Dynamic Soft String Model and Its Practical Application

Abstract

AbstractIt is well accepted that torque and drag calculations are essential for well construction applications. To the authors' knowledge, the calculations are performed based on the concepts of the soft string model. This approach enables the wellbore designer to determine torque and other forces in the drillstring. While several modifications of the soft string model have been proposed and implemented in commercial software, the model mainly used in the oil and gas drilling industry is the static soft string model. As such, not only the string bending stiffness is neglected, but also it is assumed that the string is motionless and the direction of motion changes by merely changing the sign of friction coefficient. Clearly, there is a strong need for including acceleration effects in the soft string model to permit analysis of tripping operations and more accurate evaluation of the drillstring loading and consequently, the rig equipment (hoisting system, etc.).In this paper, an improved dynamic soft string model is proposed that accounts for drillstring motion in 2D and 3D wellbores. This mathematical model explains how to compute forces along a moving drillstring or casing. The aims of this model are: 1) to analyze dynamic drillstring behavior, 2) to estimate local contact forces, and 3) to predict the effect of different tripping velocity profiles on axial and lateral contact forces. A system of equations for drillstring translational motion is solved using numerical methods. A computer code has been developed for practical design calculations.The improved dynamic soft string model can be used to determine surface load and contact force as functions of time and measured depth. This model is also applied to predict surface load and contact forces in tripping operations. In particular, the model is implemented for drillpipe in two different 2D wellbores: horizontal and S-shaped, and a 3D wellbore while tripping in and out of the hole. As expected, the surface load vs. time plot shows a similar trend of tripping accelerations. Depending on the well path shape, drillstring properties, tripping acceleration, and velocity profile, the maximum dynamic loads can be in the range of 4-40% higher compared to the conventional soft string model. In addition, results for two different tripping velocity profiles (trapezoidal and parabolic) are compared. The maximum surface load is up to 4% higher with trapezoidal velocity profile which is not significant, because both velocity profiles provide 90 ft displacement. It is noted that the governing parameter for the maximum load needed is the maximum tripping acceleration, not the maximum velocity.Improved dynamic soft string model will have significant impact on well path shape, drillstring design, drillpipe failure analysis, tripping operations optimization, and automatic control of the drawworks.