Currently I'm dealing with underground pipeline analysis. The thing is that the pipe is thermally insulated. I model the pipe specifying the insulation thickness by direct input into the dialog box and also assign cladding thickness. Insulation thickness=25mm and cladding thickness=5mm. To check the effect of thermal insulation I also ran for the case without including thermal insulation and cladding thickness. The results show that the stresses decrease significantly when I apply thermal insulation and cladding. Can anyone tell me if direct input of thermal insulation and cladding is correct way of handling and that the results are reliable?

A higher or lower stress (somewhere in your system) gives few clues.
Without such direction I can offer just a first guess.
The insulation thickness will increase the diameter used in setting bearing and friction (lateral and axial, respectively) terms modeling the soil. Perhaps the greater diameter increases the pipe-soil friction and this increase reduces the thermal thrust into some bearing point (e.g., a bend or tee); with reduced thrust, the bearing element (the other end of the bend or the tee branch) will see less bending & less stress.

kvunitsfil; You call the piping underground piping and you talk about thermal insulation and cladding application. Is it a piping act as above ground piping in a pit or buried in the soil?

If buried in the soil you are asking trouble in operation with corrosion, especially corrosion under insulation. You do not even to check, locate and repair ability during operation. Analysis with insulation and cladding as buried piping is an unusual way doing the work, and the analysis will not be as accurate as you expected. You need to go through the results and make judgement. The selection of code is going to be another issue. Are you going to be running the pipe under B31.3 or B31.4?

I would suggest to provide a pit for piping with insulation and analyse it as above ground piping by providing adequate supports.

I know that there are some contractors make the job easy and cheaper for them, but operators will suffer.

Thanks for the comments provided.
My pipeline is Flow Line from Well Pad to Meter Station. The portion under analysis is from anchor block to anchor block. It does not have any branches. The Flow Line is buried in the soil. The Flow Line is under cathodic protection and is subjected to the routine control including smart pigging. High stresses appear at bends as Dave told.

I think that the formula from American Lifelines Allians guidelines for axial soil springs confirms what Dave told since maximum axial soil force per unit length of pipe "Tu" is directly proportional to the pipe outside diameter. The question is if Caesar uses pipe outside diameter with specified insulation and cladding thickness. The results show that it does.

When you use insulation and cladding you need to answer some more questions:

1. I guess you are going to use insulation blankets to insulate the pipe.

2. Are you going to compact the back-filled soil? If you do how the cladding and insulation will behave under the soil compaction? Are you able to control the deformation on the cladding during the soil compaction?

3. Assume that the compaction, insulation and the cladding will be at the given geometry and thicknesses. The pipe under thermal expansion with the insulation and cladding will not have the same stiffness in all directions as given for the selected soil since the soil and the piping will not be in contact. Especially the axial stiffness will be so different. So, I would assume the axial soil restraint may not exist at all with some exaggeration, the restraint will only be at the change of directions with some accuracy. Therefore the pipe at the change of direction will suffer.

I suggest you to use as many nodes at the change of directions and both side of them as possible to be able simulate the soil resistance (insulation will deform but you will not be able to see this effect in the analysis, this may end with higher stresses conservatively. I am not sure but you may need to analyse the straight part of the pipe unrestrained as well as restrained in two different analyses since you cannot make sure the behaviour of the pipe in contact with the insulation surrounded by soil. If you pass in both cases I would say you are probably close with your analyses.

What I said in my previous reply for the corrosion will be valid. I hope you will provide sufficient allowance by the field experience to cover the corrosion during the lifetime of the piping. Otherwise you may cause environmental problems in addition to the unnecessary costs for replacement.

The pipe is factory insulated and as you can see in the attached photographs the insulation is stiff enough to withstand the loads from soil weight. As the pipes are welded the connection region is filled out with field repair kit to keep the same outside diameter throughout the pipeline.
The pipes are also subjected to field bending and insulation geometry stays changeless after that, this, I think, confirms the rigidity of the insulation.
My assumption is that the pipe will move with insulation when expands. This is due to the friction factor which I think on the pipe-insulation contact surface is much higher than on cladding(polyethylene)-soil surface. This of course greatly depends on the soil properties. In our case this is mostly cohesive soil in wet environment.

I wish you provided these pictures and information earlier, there was not any need for long discussions.

I do not know the maximum operating pressure and temperature. The pipe will absolutely expand in axial direction under thermal load. The bonding between pipe and insulation, and between insulation and polyethylene cladding will be effective. If their bonding resistance is greater than polyethylene-soil bonding resistance the pipe with insulation and cladding will thermally expand in the soil. If it is the other way around pipe will move axially in the insulation. I trust your company have already experience on this and I cannot add more. The compaction will not be problem for this type insulation and cladding.

You mention there is field cold bending with the insulation and cladding. I would expect the insulation inside the cladding will rubble under the concentrated forces during bending if the local forces are more than the compressible allowable of the insulation, but will still remain there. Field bending may be problem for small radius turns. I suppose you need to see adequate lifting not to cause crashed insulation in the cladding for the same reason.

You still need to provide the insulation dry for all the insulation (factory or field), otherwise the corrosion will be problem.

I did not have experience this kind of application earlier, I trust, David's comment is valid. I do not have the software to try and see it unfortunately.

We learn every day.

Good luck.

Ibrahim,

Thank you very much for all comments provided. I appreciate this. We all need to learn every day and you, personally, help people to understand this.