Mooring line stiffness matrix

Dear @Jason.Jonkman ,

In the “Definition of the floating system for phase IV of OC3 Hywind” you give linearized mooring line stiffness matrix. You said that is obtained at the linearization point.
I just want to be sure that in absence of wind and waves, linearization point is the same as the point obtained when performing static equilibrium test ???

Also, You said in your PhD thesis that Morison equation (strip theory) is only valid when the floating body is slender. Slenderness means that the ratio diameter over wavelength lower than 0.2
and you put a graph in which you show for which sea state the strip theory is available.

But i was wondering if the term slenderness is only related to the ratio diameter to wave length. (I’m a mechanical engineer and for me slenderness means that the ratio of characteristic length of the cross section to the length of the body is lower than 0.1)
If this was true, i can say that strip theory for a barge or maybe semi submersible is valid for certain sea state ???

Thank you a lot for your help !


Dear @Riad.Elhamoud,

Regarding the mooring system of the OC3-Hywind spar, the linearized matrix was calculated about the undisplaced position of the floater (not quite the static equilibrium location).

I agree that in addition to the diameter to wavelength ratio, the member diameter to length ratio is an important consideration. Strip theory generally applies to structures with small diameter to wavelength ratio and high aspect ratio (large length/diameter). Member end effects become important for low aspect ratio structures. For these structures, I would say that it is still common to apply strip theory, but it is more difficult to estimate what the appropriate hydrodynamic coefficients (Ca, Cp, Cd) should be–both transverse and for the end effects.

Best regards,

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Dear @Jason.Jonkman ,

Thanks a lot for your explanations !
But, i didn’t get well the of the linearization of mooring lines.
I know that there are multiple configurations for modeling mooring lines: Static, Quasi-Static and Dynamic model. The linearized mooring lines stiffness fall in which category ?
Also, I succeeded in running MAP standalone.
I used .map in OpenFast v3.1 in the test folder for OC3 Hywind as an input for MAP and i think i obtained the same linearized mooring line stiffness matrix. What do you think about it ?

Please find attached a screenshot of the mooring line stiffness obtained by MAP at the position (0,0,0).

Thank you for your help

Dear @Riad.Elhamoud,

I would call the linearized representation of the mooring system documented in the OC3-Hywind specifications report a linearized quasi-static model.

I agree that the linearized mooring stiffness matrix you obtained from MAP++ is very close to what is documented in the OC3-Hywind specifications report.

Bets regards,