Can anyone please help with equation for calculating the collapse of the subsea pipeline due to external pressure?
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A buckle resulting from excessive bending or another cause may propagate (“travel” along the pipeline). Offshore hydrocarbon pipelines can fail by a propagating buckle caused by the hydrostatic pressure of the surrounding seawater on a pipeline with a diameter-to-wall-thickness ratio that is too high.
The minimum wall thickness, which will prevent the formation of propagating buckles, can be calculated as follows:
Note: Buckle arrestors should be used if the minimum wall thickness does not satisfy above conditions.
The pipeline can also see buckling due to combined bending and external pressure. Combined bending strain and external pressure load should satisfy in accordance with API-1111 as follows:
To avoid buckling, bending strains should be limited as follows:
Note: A typical conservative value of 2.0 often used for safety factors f1
The collapse pressure of the pipe must exceed the net external pressure everywhere along the pipeline in accordance with API-1111 as follows:
Pi = Internal Pressure
Po= External hydrostatic pressure
Pe = elastic collapse pressure of the pipe, in psi
Py = yield pressure at collapse, in psi
f0= Collapse factor (A 0.70 Collapse factor is required for seamless or ERW pipe)
E = modulus of elasticity, in psi
v = Poisson’s ratio (0.3 for steel)
t = Pipe thickness
D = Pipe outside diameter
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