computer and odometer. Special design cups were developed that
provide two important features:
)
The cups easily collapse and give minimum friction in the small
diameter pipe. As a result, there will be minimal difference in
pressure in both pipe sections. This is important at the point
where the pig enters and leaves the reduced bore: a high
differential pressure would result in a large expansion of gas
when the pig enters the large section again and an associated
speed excursion. This has now prevented and all data collected
is useful for evaluation!
)
The cups are highly flexible and have sufficient self-centering
capability so that the pig will travel in a good inline position
through the remainder of the pipeline.
Again, before the field work was conducted, a real 1:1 mock up
model was built in the yard with an exact copy of the pipe reducer,
in order to ensure a smooth transition of the pig in both ways.
Prior to the inspection run, a cleaning pig with magnets and spider
nose was run through the line in order to ensure that especially
the smaller pipe section was free from debris that could impact
the passage of the inspection tool. A dual size caliper pig was run,
with two measurement units, in order to get proper geometrical
data from all pipe sections. Inspection data proved to be 100%
within specification and multi diameter lines are now inspectable
without the need of exorbitant prices or the need of high impact
mechanical work and sectionising of the various diameters.
High-flow pipelines
MFL tools will function properly up till velocity of approximately
4 m/s. The faster the tool moves, the stronger the induced
magnetic field that originates from eddy currents induced by
the moving magnetic field. The eddy-current-induced magnetic
field works opposite the magnetic field of the tool, reducing
the overall level of magnetisation. As a result, the magnetisation
will drop below 10 kA/m and inspection data will not meet the
required level of accuracy and confidence. Reduction of gas flow
would be required in order to achieve a successful inspection.
For several reasons, reduction of gas flow may not be feasible,
be it from economic perspective or from power strategic
considerations.
In order to move the tool through a high production
pipeline, an active speed control unit has been designed, which
regulates the amount of gas bypass through the tool while
moving through the pipeline. Again, a trial set-up is built in
order to investigate the behaviour of the tool at various speeds
and the computer unit of the speed control device is tested.
The bypass valve system is also tested in static and in dynamic
conditions in order to get a close picture of the system in the
pipeline. Finally, the fail-safe mechanism is tested in order to
ensure that, whatever the condition of the tool and the gas
flow, the inspection tool will always travel to the receiving
station. The high resolution MFL tool with inertial navigation
system and active speed control unit (ASCU) is brought to
the field to inspect a 28 in. x 250 m long distance subsea gas
transmission line. The operation for the ASCU is evaluated upon
completion of the pig run and the tool has travelled at the pre-
set conditions of 3 - 4 m/s in a gas line with velocity of 5 m/s at
launch towards 9.5 m/s at the receiving end.
Conclusion
In this article, several scenarios of pipeline conditions, either
from a constructional or from an operational point of view that
would be considered ‘non-piggable’ in the convectional way of
speaking, have been dscussed. By using new design of the MFL
module and associated technology, difficult-to pig pipelines are
now within the scope of pipeline inspection and pipeline integrity
management.
Figure 4.
The velocity profile of a low-flow pipeline: 700 hours
at an average of 0.04 m/s.
Figure 5.
MFL tool with Active Speed Control successfully
retrieved after long distance subsea pipeline inspection.
Figure 6.
Pitting corrosion accurately detected and sized by
XHR MFL tool.
52
World Pipelines
/
JUNE 2015
