Reduction in the dynamic amplitudes of moored cable systems

Cable-body systems are intelligent and cost-effective structures used in ocean engineering and also for many oceanographic missions. The demand for such systems is increasing steadily because of new types of systems like wind farms, ocean energy converters and floating airports. Mooring of such platforms to a fixed point on the earth makes the system secure. Dynamic behaviour of a moored system with a subsurface buoy is studied here. The optimum position of the buoy is found using an analytical approach. The location of the buoy along the cable is a critical aspect in tension reduction. An analytical method is developed for the solution of a typical mooring problem taking into account the presence of a subsurface buoy. The method originally proposed by Walten and Polachek (1959) is modified here to suit the present problem. The nodal accelerations and velocities at the points of lumped mass of subsequent time steps can be expressed as per the Houbolt scheme. Integration of the accelerations and velocities lead to the solutions of the mooring-line dynamic problems subject to constraints to achieve the numerical convergence of solution. Dynamic amplitudes vary with the frequency of excitation at the surface due to external disturbances like waves and current. Wave, current and water depths are considered for the dynamic load built up along the mooring line. A number of cases are worked out, and some important and significant results are presented in this paper. Dynamic tension variations with and without a subsurface buoy are determined. Effect of pre-tension is determined and presented. Dynamic amplitude for different frequency ratios and different amplitudes of motions are determined. The method is useful for deepwater systems in which tension reduction provides better efficiency for the performance of the anchor by providing better seakeeping ability in the case of a ship or a platform.