Apparatus for laser welding pipes and the like

Laser welding apparatus includes a first mirror whose angle may be adjusted but which is fixed in position and on which a laser beam is incident. The first mirror reflects the laser beam onto a second mirror which may be rotated through at least 180.degree. about the axis of the articles to be welded and which reflects the laser beam onto a third mirror whose angle may be adjusted and which may be rotated through at least 360.degree. about the axis of the articles to be welded. The laser beam then passes to a focussing mirror which focusses the beam through a welding nozzle onto the surfaces to be welded. In use, the third mirror is continuously rotated about the entire periphery of the articles to be welded and the positions and/or angles of all the mirrors are adjusted to ensure that the laser beam is transmitted along the desired pathway.

The present invention relates to apparatus for laser welding pipes and 
other structures of circular cross-section and is particularly concerned 
with welding pipe sections or the like, as are commonly used in oil and 
gas pipe lines, in situations where it is not possible to manipulate the 
pipe sections, that is to say move them relative to the laser The 
invention relates in particular to apparatus for directing a laser beam 
for welding pipes or the like. 
The sections of such pipe lines are generally made of steel whose structure 
and/or physical properties tend to be altered locally by the substantial 
heat produced when gas or arc welding a pipe section to a further such 
section or to a further component, such as a flange. The thickness of the 
pipe sections is generally such that more than one layer of weld material 
is required to complete the join using conventional welding processes thus 
producing a large zone of weld metal in the finished pipeline. By 
contrast, however, the use of a laser for such welding produces a narrow, 
deep penetration weld and the pipe sections may be joined in a single 
pass. Thus not only is the volume of weld metal at the joint reduced and 
the heat affected zone much smaller than with conventional welding 
processes, but also the time taken to weld each joint is substantially 
reduced. 
When laying a pipeline on land or at sea, pipe sections are successively 
welded together and progressively lowered into a trench or into the sea. 
Adjacent sections must of course be welded over their entire periphery but 
the sections of pipe can of course not be rotated since one of them is 
generally already connected to the pipeline under construction. It is 
therefore necessary to rotate the welding head around the pipe sections to 
be welded and whilst this presents little or no problem when gas or arc 
welding, it does present a problem when laser welding since a high power 
laser is both large and delicate and also has trailing power lines and it 
is therefore not desirable to rotate it around a large pipe section. 
Accordingly it is desirable when laser welding pipes to retain the laser 
stationary in a protected location and to provide an optical system 
arranged progressively to rotate the laser beam around the pipes whilst 
focussed on the abutting edges to be welded. The design of such an optical 
system is complicated by the fact that it must be capable of directing the 
laser beam to all locations on the pipes including that opposite to that 
of the laser, that is to say displaced around the pipes by 180.degree., 
and in this position the optical system must deflect the beam around the 
pipes and reverse its direction by 180.degree.. 
It is known that a laser or other light beam may be deflected in one plane 
by a device including a spaced pair of parallel mirrors whose planes are 
transverse to the incident beam by placing one mirror in the beam and 
rotating the other mirror. If such devices were to be used in the optical 
system of a laser welding apparatus for pipes it will be appreciated that 
at least three of them would be required, that is to say six mirrors. A 
proportion of the power is lost each time a laser beam is reflected and 
thus such a system would be associated with a substantial power loss. 
British Patent No. 1500964 discloses an apparatus for laser welding pipe 
sections in which a mirror is rotated through at least part of a 
revolution about the pipe axis to direct the laser onto the surfaces to be 
welded. Most of the constructions disclosed utilise a laser whose beam is 
directed on the pipe axis and when welding long pipe sections this means 
that the laser must of necessity be a considerable distance from the 
surface to be welded. In one construction the laser is situated externally 
of the pipe line with its beam directed perpendicular to the pipe axis and 
four movable mirrors are provided on respective tracks arranged in a 
rectangular configuration about the pipes to be welded. The laser beam is 
directed onto the first mirror which is then moved along its track whilst 
its angle is constantly altered to direct the beam progressively at about 
one quarter of the periphery of the pipes to be welded. When the mirror 
reaches the end of its track its angle is suddenly altered to direct the 
beam to the second mirror which in turn directs it to the pipes to be 
welded and is progressively moved along its track. The process is 
continued until the four mirrors between them have directed the beam over 
the entire periphery of the pipes to be welded. This construction not only 
requires four independently linearly and angularly movable mirrors but 
also produces a weld with four discontinuities, i.e. at the points at 
which the beam was suddenly deflected away from the pipes when a mirror 
reached the end of its track and directed the beam to the next mirror. 
Accordingly it is an object of the present invention to provide an 
apparatus for directing a laser beam for laser welding pipes and the like 
in which the laser beam is not required to pass through or along the pipe 
axis and the laser remains stationary as the laser beam is rotated around 
the pipes by means of an optical system, the optical system using less 
optical elements than most systems for this purpose. 
According to the present invention apparatus for directing a laser beam for 
laser welding pipes or like articles of circular section comprises a first 
mirror which is fixed in position remote from the axis of the articles to 
be welded and on which in use, a laser beam is incident, a second mirror, 
a third mirror, focussing means arranged, in use, to direct the beam onto 
the surfaces to be welded substantially perpendicular to the axis of the 
articles, first drive means arranged to rotate the second mirror about the 
articles to be welded, second drive means arranged to rotate the third 
mirror through at least 360.degree. about the articles to be welded and 
control means arranged to adjust the relative angular orientations of the 
mirrors such that, in use, the beam produced by the laser is reflected 
from the first mirror to the second mirror and thence to the third mirror 
and is substantially focussed by the focussing means onto the surfaces to 
be welded, whereby the focussed laser beam may be moved continuously 
around the periphery of the articles being welded. 
Thus the optical system in the apparatus in accordance with the present 
invention may include only three mirrors which are always so positioned 
and orientated with respect to one another that the laser beam is directed 
onto the surface or surfaces to be welded. The laser may remain stationary 
and when the laser beam impinges on the first mirror it is preferably 
directed transverse to the axis, or is provided with a beam guide to 
direct it transverse to the axis, of the pipes or the like to be welded 
and this fact coupled with the fact that the first mirror is not situated 
on the axis means that the laser may be situated close to the surfaces to 
be welded. The fact that the third mirror may be rotated through at least 
360.degree. about the axis of the articles to be welded means that the 
entire joint may be welded in a single continuous operation without 
producing any welding discontinuities. 
It is preferred that angular orientations of the first and third mirrors 
are adjustable in two e.g. perpendicular planes and the angular 
orientation of the second mirror is adjustable in not more than one plane 
but all three mirrors may be adjustable in two planes. 
It is preferred that the control means is coupled to the first and second 
drive means and ensures that the angular displacement of the second mirror 
from the first mirror is exactly one half the angular displacement of the 
third mirror from the first mirror. It is also preferred that the second 
mirror lies in the plane which passes through the mid-point of the line 
connecting the first and third mirrors and extends perpendicular to the 
said line. Thus in this construction the second mirror is always midway 
between the first and third mirrors as regards angular displacement when 
viewed in the axial direction and there is therefore no need to adjust the 
angle of the second mirror in the plane parallel to the axis of the 
articles to be welded. It is also preferred that the second mirror is so 
disposed that when all three mirrors lie in the same plane the second 
mirror is symmetrically positioned with respect to the first and third 
mirror but offset from the line connecting them. Thus in this construction 
the second mirror is always midway between the first and third mirrors in 
the axial direction and there is therefore no need to adjust the angle of 
the second mirror in the plane perpendicular to the axis of the articles 
to be welded. In the most preferred construction in which both the 
preferred features referred to above are provided the second mirror will 
always of necessity be correctly positionally and angularly located with 
respect to the first and third mirrors and thus the second mirror may be 
angularly fixed with respect to the means carrying it, i.e. the second 
mirror may rotate about the articles to be welded as a solid body and is 
nevertheless always correctly orientated. 
The focussing means may be constituted by one of the first, second and 
third mirrors, in particular the third mirror, but it is preferred that 
the focussing means comprises a separate focussing mirror onto which the 
beam incident, in use, on the third mirror is reflected and which then 
focusses this beam onto or adjacent the surfaces to be welded. 
The preferred construction includes a support member adapted to be 
positioned adjacent e.g. around the articles to be welded, the first 
mirror being connected to the support member, the second and third mirrors 
being carried by respective carriages mounted to move on respective 
circular tracks on the support member. The support member preferably 
comprises a two-part annular enclosure containing the first, second and 
third mirrors, the focussing means and the two circular tracks. Such an 
enclosure may thus be opened and then secured in position around the 
articles to be welded and will protect the optical components of the 
apparatus. 
The annular enclosure preferably includes a welding nozzle which extends 
out of the enclosure, is connected to move with the third mirror and is 
positioned so that, in use, the focussed laser beam passes out through it 
and impinges on the surfaces to be welded. Gas nozzles for inert gas may 
be provided on or adjacent to the welding nozzle to provide a locally 
inert environment around the area being welded. 
A preferred construction also includes a position error sensor associated 
with the welding nozzle arranged to produce an error signal, if, in use, 
the laser beam is incorrectly positioned with respect to the nozzle and to 
move one or more of the mirrors in dependence on the magnitude of the 
error signal.

Referring first to FIGS. 1 and 2, the apparatus includes an annular 
enclosure comprising two halves 2 which are connected together by a hinge 
4 along one longitudinal edge and which may be clamped together by a 
fastener 6 along the other edge. The enclosure has an aperture 8 in its 
outer wall adjacent which is a stationary laser 10 and through which a 
laser beam is conducted from the laser by a conventional optical system 
(not shown). The enclosure also has an annular aperture 12 in its inner 
wall through which a gas shroud nozzle 14 extends, as will be explained in 
more detail below. 
Fixed to one end wall 16 of the enclosure is a two axis servo controlled 
gimbal mounting 18 to which a first mirror 20 is connected in such a 
position that the laser beam entering through the aperture 8 impinges on 
it. Connected to the outer wall of the enclosure is a circular track 22 
which extends around the enclosure and on which a carriage 24 is carried. 
The carriage 24 carries a second mirror 26 fixed with respect to it and is 
connected to a first motor (not shown) to be moved along the track 22 
around the enclosure. Connected to the other end wall 28 of the enclosure 
is a further circular track 30 which extends around the enclosure and on 
which a carriage 32 is carried. The carriage 32 carries a further two axis 
servo controlled gimbal mounting 34 to which a third mirror 36 is 
connected. An arm 38 connected to the carriage 32 carries a focussing 
mirror 40 whilst a further such arm 42 carries the gas shroud nozzle 14 
which is of hollow tubular form and extends out of the enclosure through 
the aperture 12. The carriage 32 is connected to a second motor (not 
shown) to be moved along the track 30 together with the mirrors 36 and 40 
and the gas shroud nozzle 14 around the enclosure. The aperture 12 is 
provided with a brush seal (not shown) through which the nozzle 14 extends 
and may be moved when the carriage 32 moves. 
The two servo motors for the carriages 24 and 32 are provided with linear 
or geared rotary transducers to produce feedback signals and are connected 
to a central controller (not shown) which controls both motors to ensure 
that the angular displacement of the second mirror 26 from the first 
mirror 20 when viewed in the axial direction as in FIG. 2, is always 
exactly half that of the third mirror 36 from the first mirror 20. The two 
gimbal mountings 18 and 34 are also connected to the central controller 
and are moved by the latter after calculating the positional algorithms of 
all the mirrors such that the laser beam is always reflected from the 
first mirror to the second mirror then to the third mirror and then to the 
focussing mirror. 
It will be appreciated that the third mirror is positionally, though not 
angularly, fixed with respect to the focussing mirror and that the 
focussing mirror is aligned with the nozzle 14. Thus, if the laser beam 
impinges on the third mirror it will be reflected through the nozzle to 
focus at a point adjacent the end of the nozzle whose precise position is 
determined by the focal length of the focussing mirror. It will be 
appreciated also that the second mirror is always half way between the 
first and third mirrors as regards angular position, i.e. it is positioned 
to bisect the angle between the first and third mirrors, and that when all 
three mirrors are in the same plane the second mirror is symmetrically 
disposed with respect to the first and third mirrors, i.e. it is half way 
between them in the axial direction but offset with respect to them in a 
direction transverse to the axial direction. As a consequence, it is only 
necessary to adjust the angles of the first and third mirrors and the 
second mirror, which is automatically always orientated with its 
reflecting surface parallel to the tangent at its instantaneous position 
on the track, is automatically always correctly orientated. 
In use, the two halves of the enclosure are opened and placed around two 
pipes 50 which are in butt engagement and to be welded together and the 
two halves are then clamped together. The enclosure is then centred with 
respect to the pipes with the gas shroud nozzle directed at the junction 
of the two pipes. The apparatus is so dimensioned with respect to the 
pipes that the focal point of the focussing lens lies substantially at the 
surfaces of the pipes to be welded and the apparatus is retained in 
position with respect to the pipes by any appropriate means with the 
result that as the third mirror is rotated the focal point rotates around 
the surface of the two mating edges to be welded together. It will be 
appreciated that due to the presence of the pipes it is not possible to 
direct the laser beam around the "back" of the pipes, that is to say if 
the third mirror rotates substantially through the point where it is 
offset from the fixed mirror by 180.degree. the beam will impinge on the 
inner wall of the enclosure, the precise position at which this happens 
being determined primarily by the relative radial positions of the three 
mirrors. Welding is thus commonly begun with the third mirror offset from 
the fixed mirror and the initial entry direction of the laser beam by 
180.degree. or slightly more. The third mirror is then rotated clockwise 
as seen in FIG. 2 on its track with the mirror 40 and gas nozzle moving 
with it whilst the central controller ensures that the second mirror moves 
at precisely one half the rate of the third mirror. The mirrors then move 
through the O.degree. displacement position in which they all lie in the 
same plane and the movement continues in the same sense until the third 
mirror is again 180.degree. or more from the first mirror at which point 
the second mirror is 90.degree. away from the first mirror but displaced 
by 180.degree. from the position shown in FIG. 2. At every position of the 
mirrors the central controller ensures that the mirrors are at the correct 
angle and the laser beam is directed to and focussed on the surfaces to be 
welded. When welding is complete the enclosure is removed from the pipes 
and may be used to weld a fresh pipe to the welded pipeline. 
Whilst welding proceeds, inert gas such as 
14. argon is introduced into the enclosure which flows out through the gas 
shroud nozzle and thus ensures that the actual welding occurs in a locally 
substantially inert atmosphere and that metallic vapour is flushed away 
from the welding zone. 
In the modified embodiment illustrated in FIG. 3, the pipes 50 are shown 
encased in concrete cladding 52 and supported by rollers 54. The support 
for the mirrors comprises a wheeled trolley 55. The first mirror 20 is 
carried by the trolley and positioned so that it is impinged on by a laser 
beam which extends transverse but not perpendicular to the axis of the 
pipes. The orientation of the first and second mirrors is adjustable about 
two axes. The second mirror is connected to a carriage 24 which may be 
rotated about the pipes by a worm drive motor 21 cooperating with a rack 
23. The third mirror 36 is adjustable about two axes and is carried by an 
annular carriage 32 which may be rotated about the pipes by a worm drive 
motor 31 cooperating with a rack 33. The orientation adjustment motors of 
the mirrors 20, 26 and 36 and the motors 21 and 31 are connected to and 
controlled by a central controller comprising a computer 56. 
The mirror 26 is not positioned between the mirrors 20 and 36 in the axial 
direction and in this embodiment it is further from the mirror 36 than is 
the mirror 20 thereby facilitating assembly of the apparatus and access to 
the movable mirrors. The angular displacement of the mirror 26 from the 
fixed mirror 20 need not be exactly half that of the mirror 36, though in 
practice its displacement is generally approximately half that of the 
mirror 36. 
In use, a laser beam is transmitted onto the mirror 20 whose orientation is 
adjusted to direct the beam to the mirror 26 which directs it to the 
mirror 36 which in turn directs it to the surfaces to be welded, as in the 
first embodiment. As the carriage 32 and thus also the mirrors 36 and 40 
and the nozzle 14 are rotated, the computer 56 adjusts the orientations of 
the mirrors 20, 26 and 36 to ensure that the laser beam follows the 
desired path. Whilst the coarse control of the positions of the mirrors is 
effected by the computer in dependence on the position of the nozzle 14, a 
fine control is effected in response to feedback signals indicative of the 
beam position from each mirror which are used to adjust the position of 
the preceding mirror. Gas nozzles (not shown) discharge inert gas adjacent 
the nozzle 14 and provide a locally inert environment around the area 
being welded. In other respects construction and operation of the second 
embodiment are substantially similar to those of the first embodiment. 
It will be appreciated that a great many modifications may be made to the 
constructions described above. In particular, it may be difficult in 
practice to set the servo controlled mirrors at precisely the required 
angles using only the positional algorithms of the second and third 
mirrors and it is therefore preferred that a position error sensor be 
connected to the gas shroud nozzle and arranged to compare the actual 
position of the laser beam within the nozzle with the desired position of 
the beam and to produce a position error signal. This error signal can 
then be fed either directly or indirectly to the first and/or third 
mirrors to adjust their position to ensure that the laser beam is truly 
centrally disposed within the gas shroud nozzle. 
If the pipes to be welded have a truly circular external shape the focal 
point of the focussing lens will always be at the correct positon with 
respect to the pipes once it has been initially set up. However, pipes 
sometimes exhibit a certain degree of eccentricity and to compensate for 
this a proximity sensor may be associated with the free end of the gas 
shroud nozzle to produce a signal indicative of the instanteous distance 
between the surface to be welded and the free end of the gas shroud 
nozzle. This signal may then be used to move the focussing mirror 40 to 
ensure that the laser beam is in fact focussed at the correct spot and it 
will be appreciated that such movement will necessitate a minor angular 
adjustment of the third mirror 36 also. 
Naturally, the gas shroud nozzle may be associated with a wire feed 
apparatus for multiple pass welding or gap filling and the enclosure may 
contain more than one gas shroud nozzle and associated optical system for 
multipass welding. 
Obviously, numerous modifications and variations of the present invention 
are possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.