Pipeline Force Profile

[RandomAxe]

If I have a short pipe in pipe pipeline, say 4km long, that is surface laid (i.e. not trenched and buried) and not fully constrained/anchored and so the fully constrained effective axial force is not reached. If I wanted to plot the effective force profile for this, the maximum force is governed by the weight of the pipeline, the friction coefficient and the pipeline route, and then the gradient either side is governed by the friction force, as shown on attached sketch. Is this correct?

Or would a more accurate way of doing it be to calculate the inner pipe force (i.e. spacer friction coeff x Inner pipe Weight x L) and the carrier (carrier friction coeff x Total weight x L) - (spacer friction coeff x Inner pipe Weight x L). Then summing the two of them?

Or am I missing something?

Thanks

 

[BigInch]

I've explained a little on the attached diagram

you must get smarter than the software you're using.

 

[RandomAxe]

Thanks for the reply BigInch. So if we know the pipeline is 'short' is it then possible to draw the force profile using the weight, length and seabed friction coefficient alone (i.e. no pressure or temperature calcs)? What I was also wanting to determine is whether the maximum axial force in this case will be axial friction x weight of PIP system x (route length/2)? Or is it more accurate to calculate the inner pipe forces using spacer friction, and then the carrier pipe forces (as described above) and then summing the two? By my hand calcs the second method gives a lower max force. Thanks

 

[BigInch]

Yes. But, unless it is obviously a trivial solution, you usually have to calculate the net pressure and temperature force, considering all pipes at their respective pressures and temperatures, which for pipe in pipe is the sum of the internal pipe pressure and temperature plus the external pipe at its temperature, to know if you can assume the pipeline is short or not. If friction x 1/2 pipeline length is >= net pressure and temperature expansion force, the pipeline is long. If not, its short. If it's short, the force profile looks like your diagram where the maximum force is f x L/2. If it's long, where the sum of soil friction from end of pipeline to some point along the pipeline, it looks like mine. In any case, normally you need to know the expansion of the pipeline at the ends, or somewhere, so you will have to calculate that anyway by integrating the pressure and temperature - friction force profile over pipeline length divided by A and E.

you must get smarter than the software you're using.