Stand Off Clamp Design 1

[Taz99]

Hi All,
I usually work on Aerospace projects but have ended up sizing a new Stand Off Clamp design and wanted to run a few things past the group.

I have been advised by the customer of an pipeline operating pressure that they wish to be able to claim, they have advised that we should apply a 1.5 factor to this pressure and test the clamp at this pressure (and this seems to match what the other existing clamps claim to be tested to). However, at least in Aerospace, sometimes there are additional knock down factors we would apply in the analysis but not when doing the final test, one being an additional 1.15 factor on the bolt loads. For this case my two main allowables are bolt strength and material strength is anyone aware of additional safety factors or knockdowns that need to be applied? I have a copy of ASME B31.3-2004 but I am not convinced if the requirements are applicable to a stand off clamp design.

Next question, I have identified a 190 ksi 1/2 inch diameter bolt with 27000 lb axial ultimate strength to use. The customer would like to use a A194 Grade 2H heavy hex nut, the A194 spec Table 3 gives the proof load of 24830 lb but nowhere states the ultimate strength of the nut which should be higher than the proof strength. My customer tells me that they don't know the ultimate strength. There are calculations to determine the thread strength but a published value removes the chance of calculation error. Is anyone able to point me to published values, my Google searches have not turned up anything so far?

Thanks

 

[BigInch]

I think the problem might be that pipeline clamps are not rocket science. We've been designing clamps for offshore pipelines since 1947. This is a structural engineering problem. The construction contractors will appreciate that you do not redesign the wheel.

ASTM A193 Bolts and 194 Nuts are described in the referenced API specs, but really no need to refer to them as the typical offshore pipeline standoff clamp can be designed using methods of ASIC's "Design of Steel Structures".
You can use the allowables specified therein for both material and bolts. They will not require additional safety factors, since FS are included in the allowables given. Calculate the actual load on the riser from the design wave height and period, with the recommended factor for wave slam.

High strength bolts might be nice, but you just have to oversize them for corrosion. You would also not want high strength, high alloy bolts in contact with low alloy material. Lots of low grade material that can be covered with epoxy and cathodically protected is usually what you want.

Make the design as easy to fit up as you possibly can. If anything needs to be aligned before bolting, be sure that two pins being inserted into two hinges, for example, do not have to be aligned simultaneously. Make one slightly longer than the other, so that one will fit up first and the second is easy to follow, etc. etc. etc.




"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Taz99]

Thanks for the response, from the ASIC site I found "Specification for Structural Joints Using High-Strength Bolts" which mentions knockdown factor of 0.75 on the nominal bolt strength, so I have this answered.
The clamp is intended for above ground operations, so no wave loads, only internal pressure.
The body of the clamp will be made from fiberglass, so galvanic corrosion of the bolts should not be a problem. Is there a specific requirement for over-sizing bolts for corrosion, or just good practice?

 

[BigInch]

You could assign a corrosion allowance on the bolt diameter I suppose, but usually that is not normally required, the preference being to specify an appropriate coating. What is the nature of the corrosion you are expecting; what is expected to cause the corrosion? I wouldn't design the clamp for hoop expansion of the pipe it will hold. I don't see the point in that. It's not likely to go anywhere, is it? If it was a steel clamp, it would not be designed for ring expansion of the pipe it is holding. A vulcanized rubber pad applied to the inside of the clamp to provide electrical isolation would be provided, with a slack allowance for ring expansion of the pipe being clamped.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek