Structural behaviour of fully coupled spar–mooring system under extreme wave loading

Floating spar platform has been proven to be an economical and efficient type of offshore oil and gas exploration
structure in deep and ultra-deep seas. Associated nonlinearities, coupled action, damping effect and extreme sea
environments may modify its structural responses. In this study, fully coupled spar–mooring system is modelled integrating
mooring lines with the cylindrical spar hull. Rigid beam element simulates large cylindrical spar hull and catenary
mooring lines are configured by hybrid beam elements. Nonlinear finite element analysis is performed under extreme
wave loading at severe deep sea. Morison’s equation has been used to calculate the wave forces. Spar responses and
mooring line tensions have been evaluated. Though the maximum mooring line tensions are larger at severe sea-state,
it becomes regular after one hour of wave loading. The response time histories in surge, heave, pitch and the maximum
mooring tension gradually decreases even after attaining steady state. It is because of damping due to heavier and longer
mooring lines in coupled spar–mooring system under deep water conditions. The relatively lesser values of response
time histories in surge, heave, pitch and the maximum mooring tension under extreme wave loading shows the suitability
of a spar platform for deep water harsh and uncertain environmental conditions.
Keywords: floating structures, spar platform, fully coupled, nonlinear dynamic response, mooring tension, extreme
wave loading.