in which A(∞) is the addedmassmatrix at infinite frequency; x, ẋ and €x
are the displacement, velocity and acceleration vector in time domain,
respectively; B is the quadratic viscous damping coefficients matrix;
k(τ) is the retardation function matrix, which is based on the added
mass and potential damping matrix; and f(t, x, ẋ) is the summation of
the external force vector in time domain which is a function of the displacement
and velocity.
The equation of motion of the STC model has 12 D.O.F.s considering
the two rigid-bodies, the spar and the torus. The wave excitation forces
in the simulation include the Froude–Kryloff forces and diffraction
forces, which are calculated using the 1st order potential flow theory
by the panel method, while the drag force is simulated by a series of
slender elements with specified Cd values according to Morison's formula.
The wind thrust force is simulated by the drag force on the disk,
and it is calculated based on the measured wind velocity and the disk
area. The wind drag force on the tower is modeled by a point force applied
at the middle point of the tower above the still water plane. The
mechanical couplings between the spar and the torus are modeled as
linear spring-damper systems. Due to the mechanical coupling, a
small time step is used in the time domain calculation. Themooring system
is modeled by linear springs. The time domain model is modeled
and solved in the code Simulation of Marine Operation (SIMO)
(MARINTEK, 2007), which was developed by MARINTEK. The motion
equation for the STC model can be expanded and rewritten based on
Eq. (2) as: