A rarely recorded drilling harmonic is noted to have occurred in two North Sea applications. Torsional vibration at the second harmonic frequency of the drill string has been recorded and validated with the aid of Finite Elemenet Analysis (FEA). Heavy wear to the drill pipe hardbanding was observed in one well which is believed to be related to the second harmonic torsional vibration.

Although Stick-Slip during drilling is fairly common, little is done to understand the frequency of the Stick-Slip cycle. The typical model assumes string RPM to be constant at surface with greatest variation at the bit (first harmonic frequency). However analysis of downhole data has shown this is not always the case. Torsional vibration or Stick-Slip is typically relatable to the first harmonic frequency of the drill string, i.e. maximum RPM variance occurs at one point only (the bit). For the system to enter the second harmonic frequency (two points of maximum RPM variance) there should be an additional excitation factor. In the two wells analysed there are notably high contact forces in the casing around the upper anti-node of the drill string. It is understood that the high force of the string acting against the wellbore can create a reactive torque large enough to excite the system into the second torsional harmonic. The high contact forces can be attributed to areas of high dog leg severity (DLS) at this point in the well. Although the DLS may not be exceptionally high, its position higher up in the well is of critical importance. This is due to the magnitude of tension in the pipe at this depth created by the weight of the string below. The combination of high tension in the pipe and deviation in the well path will provide contact forces high enough to create the shift from first to second torsional mode.

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