## 1 Introduction
After neglecting the stiffness of bending moment and torque in the calculation of mooring load, [PERSON] and [PERSON] [14] developed a finite element model for a mooring line suspended in water by conjecturing Initial Boundary Value Problem (IBVP). While the boundary-value problems are resolved through the Direct Search method [15], the mooring load on the floater can be obtained accompanied with coupling relationship of 6-DOF motion. In order to calculate hydrodynamic forces of the floater, the three-dimensional potential flow method [16; 17; 18], including steady and unsteady terms, was applied. For solving steady-state problems, the translating sources are distributed over the floater by the panel method [19] and the complex plane contour integral algorithm [20]. Moreover, the pulsating sources are suggested by adopting the panel method and the series expansion method [21]. Translating sources are used to obtain the wave profiles in the steady-state, whereas pulsating sources are adopted to resolve 6-DOF wave exciting forces in the diffraction problem. Accordingly, the hydrodynamic coefficients, including added masses and damping coefficients, are calculated by simplifying dynamic boundary conditions in the radiation problem [22]. On the other hand, the wave-body-seabed interaction problems have been formulated and solved in variable bathymetry regions with different water depths by a suitable extension of the BEM to 3D [2]. In a recent study [3], it was also demonstrated that the variations of bottom boundaries have significant influences on hydrodynamic coefficients and responses of the floating body with all degrees of freedom. Since it is challenging to simulate motion responses of marine structures, this study upgraded a modular system [18] to simplify programming and calculation. Modularization can be thought of as the predecessor of object-oriented technology, which shortens the processing time and makes the program more flexible. Another advantage is that the program can be divided into multiple sub-systems, which have compatible interfaces for integration. Modularization also has the characteristics of standardization and severability. Objects that can be combined and disassembledwith the task are compatible with each other for the sake of simplified programs. Hence, the complete program can be considered a collection of modules. At present, the well-known modular system is ETAP Wind Turbine Modeling (WTM) software [23], which is used to design and monitor wind farms in a flexible graphic interface optimized for wind power simulation and analysis. [PERSON], [PERSON] [24] indicated that the Swim-Motion-Lines (SML) software suite is applicable to the maritime and offshore industries for calculating motion responses of floating platforms in winds, waves, or currents. SML is composed of three modules: the SWIM module is used to calculate hydrodynamic forces in the frequency domain; the MOTION module is adopted to simulate a slow-drift response in the time domain; and the LINES module is employed to estimate the influence of the mooring system on the floating platform. In order to resolve the hydrodynamic coefficients of floaters with complex shapes in waves, the computer program WAMIT (Wave Analysis at MIT), based on the panel method, has been applied [25]. In the following, Section 2 mainly addresses the particulars of four floating platforms and the corresponding mooring systems; Section 3 introduces the running procedure and functions of the modular system; Section 4 presents the background theory of the mathematical model; Section 5 discusses the analyzed results of hydrodynamic coefficients for four floating OWTs; Section 6 exhibits the dynamic responses of floating OWTs compared with other codes or experimental data. Finally, concluding remarks are given in Section 7.