Experimental Study of the Free Roll Decay Test for the Evaluation of Roll Damping Coefficients

Abstract

4This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.In general ships and FPSOs, roll damping is very small and consequently roll motion is very large at the roll resonance frequency. Proper evaluation of the roll damping coefficient at the resonance frequency is an important task in the study of roll motion and usually it is done by the analysis of free roll decay tests. The relative decrement method based on energy relation has been used mainly for the evaluation of roll damping coefficient from the roll decay test so far. As another method, the logarithmic decrement method based on equivalent linear decay assumption can be used for the same purpose and it is relatively simple. In this paper, both of the relative decrement method and the logarithmic decrement method are used for the evaluation of roll damping coefficient including quadratic damping from the free roll decay tests, and their results are cross-checked for verifying the obtained damping coefficients. Through applications to a box-type floating body equiped with bilge keels, it is shown that the two methods give almost the same damping coefficients in a practical view point and the cross-check of their results is to be a good tool to prevent a possible error. And also the quantitative effects of the bilge keels on the roll damping of box-type floating body are shown and discussed.

Determination of Linear and Nonlinear Roll Damping Coefficients of a Ship Section Using CFD

“…The coefficients in the figure are non-dimensionalised as the ratios to the critical damping coefficients ( 44 ′ = 44 / ). As can be seen in the figure, a good agreement was achieved between the roll damping coefficients obtained from the current simulations and the experimental results of Kim et al [20]. 0.05 (Vugts, 1978) 0.10 (Vugts, 1978) 0.20 (Vugts, 1978)…”

“…The roll damping coefficients show a fair agreement for low and moderate frequencies but the deviations increase with the frequency. Similar differences were also observed by other studies [18][19][20][21]. These discrepancies may be attributed to experimental inaccuracies at the high frequency of motions, as also questioned by Vugts [21], Bonfiglio [31] and Thilleul et al [32].…”

“…Figure 8 and Table 3 show the specifications of the model. Fig.8 Cross section of the model for validation case 2 Figure 9 compares the roll damping coefficients calculated from the current simulations and the scatter diagrams obtained from the free roll decay test performed by Kim et al [20]. The coefficients in the figure are non-dimensionalised as the ratios to the critical damping coefficients ( 44 ′ = 44 / ).…”

“…An additional validation study was performed comparing the simulation results with the free roll decay test conducted by Kim et al [20]. Figure 8 and Table 3 show the specifications of the model.…”

“…Therefore, nonlinear roll damping coefficients should be taken into account for better predictions. Accordingly, a large number of studies have been devoted to the derivations of the nonlinear roll damping coefficients [2,[14][15][16][17][18][19][20].…”