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The collapse will be governed by the
reduced wall thickness rather the OOR since the maximum water depth of the
pipeline is only 19 m. The collapse pressure for the diameter and wall
thickness and steel grade of the fuel oil pipeline, with a normal OOR of
2%, is 14.5 barg (210 psig), which corresponds to a water depth of 143 m.
An OOR of 10%, combined with a wall thickness reduction of 40% (to about
0.225 mm), would be necessary to collapse the pipeline, if it was empty.
The pipe has very low probability of collapse buckling due to excessive
OOR. Fig. 5 shows the collapse water depth for various OOR values.
4. Establishing acceptance criteria
Because of the low operating pressure of the 24-in. fuel oil pipeline, the
relatively large ratio of actual wall thickness to required wall thickness
provides a large margin of safety in the pipeline system.
4.1. Internal or external corrosion
Reviewing the guidelines presented in references (API, 2000; ASME, 1991),
wall thickness reductions due to corrosion of up to 80% of nominal do not
result in failure conditions (refer to Fig. 2). The nominal wall thickness
of the fuel oil pipeline will withstand an operating pressure of 80.7 barg
(1219 psig), approximately 8 times the operating pressure during fuel
unloading of 10.3 barg (150 psig). Unloading occurs approximately 1 day
per month. A wall thickness reduction of 80% (to 0.075 in.) reduces the
maximum allowable pressure to 16.8 barg (244 psig), still about 1.6 times
higher than the unloading operating pressure. For contiguous corroded
area, the maximum allowable longitudinal extent length is 341 mm (137/16
in.) with 1.5 mm (0.06 in.) of maximum corrosion depth. Fig. 3 presents
the allowable lengths and maximum depths of corrosion defects.
4.2. Dents and cracks
A dent and/or a crack can seriously affect the operation of the pipeline.
A dent with or without a crack can affect the operation or life of the
pipeline. |