The ROV, or subsea remotely-operated vehicle, is frequently used in marine operations such as underwater mapping, pipeline inspection and surveillance, sending payload, maintenance and operations on subsea oil and gas equipment such as BOP (blowout preventer) and Christmas tree assembly, which controls the oil/gas/water flow out of the well.
Underwater environments create various challenges for the manufacturers of the vehicle robotics. In addition to structure integrity under high pressure, complex underwater hydrodynamics characteristics due to coupling of motions in 6 degrees of freedom needs to be considered.
Most ROV’s are not symmetric in shape like a torpedo. Any surrounding seawater flow around the vehicle will induce imbalanced hydrodynamic forces on the vehicle driving involuntary motions such as noise dive, rolling, yawing and so on. These motions pose challenges to stability control of the vehicle as the ROV needs to be able to stay stationary and upright in orientation to carry out underwater operations that quite often demand high precision.
The first step in designing a subsea ROV is to understand the hydrodynamics characteristics due to different flow conditions around the vehicle.
The hydrodynamics characteristics comes into the motion equations through 3 important coefficients: added mass, linear and quadric damping coefficients. The added mass coefficient can be obtained experimentally (e.g., by a free decay pendulum motion test.) The linear and quadric damping coefficients can be extracted from the drag force vs speed relationship typically evaluated in the surge, heap, and sway directions. Instead of experiment, which is both time consuming and expansive, CFD is much more effective in evaluating the linear and quadric damping coefficients through a parametric solution of drag force vs speed. After the drag force vs speed parametric data is available, the linear and quadric damping coefficients can be easily extracted by a 2nd order polynomial curve fitting.
ANSYS Workbench is a simulation platform with parametric and optimization study in mind. In the case of parametric study of drag force vs speed on a ROV, the flow speed is set up as the input parameter and the drag force as the output parameter. ANSYS Workbench will automatically update the parametric ANSYS CFD runs throughout the flow speed range for generating the drag force vs speed curve; greatly enhances the engineering productivity. ANSYS CFD results also reveal the 3D flow patterns around the ROV. With these results at hand, ROV design engineers can further improve and optimize the shape of the ROV for a faster vehicle with less thrust.
The take home point, ANSYS CFD can help subsea remotely-operated vehicles manufacturers greatly compress the design cycle and produce superior ROV’s with less operating costs. For more information, check out our resources for Oil and Gas.