Wringing effect prevention on a piston design in a downhole drilling tool

This research studied the wringing effect present on a piston with two contact surfaces in a downhole tool and investigated piston design guidance to prevent this occurrence. A piston on the downhole drilling tool is designed to be moved by hydraulic forces on‐demand in downhole operation. The piston failed to move during an experiment at a depth of 8772 m in an oil drilling well located in the Gulf of Mexico. The wringing effect was suspected as the root cause of this failure, and subsequent laboratory experiments confirmed that wringing presence caused non‐functional piston. Then, potential influencing factors causing wringing effects were analysed and investigated in a series of laboratory experiments aiming to eliminate the wringing adhesive force between the contact surface of the piston and its mandrel. The investigated factors included the roughness and area of the contact surfaces and pre‐compression force. Based on the findings, the piston design was optimised and re‐tested, and it was found that the optimised piston eliminated wringing adherence. With experimental data, semi‐empirical design guidance regarding the influencing factors was summarised for critical piston design to eliminate possible wringing.