### Flowline thickness calculation?

What formulas do people in Flow Assurance use to calculate pipeline wall thickness?

#### 4 Answers

To calculate Buckling due to combined Bending Strain and External Pressure, use API 1111, 1999 as follows:

To calculate the external pressure of the pipe, the collapse pressure must exceed the net external pressure everywhere along the pipeline in accordance with API 1111 as follows:

Where:

f_{0} = Collapse factor (A 0.70 Collapse factor is required for seamless or ERW pipe)

P_{c}= Collapse pressure of the pipe

P_{e} = elastic collapse pressure of the pipe

P_{y} = yield pressure at collapse

E = modulus of elasticity

v = Poisson’s ratio (0.3 for steel)

Pipeline wall thickness is calculated as per API RP-1111, ASME B31.8 (multiphase pipeline) and ASME B31.4 (Water injection pipeline) using the following expression:

t_{nom} = {[(P_{i} - P_{o})D] / [2(F_{1} - L_{f} - T - s_{y})]} + t_{corr}

Where:

P_{i} = Internal Pressure

P_{o}= External hydrostatic pressure

L_{f} = Longitudinal joint weld factor (1.00)

T = 1.0, temperature derating factor (Equals 1.0 for T<250°F)

s_{y}= Specified Minimum Yield Stress

t_{corr} = Internal corrosion allowance

D = Pipe outside diameter

F_{1} = Design Factor

t_{nom} = Nominal pipe thickness

Regarding buckle resulting from excessive bending or another cause that 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, is as follows:

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