Objectives In recent years, with the continuous development of offshore engineering into deep-sea waters, remotely operated vehicles (ROVs) have been widely used in various fields such as marine scientific research, marine resource exploration, and subsea engineering due to their advantages of cost-effectiveness, high operational efficiency, and capacity for continuous operation in complex deep-sea environments. However, the complex open-frame structure of ROVs, along with various attached devices, results in a highly complex flow field during motion. To enhance the maneuverability and control precision of ROVs, mitigate the risks of offshore operations, and enhance operational safety, it is necessary to study their hydrodynamic characteristics. This study aims to conduct a systematic investigation into the hydrodynamic characteristics of open-frame ROVs and accurately determine their hydrodynamic coefficients. This research is of great significance for improving the understanding of ROV motion performance, stability, and maneuverability in water, and serve as a foundation for future motion prediction and structural optimization of ROVs.

Methods Firstly, the reliability of the STAR-CCM+ numerical simulation results was validated through direct free-flight physical modeling tests of open-frame ROVs. Then, numerical simulations were conducted for the straight, oblique, and planar forced motions of the ROV. In the numerical simulation, the RANS-based CFD software STAR-CCM+ was adopted, and the control equations consisted of the continuity and momentum equations for incompressible fluid, using the Realizable k−ε turbulence model. The ROV model was simplified while retaining its main structural features, and grid-independence and wall-independence tests were performed. Subsequently, the least-squares fitting method was used to obtain the viscous and inertial hydrodynamic coefficients of the ROV based on the simulation results.

Results In the case of straight and oblique motions, the viscous hydrodynamic coefficients Y'_vv and Y'_u|v|, which are related to the ROV's geometric port-starboard asymmetry, are relatively small, with values of 7.25×10⁻³ and 2.74×10⁻² respectively. In the planar forced motion, the inertial hydrodynamic coefficient Z'_\dotv , Z'_\dotp and X'_\dotr attributed to the ROV's port-starboard asymmetry, constitute significant portions of the overall inertial hydrodynamic coefficient matrix, with values of 1.210%, 8.850%, and 3.499% respectively. Also, in the straight-line navigation, the hydrodynamic forces acting on the ROV in the longitudinal and vertical directions show significant differences in different positive and negative motion directions, whereas the difference in the lateral direction is weak. In the oblique navigation, the longitudinal hydrodynamic force exhibits minimal variation with yaw angle, whereas the lateral and vertical hydrodynamic forces vary significantly with the angle of attack and drift angle.

Conclusions The port-starboard asymmetry of the ROV's geometry has a non-negligible impact on its inertial hydrodynamic characteristics. The hydrodynamic coefficients obtained from numerical simulations can support subsequent ROV motion prediction and structural optimization. Future research will focus on the impact of the asymmetric structure of the open-frame ROV on its viscous hydrodynamic characteristics. More accurate hydrodynamic calculation models will be developed through turning motion simulations to derive second-order and coupled viscous hydrodynamic coefficients related to angular velocity.