Elimination of Drillstring Failures
Authors: Dudman Roy L. 
Published: December 2007

Drillstring failures can be prevented by drilling a straight, useful hole free of severe doglegs with larger-than-normal drillstring components. The maximum drill-collar diameters shown in the Lubinski tables in the IADC Drilling Manual, Sec. P1,1 for straight-hole drilling are essential for drilling a straight, useful hole and preventing drillstring failures. When dogleg limits are maintained, drillpipe fatigue and associated hole/casing problems are prevented. Rate of penetration (ROP) can be greatly improved by controlling dogleg severity.

Operators have reported drillstring failures more than 50% of the time when drilling 6 3/4-in. and smaller hole sizes with conventional, fishable drillstring components. Conventional drillstrings, especially for slim holes, are torsionally weak, limber, and lightweight and have a small-restrictive bore. The lack of a stiff, heavy bottomhole assembly (BHA) for deviation control and sufficient bore size for adequate hydraulics reduces ROP and increases the probability of hole problems and drillstring failures.

Larger-than-normal drillstring components run pin-up with a fishing neck below the pin provide fishability and increased torsional strength, stiffness, weight, and bore size. The low-stress-level pin-up drillstrings have proved to be strong, heavy, and stiff enough to drill straight holes without accumulating fatigue. The larger-than-normal drillpipe connections are 40% to 60% stronger torsionally, and the drillpipe tubes have a bore that is more than 1/2 in. larger. The larger-than-normal drill collars are stronger, stiffer, and heavier. The resulting improvement in deviation control, hydraulics, and failure prevention increases ROP and the successful drilling of slim holes with 8 1/2- through 3 3/4-in. diameters. Smith Intl. performance bulletins document the increases in ROP and deviation control obtained during the last 6 years with no fatigue failures.

The correctly packed BHA consisting of three stabilization zones in the first 65 ft above the bit using one or two large-diameter short drill collars will drill a straight, useful hole and is the only recommended BHA design (Fig. 1). There is no assurance that a straight, useful hole can be drilled with a pendulum. Large-OD drill collars provide the maximum weight and stiffness to control hole deviation with a packed BHA and the maximum restoring force and drift diameter with a pendulum.

Crooked holes cause fatigue damage to drillpipe tubes, OD wear and heat checking of the tool joints, and casing wear. The Lubinski curves in API RP 7G2 show the dogleg severity that will cause fatigue damage of the drillpipe tube and wear/heat checking of the tool joints. The correctly packed BHA restricts hole-angle changes to below the critical value where fatigue damage and/or wear occur. If doglegs develop that exceed the limits, the logleg area should be renamed to reduce the severity of the hole-angle change. Casing wear results when casing is set in a crooked hole with the lateral force on the tool joints high enough to cause casing and/or tool-joint wear.

The correct low-stress-level drillstring will not accumulate fatigue in a hole drilled with the correctly packed BHA. Fatigue accumulates only when the drillstring components are weak and the stress exceeds the endurance limit of the steel.

Drilling a crooked hole and destroying a drillstring is inexcusable because the knowledge of how to prevent these problems has been available for years. The daily cost of the well might be slightly higher for the correctly packed BHA and the correct low-stress-level drillstring. The daily cost of this insurance is insignificant compared with the cost of rig time for fishing operations, lost hole sections, lost/damaged drillstring components and frequent inspections. Most downhole drillstring problems are predictable and preventable. Inspection will ensure the integrity of the drillstring but inspection will not prevent the accumulation of fatigue/failures in a weak drillstring.