Inquiry on the assessment of hydrodynamic loads in RAFT within WEIS framework

[irenegiuliani]

Dear All,
I have an inquiry regarding the evaluation and management of the platform's structural loads obtained using RAFT within the WEIS framework. I am currently working on optimizing the VolturnUS-S platform, starting from the floating version of the IEA 15MW RWT (GitHub version 1.1.13). For this optimization, I am using RAFT to model the entire FOWT system. Specifically, I am trying to do something very similar to what is shown in Example 17 of the WEIS GitHub repository, where the optimization of the floater on the IEA 22 MW is performed, but also accounting for buckling and stress in the floating platform. Furthermore I am optimizing platform only.

I would greatly appreciate any help in understanding the following points:

-If I understand correctly, RAFT calculates the system's responses. How are these responses then treated as loads acting on the system by WEIS?

-How are the hydrodynamic and hydrostatic loads (in both strip theory and potential flow models) managed when evaluating the structural stresses on the platform? I am unable to actually understand how each individual component of the floating platform is discretized, how the hydrodynamic forces are distributed across the structure, and in which way the hydrodynamic loads and discretization are linked to the structural analysis in both Morison equation and BEM model for hydrodynamics.

-In the case of using a potential flow model for hydrodynamics, how are the loads and the platform geometry integrated into the structural evaluation of the floater itself? How are the forces distributed across the floating platform?

I realize that I have asked several questions, I would be truly grateful for any help or answers.
Thank you so much in advance for your time and attention.
Best regards, Irene

[dzalkind]

Hi Irene,

The loads on the floating platform are not yet integrated with the floater geometry to calculate stress and buckling, though the libraries/methods do exist on the WISDEM side. I'm not sure yet if this is a planned development for WEIS phase II, but we would be happy to collaborate with you to develop it.

@lucas-carmo might be able to discuss how to extract the loading from RAFT that you are looking for.

I hope this helps.

Best, Dan

[lucas-carmo]

Hi Irene,

As of now, RAFT solves for the 6-dof rigid-body motions of the floating platform. We are working on a multibody and structural formulation to allow for the calculation of internal loads, but I am still not sure when that will be ready. The idea is that RAFT will be able to provide the internal loads, and I think that other parts of WEIS would take care of converting those loads to stress.

You could get member-level hydrodynamic loads from internal parts of the RAFT code. For example, you could get the loads due to the inertial part of Morison's equation acting on a single member from this part of the code https://github.com/WISDEM/RAFT/blob/0bcc3ae4ed786183ebf9c6aea81de5bccced889d/raft/raft_fowt.py#L1121 .

When using a BEM model, the potential-flow hydrodynamic loads are lumped at the platform reference point, so you wouldn't be able to get distributed loads. For that, you would need multiple potential flow bodies to at least get the loads at the connections between bodies. A more refined approach would be to use the pressure at each panel (or patch) of your BEM mesh, but that is not something that we plan to include in RAFT for now (maybe someday, you never know).

Best,
Lucas

[irenegiuliani]

Dear All, (@lucas-carmo @dzalkind)

thank you both for the very kind and complete answers!

When I run the analysis, the outputs show some values in the 'floatingse.constr_platform_global_buckling' 'floatingse.constr_platform_shell_buckling' and 'floatingse.constr_platform_stress' output variables.  Considering what you have explained to me, I suppose that these values are placeholders or default values handled by FloatingSE/WISDEM structural post-processing  on its own, without any physical meaning. 
I also noticed that these values change when the floater's geometry is modified while running the analysis under the same metocean conditions, but remain unchanged when the metocean conditions are varied while keeping the floater's geometry fixed.

Anyway, your answers have been really valuable to me and to my work!

Thank you so much again for your time and for answering me, despite your very busy schedule!

Best,
Irene