Design and Fatigue Analysis of an LWD Drill Tool

Abstract

Previous works suggest that 80% to 90% of failures observed in the rotary machines are accounted for fatigue failure. And it is observed that cyclic stresses are more critical than steady stresses when the failure occurred is due to fatigue. One of the most expensive industries involving rotary machines is the Oil and Gas industry. The large drilling tools are used for oil extracts on-shore and off-shore. There are several forces that act on a drilling tool while operating below the earth's surface. Those forces are namely pressure, bending moment and torque. The tool is designed from the baseline model of the former tool in Solidworks and Design Molder. Here load acting due to pressure and torque accounts for steady stress i.e., Mean Stress and loading acting due to bending moment account for fluctuating stress i.e., Alternating Stress. The loading and boundary conditions have been adapted from Halliburton’s previous works for the LWD drill tool to better estimate the size of the largest possible transducer. The fatigue analysis of static load cases is carried out in Ansys Mechanical Workbench 19.0 using static structural analysis. The simulation is run to obtain results for total deformation, equivalent stress, and user-defined results. The component is designed for infinite life to calculate the endurance limit. Shigley guidelines and FKM guidelines are compared as a part of a study to select the best possible approach in the current application. The width of the imaging pocket is varied from 1.25 inches to 2.0 inches to accommodate the largest possible transducer without compromising the structural integrity of the tool. The optimum design is chosen based on the stress life theory criteria namely Gerber theory and Goodman Theory.