Drill Collar

The term drill collar derived from the short sub originally used to connect the bit to the drill pipe. Amodern drill collar is about 30 ft long, and the total length of the string of drill collars may range from about 100 to 700 ft or longer. The purpose of drill collars is to furnish weight to the bit. However, the size and length of drill collars have an effect on bit performance, hold deviation, and drill pipe service life. Drill collars maybe classified according to the shape of their cross-section as round drill collars (i.e., conventional drill collars), square drill collars, or spiral drill collars (i.e., drill collars with spiral grooves).

Square drill collars are used to increase the stiffness of the drill string and are used mainly for drilling in crooked hole areas. Square drill collars are rarely used. Spiral drill collars are used for drilling formations in which the differential pressure can cause sticking of drill collars. The spiral grooves on the drill collar surface reduce the area of contact between the drill collar and wall of the hole, which reduces the sticking force. Conventional drill collars and spiral drill collars are made with a uniform outside diameter (except for the spiral grooves) and with slip and elevator recesses. Slip and elevator recesses are grooves ranging from about 1 in. deep on smaller collars to about 2 in. deep on larger sizes. The grooves are about 18 in. long and placed on the box end of the collar. They provide a shoulder for the elevators and slips to engage. They reduce drill collar handling time while tripping by eliminating the need for lift subs and safety clamps. A lift sub is a tool about 2 ft long that makes up into the box of the drill collar, which has a groove for the elevator to engage. A safety clamp is a device clamped around the drill collar that provides a shoulder for the slips to engage.

The risk of drill collar failure is increased with slip and elevator grooves because they create stress risers where fatigue cracks can form. The slip and elevator grooves may be used together or separately.

Dimensions, physical properties, and unit weight of new, conventional drill collars are specified in Tables 2.1, 2.2, and 2.3, respectively. Technical data on square and spiral drill collars are available from manufacturers.

2.1.1 Selecting Drill Collar Size

Selection of the proper outside and inside diameter of drill collars is usually a difficult task. Perhaps the best way to select drill collar size is to study results obtained from offset wells previously drilled under similar conditions.

The most important factors in selecting drill collar size are

  1. Bit size
  2. Coupling diameter of the casing to be set in a hole
TABLE 2.1 Drill Collars [1]

Drill Collar

Outside

Bore

Length

Bevel

Ref. Bending

Number

Diameter (in.)

(in.)

(ft)

Diameter (in.)

Strength Ratio

NC23-31

38

14

30

3

2.57:1

NC26-35

32

1 1

30

3 17

2.42:1

NC31-41

48

3 34

2.43:1

NC35-47

44

2

30 or 31

433

2.58:1

NC38-50

5

24

30 or 31

449 4 64

2.38:1

NC44-60

6

24

30 or 31

511 5 16

2.49:1

NC44-60

6

213 216

30 or 31

511 5 16

2.84:1

NC44-62

64

71

30 or 31

58

2.91:1

NC46-62

64

9 13

30 or 31

52 32

2.63:1

NC46-65

62

71

30 or 31

632

2.76:1

NC46-65

62

213 216

30 or 31

632

3.05:1

NC46-67

64

24

30 or 31

632

3.18:1

NC50-70

7

24

30 or 31

664

2.54:1

NC50-70

7

9 13 216

30 or 31

6 64

2.73:1

NC50-72

71

9 13

30 or 31

643

3.12:1

NC56-77

73

13

30 or 31

719

2.70:1

NC56-80

8

216

30 or 31

7 64

3.02:1

68 Reg

84

213 216

30 or 31

745 7 64

2.93:1

NC61-90

9

216

30 or 31

88

3.17:1

7 8 Reg

92

3

30 or 31

o13 816

2.81:1

NC70-97

94

3

30 or 31

932

2.57:1

NC70-100

10

3

30 or 31

911

2.81:1

88 Reg

11

3

30 or 31

102

2.84:1

TABLE 2.2 Physical Properties of New Drill Collars [1]

Drill Collar

Minimum Yield

Minimum Tensile

Elongation,

OD Range (in.)

Strength (psi)

Strength (psi)

Minimum

31-6z 3 8 6 8

110,000

140,000

13%

7-10

100,000

135,000

13%

TABLE 2.3 Drill Collar Weight (Steel) (Pounds per Foot) [4]

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

Drill Collar OD, Drill Collar ID, in.

28 19 18 16

31 26 24 22

31 30 29 27

41 46 44 42 40 38 35

41 51 50 48 46 43 41

51 75 73 70 67 64 60

6j 107 105 102 99 96 91 89 85 80

6| 116 114 111 108 105 100 98 93 89

7 125 123 120 117 114 110 107 103 98 93 84 71 134 132 130 127 124 119 116 112 108 103 93

71 144 142 139 137 133 129 126 122 117 113 102

7| 154 152 150 147 144 139 136 132 128 123 112

8 165 163 160 157 154 150 147 143 138 133 122 81 176 174 171 168 165 160 158 154 149 144 133 81 187 185 182 179 176 172 169 165 160 155 150

9 210 208 206 203 200 195 192 188 184 179 174 91 234 232 230 227 224 220 216 212 209 206 198 9| 248 245 243 240 237 232 229 225 221 216 211

10 261 259 257 254 251 246 243 239 235 230 225

11 317 315 313 310 307 302 299 295 291 286 281

12 379 377 374 371 368 364 361 357 352 347 342

*See API Specification 7, Table 13 for API standard drill collar dimensions. For special configurations of drill collars, consult manufacture for reduction in weight.

  1. Formation's tendency to produce sharp changes in hole deviation and direction
  2. Hydraulic program
  3. Possibility of washing over if the drill collar fails and is lost in the hole

To avoid an abrupt change in hole deviation (which may make it difficult or impossible to run casing) when drilling in crooked hole areas with an unstabilized bit and drill collars, the required outside diameter of the drill collar placed right above the bit can be found from the following formula [5]:

where ODbit = outside diameter of bit size

ODcc = outside diameter of casing coupling ODdc = outside diameter of drill collar

Example 2.1

The casing string for a certain well is to consist of 13| casing with a coupling outside diameter of 14.375 in. Determine the required outside diameter of the drill collar to avoid possible problems with running casing if the borehole diameter is assumed to be 17^ in.

Solution

Being aware of standard drill collar sizes (see Table 2.1), an 11-in. or 12-in. drill collar should be selected. To avoid such a large drill collar OD, a stabilizer or a proper-sized square drill collar (or a combination of the two) should be placed above the rock bit. If there is no tendency to cause an undersized hole, the largest drill collars that can be washed over are usually selected.

In general, if the optimal drilling programs require large drill collars, the operator should not hesitate to use them. Typical hole and drill collar sizes used in soft and hard formations are listed in Table 2.4.

2.1.2 Length of Drill Collars

The length of the drill collar string should be as short as possible but adequate to create the desired weight on bit. In vertical holes, ordinary drill pipe must never be used for exerting bit weight. In deviated holes, where the axial component of drill collar weight is sufficient for bit weight, heavy-weight pipe is often used in lieu of drill collars to reduce rotating torque. In highly deviated holes, where the axial component of the drill collar weight is below the needed bit weight, drill collars are not used, and heavy weight pipe is put high in the string in the vertical part of the hole. When this is done, bit weight is transmitted through the drill pipe.

TABLE 2.4 Popular Hole and Drill Collar Sizes [3]

4

21 x l4 with

3 "1 x 1 -2 with

55-6|

2| PAC or 2| API Reg 41 x 2 with NC31

2 8 PAC or 2 8 API Reg 44 x 2 with NC31 or 31 x H

62-64

44 x 21 withNC38

5 or 54 x 2 with NC38

78-78

6 x 213 with NC46

64 or 62 OD x 2 or 24 with NC46

81-84

64 x 213 with NC46 or

61 x 213 with NC46 or NC50

64 or 7 x 24 withNC50

91-9z 92 98

7 x 213 with NC50

7 x 24 with NC50

8 x 213 with NC50

8 x 243 with 6§ API Reg

IO8-II

7 x 216 withNC50or

8 x 216 with 6§ API Reg

124 I7i

8 x 216 with 6| API Reg

11 x 3 with 8 § API Reg

181-26

Drill collar programs are the same as for the next reduced hole size.

Extra care and planning must be done when apply bit weight through the pipe because buckling of the pipe can occur, which can lead to fatigue failures and accelerated wear of the pipe or tool joints.

The required length of drill collars can be obtained from the following formula:

Wdc ■ Kb ■ cos a where DF = is the design factor (DF = 1.2-1.3), and the weight on bit (lb) is determined by the following: Wdc = unit weight of drill collar in air (lb/ft) Kb = buoyancy factor Kb =1 - &

where Xm is the drilling fluid density (lb/gal), Xst is the drill collar density (lb/gal) (for steel, Xst = 65.5 lb/gal), and a is the hole inclination from vertical (degrees).

When possible, the drill pipe should be in tension. The lower most collar has the maximum compressive load, which is transmitted to the bit, and the upper most collar has a tensile load. This means there is some point in the drill collars between the bit and the drill pipe that has a zero axial load. This is called the neutral point. The design factor (DF) is needed to place the neutral point below the top of the drill collar string. This will ensure that the pipe is not in compression because of axial vibration or bouncing of the bit and because of inaccurate handling of the brake by the driller.

The excess of drill collars also helps to prevent transverse movement of drill pipe due to the effect of centrifugal force. While the drill string rotates, a centrifugal force is generated that may produce a lateral movement of drill pipe, which causes bending stress and excessive torque. The centrifugal force also contributes to vibration of the drill pipe. Hence, some excess of drill collars is suggested. The magnitude of the design factor to control vibration can be determined by field experiments in any particular set of drilling conditions. Experimental determination of the design factor for preventing compressive loading on the pipe is more difficult. The result of running the pipe in compression can be a fatigue crack leading to a washout or parted pipe.

The pressure area method (PAM), occasionally used for evaluation of drill collar string length, is wrong because it does not consider the triaxial state of stresses that actually occur. Hydrostatic forces cannot cause any buckling of the drill string as long as the density of the string is greater than the density of the drilling fluid.

Example 2.2

Determine the required length of 7 in. x 2| in. drill collars with the following conditions:

Desired weight on bit: W = 40,000 lb Drilling fluid density: Xm = 10lb/gal Hole deviation from vertical: a = 20°

Solution

From Table 2.3, the unit weight of a 7 x 2 4 drill collar is 117 lb/ft. The buoyancy factor is

Drill Factor

The section modulus, Zb, of the box should be 2 -2 times greater than the section modulus, Zp, of the pin in a drill collar connection. On the right side of the connection are the spots at which the critical area of both the pin (Ap) and box (Ab) should be measured for calculating torsional strength.

FIGURE 2.2 The drill collar connection. (After C. E. Wilson and W. R. Garrett).

The section modulus, Zb, of the box should be 2 -2 times greater than the section modulus, Zp, of the pin in a drill collar connection. On the right side of the connection are the spots at which the critical area of both the pin (Ap) and box (Ab) should be measured for calculating torsional strength.

Applying Equation 2.2 gives

The closest length, based on 30-ft collars, is 450 ft, which is 15 joints of drill collars.

2.1.3 Drill Collar Connections

It is current practice to select the rotary shoulder connection that provides a balanced bending fatigue resistance for the pin and the box. It has been determined empirically that the pin and boxes have approximately equally bending fatigue resistance if the section modulus of the box at its critical zone is 2.5 times the section modulus of the pin at its critical zone. This number is called the bending strength ratio (BSR). These critical zones are shown in Figure 2.2. Section modulus ratios from 2.25 to 2.75 are considered to be very good and satisfactory performance has been experienced with ratios from 2.0 to 2.3 [6].

The previous statements are valid if the connection is made up with the recommended makeup torque. A set of charts is available from the Drilco Division of Smith International, Inc. Some of these charts are presented in Figures 2.3 to 2.10. The charts are used as follows:

• The best group of connections are those that appear in the shaded sections of the charts.

2. DRILL STRING: COMPOSITION AND DESIGN

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Responses

  • askalu sayid
    Why use surface on drill collar bevel diameter?
    7 years ago
  • Daryl
    How to select drill collar?
    7 years ago
  • Asmarina Saare
    How to find slip size for drillcollars?
    7 years ago
  • Blanco
    How to collar a drilled foundation hole?
    6 years ago
  • Felix
    Why do drill collars part?
    5 years ago
  • Awate
    Are small drills collars the best tools for controlling crooked hole problems?
    1 year ago
  • carmen
    How many size of drill collar?
    10 months ago
  • Piera Lucchesi
    What is the length of a drill collar?
    4 months ago
  • fethawit
    How measure diameter of drill pipe and drill collar?
    2 months ago

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