End cap force / hoop stress on pipes with thick coating

[sylvaninhoo]

Any thoughts on how to calculate the end-cap force and hoop stress on submerged pipelines with thick coating material ?

Many thanks

 

[JoeTank]

If its just pressure loads and it is a cylinder/tube the answer is as follows. I don't see how the thick coating makes any difference. End cap force = Pressure*Pi*Pipe Radius^2. Hoop force = Pressure* Radius. The hoop force is compressive if the pressure is external to the leg.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com

 

[sylvaninhoo]

Thanks - I was rather thinking in terms of ext pressure effects. The pressure field is acting over the coating circumference which is in our case significantly different from the steel diameter:

The end-cap force calculated based on the overall dia. is then the total (steel + coating) force - how is that force distributed across ? Or shall we just say that end cap force in the pipe is based on the steel dia., regardless of the coating thk - in such case we have to assume no interaction between layers

Regarding the hoop stress in the pipe wall, staying conservative we could say that pressure load integrated over the coating dia. is fully taken by the steel which brings to the conclusion that with thick (watertight) coating there is an amplification of the hydrostatic pressure acting on the steel

I am actually missing some references to quantify this ...

 

[billsumner]

Sylvaninhoo is right in saying the coating makes a difference to the endcap force.

With impervious insulation coating, it is important to consider the endcap acting over the whole outside diameter. However, the force is only resisted by the steel (at the smaller diameter).

However, if the coating is concrete, then there may be a case for saying that at the field joints there is no concrete so the end cap can only act over the same cross section as the steel. Or perhaps the concrete should be deemed to crack so water acts only over the smaller diameter.

 

[iuston]

Endcap force is the difference :

PiAi-PeAe
PiAi = Internal pressure acting on the internal transversal area (Ai)
PeAe = External pressure acting on the external transversal area (Ae)

calculated at the 'end' of the pipe, considering that the pipe is capped... because, of course, if you have a pipe with internal pressure, some were it has to be capped
Thus, in my opinion, if there is a thick coating, there is a larger external transversal area in which the hydrostatic force acts, so PeAe has to take into account the coating too. But we cannot assume that the coating resists this force, so the axial stress given by the end cap effect should be calculated only with the steel thickness, not including the coating.

About hoop stress, this is more difficult to judge. The hoop stress formula :

(Pi-Pe)*OD/(2*t)
OD= nominal diameter
t = wall thickness

ONLY APPLY TO THIN WALL pressurized vessels
It is a formula derived assuming that OD/t is so small that you can neglect it, and this is generally true for pipe.
But if the wall is thick, the simplification would lead to error, because the circumferential stress (hoop stress) is not uniformly distributed across the wall.
So, if the coating is watertight, thus the hydrostatic pressure acts on the external surface of the coating, you should first check the OD/t is within the range for thin wall formula. Than you have to work out which is the hoop stress acting on the coating, and which is acting on the steel, considering that they will have the same pressure at the interface.....
anyway this is worthwhile only if Pe is relevant with respect to Pi, otherwise you will conservative by calculating hoop stress = Pi*OD/(2*wt) with wt=steel thickness.

This in my opinion, but if you think it helps, I can go back to my school books and find references