Many models in the industry use the notion of an effective length to make predictions about the bottomhole assembly (BHA). A criterion is chosen to cut the drillstring at a certain distance from the bit and use only that part for the computations. Though this method is computationally efficient, no consensus exists regarding what criteria should be chosen or a perfect method regarding the computation of that length. This study aims to compensate for the lack of in-depth analysis on this important subject. It shows the importance of understanding the strengths and limits of frequency-based models compared with more-complex time-domain modeling.
Introduction
The notion of effective length is widely used in the industry for various types of drillstring mechanical models, whether for directional purposes, survey corrections, or assessment of vibration behavior of the BHA. The drillstring is cut at a given distance from the bit, known as the effective length (see example in Fig.
