### Tech Briefs

Simulation of Multiple Rotor Behavior

Rotor behavior is of interest to engineers in the aerospace, automotive, and oil & gas industries, among many others. Simulation of rotor behavior can be challenging, especially for systems with multiple rotors, as centrifugal and Coriolis inertia forces must be included in the analysis.

In ADINA, users are able to apply inertia loads on systems with multiple rotating parts, while performing a static analysis*. This feature accounts for centrifugal and Coriolis forces, and allows users to easily model multi-rotor systems. In this News, we demonstrate the capability by simulating a tricone drill assembly in drilling motion.

The tricone drill assembly is widely used in oil well drilling. The assembly consists of a drive shaft with three rotary bits mounted on bearings, as shown in Figure 1.

Figure 1  Tricone drill bit assembly

The shaft and bits are modeled using 27-node brick elements. The bearings do not need to be modeled in detail using structural elements; instead, they are modeled using alignment elements, see Figure 2.

Figure 2  Using the alignment element to model a bearing

During the drilling process, the tricone drill assembly is subjected to a drive torque, to reaction forces from the rock, and to the inertia forces — that is, the centrifugal and Coriolis forces. In this analysis, we only focus on the response due to the inertia forces from the rotating parts.

The problem can be analysed dynamically, by prescribing the rotations through the alignment elements, as shown in the above movie. However, this solution is computationally expensive and may include significant numerical errors, unless the Bathe time integration method is used.

A better approach to solve this problem is to analyse the problem statically, using the rotational load feature in ADINA. By doing so, the problem is solved at a greatly reduced computational cost.

The rotational load feature applies the inertia forces due to the rotation of the moving parts, and accounts for centrifugal, Coriolis, and nonlinear spin softening effects.

These displacements are with respect to a body fixed frame at any snapshot in time.

Although it is frequently claimed that Coriolis effects may be neglected, in this problem, Coriolis forces are significant and must be included. Figure 4 shows that the maximum von Mises stress is nearly three times higher when Coriolis forces are included.

Figure 4  Coriolis forces are significant for the tricone problem

The rotational load feature in ADINA is a powerful capability. It allows analysts to easily model systems with multiple rotating parts by performing only a static analysis, in linear or nonlinear conditions, using all the modeling features available in ADINA.

Keywords:
Multirotor simulation, tricone drill assembly, oil well drilling, rotational loads, inertia force, centrifugal load, Coriolis load, spin softening, alignment element, gyroscopic effects

*ADINA version 8.9.1 or a later version is used