Certain known power tongs have a body, a rotary rotatably mounted in said body and at least one active jaw which, in use, is pushed against a pipe in the rotary and grips it for rotation with the rotary. In one common arrangement the pushing action is generated by a cam member which is bolted to the rotary and is shaped so that the active jaw is cammed against the pipe on rotation of the rotary relative to the active jaw in one sense and is released on rotation of the rotary relative to the active jaw in the opposite sense.

With known power tongs high torques, for example 50,000 ft.lbs, are often applied to tubulars.

It will be appreciated that if the active jaw is not applied to the tubular with sufficient force the tubular will slip.

Because activation of the active jaw relies on rotation of the rotary relative to the active jaw there is a period when the active jaw may become entrained with the rotary and rotate a small distance relative to the tubular before the active jaw is fully applied.

Whilst this is of little consequence for many types of tubulars this is extremely detrimental for certain tubulars, particularly those which have special surface coating which may be torn away.

In PCT/EP92/00648 the present applicant discloses a tong having a rotary with an active jaw which can be applied by hydraulic rams prior to rotating the rotary.

However, this arrangement cannot readily be adapted to

the myriad of tongs in the field which rely on cams to displace the active jaw into engagement with the tubu- lar.

According to the present invention there is provi- ded a power tong for rotating a tubular in wellbore operations, the power tong comprising an outer case, a rotary rotatably mounted in the outer case for rotating a tubular, apparatus for rotating the rotary to rotate the tubular, at least one active jaw movably mounted on the rotary and displaceable relative thereto to grip the tubular, the at least one active jaw having a gripping element for contacting the surface of the tubular, and wherein one of said active jaw and said rotary is provided with a cam and the other with a cam follow- er, characterized by pre-load apparatus interconnected directly or indirectly between the at least one active jaw and the rotary for, in use, displacing said active jaw with respect to said rotary so that the active jaw is cammed into gripping engagement with the tubular and so inhibit relative rotation between said active jaw and said tubular on subsequent rotation of said rotary.

The gripping element may be formed as an integral part of the jaw although the jaw will normally comprise a jaw holder on which the gripping element is mounted.

Further features are set out in Claim 2 et seq.

For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings, in which:- Fig. 1 is a simplified plan view of one embodiment of a tong in accordance with the present invention; Fig. 2A is a simplified plan view of the active jaw holder and the two passive jaw holders which form part of the tong shown in Fig. 1; Fig. 2B is a simplified plan view of the active and passive jaws which form part of the tong shown in Fig. 1 and which comprise the active and passive jaw holders shown in Fig. 2A with their respective gripping elements mounted thereon; Fig. 2C is a section taken on line 2C-2C of Fig.

2A; Fig. 2D is a section taken on line 2D-2D of Fig.

2A; Fig. 3A is a simplified plan view of an alternative jaw assembly being applied to a tubular (shown in cross- section); Fig. 3B is a front view of a gripping element which forms part of a passive jaw shown in Fig. 3A; Fig. 3C is a section of a detail of Fig. 3A; Fig. 3D is a front view of the detail of Fig. 3C; Fig. 4A shows the jaw assembly of the tong of Fig.

1 mounted on a tubular before the application of pre- loading force; Fig. 4B shows the jaw assembly of the tong of Fig.

1 mounted on a tubular after the application of pre- loading force; Fig. 5A is a side view of a second embodiment of a gripping element; Fig. 5B is a plan view of the gripping element shown in Fig. 5A; Fig. 6A is a side view of a third embodiment of a

gripping element; Fig. 6B is a plan view of a gripping element shown in Fig. 6A; and Fig. 7 is a perspective view of a modified active jaw.

Referring to Fig. 1 of the drawings there is shown a power tong which is generally identified by the refer- ence numeral 1.

The power tong 1 comprises an outer case 2 which accommodates a rotary 3 which can be rotated by a hy- draulic motor 4.

The rotary 3 accommodates a jaw assembly 5 which comprises two passive jaws 6 and 7 and an active jaw 8.

The passive jaws 6 and 7 and the active jaw 8 each comprise a jaw holder and a gripping element removably mounted thereon.

As shown in Fig. 2A the passive jaw 6 and 7 and the active jaw 8 comprises jaw holders 9, 10 and 11 respect- ively each of which is provided with stops 12 to inhibit circumferential displacement of their respective grip- ping elements 13, 14, 15 (Fig. 2B).

As can be seen from Fig. 2D the jaw holder 9 is provided with holes 16 to facilitate attachment of the gripping element 13. Similarly, the active jaw 8 is provided with holes 17 to facilitate attachment of the gripping element 15 (Fig. 2C). In addition, the active jaw 8 is provided with two aligned bores 18, each of which can accommodate a pin which is held in position by a circlip 20 which supports a cam follower in the form of a roller 19 shown in dotted lines in Figs. 2B and 2C.

Figures 3A-3D show an alternative jaw assembly to the jaw assembly shown in Figs. 2A-2D. Parts having similar features to parts shown in Figs. 2A-2D have been identified by the same reference numeral with the addi- tion of an apostrophe. The essential difference is that

the gripping elements 13', 14', 15' are mounted on respective carriers 21, 22, 23 which are mounted on their respective jaw holders 9', 10', 11'.

The carrier 21, 22 and 23 protect the jaw holder 9', 10' and 11' in the event that the gripping elements 13', 14', 15' are worn down.

Each carrier 21, 22, 23 is provided with dovetail keys 24 which slide in corresponding dovetail slots 24a in their corresponding jaw holder.

Similarly, each gripping element 13', 14', 15' is provided with dovetails 25 which slot into corresponding dovetail slots in their respective carriers.

Turning now to Figs. 4A and 4B the passive jaws 6, 7 are bolted to the rotary 3 which is provided with a cam 26 having a camming surface 27.

The active jaw 8 is mounted in the rotary 3 with its rollers (cam followers) 19 engaging the camming surface 27.

A first hydraulic piston and cylinder assembly 28 is pivotally connected between the passive jaw 6 and one end of the active jaw 8 whilst a second hydraulic piston and cylinder assembly 29 is pivotally connected between the passive jaw 7 and the active jaw 8.

The first hydraulic piston and cylinder assembly 28 and the second hydraulic piston and cylinder assembly 29 are connected to a control system which comprises hy- draulic lines 30 and 31, a spool valve 32 operable by a lever 33, a reservoir 34, a bleed valve 35, a pump 36 actuable by an air supply 37, a pneumatic accumulator 38, and non-return valves 39.

As shown in Fig. 4A the air supply 37 is switched off and the active jaw 8 is shown spaced from the tubu- lar 40.

For the purpose of this explanation it will be assumed that it is desired to tighten ("make up") a

joint by rotating tubular 40 clockwise. Air supply 37 is actuated and pump 37 pumps hydraulic fluid through hydraulic line 30 to expand the second piston and cylin- der assembly 29. At the same time hydraulic fluid in the first piston and cylinder 28 returns to the reser- voir 34 via hydraulic line 31. Contraction of the first piston and cylinder 28 is facilitated by a return spring in the first piston and cylinder assembly 28 as shown.

The expansion of the second piston and cylinder assembly 29 displaces the active jaw 8 relative to the cam sur- face 27 so that the rollers 19 roll along the cam sur- face 27. This urges the active jaw 8 against the tubu- lar 29 with a force determined by the pressure of the air supply or a pressure relief valve (not shown) in parallel with bleed valve 35.

At this point the active jaw 8 and the passive jaws 6 and 7 are in full engagement with the tubular 40. The rotary 3 may then be rotated to rotate the tubular 40.

The control system may be mounted on the rotary 3 for rotation therewith, the air supply 37 being connec- ted to the pump 36 as and when needed, the pneumatic accumulator 38 helping compensate for any seepage.

In order to release the active jaw 8, the bleed valve 35 may be opened to allow hydraulic fluid to flow through hydraulic line 30 back to the reservoir 34.

If it were desired two unscrew ("break") a joint then, starting from the position shown in Fig. 4A the spool valve 32 would be displaced to the right and the pump 36 actuated. The first piston and cylinder assemb- ly 28 would expand and the second piston and cylinder 29 assembly contract. The anti-clockwise movement of the active jaw 8 relative to the rotary 3 would cause the rollers 9 to roll along the cam surface 27 and the active jaw 8 to be urged into gripping engagement with the tubular 40 prior to activation of the rotary 3.

The gripping elements 13, 14, 15 may take the form of teeth although they preferably comprise the appli- cants "non-making jaws". A particularly suitable grip- ping element is disclosed in applicants co-pending PCT application PCT/GB97/ (based on GB97/01939).

The gripping elements can be made of a wide variety of materials, for example aluminium, zinc, copper, brass, bronze or soft steel. Optional teeth may be provided. The surface of the gripping elements may be covered to conform generally to the shape of the tubu- lar, for example as shown in Figs. 5A and 5B or of a tighter radius of curvature as shown in Figs. 6A and 6B.

If desired the gripping elements may be flexible or may comprise a plurality of rigid elements laid side by side. Although not recommended, it is conceivable that the gripping element could be formed integral with a jaw holder.

Fig. 7 shows a modified active jaw 8" which, in addition to comprising a jaw holder 11" and a gripping element 15" is also provided with rods 41 and 42 to facilitate attachment of the first and second piston and cylinder assemblies thereto.

* * * Various modifications to the arrangement disclosed are envisaged, for example the first piston and cylinder assembly 28 and the second piston and cylinder 29 could be replaced by a single double acting piston and cylin- der assembly. Similarly, a single piston and cylinder assembly could be used and moved from the position of the first piston and cylinder assembly 28 and the second piston and cylinder assembly 29 for breaking or making joints as desired.