PIPING LOADS AT TIE-IN POINTS 3

[B.L.Smith]

Dear Sirs,

If two companies are doing stress analysis for a pipe on different side of a tie-in point, they should consider effect of other side of tie-in point on their stress analysis. But other side route or supporting is not specified. How they can consider effect of other side piping on their design without knowing of route or supporting?
It should be mentioned that there is not any anchor or semi-anchor at tie-in point.

 

[MickMc]

BradSmith,

This is a common interface issue in piping where the overall project is broken down into packages constructed by different contractors e.g. Process Units, Offsites & Utilities, Pipelines etc. In my experience as normally each contractor will be at different phases of project and corresponding stress information at tie-ins not available typically piping will be anchored adjacent to tie-in point with one contractor directed to make up the final joint usually based on flow direction as part of Scope of Work.

 

[XL83NL]

How bout this; when each contractor assumes the tie-ins to be hard anchored (i.e. all 6 DOF's restrained), and finds satisfying pipe stress analysis results based on this assumption, real life situation will not pose a big problem since any support will always see some flexiblity. The flexibility will relieve the loads and therefore the calcuated loads will be higher than real life. Would such an assumption work?

 

[curtis2004]

Brad,

1. I agree with XL83NL that easiest solution would be anchor point at or near tie-in location. However, there are other ways to simulate tie-ins.
2. For instance, Contractor "A" is responsible for a main Line "A" and Contractor "B" is responsible for a branch Line "B" connecting into "A". Contractor "A" will provide displacements for Contractor "B" at various operating and occasional loads. Contractor "B" will use those displacements as a moving anchor at tie-in and design their scope. So Contractor "B" has to design it to allow those displacement without overstressing tie-in location.
3. There is another alternative. I have worked on smaller facility projects when one or two lines being replaced or added to existing piping system and this worked well on these cases. You have to model new lines with existing lines up to a next anchor or restrain. By doing this you will take into account affects of existing piping to a new piping being designed and built.

Another issue which is often overlooked different sets of operating conditions and affect of it to all lines.

Regards,
Curtis

 

[hamid38]

Brad,

In my experience the best way is setting a semi-ancor at tie-in point with a reasonable gap, such as 3mm. Then loads in Tie-in point should be summed (your loads + the other side loads).
And in the case that the other side is not done yet, we estimate the unknown load of the other side and keep at it. We usually send the other side's engineer our estimation and ask him to take that into consideration.
Usually he can do the adjustments to keep within limits of allowable load. He has reached tie-in later than us, so he has to adjust himself.

 

[SNORGY]

To me, I would ask the packager (the party working inside of the skid relative to the tie-in) to assume an in-plane' 90 degree, 2 meter + 2 meter offset of matching pipe (with an elbow) to a hypothetical 6 DOF anchor restraint at the far end. That way, a realistic and probably conservative spring rate is assumed for the skid piping to push / pull against during its expansion / contraction. If the packager's inside piping, nozzle loads and structural loads are acceptable under this premise, then the off-skid designer knows that if he builds at least that level of flexibility into the piping system on his side of the tie-in, things should work together at the end of the day. Some adjustments to this approach might be required if the lines are quite hot and thermal displacements cannot be realistically approximated from such short offsets.

Too often, the packager simply states his calculated displacements at skid edge based on free expansion, and then wants the off-skid designer to accept that growth into his side of the piping system. Then, when the off-skid designer asks the packager what spring rate has been applied throughout the movement from (0,0,0) to (x,y,z), he is either met with nebulous, evasive answers or with a package designer who doesn't understand the question. In other words, the packager expects the growth to happen first before any load is applied.

That is when the posturing starts. It's stupid.