Warning messages

ROHR2 Dynamic calculations, questions and discussions
ROHR2stoss - Structural Analysis with Dynamic Loads using Direct Integration
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jorivaldo
Beiträge: 3
Registriert: Mi 4. Nov 2015, 18:01
Wohnort: Brazil

Warning messages

Beitrag von jorivaldo »

Dear sirs,
We are analyzing a vibration problem of an alternative compressor that applies mechanical loads to its piping system. We used dynamic point loads to apply mechanical loads to the nozzle of the the pressure pulsation dampener, based on measurements we take from field (dynamic displacements transformed in dynamic forces).
We obtained the following warning messages:
Save data ...:
At node 7, a load of the type Fluid hammer load is inserted multiple times for load case AnaDin!
Load case Dead load: abs. calculation temp. (20.0 °C) < abs. operation temp. (21.0 °C) in line 6 from node 53 to node 57
Load case Operation: abs. calculation temp. (20.0 °C) < abs. operation temp. (175.0 °C) in line 6 from node 53 to node 57
Load case Analise Modal: abs. calculation temp. (20.0 °C) < abs. operation temp. (175.0 °C) in line 6 from node 53 to node 57
Load case AnaDin: abs. calculation temp. (20.0 °C) < abs. operation temp. (175.0 °C) in line 6 from node 53 to node 57
The line topology was completed automatically.
Checking boundary conditions ...:
AnaDin:
Recomended integration time step from rigidity at nonlinear support
Op Pt Comp C_exist ThDt_Recm
AR 29 X 298225.2 0.203
AR 29 Y 298225.2 0.392
AR 29 Z 298225.2 0.780
There are 3 supports with critical rigidity. See Pulsacao.out
No convergence during non-linear iteration
7 Warnings total
Analise Modal:
Internal pressure analysis:
Dead load:
Operation:
Upward load at support 57 (15.185 kN)!
Support loads:
Stress analysis:
Rp1.0 missing; Rp0.2 is taken
May you help me with references to evaluate the consequences of the bolted warning messages. Is it necessary to correct anything on the model input?
Regards,
Jorivaldo
Jorivaldo Medeiros
Master Consultant
PETROBRAS
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rohr2support
Beiträge: 455
Registriert: Mi 14. Sep 2011, 08:23

Re: Warning messages

Beitrag von rohr2support »

Hi

When you do a non-linear transient analysis, there may be a problem of convergence when you have high support stiffness.
(High support stiffness will result in high frequency effects which may be too high, compared to the integration time steps).
If you model has converged correctly you can ignore these warnings.
If your model diverges and iteration stops with an error message, than this warning gives some hints on where to search for a parameter that you can change to improve convergence:
- Option 1: reduce stiffness of the critical support (are they real or are they just stiff because you haven't specified them)
- Option 2: reduce the time step (in this case you output file size will increase unless you change the subsampling at the same time).

Best regards

ROHR2 Support
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