memory effects

[janerikhy]

What

Fluid-memory effects are a part of seakeeping theory, as opposed to maneuvering theory where there are no wave excitations. The fluid-memory effect is a dissipative force. The details can be found in Fossen 2011.

In order to include frequency-dependent added mass and damping, we must include this fluid memory effect. Fossen has implemented a system identification toolbox which approximates the retardation functions used to encompass this effect in a state space form. The state space matrices can typically be found as VesselABC configuration files.

How

1. Use the VesselABC configuration files to extract the state space models.
2. Check the dimensions of the state space models. Are they the same for each DOF and can easily be integrated for each timestep, or do we need to implement them in some other clever way?
3. Verify the Impulse Response Functions by free decay tests. Are the responses realistic?
4. Further verify the fidelity when including wave excitation. Gradually increase the number of wave components.

[janerikhy]

In the Cummins formulation, the hydrodynamic parameters should be selected for infinite frequency. When looking at the vesselABC added mass, and the vessel.json added mass, they differ somewhat. For CSAD, the vesselABC added mass is a bit larger than from the general configuration file.

• A first implementation of the memory effect in a free decay test shows a large difference when changing the damping and the added mass.