Background Art The prior art steel pipes employ a method of cutting an outside edge as an edge-cutting surface (a bevel cutting surface or an open-ended surface) for a welding surface, irrespective of great and small sizes. Although it facilitates cutting work, a difficult upward weld (an overhead weld) should be executed in the lower portion in case that pipes are arranged approximately horizontally. And, an overhead weld is very slow in working speed, and can become a cause of welding defects particularly in the narrow working space, thereby necessitating expert weldors of a very high level. Moverover, there are some problems that, in case that large-diameter steel pipes are laid under the ground, the ground should be dug deeply so as to ensure working space for the overhead weld, and that, in case that the ground comprises water or mud, the work space for the weld operation must be very inferior.

5 Disclosure of Invention Accordingly, the present invention is made in order to solve the above problems in large-diameter steel pipes into which a worker can enter, and one object of the present invention is to provide down weldable steel pipes and their welding joint method 10 in which a weld can be performed in a downward position inside the pipes by cutting an inside edge in the lower portion of the pipes so that the welding operation in the lower portion can be performed approximately in a downward position, but not in an upward position. In particular, the present invention can be 15 applied effectively to steel pipes larger in size than a weldor can enter into the pipes on a sliding plate on wheels.

To accomplish the object of this invention, down weldable steel pipes are provided in accordance with one embodiment of the present invention, which have larger size than a weldor or an 20 auto-welder can enter into the pipes and can execute welding work, the down weldable steel pipes being characterized in that, insead of cutting the whole length of an outer edge or an inner edge as an edge-cutting surface (a bevel cutting surface or an open-ended surface) for a butt-weld, an inner cutting surface is formed at 5 the lower portion of each end of the large-diameter steel pipes which cannot be welded approximately in the downward or horizontal position and outside the pipes, so that welding approximately in the downward or horizontal position inside the pipes is feasible, and an outer cutting surface is formed at other portion of each 10 end of the large-diameter steel pipes, so that welding approximately in the downward or horizontal position outside the pipes is feasible.

Furthermore, the present invention is to provide a welding joint method for down weldable large-diameter steel pipes as 15 specified above, said method being characterized by comprising the steps of: prewelding one side of a backing bar at the lower outer surface of the previously installed large-diameter steel pipe; placing the large-diameter steel pipe to be installed on other side of the backing bar; and executing welding approximately in 20 the downward or horizontal position inside and outside the pipes.

Brief Description of Drawings Fig. 1 is a front view of one pair of pipes in provisional assembly for explaining the structures of down weldable large-diameter steel pipes according to one embodiment of the 5 present invention.

Fig. 2 is a side view of Fig. 1.

Fig. 3 is a sectional view along section line 3-3 shown in Fig. 2.

Fig. 4 is a schematic sectional view illustrating a 10 downward welding appearance inside the large-diameter steel pipes for explaining a welding joint method for down weldable large-diameter steel pipes according to the present invention.

Best Mode for Carrying out the Invention Now, a preferred embodiment of the present invention will be 15 described in detail with reference to the accompanying drawings.

In general, a welding suface for a butt-weld comprises a butt surface (5 or 6) of a proper area and an edge-cutting surface (a bevel cutting surface or an open-ended surface).

In Figs. 1,2 and 4, large-diameter steel pipes (1,2) 20 according to the present invention have an outer cutting surface

(3) formed at the upper portion of each end of the large-diameter steel pipes (1,2) by cutting, etc. , as in the prior art pipes, and an inner cutting surface (4) formed at the lower portion thereof by cutting, etc. , as the edge-cutting surface for butt-welding the ends of the large-diameter steel pipes.

In such a construction, as shown in Figs. 1 and 2, a parting line (9) is formed in the right and left directions at a position where the outer cutting surface (3) and the inner cutting surface (4) are met together. The parting line (9) comprises approximately an inside surface (9a) and an outside surface (9b) similar thereto, as shown in Figs. 7 2 and 3. The parting lines (9) in the right and left directions are preferably coincidental with each other at least between adjacent ends of pipes in order to facilitate a butt-weld of the pipes. That is, as in Fig. 1, the right and left parting lines (9) of the previously installed large-diameter steel pipe (1) and the large-diameter steel pipe (2) to be installed later are preferably always coincidental with each other at ends of the pipes at the same height. For this, the outer cutting surface (3) and the inner cutting-surface are preferably formed in constant proportion to each other in

circumferential length.

Since it is easier to weld a side face in a welding position outside the pipes than in a welding position inside the pipes even if it is the same welding position, it is preferable to locate the parting line below the horizontal center line.

Therefore, the proportion between the upper cutting length and the lower cutting length is more preferably standardized according to the sizes of the pipes so that the outer cutting surface (3). may be longer than, but not the same with, the inner cutting surface (4) in circumferential length.

As shown in Fig. 4, a welding joint method using down weldable large-diameter steel pipes as specified above in accordance with the present invention has an advantage that a backing bar (, 11) can be previously welded at one side on the lower outer surface of a previously installed large-diameter steel pipe (2). After prewelding the backing bar (11) in this way, a large-diameter steel pipe (1) to be installed is placed on other side of the backing bar (11), thus welding in the upper portion can be performed approximately in the downward or horizontal position outside the large-diameter steel pipes (1,2).

Furthermore, welding in the lower portion can be performed approximately in the downward position inside the pipes as shown as a reference number"12"in Fig. 4. By such a method, the large-diameter steel pipe (1) to be installed later can be easily arranged at a welding location, thereby more simplifying and facilitating construction work.

Furthermore, as in Fig. 4, a proper blowing fan (10) and/or exhaust pipes and/or intake pipes (not shown) are installed in the pipes. The present invention can be applied effectively to steel pipes larger in size than a weldor can enter into the pipes on a rolling plate (14) on wheels (13) as shown in Fig. 4.

By virtue of the configuration and acting of the down weldable steel pipes and their welding joint method in accordance with the embodiments of the present invention described above, even the lower portion of the pipes can be welded in the downward position. Thus, a weld is more efficient, even a common weldor can perform excellent welding, it can be omitted to give the last welding in the downward position again inside the pipes in case of a large-diameter pipe, welding defects decrease, a narrow working space below the pipes is no further problem, and a weld can be performed well even for an inexpert weldor without skill of a very high level.

Although the present invention has been described in detail with respect to several preferred embodiments thereof, it will be apparent for those skilled in the art that various modifications, improvements or applications are possible with respect to the shown embodiments within the scope of the present invention.