Dynamic Responses of Truss Spar Due to Wave Actions

Abstract: Spar platforms have been used for drilling, production and storage of oil and gas in the offshore
deepwater region. The structure is installed at the deepwater locations in the sea and is exposed to continuous action
of wind, wave, current and other environmental forces. Wave force constitutes about 70% of the total environmental
force and could be considered as the most significant force affecting the dynamic responses needed for the design of
these structures. In this study, the dynamic responses of the truss spar due to wave actions including the wave force
theories and wave propagation directions are investigated. Numerical simulations are developed to investigate the
accuracy of the wave force theories i.e., Morison equation and Diffraction theory, for large structure such as truss
spar. The investigation is further expanded to study responses of the truss spar due to variations directions of the
wave propagated. The truss spar is modelled as a rigid body with three degrees of freedom restrained by mooring
lines. In the simulation, the mass, damping and stiffness matrices are evaluated at every time step. The equations of
motion are formulated for the platform dynamic equilibrium and solved by using Newmark Beta method. To
compute the wave force for truss spar, which is large compared to the wave length, Diffraction theory was found to
be more appropriate. The Morison equation was found applicable only at the high frequency range. Short crested
waves resulted in smaller responses in all the motions than that for long crested waves. Hence, it would be
appropriate to consider the short crested wave statistics for the optimum design.
Keywords: Diffraction theory, long crested waves, Morison equation, short crested waves