Because wire ropes are made up of a number of strands of wire, their outer surfaces are uneven, and the strands can move slightly relative to each other. This can make taking an accurate measurement of their outer diameter difficult. For example if a standard micrometer screw gauge is used different readings will often be obtained for the same piece of wire rope.

The present invention therefore aims to provide a method for accurately and consistently measuring the diameter of wire rope.

The present invention provides a gauge for measuring the diameter of wire rope, the gauge comprising a body having a plurality of bores therethrough the bores being of circular cross section and of different diameters whereby the wire rope can be passed through a number of the bores to determine in which bore it fits most closely, thereby to measure the diameter of the wire rope.

Preferably each of the bores is at least eight times as long as it is in diameter. This ensures that a sufficient length of the rope will be within the bore at one time for an'average'diameter of the rope to be measured.

More preferably each of the bores is at least ten times as long as it is in diameter.

Wire rope generally comprises a helical coil of, for example, six strands, around a core. The length of bore required to give an accurate measurement therefore depends on the nature of the wire rope. Preferably

the length of the bore is at least a quarter of the helical length of the wire rope, more preferably at least half of the helical length.

Preferably each bore has a main section of constant diameter and an end section which is tapered outwards to guide the end of the wire rope into the bore. This helps to guide the end of the wire rope into the bores.

Preferably the body comprises a support member and a plurality of tubes supported in the support member, each tube having one of the bores therethrough. For example each of the tubes may be supported in an aperture in the support member.

Preferably the tubes have been inserted into the apertures when the tubes were at a temperature lower than that of the support member, and the temperatures of the tubes and the support member have been allowed to equalize thereby to secure the tubes in the apertures. This method fixes the tubes very securely in place. Conveniently the support member may comprise a support plate.

Preferably the tubes are formed of hardened steel, and the support member is preferably formed of aluminium. This combination gives a good combination of strength and low total weight. Alternatively the gauge body could be formed from a single block of hardened steel.

The present invention further provides a method of measuring the diameter of a wire rope comprising passing the wire rope through a plurality of the bores of a gauge according to the invention, selecting one of the bores through which the wire is the closest fit, and using the diameter of the bore as an indication of the diameter of the wire rope.

The present invention further provides a method for passing a wire rope into a volume of high pressure fluid comprising providing a gauge according to the invention, providing a plurality of sealing tubes having different inner diameters corresponding to those of the tubes of the gauge, passing the wire rope through a number of the tubes of the gauge to select one of those tubes that the wire rope fits most closely, selecting one of the sealing tubes having a diameter corresponding to the selected tube, and forming a seal from the selected sealing tube with the wire rope extending through it such that the wire rope can be passed into or out of said volume through the seal.

The volume of high pressure fluid may be an oil well.

Preferred embodiments 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 plan view of a gauge according to the invention; Figure 2 is a side view of the gauge of Figure 1; and Figure 3 is a schematic section through an oil well having a wire rope extending into it which has been sealed according to the invention.

Referring to Figures 1 and 2 a gauge 8 for wire rope comprises a body 10 formed from a number of gauge tubes 12, in this case ten, supported in a support plate 14. Each of the gauge tubes 12 is formed from hardened steel and has a bore 16 through it comprising a main section 16a which is approximately eight times as long as it is in diameter and is of constant circular cross section along its length, and an end section 16b at each end

of the main section 16a, the end sections 16b tapering outwards towards the ends 18 of the gauge tube 12. The bores 16 are all of different diameters, in this example the diameters being at intervals of 0.002 (two thousandths) of an inch with the largest one being 3/16 (three sixteenths) of an inch. The support plate 14 comprises an annular aluminium sheet 20 having ten circular apertures 22 through it, equally spaced from each other. The apertures 22 are all of the same size which is just smaller than the outer diameter of the gauge tubes 12, which are also equal to one another.

In order to assemble the gauge, the gauge tubes 12 are cooled so that they contract and the support plate 14 is heated so that it expands, so that the gauge tubes 12 can be inserted into the apertures 22. The temperatures are then allowed to equalize so that the gauge tubes 12 expand again and the support plate 14 contracts so that the gauge tubes 12 are held tightly in the apertures.

The inner diameter of each of the gauge tubes 12 is engraved next to it on the support plate.

Referring to Figure 3 an oil well 30 comprises a well head 32 over the well bore 34, and further apparatus 36 over the well head, the details of which are not relevant to the present invention. In order to measure various parameters within the bore 34 an instrument pack 38 is lowered into the bore on a wire rope 40 which passes through a number of sealing tubes 42. The sealing tubes 42 form a seal around the wire rope 40 to contain the mud which is at very high pressures within the well bore 34.

It is therefore essential that the rope is a very good fit within the sealing tubes 42. The sealing tubes 42 are therefore provided in a number of different sizes so that they can be selected to fit the wire rope 40.

The gauge of Figures 1 and 2 is therefore designed such that the different bore sizes in the gauge tubes 12 correspond to the different sizes of sealing tube 42 which are available. Then in order to seal the wire rope 40 in position it is first pushed through a number of the gauge tubes 12 until one of them can be selected which is the closest fit around the wire rope 40. The inner diameter of that gauge tube 12 is noted and the sealing tubes 42 selected to be of the same diameter. The selected sealing tubes 42 are then put in place over the well head 32, the wire rope 40 passed through them and the instrument pack attached to the wire rope 40 so that it can be lowered into the well bore 34.

It will be appreciated that, although the example described above relates to the sealing of wire rope into an oil well, the method of using the gauge 8 to measure the diameter of the wire rope and to match it to a suitably sized bore to form a seal could be used in other applications where a wire rope needs to be inserted into a volume of high pressure fluid.

Furthermore, the gauge 8 can also be used for measuring wire rope in applications other than the forming of a seal around the wire rope.