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Output from Substructure file
Hi Developers,
I am a PhD student now, and I am interested in the structural behavior of the floater in OC4 semisub model. For example, the internal force of different submembers. According to your users guide (Fig 1), it seems possible to retrieve the force or moment of any submember. However, after defining new sensor locations (see Fig.2) and running simulations, I am unable to find any corresponding results in the final output file.
Could you please provide further instructions or clarify if there are additional steps needed to successfully retrieve this information? Thanks in advance.
Best regards,
Zhexin Lyu
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Hi Zhexin ,
you need to specify the output sensor as follows:
SUB_x_y where x is the member ID and y is the relative length along the member between 0 and 1.
After setting a sensor you find hydrodynamic outputs in the “hydrodynamic time graph” and structural outputs in the “structural time graph”. You can use the search function of the graphs to find the appropriate sensor.
The OC4 model you mention however doesnt give you any internal loads for the members, since all members are defined as “rigid”. To get internal loads you would need to setup the substructure from flexible members and also use distributed hydrodynamic forces by means of the Morison equation.
The OC4 model is rigid and uses potential flow (lumped) hydrodynamics.
Check out this part of the docs for a summary on different modeling options:
BR,
David
Thanks for your help!
Dear David,
Thanks for your suggestions last week, I successfully extract the internal loads for the members (I defined the members to be flexible, and I thought then the Morison equation would be applied right? Because the all related parameters have been set in the default of OC4 semi model)
Then I tried to validate my result according to the output from the part in QBlade Documentation – “Validation and Examples of Hydroelasticity”, Figure 166. I want to compare the local forces on the main column bottom in my model with the values provided in the documentation. However, I’ve noticed that my results differ significantly from those given in the documentation. I’m unsure as to why this discrepancy exists and how to address it.
My result and the turbine simulation setting are shown in the attachment,could you kindly provide further guidance on this issue? I am looking forward to your reply.
Best regards,
Zhexin Lyu
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Hi Zhexin Lyu,
it is hard to tell what the reasons for these differences are. Could be different material properties, different Morison coefficients, different coordinate systems etc. The part of the documentation that you mention is rather a demonstration of the “hydroelastic” feature in QBlade and not a proper validation – for this purpose the setup and test is not documented thoroughly enough.
BR,
David
Dear David,
Thank you for your response. My primary concern at the moment is to confirm whether I have correctly applied the Morison Equation.
Consider this scenario – I downloaded the OC4 semi model and kept all the default parameters, but only changed the rigid substructure members into flexible ones. If this is the case, does it imply that the Morison Equation would be applied, without the utilization of linear potential flow theory? How can I know if I used the linear potential flow theory or the Morison equation. I’m primarily aiming to verify the accuracy of the internal force I obtained through the application of the Morison equation.
Best regards,
Zhexin
Hi Zhexin,
if you want to only use Morison model based hydrodynamic forces you need to deactivate the potential flow hydro forces by setting the variables USE_RADIATION, USE_EXCITATION etc. to 0 (or false). You should remove the lumped mass, added mass and damping matrices (SUB_MASS, SUB_ADDEDMASS, etc.). In the SUBMEMBERS table you can then assign Morison coefficients to each member and also activate the distributed buoyancy calculation. The Morison (Inertia, Added Mass and Drag) forces can then be extracted for each member in the Hydrodynamic Time Graph and the internal member forces in the Structural Time Graph.
BR,
David
Dear David,
Could you explain more what is the reason to remove the lumped mass? After this removal, I’ve noticed that my model seems to be almost “flying”, which I suspect might be due to the absence of SUB_MASS. Consequently, the results have not been favorable.
Best regards,
Zhexin
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Hi Zhexin,
if you create your model with flexible elements the main reason for doing this would be to have access to distributed forces and moments at the floater substructure.
However, to be consistent with this approach all properties along the floater should be “distributed”. If you choose to use flexible elements, it would be logical to use the distributed approach for all properties along the floater. This includes assigning the mass as a property of each flexible element rather than lumping it into a single point. By doing so, you can accurately represent the flexible element’s properties, including its mass.
The reason why your model is “flying” away is that you probably have not assigned the correct mass (kg/m) to each element definition in the SUBELEMENTS table, therefore the floater is too “light” and the buoyancy force sends it flying.
BR,
David
Hi David,
Thanks for your replying. I believe I’ve already applied the correct mass (kg/m) to each element, as shown in the figures below. This is why the unexpected “flying” behavior of the model is causing me confusion.
Are there any other possible reasons that you could suggest that might be contributing to this issue?
Best regards,
Zhexin
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