The roll damping for a vessel comes from potential (wave radiation) damping, hull skin friction, eddy making and bilge keel (if applicable). The radiation damping is taken care of automatically in diffraction analysis while the remaining damping contribution are more of a viscous damping type, which is nonlinear and frequency dependent.
In MOSES there are two commands -TANAKA, TANAKA FACTOR and #TANAKA, WETSUF, OPTIONS. In -TANAKA command, only a factor needs to be defined while the #TANAKA command, we need to have it defined with the load group. The #TANAKA command allows more parameters to be defined such as bilge radius. In any cases, the viscous damping is nonlinear and I think MOSES will have it linearized in some ways when solving equation of motions and finding tee RAO
I just wonder what are the differences between these two commands and what kind of viscous damping is included in each command. From the input parameters, it seems that the -TANAKA, TANAKA FACTOR will not include the effect of bilge into the formulation while the #TANAKA, WETSUF, OPTIONS will. If the effect of viscous damping included in each command is separate, I assume that we can use both commands together otherwise we can only use once, not both,
Many thanks in advance for your feedback.
Hi Suraphong,
It is difficult for me to write a response to a post that does not have a question mark. I do not know what question I am answering. But I will attempt to write in general terms.
The commands that begin with a "#" imply that the input has data not to be calculated by MOSES. For example for the wind area of a deck. If a model is used MOSES can calculate the projected wind area based on the beam elements and plates. The client can also tell MOSES not to make that calculation and use the area with #AREA.
The same scenario is happening with "&describe piece pname -tanaka" and "#tanaka". The values used for Tanaka style damping when using -tanaka are computed. MOSES uses the extreme geometry values and determines the values needed to look up a Tanaka value. If the client does not want those calculation to take place the #tanaka command is available.
I cannot comment on when to use both, one or the other, that is a decision for the project.
I hope this helps.
Georgina Maldonado
Dear Georgina,
Many thanks for your prompt reply. Now it gives me a clearer picture. When the -Tanaka command is used, the roll damping parameters will be internally calculated form the model/panel mesh by MOSES while the # Tanaka, the input to the parametric equations for roll damping will be provided by the user.
So, it the vessel is equipped with bilge keel and if we would like to include the impact of bilge keel, on roll damping, the bilge keel must be modelled explicitly as plate element in MOSES.
My question is:
Is it possible to include the contribution from Bilge keel on Roll damping without explicitly modelling the bilge keel as structural plate element? I just notice that the #TANAKA command, on the -ROLL as an option, there are a few parameters that can be user-defined including the bilge radius. I just would like to confirm that if we correctly define this parameters, the impact of roll damping contribution from bilge keel is account for If not, the I think the only option is to include the bilge keel in plate in the model then.
I know MOSES is a smart program but to be able to determine the parameters from the topology of the panel model really exceeds my expectation!
Please see the b_keels files in the following directory.
hdesk\runs\samples\how_to
I do not know how to interpret your last comment. Again, you did not ask a question in your first post.
MOSES uses the formulation as outlined in the paper by Schmidke.