Most of the failures of drill pipes, MWDs or bits are related to whirling. This abnormal rotation of the bit or of the pipes generates high lateral vibrations, shocks or local negative speeds. It is of prime importance for the drilling industry to detect this phenomena when it starts. This may reduce drilling cost by avoiding a breakdown of the drill string or a destruction of the bit either instantaneously or by fatigue.
Unless specific and costly MWD systems are used there are up to now almost no means to detect whirling from surface measurements. However a close study of data recorded synchronously at bottom and at the top of the drill string shows that the whirling of the BHA can be detected. When whirling occurs there is a change in the mean value of the rotary torque and a specific frequency appears in the weight on hook measurement.
Using specific signal treatment procedures a software have been designed to recognize these two specific changes in these surface measurements. The first step was to verify that the surface torque and the weight on hook can be considered as random variables. Then a procedure was implemented taking into account it must work real time. It compares continuously the same probability density functions over a short and a long period of time. An alarm is raised when these two functions increase over a preset value. Furthermore a study of the weight on hook spectrum allows to estimate the severity of the whirling. Tested on a data base this software proves to be successful.
Whirling can be described as an abnormal rotation of either the drilling bit or the drill string. In case of whirling the bit or the pipe does not rotate anymore along their own axis. Their rotation is a combination of a rotation around the well centerline and of one around their axis. Therefore whirling is a complex movement. It generates lateral displacements, shocks and friction against the hole walls. This paper will focus on whirling of the drill string.
Drill string whirling occurs mainly in the bottom hole assembly (BHA). Sometimes it can happen in the drill string. While drilling the BHA is under compression and thus is susceptible to buckle and whirl. The drill string is under tension and has a lower tendency to whirl. When happening in the drill string whirling can be noticed by lateral movements of the traveling block called whipping.
Whipping is easy to detect but not whirling. When BHA whirling starts components of the BHA withstand large lateral displacements which generates bending stresses. When these displacements become too large contacts happen between some parts of BHA components and the hole walls generating lateral shocks. Sometimes there is also a contact more or less continuous with the hole walls and consequently a higher wear rate of the pipe at this point. All these phenomena increase drastically the fatigue of the BHA elements and mainly of theirs connections. As whirling is difficult to detect the fatigue accumulates while drilling and finally a failure of one component of the BHA can occur with a potential costly fishing job.
Until LWD (Logging While Drilling) systems were developed the driller has little means to detect whirling. Latter in this paper it will explain how whirling can be detected using only the surface measurements with the help of advanced signal treatment procedures. When LWD is used a shock counter can be installed [Ref 1].