Case Studies of BHA Vibration Failure

Abstract

ABSTRACT Predicting bottom hole assembly (BHA) vibrations is a complicated problem. Axial, lateral, and torsional vibrations can be coupled and effects such as stick-slip and whirl can magnify the loads. Fatigue, pipe bouncing, and tool joint washouts demonstrate the complexity of the problem. One type of vibration that can be isolated and analyzed is the rapid destruction of the BHA caused by operating at or close to resonance. At rotating speeds that reinforce the natural vibration of the BHA, the destructive harmonics generate high stresses resulting in very short fatigue life. While other factors may cause BHA failure, a significant percentage of field failures appear to be associated with harmonic vibration, particularly lateral vibration. A simplified model based on harmonic analysis using finite elements has been found to agree well with field experience. The influence of stabilizer placement, drill string forces, and mass of the drilling mud are included in the finite element vibration model. This paper consists of 8 case studies involving actual BHA failures. The model predicted lateral vibrations with critical rotation speeds that correspond closely to operating conditions at the time of failure in all of the BHA's studied. In addition, the reversing stress computed for the critical operating speeds exceeded the endurance limits of the connections and occurred at the locations in the BHA's where failures were observed. These comparisons to field data are the first steps in establishing model reliability, so that it can be confidently used as a tool to select operating speeds or evaluate BHA designs.