Vibrations are a common contributor to premature drillstring failure. The need for a better understanding of the phenomenon has driven the implementation of real-time downhole drilling mechanics measurements. As the costs associated with poor drilling performance and equipment failures increase, so does the impact of improving the modeling of these vibrations.
The onset of downhole vibrations is complex and sensitive to a wide range of factors, some of which are difficult to quantify within a typical drilling environment. Examples of these factors are the friction factor at contact points, drillstring component imbalance and rock properties and their variability. Field experience has shown that the identification of resonant conditions using frequency domain modal analysis is of value; however, additional insight can be gained through the use of a transient vibration analysis.
This paper describes a numerical modeling tool developed to enhance understanding of the transient dynamics undergone by a drillstring during the drilling operation. A Finite Rigid Body (FRB) approach has been chosen for modeling simplicity, computational cost, and physical relevance of the computed results. This approach can be used to incorporate important transient features characteristic of a drilling operation, such as the bit-rock interaction and the interaction of the drillstring with the borehole wall, as well as the relevant dynamics introduced by drilling tool components such as mud motors and bit steering devices. A number of examples and case studies illustrate the capabilities of the modeling tool.