Steel Pipe Maximum Bending Moment 2

[Chembloke99]

I am sorry for asking such a fundamental question. I have forgotten Strength of Material Basics that I studied in 1st Yr. I just want to know what is the formula for maximum bending moment for a steel pipe?
Many Thanks.

 

[Ussuri]

simply supported? encastre? continuous? curved? UDL? point? partial UDL? varying UDL?

 

[Chembloke99]

Thanks for the reply. I am not too sure exactly about the conditions. It is a 6” subsea pipeline between Well Head Platform and Production Platform with risers installed at both ends. I would like to know bending moment for subsea pipeline as well as the riser. I don’t know in which category this would come. Is it possible to determine or some more information is necessary ?

 

[Ussuri]

More than one. What you are trying to do cannot be done using a single formula such as wl^2/8

Your pipeline and riser will have loading from self weight, environmental from wave action, pressure, temperature, displacements from free spans or jacket movement. You may need to consider secondary effects such as vortex shedding, VIV, pipeline walking.

Ideally your first staring point should be the basis of design.

 

[Chembloke99]

Thanks Ussuri once again..Well I can understand lot of things are needed to be considered for accurate calculation. But I have read in a technical document, that for initial calculation Max Bending Moment can be estimated as 0.85 times SMYS. So my question is how can i get Bending Moment if I know SMYS because SMYS is a stress with units in MPa and Bending Moment units is in Kilo Newton Metre.

 

[Ussuri]

A bending moment is measure of how much something is flexing. The applied bending moment on your structure is a function of the applied loading.

Your pipe will have a limiting moment capacity but that doesn't help you if you dont know the moment it needs to resist. For example, if you calculate your pipe has an allowable moment capacity of 50kNm by limiting the maximum bending stress to 0.85 x yield stress that doesnt really help unless you know it needs to resist 400kNm.

Without that how can you tell whether it is strong enough?

 

[Chembloke99]

Ya..I understand your point..but actually I am doing wall thickness calculation based on DNV-OS-F-101 ..In the spreadsheet, for load interaction, I need to put the value of ME (Maximum Bending Moment due to Environmental load)..I know I can get the exact value once I do Dynamic Pipeline Analysis..But initially I must assume certain value to put in the spreadsheet..as based on that spreadhsheet will give the result whether the assumed thickness is ok or not..

From yielding stress, how can I get limiting moment capacity (even though it may not be too useful)

In the refernce document that I am mentioning, they have arrived at the value of Max Bending bending of arnd 50kNm for 6" pipeline (Grade X-52;SMYS=359Mpa) by taking it 0.85 X Yield Stress.

 

[patswfc]

moment capacity of pipe = yield stress x elastic section modulus of pipe.

elastic section modulus of pipe = I/y

I = pi(d4 - (d - 2t)4) / 64 , the two 4's are to the power of 4

y = d/2

 

[Ussuri]

If I have done my sums correctly that would equate to a 168.3 OD x 8.7mm WT pipe (almost). Based on linear elastic bending theory as indicated by Patswfc above.

 

[Chembloke99]

Thanks Patswfc and Ussuri for your help. I have implemented the method that you have advised !!!

 

[Ussuri]

I hope you checked first!