Resonant tangential response in laterally-excited fluid-loaded cable suspensions

Cables are used in a variety of ocean engineering applications and are often selected for their inherent flexibility. As compliant elements, cables possess vastly different stiffness properties in the tangential and lateral directions. For submerged cables, this stiffness anisotropy is augmented by drag anisotropy, heavy lateral drag versus light tangential drag. Thus, submerged cables are biased towards tangential response.; A continuum model is presented which captures tangential resonant response in submerged cable suspensions. This model incorporates the tangential inertia of the suspension (often neglected in traditional cable models) which is shown to be crucial in accurately predicting the tangential dynamic response. Two tangential resonance mechanisms are presented. Both mechanisms channel vibration energy from a directly excited but heavily damped normal dominant mode to a lightly damped tangential dominant mode. The first mechanism (1-to-1 resonance mechanism) involves coupling between a normal dominant mode and a tangential dominant mode having nearly equal natural frequencies. The second mechanism (1-to-3 resonance mechanism) involves coupling between a normal dominant mode and a tangential dominant mode possessing natural frequencies in a 1-to-3 ratio. The qualitative characteristics and existence criteria for each mechanism are established using numerical simulation of the continuum model.; To provide further understanding of each mechanism, a low-order discrete model is presented which captures each of these resonance mechanisms with sufficient simplicity to allow analytical analysis. This discrete model is shown to accurately predict both 1-to-1 and 1-to-3 tangential resonance responses (validated through comparisons with numerical results obtained from the continuum model). Approximate analytical solutions for each resonance mechanism are presented, based on traditional perturbation analysis for the discrete model. Both mechanisms are found to generate periodic cable response which is unique and stable. The asymptotic solutions are further employed to predict the influence of controlling cable/fluid parameters on each resonance mechanism.