Experimental Determination of Non-Linear Roll Damping of an FPSO Pure Roll Coupled with Liquid Sloshing in Two-Row Tanks

Wave excited roll motion poses danger for moored offshore vessels such as Floating Production Storage and Offloading (FPSO) because they cannot divert to avoid bad weather. Furthermore, slack cargo tanks are almost always present in FPSOs by design. These pose an increased risk of roll instability d...

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Bibliographic Details
Published in:Journal of marine science and engineering 2020-08, Vol.8 (8), p.582
Main Authors: Igbadumhe, Jane-Frances, Sallam, Omar, Fürth, Mirjam, Feng, Rihui
Format: Article
Language:English
Subjects:
Cargo
Cargo handling
Cargo ships
Coefficients
Computer applications
Damping
Decay
Energy
Energy methods
FPSO
Free surfaces
liquid cargo motion
Liquid sloshing
Methods
Movement
nonlinear roll motion
Offshore
Perturbation method
Perturbation methods
roll damping
roll decay
Rolling motion
Storage
Tanks
Viscosity
Weather
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Summary:Wave excited roll motion poses danger for moored offshore vessels such as Floating Production Storage and Offloading (FPSO) because they cannot divert to avoid bad weather. Furthermore, slack cargo tanks are almost always present in FPSOs by design. These pose an increased risk of roll instability due to the presence of free surfaces. The most common method of determining roll damping is roll decay tests, yet very few test have been performed with liquid cargo, and most liquid cargo experiments use tanks that span the entire width of the vessel; which is seldom the case for full scale FPSO vessels during normal operations. This paper presents a series of roll decay test carried out on a FPSO model with two two-row-prismatic tanks with different filling levels. To directly investigate the coupling between the liquid sloshing and the vessel motion, without modifying the damping, tests were performed at a constant draft. The equivalent linear roll damping coefficients consisting of linear, quadratic and cubic damping terms are analyzed for each loading condition using four established methods, the Quasi-linear method, Froude Energy method, Averaging method and the Perturbation method. The results show that the cubic damping term is paramount for FPSOs and at low filling levels, were the FPSO is more damped. Recommendations regarding the applicability of the methods, their accuracy and computational effort is given and the effect of the liquid motion on the vessel motion is discussed.
ISSN:2077-1312
2077-1312
DOI:10.3390/jmse8080582