GB2099539A - Manufacturing pipe - Google Patents

Abstract

In the manufacture of a pipe 2 from helically wound strip 1, it is arranged to impart positive (Figs. 7, not shown), zero or negative residual moment (Fig. 6, 10 not shown) to the finished pipe. By adjusting the position of forming rolls 18, 19, 20 positioned at the apexes of a triangle, the flat strip 1 is continuously bent into a helix of predetermined curvature. Then, the helically formed strip is allowed to relax toward its unbent shape (Figs. 6, 10) if necessary is either expanded from within (e.g. by rolls 41, 52) to achieve negative residual moment or pressed inwardly (e.g. by rolls 37, 55) to obtain positive residual moment. The formed strip is welded at the desired diameter of the finished pipe. <IMAGE>

Description

SPECIFICATION Method and apparatus for manufacturing spiral pipe This invention relates to a method and apparatus for manufacturing spiral pipe by bending a strip of pipe forming material into a helical form and welding together the abutting edges of the bent strip.

Methods of manufacturing spiral pipe fall into three general categories: (1) methods using external holding, (2) methods using internal holding, and (3) methods that use no holding.

In the external holding method, the strip is bent into a helix whose diameter corresponds to that of the intended circular pipe using three sets of triangularly arranged forming rolls. Relaxation of the spirally forming strip is prevented by means of a multitude of stationary external holding rolls which exert pressure on the outside of the pipe, and the abutting edges of the strip are welded together. Thus, welding is carried out with the unwelded bent strip in contact with the holding rolls. Consequently, the pipe is not welded while it is completely free of load. That is, the seam is formed without relieving the elastic strain within the material of the pipe.Accordingly, if a longitudinal slit is later cut in the pipe, the pipe springs back or relaxes in a direction tending to open the pipe because of residual moment (hereinafter called ring opening, the residual moment being defined as positive).

The internal holding method bends strip using similar forming rolls, and then welds together the abutting edges of the bent strip which is held slightly expanded by means of a muititude of stationary internal holding rolls. In this case, a longitudinal cut later made in the welded pipe causes the pipe to relax so that the edges of the slit overlap because of a residual moment ring closing; the residual amount being defined as negative).

The method using no holding rolls bends the strip so that the bent strip possesses the desired outside diameter after allowing for full relaxation back, both inward and outward. With residual moment thus eliminated, the welded pipe does not spring back even if a longitudinal slit is cut therein.

As will be understood, in spiral pipe manufactured by most conventional methods a given amount of residual moment develops in a given direction, despite the intentions of the manufacturer. But no one has heretofore put forward the idea that there might be an advantage in intentionally providing residual moment in the pipe. As the use of spiral pipe becomes more widespread, however, the inventors have noticed that pipe having the wrong residual moment may have defects or fail to give the advantages of pipe having the appropriate residual moment, which is therefore something which it is desirable to be able to control. For example, a positive residual moment in a spiral pipe for sour-gas accelerates the development of stress corrosion cracks. It is therefore desirable to provide pipe in which there is offsetting negative residual moment.Spiral pipe manufactured by the external holding method has much residual stress that develops a positive residual moment. When this type of pipe is used for a pipe line, the residual moment acts in the same direction as the pressure of the liquid carried therein and tends to expand the pipe so that it has a lower than theoretical strength. But if a sheet pile is attached to the spiral pipe which has negative residual moment or which has no residual moment at all, the crosssectional shape of the pipe tends to become warped. If a suitable amount of positive residual moment exists, the same pipe will maintain its original round cross-section. It is therefore desirable to adjust the residual moment (internal elastic strain) so that it is within a suitable range for the intended application.

But conventional manufacturing methods and apparatus are only adjustable with respect to the pipe diameter and have no means for adjusting its residual moment.

A conventional stationary forming apparatus cannot freely adjust the amount or extent of bending.

Therefore, not only is it dificult to provide positive and negative residual moment as desired but also to adjust the amount of either type of residual moment by means of a single manufacturing method and apparatus. To make spiral pipe with positive and negative residual moment as desired requires at least two different types of manufacturing equipment which respectively operate on the external and internal holding principles. Providing two different lines in a limited plant space, however, lowers the rate of equipment utilisation, entails increased capital investment and raises production costs.

An object of this invention is to provide a method and apparatus by which spiral pipe may be made in which the amount and direction of residual moment imparted to the pipe may be freely adjusted, and by which spiral pipe having positive, zero or negative residual moment may be made on a single forming line. And in a preferred feature of the invention the desired residual moment may be maintained under fluctuating material thickness and yield stress by automatic adjustment of the forming conditions in accordance with a change in the thickness and yield stress of the strip of pipe material.

In manufacturing spiral pipe according to the method of this invention, the maximum curvature to which the strip of material should preliminarily be bent in order to develop the desired residual moment is first determined based on the thickness, modulus of longitudinal elasticity (Young's modulus) and yield stress of the strip, the curvature of the pipe to be produced, and the residual moment to be imparted to the pipe. This predetermined curvature is greater, i.e. a smaller radius of curvature, than the curvature of the finished pipe. The flat strip is passed through three rows of forming rolls disposed at the apexes of a triangle, the relative position of the individual rolls being adjusted as required with respect to each other, so that the strip is continuously bent into a spiral form having said maximum curvature.

Next, the spirally formed strip is allowed to relax to or is expanded to the diameter of the finished pipe.

Then, the abutting edges of the thus formed strip are welded together.

According to the method of this invention, as described above, the strip is first bent to a curvature having a radius smaller than that specified for the finished pipe, and then the smaller radius is increased to the final radius. This permits a desired amount and direction of residual moment to be imparted to the pipe by means of a single pipe manufacturing apparatus.

Furthermore, in a preferred feature the ratio of the change in the forming load to the change in the yield stress or the amount of bending is detected while the strip is being bent into spiral form. Using this information, the relative positions of the forming rolls are adjusted when there is a change in the yield stress or thickness of the strip. This enables the desired residual moment to be imparted to the finished pipe with great precision.

The pipe manufacturing apparatus of this invention has bending moment imparting rolls disposed adjacent to the exit end of the three rows of forming rolls. These bending moment imparting rolls keep the curvature of the spirally bent strip the same as that specified for the finished pipe by restraining the springback of, or expanding, the spirally formed strip.

Various embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 a and 1 b schematically illustrate ring opening ratio, Figure 1 a showing the finished pipe, and Figure 1 b showing the pipe after a slit has been cut in it; Figure 2 is a graph showing the relationship between bending moment M (ordinate) and curvature 1/P (abscissa) and illustrates the relationship between curvature during pipe fabrication and the residual moment in the finished pipe; Figure 3 is an end elevation of a pipe forming apparatus according to one embodiment of this invention used with pipe manufacturing equipment of the external holding type; Figure 4 is a side elevation of the apparatus shown in Figure 3;; Figure 5 is a side elevation showing the structure of a stand that supports bending moment imparting rolls in the structure of Figure 4; Figures 6 and 7 are schmatic views illustrating how the bending operation is performed according to the external holding method; Figure 8 is an end elevation of a forming apparatus according to an embodiment of this invention used with pipe manufacturing equipment of the internal holding type; Figures 9 and 10 are schematic views illustrating how the bending operation is performed according to the internal holding method; Figure 11 is an end elevation of a forming apparatus according to an embodiment of this invention used with pipe manufacturing equipment having no holding rolls; Figure 12 is a schematic view illustrating the relative positional relationship of the forming rolls;; Figure 13 is a graph showing the relationship between the position of the forming roll (8$L2) and the ring opening ratio (y); Figure 14 is a diagram illustrating how the residual moment is adjusted by changing the position of the forming rolls; Figure 1 5 is a schematic illustration of a forming apparatus which includes a system that automatically adjusts the amount of bending by detecting a change in the yield stress of the strip being formed; Figure 1 6 is a flow chart showing the arithmetic operations performed by the control computer shpwn in Figure 15; Figure 1 7 is a schematic illustration of a forming apparatus which includes a system that automatically adjusts the amount of bending by detecting the ratio between the changes in the amount of bending and the forming load; and Figure 18 is a flow chart showing the arithmetic operations performed by the control computer shown in Figure 17.

For appropriate adjustment of the amount of bending of a strip of pipe forming material as it travels through bending rolls of a forming apparatus it is necessary to know what effect a change in the yield stress of the strip being formed has on the finished pipe. This information can be obtained from the theory of bending described hereinafter and from pre-manufacturing tests. These tests determine the relationship between the residual moment, yield stress and amount (or reading) of screwdown on the gauge of the forming apparatus and the amount of actual screwdown or screw-up, the same applies throughout this specification and curvature. The results of the tests may be presented in the form of a graph of table. Then if the relevant properties of the strip change, the screwdown given to the forming apparatus can be adjusted by the use of the graph or table, so that the manufactured pipe always possesses the desired residual moment.

THE THEORETICAL BACKGROUND TO RESIDUAL MOMENT IN A FINISED SPIRAL PIPE When a pipe having a residual moment is cut longitudinally a gap y is formed by a ring opening or ring closure of the finished pipe which has a direct relationship to residual moment. The following geometric relations apply to the gap y: 0 &alpha; = 2#o sin - and 2# #p = (2# - #) #0, 2 where pp (Figure 1 a) is the radius of curvature of the finished pipe, and p0 (Figure 1 b) is the radius of curvature of the cut fully relaxed pipe which has a gap in it.

From the above two equations, a ring opening ratio a defined as y r a/Dp may be expressed as follows: = eo sin = Slfl rp Po sin LU it = sin # # ... (1J where w E Pp #0 The change in curvature when a cut pipe relaxes with formation of a gap therein will now be described in terms of the curvature of the pipe and the bending moment of the pipe-forming material.In Figure 2, to form a pipe with a curvature 1/#p and with a positive residual moment, the strip of material is first bent to a point A where the maximum curvature is 1/p. Then the strip is allowed to relax partially to a point B midway towards its unstressed state and its edges are welded together. The finished pipe has a curvature of l/pp and a positive residual moment M1. If a longitudinal slit is cut in this pipe, the slit develops into a gap, releasing the residual moment. The cut pipe has a curvature 1/#0 indicated at point C (ring opening). To make a pipe with curvature l/pp but with a negative residual moment the strip is bent to a maximum curvature At greater than A to make possible imparting negative residual moment M2.In this case, as seen in Figure 2, at a zero residual moment (ie. when relaxation to the unstressed condition is complete) the radius of curvature p0 of the formed pipe is smaller than that of the finished pipe, and the pipe must be expanded to the desired radius pp and then welded up. Then if the pipe is cut, the edges of the pipe along the cut will overlap. Therefore a in Figure 1 becomes negative and, thus y also becomes negative (ring closure). To obtain the finished pipe curvature B' and C', it is therefore necessary to bend the formed strip outwardly from inside. It is apparent that in both cases the strip of material is first bent to a smaller radius of curvature than that of the finished pipe, because (as is evident from Figure 2) the stress release curve of the bent strip slopes downwardly to the left.For pipe with a positive residual moment it intersects the zero stress base line to the left of the desired pipe diameter and for pipe with a negative residual moment it intersects the base line to the right of the desired pipe diameter.

Of course if the maximum curvature is such that the line intersects the base line at the desired pipe diameter, there is a zero residual moment.

The value w is usually about 1. Therefore, y can be expressed as follows by approximation.

y = sinwTf - fl(1 12) to w Assuming that the strip of pipe forming material is perfectly elastic or plastic it follows from the fundamental theory of bending that the stress-strain relation of the strip is expressed by the equation:

where E, g,, and 2t are the Young's modulus, yield stress and thickness of the strip of material respectively.

Under normal forming conditions, the third term of the part of equation (3) in brackets is negligible. Therefore,

If the Young's modulus and yield stress (E and ay) of the strip material are known, and values are selected for the pipe dimensions (2t and pp) and maximum bending curvature 1/pj, then the value of the ring opening ratio y may be determined from equations (2) and (4).

When the residual moment is positive or zero, y > O, (# # 1), then p0 # #p #p1 When the residual moment is negative, y < O, (a) > I). thenp > p0 > p If the residual stress at the outermost surface of the finished pipe is (TRB the ring opening ratio is expressed as follows based on the fundamental theory of bending:

Furthermore, the bending moment M per unit length of pipe at a maximum curvature of 1/p; and the residual moment Mp per unit length of pipe are expressed as follows:

As is apparent from the above-described theory of bending, the strip of material is first bent on a forming apparatus to a radius of curvature p, that is smaller than the radius pp specified for the finished pipe.If this formed strip has fully relaxed to reduce the residual moment to zero, the formed strip has a radius of curvature pO The amount and direction of the residual moment in the finished pipe depends upon how much the radius of curvature pp) of the finished pipe is larger or smaller than the radius p0 at which the pipe is fully relaxed. The formed strip is then welded into the final pipe form while being externally or internally held by bending moment imparting or holding rolls that are adjustable in the direction of the diameter of the pipe being manufactured, thereby adjusting the radius of curvature p to the radius pp of the finished pipe.This not only permits the amount of residual bending moment to be adjusted at will but also provides positive and negative residual moments to be achieved.

It is therefore possible, by the present invention, to provide negative residual moment in the finished pipe, which has so far been impossible when using the external holding method, by first bending the strip to a greater extent than the final product should be and then holding the formed strip from the inside using bending moment imparting rolls. To provide a negative residual moment M2 on the moment-curvature in Figure 2, for example, the bending apparatus bends strip to point A' so that the negative moment M2 is obtained at curvature 1/pup that is specified for the finished pipe. Then, the formed strip is welded into the finished pipe form while the bending moment imparting rolls are pressing the formed strip back to the size of the finished pipe or reducing the curvature to that at point B'.By adjusting the bending carried out by the forming apparatus so as to vary the maximum strip curvature between A and A', the amount of residual moment in the finished pipe can be adjusted within the range M, to M2, and the resulting pipe may have positive, zero or negative residual moment.

On the other hand, a positive residual moment can be provided in the finished pipe by holding the formed strip against relaxation by the use of external stationary holding rolls, particularly by rolls provided immediately after the forming apparatus. The internal holding method has thus far been unable to provide a finished pipe having a positive residual moment. But it is now possible to do this by first bending the material of the strip to a smaller maximum curvature and then holding the formed strip from outside using the bending moment imparting rolls. To provide a positive residual moment M1 in the moment-curvature curve in Figure 2, the forming apparatus bends the strip to point A so that the positive moment M, is obtained at curvature 1/pup that is specified for the finished pipe. Then, the formed strip is welded into the finished pipe form while the bending moment imparting rolls are pressing the formed strip toward the size of the finished pipe, ie. reducing the curvature to that at point B. Meanwhile, a negative residual moment can be provided by holding the formed strip by use of internal stationary holding rolls, particularly those disposed immediately after the forming apparatus.

Thus, the method of this invention permits imparting a freely controlled amount of either positive or negative residual moment to the finished pipe by use of a single forming apparatus. The method obviously can be used to provide zero residual moment.

THE PIPE FORMING APPARATUS Figures 3 and 4 show a pipe manufacturing apparatus of the external holding type to which this invention is applied. As shown, an entry-side external forming stand 12 and an exit-side external forming stand 13 rest on a base 11. An internal forming stand 14 is suspended from a frame 1 5 on the base stand 11 by a support plate 1 7 on a pin 1 6. The internal forming stand 14 is positioned opposite to the entry- and exit-side forming stands 12 and 1 3.

The entry- and exist-side external forming stands 12 and 13 and the internal forming stand 14 respectively have rotatable forming rolls 1 8, 1 9 and 20 thereon. Figure 4 shows the arrangement of the entry-side forming rolls 18. As can be seen, a plurality of entry-side forming rolls 18 are disposed along the length of the pipe to be manufactured, with the axes of the rolls being skew with respect to the axis of the pipe at the same angle as the lead angle of pipe spiral. The exit-side external and internal forming rolls 19 and 20 are also arranged in the same manner as the entry-side forming rolls 1 8. When viewed from the end of the apparatus, these forming rolls 1 8, 1 9 and 20 are disposed at the apexes of a triangle.

As shown in Figure 4, the entry-side forming stand 1 2 comprises a moving table 21 that is placed on the base stand 11 so as to be movable in the direction of the axis of the pipe and a roll support table 22 that is placed on the moving table 21 so as to be movable up and down. The moving table 21 has a saw-tooth-shaped inclined surface 23 that is inclined in the direction of the axis of the pipe. An axial opening 24, extending in the direction of the axis of the pipe, is provided in the rear end of the moving table 21, and a threaded sleeve 25 is fitted in the rear part of the axial opening 24. In the threaded sleeve 25 is fitted a threaded rod 26 that is rotatably supported in a bearing 27. As the screw rod 26, which is connected to a motor 28 through a reduction gear, rotates, the moving table 21 moves back and forth.The roll support table 22 has an inclined surface 29 similar to that on the moving table 21 slidably engaged with surface 23. The roll support table 22 is placed on the moving table 21 so that the surfaces 23 and 29 are held in contact with each other. A groove 31 is cut in the front of end of the inclined surface 29 of the roll support table 22. A pin 32, which is fastened to the frame (not shown) of the base stand 11 fits in the groove 31. When the moving table 21 moves back and forth, accordingly, the pin 32 prevents the roll support table 22 from moving back and forth. Consequently, the roll support table 22 moves up and down instead. On the top of the roll support table 22 is a block 33 on which are rotatably supported the entry-side forming rolls 1 8.

The exit-side forming stand 13 has the same structure as the entry-side forming stand 12 just described. The vertical position of the forming rolls 1 8 and 1 9 is adjusted by moving the moving tables 21 back and forth. When a vertical forming load acts on the moving table 21 during the forming operation, the moving table 21 does not move back and forth because of the friction between the inclined surfaces and because of the threaded sleeve, so that the roll support table 22 does not move up and down at all. Accordingly, the entry-side forming roll 1 8 is held in the preset position. The same is the case with the forming roll 19.

As shown in Figure 3, pillars 35 stand on the base stand 11 and support a forming case 36 that is C-shaped in cross-section and extends in the direction of the axis of the pipe. The forming case 36 encloses a pipe 2 being manufactured so as to cover an area extending between the point where forming of the pipe 2 begins and the point where welding is performed. The forming case 36 carries a plurality of rotatable external holding rolls 37, which are disposed along the periphery of the pipe 2, by means of support members 38 each including a screw mechanism. The screw mechanism of the support members 38 moves the external holding rolls 37 in and out in the direction of the radius of the pipe to adjust the holding positions thereof. The external holding rolls 37 come in contact with the periphery of the pipe 2, and rotate during the spiral motion of the pipe.

A plurality of bending moment imparting rolls 41 are attached to the internal forming stand 14 and are disposed along the axis of the pipe. As shown in Figure 3, the bending moment imparting rolls 41 are positioned between the exit-side forming rolls 19 and the external holding rolls 37, ie. on the exit side in the direction of the circumference of the pipe. It is preferable that the bending moment imparting rolls 41 be positioned as close to the exit-side forming rolls 1 9 as the apparatus design permits. The bending moment imparting rolls 41 are attached to the internal forming stand 14 in the same manner as the entry- and exit-side forming rolls. That is, a base 42 on the internal forming stand supports a moving table 43 that has a saw-tooth-shaped inclined surface 44.A threaded rod 45 rotatingly driven by a motor 46 through a reduction gear moves the moving table 43 back and forth in the direction of the axis of the pipe. A roll support table 48 also has a saw-tooth-shaped inclined surface 49. The base 42 supports the roll support table 48 so that the inclined surface 49 thereof contacts the sliding surface 44 of the moving table 43. As in the case of the entry-side forming stand 1 2 described above, the roll support table 48 moves up and down as the moving table 43 moves back and forth.

A welding torch 50 projects from near the front end (ie. from near the left end in Figure 4) of the internal forming stand 14. The foremost end of the welding torch 50 is directed toward a seam 3 on the inside of the pipe 2. Although not shown, a welding torch to weld the seam on the outside of the pipe is provided near the welding torch 50.

The following paragraph describe the method of manufacturing spiral pipe having the desired residual moment by use of the above-described pipe manufacturing apparatus.

To begin with, all rolls are set in predetermined position. That is, the internal forming rolls 20 are positioned so as to hold the internal surface of the pipe 2 so that the pipe 2 attains the maximum curvature 1/pj at the position of the internal forming roll 20. The external entry- and exit-side forming rolls 1 8 and 19 are positioned so as to hold the external surface of the pipe 2 so that the pipe 2 attains a curvature substantially identical with that (1/pup) of the finished pipe at the position of the external forming roll 19. The external holding rolls 37 or the bending moment imparting rolls 41 are so positioned as to come in contact with the external or internal surfaces of the pipe 2, respectively, which has been formed by the forming rolls 18, 19 and 20 to attain the final curvature 1/pup.

A flat strip is horizontally and continuously fed, as shown in Figure 3, into the pipe manufacturing apparatus the rolls of which are all set in positions as described above. The strip 1 is increasingly bent between the entry-side forming roll 18 and the internal forming roll 20, substantially attaining the maximum curvature 1/pj at the internal forming roll 20. Because of relaxation or springback, the curvature of the strip 1 gradually decreases between the internal forming roll 20 and the exit-side forming roll 1 9, substantially attaining the final curvature 1/pup at the exit-side forming roll 19.

While the final curvature 1/pup is maintained by the action of the bending moment imparting roll 41 and the external holding roll 37, the seam 3 of the spirally formed strip is welded by the welding torch 50.

The way in which negative residual moment is imparted to the finished pipe according to the pipe manufacturing method described above will now be described. As shown in Figure 6, the forming rolls 1 8 and 1 9 are adjusted so that the strip is bent to such a curvature that it develops the desired negative residual moment. The strip thus bent as indicated by a dotted line is then pushed back from inside to the desired pipe diameter, as indicated by the solid lines, using the bending moment imparting roll 41. That is, the strip pre-formed to the curvature ilk,, at point A' in Figure 2 on the forming apparatus is pushed out from inside (C' < B'), using the bending moment imparting roll 41, until the desired finished pipe diameter is established.Then, with the strip in this state, the seam of the strip is welded to complete the manufacture of the spiral pipe. As a consequence, the finished spiral pipe has a negative residual moment M2.

On the other hand, positive residual moment is imparted as follows: the strip is first bent as indicated by the dotted lines in Figure 7, by the use of the forming apparatus, and then pressed inwardly or back by the external holding roll 37 to the desired finished pipe diameter, as indicated by the solid lines. That is, the strip pre-formed at point A in Figure 2, to such an extent that the strip expands to point C upon springback, is held at point B by the external holding roll 37. In this state, the seam of the strip is welded, leaving a positive residual moment M, in the pipe. Therefore, it is possible to impart a desired amount and direction of residual moment within the range of B to B' by setting the maximum bending curvature to that at a point betwen A and A'.

Obviously the strip can be first bent to a point between A and A' so that on springback it expands precisely to the diameter of the finished pipe, at which time it is welded.

Figure 8 is an end view of an embodiment of an internal holding type pipe manufacturing apparatus according to the invention. Parts similar to those in the apparatus shown in Figures 3 and 4 are designated by similar reference numerals, and will not be described again.

A forming apparatus 3, or a so-called three roll bender, which comprises external forming rolls 1 8 and 1 9 and an internal forming roll 20, bends a strip into a spiral pipe 2. In this spiral pipe forming operation, the pipe diameter Dp (=2pod) is maintained by a plurality of internal holding rolls 52, which are each supported on a stand 51 so as to be movable back and forth in the direction of the axis of the pipe.

The stand 51 is a combination of a roll support table and a moving table each having the same sawtooth-shaped inclined surface as described before, the apparatus further has bending moment imparting rolls 55 supported on stand 54. The bending moment imparting rolls 55 are provided behind and on the outside of the exit-side external rolls 1 9 of the three roll bender. The bending moment imparting rolls 55 are moved by a mechanism that utilises the action of a wedge the same as in the apparatus of the external holding type. The internal forming stand 14 and stand 51 are supported by an internal holding roll support table 57 that is positioned on the inside of the pipe 2 being manufactured.

To impart positive residual moment, as shown in Figures 2 and 9, the strip is first pre-formed at point A as indicated by dotted lines, and then pressed inward, as indicated by the solid lines, by the bending moment imparting rolls 55, so as to be ready for welding. The pre-formed strip which will attain a curvature as at the point C in Figure 2 at the time of complete springback, is pressed back to point B in Figure 2, thereby establishing the desired finished pipe diameter, and the seam is welded with the pipe in this state. Consequently, the finished pipe has a positive residual moment M1. For imparting negative residual moment, the strip pre-formed to a curvature indicated by the dotted lines in Figure 10 is expanded from inside, using the internal holding rolls 52, to the desired finished pipe diameter as indicated by solid lines. In other words, the strip bent to a curvature at point A' in Figure 2 is expanded back to point B' by use of internal holding rolls 52, and is welded in that state. This results in a negative residual moment M2. By thus changing the curvature of the strip in the range of curvatures between A and A', the residual moment can be varied within the range of B to B'.

Figure ii is an end view of an embodiment of a pipe manufacturing apparatus which does not use holding rolls according to this invention. As shown, two bending moment imparting rolls 61 and 62 are provided after the forming apparatus respectively lying on the inside and outside of the path of the pipe being manufactured. Residual moment is imparted by using either of or both the internal and external bending moment imparting rolls 61 and 62. Details of the operating method will not be described here since they are similar to those for the external and internal holding types of apparatus described previously.

According to this invention, the desired amount and direction of residual moment can be imparted to the product pipe at will. Furthermore, this can be accomplished on a single pipe manufacturing line, ie. without using different lines. This means that spiral pipe having positive, zero and negative residual moment can be manufactured at will on the same equipment. If desired, in addition, it is also possible to manufacture a single piece of spiral pipe in which the residual moment changes continuously from positive to negative from one end thereof to the other.

ADJUSTING RESIDUAL MOMENT (1) lnitialising the residual moment or ring opening ratio Knowing the Young's modulus and yield stress (E and (7y) of the material of the strip, the pipe dimensions (2t and pup!, the maximum curvature 1/pi to be given to the strip by the forming apparatus to achieve a desired residual moment or ring opening ratio y in the formed pipe is derived from equations (2) and (4) as follows:

Usually, p, is difficult to measure directly.Therefore, the initial value of y can be established, for example, by first determining the relationship between the curvature of the strip directly under the internal forming rolls of the forming apparatus and the position of the rolls in the forming apparatus, and then determining the position of the rolls that corresponds to p, in equation (8). The following paragraphs discuss this point in detail.

As a result of a theoretical analysis of strip deformation using three forming rolls disposed at the apexes of a triangle, the inventors have found that the ring opening ratio (y) in the formed strip can be set by adjusting the relative position of the forming rolls. This finding will be described concretely by reference to Figure 12 which shows the positional relationship between the forming rolls. As seen, there are three forming rolls; an entry-side roll 18, an internal roll 20, and an exit-side roll 1 9 which all have the same diameter 2r. In this figure, re designates the equivalent radius of each forming roll; and re = r + t (where strip thickness = 2t).The figure is drawn as if the forming rolls were in contact with the neutral line N which extends along the mid-thickness of the strip; The entry- and exit-side forming rolls 1 8 and 1 9 are spaced the same horizontal distance L from the internal forming roll 20. The vertical position of the entry- and exit-side forming rolls 1 8 and 19 is expressed by the distances at and 82 between a horizontal line H which is a tangent to the periphery of the internal forming roll 20 at the lowermost point thereof, and the periphery of each roll 1 8 and 1 9 at the highest point thereof.If the mean screwdown or screw-up distance given to the forming rolls is expressed as Sm = (, + 87)/2, the values 8m/L2 depends upon the longitudinal elastic modulus E and yield stress ay of the material of the strip, and the radius pp, wall thickness 2t and ring opening ratio y in the formed strip.

Figure 13 shows graphically the relationship between y and Am/L2 that has been empirically proven. The experiment which proved the relationship was carried out under the conditions that the pipe radius pp = 400 mm, strip thickness 2t = 9 mm, the forming roll radius Rr = 40 mm, and the yield stress at = 30 kg/mm2. As seen, the residual moment changes from positive to negative as the value of Sm/L2 increases.

Generally, the radius r of the forming rolls and the distance L therebetween are fixed in the forming apparatus, so the only value that can be varied, with respect to the positional relationship among the forming rolls, during the forming operation is the mean screwdown position dm described before. The mean screwdown distance bm can be adjusted by changing the distance S, of the entry-side forming roll 1 8 above line H by changing its screwdown position and the distance 82 of the exit-side forming roll 1 9 above line H by changing its screwdown position.The inventors have further found that satisfactory control of residual moment can be achieved if 82 is fixed at a suitable value and only 8i is changed, as described below.

Ideally, the exit-side forming roll 1 9 should be held in contact with the periphery of the strip being bent to the final curvature pp, as shown in Figure 1 2. The figure is drawn with an exit-side forming roll of equivalent radius re in contact with the centre line N of the strip at point P which is a smooth convex curve through rolls 1 8, 20, 1 9 from entry to exit-side thereof, and it is important that such a geometrical relationship should be achieved. If, for example, the exit-side forming roll 1 9 were positioned above the centre line N shown (ie. when 82 is greater than shown), that part of the strip in the vicinity of the contact point P is formed with reverse curvature which involves a waste of forming energy and makes residual moment adjustment impossible. If, conversely, the exit-side forming roll 19 is positioned too low (ie. when 82 is smaller than shown), the strip is spaced from the forming roll 19, whereby the strip fails to be bent to the desired curvature.

By reference to Figure 2, the appropriate position or optimum screwdown position to set distance 2 of the exit-side forming roll 1 9 above line H is determined as follows: If the effective bending length at the centre of strip thickness is I and the inclination angle of bending is 0, I and ocean be expressed as follows from the geometric relationship:: I = pp sinO (9) L= I + resins (10) From equations (9) and (10), L = (pp + r0) sinO (11) The effective screwdown position (or the roll height at contact point P) b* is expressed as S re (1cos0ì cos#) = #p(1 - cos#) (12) From this #2 = (#p + re) (1-cos#) (13) From equations (11) and (13)

From equation (14),

From equation (15) it is apparent that the screwdown position of the exit-side forming roll 19 should be preset to a fixed position at a distance #2 above line H.

As stated previously, there exists a certain relationship between âm/L2 and the ring opening ratio y in the formed strip or the residual moment in the product pipe. It is therefore possible to adjust the residual moment by varying the mean screwdown distance am = (8 + #2)/2 of the forming rolls. If the screwdown distance #2 of the exit-side forming roll 19 is established by equation (15) the residual moment can be adjusted by varying the screwdown distance #1 of the entry-side forming roll 18 alone.

(2) Dealing with disturbances (or changes in #y and 2t) The following paragraphs describe how the residual moment Mp can be kept constant when a disturbance occurs for example due to a change in the yield stress ay or the thickness of the strip being formed.

Because of elastic deformation of the screwdown mechanism, the housing, etc. of the forming apparatus, the apparent screwdown distance 5m and the real screwdown distance #m of the forming rolls do not agree with each other. The relationship between the two can be expressed as follows: Q= K (Sm-Am) (16) where Q is the bending load, and K is the spring constant of the forming stand.

The inventors have empirically found that the bending force Q can be considered to be substantially the function of a 2t and Xm. Therefore, Q=(ar2tm) (17) Figure 14, which will be described below is a graph that qualitatively shows the relationship O = #(#y, 2t, #m) Since S a 1/p approximately, fFm is the function of P1 Namely, Sm # # (#i) (18) From equations (16) to (18), 1 Sm = g (ay, 25 (Nm) + tP (Pi) K The way in which to adjust the residual moment will be described by assuming that ay changes to a,, during forming.

To keep the product pipe diameter 2pp and residual moment y constant, pj must be changed to p" as ay changes to av according to the relationship expressed by equation (8).

Namely, the screwdown distance of the forming rolls should be changed from Sm to Sm, according to equations (19), as follows: 5m = K sly, , 2t, Smi) + 'V(,) ... (20) Figure 14 graphically illustrates the above-described calculation. A group of straight lines Sm, Sm and 5,m" represent the elastic characteristics of the forming apparatus, and express equation 1 6, using Sm as a parameter.A group of curves Q, Q' and Q", representing the forming load, express equation 17, using yield stress zv as a parameter. Actually, the strip thickness 2t is also a variable in equation 17. But here 2t is considered to be constant. The straight line M or &gamma; indicates the desired residual moment. If the desired residual moment Mp is given, the straight line A for example, in Figure 2 can be approximately expressed by the straight line Mp in Figure 14, since Mp Q, I and, approximately, S a 1/p.

In Figure 14, point a indicates that a pipe having a residual moment Mp can be obtained by forming a strip that has a yield stress cry with a screwdown distance Sm. If the yield stress changes to av" the forming load changes, according to equation 17, to produce a load curve 0'. To obtain the desired residual moment, forming should be effected with a load at point b where the straight line M and the curve 0' intersect. At this time, the apparent screwdown distance is expressed as 5m' by the straight line that passes through the intersection point b.

When the strip thickness 2t changes, the desired residual moment can be imparted to the finished pipe in the same manner as described above.

The following paragraphs describe a method of automatically controlling the amount of strip bending in order to impart the desired residual moment to the finished pipe when the yield stress a, and/or thickness 2t of the material strip changes.

Figure 15 shows schematically a forming apparatus incorporating a control device. Before being bent, a strip 1 is flattened by a roller leveller 65. In the roller leveller, there exists a functional relationship between the load W on the rollers 66 of the roller leveller 65 and the yield stress ay of the strip 1. Therefore, if this function, ay = f(W), is empirically determined, the yield stress ay can be determined by measuring the load W. The load W is continuously measured on-line by means of a load meter 67 utilising a load cell or the like. The measured load W is inputted in a control computer 68 where the yield stress ay is determined from the function f(W).A thickness gauge 69 is provided along the path of movement of the strip for measuring the thickness 2t of the strip, and the measured thickness is inputted to the control computer 68.

The control computer 68 is programmed to compute according to two functions 0 = 5 (a,, 2t, azm) and Q = K (5m - m) as shown in Figure 14. The computer 68 calculates the screwdown distance 5m for the forming apparatus from these functions in accordance with the changes in the yield stress ay and/or thickness 2t of the material strip, following the same procedure as described previously.

Figure 16 is a flow chart showing the arithmetic operations performed by the control computer 68. The desired pipe diameter Dp and residual moment Mp are preset in the computer 68. Then, the yield stress ay and thickness 2t of material strip are measured, and supplied as initial values ay and 2to in the computer 68. Based on these initial values, the screwdown distance Sm is determined by0the calculation shown in Figure 14 and then, set in the computer 68.

When the above setting has been completed, the manufacturing of pipe is started and the values of ay and 2t are measured. If the measured values a1 and 2t1 are equal to the initial values of #y0 and 2to respectively, the pipe manufacturing is continued with the initially calculated screwdown distance Sm. If the measured and initial values are not equal, the screwdown distance Sm is corrected and, at the time, ay1 and 2t1 are set in the computer 68 as new initial values.

The thus determined screwdown distance Sm is inputted to a screwdown device 70. The time lag due to the distance between the forming apparatus and the roller leveller 65 is corrected by using the travel speed V of the strip 1 that is detected by a speed meter 72 coupled to a pinch roller 71. That is, when the part of the strip 1 where the load W of the roller leveller 65 is measured reaches the forming apparatus, the screwdown distance Sm required for that part is set in the forming apparatus.

The real screwdown Sm of the forming apparatus is detected by a screwdown detector 73 that detects the position of the roll or roll shaft. The screwdown âm thus detected is fed back to the computer 68.

Figure 17 is a schematic view of a forming apparatus incorporating a control device operating on another principle.

This device detects the load Q and real screwdown atm of the forming roll 20 using a load meter 75 and a screwdown detector 76, respectively. The detected values Q and åm are inputted to a control computer 77. The control computer 77 is programmed for the function 0 = ((wyw 2t, #m), the aimed-for value l\Q/hatm = k, and Mp shown in Figure 14. As mentioned previously, the line Mp in Figure 14 is a straight line. Therefore the real screwdown distance Am is controlled by controlling the apparent screwdown distance Sm so that AQ!Am is at all times a constant value k.

Let is be assumed, for example, that the yield stress of the strip changes from ay to a,, when forming is being carried out at point a on the forming load curve Q in Figure 14. Because of this change, the forming load curve changes from 0 to O'. Therefore, if the forming operation is continued while maintaining the screwdown position of the forming rolls at the same distance Sm, the forming condition changes from point a to point c. At this time, the ratio of the change in the forming load Q to the change in the real screwdown Am becomes AQ'/åS'm, which is wide of the aimed-for value.As a consequence, the moment in the finished pipe becomes M'p, instead of desired Mp. Therefore the computer 77 issues a command to change the screwdown distance from Sm to Slum, whereupon point c, which indicates the forming condition, moves along the curve 0' until point b is reached. At this time, the ratio SQ/ASm becomes equal to the aimed-for value k, so that the desired residual moment Mp is imparted to the product pipe.

Figure 1 8 is a flow chart showing the arithmetic operation of the control computer 77. The desired pipe diameter Dp and residual moment Mp are preset in the computer 77. Also, the preliminary measured yield stress ay and the thickness 2t of the strip are supplied as initial values. Based on these inputs, the screwdown distance Sm is determined by the calculation as shown in Figure 14, and the obtained value is set out in the computer 77.

When the above setting has been completed, the manufacturing of pipe is started, and the forming load and screwdown distance are measured, as initial values QO and 8o. While continuing the manufacturing operation, the forming load Q1 and screwdown position 8i are measured. If Q1 equals QO, the operation is continued. If Q1 is not equal to QO, the ratio AO/A is determined. If, then, AQ/hâ equals k, the operation is continued. If AQ!hâ is not equal to k, the screwdown distance Sm is adjusted by screwdown device 78 so that AQ!hao becomes equal to k.

This invention is by no means limited to the embodiments described above. For example, the forming rolls in the foregoing embodiments are of the split type; in other words, a plurality of rolls are used for performing the forming function. But a single forming roll can serve the same purpose. Also, the table having the saw-tooth-shaped inclined surface, which is used for adjusting the position of the forming roll, can be replaced with a threaded or hydraulic screwdown device.

Claims (20)

1. A method for manufacturing spiral pipe which comprises continuously advancing and forming a flat strip of pipe-forming material into a spiral which has a predetermined smaller radius of curvature than the finished pipe, expanding the spirally formed strip or allowing the spirally formed strip to relax to the radius of curvature of the finished pipe and continuously seam welding the spirally formed strip into a pipe of a predetermined residual moment.

2. A method according to Claim 1 , wherein the flat strip is formed into a spiral which if the bending force were released would relax to an unstressed bent form of radius of curvature larger than the radius of curvature of the finished pipe, a force is exerted on the exterior of the strip subsequent to the roll forming means in the direction of travel of the strip to restrain relaxation of the strip to the radius of curvature of the finished pipe.

3. A method according to Claim 1 wherein the flat strip is formed into a spiral which if the bending force were released would relax to an unstressed bent form of radius of curvature smaller than the radius of curvature of the finished pipe, and the strip is allowed to relax to its unstressed bentform and expanded by a force exerted on the strip from its interior subsequent to the roll forming means in the direction of movement of the strip so that it expands to the curvature of the finished pipe.

4. A method according to Claim 1, wherein the flat strip is formed into a spiral which relaxes, when the bending force is released, to an unstressed bent form whose radius of curvature equals that of the finished pipe.

5. A method according to any preceding Claim wherein the flat strip of pipe-forming material is formed into a spiral as it advances longitudinally through a forming apparatus having three forming roll means positioned along the circumference of the pipe being manufactured and disposed at the apexes of a triangle so that one forming roll means is inside the circumference and presses outwardly against the inner surface of the strip and the other two forming roll means are respectively ahead of and behind the inside roll forming means in the direction of movement of the strip and press inwardly against the outer surface of the strip so that the strip is continuously formed into a spiral which has a smaller radius of curvature than the finished pipe

6.A method as claimed in any preceding claim, in which said strip is formed into a spiral of maximum curvature 1/p; expressed as:

where ay = yield stress of the material strip multiplied by a predetermined coefficient E = Young's modulus of the material of the strip 2t = thickness of the material strip = = radius of the finished pipe and y is derived from 2 Et3 y Mp= > 3pp 7r+y where Mp is the desired residual moment.

7. A method as claimed in Claim 5 or 6 which comprises maintaining the internal roll forming means and the forming roll means located behind it in the direction of movement of the strip in predetermined positions and adjusting the position of the forming roll means ahead of the internal roll means to give the required maximum curvature to the strip being formed.

8. A method as claimed in any preceding claim, further comprising measuring the yield stress and the thickness of the strip being formed, and adjusting the position of the forming rolls based on the values thus determined.

9. A method as claimed in any of Claims 1 to 7, further comprising measuring one of the characteristics of the strip being formed from the characteristics of yield stress and thickness.

10. A method as claimed in Claim 8 or 9, in which the yield stress of the strip is determined by providing a roller leveller ahead of the forming roll means, measuring the levelling load applied by said roller leveller, and converting the measured load into yield stress.

ii. A method as claimed in Claim 5, further comprising measuring the bending load applied to the strip by the forming roll means and the position of the forming roll means relative to the path of movement of the strip during the forming step, and adjusting the position of the forming roll means so that the ratio of change in position of the forming roll means to change in the bending load is such that the desired residual moment is obtained.

12. Apparatus for making spiral pipe from flat strip which comprises: a continuous forming apparatus having three forming roll means disposed at the apexes of a triangle, positioned along the circumference of the pipe being manufactured and arranged to bend flat strip into spiral form, one of said roll means being an internal forming roll arranged to contact the inner surface of the strip and pressing it radially outwards and the other two of said forming roll means being external forming rolls respectively located ahead of and behind said internal forming roll in the direction of movement of the strip and arranged to contact the outer surface of the strip and press it radially inwards;; a welding apparatus provided behind said forming apparatus in the direction of movement of the strip and along the circumference of the pipe being manufactured arranged continuously to weld the seam of the spirally formed strip; and radially moveable bending moment imparting roll means located adjacent to and behind the forming apparatus in the direction of movement of the strip and adjustable to engage the strip and hold it in a position as it moves past said bending moment supporting roll means such that the diameter of the spirally bent strip equals the desired diameter of the finished pipe.

13. Apparatus as claimed in Claim 12, in which said other two external forming roll means are adjustably moveable toward said internal forming roll means, and further comprising means for sensing the characteristics of the strip being bent and means for adjusting the position of at least one of the external forming roll means in response to the sensed characteristics for applying the desired bending force to the strip.

14. An apparatus as claimed in Claim 13, in which said only one of the two external roll forming means is provided with position adjustment means arranged to respond to said sensing means.

15. An apparatus as claimed in any of Claims 12 to 14, in which said bending moment imparting roll means comprises an outer bending moment imparting roll means mounted on said apparatus for movement toward and away from the path of movement of the strip and on the outside of the circumference of the pipe being manufactured.

16. An apparatus as claimed in any of Claims 12 to 14, in which said bending moment imparting roll means comprises an inner bending moment imparting roll means mounted on said apparatus for movement toward and away from the path of movement of the strip and on the inside of the circumference of the pipe being manufactured.

1 7. An apparatus as claimed in any of Claims 12 to 14, in which said bending moment imparting roll means comprises an outer bending moment imparting roll means mounted on said apparatus for movement toward and away from the path of movement of the strip and on the outside of the circumference of the pipe being manufactured, and an inner bending moment imparting roll means mounted on said apparatus for movement toward and away from the path of movement of the strip and on the inside of the circumference of the pipe being manufactured.

1 8. Apparatus for making spiral pipe having a predetermined residual moment from flat strip substantially as hereinbefore described with reference to and as illustrated in Figures 3, 4 and 5 or Figures 9 and 10 or Figure ii of the accompanying drawings.

19. Apparatus according to Claim 1, further comprising an automatic control system constructed and adapted to operate substantially as hereinbefore described with reference to and as illustrated in Figures 15 and 16 or Figures 17 and 18 of the accompanying drawings.

20. In a method of manufacturing spiral pipe using a forming apparatus having three forming roll means at the apexes of a triangle and positioned along the circumference of the pipe being manufactured and in which method a flat strip of pipe material is fed in the longitudinal direction thereof through said three roll means and is continuously bent into spiral form by pressing one of the forming roll means which is inside the circumference against the inner surface of the strip toward the outside of the pipe and pressing the other two forming roll means which are respectively ahead of and behind said inside forming roll means in the direction of movement of the strip against the outer surface of the strip toward the inside of the pipe, and the seam of the spirally formed strip is continuously welded, the improvement which comprises the steps of:: determining, based on the thickness of the strip and the yield stress of the material of the strip and the curvature of the finished pipe, a maximum curvature of the strip which has before springback a smaller radius of curvature than the radius of curvature of the finished pipe and which is sufficiently great, when it is desired that the finished pipe have a positive residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a radius of curvature which is larger than the radius of curvature of the finished pipe, and when it is desired that the finished pipe have a zero residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a radius of curvature which is larger than the radius of curvature of the finished pipe, and when it is desired.that the finished pipe have a zero residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a curvature which is equal to the curvature of the finished pipe, and when it is desired that the finished pipe have a negative residual moment, to cause the pipe, when the force causing such bending is released to spring back to an unstressed bent form with a radius of curvature which is smaller than the radius of curvature of the finished Pine; continuously forming the flat strip into a spiral having said maximum curvature by adjusting the positions of said forming roll means in a direction transverse to the thickness of the strip; and, prior to the welding step, when it is desired to impart a positive residual moment to the finished pipe, exerting a force on the exterior of the strip subsequent to the forming roll means in a direction of movement of the strip for restraining the springback of the strip to the curvature corresponding to the curvature of the finished pipe, and when it is desired to impart a negative residual moment to the finished pipe, allowing the pipe to spring back to the unstressed bent form and then exerting a force on the strip from the interior of the strip subsequent to the forming roll means in the direction of movement of the strip for expanding the strip to a curvature corresponding to the curvature of the finished pipe.