Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering ConferenceShanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

EXPERIMENTAL STUDY ON THE EFFECTS OF WAVE AND CURRENTINTERACTION ON VERTICAL MOTIONS OF A CYLINDRICAL FPSO
Shi Deng1, Yuanlang Cai*1, Xiaolong Yang1, Zhengrong Song1, Guang Yang1,1.

Offshore Oil Engineering Co. Ltd. Tianjin, China*, Corresponding Author

ABSTRACTmooring system of the CFPSO, and the difference frequency
wave loads, since the natural frequency of the surge or sway are
well within the content in the spectra of the slowing vary wave
loads (Afriana, 2011; Vidic-Perunovic et al., 2017; Wang et al.,2016). They studied in-plane motions (such as surge and sway)of the structure in different sea state and loading conditions.

Cylindrical floating production storage offloading (CFPSO) is analternative for oil filed development drawn many attentions
recently due to its low construction and transportation cost,especially for the mid-water-depth. However, for its circularwater plane, the hydrodynamic features of a CFPSO are quite
different from the conventional offshore ship-type floating
structures, especially in the coupled wave and current loadcondition. A scaling model test has been conducted with regular,bichromatic waves and current to study this phenomenon. It isobserved that the out-plane motions (including heave androll/pitch) amplitude in the coupled load condition is a little
larger than those observed from the pure wave cases. A quasilock-in phenomenon and nonnegligible mean displacement inheave is observed in the coupled load case, and vortex-induced
force is supposed as the reason for the phenomenon. Moreover,large frequency components, far away from the eigenfrequency
cannot be neglected in the coupled load case (bichromatic)
compared with that in the pure wave case. Findings in this paper
provide a basic reference for the initial strength and fatigue
design of the CFPSO considering coupled wave and current
effects.

For out-plane motions (heave, roll and pitch), former researchers
mainly focused on the 1st order wave loads and damping features.
Feasibility of the CFPSO in different wave conditions anddimensions were experimentally studied, they found that it issuitable in most conditions with acceptable motion amplitude(Siow, 2019). Compared with the numerical simulation result,researchers proposed empirical method to improve the accuracyof the widely used boundary element method (BEM) by roughly
considering viscous effect (Afriana, 2011). Viscos damping
effect on the wave motions due to bilge structure at the bottomof CFPSO is another important point. Ji et al., Jiang et al. andAmin et al. studied the effect of dimensions of the bilge box onthe damping (Amin et al., 2022; Ji et al., 2019; Jiang et al., 2020)
numerically. They found the vortex shedding at the edge of thebilge box is the main source of the damping, thin and long bilge
will induce a leger damping. Shao et al further proposed astochastic linearization method to consider this viscous damping
in the potential theory based frequency domain simulation (Shaoet al., 2016).

KEY WORDS: Cylindrical FPSO, Model test, coupledwave and current, quasi-lock-in, mean displacement, large
frequency component
INTRODUCTIONFor the high-cost efficiency requirement of the oil fieldexploitation, the cylindrical floating production storage andoffloading (CFPSO) unit had drawn more attentions in recentyears for its low construction cost. Similar as the conventional
ship-type FPSO, this new concept is normally moored in theopen sea, as shown in Fig. 1. It sometimes works in a strong
coupled load condition (combined wave and current), especiallyin South China sea. Previous scholars normally focused on theFig. 1 Typical Cylindrical FPSO working in the coupled loadcondition.

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