Water Kinematics in MoorDyn

Wind & Water Computer-Aided Engineering Software Tools
1

Dear everyone,
I noticed that Moordyn has updated the Water Kinematics section, as shown in the following picture,

image
image1058×501 74.6 KB

I have a few questions about the update section
1.Is the result obtained by the three methods the same, or is the new method considering something new? In my opinion, is more like directly obtaining wave height and current velocity from SeaState

2.Previously, when I was considering the Water Kinematics of mooring systems, I used the first old method. The old method felt more like obtaining the force induced by velocity as the Morrison equation acting on the mooring iron chain. I wonder if my understanding is correct?

  1. I am interested in the SeaState method. If I were to divide the grid from the water surface to the seabed and include the entire mooring system, does this mean that my mooring system can also be affected by high-order wave forces like floating platform? I’m not sure if the old method can calculate the impact of high-order wave forces on the mooring system

4.If the above statement is not valid, is there any way to consider the impact of high-order wave forces on the mooring system?

Best regards,

2

image
image1063×182 23.7 KB

By the way,I tried following the prompts to use the third method. I used the SEASTATE flag in Moordyn, but as shown in the picture, an error occurred. It seems that a number needs to be entered, but I am not sure what to enter

image
image747×104 5.33 KB

3

Hi @Yushun.Fu,

You’re quick to read the new docs, those were just published yesterday. Those instructions correspond to some ongoing work we are doing that will be released in a future OpenFAST version. For now, refer to the Water Kinematics instructions on the ‘master’ version of the docs (see attached screenshot).

Screenshot 2025-04-24 at 10.30.23 AM
Screenshot 2025-04-24 at 10.30.23 AM1179×829 160 KB

4

To answer your questions directly:

  1. The results can be the same, but it is dependent on if you provide the equivalent inputs to the three methods. This is a feature in development and is not yet released so you won’t be able to use the Hybrid or SeaState method yet.

  2. Your understanding is correct. All three of these methods provide the water velocity, acceleration, and dynamic pressure at a given point in space, which is then used to calculate forces on the moorings via the Morrison equation.

  3. The SeaState method is not yet released. Once it is released, then yes it will allow for simulating higher order forcing on the mooring. The old method cannot calculate this, but generally in practice the higher order forces on the moorings are negligible compared to platform loads on the moorings.

  4. Currently, higher order wave forcing is not supported in MoorDyn. In most cases those effects are negligible

5

Dear @Ryan.Davies ，

Thank you for your reply. Do you remember the example you mentioned where high-order wave forces have little impact on mooring systems? I couldn’t find it online and hope to receive some keyword reminders.

Best regards,

6

Hi @Yushun.Fu,

Here is an example paper that compared MoorDyn to OrcaFlex: https://doi.org/10.1115/IOWTC2021-3565

I would stress that for your specific case you would need to verify yourself that the difference in impacts on your system is minimal.