How to model a pipe in pipe ?

[Anonymous]

Hi,

I am analysing a Davit Crane used on offshore rigs. 

I have a piep (pipe2) going inside another pipe (pipe1). Pipe2 is free to rotate 360 degrees. Pipe1 is stationary.

How to model this pipe in pipe configuration ?!

Thanks

 

[Rafacascudo]

If you release RX rotation on Pipe 2, structure becomes unstable.

[Anonymous]

Yes, it will.

But i can't model pipe2 till the bottom point of pipe1, can I?.
It is saying overlapped beams and they are excluded from the calculations.

I just need to check the deflection for the stationary condition. There will be some clearance between pipe1 and pipe2, when the pipe2 is inside pipe1.

But by not modelling it, i am loosing the weight of that lower portion of the pipe2.

Any workaround?!

[Rafacascudo]

You will loose the weight and the increased Inertia if they are connected horizontally(Ux,Uy).

Maybe you can create a custom  section( tools/ section definition) with the 2 tubes. Never done , don´t know if it´s possible.

Another option  is creating a generic section Ax,Iy,Iz...( last tab on " new section) with the properties you want for the section.But you lose the graphic representation

- Another one is create a tube section in tolls /section definition send it to user database and edit its properties on tools / section database.

if your problem is really only the weight you can create a material with an increased "Force density"

[teixeiranh]

Why not using a linear (local OX) force to model that weight?

[StefanoPasquini6790]

Hi everyone, one more option is to model the structural double pipe with shell element, with all the overlapping that you need, and connect with rigid links (made for your specific needed) all the node in the overlapping area.

Good luck

[Rafal.Gaweda]

My 5 cents to discussion:

in case of applying load as shown picture you  can ignore RZ rotation of top part 

simplified model : top part as it is; bottom part assumtion: no clearence between pipes ->  custon circular section with inner diameter = inner diameter of P2 and outer diameter = outer diameter of P1

model as described above by Stefano - beacause of rigid links the assumption will be the same as above : no clearence between pipes

model as described above by Stefano - but WITH clearence between pipes -> rigid links should be replaced by "rigid bars" with gap releases

 In none of these approaches you will loose weight.

[Anonymous]

Below is the wroking schematics intended. 

I can not model a single circular section for the bottom part, as there is a gap between the pipes. 

[HoshangMustafa]

Hi

You can use two free body diagrams:

analysing one free body diagram for the forces for the inner pipe

inserting the forces (got from the analysis of inner pipe) to the free body diagram of the outer pipe

[Anonymous]

Hi Mustafa,

Can I do that in Robot !?

[Pawel.Pulak · Accepted solution]

It is possible to use overlapped bars in Robot and to model such crane using them but it is necessary to model it in the appropriate way.

Your drawing suggests that the internal (small) pipe at the bottom is connected in the continuous way with the big top pipe.

Moreover he internal (small) pipe at the bottom is placed inside the bottom big pipe, which is fixed at the bottom.

The inner and outer pipes make contact only at their ends (clearance between the pipes).

It is not clear where the inner pipe is supported vertically - I have assumed that at the bottom.

See this video:

Attached the model corresponding to it.

The main modeling steps visible on the video:

1. defining section labels
2. defining the inner bottom pipe and the top pipe
3. defining the outer bottom pipe - composed of 2 parts to avoid identifying it with the already existing inner bottom pipe
4. defining releases and assigning them to appropriate bars to avoid moment transfer between the outer and the inner pipe and to obtain the desired transfer of forces and moments
5. defining some loads
6. test run of analysis, observing results
7. setting for the inner bottom pipe to be neglected in generation - to avoid connecting it with the outer bottom pipe in the internal node
8. running analysis again

As you can see step 7. resulted in stopping to display the verification error related to overlapped bars - Robot "guessed" that it is an intended action.

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