Effective Force and True Force: What are They?

Abstract

Abstract The predictions of hookload, tension, and stresses are important for the successful completion of any well operation, as well as for the selection of optimum operating parameters. The terms effective axial force and true force or effective tension and true tension are used for various tubular designs, including riser designs. To further complicate matters, they are also defined as buoyancy and pressure area methods of calculating forces. Discussions of these two forces occasionally raise doubts regarding how to treat the internal and/or external pressures. The main problem stems from the perception that effective force as a fictitious force, and the inclusion of the internal and external pressures as counter intuitive source of misunderstandings and wrong designs. Although several authors have used the calculation and concepts correctly, other previously published papers failed to address the issue clearly and in a simple way; as a result, confusion still exists. These two forces further result in two neutral points, which raises the questions of “when you apply pressure, does the neutral point change?” and “which neutral point should be used for stuck pipe calculations?” The working envelop using yield limit with combined loading of uniaxial force and pressure forces are important. Using the wrong force/tension will result in an incorrect interpretation of the burst and collapse limits. This paper provides the mathematical treatise and discusses the theoretical basis of these two forces, including how they are used in torque, drag, buckling, stress, and limit calculations. This paper describes the details, apparent definition, pitfalls, and context in which these forces can be used. Simple guidelines are presented, as well as several example calculations to help explain various tubular modeling aspects for future generations of drilling engineers.