The Joint Branch of RINA and IMarEST (Singapore), Society of Naval Architects and Marine Engineers, Singapore and the Centre for Offshore Research and Engineering will be organizing a Technical Talk on Wednesday, 31 July 2013, 7.00 pm at EA-02-11, the Executives Seminar room, Faculty of Engineering, NUS. The topic for the lecture is “Roll Damping of Floating Elongated Bodies: From Froude’s to PIV and CFD approaches” by Dr. ANTONIO CARLOS FERNANDES, Professor in the Naval Architecture, Marine and Ocean Engineering Program, UFRJ (Federal University of Rio de Janeiro), Brazil.

The roll motion of a floating elongated vessel, like a ship or, more recently, like a FPSO (Floating Production, Storage and Offloading) platform, is an important mode that may be tuned to typical seas all over the world. Particularly, there is news that FPSO hulls are tuned to incoming waves in the Brazilian and Australian coasts reaching in some cases 35 degrees pick to pick roll responses. More than these particular cases, in fact, it is a common sense that floating unit has been observed to roll significantly. The reason is that, unlike the pitch mode, for instance, the radiated waves are very small due to the usual vertical walls of a vessel that should transport/store goods. This is why the bilge keels are so popular. For vessel that should sail, a typical 45 cm bilge keel is a standard device for VLCC (Very Large Crude Carrier) but for stationary units, 2 m wide bilge keel is being considered/installed in FPSOs. In summary, the roll damping may be tuned and no natural wave radiation damping is available from typical hulls what leads to the need of bilge keels or other devices like the dynamic absorbers U-Tubes and Stabilizing Tanks installed. This roll common occurrence is also the reason that this subject has been addressed a long time. It has been studied by William Froude himself at least since 1861 (FROUDE, 1861). On the other hand, for some, after the comprehensive research by Ikeda (HIMENO, 1981), the subject is well established and there is nothing more that could be done about it. In fact, several text books including (FALTISEN, 1990) spread the idea that for all floating elongated bodies the roll damping, being nonlinear, it is a quadratic expansion of the average roll angle. The present work, alternatively, criticizes this quadratic reasoning. Based on global experimental observation of decaying tests, a bilinear approach was proposed. This bilinear idea was a breakthrough idea that proposed that the nonlinearity instead of a quadratic expansion should be addressed as two linear approximations one from the small angles region and another from the large angles region. Later, a smother connection between these two regions has lead to a continuous adjustment using now a hyperbolic tangent matching. The research, still analyzing decaying tests, but using advanced PIV (Particle Image Velocimetry) came up with physical reasons for these bilinear characteristics. In the large angle region a stronger (say counter-rotating) vortex is send away from the hull with bilge keel and arrests a chunk of the (say rotating) vortex. In the small angle region, the stronger vortex pulls the weaker vortex and is pushed back from it forming a 450 street similar to the Von Kármán Street. Finally, any modern topic should be investigated using CFD (computer Fluid Dynamics) approaches. This was done and the same behavior from the PIV has been simulated very successfully, confirming the above systematization, if one may say so. From the more practical side, these studies make clear that the quadratic damping idea is indeed unsafe, since it predicts higher damping for large angles than that happening in reality. The main lesson learned so far is that our field is so dynamic that even classical theories may be revisited. More research has been carried out addressing now the forced oscillation tests but the research for a model to be used in random seas, the real challenge, is also in the agenda.

To register your attendance, please click on the link HERE by Monday 29 July 2013.
For more details, please contact Ms Norela Buang at: Tel No. 65164314; Email: nor@nus.edu.sg