Page 166 - Pipeline Pigging Technology

Inspection of the Forties sea line

of individual corrosion pitting, general corrosion containing pitting, selective
corrosion attacks of girth welds and also areas of relatively-uniform metal loss,
which in appearance would be similar to general wall thinning but with a
rough internal surface texture. Fig.2 shows an example of the type of
corrosion in the replaced riser.

INSPECTION VEHICLE DETAILS

The 32-in inspection vehicle produced for BP is based on the magnetic flux
leakage principle, and is shown in Fig.3.
The design is based on two pressure vessel assemblies linked by a flexible
coupling. The leading pressure vessel carries the strong permanent magnets
onto which are bolted flexible carbon steel bristle assemblies to transfer the
magnetic field to the pipe wall. The main sensing system, containing several
hundred sensors, is situated between the bristle assemblies. It is designed to
maintain close contact with the pipe wall even under the most difficult
dynamic situations, enabling the sensors to. maintain contact with the wall
even at the girth weld areas, thus ensuring that all areas of the pipe are
inspected.
A second sensor system is carried by the trailing pressure vessel to enable
discrimination between internal and external metal loss to be obtained.
Both pressure vessel modules have the on-board signal processing units,
batteries and digital recorders, required to format and store the vast quantities
of information obtained during an inspection operation.
The performance specification of the inspection system was that of the
standard British Gas specification, as given in Fig.4. However, the adaptations
carried out to the sensing systems expanded the specification to include pipe-
wall thickness assessment and sizing of specific girth weld corrosion.
These adaptations meant that all the types of corrosion damage evident on
the replaced riser could be unambiguously identified and accurately sized.

INSPECTION PROGRAMME

To date three inspection operations have been performed in the Forties
sea line, having been undertaken in June, 1988, March, 1989 and October,
1989.

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