where Fiðω þ μ=2Þ is the mean wave load in direction i for frequency
ω + μ/2; S(ω) and S(ω + μ) are the wave spectral values for the frequencies
ω and ω + μ, respectively; ζa is the incoming wave's amplitude;
and Fiðω þ μ=2Þ=ζ2a
is the mean drift force transfer function. The
second order forces for the STC were calculated by considering the
model as one body, and the forces were applied on the spar in the SUB
mode and on the torus in theMWL mode.