Drill Collars

What collars do in the string

Drill collars are thick-wall bars run above the bit. Their buoyed weight puts load on the bit while the neutral point stays in the collars or heavy-weight pipe, which keeps the drill pipe above in tension. Their stiffness resists buckling and helps hold hole direction with the stabilizers between them.

Non-magnetic collars are made from austenitic stainless steel so a magnetic survey tool inside reads the earth's field without distortion from the steel string. Their number and placement come from your survey company's magnetic interference calculation.

Configurations

• Slick collars, with a plain round body and the most weight per foot.
• Spiral collars, with grooves that cut wall contact and reduce the risk of differential sticking, for a few percent less weight.
• Slip recess and elevator recess, which let you handle collars with slips and elevators without a lift sub or safety clamp.
• Non-magnetic collars, slick or spiral, usually one to three joints above the survey tool.
• Short and pony collars for spacing out a BHA.

Key specifications

Standards and what they ask of a used collar

API Spec 7-1 sets dimensions, mechanical properties and markings for new steel and non-magnetic collars. For non-magnetic collars it caps relative magnetic permeability at 1.010 and limits magnetic hot spots in the bore. API Spec 7-2 governs thread form and gauging. API RP 7G gives make-up torque and bending strength ratio guidance, which you need when you mix collar sizes or connection types. Used collars are inspected to API RP 7G-2 or DS-1.

Failure modes and inspection

The BHA bends as it rotates, so every turn cycles the connection. Fatigue cracks start at the last engaged thread of the pin or the box, depending on the bending strength ratio.

• Connection cracks. Wet fluorescent magnetic particle inspection on steel collars, liquid penetrant on non-magnetic collars.
• Box swell and pin stretch from over-torque, measured against Spec 7-2 limits.
• Shoulder damage and galling, worst on non-magnetic material.
• OD wear near the connections, which lowers bending strength, and body straightness.
• Non-magnetic collars only. A magnetic hot spot survey along the bore with a fluxgate probe, and a check for chloride stress corrosion cracking in the bore.

Documentation

• Material certificate with heat number and hardness, or permeability data for non-magnetic collars.
• The latest inspection report, with standard, category and results per serial number.
• Current length against original length, and a recut log.
• Stress relief feature dimensions and whether threads were cold rolled.
• For non-magnetic collars, the last magnetic survey report and any galling repairs.

Integration with your BHA

Calculate weight on bit with buoyed collar weight and a design factor so the neutral point stays below the drill pipe. Check the bending strength ratio at every collar to collar change and at the bit sub. Confirm your slips, safety clamps, elevators and pipe racker can grip the OD. Collars without an elevator recess need a lift sub matching their connection, covered under Drilling Tools and Accessories.

Refurbishment and recertification

A shop can recut connections with a pin stress relief groove and box boreback, cold roll the thread roots, apply an anti-galling phosphate coating and re-hardband the body. Each recut costs several inches of length. Bent collars can sometimes be straightened. Non-magnetic collars with hot spots may need the bore re-machined, and bore peening restores the compressive layer that resists stress corrosion cracking.

Steel collars last a long time, so a used collar with freshly cut and inspected connections does the same job as a new one. Unused surplus non-magnetic collars are worth the most attention, since the material is expensive and a clean survey is easy to verify. For the stabilizers and jars that run between collars, see Bottom Hole Assembly (BHA) Tools.