Drill string oscillations during connections when drilling from a semi-submersible platform

Abstract

As the operational window is getting narrower, pressure control is becoming more important. Drilling from a semi-submersible platform can in this context offer new challenges as the top of the drill string will follow the semi-submersible platform's heave response to ocean waves during a connection. The heave movement can travel down the drill string and create pressure fluctuations around the drill bit. The movement of the drill string has been numerical simulated with a numeric program created in Matlab. The movement of the semi-submersible platform was simulated using a combination of two sinus functions. The results from the simulations show that the drill bit velocity and amplitude is generally increasing with heave amplitude, and generally decreasing with increased heave period and deviation angle. For the drill strings in vertical wells, the amplitudes are increasing with drill string length. However, as the deviation angle is increasing, more of the energy in the drill strings is lost due to contact friction, leading to a non-linear behavior and less distinctive oscillation patterns. The drill bit amplitude in deviation wells is then small up to a certain heave amplitude, where the drill bit amplitude increases rapidly. The simulations show that some of the drill strings start to resonate at a certain heave period, but the resonance is terminated by contact friction if the deviation angle is larger than approximately 7 degrees. Simulations also showed that the surge and swab pressures during normal weather conditions and drill string oscillations are up to approximately 5 bar. The pressure calculations were done with a relatively large flow area between the BHA and wellbore wall, but preliminary calculations have shown that the pressure fluctuations can be drastically increased if the flow area between the BHA and the wellbore wall is decreased. However, the calculations performed during the simulations and pressure estimations are probably conservative.