Study of tension in mooring cables under parametric excitation for submerged floating tunnels

The coastal highway E39 project in Norway is considering the construction a Submerged Floating Tunnel (SFT). Some of the preliminary designs use a mooring system to maintain the structure in the prescribed position. The main environmental load in such a system will be long period swell waves. This low frequency excitation produces a parametrically excited system. Certain situations might lead to parametric resonance or dynamic instability that causes excessive vibrations of the cable and the SFT. However, it is unclear how parametric excitation affects the cable tension of a mooring system. Correct design should avoid excessive forces that might break the line or very small forces that lead to momentarily slack cables. The aim of this paper is to assess the validity of using the stability charts for the design of mooring lines with respect to safe levels of cable forces. This is achieved through the numerical analysis of a cable model using Abaqus. Cable vibrations and cable forces are analysed and compared for a range of amplitudes and frequencies of the support motions. Also the dry and submerged cases are compared, clearly exposing the repercussion of the hydrodynamic effects.