Logged: Nov. 2014, AutoPIPE V8i 09.06.01.10

Problem:

I would like to request advice on a modeling approach. We want to use FRP piping for a large (DN900) buried irrigation scheme. The FRP system is prefabricated pipes lengths that are joined with sliding couplings. These couplings offer no axial restraint for the pipe therefore at the bends there is a pressure thrust force that must be resisted to stop the pipes sliding from the couplings. This is not uncommon and usually would be solved by installing a concrete thrust block, however, this application is remote and the quantity of bends are high. We would like to use a straight length of pipe either side of the bend and utilize friction and bearing from the pipe/soil interface to overcome the thrust force instead.

I could calculate the straight length of pipe required to provide enough friction and bearing to ensure the pip e remains in place within the coupling and is not over stressed. However, I would like to use AutoPIPE to carry out this calculation as it: - Can easily solve the complex non-linear interaction of the pipe and soil to resolve bearing forces - Has ISO 146 92 for calculation the FRP stress values

We are currently struggling with how to approach the modeling for this situation. Primarily how to define the boundary conditions of the model since the piping is sliding in the couplings and therefore there is no fixed point. The couplings can tolerate a certain amount of axial pipe movement before the pipe slides out and fluid is released. Therefore we would like to be able to iterate my model by increasing pipe length until the movement below the maximum amount of axial movement required at the coupling is detemined.

Q1. Can AutoPIPE be used to model this scenario or does the lack on a fixed point on the model make it impossible? -

Q2. If it is possible, how should the conditions at the coupling ends of the piping be modeled?

Q3. Is there any other advise that would help with the accuracy of my model?

Solution:

AutoPIPE does not support this type of optimization where a maximum displacement is a boundary condition for determining the length of a run. The lengths of the pipe are the initial conditions to which the displacement and stresses are calculated. This would have to be a trial-and-error on the user end. The user would have to create a series of lengths and review the resultant displacement at the piping end node points where displacement is desired to be = 0.00. 

In addition, one of the components of resistance in the axial direction can be approximated from virtual anchor length calculations i.e. length at which there will be minimal pipe strains. However, as the calculations are required near a bend point, so the lateral bearing forces will also provide reaction to the pipe. It is advise the user to go through following AutoPIPE online information:

Please see the following AutoPIPE help section: 

Help > Contents> Contents Tab> Modeling Approaches> Example Systems> PIPE-SOIL Interaction: Transition Example (particularly the "Determination of Zone Locations" document)

and

Help > Contents> Contents Tab> Reference Information> PipeSOIL> (particularly the "Model Discretization" document)

Knowing the virtual anchor length may help the model converge quickly.

Another thing, to better model soil by keeping the maximum soil spacing small in the length Lb (calculated from AutoPIPE virtual anchor length dialog) near the bend. This will increase accuracy of the calculated forces and displacements. Apart from these two things, again to suggestion modeling a couple of length of pipe from the sliding coupling in each direction from the bend and through trial and error determine> the pipe length needed for zero node point movement..

See Also

Model Soil Properties with Soil Calculator

Bentley AutoPIPE