Effect of slowly varying drift forces on the motion characteristics of truss spar platforms

Spar platform has been regarded as a competitive floating structure for deep and ultra deep water oil
and gas production. In this paper, an efficient methodology has been developed to determine the slow
motion responses of slender floating offshore structures due to wave forces. Based on this methodology,
a MATLAB program named ‘TRSPAR’ was developed to predict the dynamic responses in time domain
and it was used in this study to obtain the numerical results of a typical truss spar platform connected
to sea bed using nine taut mooring lines. The difference frequency forces were calculated using the
principles of the extension of Morison equation for an inclined cylinder and the wave kinematics were
predicted using hyperbolic extrapolation. Mooring lines were modelled as nonlinear springs and their
stiffness was obtained by conducting the static offset simulation. Because of the lack of detailed
calculations in literature, most of the equations used were derived and presented in this paper. The
effects of the different sources of the second order difference frequency forces were compared for
inertia and drag forces in terms of response spectra. To validate the TRSPAR code, its results were
compared to results of a typical truss spar model test.