ANSYS Cargo Hold Strength Analysis - Boundary Conditions

[lalis98]

Hello,

Recently I am trying to implement a FEA according to Common Structural Rules (CSR) for Strength Assesment for a Bulk Carrier, using ANSYS 2024R2.

Currently, the geometry was created in Rhinoceros and was transferred then to ANSYS Spaceclaim. In Spaceclaim, the geometry in consisted of Shell & Beam elements, where the geometry model was succesfully created.

The Geometry then was opened in ANSYS Mechanical to implement the Meshing, the Loadings and the Boundary Conditions (BC) of the model.

1. For Meshing, the command Batch Connections was used to create the connectivity between the Shell & Beam elements of the structure. The mesh succesfully created.
2. For the Loadings, a Mapping was created through Python for the Dynamic Load Cases and Loading Condition I would examine with (X, Y, Z, Pressure) for each selected compartment / surface. These were brought in by the component of ANSYS Workbench External Data, which some CSV files were imported with the (X, Y, Z, Pressure). The Loadings were succesfully applied also.

The issue seems to be with the Boundary Conditions. The CSR prescribe that the Boundary Conditions should be applied as follows:

For the Cross Sections, Beams with the mechanical properties described in [2.5.4] were built, for both Aft & Fore End.

Currently I am struggling to apply the Boundary Conditions. I tried the Remote Displacement (by using Rigid behaviour) for each Independent point, but the analysis gives me warnings that there might be a rigid body displacement. Also the Intersection of centerline and the inner point displacement fixing was created.

My question is, how I apply properly the above Boundary Conditions, though ANSYS' environment?

Thank you.

 

[Ad Hoc]

Welcome to the forum Lalis98

.... but the analysis gives me warnings that there might be a rigid body displacement. Also the Intersection of centerline and the inner point displacement fixing was created.....

This means you have insufficient restraints in your model.

What this basically means is that for most FEMs the displacement is used to describe the support or constraints on the model, and hence the prescribed displacement values are often zero.
So, if the structure has insufficient constraints to prevent rigid body motion, then an infinite number of solutions to the model (equations) exist, each having the same deformation, but a different location in space. Hence it 'looks' like it is moving without being held.

A picture of your model and showing the location and type of restraints would assist.

 

[lalis98]

Hello,

Thank you for your answer.

It seems that the issue is due to the constraints and the mesh. I have created my mesh with the command Batch Connections of ANSYS Mechanical.

By applying the Boundary Conditions, I noticed that some constraints were not created by me, but most probably were occurred due to the mesh (or any other reason which I am not able to identify now). Please, if you see the screenshot below, you can notice that apart from the 2 ends BCs (Fore and Aft), some other appear also in some areas, that should not have Constraint Equations (see in red colour):

In the above screenshot, some small equation constraints may not be seen clearly, but they exist in some areas except the two large areas that have the constraints created.
This probably has to do with Meshing. In my analysis, through Contact Tool under Connections, I found all these issues:

Regarding my Geometry, I have run in ANSYS SpaceClaim a lot of checks, repairs and preparation. In fact, I did I also contacted the Check Geometry command, with no issues. However I cannot be 100% that it is not the Geometry, but the issue seems to be from the Connectivity and the Meshing.

Batch Connections command may be the issue and I am not sure how to contact properly the connection, as the above problems have risen.
Please find below also the BCs I have applied to this model:

Thank you for your reply above, if you have any recommendation or advise, it would be greatly appreciated!

 

[Ad Hoc]

.... I have created my mesh with the command Batch Connections of ANSYS Mechanical....

I think therein lies your problem.
This command is saving you doing proper modelling. As it is allowing a tolerance between surfaces/edges to me 'merged' as it if joined, when in the model exists a gap.
So it is saving you time to remodel the join as no gap.

In doing so,, issuing this batch command, you have lost control of what is being joined or not, unless you specify a tolerance to the regions/edges that you do not want to me merged or joined.
Which is why you are probably getting constraints here:

Where should not be any.

You need to debug the model to understand what is being joined and where...and which are the locations that are to be constrained, and only those locations.

It is not a simple matter of click click and click and hey presto.
You need to understand your model, and use minimal automation as possible, to get to what you want. Do not let the programme do it for you.

 

[baeckmo]

...........and use as minimal automation as possible, to get to what you want. Do not let the programme do it for you.

That sentence should be nailed to the wall behind every computer used for engineering purposes!

 

[lalis98]

Thank you very much all for your comments.

I now see my geometry again in order to fix it. As I mentioned above I have imported the geometry from the Rhinoceros, because of the compatibility and the the fact that I know to use it better than SpaceClaim. When importing my geometry from Rhinoceros to Spaceclaim, I noticed that some edges of surfaces do not much (very little gaps, undetectable from the repairing tools of Spaceclaim). I can understand that many times there issues with cad geometry transferring, but by meshing in ansys the outter surface with this issues, still did not have any trouble. Is this something I need to be worried?

 

[baeckmo]

Switching 3D models between cad-systems can be very tricky. I often do modeling in KeyCreator (ex "CadKey"), then save in STEP-format for import into customer's system using Solidworks or Creo. Models that are perfectly tight in KC come out with "leaks" after translation and vice versa. Just those little edges that don't meet exactly work fine after healing in one system, but the healing is not always following through the journey.

 

[Ad Hoc]

....I can understand that many times there issues with cad geometry transferring, ...

I only ever import the basic global dimensions of the structure. Never the whole model, created in another software.
I away create the model and the details associated with it, inside the software of the FEM, never outside.

This way I can totally control not just the model, but the mesh and how the constraints and application of forces are applied as well.

Does this take time - yes. Does it work - yes!

 

[lalis98]

Hello All,

After addressing the geometry issues, I am now encountering a problem with the Boundary Conditions (BCs).

BCs are applied to nodes on both sides (Aft & Fore) at the Cross Sections. At the End Cross Sections, I need to apply BCs and Vertical Bending Moment loads to specific beams with pre-defined mechanical properties. These should be set as follows:

• Aft End: δy=0, δz=0, θx=0 as rigid links, referenced from an independent point at (x_aft, 0, z_neutral),
• Fore End: δy=0, δz=0, θx=0 as rigid links, referenced from an independent point at (x_fore, 0, z_neutral),
• Intersection Point at the Fore End: δx=0 at (x_fore, 0, h_DB), where h_DB represents the height of the Double Bottom from the base line.

I implemented these constraints using Remote Displacement in ANSYS, setting the appropriate DOFs for each cross section and remote point with rigid behavior. However, I am receiving the error "Not enough constraints to prevent rigid body motion", although stress results are still provided. If I apply fixed constraints at both ends, the error disappears, and stress results are obtained without issue. It is worth mentioning that the stress results in both cases are relatively close.

I understand that without the correct BCs, the structure may form a singular matrix, indicating infinite possible solutions due to insufficient constraints.

It is possible that I may have misunderstood the BCs, so please do not take the above interpretation as definitive, but the below table is given by the rules:

Any guidance on accurately translating these BCs, that are prescribed in CSR, to ANSYS would be greatly appreciated.

Thank you.

 

[Ad Hoc]

....I understand that without the correct BCs, the structure may form a singular matrix, indicating infinite possible solutions due to insufficient constraints.....

If you have 'somewhere' within your model, a node or a series of nodes that have with x, y and z displacements as zero, then the model cannot move. In other words, no rigid body displacement.

Thus, it then comes down to correctly applying the boundary conditions at either end of the model, as conditions of symmetry.