Dynamic Analysis of a Symmetrically Moored Semisubmersible Under Various Possible Mooring Combinations

Abstract

In this technologically advancing world, the demand for more energy, oil and gas production is rapidly escalating. To accomplish this, people have inclined more towards completely floating offshore structures, deployed in deep waters. A semi-submersible is selected in the present study, due to its better response characteristics and stability under harsh environmental conditions. The semi-submersible is position restrain with spread mooring lines incorporated with submerged buoy at different locations has been studied. A detailed numerical analysis is carried out using Ansys Aqwa for dynamic response analysis of semi-submersible under the combination of wind, wave, and current forces for 0°, 45°, and 90° directions. It was observed that damping ratios and natural periods had been affected based on the position and number of submerged buoys in the mooring system. Also, reduction in mooring force after incorporating buoy in the mooring lines was observed. Subsequently, a Matlab code based on the S-N curve approach was generated and employed to investigate the fatigue damage of mooring lines under dynamic variation of mooring forces. When pegged with submerged buoys, fatigue life of mooring lines is increased under intact and postulated damaged mooring conditions. Moreover, coupling of motion responses of semi-submersible is observed, and unbounded response is not seen in any degrees-of-freedom, even during damaged condition of mooring lines.

Keywords

Dynamic analysis / Fatigue damage / Offshore Structures / Postulated failure / Submerged buoy / Mooring lines