Field of the invention

The present invention relates to a method of manufacture of a reinforced pipe sector.

Background

The offshore floating wind industry is growing. New wind turbines designs are being developed with increasing size. Thus, new buoyancy tanks that are both light weight and strong have to be provided in an efficient manner in order to produce floating structures having enough buoyancy at a lowest possible cost.

The present invention relates to cylinder fabrication methods, more particularly to methods for fabricating reinforced pipe sectors which may be used as buoyancy members of an offshore floating wind turbine structure or other large diameter thin shell structure. One solution so far has been to reduce the wall thickness to reduce the steel weight and thereby the costs of floating offshore structures.

Conventional techniques for fabrication of ring stiffened structures comprise bending a metal plate to form a structure, such as a pipe, and thereafter welding ring elements on the inside of the pipe. For structures, such as pipes, with relatively large diameter to wall thickness ratios, this method has been fraught with problems pertaining to buckling during the bending of the metal plate due to the plate’s own weight. It is also expensive to weld in the ring stiffening elements after the plate has been bent as this will pose an extra operation including extra handling during the manufacturing of the stiffened shells. One solution to this has been to design for flat plates buoyancy members instead of circular cylinders. Flat plated structures can easily be stiffened with T-beams by welding the T-beams to the hull plate when the plate is in the flat position on the floor. This is a well-known manufacturing method in the shipbuilding industry. Flat plated stiffened panels are therefore less costly to produce per kg steel than circular stiffened panels. However, flat panel buoyancy members will have higher hydrodynamic excitation forces and thus more steel is needed for the same wind turbine carrying floating foundation.

Therefore, new methods of production have been developed in order to alleviate the problems of the existing methods for fabrication of stiffened thin-walled shells. SUMMARY OF THE INVENTION

The present invention is defined by the appended claims and in the following:

In a first aspect, the invention relates to a method of manufacture for a reinforced pipe sector including the steps of, a) providing a first metal plate, having a thickness t, b) providing at least a first metal stiffening element and placing the at least a first metal stiffening element on the first metal plate; c) bending the first metal plate together with the at least a first metal stiffening element to form a pipe sector, and; d) welding the at least a first metal stiffening element to the first metal plate wherein steps c and d can be performed in any order.

In an embodiment of the method, the pipe sector has a bending diameter D, the ratio D/t may be at least 100, at least 150, at least 200, at least 250, at least 300 or at least 500.

The skilled person will understand that the bending diameter is the diameter of the circle along which the metal plate is bent.

In an embodiment, the ratio D/t may be comprised between 100 and 1500; preferably between 200 and 800.

In an embodiment, the order of the steps in the method is a), then b), then d) and then c). In other words, the at least a first metal stiffening element is first welded to the first metal plate, forming a reinforced metal plate, and then bending the reinforced metal plate to form a reinforced pipe sector.

In an embodiment, the order of the steps in the method is a), then b), then c) and then d). In other words, the first metal plate together with the at least a first metal stiffening element are first bent forming a pipe sector, and then the at least a first metal stiffening element is welded to the pipe sector forming a reinforced pipe sector.

In an embodiment, the welding step d) is done after step c) and within 360° of the bending line, i.e. the line on the metal plate along which the metal plate is bent; and the angle is defined between a radius of the circle along which the metal plate is bent starting from the bending line, and a second radius of the circle along which the metal plate is bent starting from the welding point.

In an embodiment, the welding step d) is done after step c) and within 270°, within 180°, within 135°, within 90°, within 60°, within 45° or within 30° of the bending line

In an embodiment, the thickness t may be 50mm or less, 40 mm or less, 30 mm or less, 25 mm or less, 20 mm or less, 15 mm or less, or 10 mm or less.

In an embodiment, the thickness t may be comprised between 10 mm and 500 mm, preferably between 15 mm and 200 mm.

In an embodiment, the diameter D may be comprised between 5 m and 50m; preferably between 8 m and 30 m.

In an embodiment the width of the metal plate is 10 times, 20 times, 50 times, 100 times, 500 times, 1000 times the width of the first metal stiffening element.

In an embodiment, the at least a first metal stiffening element is placed on the first metal plate in the bending direction. For example, if the at least a first metal stiffening element is a beam, it is placed on the metal plate perpendicularly to the axis of rotation around which the metal plate and the at least a first metal stiffening element are bent.

In an embodiment, the welding step b) may comprise welding at least two, at least three, at least four, at least five or at least ten first metal stiffening elements on the first metal plate.

In an embodiment the welding step b) may comprise welding at least two first metal stiffening elements on the first metal plate and the at least two first metal stiffening elements are parallel to each other.

In an embodiment, the at least two first metal stiffening elements are parallel, and the distance between the at least two first metal stiffening elements is between 1/50 and 1/4 of said pipe diameter D, preferably between 1/30 and 1/6 of said pipe diameter D.

In an embodiment, the method may further comprise the step of e) welding at least a second metal stiffening element on the first metal plate, at an angle to the at least a first metal stiffening element.

In an embodiment, the at least a second metal stiffening element is perpendicular to the at least a first metal stiffening element. In an embodiment, the welding step d) may comprise welding at least two, at least three, at least four, at least five or at least ten second metal stiffening elements on the first metal plate.

In an embodiment, the pipe sector is a full pipe, a half pipe, or a quarter pipe.

In an embodiment the pipe sector is a cylinder or a tapered cylinder having a substantially circular or oval base.

In an embodiment, the pipe sector is a helix, or a tapered helix having a significantly circular or oval base.

In an embodiment, the method may further comprise the step of f) bending at least an end portion of the first metal plate between step a) and b) to the substantially same curvature as the rest of the pipe sector, and cutting at least an end portion of the at least a first stiffening element, such that the end portion of the at least a first stiffening element has substantially the same curvature as the end portion of the first metal plate.

In an embodiment, the method may further comprise the step of f) bending both end portion of the first metal plate between step a) and b) to the substantially same curvature as the rest of the pipe sector, and cutting both end portion of the at least a first stiffening element, such that the end portion of the at least a first stiffening element has substantially the same curvature as the end portion of the first metal plate.

Here, the skilled person will understand that the end portions of a metal plate or of the at least a first stiffening element are the portions of the metal plate or of the at least a first stiffening element that are close to the edges of the plate that will come into contact with each other when the cylinder is formed.

In an embodiment, the first metal plate may have a yield strength of at least 200MPa, at least 250MPa, at least 300MPa or at least 400MPa.

In an embodiment, the first metal plate may be made of steel, a steel alloy, aluminium or an aluminium alloy.

In an embodiment, the at least a first stiffening element may be a T-beam or a U- beam.

In an embodiment, the at least a first stiffening element may be made of steel, a steel alloy, aluminium or an aluminium alloy. In an embodiment, the at least a second stiffening element may be made of steel, a steel alloy, aluminium or an aluminium alloy.

In an embodiment, the at least a first stiffening element and the first metal plate are made of the same material.

In an embodiment, the at least a second stiffening element and the first metal plate are made of the same material.

In an embodiment the ratio of the structural capacity of the reinforced pipe sector to the structural capacity of the first metal plate may be at least 5, preferably at least 10.

In an embodiment, the at least a first stiffening element may be oriented on the first metal plate in a direction so that when the cylinder is formed the at least a first stiffening element forms a full ring, or substantially a full ring if a small gap is left for final welding at a later stage, along the internal periphery of the cylinder.

In an embodiment, at least a second stiffening element is welded in an orthogonal direction to the at least a first stiffening element.

In an embodiment, the method further comprises welding a second metal plate on the at least a first metal stiffening element, wherein the second metal plate is parallel to the first metal plate.

In an embodiment, the second metal plate may be made of steel, a steel alloy, aluminium or an aluminium alloy.

In a second aspect, the application describes to a reinforced pipe, comprising a first metal pipe, having a thickness t, and a base of diameter D, at least a first metal stiffening element welded on the inside surface of the reinforced pipe, wherein the ratio D/t is over 100.

In an embodiment of the second aspect, the ratio D/t may be between 100 and 1500, preferably between 200 and 800.

In a third aspect, the application describes to a bending machine for making a reinforced pipe sector, the reinforced pipe sector comprising a metal pipe sector, having a thickness t and a base of diameter D, and at least a first metal stiffening element welded on the inside surface along a circumference of the metal pipe sector, wherein the bending machine comprises one inner roller and two outer rollers, wherein the outer surface of the inner roller or of the outer rollers comprises at least a first groove, to accommodate the at least a first metal stiffening element.

In an embodiment of the third aspect, the ratio D/t is at least 100.

In an embodiment of the third aspect, the at least a groove has a depth that is at least equal to the height of the at least a first metal stiffening element.

In an embodiment of the third aspect, the at least a groove may have a depth that is substantially equal to the height of the at least a first metal stiffening element.

In an embodiment, the bending machine may further comprise a means for welding the at least a first metal stiffening element to the metal pipe sector, such as a welding arm for automatic welding.

In an embodiment, the bending machine may further comprise at least two spacers. Spacers are objects that may be placed on each side of the at least a first stiffening element and between the first metal plate and the inner roller.

In an embodiment, each spacer may comprise a vertical acting spacer roller. A vertical acting spacer roller transmits the pressure of the inner roller to the first metal plate

In an embodiment, each spacer may comprise at least a lateral acting spacer roller. The lateral acting spacer roller are placed on each side of the at least a first stiffening element, preferably in contact with the at least a first stiffening element, applying a sufficient pressure on the at least a first stiffening element to prevent the at least a first stiffening element from buckling.

In an embodiment, each spacer may comprise two lateral acting spacer rollers.

In an embodiment, each spacer may be freestanding. That is, each spacer need not be attached to another structure.

In an embodiment, the bending machine may further comprise a support roller. A support roller is a roller that supports the formed reinforced pipe, to further help avoid buckling or bending of the helix, pipe or reinforced pipe under its own weight.

In an embodiment, the support roller may be adjustable in height and position. This allows advantageously of modifying the orientation of the longitudinal axis of the reinforced pipe during production of the reinforced pipe/ during operation of the bending machine. In an embodiment, the orientation of the inner and outer rollers may be adjustable relative to the longitudinal axis of the reinforced pipe during operation of the bending machine. This allows advantageously of modifying the orientation of the longitudinal axis of the reinforced pipe during production.

In an embodiment, the first groove of the inner roller of the bending machine may be a helicoidal groove.

In an embodiment, the inner roller may comprise a series of inner rollers and lateral rollers arranged on an inner roller support arm or the outer rollers comprise a series of outer rollers and lateral rollers arranged on an outer roller support arm.

In a fourth aspect, the application describes to a method of manufacture for a joined reinforced pipe sector comprising the steps of, a) providing a first metal plate, having a thickness t; b) bending the first metal plate forming a first pipe sector of at most 340°; c) providing a pre-manufactured reinforced pipe sector, aligning the premanufactured reinforced pipe and the first pipe sector along their longitudinal axis and welding the first pipe sector to the pre-manufactured reinforced pipe sector along their circumference; d) providing a pre-manufactured at least a first metal stiffening element; e) positioning the pre-manufactured at least a first metal stiffening element along the internal or external periphery of the first pipe sector and welding the pre-manufactured at least a first metal stiffening element to the first pipe sector forming a joined reinforced joined pipe sector.

In an embodiment of the method according to the fourth aspect, the premanufactured at least a first metal stiffening element forms a full circle or substantially a full circle.

Here the skilled person will understand that “a first metal stiffening element forming full circle or substantially a full circle” means that the stiffening element forms a circle or substantially a circle in cross section.

In an embodiment of the method according to the fourth aspect, the premanufactured at least a first metal stiffening element is positioned along the internal periphery of the first pipe sector.

In an embodiment of the method according to the fourth aspect, the premanufactured at least a first metal stiffening element is positioned along the external periphery of the first pipe sector. In an embodiment of the method according to the fourth aspect, the premanufactured reinforced pipe sector is a pipe (i.e. a pipe sector of 360°), and the method may further comprise the step of f) further bending the first metal plate forming a first pipe sector of substantially 360°.

Here the skilled person will understand that the first pipe sector and the premanufactured reinforced pipe sector have substantially the same diameter and are aligned so that the longitudinal axis of the first pipe sector and the premanufactured reinforced pipe sector are aligned and placed next to each other. The first pipe sector and the pre-manufactured reinforced pipe sector are welded along at the contact point, in other word a circumference of the base of the pipe sector.

In an embodiment the pre-manufactured reinforced pipe sector is obtained according to the first aspect of the invention.

In an embodiment of the method according to the fourth aspect, the joined reinforced pipe sector and the first pipe sector have a bending diameter D, the ratio D/t may be at least 100, at least 150, at least 200, at least 250, at least 300 or at least 500.

The skilled person will understand that the bending diameter is the diameter of the circle along which the metal plate is bent.

In an embodiment, the ratio D/t may be comprised between 100 and 1500; preferably between 200 and 800.

Short description of the drawings

In the following description this invention will be further explained by way of exemplary embodiments shown in the drawings:

Fig. la is a lateral view of a first embodiment of a series of first metal stiffening elements welded to a first metal plate.

Fig. lb is a perspective view of a first embodiment of a series of first metal stiffening elements welded to a first metal plate.

Fig. 2a is a lateral view of a second embodiment of a series of first metal stiffening elements welded to a first metal plate and welded to a second metal plate.

Fig. 2b is a perspective view of a second embodiment of a series of first metal stiffening elements welded to a first metal plate and welded to a second metal plate.

Fig.3 is a lateral view of a first metal plate and first metal stiffening elements during bending and the subsequent welding at an angle a.

Fig. 4 is a perspective view of a series of first metal stiffening elements welded a first metal plate and bent into a helix.

Fig. 5 is a lateral view of an embodiment of a reinforced metal plate being bent into a helix and the subsequent welding of the seam at an angle a.

Fig. 6 is a perspective view of a first embodiment of the bending machine

Fig. 7 is a perspective view of a second embodiment of the bending machine

Fig. 8 is a perspective view of a series of first metal stiffening elements welded a first metal plate with a first embodiment of spacers

Fig. 9 is a perspective view of a series of first metal stiffening elements welded a first metal plate with a second embodiment of spacers

Fig. 10 is a front view of a bending machine comprising spacers

Fig. 11 is a perspective view of a series of first metal stiffening elements and a first metal plate going through a bending machine comprising spacers

Fig. 12 is a detail view of a series of first metal stiffening elements and a first metal plate going through a bending machine comprising spacers

Fig. 13 is a detail view of a series of first metal stiffening elements and a first metal plate going through a bending machine comprising a series of vertical rollers and lateral rollers arranged on an inner roller support arm. Fig. 14 is a perspective view of a series of first metal stiffening elements welded to a first metal plate, bent into a helix and welded to a second metal plate.

Fig. 15 is a perspective view of a series of first metal stiffening elements and a first metal plate, bent into a helix welded together and welded to a second metal plate.

Detailed description of the invention

In the design of offshore floating wind farms, reducing the weight of the materials is essential to ensure low cost of the energy produced. At the same time, the structures need to keep their strength, in order to be able to tolerate the environmental mechanical stresses.

Therefore, new structures, especially buoyancy tanks, having higher and higher pipe diameter-to-wall-thickness ratios have been developed. For example, it is not unusual to use buoyancy tanks with more than 11m diameter and only approximately 20mm wall thickness.

For pipes with relatively high pipe diameter-to-wall-thickness ratios, the conventional production method has been fraught with problems pertaining to buckling of the plates during bending due to the gravity forces acting on the plate. In addition, welding the rings after forming the full pipe is a complex, time consuming and expensive operation.

Therefore, new methods of production have been developed in order to alleviate the problems of the existing methods.

Conventional techniques for fabrication of reinforced pipe sectors or pipes comprise first bending a metal plate, then forming a pipe and thereafter welding metal stiffening elements on the formed cylinder.

Here the skilled person will understand that the method may be used to form pipes such as cylinders, and also tapered cylinders. A tapered cylinder is equivalent to a truncated cone. The method is also adapted to form partial cylinders or partial tapered cylinders.

The skilled person will also understand that any type of welding may be used here.

An example of such a method is for example illustrated in figure 5. In detail, the inventive method proposes to bend together at least a first stiffening element 2 and a metal plate 1, to form a pipe sector 10.

Here the skilled person will understand that the stiffening element 2 may typically be any stiffening element traditionally used in the oil and gas industry, especially beams, such as T-beam & U-beam.

Thanks to this inventive method the metal plates 1 will be able to support their weight during the bending step, and the risk of buckling will be at least reduced or even eliminated. In addition, this method allows for a continuous process for making a reinforced pipe sector 10. In particular it allows making a continuous process for making a full reinforced pipe, i.e. a pipe of any desired length.

First example:

In a first step a series of parallel stiffening elements 2, here T-Beams, were first welded to a flat plate 1, forming a reinforced flat plate 3 as illustrated in figure la and lb. The reinforced flat plate 3 is then run through a bending machine 50 to form a circular ring reinforced cylindrical pipe sector 10, and where the T-Beams form circle sections.

In this example, the welding step is realised before the bending step, in some examples, it may be advantageous to have first the bending step, and then the welding step.

Second example

In a first step a series of stiffening elements 2, here shear webs are welded to a first flat aluminum plate 1, and then to a second aluminum plate 5, so that the shear web is comprised between both the first and the second plate 1,5, and so that the first and second aluminum plate 1,5 are parallel. This produces a double flat deck (or double hull deck), as illustrated on figures 2a and 2b. Then the double flat deck is run through a bending machine to form a circular ring reinforced cylindrical pipe sector 10.

Third example

In a first step a series of parallel first stiffening elements 2, here T-Beams, were welded to a flat plate 1. A series of parallel second stiffening elements 4 orthogonal to the series of first stiffening elements 2, here also T-Beams, were also welded to a flat plate 1, perpendicular to the first stiffening elements 2, forming a reinforced metal plate 3.

The reinforced metal plate 3 is then run through a bending machine 50 to form a circular ring reinforced cylindrical pipe sector 10, as illustrated in figure 4. The reinforced pipe sector 10 is supported by a support roller 70.

Having a series of second stiffening elements in an orthogonal direction also stiffen the reinforced pipe in the orthogonal direction. This is not a necessity but is advantageous for the final reinforced pipe as installed to increase the strength of the pipe. This is also valid for a pipe sector.

Fourth example In a fourth example, a series of first stiffening elements 2 are placed on a metal plate 1 (without welding). Then the metal plate 1 and the stiffening elements 2 are run through a bending machine 50 and welded shortly after the plate 1 and stiffening elements 2 are bended as illustrated in figure 3. In other words, at least tack welding will beneficially take place at a position close to the point of plastically bending the plate 1 and the stiffening elements 2 and final welding can take place close to the point of plastically bending the plate 1 and the stiffening elements 2 or at a position further away, for instance if vibration from the roller cold forming operation is affecting the weld quality negatively. In other words welding takes place at an angle a of the bending line, i.e. the line on the metal plate along which the metal plate is bent; and the angle a is defined between a radius of the circle along which the metal plate is bent starting from the bending line, and a second radius of the circle along which the metal plate is bent starting from the welding point. Welding the stiffening elements 2 to the plate 1 after the plastic bending reduces the combined bending stiffness of the metal plate 1 and the stiffening elements 2 during the bending operation, thus reducing the required bending forces and the risk of buckling of the stiffening elements during the bending operation. This is also advantageous as it will reduce the built-in material strain in the metal plate and the first stiffening elements during and after the plastic bending operation.

Fifth example

In a first step a single stiffening element 2, here a T-Beam, was first welded to a flat plate 1, forming a reinforced flat plate 3. The reinforced flat plate is then run through a bending machine 50 to form a helicoidal reinforced pipe, as illustrated in figure 5. The reinforced pipe 10 is supported by a series of support roller 70.

After forming a pipe of suitable dimensions, here for example a pipe of 10 m in diameter, 25 mm thickness, and 40 m in length; the ends are cut so that the pipe has a cylindrical shape. Alternatively, the plates 1 may be pre-cut prior to entering the bending machine so that the ends of the helicoidal reinforced pipe becomes a cylinder with an orthogonal end relative to the longitudinal axis of the pipe.

Here the distance between two first metal stiffening elements 2 may be typically 500-2000mm.

Sixth example

In a sixth example, as illustrated in fig. 12, a series of lower web members 64 are welded to a metal plate 1, a series of first stiffening elements 2 are placed on top of web members 64 (without welding). Then the metal plate with the web members 64 and the stiffening elements 2 are run through a bending machine and welded together at intersection 63 shortly after the plate 1 with lower web members 64 and stiffening elements 2 are bended (in other words, the welding takes place at a position not long after the point of plastically bending the plate 1 with lower web members 64 and the stiffening elements 2), as illustrated in figure 3. In other words at an angle a of the bending line, Welding the stiffening elements 2, which in this example has a reduced web height, to the plate 1 comprising the lower part of the lower web membrane 64, after the plastic bending takes place further reduces the combined bending stiffness of the metal plate 1 and the stiffening elements 2 during the bending operation, thus reducing the total required bending forces and the risk of buckling of the stiffening elements during the bending operation.

In this example, a part of the welding step is realised before the bending step, and another part is realised after the bending step in order to reduce the stiffness and risk of web and flange buckling during the bending operation as well as reducing the post bending plastic (and permanent) strains in the bended materials. High plastic strain levels could initiate micro cracks in the material and therefore reduce the fatigue strength of the bended materials and should therefore desirably be reduced as much as possible during the fabrication.

Seventh example

In a first step a first flat metal plate 1 is run through a bending machine to form a circular pipe sector of 340°. The formed pipe sector is welded to a previously produced (or pre-manufactured) reinforced pipe sector. The first metal plate 1 is further bent for a total of substantially 360° using the bending machine and welded to the reinforced pipe sector. An at least a metal stiffening element 2 is independently manufactured to form a full circle, or substantially a full circle, and positioned inside pipe sector. Then, the at least a first metal stiffening element 2 are welded on the pipe sector forming a joined reinforced pipe sector. Welding the stiffening elements 2 to the plate 1 after the plastic bending reduces the combined bending stiffness of the metal plate 1 and the stiffening elements 2 during the bending operation, thus reducing the required bending forces and the risk of buckling of the stiffening elements during the bending operation.

Eighth example

In a first step, a series of parallel stiffening elements 2, here T-beams, are first welded to a flat plate 1, thus forming a reinforced flat plate 3, as illustrated in figures la and lb. In addition, a series of parallel second stiffening elements 4, here also T-beams, may be welded to the flat plate 1, perpendicular to the first stiffening elements 2 to form a reinforced flat plate 3. The reinforced flat plate 3 is then run through a bending machine 50 to form a reinforced cylindrical pipe sector 10. In an eighth example, the stiffening elements 2 form circle sections on the outer surface area of the formed reinforced cylindrical pipe sector 10, as illustrated in figure 14.

An advantage of having the stiffening elements 2 on the outer surface area of the formed reinforced cylindrical pipe sector 10 is that a second metal plate 5 may be independently bent and more easily welded onto the stiffening elements 2 to produce a double flat deck (or double hull deck), which further reinforces the pipe sector 10.

Ninth example

In a ninth example, the stiffening elements 2 are placed on the flat plate 1 without welding initially. Then, the flat plate 1 and the stiffening elements 2 are run through the bending machine 50 and the bent stiffening elements 2 may be welded onto the outer surface area of the bent metal plate 1 shortly after, as illustrated in figure 15.

Tenth example

In a tenth example, a reinforced pipe sector 10 is prepared as in the eighth or ninth example. Thereafter, a second metal plate 5 is bent and then welded to the stiffening elements 2 formed on the outer surface area of the reinforced cylindrical pipe sector 10 to form a double deck (or double hull deck), as illustrated in figure 14 and 15.

Independently from the examples, when the at least two first metal stiffening elements 2 are placed on the first metal plate 1, the at least two first metal stiffening elements 2 are advantageously parallel, and the distance between the at least two first metal stiffening elements 2 is between 1/50 and 1/2 of said pipe sector bending diameter D, preferably between 1/30 and 1/3 of the pipe sector bending diameter D.

In the same spirit when the at least a first metal stiffening elements 2 is a helix, the pitch of said helix is between 1/50 and 1/2 of said pipe sector bending diameter D, preferably between 1/30 and 1/3 of the pipe sector bending diameter D.

These sections can then later be mounted together and welded to form a longer cylindrical or tapered cylindrical section.

Bending machine

In figure 6 and 7, two examples of bending machine 50 are illustrated. The bending machine 50 comprises a pair of outer rollers 52 and an inner roller 51. In the inventive bending machine 50, the inner roller 51 is adapted to bend the reinforced bending plate 3, i.e. the metal plate 1 welded to at least a first stiffening element 2.

As illustrated in figure 6 (and 7), instead of being flat, the inner roller 51 is a cylinder (or a tapered cylinder) comprising at least a first groove 55, to accommodate at least a first stiffening element 2. The depth of the at least a first groove 55 is preferably equivalent to the height of the at least a first stiffening element 2. This way the bending force applied by the inner roller 51 is distributed on both the metal plate 1 and the at least a first stiffening element 2, instead of just on the at least a first stiffening element 2, thereby reducing the risk of buckling in the at least a first stiffening element 2.

The at least a first groove 55 is arranged on a circumference around the inner roller 51.

The inner roller 51 may be a solid piece, or it may alternatively be a cylinder covered by a series of discs.

It is to be understood that when forming a reinforced pipe sector 10 where the stiffening elements 2 are shaped on an outer surface area thereof, such as in the above examples 8, 9 and 10, the at least a first groove 55 must be arranged on the outer rollers 52 instead of the inner roller 51, to accommodate the at least a first stiffening element 2.

As illustrated in figure 7, if the reinforced plate 3 comprises at least a first stiffening element 2 and at least a second stiffening element 4, perpendicular to the at least a first stiffening element 2, then the inner cylinder 51 comprises at least a first groove 55, to accommodate at least a first stiffening element 2 and at least a second groove 56, to accommodate at least a second stiffening element 4. The depth of the at least a first groove 55 and of the at least second groove 56 is preferably equivalent to the height of the at least a first stiffening element 2 and of the at least a second stiffening element 4, respectively. This way the bending force applied by the inner roller 51 is distributed on both the metal plate 1, the at least a first stiffening element 2 and the at least a second stiffening element 4, instead of just on the at least a first stiffening element 2 and/or the at least a second stiffening element 4, thereby reducing the risk of buckling in the at least a first stiffening element 2 and/or the at least a second stiffening element 4.

The at least a first groove 55 is arranged on a circumference around the inner cylinder (or of the tapered inner cylinder). The at least a second groove 56 is arranged longitudinally along the outer surface of the inner roller 51. The bending machine 50 may be advantageously mounted so that the central axis of the inner roller is orthogonal or parallel to the axis of gravity.

It is to be understood that when forming a reinforced pipe sector 10 where the stiffening elements 2 are shaped on an outer surface area thereof, such as in the above examples 8, 9 and 10, the at least a first groove 55 and the at least a second groove 56 must be arranged on the outer rollers 52 instead of the inner roller 51, to accommodate the at least a first stiffening element 2 and the at least a second stiffening element 4.

In a third example of bending machine 50 the bending machine is arranged in the same way as described in the examples of bending machine 50 above but with the inner roller 51 comprising a series of inner rollers 61 and lateral rollers 62 arranged on an inner roller support arm 73 as shown in figure 13. Individual inner rollers 61 are arranged on inner roller support arm 73 spaced apart along a bending line substantially parallel with the longitudinal axis of helix 30. Said individual inner rollers 61 are further arranged on inner roller support arm 73.

As shown on figure 13 lateral spacers comprising lateral rollers 62 are arranged on inner rollers 61 to support the at least first metal stiffening element 2 during plastic bending of said first metal stiffening element 2. In order to ensure stability of the rollers at least two lateral rollers 62 are arranged on each side of each of inner roller 61.

It is to be understood that when forming a reinforced pipe sector 10 where the stiffening elements 2 are shaped on an outer surface area thereof, such as in the above examples 8, 9 and 10, it is the outer rollers 52 that comprise a series of outer rollers and lateral rollers arranged on an outer roller support arm.

When bending the metal plates 1 with stiffening elements 2 there is a risk that the stiffening elements may buckle. One way to reduce the buckling risk is to increase the width between the 2 outer rollers 52 in order to increase the efficient bending arm, thus reducing the required roller 51,52 forces needed to plastically bend the plate 1 with stiffening elements 2.

In other words, to reduce the force applied to the plate 1 with welded stiffening elements 2, the space between the outer rollers 52 of the bending machine 50 may be increased.

Another alternative to reduce the risk of buckling is using at least two spacers 60. Spacers are objects that may be placed on each side of the at least a first stiffening element 2 and between the first metal plate 1 and the inner roller 51, as illustrated on figures 8 and 9. Preferably these at least two spacers 60 cover the width of the first metal plate 1, along the bending line, and even more preferably on each side of the bending line, wherever the inner roller will be in contact with the first metal plate and the at least a first stiffening element.

There may be multiple spacers of different width depending on the number of first stiffening elements 2, and their placement on the first metal plate 1. In other words, the width of a spacer 60 is the distance between two stiffening elements 2, or between a stiffening element 2 and the side of the first metal plate 1.

Preferably, it is intended that each of the at least a first stiffening element 2 is comprised between at least two spacers, so that the spacers will prevent buckling during the bending step c).

The at least two spacers 60 may be any suitable shape that complements the shape of the at least a first stiffening element 2, for example so that the cross section of the shape of the at least two spacers 60 and the shape of a first stiffening element 2 may be a substantially rectangular, as shown in figs 8 and 9, of substantially the same width as the first metal plate 1 and of substantially the same height as the at least a first stiffening element 2.

In the case of the at least a first stiffening element 2 being a T-Beam or a U- Beam, the at least two spacers 60 may be cuboids.

The at least two spacers 60 may be freestanding. That is, they are not necessarily attached to a support structure or the like.

Preferably the at least two spacers 60 may comprise vertical acting spacer rollers 61 that transmit the pressure of the inner roller 51 to the first metal plate 1, as illustrated in figures 10 and 11. In order to hold back the spacers 60 during the rolling/bending operation a spacer hold-back support 71 may be arranged in front of the inner and outer rollers 51,52.

Preferably the at least two spacers 60 may further comprise lateral acting spacer roller 62 to keep the at least a first stiffening element 2 from buckling.

Each roller 61,62 may be free-rolling or may have a motor driving them.

In the case of the at least a first stiffening element 2 being T-beams, the at least two spacers 60, should be designed to avoid the buckling of all the part of the T-beams, i.e. both the shear web and the flange, as in the examples above using roller(s) (fig. 9, 10 or 11) or cuboids (fig. 8).

It is to be understood that when forming a reinforced pipe sector 10 where the stiffening elements 2 are shaped on an outer surface area thereof, such as in the above examples 8, 9 and 10, the at least two spacers 60 are preferably placed on each side of the at least a first stiffening element 2 and between the first metal plate 1 and the outer rollers 52. A bending machine 50 is not always able to apply a bending moment to the end of the item to be bent and it is therefore not possible to permanently and plastically bend the very ends. Due to this difficulty, the end parts of the metal plates 1 are often not bent when bending the plates 1 with stiffening elements 2. Usually, the non-bent end portions of the plate 1 are cut away before being welded together to form a pipe, such as a cylinder.

To further improve the method one or both end portions of the metal plate 1 may be bended in a first step, before placing the stiffening element 2 on the plate. In addition, the at least a stiffening element 2 may be shaped or cut at one or both end portions so that the at least a stiffening element 2 may be easily placed on the plate 1 (with bended one or both end portions).

Thus, the metal plate 1 has ski tip like end portions which do not need to be plastically bent to become part of the final circular or substantially circular shape of the produced pipe sector, as they have a substantially identical curvature.