Method for welding aluminum plates

A method for manufacturing a large basically spherical vessel from plane aluminum plates having a thickness in the range from about 20 mm to about 70 mm, comprises (a) forming at least first and second plane plate assemblies by downhand welding together plane plates controlled by a welding device including an adjustably lockable portal, (b) cutting each welded plane plate assembly to a peripheral shape that fits into the structural pattern of the vessel except for an allowance for machining in step (d), (c) bending each plate assembly to part-spherical configuration, (d) machining respective edges of the first and second bent plate assemblies to a profile suitable for welding in step (e), and (e) downhand welding the first and second plate assemblies together along the machined edges using a manipulator device that includes a tiltable support.

BACKGROUND OF THE INVENTION 
This invention relates to a method for welding aluminum plates for 
manufacturing a large spherical structure and to a large spherical 
structure manufactured by this method. 
A ship-borne tank for liquified natural gas (LNG) may be manufactured by 
welding together large and thick aluminum plates and bending the resulting 
plate assemblies to form them to portions of the desired spherical 
LNG-tank. The same kind of tank may be used elsewhere for the storing of 
LNG and other substances. Because a large part of the manufacturing costs 
are welding costs, the plate blanks that are welded together should be as 
large as possible. Joining together this kind of large plate blanks and 
welded plate assemblies by further welding is demanding work, because all 
weld joints must be of high quality. Also the plates and the plate 
assemblies must be precise in shape and dimension for achieving a proper 
final result. 
U.S. patent application Ser. No. 08/061,193, the disclosure of which is 
hereby incorporated by reference herein, discloses a method of 
manufacturing a large spherical LNG-tank by welding together several plane 
aluminum plates to form a planar plate assembly and subsequently forming 
the plate assembly in a mold to part-spherical configuration and using the 
formed plate assembly in the following stage of manufacture. However, 
during bending of the plate assembly in the mold, the plate assembly 
stretches somewhat, which makes it difficult to use modern high power 
MIG-welding equipment for welding in one pass per side because there are 
shape defects in the welding groove and at its root. 
It is known also to manufacture spherical tanks of plates that are welded 
together to form larger plate assemblies by a so-called tractor type 
welding carriage. However, use of a tractor type welding carriage is 
difficult and time consuming, especially at start and finish of the 
welding operation. Known manufacturing methods using tractor type welding 
equipment for welding together plates that have been cut to a particular 
peripheral shape and/or bent to a desired configuration also require twice 
as many assembly stations. It is necessary to remove the weld 
reinforcement before forming in order to allow the inspections that are 
included in many methods and because of the forming itself. Special 
attention must be given to the automation of the work and to the 
dimensional precision of the final work. 
SUMMARY OF THE INVENTION 
The object of the invention is to achieve a high level of automation in the 
manufacture of large spherical LNG-tanks and to improve the quality of 
both the welds and the manufactured spherical tank. 
A large spherical LNG-tank or the like having a diameter of more than 20 
meters, even more than 40 meters, is usually manufactured of aluminum 
plates that are 20-70 mm thick. The plates to be welded are fixed during 
welding by jigs and suction fastening devices. In a preferred embodiment 
of the invention, large plane plate assemblies are formed by downhand 
welding of plane plates using manipulator devices equipped with an 
adjustable and a lockable portal. The manipulator device comprises, in 
addition to the welding equipment, a device for brushing the weld groove 
in order to remove impurities present in the groove and the oxide layer 
present on the surface of the aluminum. Removing the oxide layer reduces 
the possibility of defects in the weld. The groove brushing device moves 
along the weld groove in front of the welding device most suitably at a 
distance of half a meter from the welding device. The manipulator is also 
provided with a milling cutter for removing the weld reinforcement. The 
cutter moves in the direction of the weld groove behind the welding device 
and removes the weld reinforcement immediately after the welding. Because 
the cutter is provided as part of the manipulator device, the cutting is 
accurate and fast and the repeatability of the cutting is excellent. The 
plates and plate assemblies welded together are plasma cut to a size and 
peripheral shape suitable for a spherical LNG-tank, but working allowance 
is left, because the machining of the edges of the bent plate assembly to 
a suitable profile for the subsequent welding is done with a form milling 
cutter. Accurate forming of the groove and the production of smooth groove 
surfaces are important for achieving a good enough weld and for achieving 
compatible edge surfaces of the plates to be joined. The plate assemblies 
are bent to spherical configuration and two such bent plate assemblies are 
joined by a flat butt weld using a manipulator device and a tilting 
device. The spherical LNG-tank to be manufactured is assembled of large 
sections at the final building site. 
The aluminum plates are welded together with the help of a manipulator 
device with only one pass per side irrespective of material thickness. For 
this purpose, a high power welding device may be used. Fastening and 
turning equipment is provided for turning the plate assembly upside down 
so that the weld of the other side may be accomplished. For speeding up 
the work it is advantageous to employ a production line in which turning 
equipment is located between two manipulators of the line. 
For welding plane plates together to form a large plate assembly, the 
suction fastening device holding the plane plates is tilted so that the 
inclination of the groove in the welding direction is 4.degree.-7.degree., 
preferably about 5.degree., upwards. It is then possible to use a high 
power welding device and complete the weld joint at one side in one pass, 
because full control of the melt is easily achieved when the weld groove 
is in a slightly inclined position. 
The plate assemblies that have been bent to part-spherical configuration 
for subsequent welding together are fixed to a tiltable table, by means of 
which the inclination of the plate assemblies at the welding location is 
set to a value of 4.degree.-7.degree., preferably to the value of 
5.degree., upwards in the welding direction by continuously adjusting the 
tiltable table during welding. This makes it possible to use a high power 
welding device and complete the weld joint at one side in one pass for the 
reason mentioned above. 
It is advantageous to support at least two plate assemblies of 
part-spherical configuration by a support device mainly in the shape of a 
circular ring. The plate assemblies may be disposed with either the convex 
side or the concave side upwards for welding. The diameter of the circular 
ring should be as large as possible for achieving a good support, but 
small enough that the plate assemblies totally cover it. One may also use 
a suction fastening device, which may be of a known type, with suction 
heads that face the plate assembly and are easily adjustable and are 
lockable for the time of the work in a vertical position suitable for 
either the convex side or the concave side of the plate assembly. 
The height of the support structure in the middle area of the support 
assembly is adjustable depending on whether the plate assemblies to be 
welded are positioned with the concave or the convex side upwards. 
According to a preferred embodiment, the support structure in the central 
point of the circular ring comprises a smaller circular ring with a 
diameter of, for example, approximately 1250 mm and the height of which is 
adjustable so that it supports either the convex or the concave side of a 
plate assembly. Another way is to provide in the central area of the 
circular ring, an upwardly convex general support area preferably having a 
part-spherical shape. The plate assemblies to be welded are supported by 
this area upon adjusting it to a suitable height. For welding upwardly 
concave plate assemblies one may arrange, in the central support area, an 
upwardly concave intermediate structure, supporting with its top the 
central area of the plate assembly. 
The weldable edges of the aluminum plates are prepared by a form milling 
cutter that forms both the root portion and the sides of the groove 
simultaneously. A cylindrical disc cutter with a relatively large diameter 
can be used. It is important that the disc cutter moves along such a line 
and at such an angle to the plate that a weld root surface is produced 
allowing easy positioning of one plate against another, so that no 
substantial slot occurs between adjacent root surfaces. If irregularities 
occur, a slot of about 1 mm between the root surfaces may be acceptable. 
The invention also relates to a large spherical LNG-tank or the like, the 
plate sections of which are manufactured by using the method according to 
the invention.

DETAILED DESCRIPTION 
In the drawings, a production line for joining plane plates to form plate 
assemblies is formed by a welding and manipulating device 1, a turning 
device 2, and another welding and manipulating device 3 at successive 
stations. At the first welding station, plane plates 4 and 5, already cut 
to the desired slightly tapered peripheral shape, are joined by welding. 
The plate thickness used in the case of a spherical LNG tank is as a rule 
between 20 and 70 mm, the thinner plates being used in the upper regions 
of the tank. The plate edges 6 and 7 at which the plates are to be welded 
are machined to the proper profile by a cylindrical cutter so that a weld 
groove is formed. The weld groove includes double, symmetrically 
positioned V-grooves with root face. The root area takes up around 40-50 
percent of the plate thickness. The groove angle varies from approximately 
90.degree. with thinner plates to approximately 70.degree. with thicker 
plates. A beam 8 of a portal carries a welding device 9 movable along the 
beam. The welding device moves to the right of FIG. 1 for welding the 
plates 4 and 5, and the portal is inclined 4.degree.-7.degree. upward to 
right of FIG. 1. The inclination may be adjustable and lockable. A tilt 
angle of 5.degree. is normal. Under the beam 8 there is a suction 
fastening device (not shown in detail) holding the plates 4 and 5 firmly 
in place. The suction fastening device is also tilted for welding so that 
the weld groove is parallel to the portal 8. Other parts are shown in more 
detail in FIG. 4. 
The plane plate assembly formed at the first welding station is turned 
upside down at the turning station and is delivered to the second welding 
station. 
The welding and manipulating device 3 at the second welding station 
includes a portal having a beam 8a and a welding device 9a of the same 
kind as at the first welding station. At the second welding station, 
welding of the second side of the plane plates 4 and 5 takes place. It is 
also possible to omit the second welding station and weld the second side 
of the plate assembly 4, 5 at the first welding station after turning the 
plate assembly upside down at the turning station. 
After welding at both sides, the plate assembly is delivered to a cutting 
station at which the plate assembly is cut to the proper peripheral shape 
to conform to the selected spherical pattern, except for a machining 
allowance. The plate assembly is then bent to spherical configuration, for 
example in the manner described in U.S. patent application Ser. No. 
08/061,193, and the bent and cut plate assembly is delivered to a 
machining station. At the machining station, a form milling machine is 
used to machine the edges of the plate assembly to the proper profile, as 
mentioned above in connection with the plane plates, and concurrently 
bring the plate assembly to the proper peripheral shape. This is done 
after the plate assembly has been bent, in order to ensure that the 
bending operation does not introduce errors. 
From the machining station, the bent plate assemblies of the proper 
peripheral form and having the proper edge profile are delivered to a bent 
plate welding station at which two such bent plate assemblies are welded 
together. 
FIG. 2 shows three embodiments 12a, 12b, and 12c of a tiltable welding 
table that is provided at the bent plate welding station. The welding 
table is provided with support and fastening equipment, which is shown in 
more detail in FIG. 3. Table 12a is used for the welding of a spherical 
curved structure composed of a two-plate assembly 4+5 that has been bent 
to part-spherical configuration and another similar plate assembly 14+15. 
A manipulator device 16 carries a welding device 17 on a horizontal beam 
20 that is movable vertically on a pillar 19, which in turn is movable 
horizontally on guides 18a and 18b. The plate assembly 4+5, 14+15 is 
placed on the table 12a, tiltable through at least .+-.20.degree., 
preferably .+-.30.degree., from its horizontal position. By tilting the 
table 12a, the inclination of the plate assembly 4+5, 14+15 can be 
adjusted during the welding operation so that the groove is inclined at an 
angle of 4.degree.-7.degree. at the location of the welding device as the 
welding device passes along the entire weld. 
The plate assembly 4+5, 14+15 is fixed on the table 12a by means of suction 
fastening equipment. The weld groove formed by plate edges 21 and 22 
extends along an arc of a great circle on the spherical structure, and the 
plate assemblies are positioned so that the arc lies in a vertical plane. 
The weld groove is therefore substantially straight when viewed from 
above, and the plate assemblies are positioned so that the weld groove is 
set parallel to the guides 18a and 18b. The guides 18a and 18b define an 
axis of horizontal movement that lies in a vertical plane that is 
perpendicular to the tilting axis of the table. The tilt of the table is 
continuously adjusted so that the welding always takes place in a groove 
inclined about 5.degree. upwards. While welding progresses, the pillar 19 
moves on the guides 18a and 18b. Should the weld groove not be parallel to 
the guides 18a and 18b, when viewed from above, correction can be made by 
moving the welding device 17 on the beam 20 in response to a signal 
provided by a groove sensor that is described with reference to FIG. 4. 
The distance of the welding device 17 from the welding groove is adjusted 
by moving the beam 20 vertically along the pillar 19. 
The manipulator device 16 is preferably equipped with a device for removing 
weld smoke and with groove cleaning devices and a milling device for 
removing the weld reinforcement. It is also possible to provide quality 
monitoring devices for monitoring the weld joint. 
The tiltable table 12b has two cross-wise arranged tilting axes allowing 
tilting as indicated by the arrows 11 and 13. Table 12b is intended 
especially for the plate assemblies of the upper and lower calottes of the 
spherical tank. These calottes include plates 23, 24, 25, and 26. The 
welding device 17b of the table 12b has the same freedom of movement as 
the device 17. The two tilting axes of table 12b make it possible to carry 
out all welds without releasing the fixation of the workpiece. 
The pieces to be welded on table 12c are intended for welding by the 
welding device 17. Table 12c is an alternative to table 12a. Support and 
fastening means for the workpiece are indicated by numerals 29, 30, 30b, 
and 31b, explained below with reference to FIG. 3. 
FIG. 3 shows fastening and support equipment located on a tiltable table 12 
(table 12a or table 12c) for receiving the concave or the convex side of a 
part-spherical workpiece. The main support member is a circular ring 29, 
the diameter of which may be around 10 m when manufacturing portions of a 
spherical tank with a diameter of about 40 m. At the center of the support 
ring 29 there is a support 30 provided with a support member 30b, which is 
higher than the circular ring 29 and is used when welding the convex side 
of a plate assembly. A corresponding support 30a, of lesser height than 
the ring 29, is used when the concave side of the plate assembly is 
welded. Vertically adjustable suction fastening devices 31a, 31b are used 
for holding the plate assembly against the support ring 29. Instead of the 
central support 30, with the members 30a and 30b, another annular support 
of smaller diameter than the ring 29 may be used. This smaller annular 
support should be vertically adjustable to suit the concavity or convexity 
of the plate assemblies to be supported. 
The plate assembly 4+5, 14+15 is placed on the fastening and support 
equipment so that when the plate assembly is being welded from its convex 
side, the two component plate assemblies 4+5 and 14+15 slope away from the 
weld groove, whereas when the plate assembly is being welded from its 
concave side, the two component plate assemblies slope toward the weld 
groove. 
FIG. 4 shows a welding device, for instance, a high power MIG-welding 
device, using a welding wire provided from a roll 32. A cleaning device in 
the form of a rotating brush 33 is provided for cleaning the groove of 
impurities and harmful particles and removing oxide film from the aluminum 
plate. Matter removed by the brush 33 is sucked into a pipe 34, in which 
an air flow is maintained for removing said matter. A groove sensor device 
35 is arranged between the brush 33 and the welding device. The groove 
sensor device therefore runs in a clean groove, which aids in accurate 
guiding of the welding device. A cutter 36 is provided for removing the 
weld reinforcement. The cutting chips are sucked away through a pipe 37. A 
similar suction pipe 38 removes the weld smoke produced at the welding 
point 39. It is not necessary in all cases that the welding device should 
include all the auxiliary devices sown in FIG. 4. 
In the case of the welding device shown in FIG. 4 being used at the first 
welding station, the groove sensor device 35 does not directly control 
positioning of the welding device in a horizontal direction perpendicular 
to the beam 8, but is used to test whether the plates are properly 
positioned with the weld groove parallel to the beam 8. This might be 
accomplished in either of two ways. In accordance with a first 
alternative, the welding device is run along the beam 8 without welding, 
and if the groove sensor does not indicate mispositioning, the welding 
device is run a second time with the welding electrode active, whereas if 
the groove sensor indicates mispositioning, the plates are repositioned. 
In accordance with the second alternative, the welding electrode is 
activated and the signal provided by the groove sensor stops the welding 
operation in the event that the welding device does not follow the groove 
accurately enough. 
The welding device shown in FIG. 4 may also be used at the bent plate 
welding station. The configuration that is shown in FIG. 4 is particularly 
applicable to the welding device 17b, when used to weld along the beam by 
which it is supported. In this case, the signal provided by the groove 
sensor device controls positioning perpendicular to the beam. When used to 
weld perpendicular to the support beam, the signal provided by the groove 
sensor device controls positioning along the beam. 
After two two-plate assemblies have been welded together at the bent plate 
welding station from, for example, the convex side, the resulting 
four-plate assembly is delivered to a turning station at which the 
assembly is turned upside down, so that the convex side is down, and the 
assembly is then returned to the bent plate welding station and is placed 
on a tiltable table with the support and fastening means set to receive 
the convex side of the assembly. The plate assembly is downhand welded 
from its concave side in a single pass while adjusting the tilt of the 
table as described with reference to FIG. 3. 
The invention is not restricted to the embodiments shown, but several 
modifications thereof are feasible within the scope of the attached 
claims.