US3026668A

US3026668A - Method and apparatus for handling cables

Info

Publication number: US3026668A
Authority: US
Publication date: 1962-03-27
Anticipated expiration: 1979-03-27

Description

March 27, 1962 K. E. LATIMER METHOD AND APPARATUS FOR HANDLING CABLES Filed Dec. 24, 1957 2 Sheets-Sheet 1 March 27, 1962 K. E. LATIMER METHOD AND APPARATUS FOR HANDLING CABLES 2 Sheets-Sheet 2 Filed Dec.

3,026,668 METHOD AND APPARATUS FOR HANDLING CABLES Kenneth Eric Latimer, London, England, assignor t Submarine Cables Limited, London, England, a corporation of Great Britain Filed Dec. 24, 1957, Ser. No. 705,049 Claims priority, application Great Britain Jan. 1, 1957 3 Claims. (Cl. 57-77.4)

The present invention relates to the picking up or handling of cables, hawsers and the like and especially concerns the picking up of submerged armoured submarine cables.

When submarine cables armoured with a single layer of armour wire are being picked up from a considerable depth of water the cable is subjected to substantial tension. This tension is sustained by the armour wire, the effect of which is to cause this to unlay or untwist. This unlay must be corrected as the cable is brought inboard else damage may be caused not only to that portion of the cable in the storage tanks, but also to the cable at the point where it is being lifted from the ocean bed.

In practice the cable is picked up by the cable ship over a sheave positioned at the bow of the ship, which sheave normally has a profile consisting of a V with a rounded base, the sides of the V being much steeper than the angle of friction between the cable and the sheave. As the cable passes over this sheave and if it is at an angle to the sheave, a torque is imparted to the armour wire due to the rolling of the cable into the rounded base of the sheave.

This torque, which is in addition to that caused by the tension in the cable, may either increase the tendency of the armour Wires to unlay or may, if in the reverse direction, counteract the torque due to the tension in the cable, the latter being the effect usually desired.

Previously the adjustment of the relative inclination of the bow sheave and cable, to ensure that the desired torque is obtained at the bow sheave, has been achieved by steering the ship on such a course that the cable reaches the sheave at the correct angle thereto to ensure the amount of roll necessary. Proposals have also been made for the bow sheave to be mounted in a movable mounting with its axis inclined at an angle to the horizontal so as to ensure that the cable will roll on the side of the sheave the desired amount without having to steer the ship to achieve this result.

The adoption of the steering technique entails much skill in order to obtain satisfactory results and the movable mounting of the bow sheave has been found both expensive and too complicated to be satisfactory in use.

According to the present invention, the means for imparting twist to an armoured cable, hawser or the like comprises a cone or a truncated cone element capable of rotation about its longitudinal axis, the half angle of the cone being not less than that given by the following formula and not greater than A where A is the angle of friction between the cable or the like and the surface of the cone:

0 X rs in which a=the minimum angle in radians of the taper (the half angle of the cone).

0=the angle of lay in radians, that is the angle between the armour wire and the longitudinal axis of the cable.

nited States 1 atent ra=the means radius of the armour wire Wltl'l. respect to the axis of the cable.

mean radius ra.

3,Z,h63 Patented Mar, 27, 1952 rs=external radius of the cable including any serving which may be provided on the outside of the armour wire.

R=the mean radius of the central portion of the cone over which the cable passes.

X =the length of the arc in which the cable is in contact with the surface of the cone.

In further accordance with the invention a sheave for use in the picking up or handling of hawsers, armoured submarine cables and the like comprises a centre portion or belly for rotation about a longitudinal horizontal axis and two flanges extending from said central portion and defining a groove, wherein that surface portion of the belly forming the base of the groove is a cone, tapered, that is inclined in one direction with respect to the axis of rotation, and is sufliciently Wide to allow of movement of the cable thereacross.

Preferably the cable is guided onto the cone of the sheave at one point of the surface thereof and guided oil at another point by guide means, whereby the cable is forced to roll'a fixed distance in the axial direction of the cone, down the latter, to control the rate at which the cable is twisted per unit length thereof.

The guide means may comprise two small pulleys which are conveniently mounted with their axes roughly perpendicular to each other. Alternatively a fixed guide or fleeting knife-may be provided.

The cable is preferably passed around the sheave for an angle of approximately In this case substantially equals and thus, for a given cable a may be calculated.

The maximum angle of the cone is the angle of friction since for a greater angle, the cable would slip down the cone rather than roll.

The desired minimum angle of the central conical portion of a sheave or of a drum which will give the required amount of twist to a given cable will now be envolved with reference to the accompanying drawing in which FIGURE 1 is a plan view of a sheave according to the present invention for use in the picking up of cables or the like,

FIGURE 2 is a diagram showing the cable tension and the resultant torque'acting on the armoured wires thereof,

FIGURE 3 is -a diagram showing an end elevation of the sheave with the flanges thereof removed and indicating the forces acting on a length of cable when passed therearound,

FIGURE 4 is a force diagram corresponding to FIG- URE 3,

FIGURE 5 is a side elevation of the sheave showing the cable in an equilibrium position thereon.

FIGURE 6 is a force diagram corresponding to FIG- URE 5.

FIGURE 7 is a schematic view of part of a cable laying and picking up ship incorporating a conical bow sheave.

Referring to FIGURE 2 the armour wires A have a small angle of lay 0 with respect to the longitudinal axis of the cable C and are wrapped around the cable at a If the tension of the cable is T (this is substantially the tension in the armour wires) and the torque acting on the wires is t then t=Tra6 Referring now to FIGURE 3 the sheave S which is mounted on a shaft s for rotation about a horizontal axis has a mean radius R on the central conical portion there- .at 3 around which the cable is given several turns.

of and the cable is wrapped around it for an arc of length X subtending an angle as at the axis of the sheave.

Consider an elemental length dX of arc subtending an elemental angle dqb at the centre of the sheave, then from the triangle of forces (FIG. 4) it will be realised that this elemental length of cable exerts a force Tdqfi on the sheave.

If the total torque to be provided by the sheave is t and if this is to be exerted at the outer periphery of the cable including the servings thereon at an external radius rs then the total tangential force F to be provided over the length of the cable X must be since t=Frs and the force dF to be provided in the element of length dx is:

As may be seen in FIGURE 6 the forces acting on the element of cable dx are Tdq), dF and the force acting normally to the cone, which is slightly less than Td. If the half angle of the cone is a (in radians):

dF=Tda since a is a small angle.

Also (see FIGURE 3) dx=Rd and the following expression may be obtained by substituting for dF using Equation 2 and for t using Equation 1:

If the cable is laid around the sheave for an angle of 90 R/X has the value 2/ 11- and the value of ra/rs will be somewhat less than unity. Hence a is somewhat smaller than but of the same order. The angle of lay 0 of the armour wire A does not vary much, nor does the ratio ra/rs, so that it is quite practicable to build a sheave with a fixed angle. It will be noted that a is not a function of tension, so that the same sheave may be used for different depths and for different weights of armour so long as 0 is kept constant.

It will be realised therefore that the cable C can remain in torsional equilibrium in any part of the cone irrespective of the depth of water.

The angle a is the minimum effective half angle of the conical portion of a sheave to be used for handling cables. The maximum practical angle is the angle of friction between the surfaces of the conical portion of the sheave and the cable being handled. In practice the sheave is given a half angle between these two limits, for example, a preferred half angle of l /za, the increase over the theoretical angle on having been found to be necessary in some cases in order to compensate for a fairly large resistance to the twisting of the cable due to internal friction (flattening and bending).

When the cable C is being picked up by a cable ship as indicated at 1 (FIG. 7) it is fed from the conical bow sheave S over a dynamometer 2 which measures the tension in the cable, to a power driven capstan indicated The capstan applies the pulling force for hauling in the part of the cable extending from the conical surface of the sheave S and remaining to be hauled in and over the sheave. The cable then passes from capstan 3 to a drawolf drum 4 and over groups 5, 6 of guide rollers into a cable tank 7 in the hold of the ship.

The torque due to the bow sheave S produces a laying up of the armour on the cable C so that normal lay is restored already after passing the bow sheave. In other words the bow sheave S exerts a torque just equal and opposite to that produced by the tension of the armour wires, thus restoring the lay to its normal or original value. The torque produced by the bow sheave and which is present in the section 1 of the cable between the bow sheave and the capstan, is resisted by the capstan since the cable is prevented from rotation by the friction present at the surface of the capstan. Thus the portion of the cable C passing into the cable tank 7 is of normal lay and is free from torque. Only that section of cable between the bow sheave and the capstan is subjected to torque by the bow sheave and there is no tendency for this torque to appear in the section of the cable between the bow sheave S and the ocean bed when a steady condition of pickup of cable is maintained. This is because for each elemental length of cable of normal lay free from tension picked up from the ocean bottom, there will be a similar length of cable of normal lay free from tension passing into the cable tanks. There is, thus, no tendency for turns to accumulate in the suspended cable. It will be understood however that the section of cable between the sheave and the ocean bed will have its armour wire partially unwound, which unlaying is due to the tension in the cable.

To some extent a device of this kind is self-compensating. Assuming for example that the cable has been previously picked up with a torque present in the suspended portion thereof, this will result in the section of the cable at the sea bed being laid up, i.e. tending to assume a greater twist, and that at the bow being less unlaid, i.e. untwisted, than it should be. This will result in the cable rolling down the cone further than it otherwise would, so that the cable going into the tanks would initially have abnormally short lay and would be springy. This would remove turns in the suspended portion of cable and eventually reduce the torque at the sea bed to zero, at the expense of some damaged cable in the tanks. Such a sacrifice to avoid cable faults is highly advantageous.

However, if it is desired to control the extent of the unlay of the cable, guide pulleys G, G (see FIGURE 1) mounted respectively on shafts g, g with their axes perpendicular to each other are provided to define the extent to which the cable can roll on the cone. The guide pulleys are so positioned as to be contacted by the cable at points which are mutually displaced axially and circumferentially of the conical surface of the sheave S, so that the cable is constrained to run in contact with a predetermined limited axial extent of the conical surface and at an inclination from the greater diameter portion of the surface toward the smaller diameter portion of the surface. Since the cable is nearly in equilibrium the guide pulleys do not exert any great force on the cable and there is little chance therefore of damage being done to the servings of the cable.

What is claimed is:

1. In a method of hauling in an armoured cable, hawser or like member having armour wires laid helioally with respect to the longitudinal axis of the member whereby tensioning of said member while hauling it in produces a torque therein tending to twist and thus unlay said wires: applying a pulling force to said member to haul said member in and thereby tension it and produce such a torque therein; and counter-acting said torque by passing the tensioncd member over a conical surface rotatable about an axis generally transverse to a plane in which said member is hauled in with said member in contact with said conical surface between the point of application of said pulling force to said member and the part of the member remaining to be hauled in, rolling said member about the longitudinal axis thereof down said conical surface generally transversely to the direction of hauling in passage of said member over said conical surface, the angularity of said conical surface in contact with said member being less than the angle of friction between said member and said conical surface and exerting on the latter a torque equal and opposite to the torque 5 produced by tensioning said member, whereby said member is in torsional equilibrium on said conical surface.

2. In a method according to claim 1, guiding said member by contacting it on opposite sides of said member at points which are mutually displaced axially and circumferentially of said conical surface to constrain said member to run in contact with a predetermined limited axial extent of said surface and at an inclination from the greater diameter portion of said surface toward the smaller diameter portion thereof.

3. A sheave apparatus for use in the handling of armoured submarine cables, hawsers and the like, comprising a sheave mounted for rotation about a longitudinal substantially horizontal axis and having a central portion and two flanges extending from said central portion and defining a groove, that part of the central portion forming the base of the groove being tapered in a longitudinal axial direction and being sufficiently wide to allow slight movement of the cable across its surface, and guide means, consisting of two small pulleys mounted with their axes approximately perpendicular to each other, mounted at two locations adjacent the tapered base of the sheave to define the points at which the cable is guided 0n and off the sheave, whereby the cable is constrained to roll by said slight movement across the tapered central portion of the sheave a fixed distance in the axial direction of the central portion to control the rate at which it is twisted per unit length thereof.

References Cited in the file of this patent UNITED STATES PATENTS