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4.2.1. Dents without cracks
Fig. 4 shows that the allowable operating pressure is very sensitive to
the dent depths.
4.2.2. Dents with cracks
A dent with a crack seriously affects the operation or life of the
pipeline. A 6-in. dent with a 0.1-in. crack is acceptable. If the crack
increases to 5.1 mm (0.2 in.), a 101.6 mm (4 in.) dent is no longer
permissible. For a crack depth of 7.62 mm (0.3 in.), no dent is allowed
(refer Fig. 4). Fig. 4 shows the reduced allowable operation pressure in a
pipeline containing dents with cracks.
4.3. Corrosion
Due to the low operating pressure and the chosen wall thickness, a very
large corrosion allowance exists. Sacrificial anodes protect the exterior
of the pipeline from corrosion. The light crude purchase specification
limits potential corrosive elements in the fuel oil.
4.4. External pressure
Although the external pressure on the pipeline is not great, a beneficial
effect is the reduction of hoop stress in the pipe wall.
4.4.1. Collapse/buckling
The pipe has very low probability of collapse buckling due to excessive
OOR. Assuming that the pipeline is empty of crude, an OOR up to 2% and a
wall thickness reduction of 50% is acceptable. OOR up to 10% plus a wall
thickness reduction of 40% is acceptable. Fig. 5 contains the results of
calculations of the collapse pressure of the pipeline assuming OOR values
of 1, 2 and 10% and varying the wall thickness between 4.45 mm (0.175 in.)
and 9.53 mm (0.375 in.), the nominal pipeline wall thickness. |