Introduction To Tubulars Page 10 Of 10

TUBULARS The Drill Collar The term drill collar derived from the short sub originally used to connect the bit to the drill pipe. A modern drill collar is about 30 ft long, and the total length of the string of drill collars may range from about 100 to 700 ft or longer. The purpose of drill collars is to furnish weight to the bit. However, the size and length of drill collars have an effect on bit performance, hold deviation, and drill pipe service life.

Drill collars may be classified according to the shape of their cross-section as round drill collars square drill collars, or spiral drill collars (i.e., drill collars with spiral grooves)

Selecting Drill Collar Size

Stress Relief Features for Drill Collars

Selection of the proper outside and inside diameter of drill collars is usually a difficult task. Perhaps the best way to select drill collar size is to study results obtained from offset wells previously drilled under similar conditions. The most important factors in selecting drill collar size are. Box Stress Relief Feature Bit Size Coupling diameter of the casing to be set in a hole Formation's tendency to produce sharp changes in hole deviation and direction Hydraulic program Pin Stress Relief Feature

Spiral DC Selecting Drill Collar Length Drill Collar Range

Minimum Yield Strength (psi)

Tensile Strenght (psi)

3 1/8” – 6 7/8”

110,000

140,000

7”-10” greater

100,000

Elongation

13%

135,000

Physical Properties of Drill Collars

13%

The required length of drill collars can be obtained from the following formula:

Ldc = DF x WOB Weight of DC x BF x Cos Ө

Selection of Connection Bending strength ratio charts may be used for determining the most suitable connection to be used on new drill collars or for selecting the new connection to be used on collars which have been worn down on the outside diameter.