New depth-independent,
high resolution subsea pipeline inspection tool released
Matthew Kennedy -
AGR Integrity UK
Nick Terdre -
Contributing Editor
External scanning of
pipelines traditionally is undertaken by divers who require support vessels.
AGR Group’s Neptune system, however, provides inspection without diver
intervention and associated availability issues and depth limitations.
Neptune combines an
external state-of- the-art ultrasound scanner with a small ROV. The system can
be mobilized anywhere in the world to examine and predict the remaining life of
subsea tubulars. The system delivers high-resolution ultrasonic data in real
time, which is used to underpin the detailed finite element analysis (FEA)
calculations used in industry-standard, fitness-for-service (FFS)
determinations.
The neutral buoyant
Neptune system, weighing 150 kg (331 lb) in air but neutrally buoyant in water,
is deployed via an inspection class ROV to the work site. The scanner comprises
a hydraulically opening and closing twin collar, 600-mm (23.6-in) wide
construction containing a fully automated X-Y scanner. This clamshell construction
is self-aligning to allow rapid installation by the ROV.
Self-centering rams
within the clamshell hold the scanner firmly on the pipe, creating a stable
platform for the X-Y probe carriage. The probe carriage has an axial range of
500 mm (20 in.) and a circumferential movement of over 360º. It is configured
to deploy Time of Flight Diffraction (TOFD) transducers for volumetric weld
inspection, and compression wave transducers to perform color graphic material
mapping.
The historic
restriction of analogue data transmission has been removed by locating the AGR
Technology Design ultrasonic digital flaw detector on the Neptune scanner. This
allows the inspection data to be digitized and processed at the subsea
worksite, then sent through the ROV umbilical to be viewed in real time on the
surface.
Currently, the
Neptune system is configured to operate in water depths of up to 1,000 m (3,280
ft), but this could be extended. The system’s ultimate working or depth range
is equivalent to the ROV umbilical length: some ROVs today operate to a range
of 6,000 m (19,685 ft).
The ROV pilot and
Neptune operator sit together during operations to ensure optimum operational
interface. The objective of any examination performed with the Neptune system
is to obtain high quality graphical images of parent material, welds, and
adjacent HAZ material.
As the probe
carriage rasters around the pipe, the data is stored and viewed in real time
for both mapping and weld inspection. In TOFD mode, the two transducers
straddle the weld at a pre-set standoff to allow volumetric imaging of the weld
in one pass.
There are a
multitude of ROVs in service around the world, hence the importance of being
able to interface mechanically and electronically with any type of inspection
class ROV. The size and weight of the self-contained Neptune system allow
deployment from, small supply vessels or fixed offshore installations to
monitor risers and caissons.
The system also can
check pipeline areas following subsea impact, anomaly verification and quantification
following IP runs, and to assess potential hot-tap locations. In its current
configuration the double-collar scanner is ideal to examine straight pipe and
upstream and downstream of bends.
The examination is
performed on production pipelines from the external surface. The cleaner the
surface, the higher quality the resulting images. Thanks to an existing range
of cleaning, excavation, and dredging options, some residing within the AGR
group, each proposed inspection site can be addressed individually to optimize
the data quality.
Gaining direct
access to the pipeline wall may be difficult if the line is concrete-coated,
buried, or rock-dumped. In such cases, internal inspection techniques may offer
a more cost-effective solution, which AGR again can address via its suite of
inspection tools.
Neptune’s current
inspection diameter range is 12-18-in. (30-46 cm), with plans to build both
smaller and larger diameter collars deploying the same techniques. There are
further plans to use the system’s scanner as a platform for other techniques
such as ACFM, eddy current, and phased array.
AGR embarked on the
development of this technology in the mid 1990s aiming to inspect pipelines not
designed for pigging. There are a number of reasons why such services may be
required. Many non-piggable lines have reached the limit of their design life,
so their integrity needs to be demonstrated if they are to remain in operation.
Again, operators in
general are giving greater priority to ensuring the integrity of their
pipelines, of any age. Production downtime resulting from loss of a pipeline
due to corrosion or a defective weld more than outweighs the cost of regular
inspection. And operators also find themselves facing more stringent
regulations as authorities seek to avoid environmental damage from pipeline
leaks.
Crack detection
Demand has grown for
internal and external inspection of pipelines and welds the past year. Last
fall, AGR introduced Claycutter X, a technology to excavate the sea bottom and
to remove soil from old pipelines. AGR plans to provide the Neptune Subsea
Inspection system and Claycutter X as a package to combine excavation,
examination, and recovering.
Another development
is the WeldScan tool, which the AGR PipeTech division says it aims to promote
in the Gulf of Mexico and West Africa. To date the system has been applied only
in the Norwegian sector of the North Sea.
|
A pipeline inspection train is
readied, with AGR’s PipeIntruder, which supplies the motive force,
at the front. |
Like its predecessor
PipeScan, WeldScan is equipped with ultrasonics to measure wall thickness and
to detect weld defects. However, using TOFD takes accuracy to new levels,
capable of detecting cracks in welds of less than a millimeter for both width
and depth. In other words, cracks can be identified much earlier.
This meets the needs
of increasing application of exotic and high-grade steels in pipelines and
risers to cope with multiphase flows and corrosive wellstreams. These materials
are often difficult to weld, so regular monitoring of welds is required.
The move into deeper
waters also places a premium on reliable integrity monitoring techniques, i.e.
for inspecting steel catenary risers which are exposed to severe loadings.
WeldScan has proved
its worth in examining pipelines made of high-grade steel – in this case 13%
chrome – in a number of assignments carried out for an operator in the
Norwegian sector.
AGR also has
developed a method to transport its inspection tools through the pipeline. This
is self-propelled pig, known as PipeIntruder, incorporates a seal disc with an
internal bypass. Water is pushed through the seal disc by a pump at the front,
creating back-pressure to push the tool forward. Pumping can be reversed,
sending the tool backwards.
An odometer wheel
tracks PipeIntruder’s position in the pipeline. The tool also has axial and
circumferential motors to position WeldScan alongside a weld with ±1mm (0.04
in.) axial accuracy. Video cameras monitor this operation. Data from WeldScan
is transmitted to the surface via fiber-optic cable in real time.
The PipeIntruder is
available for pipe diameters from 8-30 in. (20.3-76.2 cm). Above 30 in. (76
cm), electro-hydraulic tractors are available. The pig hauls all combinations
of inspection tools, and can travel up to 10 km (6.2 mi), the maximum range of
the umbilical winch.
The string made up
of the PipeIntruder and inspection tools is inserted into the pipeline at the
host platform. The tools can be used to inspect other tubular structures such
as risers, J-tubes, and loading lines.
04/01/2008
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