Due to the difference in modulus of elasticity of aluminum compared to steel, an aluminum drill string will exhibit more stretch than a steel string in air. In a liquid, an aluminum string will develop greater buoyancy than steel, which will nullify a portion of the aluminum string weight, thereby reducing the stretch of the string.
The difference in length of a steel string, compared to an aluminum string in a liquid, depends on several factors:
- String length
- Weight of drill collars
- Weight of other bottom hole assemblies in use (pipe sizes, mud weight, and straightness of the hole)
Arconic aluminum drill pipe, under its own weight without collars, etc., has less stretch in 10 lb. mud (or a heavier mud) than a comparable steel string.
Aluminum drill pipe users have reported no problems associated with stretch. Although aluminum drill pipe strings exhibit more axial displacement than steel drill pipe on a pull, no cases of stretched pipe (permanent axial elongation of the joints) have been observed or reported on strings in service.