Uncertainty Reduction in Fatigue Life Validation Testing for Drilling Tools with a Universal Runout Compensator

Abstract

Rotating bending fatigue (RBF) life is a critical performance index for oil drilling equipment. It can be measured via cantilever-type RBF testing, with a sideload applied on the specimen to generate the required bending moment at the point of interest. Multiple factors cause runout or eccentricity of the test specimen at the sideload point, leading to deviations in the effective sideload acting on the specimen and, consequently, deviations in the bending moment at the point of interest. Runout generates greater uncertainty in fatigue life test results. A Universal Runout Compensator (URC) was developed to reduce this uncertainty by mitigating the test specimen runout. It consists of two eccentric bushings, one assembled inside the other. Depending on the relative orientation of the two bushings, the total eccentricity of the URC can be adjusted. With the properly set URC installed on the specimen, the final runout at the URC where the sideload is applied becomes negligible. Finite element analysis (FEA) was used to confirm the URC structural integrity. Full-scale RBF tests were conducted to validate the URC design and FEA studies. With the URC used in the tests, bending moment variation decreased by up to 15%, reducing life cycle uncertainty by up to 44%.