This invention relates to anti-rotation devices for use with well tools and, more particularly, but not exclusively, is concerned with anti-rotation devices for plugs, float collars and float shoes. The present inven¬ tion also relates to well tools provided with said anti- rotation devices.

During cementation, plugs are lowered into a well bore. Once the cementation operation is completed these plugs, together with the associated float collar and/or float shoe are drilled out.

One of the difficulties in drilling.out plugs is that they can rotate with the drill thereby seriously delaying progress. Various proposals have been made for inhibiting the rotation of plugs in the well bore during drilling out. Typically, these comprise providing the plug and/or the float collar/shoe with anti-rotation devices which are intended to inhibit rotation of the plug relative to the float collar/shoe which is non-rotatably set in the casing.

Early anti-rotation devices included protrusions which had the disadvantage that the full weight of the drill string was applied to the protrusions via the plug with the result that the protrusions often broke under the axial load and were rendered largely ineffective.

Applicants WO-91/17340 discloses an anti-rotation device in which the axial load is carried by circular load bearing seals mounted on the upper surface of the float collar and the lower surface of the plug and both the plug and the float collar are provided with teeth which inter-engage but are not subject to any axial load. As a result, the teeth are used solely to transmit rotational forces. Whilst this arrangement works generally acceptably

it has a disadvantage that in deviated wells the protru¬ sion on, for example the plug may land on the load bearing member of the float collar rather than inter-en¬ gaging the teeth thereof. On attempted rotation of the plug these protrusions may then be broken off.

The present invention aims to provide an anti- rotation device which can be used for both straight and deviated wells.

According to one aspect of the present invention there is provided an anti-rotation device for use in a well tool, said anti-rotation device comprising a ta¬ pered male member having a corrugated outer surface.

Advantageously, said anti-rotation includes a load bearing surface circumjacent said tapered male member. Preferably, said load bearing surface is also a sealing surface.

Preferably, said well tool is a plug and said anti- rotation device is attached to or formed as an integral part of said plug. Advantageously, said corrugations extend generally parallel to the longitudinal axis of said plug.

Preferably, said corrugations comprise mounds and recesses which, when viewed in cross-section in a plane perpendicular to the longitudinal axis of said plug, are bound by semi-circles.

Advantageously, said corrugations comprise mounds and recesses and the outer extremity of said mounds lie on an imaginary sphere having its centre substantially on the longitudinal axis of said plug. Preferably, said corrugations comprise mounds and recesses and the inner extremity of said recesses lie on an imaginary sphere having its centre substantially on the longitudinal axis of said plug.

Advantageously, the centres of said imaginary spheres are displaced from one another along the long-

itudinal axis of said plug.

The present invention also provides an anti- rotation device for use in a well tool, said anti- rotation device comprising a female socket having a corrugated surface complementary to the corrugated outer surface of the tapered male member of the aforedescribed anti-rotation device in accordance with the invention.

Preferably, said anti-rotation device includes a load bearing surface circumjacent said female socket. Advantageously, said load bearing surface is also a sealing surface.

Whilst it is preferred that a well tool such as a float shoe or a float collar be provided with a female socket and the bottom of a plug with a tapered male member it is also possible for the float shoe or float collar to be provided with an upwardly tapered male member and the bottom of the plug with a female socket. However, the former arrangement is much preferred as any debris is directed downwardly through the float collar whilst an upwardly tapered male member would direct the debris onto the upwardly facing surface of the float collar/shoe circumjacent the upwardly tapered member.

Preferably, said anti-rotation device comprises a load bearing surface circumjacent said female socket. Advantageously, said load bearing surface is also a sealing surface.

Preferably, the projection of a tapered male member beyond its load bearing surface is less than the depth of the female socket so that when the tapered male member is fully inserted in the female socket substanti¬ ally all axial load is transmitted via said load bearing surfaces.

For a better understanding of the invention refer¬ ence will now be made, by way of example, to the accom¬ panying drawings, in which:-

Figure 1 is a vertical cross-section through one embodiment of a top plug in accordance with the inven¬ tion;

Figure 2 is a vertical cross-section through one embodiment of a bottom plug in accordance with the invention; Figure 3 is a view taken on line III-III of Figure 1;

Figure 4 is a perspective view taken in the direc¬ tion of arrow IV in Figure 1; and

Figure 5 is a vertical cross-section showing the top plug and bottom plug approaching a float shoe in a casing.

Referring to Figure 1 of the drawings, there is shown a top plug which is generally identified by the reference numeral 1. The plug 1 is made of plastics material and comprises a core 2 of rigid polyurethane in an outer casing 3 of elastic polyurethane. The outer casing 3 includes a plurality of wipers 4, a sealing fin 5 and a top 6.

The top plug 1 includes an anti-rotation device in the form of a tapered male member 7 which, as is more clearly shown in Figure 4, has a corrugated outer sur¬ face 8 comprising alternate mounds 9 and recesses 10.

As shown in Figure 1, the outer extremity of each of the mounds 9 lies on an imaginary sphere having its centre on the longitudinal axis 11 of the top plug 1 and a radius r^. Similarly, the inner extremity of each of the recesses 10 lies on an imaginary sphere having its centre on the longitudinal axis 11 of the top plug 1 and a radius ^. It will be noted that r2 is displaced from r-^ along

the longitudinal axis 11 of the top plug 1.

As can be seen from Figure 3, if a section is taken through the tapered male member 7 perpendicular to the longitudinal axis 11 of the top plug 1 the mounds 9 and recesses 10 have the outline of substantial semi-circles which flow smoothly into one another.

The tapered male member 7 is surrounded by an annular load bearing and sealing surface 12.

The core 2 is provided with a cavity 13 which reduces the overall weight of the top plug 1 and facili¬ tates drilling out of the top plug 1 after use.

Referring now to Figure 2, the bottom plug 101 is in many respects similar the top plug 1 and parts having similar functions have been identified by similar refer- ence numerals in the 100 series.

The bottom plug 101 differs from the top plug 1 in that the top surface 106 is provided with an anti- rotation device in the form of a female socket 114 having a shape which is complementary to the tapered male member 7.

In addition, the cavity 113 extends the full axial length of the bottom plug 101 and is provided with a removable bursting disk 115.

Turning now to Figure 5, during the construction of a typical well a hole is first drilled to a depth of, say 1000m. A float shoe 201 provided with a female socket 214 is secured onto the end of a string of casing

202 which is lowered to within a few metres of the bottom of the well. The bottom plug 101 is then placed in the casing

202 and the calculated quantity of wet cement pumped onto the top of the bottom plug 101 which is slowly forced down the casing 202 by the weight of the cement and the pressure applied thereto by the pump. Figure 5 shows the bottom plug 101 approaching the

float shoe 201. As the bottom plug 101 approaches the float shoe 201 the tapered male member 107 engages in the female socket 214 and the annular load bearing and sealing surface 112 sealingly engages a similar annular load bearing and sealing surface 212 circumjacent the female socket 214.

Once the bottom plug 101 seats on float shoe 201 the pressure of the wet cement builds up and fractures the bursting disk 115 thereby allowing the cement to pass through the float shoe 201, outwardly to the walls of the hole and upwardly between the outer wall of the casing 202 and the wall of the hole.

When the desired amount of cement has been pumped into the casing 202 the top plug 1 is placed in the casing 202. The top of the casing 202 is then closed and drilling mud is admitted to the top of the casing 202 to drive the top plug 1 and the remaining wet cement down the casing 202.

Cement continues to flow until the tapered male member 7 on the top plug 1 enters the female socket 114 on the bottom plug 101 and the load bearing and sealing surface 12 abuts the load bearing and sealing surface 112' circumjacent the female socket 114.

Hydraulic pressure may be maintained on the top plug 1 whilst the cement dries. At this stage the casing 202 is depressurised, opened and a rotating drill is lowered down the casing 202 until it engages the top plug 1.

Because the tapered male member 7 is engaged in the bottom plug 101 the top plug 1 will not rotate independ¬ ently of the bottom plug 101. Similarly, since the tapered male member 107 on the bottom plug 101 is en¬ gaged in the female socket 214 in the float shoe 201 neither the top plug 1 nor the bottom plug 101 will rotate. This facilitates rapid drilling out.

The shape of the tapered male member 7 and the complimentary female socket have several advantages. In particular, the tapered male member 7 will engage the female socket 114 even though they may not be in perfect axial alignment immediately prior to engagement. This is particularly important for use in deviated wells. In addition, because the female socket 114 slopes contin¬ uously and smoothly downwardly, there is little risk of debris becoming lodged between the tapered male member and the female socket. The height of the tapered male member is slightly less than the depth of the female socket 114 so that the tapered male member 7 is not subject to axial compressive stresses. Whilst this fea¬ ture is most highly recommended it is not however essen- tial although we would recommend the provision of two or more axially extending slots in the tapered male member 7 to allow for radial compression if the tapered male member is to be subject to compressive forces.

Various modifications to the embodiments described are envisaged. For example, a float collar may be used above the float shoe. In this case the float collar would be provided with the female socket whilst this would not be necessary for the lower float shoe. If desired, the outer casing and/or the entire plug could be made from rubber. Also the tapered male member could be made as a separate part which could be mounted on the plug. Similarly, the female socket 114 could be formed in a separate disk which could be secured, for example bolted, to the top of the bottom plug. Similarly, the female socket 214 could be formed as an integral portion of the float collar or float shoe. In such an embodiment the female socket 214 could be surrounded by concrete which could optionally be covered with a layer of plas¬ tics material.

If desired, the wipers 4 and sealing fin 5 may be replaced by the wipers and sealing fins disclosed in our co-pending application number of even date.

Whilst the embodiments shown in the drawings have eight mounds 9; 109 and eight recesses 10; 110, it will be appreciated that the number of mounds and recesses may be varied. Thus, whilst a plug for 9.5/8" casing, as shown in the Figures, may have six or eight mounds and recesses it is envisaged that plugs of smaller diameter might be provided with fewer mounds and recesses, for example four or even three mounds and recesses. Similar¬ ly, it is envisaged that larger diameter plugs might be provided with additional mounds and recesses, for exam¬ ple ten, twelve, fourteen or sixteen mounds and reces- ses. The optimum number of mounds and recesses for any plug will be determined by trial and error. Essentially, the greater the number of mounds and recesses the easier the bottom plug will engage the socket in the float collar or float shoe. However, as the number of mounds and recesses increases the ability of the interaction of the male member and socket to resist rotational stresses decreases. For general purposes plugs having six or eight mounds and recesses should be quite satisfactory.