Modeling of Drillstrings

In this paper we show how a drillstring can be modeled in terms of limited numbers of masses and springs for the purpose of building a test rig. The model should represent the drillstring dynamics to a reasonable degree of accuracy. We will use the real component of the Frequency Response Function and the stability diagram as measures of dynamic similarity between the model and drillstring. We will also show how the chosen modes can be decoupled and used in obtaining the bit displacement. The decoupled modes will be used in a proposed test rig configuration that would increase flexibility in adding or removing modes from the system. Obtaining a representative model of the test rig is critical to our ability to extrapolate laboratory data into field applications.