Convergence problem during analysis

[Kat_kat]

Hello, I am trying to run a Riks analysis on a straight pipeline with an initial small imperfection. I define the nodes, apply pressure and weight loads and then I start the Riks analysis. I have tried many many times but the model doesn't seem to converge. Not even when I apply the pressure or weight loads. I get the note: "***NOTE: THE SOLUTION APPEARS TO BE DIVERGING. CONVERGENCE IS JUDGED UNLIKELY" and it stops after a few attempts. I also tried a *Dynamic step but that didn't work either. I can't understand what I am doing wrong. I have tried with a very small step but the error persists.

The BC are:
left/right node: 1,2,3,4,6
all pipe nodes: 2,4,6

There are also PSI34 elements along the pipe's length to model soil interaction.

And the Riks step is:

*STEP, NAME=BUCKLING, NLGEOM=YES, INC=30
*STATIC,RIKS
1e-5, 1., 1e-15, 1., ,
*Temperature
ALLNODES, 300
*OUTPUT, FIELD, FREQUENCY=1
*NODE OUTPUT
U,RF
*END STEP

Thank you in advance!

 

[FEA way]

Are imperfections taken from previous linear buckling analysis ? Have you tried adjusting their scale factors ?

 

[oldNail]

What is causing the divergence? Take a look at

https://msgfile.info

for help debugging.

 

[g.alshamsi]

How did you define the imperfection? Like the previous user stated, you might have chosen a large scaling factor (from my experience, I always used a small % of the shell thickness to define imperfections). Check the status and msg files for more clues on what might be causing the convergence issues.

 

[Kat_kat]

The imperfection isn't from a buckling analysis. I calculated the node coordinates based on a given f(x) and defined each node in the .inp file. I also tried adjusting the amplitude of the imperfection but with no result.

Thank you for the link to read msg files, I didn't know something like that existed.
From what I understand from the msg file, in each iteration it checks for convergence and although in some increments it converges, I get a warning:
"***WARNING: THERE IS ZERO MOMENT EVERYWHERE IN THE MODEL BASED ON THE DEFAULT CRITERION. PLEASE CHECK THE VALUE OF THE AVERAGE MOMENT DURING THE CURRENT ITERATION TO VERIFY THAT THE MOMENT IS SMALL ENOUGH TO BE TREATED AS ZERO. IF NOT, PLEASE USE THE SOLUTION CONTROLS TO RESET THE CRITERION FOR ZERO MOMENT."

 

[Kat_kat]

Because I am a beginner, I am not sure how to interpret some of the results in the .msg file so I'm uploading it here in case someone wants to take a look.
Thank you.