Subsea Lifting Operations Considering Taut-Slack Cable Conditions and KC-Dependent Hydrodynamic Coefficients

The assessment of the dynamic behavior of vessel-cable-equipment systems can be complex due to the various involved parameters. For this kind of system, the practical point of view is commonly interested in maintaining the taut cable condition, avoiding the slack one. Compared to models with constant hydro-dynamic coefficients, the ones that consider Keulegan-Carpenter number (KC) dependent hydrodynamic coefficients in subsea lifting operations have different dynamical behaviors. However, there is no evidence of how the taut-slack limits change when considering KC-dependent hydrodynamic coefficients. To better understand this issue, the present study assesses three cable-equipment dynamic models with constant and KC-dependent hydrodynamic coefficients: (1) a single-degree-of-freedom, (2) a multi-degrees-of-freedom, and (3) a continuous model. Herein, we demonstrate that for a given model, at low-frequency ratios, considering KC-dependent hydrodynamic coefficients increases the region for slack in the (Λ, τ) domain, where Λ and τ stands for displacement and frequency ratios, respectively. In contrast, for high-frequency ratios, there are no significant differences. Also, we found that the taut-slack regions may differ significantly between the models, for example, surpassing a 20% difference in the frequency ratio. Furthermore, increasing the equipment mass or damping coefficients reduces the region for slack in the (Λ, τ) domain, while the opposite occurs when increasing the added mass coefficients. This study sheds light on the dynamical behavior of subsea lifting operations contrasting models with constant hydrodynamic coefficients and models considering KC-dependent hydrodynamic coefficients, showing that the response of the models may change significantly depending on the assumptions, which impacts the maintenance of taut cable conditions in subsea lifting operations.