In addirtion, they are manufactured from chrome-molybdenum alloy 4140 (4140 is a code that indicates the chemical makeup of the metal). Furthermore, they are fully 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?
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
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.
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)
- D = Outside diameter in mm/inches
- d = Inside diameter in mm/inches.
The below table will help you in the weight calculations in the drilling fluid after considering the buoyancy factor.
Drill Collar Length
Drill Collar Connection 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 the 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.
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, therefore, more resilient to thread root cracking.
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 in 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 in frequent use.
- 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.
- Stresses in DC connections are concentrated at the base of the pin and at the bottom of the box.
- DP body bends easily and takes up most of the applied bending stress; DP connections are therefore subjected to less bending than the DP body.
- DCs and other BHA components are, however, much stiffer than the DPs, and much of the bending stresses are transferred to the connections.
- These bending stresses can cause fatigue failure at the connections.
As the threaded connection of the drill collars is the weakest part, bending is most likely to occur in the connection. At the bottom of the box and in the widest part of the pin there are a few threads which are not engaged. These threads form notches which could accelerate fatigue failure.
As these threads do not strengthen the connection but rather weaken the joint, it is necessary to remov 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.
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 effective commercial life of the component.
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
We commonly use this type. They have spiral flats, or shallow depressions that is 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.
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 for that to provide maximum stabilization and prevent deviation from the existing course of the hole. Their use is, however, not commonplace.
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
- Alternating stresses.
If the drill collar connection is a suitable one, and we applied 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.
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.
Lifting & Make 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 for 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.
To prevent a DC from slipping through the slips, we install 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).
We obtian 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 pipe in tension. The neutral point. is the crossover point of zero tension in the string.
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,
Generally, for the best Drill String Design, we select the most significant drill collar size that we can wash over and fish out. WHY
- Using Fewer collars
- Less tripping pipe time
- 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
- D/C String Design And Its Effect On Drilling M.E.R.
- D/C Length is a Major Factor in Vibration Control
- Quartz School. Module 2: Drill String Design & BHA Selection / Section 1