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STRESS ANALYSIS BASED DESIGN OF PIPELINES
TUTORIAL on Session 5
Q1. You have just been given the engineering (or nominal) stress, S, and engineering (or
nominal) strain, e, data obtained from a uniaxial tensile test of a pipeline material.
(a) Show how you would obtain the corresponding true stress and true strain . (b) Hence, show that the nominal measure of the ultimate tensile strength, Su, and the
true ultimate tensile strength, u, are related by )exp(. uuu S
where u is the true strain at u.
(c) The true stress and the true plastic strain p data can be fitted to the power law
equation npK
where n is the strain hardening index and K is the strength coefficient. It is possible to
determine the material constants from a ln() - ln(p) plot of the experimental data. But
the accuracy of this approach depends on the stress range used in the plot. Hence show
that the strain hardening index n and the strength coefficient K can be alternatively be
obtained from the relations : n = u; and K = Su (e/n)n , where e = 2.718 is the base of
natural log.
Q2. A subsea pipeline with outer diameter 220 mm is to be used at a water depth of 500 m
to transport non-flammable fluids at a pressure of 25 MPa and temperature of 80 oC . The
material from which the pipe is made has a room temperature yield strength of 448 MPa and
tensile strength of 610 MPa, and the pipe is be manufactured to a wall thickness tolerance of
0.5 mm. The estimated loss of thickness due to corrosion over the design life is 0.25 mm.
Assume the pipe is straight, the deformation is elastic, and there is no restraint to the axial
movement. Determine the minimum nominal thickness of the pipe using the hoop-stress
based approach. Assume thin-walled pipe and water density of 1000 kg/m3.
Ans: [7.57 mm by PD8010; 6.67 mm by DNV]
Q3. The pipe in Q2 is to be concrete-coated. The concrete is 30 mm thick and has a
Young’s modulus of 20 GPa and Poisson’s ratio of 0.2. The pipeline steel has a Young’s
modulus of 200 GPa and Poisson’s ratio of 0.33. Using the nominal thickness of the pipe
based on BS PD8010-2 obtained from Q2 and assuming plane sections remain plane,
determine
(a) the interface pressure at the pipeline-concrete interface after the internal pressure is
applied, and [Ans: 8 MPa]
(b) the minimum allowable tensile strength of the required concrete material, if the
concrete in most likely to fail in tension. [Ans: 12.7 MPa].
Assume the deformation is linear elastic.
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