Method of controlling a downhole operation

A method for controlling a drilling or cutting operation performed by a wireline tool downhole, comprising the steps of:

• commencing a drilling or cutting operation in a downhole object, such as a casing or valve,
• detecting vibrations in a tool housing produced during the drilling or cutting operation in the downhole object using a vibration sensor being an accelerometer arranged on the tool housing,
• processing a vibration signal from the vibration sensor to produce a reference frequency spectrum comprising actual measurements taken during a first part of the drilling or cutting operation,
• processing a vibration signal from the vibration sensor to produce a real-time frequency spectrum comprising actual measurements taken during a second part of the drilling or cutting operation,
• comparing the real-time frequency spectrum to the reference frequency spectrum,
• calculating and detecting a completion or failure of the operation, such as a completion in which the casing has been cut into two casing sections or a failure in which the bit is stuck, based on the comparison of the real-time frequency spectrum and the reference frequency spectrum, and
• controlling the operation to terminate the operation when completion or failure of the operation has been detected,
• wherein the step of calculating and detecting a completion of the operation comprises a step of calculating a centre of gravity of oscillation of an area of peaks in the frequency spectrum, which area is above a certain value and comparing that centre of gravity with a centre of gravity of an area of peaks in the reference frequency spectrum, which area is above the same certain value to determine a discrepancy between the two centres of gravity, and wherein the step of controlling the operation is based on the centre of gravity comparison.