Nonlinear Dynamic Coupled Behavior of Integrated Spar- Mooring System in Deep Water Region

Offshore oil and gas industry is blooming up towards the deep-water region of ocean deposit. Far off the continental shelf, the traditional jacket type fixed platform and bottom supported compliant platform are not suited to be economic offshore structure in deep water oil and gas exploration. Floating Spar platform is essentially most economic and efficient in these sea states. In the present study coupled analysis of integrated Spar platform has been carried over. A fully coupled integrated model is developed which includes the contribution of mooring lines and risers with Spar hull in a consistent manner. This model is capable in matching the forces and displacements at fairlead position along with all structural and environmental nonlinearities. Nonlinear dynamic responses have been evaluated for long span of wave hitting under unidirectional regular wave of varying heights. An automatic Newmark-β time integration technique has been adopted to conduct the analysis in time domain. Wave kinematics follows Airy’s wave theory and the Morison’s equation is utilized to cope with the forces. The damping effect of mooring lines in coupled model is significant and increases with the depth of sea bed. Displacement, rotation, velocities at surge, heave and pitch direction along with mooring line tension has been evaluated for 6 m and 7 m wave height in 1018 m water depth. The responses indicating spar movements follow similar pattern. But the greater peak values occur earlier for larger wave height case. Mooring line tensions are showing sensitive result showing significant value for higher wave period. Sudden increase of mooring tension at some time stations indicates the possibility of instability and failure of the spar-mooring system. Hence, associated with the coupling effect the response behaviours alter at wave of larger height with respective probabilities of occurrences. The design consideration should include the optimum and worse conditions to avoid severity.