Motion responses of a catenary–taut–tendon hybrid moored single module of a semisubmersible-type VLFS over uneven seabed

Abstract

Hydrodynamic performances of very large floating structure (VLFS) proposed for floating runway are crucial for its serviceability and survivability. This paper adopts time-domain approach to investigate the motion responses of a catenary–taut–tendon hybrid moored single module (SMOD) of a semisubmersible-type VLFS in the coastal region. Hydrodynamic coefficients of the SMOD were obtained by utilising conventional panel method. Time-domain simulation was performed and validated against corresponding scaled model tests. Mooring tensions were simulated using lumped mass method. In consideration of the coastal space limitation, a catenary–taut–tendon hybrid mooring system was utilied to constrain the SMOD motion. The catenary mooring lines with considerable anchor radius were deployed on the deep water side of SMOD and taut mooring lines on the other side. Tendon system was also applied in order to meet the strict heave motion requirement of the floating runway. Numerical results agree with the experimental data well. Dependency of the motion responses on hybrid mooring system and the seabed topography was also discussed in this paper. Response amplitude operators over sloped seafloor were compared with those over the level bottom. The comparison results indicate that the mooring system and the seabed topography both have significant influence on the SMOD motion responses in the coastal region. The influences of tendon and taut mooring stiffness on the SMOD motions and mooring safety were further specifically studied by sensitivity analyses. The results reveal that the SMOD with larger tendon and taut stiffness have smaller sway and roll motion responses in severe environmental condition. However, risk of breaking the mooring lines is increased as the axial stiffness increases.