This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18836, “How To Design a BHA Rotary-Speed Sweet Spot,” by Jeffrey R. Bailey, SPE, and Andrius Minkevicius, SPE, ExxonMobil; Alexander Bolzan and Victoria R. Wilkins, Hibernia Management and Development; and Joshua S. Pokluda, Esso Exploration and Production Nigeria, prepared for the 2016 International Petroleum Technology Conference, Bangkok, Thailand, 14–16 November. The paper has not been peer reviewed. Copyright 2016 International Petroleum Technology Conference. Reproduced by permission. Correct placement of the rotary-speed sweet spot of a bottomhole assembly (BHA) provides multiple benefits toward achieving the ultimate goal of drilling to section total depth in a single trip. The process of BHA redesign and tuning for the desired operating parameters provides greater control over the drilling process. Vibration minimization through proper design and operation of the BHA facilitates drilling objectives that include reaching section depth in fewer trips and providing acceptable borehole quality to run casing to depth. Introduction A musician understands the basic principle of a stringed instrument: The frequency of the note made by the instrument varies with the length of the string span. Other parameters play a role, including the string properties and tension. However, once the instrument is tuned and the concert has begun, the musical notes are played principally by changing the length of the vibrating string. As the finger position is changed on the fingerboard, the pitch of the note changes. Although this is a basic concept to a musician, it is not common today in the oil and gas industry for engineers to apply this concept to BHA design selection. A BHA is a beam that may be rep-resented by a fourth-order differential equation. Model calculations proceed by subdividing the BHA into elements comprising short sections of pipe in a lumped-parameter model. A 2D model uses a state vector at each mass node comprising the lateral displacement in a plane, tilt angle relative to the center-line, bending moment, and beam shear load. In the lateral bending flex mode, a dynamic side force is applied at the bit at integral multiples of the rotary speed. In the rotary twirl mode, offset mass elements are used to investigate the stability of the BHA to eccentric mass and centrifugal forces. The dynamic response is considered to be a perturbation from the static solution. The bending strain energy is related to the square of the bending moment in the beam, and calculating the length-averaged BHA bending energy provides a BHA bending-strain-energy vibration index. Lower values of vibration indices are sought because the dynamic response to a reference input is then minimized (i.e., lower vibrations are then predicted by the model). All such modeled designs are perturbed by identical inputs, and the model results may then be compared on an apples-to-apples basis.