PIPE UNDER EXTERNAL PRESSURE
PIPE UNDER EXTERNAL PRESSURE
(OP)
Hello, I am looking for a formula (in International Units) for checking a pipe of 14" under external pressure. For internal pressure I use Barlow's formula. Is there any formula for determining the spacing between buckle arrestors?
Thank you very much in advance.
Thank you very much in advance.
RE: PIPE UNDER EXTERNAL PRESSURE
* BS 8010, Pt 3, Annex C
* Timoshenko and Gere, Theory of elastic stability (Book)
* Murphy and Langer, Ultimate pipe strength under bending (technical paper, maybe OTC)
* DNV 1996 and DNV 2000
These are not in any particular order other than as I found them within my reference papers.
For spacing of buckle arrestors, it is a matter of cost and risk optimisation. If the spacing is too short, you pay up front for a lot more buckle arrestors. If the spacing is too long, then if a propagating buckle were to occur, you need to recover and replace a lot of pipe. In simplistic terms, the cost equation can be of the form :
Cost(S) = L*C1/S + K*C2*S
where
L = Pipeline length
C1 = Cost of buckle arrestor
S = Buckle arrestor spacing
K = Probability of propagating buckle
C2 = Cost of recovery and replacement of buckled section
The curve has a U-shape and the optimum point is obviously at its minimum. The optimum spacing has to be derived for each individual situation (C1, K, C2) and is not really dictated by any codes but by economic risks. In my last project, the spacing was selected at 500m.
Hope this is of some use.
RE: PIPE UNDER EXTERNAL PRESSURE
Regarding the buckle arrestors, I suppose they are designed for the laying time and that the external pressure for 900 m is negligible compared with the stresses coming from the laying ?. Am I correct?
I suppose also that the probability of buckling depends in the laying technique S or J and water currents. Is there any reference value for c1, k & c2 for 14 “ pipe at 900 m.?
Thank you again, in advance.
RE: PIPE UNDER EXTERNAL PRESSURE
p=0.73Et3/D3
p=allowable external pressure (with this formula it equals 1/3 of the elastic buckling pressure)
E=elastic modulus
t=pipe thickness
D=pipe OD (use consistent units)
If the thickness is relatively high, you should also check the following
p=1.875tS/Dk
S=allowable stress of pipe material
k=1+0.015D/t
Max.ovalisation should be limited to 1%.
prex
motori@xcalcsREMOVE.com
http://www.xcalcs.com
Online tools for structural design
RE: PIPE UNDER EXTERNAL PRESSURE
If you have trouble finding the references, let me know your fax number or email address and I can send over some pages.
With regard to the costs and risks, I cannot give you specific numbers but point out the following :
The cost of the buckle arrestor (C1) will depend strongly on its design, such as wall thickness, method of manufacture, operating condition (e.g. sour service ?), etc. The only way to get this is to go for quotes from suppliers.
The cost of repair (C2) is directly related to the construction vessels involved. As the number of players able to lay this line will be limited, the costs can become distorted depending on market conditions, vessel availability, etc. You should have some estimate of this from your overall project cost database.
K is purely a guess factor, there are no historical data that you can base your values on. However, unless the pipeline is extremely long, I would suggest that you allow for the possibility of one buckle. Less than that, you are exposed to some risks. More than that, you could end up with a lot of spare pipe left over at the end requiring an embaressing explanation to the bean counters.
Regards
RE: PIPE UNDER EXTERNAL PRESSURE
For my feasibility study it is enough. I have asked for references it will give culture for next occasions.
Thanks again.