Cylinder under external pressure analysis

[rattler]

Hi,

I'm doing some check analysis on a cylindrical housing under external pressure (subsea); the radius / wall thickness ratio is approximately 8.

I have the capability to do static FE analysis which will predict stresses however I'm not convinced that if I compare these with material yield strength or UTS it will give me relevant a safety factor against "failure". I believe the failure mode is likely to be buckling.

I don't have the software to undertake buckling analysis however I've found a formula for critical buckling pressure for a thin tube under external pressure (roark Table 35 case 19); this only applies for thin cylinders with r/t > 10.

Can anyone advise please if the following approach is correct?

1. Calculate the critical pressure using a lower value of thickness such that r/t = 11
2. Assuming that gives me a good margin against my actual pressure, conclude that my cylinder has a greater margin because it is actually thicker.

Or is the reality more complicated?

Thanks.

 

[prex]

Your procedure may be used, but it may be unnecessarily conservative, also because Roark's formula, if I recall correctly, is for a cylinder of infinite length. Also consider that the factor of safety against buckling can be as high as 3.
Here, from the first site below, a sheet for calculating the allowable external pressure according to ASME VIII Div.1.

prex
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[Ussuri]

I might suggest getting a copy of API RP 2A "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms—Working Stress Design".

It gives an approach for assessing buckling under hydrostatic pressure, and also how to combine the effects of hydrostatic pressure in conjunction with member loads due to other applied forces (eg tension, compression, bending).

In the event that you do have a collapse issue it also gives guidance on the design of ring stiffeners.

 

[rattler]

Hi Prex,

Thanks very much for your reply.

Cheers

 

[rattler]

Hi Ussuri,

Thanks also for your reply; I've just got that spec and their treatment of the calc looks comprehensive.

Cheers

 

[Denial]

An analysis method is given in the book "Buckling of steel shells — European design
recommendations" published by the European Convention for Constructional
Steelwork (ECCS), fifth edition, published 2008.[ ] I think this would be applicable to your problem, but you would need to check on that.

The method is quite elaborate, and tedious to carry out.[ ] I have written a spreadsheet that attempts to implement the method, and this can be downloaded from my website (rmniall.com).[ ] Do not attempt to use the spreadsheet without first having read the appropriate parts of the book.[ ] The method also allows for regularly placed ring stiffeners.