Restoring Force upon Steady state in WEC-Sim

Wind & Water Computer-Aided Engineering Software Tools
1

Hi all,
I am running a simulation of an offshore WEC with its mooring line, analyzed using MoorDyn. I am trying to implement a delayed wave ramp function that applies waves onto the structure after a certain time has elapsed, i.e. 120 seconds. I expect that there is some free decay in the beginning, and that it would stay at its natural position (given that there is no wave excitation), yet there are small oscillations that arise. It seems these contributions are from the restoring/mooring force. This does not damp out over time as well. Is there a way to reach full steady state of the object, where no motions are occurring, and then apply the wave?

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Thanks for the help!

Best,
Elaine

2

It looks like there might be an issue with the hydrodynamic coefficients generated by your BEM solver. In the free decay test, the motion does not decay to zero, which suggests a potential problem with the hydrodynamic stiffness or radiation damping coefficients.

I recommend checking the following:

  1. Mesh Quality: Ensure your mesh has low skewness and is of good quality. A highly skewed mesh can lead to inaccurate results.
  2. Hydrodynamic Coefficient Convergence: Use BEMIO to check whether your added mass and radiation damping plots converge to constant values. If they do not, try increasing the maximum frequency input in your BEM solver until convergence is achieved.
  3. Free Decay Test: Run a separate free decay test using: wave(1) = waves('noWaveCIC'); before testing your model in wavy environment.
3

Hi Mohamed,
Thank you for the insightful comments. I ran a free decay test with ‘noWave’ and here are my results. It seems that it does in fact reach steady state at a neutral position as shown in the heave direction. This means the oscillating behavior after I delay the wave ramping is strange. Do you think this still has to do with the hydrodynamic coefficients? Thanks!

Screenshot 2025-03-19 at 7.44.08 PM
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Best,
Elaine

4

Hi,
Just following up on this to see if you had any insight! I’m happy to send over my simulink model if that would be easier to visualize.

Thanks,
Elaine

5

I assume you didn’t add an initial displacement? If no initial displacement was applied, the platform shouldn’t exhibit a -0.12 m motion. This suggests a potential issue with the system setup.

Additionally, your stiffness values appear to be quite small or negative in all directions except the heave motion. It’s well known that BEM tends to produce low stiffness values for yaw motion, which could explain why your floater continues to yaw indefinitely, even in the absence of an applied torque.

If you are confident that your BEM code is functioning correctly, consider the following:

  1. Adjust the Center of Gravity (C.G.) – Ensure that the C.G. is set correctly to match the actual weight distribution. This should prevent any unintended motion in the heave direction.
  2. Modify the Constraints – Try replacing the 6DOF constraint in your Simulink model with a 3DOF constraint that allows only x, y, and z motions. This will help eliminate excessive yaw motion.
  3. Verify the BEM Code – If the problem persists even with a 3DOF constraint, there may be an issue with the BEM results that needs to be investigated further.

Let me know if you need further clarification or assistance in debugging the issue!

6

Hi Mohamed,
That all makes sense, I will go ahead and investigate your suggestions, thanks!

Elaine