Local Membrane (Contact) Stresses due to Buoyancy Weights

[mecheng1t0]

k so here it is:

We have a buoyant section of pipeline where we are putting screw anchors. We have been asked to also consider local membrane (contact) stresses due to the presence of the screw anchor (anchor) on the pipe. For analysis this is what I did:

1) Calculated hoop stress (Sh)
2) Calculate longitudinal stress (stress due to Poisson and Thermal) (SL)
3) Calculated bending stress (Sb)
4) Calculated the contact stress (Sc)

Now for finding out the Net stress I am told that we have to do the following:

Net Longitudinal Stress at Top Fibre= SL +/- Sb
Net Longitudinal Stress at Bottom Fibre = SL +/- Sb
Net Longitudinal Stress at Neutral Axis = SL + Sc
Net Circumfrential Stress at Neutral Axis = Sh + Sc


Now I am confused
- why are we only adding Sc to circumferential and longitudinal stress at the neutral axis?
- why are we not adding Sc to net longitudinal stress at top and bottom fibre ?

 

[LittleInch]

On the basis that no ones given an opinion so far and it is a little difficult to guess based on your description, but my assumption here is that the screw anchors are attached to the pipe by some sort of circular clamp? Hence they are applying some force to the pipe in a cicumferential manner and some small axial force (not much I wouldn't have thought). However they are not themselves adding any bending stress. This is being caused by the uplift of the pipe being resisted at point locations by the anchors inducing a bending moment that way.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rconner]

[At least a sketch of exactly how the holddown forces are intended to be transmitted to the pipe "membrane" (or sort of ring girder-like hickey, as apparently LittleInch has guessed), and what material of pipe is involved may help to get you better guidance. While at least some language (inclduing "hoop stresses") in the OP tells me this is likely pressure pipe, I guess could also ask if there are any thrust foci nearby to these anchors, and if so what?]