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2.5. Operational fluid
characteristics
The pipeline transports light crude oil for turbine
fuel and no temperature stresses result. The pipeline system has been
operating for 4.5 years with no reported problems since December 1997.
Several external pipeline inspections have been done by divers and
side-scan sonar to collect pipeline data. Divers conduct periodic span
checks. Spans exceeding prescribed limits have been corrected by
installing grout bags under the pipeline. The maximum internal pressure
occurs during tanker offloading. The tanker discharge pressure is 10.3
barg (150 psi). The internal pressure of the pipeline when not offloading
crude is equal to the static head of oil in the storage tanks on shore.
The maximum external pressure occurs near the offshore end of the fuel oil
pipeline and is equivalent to a seawater head of 19 m.
3. Factors of acceptance criteria
The method of inspection will, to a certain extent, affect the acceptance
criteria. Imposing limitations at or beyond the limits of detection
capability would obviously be counter-productive. The following sections
discuss the inspection technique to be employed and some limitations.
3.1. Inspection technique
The 609.6 mm (24 in.) pipeline is to be inspected by RTD using its
internal inspection crawler to check for damage using the ultrasonic
standoff method (Agthoven, 1998, 2001). Reliable sizing of the flaws by
inspection is important. Parameters to be considered in the uncertainties
of flaw sizing include the crack length, depth, orientation of the flaw,
whether or not the flaw is surface breaking, and number of flaws (i.e.
single flaw or multiple flaws). The crack dimensions required are the
crack depth, crack length, crack angle with the plate surface, crack
location from the surface, and spacing between the cracks if the component
has multiple cracks. Consideration should be given in the uncertainties in
corrosion rate and measurement accuracy. These uncertainties may be
modeled by a probabilistic method. The internal inspection tool must be
calibrated using standard defects prior to insertion into the pipeline and
following the inspection. Wax build-up on the interior wall of the
pipeline
can affect the operation of the inspection pig.
3.2. Corrosion defect measurement
The actual size (depth, width, and length) and shape of any corrosion
damage is to be defined by an adequate number of measured thickness
profiles. The measurements should be performed in accordance with Section
5.3 of API 579 (API, 2000)
or equivalent. Refer to API 579 Section 4.3 for guidance on extracting the
critical profile from measured thickness profiles. If several corrosion
defects in a small area are detected during an inspection, appropriate
determination of defect interaction is
to be conducted, accounting to the following factors:
angular position of each defect;
axial spacing between adjacent defects;
length, depth, and width of defects. |