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Rotary friction welding for geothermal applications

Wed, 19 August, 2020

Drill pipes are employed in high torque environments deep within the earths crust, they must withstand high pressure and torque. As such drill pipe tends to be manufactured from high strength steel. Modern drill pipe is manufactured from separate pieces, most commonly the tool joint and tube body. The tool joint provides high strength, high pressure connections between drill pipes and therefore tend to be produced with higher strength steel than the tube body. The dissimilar steel sections required welding together to produce a drill pipe. Rotary friction welding (RFW) has long been the joining method of choice for drill pipes used in the oil and natural gas industry. So much so that the use of RFW is enforced joining method to meet the current industry standards (API 5DP & BS EN IS ISO 11961).

RFW is a solid-state welding technique where two parts, in this case the pipe and the tool joint, are forged together using mechanical motion and heat generated by friction. One part is rotated while the other remains stationary and an axial force is applied between the two parts. RFW is the preferred joining method for drill pipes and other critical applications due to the advantages it displays in comparison to fusion welding techniques. The process has lower peak temperatures than fusion welding, reducing intermetallic formation and allowing for a range of dissimilar materials to be joined. The process is easily automated, and therefore highly repeatable. Defects associated with melting and solidification are avoided and the resulting microstructure often displays higher strength than the starting base material.

RFW will be employed in the Geo-Drill project to produce full size demonstrator parts.

Schematic of rotary friction welding process
Schematic of rotary friction welding process