Abstract
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.
Highlights
Roll motion is the most dangerous motion amongst the six degrees of freedom because it can lead to capsizing [1,2], causes crew discomfort, and reduce the vessel’s efficiency [3]
Roll motion is especially critical for offshore floating vessels such as Floating Production Storage and Offloading (FPSO) units because they are expected to operate at a location for extended duration and as such cannot avoid severe weather conditions
This paper investigates the equivalent linear damping coefficients obtained using four methods and two mathematical models for roll damping from a roll decay test of a model scale FPSO
Summary
Roll motion is the most dangerous motion amongst the six degrees of freedom because it can lead to capsizing [1,2], causes crew discomfort, and reduce the vessel’s efficiency [3]. Oscillations in slack cargo tanks can effect the roll motion characteristics of the vessel, it is important to consider the free surface effect on roll damping [5]. Linear damping does not consider the viscosity of the liquid and it is described using linear radiation/diffraction theory [6]. The vessel’s roll damping can be described as linear for vessels with small roll amplitudes [8] but linear models are insufficient for vessels with large roll amplitudes, nonlinearities need to be accounted for when the roll amplitude is large enough to capsize the vessel [9]. Nonlinearities are further important when considering the effect of bilge keels [10] or liquid cargo motion [5]
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