Drill collar – Drilling Equipment – Readyzone

Drill collar

Drill collars are used to provide weight on bit and to keep drill pipe in tension. Drill pipe has a comparatively low stiffness, which makes it susceptible to bucking when under compression. Repeated bucking will eventually lead to drill pipe failure.

Since elastic members can only buckle in compression, fatigue failure of drill pipe can be eliminated by keeping it in constant tension. In practice, 85% of the buoyant weight of drill collars ( giving a safety factor of 1.15 ) is used as weight-on-bit, to ensure that the drill pipe is kept in tension. This practice also places the neutral point ( point of zero tension and zero compression ) in the drill collars.

Table 1 lists the sizes and weight per foot of available drill collars. From this table, the reader can observe that the inside diameter of drill collars is quite small, e.g. for a 5 in OD drill collar, the ID varies from 1.5 to 2.5 in. The reduced diameter of a drill collar results in a high pressure loss during the circulation of drilling mud and, for this reason, drilling engineers often select the largest available bore to limit pressure losses.

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Fig. 1 – Drill collar connection.

Drill collars are normally manufactured in an average length of 31 ft. Owing to the large thickness of pipe body, drill collars are not provided with tool joints and, instead, pins and boxes are cut from the pipe body. The drill collar connection has a tapered, threaded jack screw that forces the shoulders of individual drill collar joints together to form a metal seal ( as shown in Figure 1 ) at the shoulders.

The drill collar threads are normally strengthened by cold-working the thread roots. Cold-working involves the prestressing of the thread root with a roller driven by a hydraulic ram. This results in a thread surface with a greater resistance to cyclic stress reversals.

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Fig. 2 – Special features available on drill collar (a) fishing neck; (b) stepped bore; (c) slip and elevator recesses; (d) spiral grooving.

Drill collar features

Drill collars possess the following features

   Fishing necks

Large-sized drill collars with OD in excess of 8 in are manufactured with reduced diameters near the pin and box ends ( Figure 2a). This feature is introduced to allow drill collars to be filled with smaller connections which can be properly torqued with the available rig tongs and line pulls.

As outlined by Wilson, the name ‘fishing neck’ comes from the original use, which was to receive an overshot and grapple in case of joint failure in hole which demanded a fishing job.

Stepped-bore drill collars

In small and medium-sized drill collars where large bores are required, to reduce pressure losses, the pin strength can be increased by stepping its bore hole as shown in Figure 2(b). For example, in a 6.25 in OD drill collar with a 2.8125 in bore, the pin bore is reduced to 2.25 in, to increase its strength.

Slip and elevator recesses

Slip and elevator recesses are introduced to allow drill collars to be handled like drill pipe by simply changing the drill pipe elevators and slips. Figure 2(c) shows in drill collar with slip and elevator recesses.

Spiral grooving

Differential sticking is more prevalent with drill collars than with drill pipe. A drill collar with spiral grooving has a much reduced contact area; this greatly reduced the magnitude of the differential sticking force. Spiral grooving only reduces the weight of the drill collar joint by 4%( Figure 2(d) ).

Square drill collars

Squared-section drill collars are used for special drilling purposes such as reducing deviation in crooked hole formation and to maintain the existing hole direction in directional drilling.

K-Monel drill collars

K-Monel drill collars are manufactured from nonmagnetic steel alloys, and are used to shield directional survey instrument from the magnetic effects of normal steel drill collars.

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Table 1 – Drill collar weight ( steel: lb/ft ).

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Reference : Mc Cray & Cole, Oil Well drilling Technology, New India Publication.


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