This invention relates to an elevator for raising and lowering a pipe string and a drilling rig provided with such an elevator. During the construction of oil and gas wells suc¬ cessive lengths of pipe are attached to one another to form a pipe string which is progressively lowered into the well. The pipe string is usually supported by an elevator which is attached to a travelling block. When it is desired to add an additional length of pipe slips are applied to the existing pipe string. The elevator is then disconnected from the pipe string and raised. One or more lengths of pipe are then connected to the exist¬ ing pipe string and the elevator is reconnected to the uppermost pipe.

The whole pipe string is then lifted slightly to allow the slips to be released whereafter the entire pipe string is progressively lowered into the well until the elevator is just above the slips whereafter the process is repeated. The upper end of each pipe is provided with a threaded socket, referred to in the art as a "box", and the elevator is provided with wedges which, ideally, should engage the pipe immediately below the box. Various problems can arise with known elevators. In particular, it is often difficult to see on which part of the pipe the wedges come to rest. If they grip the outer surface of the box then there is a serious risk that the pipe string will slip, perhaps allowing the entire pipe string to plummet down the well. Alternat¬ ively, if the wedges grip the pipe significantly below the box the pipe string could again slip resulting in the wedges marking the outer surface of the pipe and damaging the box on impact therewith. In this connection it should be appreciated that pipe strings may weigh up

to 500 tonnes .

The former problem is particularly serious and with this in mind, the present invention provides an elevator for raising and lowering a pipe string, comprising a plurality of pipes each provided with a box, which elevator is provided with at least one wedge for grip¬ ping said pipe string, characterized in that said eleva¬ tor further comprises a safety device to inhibit closure of said wedge on a box. Preferably, said safety device comprises a valve and a sensor which is associated with said valve and which, on engagement with said box, allows said at least one wedge to be applied.

Advantageously said sensor and said at least one wedge are juxtaposed so that when said sensor engages said box said at least one wedge will be applied to said pipe close to the bottom of said box immediately below said box.

Preferably said elevator includes a guide plate having an opening for locating said box with respect to said sensor.

Advantageously, said elevator includes an indicator responsive to actuation of said valve.

Preferably, said indicator is responsive to engage- ment of said at least one wedge with said pipe.

Advantageously, said indicator gives a visual and/or audible signal.

Preferably, said sensor comprises a wheel.

In one embodiment, said wheel is provided with a cam surface, the arrangement being such that as said box enters said elevator said wheel rolls along said box and said cam surface actuates said valve after said wheel has rolled a predetermined distance along said box.

Advantageously, said wheel is mounted on an inner housing which is slidably mounted in an outer housing

secured to said elevator.

In another embodiment, said sensor comprises a pivotally mounted trip, a spring biasing said trip towards (in use) a box in said elevator, and a connect- ing member connecting said trip to said valve so that when said box displaces said trip said connecting member actuates said valve.

In a further embodiment, said sensor comprises one or more bands resiliently mounted across said elevator so that when said elevator is lowered onto said box, said band(s) are displaced upwardly to actuate said valve.

Preferably, said elevator includes a spring acting to restore said band(s) to their home position when said elevator is lifted from said box.

Preferably, said elevator includes means to adjust the height of said safety device with respect to said elevator.

The present invention also provides a drilling ring comprising an elevator according to the invention and slips for gripping a pipe string whilst said elevator is disconnected from said pipe string, characterized in that said drilling rig further comprises means connect¬ ing said safety device of said elevator and said slips, said means functioning to release said slips after application of said at least one wedge.

In one embodiment, said means functioning to re¬ lease said slips after application of said at least one wedge comprises a spool valve actuable to allow pneu- matic fluid to release said slips and a time delay circuit which includes an orifice which, on application of pneumatic fluid to apply said at least one wedge to said pipe, allows pneumatic fluid to flow to means for actuating said spool valve at a restricted rate so that said actuation of said spool valve is delayed.

In another embodiment, said means functioning to release said slips after application of said at least one wedge comprises a valve which opens to release said slips only when the pressure applied to close said at least one wedge reaches a predetermined level.

For a better understanding of the present inven¬ tion, reference will now be made, by way of example, to the accompanying drawings, in which:-

Figure 1 is a side view, with parts cut away, of one embodiment of an elevator in accordance with the present invention;

Figure 2 is a side view with parts cut away, of a second embodiment of an elevator in accordance with the present invention; Figure 3 is a side view with parts cut away, of part of a third embodiment of an elevator in accordance with the invention;

Figure 4 is a side view, with parts cut away, of a fourth embodiment of an elevator in accordance with the invention;

Figure 5 shows a pneumatic circuit diagram respon¬ sive to the actuation of the safety device in the embo¬ diments described above;

Figure 6 shows one embodiment of a pneumatic cir- cuit diagram for linking the actuation of the safety device in the embodiments described above to the opera¬ tion of the slips; and

Figure 7 shows another embodiment of a pneumatic circuit diagram for linking the actuation of the safety device in the embodiments described above to the opera¬ tion of the slips.

Referring to Figure 1, an elevator 1 comprises a housing 2 containing wedges 3, which can be applied by a pneumatic cylinder 4 against the outer surface of a pipe 5.

Following the attachment of the pipe 5 to the already existing pipe string, the elevator 1 is lowered over the box 6 located at the upper end of the pipe 5.

This operation is facilitated by a guide bell 7 formed on the bottom of the elevator 1 and having a converging

guide surface 8.

On the top side of the housing 2 there is disposed a guide plate 9 having a centring opening 10 to enable the box 6 and hence the entire pipe 5 to be held sub- stantially concentrically in the elevator 1. For this purpose, the annular surface 11 of the centring opening 10 is tapered downwardly.

A safety device 13 is mounted on the annular plate 9. In the embodiment shown in Figure 1, the safety device 13 comprises an outer housing 14 in which is slidably mounted an inner housing 15. A spring 16 acts between the inner housing 15 and the outer housing 14.

A control valve 21 is mounted on the inner housing 15 and is provided with a pin 20.

A control wheel 17 is rotatably mounted on the inner housing 15 and bears a cam 18, which is constantly in contact with the contact roller 19 of the pin 20 of control valve 21. The position of the control valve 21 within the inner housing 15 can be varied by an adjust¬ ing screw 22. In use as the box 6 is inserted in the elevator 1, the inner housing 15 is displaced to the left as shown in Figure 1. As the elevator 1 is lowered further, the control wheel 17 rotates, so that the cam 18 progressively displaces the control roller 19 to the left until the control valve 21 is actuated. A spring (not shown) ensures return of the control wheel 17 to a home position when the elevator 1 is lifted off the box 6. A safety device having a control wheel 30 without a cam is shown by Figure 2, in which, as in all other Figures, the same reference numerals are used for parts having similar functions.

The control wheel 30 is rotatably mounted in a housing 31, which is disposed on a vertically adjustable

mounting plate 32. The vertical adjustment is achieved by adjusting screws 33, which extend through threaded openings in the mounting plate 32. The control valve 21 is also mounted on the mounting plate 32. The pin 20 of the control valve 21 is biased against the rear of the housing 31 by a spring (not shown). The housing 31 is biased by a spring 34 in the direction of the elevator and, in use, is deflected back from this position as a box 6 enters the elevator 1 as a result of which the control valve 21 is switched by means of the pin 20.

The adjusting screws 33 rest on a baseplate 35 having slots 36 which enable the entire device to be adjusted according to the diameter of the box 6. The baseplate 35 is screwed to the guide plate 9, the fastening screws being identified by reference numeral 37.

A further embodiment is shown by Figure 3, having a safety device incorporating a trip 40. This is arc- shaped and is pivoted to the holding plate 32 on a shaft 43. A connecting bar 41 is pivotally connected between the trip 40 and the pin 20. A restoring spring 42 biases the trip 40 towards the box 6. As a box 6 enters the elevator 1 the trip 40 is displaced resulting in the pin 20 actuating the control valve 21. A particularly simple embodiment of a safety device is shown by Figure 4. The control valve 21, which, just as in the embodiments according to Figures _. 2 and 3, rests on a vertically adjustable mounting plate 32, can be actuated by one or more elastic contact bands 50 extending transversely across the centring opening 10 of the guide plate 9. The home position of the contact bands 50 is identified by the broken line 50\'. When the box 6 of a pipe is guided through the elevator, the contact bands 50 are raised. They yield elastically, in the upward direction withdrawing the pin 20. A spring 52

ensures the return of the pin 20 into its home position as soon as the contact bands 50 are released when the elevator 2 is lifted off the box 6.

This embodiment offers particularly simple con- structional features. The contact bands 50 are merely rigidly fastened, on the side lying opposite the safety device, to a suitable vertically adjustable plate or the like.

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The elevators described with reference to Figures 1 to 4 may be used with control circuits of differing degrees of sophistication. Figure 5 shows the pneumatic circuit diagram of a simple system in which the pneu¬ matic fluid is compressed air.

In use, the rapid-action coupling 60 is coupled to a supply of compressed air (Figure 5). Compressed air initially bears against the control valve 21 and spool valve 61, which is spring-loaded and actuable by com¬ pressed air. As soon as the pin 20 is actuated by the proximity of the box 6, compressed air passes through line 62 and displaces the spool of spool valve 61 to the right as shown in Figure 5. This allows compressed air to pass through spool valve 63 and actuate the pressure cylinder 4 to apply the wedges 3. Simultaneously, com¬ pressed air is applied to indicator 23 to extend bar 24.

If desired, bar 24 may be physically connected to the pressure cylinder 4 so that the bar 24 indicates the true position of the clamping wedges 3.

When the spool of spool valve 63 is depressed the wedges 3 are released. Care must be taken to ensure that spool valve 63 is not activated when the elevator 1 is under load. The circuit diagram according to Figure 6 shows one

way in which the actuation of the elevator may be linked to the actuation of the slips. In the example shown, the spool valve 63 (Figure 5) is replaced by an electromag- netically actuable spool valve 64. The pneumatic cylin- der 4 for applying the wedges 3 of the elevator 1 and a pneumatic cylinder 65 for releasing the slips (not shown) are connected so that compressed air is not supplied to the pneumatic cylinder 65 to release the slips until after the wedges 3 are applied. In particu- lar, when compressed air is applied to line 70 pneumatic cylinder 4 applies the wedges 3.

Simultaneously, pneumatic fluid flows through line 68 to a time delay circuit 67.

More particularly, the pressure fluid flows through line 68\' to an adjustable orifice 67\' which restricts the flow and thus the actuation time of spool valve 67" to permit compressed air to flow into the pneumatic cylinder 65. In practice it will normally be necessary for the elevator 1 to be raised to lift the pipe string slightly before the slips will release.

Conversely, when spool valve 64 is depressed com¬ pressed air flows through line 168 to pneumatic cylinder 65 to apply the slips whilst the flow of compressed air to the pneumatic cylinder 4 via lines 169 and 69 is delayed by delay circuit 66 which is similar to delay circuit 67.

In an alternative embodiment, valve 67" may be operable in response to an upward thrust on the slips, i.e., in response to the pipe string being lifted by the elevator.

In the alternative embodiment of Figure 7, compres¬ sed air is applied to pneumatic cylinder 4 which applies the wedges 3. When the pneumatic cylinder 4 reaches the limit of its travel the pressure rises and opens adju- stable valve 72 at which time compressed air is avail-

able to spool valve 73 to release the slips.

Various modifications to the embodiments described are envisaged, for example, it would be desirable to have sensors to confirm engagement of the wedges 3 with the pipe and the slips.

In such an embodiment release of the slips would be inhibited until engagement of the wedges 3 of the eleva¬ tor 1 with the pipe is confirmed by the sensor. Similar¬ ly, release of the wedges 3 of the elevator 1 would be inhibited until engagement of the slips is confirmed by the appropriate sensor.

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