Analysis on ROV dynamic responses under considerations of moving boundary and coupling hydrodynamic force

Due to the interactions between ocean environmental loads, remotely operated vehicle (ROV), suspending cable and top-end vessel motions, the dynamic responses of ROV system have strong coupling and nonlinear characteristics. Firstly, the FEM numerical simulations of ROV system, which include the vessel surge and the hydrodynamic force related to ROV motion, are developed to examine dynamic responses of ROV system. It is found that, the ROV response can be amplified when frequency ratio is larger than 0.7, up to 5.95 times of the vessel surge amplitude. As the surge amplitude increases, the ROV displacement gets larger. However, the response amplification factor (RAF) becomes smaller owing to nonlinear damping coming from hydrodynamic force, particularly when the frequency ratio is close to 1.0. Furthermore, to solve the nonlinear governing equation of the ROV dynamics, an equivalent damping model is developed based on ROV displacement attenuation, and a semi-analytical expression as a function of the ROV displacement along with surge amplitude and frequency, is obtained. And the method of multiple scales is employed to obtain the asymptotic solution. It is seen that in addition to the primary resonance at the frequency ratio of 1, secondary resonances occur at frequency ratios of 1/3 and 3. Finally, the implicit and explicit formulas of the RAF, depending on the excitation frequency, amplitude and the developed equivalent damping, are given respectively, so that we could have a deeper insight into the nonlinear behaviors of the ROV response.