Abstract:
A protection plate is welded to a metal structure in water so that the protection plate covers the surface of the structure including a portion to be repaired. The welding is carried out such that the periphery of the protection plate is welded in water by laser welding while inactive gas is supplied to portion to be welded to seal the portion to be repaired by the protection plate. Welding is carried out while the inactive gas is supplied to the portion to be welded coaxially with a laser beam during the laser welding. The plate is provided therein with an opening in advance so as to release vapor produced from water remaining during welding between the protection plate and the structure and the opening is sealed after the periphery of the protection plate is welded to the structure.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This is a continuation-in-part of Application PCT/JP2007/053741, filed on Feb. 28, 2007, now abandoned. This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-053748, filed in the Japanese Patent Office on Feb. 28, 2006, the entire content of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to an underwater repair welding method for carrying out an underwater weld repairing operation by means of a laser beam without discharging water. 
         [0003]    A method of laser-welding an underwater structure by applying a highly watertight cover (chamber) to the object of welding in order to produce a gaseous environment at the site of the object of welding and forcing out water in the chamber by means of inactive gas is known (See Japanese Patent Application Laid-Open Publication No. 05-031591, the entire content of which is incorporated herein by reference). However, with the above-described known underwater laser welding method and an underwater laser welding apparatus adapted to employ the method, the entire arrangement becomes inevitably huge because a complex and large drying apparatus is required to by turn give rise to a problem of a poor accessibility to the inside of the complex structure including narrow areas and a poor operability. 
         [0004]    A technique of realizing underwater welding by ejecting inactive gas from a laser welding head nozzle and holding only the site of welding temporarily in a gaseous environment by the energy of the ejected inactive gas is known as a technique for coping with the above identified problem (See Japanese Patent Publication Nos. 3012175 and 3619286, the entire contents of which are incorporated herein by reference). However, this technique mainly relates to a nozzle profile and making a gas flow to stay at the underwater site of welding in order to hold it temporarily in gas without any chamber. 
         [0005]    A welding head of the above-described type can satisfactorily operate on the surface of an underwater structure for laser welding. However, when a protection plate is applied to the surface of the structure, defective welding can arise frequently to make it difficult to provide a good welding quality because water cannot be discharged sufficiently from the area between the protection plate and the welding site and the water staying in the welding site evaporates and bursts out. In short, the conventional art does not specifically define the laser beam converging method, the protection plate profile, the protection plate aligning procedures and the welding process for underwater welding using a protection plate and hence the method of welding/bonding the protection plate and the underwater surface to be repaired is accompanied by problems including that the water staying in the gap between them evaporates and bursts out. 
         [0006]    Additionally, while the protection plate and the surface to be repair-welded need to be completely put together for welding using a protection plate, a large protection plate holding apparatus needs to be introduced in order to remotely carry out the underwater welding operation. Therefore, there is a demand for simpler operation jigs that can improve such underwater welding operations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    In view of the above-identified circumstances, it is therefore an object of the present invention to suppress or prevent the problem of defective welding due to burst out of the welding metal that may arise in underwater weld-repairing when the residual water evaporates and bursts out while using a laser beam without discharging water. 
         [0008]    In order to achieve the object, according to an aspect of the present invention, there is presented an underwater repair welding method for welding a protection plate to a metal structure so as to cover a surface of the structure including a part to be repaired underwater, wherein the part to be repaired is hermetically closed by the protection plate by welding the periphery of the protection plate underwater, while supplying inactive gas to a welding part coaxially with the laser beam for laser welding. 
         [0009]    According to another aspect of the present invention, there is presented an underwater repair welding method wherein the part to be repaired of a metal structure having a defective part is covered by a shield cover and the defective part is welded and closed by irradiating a laser beam from below to above of the defective part. 
         [0010]    According to yet another aspect of the present invention, there is presented an underwater repair welding method for repairing a defective part of a lap plate itself arranged along a surface of a metal structure so as to cover part thereof or a defective part of a welding part of the lap plate underwater, wherein the method comprises: a through hole forming step of forming through holes through the lap plate; a defective part welding step of irradiating a laser beam coaxially with a nozzle, ejecting inactive gas from the nozzle to the defective part, to repair the defective part underwater by laser welding, after the through hole forming step; and a through hole closing step of tightly closing the through holes underwater by irradiating a laser beam coaxially with the nozzle, ejecting inactive gas from the nozzle to the through holes, after the defective part welding step. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above and other features and advantages of the present invention will become apparent from the discussion hereinbelow of specific, illustrative embodiments thereof presented in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic longitudinal cross-sectional view showing a state where a first embodiment of underwater repair welding method of the present invention is being employed; 
           [0013]      FIG. 2  is a schematic plan view showing a welding part where the first embodiment of underwater repair welding method of the present invention is being employed; 
           [0014]      FIG. 3A  is a schematic plan view of a protection plate where a second embodiment of underwater repair welding method of the present invention is being employed, showing a possible slit position thereof; 
           [0015]      FIG. 3B  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing another possible slit position thereof; 
           [0016]      FIG. 3C  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing still another possible slit position thereof; 
           [0017]      FIG. 3D  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing still another possible slit position thereof; 
           [0018]      FIG. 3E  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing still another possible slit position thereof; 
           [0019]      FIG. 3F  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing another possible slit position thereof; 
           [0020]      FIG. 3G  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing still another possible slit position thereof; 
           [0021]      FIG. 3H  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing still another possible slit position thereof; 
           [0022]      FIG. 3I  is a schematic plan view of a protection plate where the second embodiment of underwater repair welding method of the present invention is being employed, showing still other possible slit positions thereof; 
           [0023]      FIG. 4  is a schematic plan view showing a welding part where the second embodiment of underwater repair welding method of the present invention is being employed; 
           [0024]      FIG. 5  is a schematic longitudinal cross-sectional view showing a state where the second embodiment of underwater repair welding method of the present invention is being employed; 
           [0025]      FIG. 6  is a schematic longitudinal cross-sectional view showing a state where a modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0026]      FIG. 7  is a schematic longitudinal cross-sectional view showing welding beads where the second embodiment of underwater repair welding method of the present invention is being employed; 
           [0027]      FIG. 8  is a schematic longitudinal cross-sectional view showing a state where another modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0028]      FIG. 9  is a schematic longitudinal cross-sectional view showing a state where still another modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0029]      FIG. 10  is a schematic longitudinal cross-sectional view showing a state where still another modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0030]      FIG. 11  is a schematic longitudinal cross-sectional view showing a state where still another modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0031]      FIG. 12  is a schematic longitudinal cross-sectional view showing a state where still another modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0032]      FIG. 13  is a schematic longitudinal cross-sectional view showing a state where still another modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0033]      FIG. 14  is a schematic longitudinal cross-sectional view showing a state where still another modified second embodiment of underwater repair welding method of the present invention is being employed; 
           [0034]      FIG. 15A  is a schematic plan view of a protection plate where a third embodiment of underwater repair welding method of the present invention is being employed, showing possible through hole positions thereof; 
           [0035]      FIG. 15B  is a schematic plan view of a protection plate where the third embodiment of underwater repair welding method of the present invention is being employed, showing another possible through hole positions thereof; 
           [0036]      FIG. 15C  is a schematic plan view of a protection plate where the third embodiment of underwater repair welding method of the present invention is being employed, showing still another possible through hole positions thereof; 
           [0037]      FIG. 15D  is a schematic plan view of a protection plate where the third embodiment of underwater repair welding method of the present invention is being employed, showing still another possible through hole positions thereof; 
           [0038]      FIG. 15E  is a schematic plan view of a protection plate where the third embodiment of underwater repair welding method of the present invention is being employed, showing still another possible through hole positions thereof; 
           [0039]      FIG. 15F  is a schematic plan view of a protection plate where the third embodiment of underwater repair welding method of the present invention is being employed, showing still another possible through hole positions thereof; 
           [0040]      FIG. 16A  is a schematic plan view of a protection plate where a fourth embodiment of underwater repair welding method of the present invention is being employed, showing possible positions of a slit and through holes; 
           [0041]      FIG. 16B  is a schematic plan view of a protection plate where the fourth embodiment of underwater repair welding method of the present invention is being employed, showing another possible positions of a slit and through holes; 
           [0042]      FIG. 16C  is a schematic plan view of a protection plate where the fourth embodiment of underwater repair welding method of the present invention is being employed, showing still another possible positions of a slit and through holes; 
           [0043]      FIG. 16D  is a schematic plan view of a protection plate where the fourth embodiment of underwater repair welding method of the present invention is being employed, showing still another possible positions of a slit and through holes; 
           [0044]      FIG. 16E  is a schematic plan view of a protection plate where the fourth embodiment of underwater repair welding method of the present invention is being employed, showing still another possible positions of a slit and through holes; 
           [0045]      FIG. 16F  is a schematic plan view of a protection plate where the fourth embodiment of underwater repair welding method of the present invention is being employed, showing still another possible positions of a slit and through holes; 
           [0046]      FIG. 17  is a schematic longitudinal cross-sectional view showing a state where a fifth embodiment of underwater repair welding method of the present invention is being employed; 
           [0047]      FIG. 18  is a schematic plan view showing a welding part where the fifth embodiment of underwater repair welding method of the present invention is being employed; 
           [0048]      FIG. 19  is a schematic plan view of a protection plate where the fifth embodiment of underwater repair welding method of the present invention is being employed, showing a possible welding direction; 
           [0049]      FIG. 20  is a schematic plan view of a protection plate where the fifth embodiment of underwater repair welding method of the present invention is being employed, showing another possible welding direction; 
           [0050]      FIG. 21  is a schematic longitudinal cross-sectional view showing a state where a modified fifth embodiment of underwater repair welding method of the present invention is being employed; 
           [0051]      FIG. 22  is a schematic plan view showing a welding part where the modified fifth embodiment of underwater repair welding method of the present invention is being employed; 
           [0052]      FIG. 23  is a schematic longitudinal cross-sectional view showing a state where another modified fifth embodiment of underwater repair welding method of the present invention is being employed; 
           [0053]      FIG. 24  is a schematic plan view showing a state where a sixth embodiment of underwater repair welding method of the present invention is being employed; 
           [0054]      FIG. 25  is a schematic plan view showing a state where a seventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0055]      FIG. 26  is a schematic longitudinal cross-sectional view showing a state where the seventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0056]      FIG. 27  is a schematic plan view showing a state where the seventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0057]      FIG. 28  is a schematic plan view showing a state where a modified seventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0058]      FIG. 29  is a schematic longitudinal cross-sectional view showing a state where an eighth embodiment of underwater repair welding method of the present invention is being employed; 
           [0059]      FIG. 30  is a schematic plan view showing a state where the eighth embodiment of underwater repair welding method of the present invention is being employed; 
           [0060]      FIG. 31  is a schematic longitudinal cross-sectional view of a nuclear reactor, showing the inside thereof where a ninth embodiment of underwater repair welding method of the present invention is being employed; 
           [0061]      FIG. 32  is a schematic longitudinal cross-sectional view of a principal part of  FIG. 31 ; 
           [0062]      FIG. 33  is a schematic plan view showing a state where the ninth embodiment of underwater repair welding method of the present invention is being employed; 
           [0063]      FIG. 34  is a schematic longitudinal cross-sectional view showing a state where a tenth embodiment of underwater repair welding method of the present invention is being employed; 
           [0064]      FIG. 35  is a schematic plan view showing a welding part of where the tenth embodiment of underwater repair welding method of the present invention is being employed; 
           [0065]      FIG. 36  is a schematic longitudinal cross-sectional view showing a state where an eleventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0066]      FIG. 37  is a schematic longitudinal cross-sectional view showing a state where a modified eleventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0067]      FIG. 38  is a schematic longitudinal cross-sectional view showing a state where another modified eleventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0068]      FIG. 39  is a schematic longitudinal cross-sectional view showing a state where still another modified eleventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0069]      FIG. 40  is a schematic longitudinal cross-sectional view showing a state where still another modified eleventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0070]      FIG. 41  is a schematic longitudinal cross-sectional view showing a state where still another modified eleventh embodiment of underwater repair welding method of the present invention is being employed; 
           [0071]      FIG. 42  is a schematic plan view showing a welding part where a twelfth embodiment of underwater repair welding method of the present invention is being employed; and 
           [0072]      FIG. 43  is a schematic longitudinal cross-sectional view showing a state where a thirteenth embodiment of underwater repair welding method of the present invention is being employed. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0073]    Now, preferred embodiments of underwater repair welding method of the present invention will be described in greater detail by referring to the accompanying drawings. The parts common or similar to other embodiments are denoted respectively by the same reference symbols and will not be described repeatedly. 
       First Embodiment 
       [0074]    Referring to  FIGS. 1 and 2 , the first embodiment of the present invention employs a protection plate  3  for isolating a defective part  1  of a structure  2  that may actually exist or eventually come to exist and a laser welding head  4  for welding the protection plate  3  and the structure  2 . The welding head includes a condenser lens  8  for converging the laser beam  7  irradiated from an oscillator  5  and transmitted through a fiber  6  and a nozzle  9  arranged coaxially with the laser beam  7 . The protection plate  3  is arranged so as to cover the defective part  1  of the structure  2  and its surrounding. The welding operation is carried out around the outer periphery of the arranged protection plate  3  by irradiating the laser beam  7 , while supplying inactive gas  10  from the nozzle  9 . 
         [0075]    With this embodiment, the surface of the structure  2  having the defective part  1  is covered by the protection plate  3  and the outer periphery of the latter is welded to the structure  2  by means of the laser beam  7 , while supplying the inactive gas  10  from the nozzle  9  to isolate the defective part  1  of the structure  2  and suppress the erosion and prevent it from being worsening. 
       Second Embodiment 
       [0076]      FIGS. 3A through 3I  show protection plates  3  provided with one or two slits (apertures)  12  for releasing steam produced from water between a structure  2  and the protection plate  3  during a welding operation. For example, the slit  12  is 0.1 to 3 mm width and 0.3 mm long or longer and arranged to run along one of the sides and near a related corner of the protection plate. The slit  12  may be open to the end facet of a side of the protection plate. The slit  12  may have an oblong profile or a T-shaped or L-shaped profile. Two or more than two slits may be arranged horizontally. 
         [0077]    As shown in  FIG. 4 , the outer periphery of the protection plate  3  is welded by laser welding in such a way that the end facet thereof located close to the slit  12  is the last welding position, while supplying inactive gas  10  from the nozzle  9 . 
         [0078]    As shown in  FIGS. 5 and 6 , when welding the outer periphery of the protection plate  3 , the laser beam  7  is converged onto the surface of the protection plate  3  in order to realize keyhole welding. As shown in  FIG. 6 , the part of the protection plate  3  being welded may be covered by a shield cover  13 , while supplying inactive gas  10 . The outer periphery of the protection plate  3  may be welded by one pass welding or two pass welding. 
         [0079]    As shown in  FIG. 7 , when the outer periphery of the protection plate  3  is welded by two pass welding, the protection plate  3  is welded firstly at a position separated from an end facet thereof by 1 mm to 5 mm as indicated by a welding bead  11   a , and then subsequently at an area separated from the end facet by 2 mm as indicated by another welding bead  11   b . When thermal conduction type welding is employed alternatively for welding the outer periphery of the protection plate  3 , the laser beam  7  is converged to show a beam diameter of not less than 1.0 mm on the end facet of the protection plate  3  for the welding operation in a manner as shown in  FIG. 8  or  9 . Still alternatively, the outer periphery of the protection plate  3  may be welded, while supplying a filler wire  14  to the part being welded as shown in  FIG. 10  or  11 . 
         [0080]    The end facets of the protection plate  3  may alternatively be made to show an inclined profile as shown in  FIG. 12  instead of supplying a filler wire  14 . Still alternatively, the end facets of the protection plate  3  may be made to show a lip-like profile as shown in  FIG. 13 . When thermal conduction type welding is employed for welding the outer periphery of the protection plate  3 , a TIG arc (tungsten inert gas arc)  15 , an MIG arc (metal inert gas arc) or a plasma arc may be used as heat source as shown in  FIG. 14 . 
         [0081]    With this embodiment having the above-described arrangement, the steam produced from water between the protection plate  3  and the structure  2  by heat generated as a result of welding is discharged when the laser welding is terminated at the position of the slit  12 . The welding bead  11  can be made to show a smooth surface by way of keyhole and two pass welding when welding the outer periphery of the protection plate  3 . The welding bead for welding the protection plate  3  and the structure  2  can be made to show a smooth surface by providing the end facets of the protection plate  3  with inclination or a lip-like profile. A similar effect can be achieved by welding, while supplying a filler wire  14 . 
       Third Embodiment 
       [0082]      FIGS. 15A through 15F  show protection plates  3  provided with circular through holes (apertures)  16  for releasing steam produced from water between the structure  2  and the protection plate  3  during a welding operation. Two or more than two through holes  16  having a diameter of not less than 0.1 mm and separated from each other by a distance not less than 1 mm are vertically bored through the protection plate  3  near a corner of an end facet thereof. The outer periphery of the protection plate  3  is welded by laser welding in such a way that the end facet thereof located close to the through holes  16  is the last welding position. 
         [0083]    With this embodiment having the above-described arrangement, the steam produced during the welding operation from water remaining between the protection plate  3  and the structure  2  is discharged through the through holes  16 . Therefore, any metal is not blown off by steam pressure as a result of welding so that the protection plate  3  can be soundly welded to the structure  2 . 
       Fourth Embodiment 
       [0084]      FIGS. 16A through 16F  show protection plates  3  provided with a slit  12  and circular through holes  16  for releasing steam produced from water between the structure  2  and the protection plate  3  during a welding operation. The slit  12  and the through holes  16  are bored through the protection plate  3  near a corner of the side surface thereof. The outer periphery of the protection plate  3  is welded by laser welding in such a way that the end facet thereof located close to the slit  12  or the through holes  16  is the last welding position. 
         [0085]    With this embodiment having the above-described arrangement, the steam produced during the welding operation from water remaining between the protection plate  3  and the structure  2  is discharged through the slit  12  and the through holes  16 . Therefore, any metal is not blown off by steam pressure as a result of welding, so that the protection plate  3  can be soundly welded to the structure  2 . 
       Fifth Embodiment 
       [0086]      FIGS. 17 and 18  show a welding process of closing the slit  12  and the through holes  16  of a protection plate  3  for releasing steam. As shown, the protection plate  3  for isolating the defective part  1  of the structure  2  and a laser welding head  4  for welding the protection plate  3  and the structure  2  are arranged. The protection plate  3  has a slit  12  or both a slit  12  and circular through holes  16  for releasing steam. The welding head  4  is equipped with a shield cover  13  for covering an area including the slit  12  and the through holes  16  for releasing steam such that inactive gas  10  can be supplied to the inside of the shield cover. A filler wire  14  is supplied to the position where a laser beam  7  is irradiated. 
         [0087]    The laser beam  7  to be irradiated is so defined as to show a beam diameter of not less than 1.0 mm in order to realize thermal conduction type welding. When welding the slit  12  and the through holes  16 , firstly inactive gas  10  is supplied into the shield cover covering the slit  12  and the through holes  16  to discharge the water remaining in the gap between the protection plate  3  and the structure  2 . Subsequently, the laser beam  7  is irradiated, while supplying a filler wire  14  as indicated by an arrow or arrows in  FIGS. 19 and 20  to close the slit  12  and each of the through holes  16  by welding from below to above. The welding may be uphill welding. 
         [0088]    The laser beam  7  may be focused onto the surface of the protection plate  3  so as to realize keyhole welding as shown in  FIGS. 21 and 22 . In the case of thermal conduction welding as shown in  FIG. 23 , a TIG arc  15 , an MIG arc or a plasma arc may be employed as heat source. 
         [0089]    With this embodiment having the above-described arrangement, the slit  12  and the through holes  16  are covered by the shield cover  13  and the water remaining between the protection plate  3  and the structure  2  is discharged to below the lower surfaces of the slit  12  and the through holes  16  by inactive gas, so that the water remaining between the protection plate  3  and the structure  2  is prevented from turning to steam and blowing off welding metal when the slit  12  and the through holes  16  are welded. As a result, the protection plate can be welded soundly. 
       Sixth Embodiment 
       [0090]      FIG. 24  shows another welding process of closing the slit and the through holes  16  for releasing steam. As shown in  FIG. 24 , a shield cover  13  for covering an area including the slit  12  and the through holes  16  for releasing steam is arranged such that inactive gas  10  can be supplied to the inside of the shield cover. Additionally, a high frequency heating source  20  is arranged to heat the surface of the protection plate  3 . 
         [0091]    With this embodiment having the above-described arrangement, the slit  12  and the through holes  2  are covered by the shield cover  13  and the inactive gas  10  is supplied, while the protection plate  3  is heated from the surface thereof by the high frequency heating source  20 . As a result, the water remaining between the protection plate  3  and the structure  2  is discharged to below the lower surfaces of the slit  12  and the through holes  16  so that the water remaining between the protection plate  3  and the structure  2  is prevented from turning to steam and blowing off welding metal when the slit  12  and the through holes  16  are welded. As a result, the protection plate can be welded soundly. 
       Seventh Embodiment 
       [0092]      FIG. 25  shows a process of welding the outer periphery of a protection plate  3  when neither a slit nor a through hole for releasing steam is arranged in the protection plate  3 . As shown in  FIG. 25 , a structure  2  having a defective part  1 , a protection plate  3  for isolating a structure  2  having a defective part  1  and a laser welding head  4  for welding the protection plate  3  and the structure  2  are provided. The outer periphery of the protection plate  3  is welded except an unwelded part  30  that is not less than 1 mm long, while supplying inner gas  10  to the part to be welded, from a nozzle  9 . 
         [0093]    Subsequently, as shown in  FIGS. 26 and 27 , the unwelded part is covered by a shield cover  13  and the inactive gas  10  is supplied to discharge the water remaining in the gap between the protection plate  3  and the structure  2 . Then, the unwelded part is welded from below to above. Before welding the unwelded part, the water remaining in the gap between the protection plate  3  and the structure  2  may be discharged by heating the surface of the protection plate  3  by means of a high frequency heating source  20  in addition to covering the unwelded part by a shield cover  13  and supplying the inactive gas  10  as shown in  FIG. 28 . 
         [0094]    With this embodiment having the above-described arrangement, an unwelded part is left at some position of the outer periphery of the protection plate  3 . Then, the unwelded part is covered by the shield cover  13  and the inactive gas  10  is supplied, or the unwelded part is covered by a shield cover  13  and the surface of the protection plate  3  is heated by the high frequency heating source  20 , in order to discharge the water remaining in the gap between the protection plate  3  and the structure  2 . As a result, the protection plate can be welded soundly. 
       Eighth Embodiment 
       [0095]      FIGS. 29 and 30  show a process of welding the outer periphery of a protection plate  3 , arranging a side shield nozzle  21  in addition to a nozzle  9  and a shield cover  13 . 
         [0096]    With this embodiment, the surface of a structure having a defective part  1  is covered by a protection plate  3  and the end facets of the protection plate  3  are welded, while driving off the water remaining in the gap between the protection plate  3  and the structure  2  by supplying the inactive gas  10  from a side shield nozzle  21 . Thus, the part being welded and its vicinity are prevented from being wet by splashing water and any metal is not blown off by steam pressure as a result of welding so that the protection plate can be soundly welded to the structure. 
       Ninth Embodiment 
       [0097]    With the ninth embodiment of underwater repair welding method according to the present invention, a laser welding head  4  is arranged for a reactor internal structure  22  having a defective part  1  in order to weld the defective part  1  as shown in  FIGS. 31 ,  32  and  33 . The welding head  4  has a condenser lens  8  for converging a laser beam  7  irradiated from an oscillator  5  and transmitted through a fiber  6 , a nozzle  9  arranged coaxially with the laser beam, and a shield cover  13  for covering the defective part  1 . The defective part  1  is covered by the shield cover  13 , and the laser beam  7  is irradiated horizontally toward the defective part  1 , while supplying the inactive gas  10 . Then, the irradiated part (part being welded) is moved upward. 
         [0098]    With this embodiment having the above-described arrangement, the water remaining in the inside of the defective part can be discharged as the surface of the reactor internal structure  22  having the defective part  1  is covered by a shield cover  13  and the laser beam  7  is moved upward for welding. Thus, any metal is not blown off by steam pressure as a result of closing the surface of the defective part by welding, so that the defective part can be soundly repair-welded. 
       Tenth Embodiment 
       [0099]    The tenth embodiment of underwater repair welding method according to the present invention will be described below by referring to  FIGS. 34 and 35 .  FIG. 34  is a schematic longitudinal cross-sectional view showing a state where the tenth embodiment of underwater repair welding method is being employed and  FIG. 35  is a schematic plan view showing a welding part therefor. 
         [0100]    The tenth embodiment is for repairing a welding bead  11  having a defective part  1  or a lap plate (protection plate)  3 . 
         [0101]    The lap plate  3  is arranged along the surface of a metal structure  2  and welded airtight along the outer periphery thereof by means of a welding bead  11 . The lap plate  3  may typically be a protection plate for protecting a scar or some other defective part (not shown) produced on the surface of the metal structure  2 . It may be a protection plate  3  employed in any of the above-described first through ninth embodiments. 
         [0102]    With the tenth embodiment, the defective part  1  that is produced on the welding bead  11  or the lap plate  3  is repaired underwater by welding. The defective part  1  is produced on the welding bead  11  in the illustrated example. A gap  40  is formed between the lap plate  3  and the surface of the structure  2 . 
         [0103]    When carrying out the tenth embodiment of underwater repair welding method, firstly steam release holes  12  are formed through the lap plate  3  at positions near the defective part  1 . A specific example of forming the steam release holes  12  will be described below by referring to the thirteenth embodiment. 
         [0104]    Then, the defective part  1  is repaired by means of a laser welding head  4 . The laser welding head  4  is connected to an oscillator  5  by way of a fiber  6  and provided with a condenser lens  8  and a nozzle  9  arranged coaxially with the laser beam  7 . The laser beam  5  irradiated from the oscillator  5  is transmitted through the fiber  6  to the laser welding head  4  and converged by the condenser lens  8 . At the same time, inactive gas  10  is supplied to the part being welded from the nozzle  9 . 
         [0105]    Thereafter, the steam release holes  12  are closed by welding in a manner like the repair of the defective part  1  described above. At this time, the water remaining in the gap  40  between the lap plate  3  and the surface of the structure  2  is turned to steam and discharged through the steam release holes  12  before the steam release holes  12  are tightly closed. 
         [0106]    As described above, with this embodiment, the water remaining in the gap between the lap plate  3  and the surface of the structure  2  is turned to the steam and discharged, and any metal is not blown off by steam pressure, so that the defective part can be soundly repair-welded. 
       Eleventh Embodiment 
       [0107]    The eleventh embodiment of underwater repair welding method according to the present invention will be described below by referring to  FIGS. 36 through 41 .  FIGS. 36 through 41  are schematic longitudinal cross-sectional views showing a state where the eleventh embodiment of underwater repair welding method is being employed in so many different situations. 
         [0108]    In the instance illustrated in  FIG. 36 , when welding a welding bead  11  having a defective part  1  like the one described above as the tenth embodiment, a laser beam  7  is converged to the surface of the welding bead  11  so as to realize keyhole welding. 
         [0109]    Steam release holes  12  are formed near the defective part  1  so that the water remaining in the gap  40  between the lap plate  3  and the surface of the structure  2  is turned to steam by the heat generated as a result of welding and discharged during the welding operation. The defective part  1  can be completely closed by employing keyhole welding for welding the defective part  1 . 
         [0110]    With this embodiment, since the water remaining in the gap  40  between the lap plate  3  and the surface of the structure  2  is turned to the steam and discharged from the steam release holes  12  during the welding operation, so that any metal may not be blown off by steam pressure. Additionally, the defective part  1  can be completely closed without leaving any defect in the inside, although the defective part  1  is not removed. 
         [0111]    As a modification to the above-described eleventh embodiment, the part to be welded may be covered by a shield cover and inactive gas  10  may be supplied during the welding process, as shown in  FIG. 37 . 
         [0112]    As another modification to the eleventh embodiment, the diameter of the laser beam  7  irradiated on the surface of the welding bead may be defined to be not less than 1.0 mm as shown in  FIG. 38  when the welding bead  11  having a defective part  1  is welded by means of thermal conductive type welding. 
         [0113]    As still another modification to the eleventh embodiment, a filler wire  14  may be supplied to the part being welded of the defective part  1  as shown in  FIG. 39 . 
         [0114]    As still another modification to the eleventh embodiment, a TIG arc  15 , an MIG arc or a plasma arc may be employed as heat source as shown in  FIG. 40  when the outer periphery of the lap plate  3  is welded by means of thermal conductive type welding. 
         [0115]    As still another modification to the eleventh embodiment, the welding bead  11  having a defective part  1  may be removed mechanically in advance instead of employing keyhole welding and subsequently the defective part may be repaired by re-welding, while supplying a filler wire  14  as shown in  FIG. 41 . 
       Twelfth Embodiment 
       [0116]    Now, the twelfth embodiment of underwater repair welding method of the present invention will be described below by referring to  FIG. 42 .  FIG. 42  is a schematic plan view showing a welding part where the twelfth embodiment of underwater repair welding method is being employed. Steam release holes  12   a  are formed along the lower edge in  FIG. 42  of a lap plate  3  in addition to steam release holes  12  formed near a defective part  1 . The defective part  1 , the steam release holes  12  formed near the defective part  1  and the steam release holes  12   a  formed along the lower edge in  FIG. 42  of the lap plate  3  are welded sequentially in the above mentioned order. 
         [0117]    With this embodiment, steam generated during the process of welding the defective part  1  is discharged through the steam release holes  12 ,  12   a  and all the water remaining in the gap  40  can be discharged before welding the lower steam release holes  12   a . As a result, any metal is not blown off by steam pressure, so that the defective part  1  can be soundly repair-welded. Additionally, all the water remaining in the gap  40  can be discharged before welding the lower steam release holes  12   a  of the lap plate  3  for the repair welding operation. 
       Thirteenth Embodiment 
       [0118]    Now, the thirteenth embodiment of underwater repair welding method of the present invention will be described below by referring to  FIG. 43 .  FIG. 43  is a schematic longitudinal cross-sectional view showing a state where the thirteenth embodiment of underwater repair welding method is being employed. This embodiment employs a laser welding head (laser processing head)  4 , a fiber  6 , an oscillator  6  and a nozzle  9  that are similar to those employed for repair-welding and closing steam release holes with the tenth embodiment when processing the steam release holes  12  like the ones described above by referring to the tenth through twelfth embodiments by laser welding (see  FIG. 34 ). 
         [0119]    A laser beam  7  irradiated from the oscillator  5  is transmitted to the laser welding head  4  by way of the fiber  6  and converged by a condenser lens  8 . At the same time, inactive gas  10  is supplied to the part to be welded from the nozzle  9 . 
         [0120]    Thereafter, the steam release holes  12 ,  12   a  are tightly closed by repair-welding the defective part  1  by means of the laser welding head  4  as in the case of the above-described first through twelfth embodiments. 
         [0121]    With this embodiment, the laser welding head (laser processing head) that is employed for processing the steam release holes  12 ,  12   a  can also be employed for the subsequent repair-welding and also for closing the steam release holes. 
       Other Embodiments 
       [0122]    The above-described embodiments are merely examples and the present invention is by no means limited thereto. While the lap plate  3  employed with each of the tenth through thirteenth embodiments is the same as a protection plate to be fitted to a defective part such as a scar produced on the surface of a metal structure  2  in the above description, the lap plate may not necessarily be the same as a protection plate so long as it is a plate to be laid on and welded to the surface of the metal structure  2 . 
         [0123]    Either or both of the characteristic feature of the twelfth embodiment ( FIG. 42 ) and that of the thirteenth embodiment ( FIG. 43 ) may be combined with any of the specific examples ( FIG. 36  through  FIG. 41 ) described above for the eleventh embodiment.