Casing/Wellhead Max Stress

[cnuk]

I have a device that is attached to the wellhead. It transmits a large bending moment, axial load, and some shear to the casing and wellhead. I would like to know what the max allowable load/stress for a given casing / wellhead flange combination is? I have looked over some of the API specs and haven't found anything relevant. I have found some casing/tubing rating for axial load and internal pressure but nothing that fits my case. I was hoping for a max allowable von mises stress rating but that would be too convenient

Any help on where to start would be greatly appreciated. I don't have the API specs in front of me to tell you which ones I've reviewed.

Thank You to all who reply.

 

[roadapple]

cnuk,

You have to follow the design portion of API 6A -look on page 35 and 37.

API 6A only allows von Mises for sizing basic sections at test pressure - I don't understand the logic either. However, if you calculate around test pressure you should should be OK if you want to go that route. Remember Se=Sy for von Mises at test pressure.

However, your situation sounds like classic Stress Intensity. Look at section 4.3.3.2 - determine whether you are using standard (75 ksi yield or less) or non-standard materials (above 75 ksi or greater). Look on 4.3.3.6 if you are using non-standard material. These are your allowable stresses.

Calculate your direct stresses -and then your principal stresses- and determine you stress intensity (P1-P3) - the largest principal stress minus the smallest principal stress.

Remember for bending - this may either be a primary or secondary membrane stress and will affect what allowable stress can be applied. The classification is up to you - if the bending is a reaction due to pressure loading - then (typically but not always) it will be secondary and will therefore have a higher allowable value.

Also if you have combined loading with shear and bending:

Stess Intensity=(Bending Stress^2+ 4*Shear Stress^2)^.5

for von Mises

von Mises=(Bending Stress^2+ 3*Shear Stress^2)^.5

roadapple

 

[cnuk]

Thanks roadapple.

I started looking into API 6A but it quickly (section 4.3.3.2) refers to the ASME boiler pressure vessel code (ie: stress intensity). I don't have a copy of ASME so am still trying to figure out what exactly "stress intensity" is. It sounds like it is max principal - min principal stress but I need to do a lot more reading before I'd claim I fully understand it and why it is used as a design criteria. I also need to understand why there is such a distinction between primary and secondary stresses. My problem is that I already have a very detailed FEA analysis on my structure and am trying to tie the FEA results back to the codes. This is proving much harder than I originally thought. Again, it would be too easy to have a safety factor as commonly used in mechanical design.

More info on my structure. It is actually a large HSS structure that mounts on top of the wellhead, but contains no pressure..only supports other equipment. Basically a mini-mast structure. Some have told me to use the AISC or CSA code for steel buildings whereas others have suggested API 6A because it is mounted on a wellhead. In the end, either way I am getting a rough introduction to dealing with codes. I have designed equipment for several years but there has rarely been a relevant code that applied and if it has it was much easier to understand than AISC/API/ASME.

Thanks Again.

 

[roadapple]

cnuk,

API 6A in section 1.2 Applicability - your structure does not fall under this jurisdiction based on what I understand about your product.

Most of the oil field structure would be expected to be welded to AWS D1.1 - but the design code - that is a question. I doubt that AISC or CSA would cover this, but I don't know anything about those codes.

Your analysis of FEA to the FEA code is possible - through a process called linearization. I have done it - but it is a real bear because it requires modifying the mesh and pulling the actual element values of each element.

I don't think you will be working to this result based on the fact that I doubt seriously that anyone will say that structure is controlled by API 6A.

If you want to learn the inns and outs of API 6A - try on-line resources in Houston. I suffered many headaches until I took a API 6A class from them. The course would be worth years of experience. I would bet that there are others available but I haven't found them. I am not associated with this group in any way.

roadapple

 

[jte]

cnuk-

You're right as far as defining stress intensity. It is defined as the algebraically highest principal stress minus the algebraically lowest principal stress. Also known as twice maximum shear stress or Tresca stress. It's just another reasonable material model for ductile materials. Similar to von Mises.

The distinction between primary and secondary stresses is important for linear analyses. If you take a nonlinear analysis approach you will not need to worry so much about the distinction.

For what its worth, the new ASME VIII Div. 2 has abandoned Stress Intensity (see Appendix 4 in 2006 and earlier editions) and now uses von Mises for the analysis criteria (see 5.2.2.1b). It also has switched from an Allowable Stress Design approach to a Load and Resistance Factor Design approach.

I'd suggest you get a copy of the new (2007 ed) VIII Div. 2 and take a close look at Part 5. Alternatively, if you have a copy of the 2007 API 579 / ASME FFS-1 the Appendix B approach is nearly identical to the new Div. 2 Part 5, just a different perspective (FFS vs new design).

jt

 

[cnuk]

jte,

Thanks for the response. I'm always impressed my the amount of knowledge shared on these forums.

Greatly Appreciated.