Dynamic coupling analysis for the design of hybrid mooring lines of truss spar in deep sea

Abstract

Truss spar platform is one of the most optimal solutions to the oil and gas exploration. Owing to the severe environment, the design of mooring lines is vital for the platform. As traditional steel chain is very heavy, synthetic rope is attractive for its lightness. Considering the dynamically coupled interaction between the hull of truss spar and mooring lines, the dynamic coupling analysis is performed to get the motion and line tension responses. The dynamic stiffness of synthetic rope is modelled as upper-lower critical stiffness model. The upper critical stiffness is used to calculate the motions, while the lower critical stiffness is for the line tension. Results show that the hybrid mooring lines could secure the platform sufficiently. Higher pre-tension could improve the stiffness of the system, make full use of the mooring lines and limit the motions. From the responses, we can conclude that the energy dissipation is located in the low frequency and middle frequency. The above observations made based on a truss spar may have important implications for other floating structures moored by a synthetic rope mooring lines.