Comparison of Direct and Indirect MWD Measurements

Abstract

Drilling boreholes for geotechnical purposes such as sampling and in situ tests is usually performed with the aid of drilling machines. In France, typically, the machine’s drilling parameters such as applied torque and thrust are monitored in realtime and registered so that information about the subsoil’s structure can be acquired. As these machines tend to use hydraulic systems, the oil pressures fed into each motor and actuator are the parameters commonly monitored. Recently, a new type of sensor and its application in geotechnical investigations through the measuring of drilling parameters was presented. This sensor is directly mounted on top of the drill string of a drilling machine and records the actual thrust, torque, and rotation speed transmitted to the drill string and bit. This bypasses the energy losses present in the hydraulic circuit between the machine’s pressure sensors and the hydraulic actuators. This mounting position allows a more accurate measurement of the forces and, furthermore, the effective rotation speed applied to the drill bit. The data is transmitted through a wireless Bluetooth connection enabling real time monitoring. Nevertheless, finding the right balance between a soil’s resistance, the drill rig’s power and the sensor’s optimal measuring range remains a topic for further development. This analysis of several worksites with different soils throughout France contributes to a better understanding of the last generation’s sensor’s precision and application range.

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