A method is presented which gives general guidelines for predicting fatigue life of Measurement-While-Drilling (MWD) tools. The method has been particularly useful as a well planning aid for determining how much (if any) drillstring rotation should be permitted in the angle build sections of medium-radius horizontal wells. The results show that, under most conditions, drillstring rotation should be avoided in doglegs greater than 15 deg/100 ft [15 deg/30 m]. In some cases, even smaller upper dogleg limits should be observed.
The angle build sections of medium-radius wells usually have dog legs which range from 6 to 30 deg/100 ft [6 to 30 deg/30 m]. The drillstring is occasionally rotated through these large doglegs for various reasons which include: reaming tight hole, improving hole cleaning, or reducing the rate of angle build which occurs during sliding. Rotation through such doglegs causes large cyclic bending stresses in MWD tools, which can result in a fatigue-failure.
Before 1990, Sperry-Sun negative-pulse MWD tools had not been used to drill medium-radius wells. Since that time, their use has been increasing. Because these MWD tools were originally designed for long-radius wells (dog legs less than 6 deg/100 ft) [6 deg/30 m], we were initially unable to provide guidelines for their use in medium-radius wells. Although fatigue is a primary cause of bottom-hole assembly failures, there were no public guidelines which met our needs.
Fatigue life is difficult to predict accurately. One way to predict fatigue life would be to track the stress and rotation history for each MWD tool for its entire life. This is beyond the scope of the current work. Instead, we choose to provide some general guidelines for the design limit of our MWD tools. In particular, we are trying to define the rotary conditions under which our MWD tools can not operate, regardless of our handling of the controllable variables (e.g. inspection frequency, stabilization, etc.). The presence of chlorides in water-base mud makes accurate prediction more difficult, due to stress corrosion cracking. The initiation of a potential fatigue crack is affected by the corrosiveness of the environment although the crack growth phase is not.
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