| CPC E21B 1/24 (2020.05) [E21B 4/14 (2013.01)] | 16 Claims |
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15. A method of controlling an excitation frequency of a drilling device for forming a drilled hole, the drilling device including at least one drill rod, the or each drill rod being disposed in the drilled hole during use and having a first cylindrical wall defining an elongate chamber for receiving a working fluid to form a fluid column, the length of the fluid column being equal to a total length of the elongate chambers of the or each drill rod, a displacement excitation device arranged at a proximal end of the fluid column and configured to excite the fluid column to cause the working fluid in the fluid column to oscillate, wherein the excitation device is configured to excite the fluid column at an excitation frequency at or within 10% of a natural frequency of the fluid column determined based on the fluid column having a fixed boundary condition at the proximal end thereof, and a tool piston moveably mounted at a distal end of the fluid column and a drilling tool connected to the tool piston such that the oscillation of the working fluid in the fluid column imparts an oscillating force to the drilling tool, the method comprising:
setting an input torque for the displacement excitation device, whereby the excitation frequency is determined based at least on the input torque and a frequency response of the fluid column,
wherein the displacement excitation device reciprocally moves the working fluid in the fluid column in a longitudinal direction of the fluid column.
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