we are currently trying to run a coupled model of a RNA mounted on a TLP using FAST, HydroDyn and MoorDyn.
At this point we try to run a model without Potential Flow input Data from WAMIT so we are focussing now
on strip theory. (Later a more sofisticated model will include Potential Flow output from AQWA which will be
formated into WAMIT format)
I got some uncertainties regarding the input files for HydroDyn on the data necessary for using Strip Theory in HydroDyn.
- There is a section called “Axial Coefficients”. Hereby AxCd, AxCa and AxCp can be defined for the calculation of the lumped forces at the member ends.
Furthermore there are 3 sections for for “Hydrodynamic Coefficients” Model1/2/3 Hereby the distributed forces over each member will be calculated.
When we calculate the distributed loads on the members by i.e. Model 3 (Member-Based), what do we need the lumped forces at the member ends (joints)
in the “Axial Coefficients” section for?
In 6.3.2 in the manual it says “For example, if you want axial effects at a marine-growth boundary, you must explicitly set a joint at that
location”. But still I don´t exactly understand what the lumped loads are for.
- In the HydroDyn manual in the “Hydrodynamic Coefficients” sections it says:
“All of these models require the specification of both transverse and axial hydrodynamic
coefficients for viscous drag, added mass, and dynamic pressure (axial viscous drag is not yet
So I guess the lateral (e.g. MemberCa1) and axial (e.g. MemberAxCa1) motions shall be defined here. In the
FAST model of the OC3 Floater however the axial coefficients are set to 0. Are they ignored here? Why?
When I read about Morison equation, I only learned about lateral Drag- and Added-Mass-Coefficients. Are there publications
on axial Drag- and AM-Coefficients available?
For me this sounds contractictory to what is said in the next paragraph “… and
Ax identifies the axial coefficients to be applied for tapered members (the transverse coefficients
are identified without Ax)…” So the “Axial” coefficients apply only for tapered members?
I am not completely sure what C_P is. I know the Morison equation can be seperated into a drag and an added mass term.
And the first includes a drag and the latter an added mass coefficient. But I don´t remember a pressure coefficient.
When HydroDyn divides single members by the value assigned by “MDivSize” the distributed Strip Theory loads will be applied with a resolution depending on MDivSize.
Why does it not integrate over the member length? Is this a question of computation time?
The AddCLin Matrix for the OC4 Floater looks like this (in accordance to the HydroDyn Manual):
I understand that because of symmetry the green marked terms become 0. But as far as I understood the OC4 Floater is only symmetrically in one axis, therefore the orange marked terms shouldn´t be zero. Can you please explain, where is my mistake here?
Furthermore the red marked term shouldn´t be 0 as it is the product of rhogA. Is this value so small and can therefore be neglected? Or do all the neglected and set to 0 terms are not necessary here because there is a WAMIT input file for the OC4 model?
Thank you very much,