Drill-collar (D/C) are large diameter-small bore steel pipes that possess a great weight and great stiffness. While designing BHA, drilling engineers place D/Cs above the Drilling Bit as D/Cs are the predominant component of the Drilling BHA. To select the proper D/C you will have to have all information about Drill collar Weights, Size, Specs, Definition, lengths & Types. This article is a part of the series explaining Drilling Bottom Hole Assembly Components, Types & mechanism
What is the functions of Drill Collars:
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- Provide weight on the bit (WOB)
- Minimize bit stability problems from vibrations, wobbling and jumping
- Minimize directional control problems by providing stiffness to the BHA.
- Keep the drill string in tension, so that it will reduce bending stresses and failures due to fatigue
Drill Collar Thread Types
Since the D/C have such a large wall thickness Tool Joints are not necessary and the connection threads can be machined directly onto the body of the collar. The weakest point in the D/Cs is the connection and therefore the correct makeup torque must be applied to prevent failure. The external surface of a regular collar is round (slick), although other profiles are available.
The D/C number (column 1) consists of 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 Drill Collar 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 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 with those in the NC style.
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 there are no NC connections in the recommended bending strength ratio range.
The drill collar sizes listed in Table 1 were adopted in order 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
Purchase orders for collars with the 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.
Drill Collar Length
D/C are normally supplied in Range 2 lengths (30-32 ft). The collars are manufactured from chrome-molybdenum alloy, which is fully heat-treated over the entire length. The bore of the collar is accurately machined to ensure a smooth, balanced rotation.
Drill Collar Weights & Size
D/Cs are produced in a large range of sizes with various types of joint connections. The Drill collar size and weights per foot of a range of DC size are shown in Table 2. The weights that are quoted in Table 2 are the “weight in the air” of the D/Cs. It is very important 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).
Like drill pipe, collars are subjected to stresses due to:
- Buckling and bending forces
- Alternate compression and tension.
However, if properly made up, the shoulder/shoulder connection will be sufficient to resist these stresses. The figure below shows how numbered connections should be selected to provide an efficient seal and adequate strength.
Drilling Collar Types
Anti-wall stick / Spiral Drill Collar
When drilling through certain formations the large diameter D/Cs can become stuck pipe against the borehole (differential sticking). This is likely to happen when the formation is highly porous, a large overbalance of mud pressure is being used and the well highly deviates. One method of preventing this problem is to reduce the contact area of the collar against the wellbore. Spiral grooves can be cut into the surface of the collar to reduce its surface area.
What is the application of the D/C square type:
- they provide good centralization all over its length.
- They increase bending resistance (stiffness) to the max
- They increase torsional damping
- They decrease axial vibrations.
Why not use them all the time?
- This drill collar type is expensive to purchase and to do the regular maintenance.
- They usually create high rotary torque.
- Due to their shape, they grind drilling cuttings and well caving’s to fines which will increases mud treatment and cost.
- The fishing of this drill collar type are difficult.