AU2005235770A1 - Drilling method and assembly for drilling large-diameter holes - Google Patents

Description

CERTIFICATE OF VERIFICATION I, BERTRAND Didier of S.A. FEDIT-LORIOT ET AUTRES CONSEILS EN PROPRIETE INDUSTRIELLE 38, avenue Hoche 75008 Paris (France) state that the attached document is a true and complete translation to the best of my knowledge of International Patent Application No. PCT/FR02005/00920. Dated this 22nd September 2006 ature: WO 2005/103438 PCT/FR2005/000920 Drilling method and assembly for drilling large diameter holes 5 The present invention relates to a drilling method and assembly for forming large-diameter holes. An example of a possible field of application is that of offshore oil exploitation in which large holes are formed in the sea bed to create suitable 10 foundations for mooring platforms. A known drilling assembly comprises a drill bit and a drill rod for forming boreholes, said drill rod being rotated to drive said bit so as to form said hole. These boreholes are formed in the ground, and 15 drill rods are added at the surface as the depth of said hole increases and the bit advances into the ground. At the same time, cuttings from the drilled ground are raised to the surface. Additionally, a fluid mixture, for example 20 drilling mud having a specified composition, is injected into the drill bit, in order to cool the drill bit and also to carry the cuttings from the drilled ground, the drilling mud being returned from the bottom of the hole to the surface. 25 Two methods can be used to guide the raising of the fluid mixture or drilling mud. A first method, illustrated for example in US 4,341,272, called "direct circulation", in which the drilling fluid, for example sea water, mud, viscous 30 gels or a combination of fluids suitable for each stage of the drilling, is injected through the drill rods and into the drill bit. The drilling fluid then rises again from the bottom of the hole towards the surface, in the annular space between the wall of the borehole and the 35 drill rods, carrying the cuttings which are in suspension. The drilling fluid is selected according to the ground, the stability, depth and diameter of the borehole, and the flow velocity.

WO 2005/103438 - 2 - PCT/FR2005/000920 This first method limits the risks of collapse of the hole wall, because it enables the wall to be lined with the drilling mud or the viscous gel, for example, which consolidates it. 5 However, when large-diameter holes are to be formed, for example holes larger than 36 inches (91.44 cm), the cross section of the annular space becomes too great with respect to that of the drill rods into which the drilling fluid is injected. These rods are 10 standard, and their section is therefore fixed. Consequently, the drilling fluid does not have enough energy to hold the cuttings in suspension and carry them from the bottom of the hole to the surface. To overcome this drawback, a second method, 15 called "reverse circulation drilling", consists in injecting the drilling fluid from the surface into the annular space and sucking the mixture of fluid and cuttings through the drill rods by means of pumps located at the surface. However, this method requires 20 extremely powerful pumps, without which the mixture cannot be entirely brought to the surface, and furthermore the hole wall is not consolidated by the sediments contained in the cuttings. Moreover, the quality of consolidation depends on the nature of the 25 drilled ground. In particular, when the ground is relatively loose, the pressure drop created by the suction of the mud can cause instability or dislocations which are unfavorable to the stabilization of the wall. 30 Thus one problem which arises and which the present invention is intended to resolve is that of providing a drilling assembly for forming large diameter holes, while injecting drilling mud through standard drill rods. 35 For this purpose, the present invention proposes, according to a first object, a drilling assembly comprising a drill bit and a drill rod, said drill rod being designed to drive said drill bit to form a borehole having a cylindrical wall, said WO 2005/103438 - 3 - PCT/FR2005/000920 assembly also comprising means for circulating drilling mud through said drill rods with a downward flow to said drill bit, said drilling mud subsequently flowing out along said wall in an upward flow; said drilling 5 assembly also comprising a sealed cylindrical sleeve fitted around said drill rods to guide the mud flowing upwards between said cylindrical sleeve and said cylindrical wall. Thus, a characteristic of the invention 10 consists in the placing of the cylindrical sleeve around the drill rods, making it possible to reduce the section of the annular space, which then extends between the sleeve and the wall of the borehole. Consequently, the pressure of the drilling mud in the 15 drill rods enables this mud and the cuttings to be carried through the annular space towards the surface. Therefore, as a result of the application of this drilling assembly, not only is the wall of the borehole lined and consolidated by the mud mixed with the 20 cuttings, but it is also possible to dispense with the costly suction pumps used in the prior art of "reverse circulation" for large-diameter holes. In a particularly advantageous embodiment of the invention, said drill bit has a cross section with 25 a first diameter and said cylindrical sleeve has a section with a second diameter, from 10 to 30 cm for example, which is smaller than said first diameter and of course larger than the diameter of the drill rods, thus reducing the section of the annular space. 30 Additionally, a sealed joint is advantageously interposed between the sealed cylindrical sleeve, which extends from the surface to the bottom of the hole, and the drill bit, to prevent the drilling fluid and the cuttings from being discharged between the drill rods 35 and the cylindrical sleeve. In another embodiment, the cylindrical sleeve terminates, in the proximity of the drill bit, in a joint extending between the sleeve and the drill rods.

WO 2005/103438 - 4 - PCT/FR2005/000920 Additionally, in a preferred embodiment of the invention, said cylindrical sleeve is centered on an axis of the drill rods by spacing means. Thus the cylindrical sleeve is kept spaced apart from the drill 5 rods which pass through it. Thus the drill rods, which are rotated, are not made to rub against the cylindrical sleeve which is fixed with respect to rotation. Furthermore, the cylindrical sleeve is thereby 10 centered with respect to the drill bit, enabling an annular space of constant section to be formed as the drill bit advances and forms the borehole. The drilling assembly also includes guide means, fitted around said cylindrical sleeve and kept 15 in a fixed position with respect to the surface, so as to provide longitudinal guidance for the cylindrical sleeve, and consequently the drill rods. Advantageously, the cylindrical sleeve is fixed to the drill rods with respect to rotation, the rods 20 being rotated to drive the drill bit. In another embodiment, the cylindrical sleeve is freely rotatable with respect to the drill rods, and is rotated independently by its own means. According to another object, the present 25 invention proposes a drilling method which is applied by means of the drilling assembly described above to form boreholes. Other characteristics and advantages of the invention will be made clear by the following 30 description of a specific embodiment of the invention, provided for information and without restrictive intent, with reference to the attached drawings, of which - Figure 1 is a schematic front elevation of a 35 drilling assembly according to the invention; and - Figure 2 is a schematic detail view of the drilling assembly shown in Figure 1. Figure 1 shows a drilling assembly 10 according to the invention, partially engaged in a hole 11 formed WO 2005/103438 - 5 - PCT/FR2005/000920 in the ground 12. The hole 11 has a cylindrical wall 13 and extends between a bottom 14 and a surface 16. The drilling assembly 10 comprises a drill bit 18 having a cutting bit 19, which can be rotated by 5 drill rods 20 coupled to each other and extending from the drill bit 18 to rotary drive means. Each drill rod 20 also has an inner channel 21 into which the drilling fluids, mud for example, are injected under pressure from the surface, in order to 10 cool the drill bit 18 and also carry the cuttings. This is because, when the cutting bit 19 is rotated, it excavates the ground and produces cuttings which are removed by means of the drilling mud, in an annular space 22. 15 The drilling assembly 10 also comprises a sealed cylindrical sleeve 24 which is fitted around drill rods 20, from the drill bit 18 to the surface 16 and above. This sealed cylindrical sleeve 24 has an outer surface 23 extending so that it faces the wall 13 20 of the hole 11, and an inner surface 25 facing the drill rods 20. This sealed cylindrical sleeve 24 is formed by a plurality of cylindrical portions 26, which are adjusted together as the hole is formed and the drill 25 bit 18 advances, when drilling is carried out on land. However, the drilling assembly can be used equally well to drill the ground in sea beds, in which case the cylindrical elements 26 of the sleeve 24 are assembled on a drilling platform. 30 The cylindrical portions 26 are linked end-to end by centering rings 28 which also form spacers between the drill rods 20 and the cylindrical sleeve 24. For this purpose, these centering rings 28 have a cylindrical part into which the two portions of 35 cylinder are fitted, and a central part forming a plate in the center of which is formed a hole for the passage of the drill rods. The centering rings 28 extend outside the envelope defined by the cylindrical sleeve WO 2005/103438 - 6 - PCT/FR2005/000920 24, thus also enabling the cylindrical sleeve to be centered in the hole 11. In this way, the cylindrical sleeve is both completely sealed at the joint between the cylindrical 5 portions 26 and kept completely centered with respect to the drill rods 20 by means of the central parts forming plates. The joint between the drill bit 18 and the sealed cylindrical sleeve 24 will now be described more 10 fully, with reference to Figure 2. Figure 2 shows the drill bit 18 and its cutting bit 19 which enable the rocks to be broken mechanically and enable the assembly to penetrate into the ground 12. The drill rods 20 extend to an end 30 which is 15 fixed to the cutting bit 19 with respect to rotation and can drive the latter. Additionally, a sealing joint 32 between the drill bit 18 and the sealed cylindrical sleeve 24 is adapted to form a barrier to the cuttings created by the mechanical breaking of the rocks and to 20 the drilling mud which are carried under pressure along the inner channel 21, so as to prevent them from penetrating into the space between the sealed cylindrical sleeve 24 and the drill rods 20. Thus, a mixture of drilling mud and cutting 25 sediments in suspension is formed ahead of the cutting bit 19, and is carried back toward the rear of the cutting bit as the latter advances and as the mud is injected. Owing to the pressure of the drilling mud, the mixture is discharged along the wall 13 in the 30 annular space 22, toward the surface in the direction of the arrows F. This annular space 22 is also limited by the sealed cylindrical sleeve 24 which enables a free longitudinal space 36 to be formed between the drill 35 rods 20 and the sleeve 24, in which the mixture of muds cannot flow. In this way, if the cross section Seff of the drill bit 18 is greater than one meter, for example, and if standard drill rods 20 with a calibrated inner WO 2005/103438 - 7 - PCT/FR2005/000920 channel 21 are used, the mixture of drilling muds can be removed through the annular space 22 which is thus formed, since this space is relatively small by comparison with the inner channel 21 which, for its 5 part, makes it possible to provide a limited flow of drilling mud at a specified pressure. In the absence of the sealed cylindrical sleeve 24, the annular space through which the drilling mud mixture could be removed would be equal to the space 10 between the wall 13 of the hole 11 and the drill rods 20. In this case, and particularly if the hole diameter is more than one meter, the space would be too large to allow the pressure and flow of drilling mud in the inner channel 21 to bring the cutting sediments into 15 uniform suspension in the mud, or to carry this mixture toward the surface. This is because the pressure of the mixture which would be created behind the drill bit 18 would rapidly become greater than the pressure of the drilling mud flowing in the inner channel 21. 20 In one embodiment, if a drill bit 18 with a cross section suitable for drilling holes with a diameter of 167 cm is used, it is necessary to fit a sealed cylindrical sleeve centered with respect to the drill rods 20 and having a diameter greater than 140 25 cm, in order to enable the drilling mud mixture to be raised to the surface. A rule for calculating the diameter d of the sleeve with respect to the diameter D of the borehole can be provided, by way of illustration, by the formula d2 > D2 - 8200. This rule 30 is applicable, for example, where the diameters of the boreholes are in the range from 1 to 2 meters. As shown in Figure 1, the drilling assembly 10 advantageously includes a tubular guide element 40 kept in a fixed position in the vicinity of the ground 35 surface 16, for guiding the translation of the sealed cylindrical sleeve 24, while the drill rods 20 are driven so as to be simultaneously translated and rotated.

Claims (6)

1. A drilling assembly comprising a drill bit (18) 5 and a drill rod (20), said drill bit (18) having a cross section adapted to drill a borehole with a diameter D, said drill rod being intended to drive said drill bit to form said borehole (11), said borehole (11) having a cylindrical wall (13), said assembly 10 additionally comprising means for circulating drilling fluids through said drill rods (20) in a downward flow to said drill bit (18), said drilling fluids subsequently flowing along said cylindrical wall (13) in an upward flow; 15 characterized in that it also comprises a sealed cylindrical sleeve (24) with a diameter d, fitted around said drill rods (20) to guide the drilling fluids flowing upward between said cylindrical sleeve (24) and said cylindrical wall (13), 20 and in that the diameter of said sealed cylindrical sleeve conforms to the relation d2 > D2 8200.

2. The drilling assembly as claimed in claim 1, 25 characterized in that the said drill bit (18) has a cross section with a first diameter, and said cylindrical sleeve (24) has a cross section with a second diameter smaller than said first diameter. 30

3. The drilling assembly as claimed in claim 1 or 2, characterized in that said sealed cylindrical sleeve (24) and said drill bit (18) are connected to each other by a sealed joint (32). 35

4. The drilling assembly as claimed in any one of claims 1 to 3, characterized in that said sealed cylindrical sleeve (24) is centered on an axis of the drill rods (20) by spacing means (28). .WO 2005/103438 - 9 - PCT/FR2005/000920

5. The drilling assembly as claimed in any one of claims 1 to 4, characterized in that it also comprises guide means (40), fitted around said sealed cylindrical sleeve (24). 5

6. A drilling method for forming a borehole, characterized in that a drilling assembly as claimed in any one of claims 1 to 5 is used.