How many members VS discretization

Hi, I wanted to ask a clarification.
In the OpenFAST readthedocs manual, in modeling considerations for subdyn Link and hydrodyn (strip theory) Link, the following is suggested:

Due to the exponential decay of hydrodynamic loads with depth, a higher resolution near the water free surface is required to capture hydrodynamic loading as waves oscillate about SWL. It is recommended, for instance, that the HydroDyn discretization not exceed element lengths of 0.5 m in the region of the free surface (5 to 10 m above and below SWL), 1.0 m between 25 and 50 m depth, and 2.0 m in deeper waters. When HydroDyn is coupled to SubDyn through FAST for the analysis of fixed-bottom systems, it is recommended that the length ratio between elements of HydroDyn and SubDyn not exceed 10 to 1.

My question is:

  • does this mean that I need to have a subdyn member for every 20m between seabed and -50m water level, for every 10m between -50m and -25m water level, etc, if I set NDiv to 1 in subdyn? BUT that I do not have to do so in hydrodyn.dat, i.e. I can have a single member from seabed to transition piece, and specify MDivSize equal to 0.5m?
  • OR i need to have a similar discretization (but with members 1/10th of the subdyn members as suggested)? This would be more cumbersome, and keeps giving the error “The lowest element of a member must not cross the free surface…”

Many thanks

Dear @Maurizio.Collu,

I would say that HydroDyn is a bit more user-friendly than SubDyn, with HydroDyn allowing you to subdivide members into elements via MDivSize specified per member and SubDyn allowing you to subdivide members into elements only via NDiv universally across all members. Given the limitation in SubDyn, it is often simpler to set NDiv = 1 with more joints and members.

The first approach you mention would be sufficient but would require finer resolution of HydroDyn than warranted for the deeper nodes.

Regarding the error you mention in the second approach, you can avoid this by specifying a single member in the vicinity of the free surface in HydroDyn with MDivSize = 0.5 m.

To follow this modeling guidance exactly for a spar with 110-m draft:

In HydroDyn you can create a model with NJoints = 4 and NMembers = 3 with:
Jointzi = 10, -25, -50, -110 m
MDivSize = 0.5, 1.0, 2.0 m

In SubDyn you can create a model with NJoints = 14 and NMembers = 13 with:
JointZss = 10, 5, 0, -5, -10, -15, -20, -25, -30, -40, -50, -70, -90 m, -110 m
NDiv = 1

Best regards,

Dear @Jason.Jonkman ,
perfect, crystal clear as always!

Many thanks

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