EP3014017A2

EP3014017A2 - Pressure resistant strands

Info

Publication number: EP3014017A2
Application number: EP14732610.2A
Authority: EP
Inventors: John Mawson Walton
Original assignee: Bridon Ltd
Current assignee: Bridon International Ltd
Priority date: 2013-06-27
Filing date: 2014-06-19
Publication date: 2016-05-04
Also published as: GB2517404A; WO2014207437A3; GB2517404B; WO2014207437A2; GB201311497D0

Abstract

The present invention provides a spiral strand formed of a plurality of layers, each layer being formed of a plurality of individual wires. At least one inner layer is adapted to have a greater resistance to radial forces than other layers, such that the inner layer is better able to resist the higher radial forces that are generated within the inner layers of relatively larger spiral strands, for example strands that have seven or more layers provided radially outwardly of the adapted inner layer. The inner layer may be adapted by including shaped wires that provide contact surfaces between layers that have a greater surface area.

Description

Pressure Resistant Strands

The present invention relates to an open spiral strand. The present invention also relates to a method of manufacturing an open spiral strand.

Spiral strands are formed from concentric layers of wires that are helically wound around a strand core. The wires are traditionally produced by cold drawing from high- carbon steel rods and may be subsequently galvanised with a corrosion resistant coating, such as a zinc coating.

Open' spiral strands have an outer layer of wires that consists of wires of a substantially circular cross-sectional shape. The outer surface of the strand is formed from the exposed outer surfaces of each wire in the outer layer. Open spiral strands are used, for example, in quasi-static structural applications such as mast stays, bridge cables and offshore mooring lines.

When the spiral strands are subjected to a tensile load, radial forces are generated between the wires of the strand. This is due to the helical path taken by the wires within the strand. The radial forces generated by each wire are proportional to the helical curvature of the wire, as well as to the tensile load. Wires in radially adjacent layers have differing helical pitches and so cross over each other, thereby forming areas of contact between the wires. The radial forces act on these areas of contact, which creates transverse (contact) stresses on the wires. These transverse stresses can influence the way in which a spiral strand fails under tension. Accordingly, the maximum tensile strength of the spiral strand is often limited by the maximum transverse contact stresses that the wires can withstand and these stresses are worthy of more detailed consideration. The radial forces are resisted by the points of contact with underlying layers and the contact forces between each of the layers can be calculated with reasonable accuracy, which allows different design solutions to be compared, at least in a qualitative sense. The resulting contact stresses, are much more difficult to determine but it will be understood that for a given contact force, the resulting local contact stresses will depend on the area of contact developed between the two surfaces. The geometry of the helical paths taken by each of the wires in a spiral strand in its unloaded state are defined by the respective cover diameters and lay lengths given in the design specification. In reality, under load, the wires will be subjected to secondary bending effects about the points of contact, but for the purposes of comparison the helical radius of curvature will be assumed to be uniform, proportional to the mean cover diameter and inversely proportional to square of the sine of the lay angle. Similarly, since the emphasis here is on the breaking performance of the product, it will be assumed that all of the wires are simultaneously loaded to their full nominal breaking load, based on the minimum tensile strength of the grade. Neither of these assumptions is considered to be unrealistic for the limited purposes of this analysis.

The next step is to calculate the number of contact points between each of the layers, and to apportion the radial forces (uniformly) between them. The support situation for the outer cover of wires is simply defined by its own geometry relative to that of the underlying cover. It will be appreciated however that there will be a cumulative effect as we move down into the strand, where each successive layer is supporting a greater number of covers. If adjacent covers are cross-laid in the same direction, the number of contact points between them will be relatively small and the contact forces will be much larger. In these cases however the crossing angles will be relatively small, so that the conditions approximate almost to line contact.

These points are illustrated in the following Table 1 for the case of a 127mm spiral strand that has been designed with an 18 degree lay angle. Typical average contact forces are given for each layer of wires in each strand, together with an estimate of the crossing angle at which the wires meet with those in the underlying layer. In this example succeeding layers are laid in opposite directions to one another (contra-laid) with the exception of the innermost layer [#12], where the six wires are in line-contact with the central king wire. The individual wires are each assumed to be of the order of 5.0mm diameter and to have an ultimate tensile strength of 2.0 GPa.

It is apparent that the contact forces seen by the inner layers are considerably higher than those at the outer cover, in this case by almost two orders of magnitude, largely due to the cumulative effects of adding a succession of layers. It follows that the highest contact forces will always be seen by the innermost covers and that the magnitude of the resulting forces will be related to the number of overlying wire layers. In this particular example it can be shown that the relationship is an exponential one that is roughly represented by the equation: y = 0.5 e^a4x where y is the contact force in kN and x is the layer

The above contact forces act at each cross-over point with the radially inward adjacent layer and assume that a wire tensile stress of 2000 MPa will be reached at the breaking point of the strand. The axial breaking strength of each 5.0mm diameter wire is therefore 39.2 kN, which is less than the maximum compressive force acting between the 12- and 6-wire layers (#1 1 & #12) in the above example. Clearly these are forces to be reckoned with in estimating the axial performance (and tensile efficiency) of the strand. Tests on individual wires in the laboratory suggest that when such compressive forces exceed about 20% of the tensile strength of the wire, the load carrying capacity of those wires can be significantly impaired. In response to market demands, it is desired to produce open spiral strands of increased unit breaking strengths. One option to do this is to increase the number of layers of wires to produce larger diameter open spiral strands. However, as can be seen from Table 1 , as the number of layers of wires is increased, the radial forces on the wires increases, which thereby effectively limits the number of layers that can be used in a spiral strand. Figure 10 shows this dilemma graphically with the radial stresses increasing very rapidly when there are seven, eight or more layers.

It will be appreciated that the above example refers to the general case where the potentially damaging radial forces are self-generated internally by the strand itself being tensioned in free space. There are also situations where additional pressures may be applied externally to the strand in the course of its usage or duty. The most obvious example lies in the terminations or end fittings when the strand is invariably compressed, either by the fitting of a swaged ferrule or more typically a conical spelter socket. If an appreciable length of undisturbed strand is led into the conical area of the socket, then the additional radial forces applied as the cone pulls in under load may be sufficient to provoke wire failures within the termination.

Alternatively the radial force situation may be materially worsened in the case of sheathed spiral strands that are being used offshore in considerable depths of water. In this scenario, some air may remain in voids within the strand such that the strand will be subjected to additional radial forces from the prevailing hydrostatic pressure of the sea-water. Strands may sometimes be subjected to even higher external radial pressures if they are bent around a curved surface such as a sheave or drum or fairlead under tension, particularly if that surface is not grooved to suit the strand diameter. Alternatively very high external pressures may be generated if a wedge-type gripper is used for tensioning purposes at one end of the strand. These external pressures will all have an additive effect on the radial forces already being experienced within the strand.

One solution to this problem would be to limit the number of layers of wires that a strand may comprise and to vary the wires size in proportion to the intended strand diameter, but this would inevitably result in some reduction in tensile grades. This would impact on both the strength :size ratio and the strength :weight ratio of the product which may be less attractive in the more demanding applications. An alternative approach would be to look at tubing one or more of the inner covers so that the radial forces are resisted by adjacent wires within a layer coming into contact as the strand approaches its breaking strength. This would have the effect of generating hoop stresses within the said layer(s) and alleviating the radial forces on the underlying layers of wires. However it would be very difficult to ensure that the appropriate level of wire interference was achieved in order to control the level of hoop stress and there is a risk that the normal working and flexibility of the strand might be put at risk. A more preferable solution would be to extend the use of equal-laying from the innermost six-wire core to several layers to give greater areas of line contact. For example, equal-lay strands of 31 -wires, 36-wires, 41 -wires & 49-wires are commonly used, based on the Warrington Seale principle of construction. Similarly there is a case for introducing yet larger equal-lay core strands, containing 55-wires, 61 -wires or even more, but there are practical limits to the number of wires that can be spun together in a single operation and the beneficial effects of this are still confined to the central core strand.

Equal-laying the first two layers over the core strand as a separate (secondary) operation has been considered. This would put two of the most highly stressed layers into partial line contact, which may have a beneficial effect in terms of spinning losses (as well as steel area). However, the contact between the core strand and the succeeding layers would result in cross-over points that would be just as critical if not more so. A means of improving the contact conditions with and within the subsequent layers is also desirable.

An alternative option, to increase unit breaking strengths, is to increase the tensile grade of the wires of the strand. However, there is a practical limit to the tensile grades that can be achieved.

It is an object of the present invention to provide an open spiral strand of high unit strength that is of relatively high tensile efficiency compared to the known prior art.

According to the present invention there is provided a spiral strand comprising a plurality of layers wherein each layer is formed of a plurality of individual wires, wherein at least some of the wires comprising at least one inner layer are shaped so as to provide a comparatively greater surface area contacting an adjacent layer than if said at least some wires were circular in cross-section, wherein said strand further comprises at least seven layers provided radially outwardly of said at least one inner layer.

Preferably the shaped wires are provided in a layer that is anticipated to experience particularly high radial forces, e.g. radial forces that are higher than those experienced by the majority of layers. More preferably still, at least the layer that experiences the highest radial forces of all is provided with the shaped wires.

Preferably the spiral strand may be provided with an equal-lay core. The equal-lay core may be provided with at least 49 wires, more preferably at least 55 wires. The core is provided the core is preferably is compacted.

When an equal-lay core is provided the shaped wires are preferably provided in the first layer provided over said core, and may also be provided in the third layer provided over said core. In preferred embodiments of the invention the layer(s) comprising shaped wires may preferably comprise alternating half-lock and round wires.

In embodiments of the invention the strand may comprise an outer plastics sheathing, and when a sheathing is provided voids in the strand are substantially filled (e.g. filled to at least 95%, preferably 98%, of the void volume) with an incompressible medium.

According to another broad aspect of the present invention there is provided a spiral strand comprising a plurality of layers wherein each layer is formed of a plurality of individual wires, wherein at least one inner said layer is adapted to have a greater resistance to radial forces than other said layers, wherein said strand further comprises at least seven layers provided radially outwardly of said at least one adapted inner layer.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a front perspective view of a spiral strand according to a first embodiment of the present invention;

Figure 2 is a lateral cross sectional view of the spiral strand shown in figure 1 , viewed along the longitudinal axis of the spiral strand;

Figure 3 is an enlarged view corresponding to a part of Figure 2;

Figures 4(a) and (b) are enlarged partial (a) cross-sectional and (b) perspective views showing an equal-lay core with a layer comprising shaped wires according to an embodiment of the invention;

Figures 5(a)-(c) are cross-sections of wires that may be used in embodiments of the present invention;

Figure 6 is a perspective view of a core strand according to an embodiment of the present invention;

Figure 7 is a lateral cross-sectional view of the core strand of Figure 6;

Figure 8 is a perspective view of a core strand according to a further embodiment of the present invention;

Figure 9 is a perspective view of a core strand according to a still further embodiment of the present invention; and

Figure 10 is a plot of radial forces against the number of layers experienced in an exemplary 127mm diameter open spiral strand according to the prior art.

Referring now to Figures 1 to 4 there is shown a spiral strand 1 according to a first embodiment of the present invention. The spiral strand 1 comprises a plurality of wires 2 arranged in concentric layers about a longitudinal axis 4 of the strand 1 . Figure 1 shows a perspective view of the strand 1 and Figure 2 a cross-sectional view, while Figure 3 shows an enlarged partial cross-sectional view and is presented such that the core and the first few layers of wires radially outward of the core can be seen more clearly than in Figure 2.

The spiral strand 1 comprises a central core 5 which comprises a plurality of core wires 7 that are helically wound about a central king wire 6 that extends along the longitudinal axis 4 of the strand 1. The longitudinal axis 4 of the strand 1 is substantially straight, with the central king wire 6 extending in a substantially straight direction. The core wires 7 are helically wound about the central king wire 6, i.e. about the longitudinal axis 4 of the strand 1 , being arranged in a plurality of concentric layers about the longitudinal axis 4. The layers of core wires 7 are distributed in the radial direction of the spiral strand 1 . The layers of core wires 7 are disposed radially adjacent to each other and in the abutment with each other. The further the layer of core wires 7 from the longitudinal axis 4, the greater the diameter of the layer and the greater the number of wires 7 in the layer. It will be understood that in this description the term "radially inner layer" and the like refers to a layer that is closer to the axis of the strand, and the tem "radially outer layer" and the like refers to a layer that is further away from the axis of the strand.

The core wires 7 are helically wound about the longitudinal axis 4 with equal pitch. Accordingly, the core wires 7 extend in a parallel direction to each other. Therefore the core wires 7 do not cross each other, but form lines of contact with each other along opposed radially inner or outer surfaces respectively. Such a core is known as an equal-lay core. The core 5 comprises an outermost layer 8 of core wires 7. The spiral strand 1 comprises a plurality of layers of non-core wires disposed radially outwardly of the core 5. The non-core wires comprise a plurality of layers and in particular may include at least seven, and possibly more than seven, layers laid over the core. In the embodiment shown in Figures 1 and 2 there are a total of ten layers laid over the core. The layers have a radially outermost layer 30 that defines an Open' outer surface of the strand 1 . Open' spiral strands are used, for example, in quasi- static structural applications such as mast stays, bridge cables and offshore mooring lines. As will be explained further below a plastics sheath 100 may be applied over the outer surface of the strand. The sheath 100 protects the wires 2 and is impervious to moisture or other corrosive agents.

For the sake of clarity and better understanding of the invention, Figure 3 is an enlarged view of part of the strand shown in Figures 1 and 2 and, in particular, shows in cross-section the core and the first few non-core layers including in particular (counting outwardly from the core) the first three layers 25, 26 and 27, and the fourth layer partially. The first inner layer 25 is disposed radially outwardly of, and adjacent to, the outermost layer 8 of the core 5, the second layer 26 is laid over the first layer 25, and the third layer 27 is laid over the second layer 26. It will be understood that layers radially outward of the fourth layer are omitted only for clarity of illustration. In the currently described embodiment, with the exception of certain layers as will be explained below, the layers of non-core wires all have a substantially circular lateral cross-sectional shape, i.e. the cross-sectional shape defined on a plane that is perpendicular to the longitudinal axis of the wire. In other words, the lateral cross- sectional shape is the cross-sectional shape when viewed along the longitudinal axis of the wire. The exception lies in the first 25 and third 27 layers which comprise at least some wires which have been shaped such that they provide a comparatively greater surface area contacting an adjacent layer than if they were circular in cross-section. In particular, in the embodiment of Figures 1 to 4, the first 25 and third 27 layers comprise alternating round wires 33 and half-lock wires 34 as is best seen in Figure 3 in conjunction with Figure 5(a) which shows one half-lock wire 34 in cross-section. The half-lock wires 34 have side surfaces 40,41 that are concave such that they generally receive the side surfaces of adjacent round wires 33. The radially inner and outer surfaces of the half-lock wires, 42,43 are curved to substantially conform to the inner and outer (cylindrical) surfaces of the layer. The effect of providing alternating half-lock wires and round wires is best seen in Figures 4(a) and (b) which show the first layer 25 laid over the outer core layer 8. As can be seen from Figure 4(b) in particular the radially outer surfaces 43 of the half-lock wires 34 present a greater surface area than the round wires 33 for supporting the second layer which is not shown in Figure 4(b) but which would be laid on the radially outer surface of the first layer. Generally, the combination of alternating round wires and half-lock wires presents a much smoother radially outer surface than would be achieved by a layer comprising only round wires, and as such the radial stresses at the points of contact where wires in the respective layers cross are substantially reduced. It will also be understood that the radially inner surface of the first layer 25 which contacts the outer layer 8 of the core likewise has a smoother surface with the radially inner surfaces 44 of the half-lock wires 34 presenting a greater surface area for contact the outer core layer 8 than the round wires 33. The applicant has identified that adjusting in this way the shape of at least some wires of radially inner layers, which are the layers where the highest radial forces are found, produces a disproportionately large reduction in contact stresses within the strand. Accordingly, adjusting the shape of only radially inner layers of wires produces spiral strands that are able to withstand a higher tensile load with a relatively low impact on weight, cost and complexity. Referring back to Figures 1 to 3 it will be seen that the third layer 27 also comprises alternating round wires 33 and half-lock wires 34 so as to produce relatively smoother inner and outer surfaces for contacting the adjacent second layer 26 and fourth layer. Similarly, the applicant has identified that adjusting the shape, in this way, of wires in alternate layers in the radial direction (e.g. the first and third layers 25, 27 separated by the second layer 26) is advantageous in that it produces a large reduction in contact stresses, that is more cost effective than when the shape of radially adjacent layers of wires is adjusted in this way.

In the embodiment of Figures 1 to 4 the layers 25,27 comprise interlocking round wires and half-lock wires. However, the wires in these layers may have other cross-sectional shapes. In particular as shown in Figure 5(b) the wires may have a Z-shape, also known as a full-lock, or as shown in Figure 5(c) the wires may have a trapezoidal wedge-shape cross-section. With the cross-sectional shapes shown in Figures 5(b) and 5(c) the wires interlock with each other in a circumferential direction such that the layers 25,27 may be formed in part or in their entirety by the second wires alone.

In order to further increase the strength of the strand the core of the strand may be compacted as illustrated in Figures 6 to 9. The strand core may consist of a number of wires laid helically with an equal pitch. A number of known configurations for equal-laid cores are known which may comprise, for example, 36, 41 or 55 individual wires. Ideally a greater number of equal-laid wires would provide greater strength but there are practical limits to the number of wires that can be spun together in a single operation. In embodiments of the invention, however, the central core may be compacted by rolling or die-drawing. This has the effect of flattening the contacting surfaces of the wires within the core, and increasing the areas of contact, but also flattening the crowns of the outer wires of the core to present a much larger contact surface to the succeeding layer. This could then allow a further two layers of Seale-laid wires to be added over the compacted core, to good effect. For example it would be possible to replace the 20- and 26-round wire layers with a double layer of equal-lay wires such as 24/24 that would be in line contact with one another. Figures 6 to 9 show respectively 36 (shown in perspective in Figure 6 and in cross-section in Figure 7), 41 (Figure 8) and 55 (Figure 9) wire cores that have been compacted. The Figures show in particular the flattening of the outer surfaces but flattening of inner inter-wire contact surfaces will also occur.

It will also be understood that when a core is compacted at least some of the wires in at least the outer layer will be shaped such that they present a greater surface area to an adjacent layer, in particular the first non-core layer, such that it may not be necessary to additionally provide shaped wires in a non-core layer.

Roller compaction of these and subsequent layers could also be considered, at least to the extent of increasing the contact patches at the points of contact between the layers (without significantly reshaping the bulk of the wires) but this will have the adverse effect of locking up the cross-over points between the layers and inhibiting the normal wire movements when the strand is bent, giving rise to a substantial increase in flexural stiffness.

In view of the above, it can be seen that the open spiral strand of the invention provides an open spiral strand that is of relatively high maximum tensile strength with a relatively low impact on weight, cost and complexity. The described and illustrated embodiments are to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected. For example, in the above embodiments there are ten layers of non-core wires. Alternatively, the number of layers of wires may be higher or lower in number, provided that at least one inner layer is provided with at least some wires shaped as discussed and with at least seven further layers radially outward of that layer. There will also always be at least one outer layer of wires having a circular cross-sectional shape, so that the strand 1 is an open spiral strand.

In the above embodiments, each of the first and third layers 25, 27 comprises a plurality of pairs of round and half-lock wires 33, 34, and arranged such that the first and second wires 33, 34 alternate with each other in the circumferential direction of the layer 25, 27. Alternatively, only a section or a number of sections of the first and third layers may comprise shaped wires and other arrangements are possible. For example, there may be more than one round wire 33 disposed between two half-lock wires 34.

Furthermore, only one of the first, second and third layers may comprise shaped wires.

Furthermore in embodiments in which an outer sheath 100 is provided consideration may need to be given to the effects of the sheathing itself. In particular with plastic sheathed strands the radial forces referred to above may be further aggravated by the pressure exerted by the sheathing itself, due to residual stresses from the plastics extrusion process and due to external hydraulic pressures if used at considerable water depths. These effects can be mitigated by substantially filling the voids of the strand with an incompressible fluid or blocking material such as a petrolatum based lubricant, to ensure that the sheathing pressures are resisted by the hydrostatic pressure within the blocking material rather than by radial forces from the strand. This can be achieved by ensuring that the volume of air that is entrained within the sheathing is always less than the reduction in volume of the strand at its working load or more importantly at its breaking load. For example if it is known that when the strand elongates by 5% under load, it will concomitantly reduce by 5% in diameter, then the volume enclosed by the sheathing will reduce by 5%, which is the basis for requiring that in embodiments of the invention at least 95% of the void volume, and preferably at least 98% is filled.

It will thus be seen that the present invention, at least in preferred embodiments, proposes a solution to the problem of particularly high radial forces in the inner layers of an open strand, or at least a mitigation thereof, by introducing shaped wires into the most highly compressed inner layers, which serve to lower the contact stresses by substantially increasing the bearing area at the crossover points.

As can be seen from the above description, in particularly preferred embodiments of the invention enhancement of the contact conditions of the succeeding layers of wires can best be achieved by introducing wires that are themselves shaped to present much smoother surfaces to other adjacent layers of wires. Emphasis is placed on the first layer of wires over the (equal-lay) core strand, since this also sees the highest radial (cross-over) forces. There are a number of alternative shaped wire sections that could be considered but the preferred solution is half-lock and round, because this maintains the highest round wire content, whilst dramatically improving the arc of contact with adjacent covers. The alternative wedge or full-lock sections would also enhance the contact conditions with adjacent layers.

It will be appreciated that this solution not only benefits the wires in the shaped wire layer itself, but also improves the contact stress distribution to the underlying and overlying layers. Of course the benefits of using shaped wires can be applied to other layers of the strand, and particularly to alternate layers, since this confers the maximum benefit from the minimum number of additional shaped wires. A further preferred embodiment of this invention would therefore be to add shaped wires to the third layer of wires above the core strand, (as illustrated in the above drawing), and so on. This would have the effect of ameliorating the contact stresses in both the underlying and overlying layers, extending the protection to five of the most highly stressed layers in the strand. It will be understood that the Figures provided, and in particular the cross-sectional views, are for illustrative purposes only and in practice a real cross-section may differ. What is important to recognise, however, is that at least one layer has a relatively smoother layer for supporting other layers. It will also be understood that the shape of the individual wire sections is of secondary importance and that the inter-locking feature per se is not essential. Adding the half- lock sections to a particular layer serves not only to smooth out the undulations in the inner and outer surfaces of that layer, but also increases the number of wires in it, so that the radial force carried per wire is correspondingly reduced. The effect of transforming the cylindrical surfaces in this way, is to dramatically reduce the extent to which the wires interfere or cut into on another at the inter-layer cross-over points. The strand will therefore be more stable and reduce less in diameter under load.

Claims

CLAIMS:

1. A spiral strand comprising a plurality of layers wherein each layer is formed of a plurality of individual wires, wherein at least some of the wires comprising at least one inner layer are shaped so as to provide a comparatively greater surface area contacting an adjacent layer than if said at least some wires were circular in cross-section, wherein said strand further comprises at least seven layers provided radially outwardly of said at least one inner layer.

2. A spiral strand according to claim 1 wherein said shaped wires are provided in a said layer that is anticipated to experience the highest radial forces.

3. A spiral strand according to either of claims 1 or 2 comprising an equal-lay core.

4. A spiral strand according to claim 3 wherein said equal-lay core comprises at least 49 wires.

5. A spiral strand according to claim 3 wherein said equal-lay core comprises at least 55 wires.

6. A spiral strand according to any of claims 3 to 5 wherein said equal-lay core is compacted.

7. A spiral strand according to any of claims 3 to 6 wherein said shaped wires are provided in the first layer provided over said core.

8. A spiral strand according to claim 7 wherein said shaped wires are provided in at least the first and third layers provided over said core.

9. A spiral strand according to any preceding claim wherein said layer comprising shaped wires comprises alternating half-lock and round wires.

10. A spiral strand according to any preceding claim wherein said strand comprises an outer plastics sheathing. A spiral strand according to claim 10 wherein voids in said strand are substantially filled with an incompressible medium.

A spiral strand substantially as hereinbefore described with reference to the accompanying drawings.