The changes of dynamic response for a spar platform due to mooring lines failure are evaluated. The wave and low frequency responses are displayed in motion and tension spectrums. The development of the hydrodynamic behaviour subjected to the heaviest load on the mooring lines is quantitatively assessed by comparing response spectrums and statistical results. It is shown that the changes vary greatly for different structure motions and line tensions under various environment loads or line failure situations, though the dynamic responses tend to increase significantly.
As exploitation activities for offshore energy move into deeper seas, various innovative floating offshore structures have been proposed and developed to comply with these severe environments. Among these, a spar platform is an attractive design solution due to its excellent hydrodynamic performance. A typical spar platform system consists of a cylindrical structure, a station keeping system consisting of several spread mooring lines. The spar hull generally has helical strakes to prevent VIV (vortex induced vibration) in current (Rho et al., 2002). Owing to its deep draft and small water-plane area, its natural frequencies are quite different from the peak frequency of an incident wave. The failure of mooring lines and riser is liable to occur due to overload, fatigue and corrosion in severe ocean environments. After one single line failure, the optimum configuration of the station keeping system is disturbed and the remaining lines are overloaded in sequence. This is quite like the so-called "knock-on" effect resulting in more lines failing. Multiple line failures always lead to a complete loss of station keeping. The natural periods, typically 160s for surge, 60s for pitch and 30s for heave (Converse and Bridges, 1996), of classical spar platforms are longer than wave frequencies. However, the second-order wave effects can excite large-amplitude slowly varying resonant motions and the corresponding riser and mooring line loading.
