Reducing stick-slip oscillations in oilwell drillstrings

Torsional drillstring vibrations, also known as "stick-slip" oscillations appearing in oilwell drillstrings are a source of failures which reduce penetration rates and increase drilling operation costs. An important problem to solve is the determination of control laws that reduce stick-slip phenomenon. A distributed model for the drilling system is given, this model is reduced to a neutral type retarded model through two different approaches: the d\´Alembert transformation and Laplace transforms techniques. Some experience-based control strategies are evaluated in order to reduce stick-slip oscillations. The use of the angular velocity at the drillstring upper part, the torque on the bit and the weight on the bit is shown to have a key effect in the reduction of drillstring torsional vibrations.