Methods of this type are used for e. g. the manufacture of flexible pipes for use in the transport of fluids between various types of offshore installations, such as floating production units, fixed platforms, sub-sea completions and the like.

These flexible pipes are usually composed of several layers having various functions, such as resistance to corrosive liquids, protection against internal and external pressure, axial loads, etc. One of the layers is in the form of a so-called carcass, consisting of a wound steel strip having the purpose of protecting a polymeric liner, which is placed on the outside of the carcass, against mechanical impacts.

This type of pipes is well known in the literature, and as an example of a more recent type reference may be made to the description of WO 00/36324 A1.

EP Patent No. 0 335 969 B1 discloses an apparatus for the manufacture of a carcass for flexible pipes, in which apparatus a forming unit for the profil- ing of a steel strip and a winding unit for the winding of the profiled strips have interposed between them a loop of steel strip whose length may vary as a function of the speeds between the forming unit and the winding unit.

Thus, the loop serves as a buffer for the steel strip present at any time in the loop, which occurs because of the speed deviations that exist between

the steel strip pay-off of the forming unit and the steel strip take-up of the winding unit.

A slack sensor is inserted along the path of the steel strip for adjusting the length of the strip, said slack sensor being adapted to signal the forming unit to adjust its speed in dependence on the size of the loop. In other words, a"small"loop will mean that the speed of the forming unit is to be increased, while a"large"loop means that the speed of the forming unit is to be reduced. Hereby, it is attempted to maintain a constant length of steel strip between the forming unit and the winding unit.

An objective of the invention is to provide a method for the manufacture of a helically wound pipe, by use of which method the optimal speed of the forming unit can relatively be obtained.

A further objective is to provide a method for the manufacture of a helically wound pipe, which method is simple and effective.

Yet a further objective of the invention is to provide a method for the manufacture of a helically wound pipe, which method includes a step of recording and adjusting the operational conditions of the forming unit relative to the winding unit in a simple and effective manner.

The object of the invention is achieved by the method as defined in the claims.

In the method of the invention a sensor is adapted to measure the force or load by which the forming unit pushes the strip forward to the winding unit, said force being sensed or recorded by a force-sensing cell which is adapted to record a force applied by the steel strip due to the pushing pressure.

The method according to the invention thus includes the steps of i. providing a steel strip ii. passing the steel strip from a feeding reel to a forming unit, iii. passing the strip further onto a winding unit, and iv. sensing the force by which the forming unit pushes the strip forward to the winding unit, in the following denoted the pushing force.

The invention thus includes passing a steel strip from a feeding reel to a forming unit and further onto a winding unit. The feeding reel, the forming unit and the winding unit may e. g. be of any type e. g. as disclosed in EP 335969 which is hereby incorporated by reference. As it is generally known the strip is preshaped by a forming unit into a formed strip having a predetermined cross-sectional shape, and in the winding unit the strip may be further shaped so that the adjacent turns of the wound strips are interlocked. The strip is optionally wound around a mandrel, which the pipe passes along. In the method of the present invention the individual steps of preshaping, winding and the interlocking may e. g. be as disclosed in WO 0036324, which is hereby incorporated by reference.

The pushing force is detected by a force-sensing cell provided between the forming unit (2) and the winding unit (3).

The pushing force may in principle be detected by any type of force-sensing cells. By detecting the pushing force a very fast control can be obtained, as the pushing force may be detectable prior to any change of the length of the strip between the forming unit and the a winding unit.

The method may e. g. comprise a further step of adjusting the speed of the forming unit in relation to the pushing force so as to keep the pushing force substantially constant.

By keeping the pushing force substantially constant, any variation in length of the strip between the forming unit and the winding unit may be avoided.

The term"substantially constant pushing force"means pushing force with a variation within ordinary measuring uncertainty, preferably less than 5%, more preferably less than 1% or even 0.5% The adjustment may e. g. be performed manually or using automatic control devices, said adjustment particularly being performed in the start-up phase until a stable winding of the strip is obtained. The method is particularly useful in a start-up phase, i. e. the winding of the first 2,3 or up to about 10 meters, because the adjustment of the speed of the forming unit may be very simple and easy based on the measurement of the pushing force. In many cases the adjustment is made manually due to being most effective, and in these cases expensive control equipment may be saved.

The method may also include sending a control signal back to the forming unit so as to obtain and keep the pushing force substantially constant.

In one embodiment of the invention an adjustment is performed using of a feed back transmission, from the force-sensing cell to the forming unit.

In a preferred embodiment the pushing force detected by the force-sensing cell is adjusted to a preselected level. The level of pushing force may e. g. be selected in relation to the speed of the winding unit or it may be selected based on the experience of an operator.

When using equipment for producing helically wound pipes, different types of strips and pipes may result in different speeds of the forming unit and winding unit, respectively. However, the optimal pushing force of the strip depends basically on the speed of the winding unit. Therefore, by

determining the speed either exactly or by correlating it to other production parameters, e. g. the type of strip to be wound or the diameter of the pipe, the optimal pushing force may be determined. The optimal pushing force may alternatively be selected by experience.

Having selected the pushing force, the speed of the forming unit can be quickly adjusted to obtain this pushing force. When the pushing force is well selected the process maintains stable, and in principle no further adjustments should be needed.

Due to this very fast setting or adjustment of the production based on the setting or adjustment of the pushing force, the distance of the strip between the forming unit and the a winding unit may be shorter than when using prior art methods.

The force-sensing cell may preferably be in the form of a weighing cell. The term"weighing cell"means a sensor unit that can detect a weight or contact pressure. In this case, the strip is acting on the weighing cell, which is thereby registering the pushing force. The weighing cell may be so arranged relative to the steel strip that a force applied by the strip becomes proportional to the force by which the strip is pushed forwardly by the forming unit. A skilled person will easily be able to adjust the weighing cell to fulfill this requirement. By this arrangement, it is thus ensured that a force applied by the strip may be detected in a simple manner.

The force-sensing cell may e. g. display the measured force. Alternatively the force-sensing cell may be adapted to send a signal at different force levels, e. g. a warning signal. The force-sensing cell may continuously display or send the sensed force, or it may be adapted to only register or send the sensed force the force stepwise.

In one embodiment the force-sensing cell is a weighing cell, which comprise a sensing pin. The weighing cell can detect a force, which is applied onto the pin.. The sensing pin may be arranged so that it actually breaks when the force exceeds a certain level. Thereby, the operator will know that immediate adjustment of the speed of the forming unit is required. The weighing cell may preferably be positioned so that the pin is in contact with the strip at a point where said strip follows a curve. The pin should preferably be in contact with the surface of the strip constituting the outer surface of the curved strip. A useful weighing cell is marketed by Eilersen Electric A/S under the type name SDM70 1000Kg.

For easy mounting of the weighing cell it is an advantage if the steel strip between the forming unit and the winding unit extends in a partly open, fixed channel, which has the shape of a loop or an arc.

In an alternative embodiment, it is expedient if the steel strip extends in a substantially straight line between the forming unit and the winding unit, and that the forming unit or the winding unit is arranged on a movable support which is connected via a probe with a weighing cell for measuring the force by which the forming unit pushes the strip forwardly to the winding unit.

The term"substantially straight line"means that the length of the strip between the forming unit and the winding unit is between 1 and 1.2 times, preferably between 1 and 1.1 times the shortest distance between the exit of the strip from the forming unit and entrance of the strip to the winding unit. In this embodiment it is further preferred that one or both of the forming unit and the winding unit are arranged on a movable support which is connected via a probe with a weighing cell for measuring the force by which the forming unit pushes the strip forwardly to the winding unit.

The method may include the step of detecting the pushing force using two or more weighing cells arranged between the forming unit and the winding

unit.

In one embodiment the steel strip extends in a curve between the forming unit and the winding unit, and the length of the curve is constant.

The invention will now be more fully explained with reference to the example of the invention shown in the drawing.

In the drawing: fig. 1 shows the basic structure of a known apparatus for use in the manufacture of carcass pipes, fig. 2 shows the basic structure of a first embodiment of an apparatus for use in a method according to the invention, fig. 3 shows the structure of a path for the embodiment of fig. 2, while fig. 4 shows an alternative embodiment of the invention.

In fig. 1, the numeral 1 designates a schematically shown steel strip reel, from which a steel strip 5 runs via a roller to a forming unit 2, where the steel strip 5 is profiled by means of pre-forming rollers 6a, 6b, 6c and 6d and is conveyed via a guide roller 7 in a loop 9 to a winding unit 3, where a wound pipe (not shown), such as a carcass, is manufactured perpendicularly to the plane of the paper.

As will be appreciated, it is imperative that the speed at which the forming unit 2 pays off the profiled steel strip is close to the speed at which the winding unit takes up the profiled strip. However, the speeds of the forming unit 2 and of the winding unit 3 cannot be synchronized completely, which

means that the length of the loop 9 will vary as a function of these differences in speed.

Figure 1 shows with a dashed line a case where the size of the loop 9 has been reduced, because the forming unit has run at a lower speed than the winding unit 3. As mentioned, slack sensors have been used to compen- sate for these undesired speed differences, as discussed in connection with EP Patent No. 0 335 969. These slack sensors are not shown in fig. 1, but may be secured at any location along the loop 9, in principle. Thus, it is possible on the basis of the sensing of the slack of loop 9 to adjust the speed of the forming unit or of the winding unit, thereby maintaining the acceptable limits as to how long and how short the loop may be during operation of the apparatus.

Fig. 2 shows the apparatus similar to the one in fig. 1, but now equipped with the features of the invention, noting that the same reference numerals are used in fig. 2 as in fig. 1 for the same structural details, and consequently, will not be described further.

In contrast to fig. 1, sensors 11 and 12 in the form of weighing cells are ar- ranged along the loop 9, which are adapted to sense the forces applied by the loop 9, i. e. the force by which the forming unit 2 pushes the strip for- wardly to the winding unit 3.

As will be seen, the loop 9 extends in a path, which is composed of guide sections 13, cf. also fig. 3, which clearly shows the shape of the guide sec- tions. The guide sections may generally be interconnected by means of a path (not shown in detail) along the straight, upper portion of the guide sections shown in fig. 3.

Fig. 4 shows an alternative embodiment of the invention. This figure also

shows the forming unit 2 and the winding unit 3 of fig. 1 and of fig. 2. As will be seen, the steel strip 19 extends in a straight line between the forming unit 2 and the winding unit 3. The forming unit 2 is placed on a support 16 having wheels 17, which can run on a table 18. The table 18 has arranged thereon a force meter in the form of a weighing cell 15 which can detect the pushing force in the strip via a rod connection 20.

The signal from the weighing cell may be used for controlling the forming unit so as to keep the pushing force constant.

Although not shown, nothing, of course, prevents the winding unit from be- ing placed on a support with wheels in the same manner as the forming unit 2.