Everything You Need To Know About Drill Collar

A Drill Collar (DC) is a heavy, thick-walled steel tube with threaded connections cut (NB not welded) on both ends that we use in oilfield rigs. They are designed to withstand downhole conditions while under compression and tension. While designing BHA, Drilling engineers place DCs above the Drill Bit as DCs are the predominant component of the Drilling BHA. To select the proper DC, you must have all the information about Drill collar Weights, Size, Specs, Definition, lengths, and types.

In addition, they are manufactured from chrome-molybdenum alloy 4140 (4140 is a code that indicates the chemical makeup of the metal). Furthermore, they are heat-treated over their entire length, with a Brinell hardness range of 255 to 341. Their bore is accurately machined to ensure smooth, balanced rotation. Collars are manufactured in a wide range of sizes (ODs) and are available in length ranges 2: 9.15 – 9.76 m (30 – 32 ft); and 3: 12.8 – 13.27 m (42 ft-43 ft).

DC can also be manufactured using magnetic steel or spiral type, including complex threaded connections, weight increases, and spiral grooves on the outside surface for better drilling mud type flow control.

What are the Functions of Drill Collars?

• Provide weight on bit (WOB)
• Minimize bit rotational stability problems from drill string vibrations, wobbling, and jumping.
• Minimize directional control problems by providing stiffness to the BHA.
• Keep the drill string in tension to reduce bending stresses and failures due to fatigue.
• Provide pendulum effect. (BHA Types)
• Reduce keyseats, doglegs & ledges.

Drill Collar Specs

The parameters used to specify them are:

• OD: 9 1/2″, 8″, 6″, 4 3/4″ etc.,
• ID: 3 1/2″, 3″, 2 1/2″”, 2 3/8″ etc.,
• Length: Usually range II
• Material: Steel, Monel
• Connection: 6 5/8″ REG etc.

Drill Collar Weights & Size

Figure 1: Weights

D/Cs are produced in an extensive range of sizes with various types of joint connections. The Drill collar size and weights per foot of a range of DC sizes are shown in Table 2. The weights quoted in Table 2 are the “weight in the air” of the D/Cs. It is vital that proper care is taken when handling D/Cs. The shoulders and threads must be lubricated with the correct lubricant (containing 40- 60% powdered metallic zinc or lead).

Figure 2: Drill Collar Weight in Kg/m

Their sizes range from 2 3/8″ to 12″. In the below table, you will find the weights for each size. By using the collar OD & ID, you will get its weight.

Figure 3: Weight in lbs/ft

Drill Collar Weight Calculations

As DCs are uniform tubes of steel with few upsets (changes in ID or OD) across their length, it is easy to calculate their weight. We can calculate the weight of drill collars in kg/m or lbs/ft using the following formula:

SI units: 6.16 x 10-3 x (D2 – d2)

Field units: 2.67 x (D2 – d2)

Where :

• D = Outside diameter in mm/inches
• d = Inside diameter in mm/inches.

The table below will help you with the weight calculations in the drilling fluid after considering the buoyancy factor.

Figure 3: Buoyancy Factor

Drill Collar Length

D/C are usually supplied in Range 2 lengths (30-32 ft). The collars are manufactured from chrome-molybdenum alloy, which is heat-treated over the entire length. The collar bore is accurately machined to ensure a smooth, balanced rotation.

Drill Collar Connection Thread Types

Since the DC has such a large wall thickness, Tool Joints are unnecessary. In addition, we can machine the connection threads directly onto the collar body. The connection is the weakest point in the D/Cs; therefore, we must apply the correct makeup torque to prevent failure. The external surface of a regular collar is round (slick), although other profiles are available.

Table 1: Standard Drill Collar Thread Types

• The D/C number (column 1) comprises two parts separated by a hyphen.
  • The first part is the connection number in the NC style.
  • The second part, consisting of 2 (or 3) digits, indicates their outside diameter in units and tenths of inches.
• The connections shown in parentheses in column 1 are not a part of the D/C number; they indicate the inter-changeability of D/Cs made with the standard (NC) connections, as shown.
• If the connections shown in parentheses in column 1 are made with the V-0.038R thread, the connections and D/Cs are identical to those in the NC style.
• The drill collar sizes listed in Table 1 were adopted to provide a full range of DCs with improved connections as a replacement for the DCs with the various connections specified in previous editions of API Spec.

Regular Connections

For some reason, Regular connections remain the most common choice for BHA components even though an NC connection of the same bending strength is superior due to its thread having a less sharp root radius and being more resilient to thread root cracking.

H90 Connections

These connections were originally a proprietary product of Hughes Tool Company and are easily recognizable by their shallow 90° thread angle. They are about equivalent performance to Regular connections except that the shallow thread angle causes high box hoop stresses at high makeup torques. Again, NC is a superior thread in almost all applications, but the H-90 type is still frequently used.

Notes

• The shoulder provides the only positive seal against fluid leakage.
• D/Cs with 8-1/4 and 9-1/2 inches outside diameters are shown with 6-5/8 and 7-5/8 REG connections since no NC connections are in the recommended bending strength ratio range.
• The connection is the weakest part of the entire BHA.
• Improper M/U torque, improper or insufficient lubricant, and galling can all lead to connection failure.
• Purchase orders for collars with improved connections should state the D/C number or size and style, bore, and length. Purchase orders for collars with optional connections should state the outside diameter, bore, length, connection size and style, and bevel diameter.
• The DC connections go through tension-compression cycles and are subject to bending stresses.

Connection Stress

• Stresses in DC connections are concentrated at the base of the pin and the bottom of the box.
• DP body bends easily and takes up most of the applied bending stress; DP connections are subjected to less bending than the DP body.
• However, DCs and other BHA components are much stiffer than the DPs, and the bending stresses are transferred to the connections.
• These bending stresses can cause fatigue failure at the connections.

Stress Relief

As the threaded connection of the drill collars is the weakest part, bending is most likely to occur. At the bottom of the box and in the broadest part of the pin, a few threads are not engaged. These threads form notches, which could accelerate fatigue failure.

Figure 5: Stress relief groove on the pin of a crossover.

As these threads do not strengthen the connection but weaken the joint, it is necessary to remove them. We call the grooves that now appear the stress relief grooves. In boxes, the bottom is often bored out over a greater length. The length of the uniform wall section now distributes bending stresses over a longer interval.

Figure 6: Stress relief groove on the pin of a crossover.

Some suppliers of components have in the past avoided putting stress-relieving grooves onto pins as to reduce the number of re-cuts to the thread to about three, depending on the component in question. This reduces the practical commercial life of the component.

Figure 7: Lack of stress relief groove on the pin of a drilling stabilizer.

Occasionally, BHA components have bore back boxes but no stress-relieving groove. This shows that the manufacturer knows the requirements but saves costs by not cutting a stress-relieving groove. We should return these components to the supplier.

Drilling Collar Types

Anti-wall stick / Spiral Drill Collar

When drilling through certain formations, the large diameter D/Cs can become stuck (Pipe sticking problem) against the borehole (differential sticking). This is likely to happen when the formation is highly porous, using a large overbalance of mud pressure, and the well deviates. One method of preventing this problem is to reduce the contact area of the collar against the wellbore. We can cut the spiral grooves into the collar surface to reduce its surface area.

Figure 4: Spiral Collar Type

We commonly use this type. They have spiral flats, or shallow depressions, that are machined in their surface along the more significant part of their lengths. This reduces their effective weight per unit length by approximately 4%. Reducing the wall contact area between the DC and the hole wall dramatically reduces the possibility of differential wall sticking. 

Square Type

These are drill collars with a square section and a diagonal dimension 1.6 mm (1/16″) less than the bit size, with hard-facing material applied to the corners. We generally run this type in the string when drilling crooked hole formations. The reason is to provide maximum stabilization and prevent deviation from the existing course of the hole. Their use is, however, not commonplace.

What is the application of the D/C square type:

• Firstly, they provide good centralization all over their length.
• Secondly, they increase bending resistance (stiffness) to the max
• Thirdly, they increase torsional damping.
• Fourthly, they decrease axial vibrations.

Why not use them all the time?

• This DC type is expensive to purchase and to do regular maintenance.
• They usually create high rotary torque.
• Due to their shape, they grind drilling cuttings and well caving to fines, increasing mud treatment and cost.
• The fishing (fishing in drilling) of this type is difficult.

Non-Magnetic Type

The most common type of DC is the non-magnetic drill collar. It is made from alloy steel with an outer surface machined to a specific smoothness and an elevator recess radius that meets dimensional requirements. We should pay special care when making up NMDCs. This is because the material is more susceptible to the galling of the threads and shoulders than normal DCs. They aim to isolate directional Survey Instruments from magnetic distortion due to the steel Drill String.

API Drill Collar Make-Up Torque

During drilling, DCs are subject to the following:

• buckling and bending forces
• torsion
• vibration
• Alternating stresses.

If the drill collar connection is suitable, and we apply the correct make-up torque, the joint should absorb the bending stresses encountered. In addition, the shoulder-to-shoulder seal will effectively contain the internal pressures.

Drill collar connections are never referred to as tool joints, although thread profiles may be the same as drill pipe tool joints. The OD and ID are different, so the pin and box areas are more significant, and therefore, the required make-up torque is more incredible.

If make-up torque is less than, say, 90% of the recommended value, the tool joint may not develop enough strength to:

• prevent wobbling
• resist bending loads
• form an adequate seal
• resist excessive makeup due to impact torque while drilling.

On the other hand, over-torquing is likely to distort and weaken the threaded connection, stretch the pin, or expand the box.

Correct make-up torque is essential for trouble-free performance, and we should always measure the applied torque conscientiously.

Table 2: Drill Collar Make-Up Torque

As collar make-up torques are high it is especially important to be careful when making up or breaking out connections. Should a tong slip or a tong line break the crew not directly involved could be hurt, so make sure they remain outside the danger area.

Table 3: Drill Collar Make-Up Torque

Lifting & Making Up Facilities For Drill Collars

DCs should have Elevator recesses (Figure 8) for lifting and suspending them by slip and rig elevator. When handling flush collars, we should use a lifting nipple or lifting head (using the same torque as an ordinary DC connection). The threads and shoulders of the lifting nipples should be treated and inspected with the same care as the drill collar threads.

Figure 8: Recesses machined at the box ends of DCs

To prevent a DC from slipping through the slips, we installed a safety clamp around the drill collar above the slips. This clamp comprises links with spring-loaded inserts/dies with tapered backs (see Figure 9).

Figure 9: Drill collar safety clampFigure 10: lifting equipment

Neutral Point

We obtain the total weight on the bit by having the lower section of the DC in compression, leaving the upper section of the collars and the drill pipe in tension. The neutral point is the crossover point of zero tension in the string.

There should never be less than ten percent of the length of the DCs in tension, and the neutral point should never be at the drilling jar.

API Drilling Collar Selections

Woods and Lubinski pointed out that using an unstabilized Drill Bit and a small OD DC size can cause an undersized hole, making it difficult for Casing Running. They determined that the actual drift, or proper diameter, of the hole would be equal to the bit diameter plus the DC diameter, divided by two,

We use the above equation to determine the minimum drill collar OD to ensure passage of the larger Casing & Casing Coupling diameter. Substituting casing pipe coupling diameter for drift diameter,

Generally, for the best Drill String Design, we select the most significant drill collar size we can wash over and fish out.  WHY

• Using Fewer collars
• Less tripping pipe time
• It will be more Stiffer.
• Have less tendency to buckle or bend.
• Good load distribution on the bit for better Roller Cone Bit Performance and PDC Drill Bits Performance and lessens hole deviation problems.

References & Related Papers