the outrigger  236  to the auxiliary installation rail  140 . 
       FIG. 25  is a process diagram for explaining an overpass installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention, and  FIG. 26  is a process diagram for explaining the overpass installation step in the method for installing the gantry tower crane for use in the cargo hold inspection according to the preferred embodiment of the present invention. 
     As shown in  FIGS. 25 and 26 , in step S 600 , the overpass  400  may be installed between the prefabricated towers  200 . Since steps from the step of installing the installation rail  130  to the step of installing the basket  300  on the prefabricated tower  200  are the same, the duplicated description will be omitted. 
     The overpass installation step may include: a step S 610  of assembling and descending the overpass  400  separated into a plurality of pieces; a step S 620  of connecting (installing) one side of the overpass  400 , which is inserted into the cargo hold  10 , to one of the prefabricated towers  200 ; and a step S 630  of connecting (installing) the other side of the overpass  400 , which is inserted into the cargo hold  10 , to the remaining one of the prefabricated towers  200 . 
     In step S 610 , the overpass  400  mounted on the unit descending device  500  is brought into alignment with the overpass  400  moved by the crane, and the overpasses  400  are lowered while being assembled by the fixing devices such as the fixing pin. 
     In this manner, in step S 620 , one side of the overpass  400  inserted into the cargo hold  10  may be connected (installed) to the first prefabricated tower by the winch  253 . In addition, the other side of the overpass  400  may be connected (installed) to the second prefabricated tower. 
     The installed gantry tower crane may move along the running rail  150  in the longitudinal direction of the cargo hold  10 , and the basket  300  may be vertically moved up and down by the winch  253  installed in the prefabricated tower  200 . 
     In addition, the boom support  330  of the basket  300  may have a length varied along the wall surface of the cargo hold  10 , and the work table  350  may be freely rotated by the rotation boom support  331 . 
     In addition, the upper truss unit  242  is opened to communicate with the overpass  400 , so that the operator can freely move as necessary. 
     Although the present invention invented by the present inventor has been described in detail with reference to the embodiments, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the scope and spirit of the present invention.