Two types of frequency-induced whirling are possible in the rotating BHA, namely forward whirl and backward whirl. When the BHA center of gravity Kainic acid not located along the center of the hole, a centrifugal force causes the collar to bend. Forward whirl is the rotation of the deflected drill collars around the borehole axis in the same direction as spin rotation (Leine et al., 2002). Backward whirl is a rolling motion of the drill collars over the borehole wall in the direction opposite to spin. Whirling modes are excited due to wellbore contact in low strength formations and as a result, an over gauge hole can be drilled (Leine et al., 2002). Eccentricity, bent drill collars and high compressive loads at the bit, nonlinear effects of mud forces, and stabilizer clearance are other sources of whirl excitation. Vandiver et al. (1990) and Jansen (1991) studied the whirling motion of the drillstring. The heavier collar section is easily excited by contact with the wellbore in the lower modes. In pistil modes, the collars are vibrating transversely, while the pipes do not vibrate and remain approximately motionless. This is due to the axial load distribution along the drillstring, the collars of which are mainly under compression, while the pipes are under tension. As a result of the tension, the natural frequencies of the pipe section increase, while the natural frequencies of the collar section are reduced (Thomsen, 2003). The lateral vibration behavior of the drillstring is strongly influenced by BHA vibration.