Anti-motion structures are commonly used to improve motion performance of cylindrical FPSO. In present work, the self-developed naoe-FOAM-SJTU solver, which was validated by comparing numerical and experimental results, was used. The effects of the free-flooding anti-motion structure on heave natural period and damping performance of the FPSO were studied by two methods of free decay and forced motion. The results show that the free-flooding structure has little effect on heave natural period, but the damping coefficient can be significantly increased due to the generation of vortices around the structure and in the internal fluid due to the opening.
Cylindrical floating production storage and offloading system (FPSO) has many advantages, but its heave motion performance is poor (Ji, Li, Tang and Tong, 2019). It is often used to install an anti-motion structure in the lower part of the FPSO main cylinder body to improve movement (Ji, Li, Tang and Tong, 2019; Ji, Li, Tang, Zhu and Hu, 2019).
In present work, the effects of the free-flooding anti-motion structure on heave natural period and damping performance of the FPSO were studied by two methods of free decay and forced motion, which are commonly used to determine the hydrodynamic performance (Rao, Seeninaidu and Bhattacharyya, 2014; Igbadumhe, Sallam, Fürth and Feng, 2020).
Under the assumption of linear damping, the equation of free heave decay can be given as
(Equation)
where m is mass of FPSO, ma is heave added mass, b is the damping coefficient, and c is the restoring stiffness coefficient. z(t), ż(t) and (equation) are the displacement, velocity and acceleration in heave direction respectively. in the case of small-amplitude motion, the water plane of the cylindrical FPSO is a circle, and the area of water plane Aw is constant, which is defined in Eq. 2
(Equation)
where D is the diameter of the water plane. So that c can be written as
(Equation)
where
(Equation)
In Eq. 4, ρ and g are respectively the density of water and the acceleration due to gravity.
