An improved quasi-static model for mooring-induced damping estimation using in the truncation design of mooring system

The horizontal motions of floating oil and gas structures by a mooring system can be modeled as a resonant oscillator whose natural frequencies may be much lower than wave frequencies. Thus, the low frequency (LF) excitation caused by the irregular wave results in resonant motion responses in the horizontal plane. For instance, mooring lines may provide 80% of the total LF surge motion damping of a moored tanker in water depth of 200 m, thus reducing the floater resonant excursion significantly. Therefore, when designing truncated mooring system for model testing, mooring damping equivalence should be considered. Thus, simple but effective method for predicting mooring line damping is required for designing truncated mooring system. Different with other quasi-static methods for estimating mooing line damping, besides normal drag forces, tangential drag forces and seabed friction are also considered in the improved quasi-static model presented in this paper. The analytical results based on the improved method are compared to other quasi-static and time domain results for a chain mooring line and a wire line. In addition, model tests of a 1:70 scaled catenary mooring line are conducted. Then, the improved quasi-static results, the AQWA results and the experimental results are compared.