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THE APPLICATION OF LONG RANGE GUIDED ULTRASONICS FOR THE
INSPECTION OF RISER PIPES
J. McGregor, B. Nooteboom, N. Ivory
PTAS-ITS Lot 1 and 2 Tapak Perindustrian Pekan Belait,
Jalan Setia Di-Raja, Kuala Belait, Brunei Darussalam
Abstract: Corrosion and other defects that reduce the cross-sectional area of pipes cause major
problems in the oil, chemical and the other industries. Most standard NDT methods measure the
remaining pipe wall accurately, but over a small area so it is almost always uneconomic to inspect
100% of the pipe. In practice a pragmatic approach is usually adopted where the wall thickness at
a number of points is measured and this information is used to determine the fitness for purpose
of the pipe. However, this approach generally requires unrestricted access to the outer surface of
the pipe and this is not possible, if the pipe is insulated, has protective coatings or is buried. This
means that even external corrosion cannot be seen so that accurate measurements of the
remaining wall can be made at the correct location on the pipe. Ultrasonic guided wave inspection
using the Wavemaker Pipe Screening System (WPSS) offers a novel solution to many such
inspection problems. Guided waves can be excited from easy to access locations on the pipe and
will propagate many metres along the pipe, the returning echoes indicating the presence of
corrosion or other pipe features. PTAS-ITS now use the WPSS routinely in Brunei and Indonesia;
this paper discusses our operational experience in proving the technique to our client and presents
recent inspection results on riser pipes inspected in Indonesia.
Introduction:This paper discusses our operational experience in proving the technique to our
client and presents recent inspection results on riser pipes inspected in Indonesia.
The application of
long range guided
ultrasonic waves for
the inspection of riser
pipes
Long Range Guided
Ultrasonics
Wavemaker Pipe Screening System Slide
1
Guided Ultrasonics makes the Wavemaker
pipe screening system. It is important to
understand from the beginning that this is a
screening tool. It is not a magic solution to
all inspection problems. What is does do is
provide a method of rapidly determining
where there are problem areas on a pipe.
What is Long Range UT (LRUT)? Slide
2
What is long range UT : Instead of scanning
the region directly below or near to the
transducers, guided waves travel down the
length of the pipe. This allows 10's of
meters to be inspected from a single
location. In this picture you can see a ring
attached to a section of EPDM wrapped
pipe. From this single location waves are
sent in each direction.
How Does It Work? Slide
3
To actually perform the test, a ring of
transducers is placed around the pipe. No
couplant is needed. The transducers dry
couple on to the pipe. The surface of the
pipe usually does not need to be prepared.
Any loose flaking paint or corrosion needs
to be scraped off of the pipe, but otherwise,
no preparation is necessary. In this picture
the area of test is the straight piece of pipe
after the bend under the EPDM wrap.
Clean Pipe
Factors affecting the results:
General Condition of Pipe
Generally Corroded Pipe
The propagating guided waves reflect from all changes in cross section
no matter how small. The reflections from small amounts of corrosion
appear as a 'noise' floor on the trace. For example in the trace above,
the pipe section on the left is a clean pipe, but the section on the left is
generally corroded like the pipe shown on the bottom right. Slide
5
The propagating guided waves reflect from
all changes in cross section. The reflections
from small amounts of corrosion appear as a
'noise' floor on the trace. For example in
the trace above, the pipe section on the left
is a clean pipe, but the section on the left is
generally corroded like the pipe shown on
the bottom right.
Slide
4
What Can It do?
* Can be performed without taking the pipe out of service
* Can be performed at elevated temperatures up to 120°C
* Can be performed under coatings, Ground, Concrete, Cladding, Bitumen,
Ethylene Propylene Diene Momomer (EPDM) etc
* It is sensitive to corrosion anywhere on the circumference of the pipe
* It is equally sensitive to both internal and external corrosion
* The entire volume of the pipe is inspected (within the diagnostic length of a
test). Not only spot locations
* Pulse echo type operation provides information on feature position and
approximate size
* Sophisticated analysis aids interpretation of results The inspection can be performed without
taking the pipe out of service. The entire
volume of the pipe is inspected. It does not
sample at only spot locations, it is sensitive
to corrosion anywhere on the
circumference. It is equally sensitive to
both internal and external corrosion. The
technique works by looking for reflections
from changes in cross section of the pipe.
Long ranges of pipe can be screened from a
single location.
Slide
6
Factors affecting the results:
Type of defect
* More difficult
- Single isolated pit
- Smooth gradual defects
- Axial cracks
- Small pits in welds
* Easier
- A cluster of pits
- Sharp corrosion
- Circumferential cracks
- Large cracks in welds Factors affecting the results - Type of
defect.
What Are The Benefits?
* Access is not required to
the area requiring
inspection
* Ability to test metres of
pipe at a time
* Usually no surface
preparation required
* Difficult to inspect areas
such as insulated or
buried sections of pipe
can be screened for
defects
No access to
Area of test Slide
9
Access is not required to the area requiring
inspection, it has the ability to test many
metres of the riser at one time. Usually no
surface preparation is required. Difficult to
inspect areas such as splash zone or buried
sections of the riser can be inspected.
What Are The Limitations?
* Best employed as a
screening tool to
identify troubled areas
* Does not give finite
sizes
* Cannot resolve small
pits
* Cannot differentiate
between internal and
external corrosion Slide
10
LRUT can be best employed as a screening
tool that is used to identify trouble areas.
Once these trouble areas have been
identified they can be targeted using other
methods of examination. This allows for
100 percent coverage at a fraction of the
price of many other methods.
Slide
7
Factors affecting the results:
Effect of Wrapping
Exposed side shows
little attenuation
Strong attenuation is caused
by wrapping
The type of wrapping that surrounds the pipe strongly affects the results. Bitumen
causes a very high amount of attentuation as
can be seen in the result above. The guided
wave can easily propagate in the non-
wrapped side. However, the wave quickly
dies away in the bitumen wrapped section.
Usually only the first few meters of a
bitumen wrapped section can be inspected.
These first few meters are usually the most
likely to corrode.
Slide
8
Typical Ranges
(in each direction, using standard transducers)
* The range that can be inspected in a single test varies greatly depending on such
items as the condition of the pipe and any coatings that have been applied. The
list below shows some typical ranges that can be expected when the standard
transducer configuration is used. The range that will be inspected in a single
test varies greatly depending on such items
as the condition of the pipe and any coatings
that have been applied. The slide above
shows some typical ranges that can be
expected when the standard transducer
configuration is used. If the low frequency
transducers are used, these ranges can
normally be doubled (or more).
What was the job?
Ring position
Test
area Slide
11
This slide is typical of the working
conditions; this is in the tidal zone of a river
estuary. For larger diameter pipes, an
inflatable belt system is used. The system
allows for a light flexible application of the
technique to large diameter pipes. The
normal range of inflatable rings is from 6 to
24 inches in diameter. (Larger pipes can be
inspected, but on large pipes only large
corrosion patches can be found.)
Description of Work
The work scope consisted of carrying out Long Range UT on
13 off risers ranging from 6" to 30" diameter without
disrupting ongoing repair work or affecting production. Slide
13
This slide shows another typical location of
the worksite. The job entailed carrying out
LRUT on 13 risers from 6 to 30 inch
diameter in the splash and ground entrance
area.
Slide
12
Client Requirements
Our client, a major international oil producer, had a corrosion problem on a
gas riser resulting in an explosion.
The problem was related to external corrosion on the riser in the tidal zone
interface.
Their requirement was to carry out non intrusive inspection on the risers
without removing corrosion product, excavating or shutting down the line to
determine whether corrosion was present in the tidal area and give an
estimate of its size.
The work was to be carried out in conjunction with a maintenance
programme on the risers. The risers were first tested by LRUT to determine
the extent of corrosion present and only then were the maintenance crew able
to disturb remaining surface corrosion for repair purposes.
The work was to be carried out in conjunction with a maintenance programme
on the risers. The risers were first tested by
LRUT then only were the maintenance crew
able to disturb remaining surface coatings
for repair purposes.
Slide
14
The problems
Site : Location of work was in a tidal range of a river delta.
Risers : Ranged in size from 6" to 30" diameter
Access : Access was restricted on some risers to locations that did not
provide for the best test parameters.
Timing : Timing was restricted in some locations as the risers were
situated on the tidal flats.
Coatings : Riser coatings consisted of up to 12mm thick EPDM coating
and or epoxy polyproplyene covering the tidal zone area.
Proving : We had to prove to the clients satisfaction that the system
was capable of detecting the size and type of corrosion he was
interested in. The Client was interested in detecting
corrosion equivalent to 5 % change in cross
sectional area.
Proving the system
Our client was concerned that
the guided waves would not be
able to penetrate the EPDM
coating to the area of interest,
which was the tidal zone of the
riser.
This area could be up to 8
metres away from the start of
the wrapping. We had to prove
to the client that the system had
the capability to detect a 5%
loss of CSA defect as far away
as possible from the ring
location.
Concrete
coating
EPDM
coating Slide
15
Our client was concerned that the guided
waves would not be able to penetrate the
EPDM coating to the area of interest, which
was the tidal zone of the riser. This area
could be up to 8 metres away from the start
of the wrapping. We had to prove to the
client that the system had the capability to
detect a 5% loss of CSA defect as far away
as possible from the ring location.
The Test Riser
A 20" diameter by 25.4mm thick test
riser was made available for proving the
technique.
This riser had 12metres of EPDM
coating on the pipe and 3.1metres of
epoxy fiberglass coating on the bend.
The object of the testing was to
determine if a 5% loss of cross sectional
area on the elbow could be reliably
detected through the coating from the
other end of the pipe.
We were supplied with a redundant test Slide
16
riser to prove the system to the client. We
were able to introduce artificial defects into
the riser at various locations.
Scanning the Riser
Testing using standard frequency
transducers at 35mm spacing resulted
in better than expected results. The
best results were obtained from a
frequency of approximately 26 to
30kHz and an excitation signal of 8 to
10 cycles Hanning. This resulted in the
reliable detection of an artificial defect
in the outside of the elbow 100mm
square by 6mm deep equating to a 2%
loss of cross sectional area, from a
location 13 metres away, through an
EPDM coating length of 11.5m and
1.5m of Epoxy fibreglass. The next
slide shows the results achieved from
the location at defect 1
Defect 4 Slide
18
The riser was scanned from a variety of
locations whilst the defects were being
introduced utilising both low frequency and
standard probes. The testing was stopped
when we were able to prove to the client
that we were able to consistently record
defect 4 located in the outside of the elbow
and equivalent to a change in CSA to 2%
through more than 11metres of EPDM
coating.
Slide
17
The Defects 4 defects were introduced by grinding,
4 defects were introduced into the riser at a a variety of locations
1:100mm x 300mm x 11mm deep = 9% change in CSA
2:100mm x 300mm x 11mm deep = 9% change in CSA
3:50mm x 300mm x 11mm deep = 9% change in CSA
4:100mm x 100mm x 6mm deep = 2% change in CSA ranging from 2% to 9% change in CSA.
Standard frequency scan
This slide shows the results of one of the Slide
19
final scans. The location of test is adjacent
to defect 1. All the other defects can be
clearly seen even defect 4 which is
equivalent to a 2% change in CSA and
defect 3 which is only 50mm to a weld.
Close up of defect 3 and weld
Defect 3 was introduced into the pipe adjacent to a weld (within 50mm)
in order to determine whether it could be detected. Using the higher
frequency transducers and a hanning tone burst, this defect is able to be
resolved. This slide shows the resolution of the system, as the defect can
be detected, but the sizing is not as accurate as if the defect was further
away.
Defect 3
Weld Slide
20
Defect 3 was introduced into the pipe
adjacent to a weld (within 50mm) in order
to determine whether it could be detected.
This slide shows the resolution of the
system, as the defect can be resolved but the
sizing is not as accurate as if the defect was
further away.
Slide
22 8" riser location This riser was located offshore, in shallow
water.
Slide
21 The test risers
* A risk analysis carried out by the client revealed 13
risers which required urgent inspection.
* These risers ranged from 6" to 30" diameter, of
which 10 were found to have reportable corrosion
of above 10% loss of cross sectional area. One was
determined to have more than 50% loss of CSA.
* The following slides detail a selection of these tests. A risk analysis carried out by the client
revealed 13 risers, which required urgent
inspection.
8" riser report
This scan shows clean
pipe with no significant
corrosion present. The
double echos at positions
3 and 4 are due to the
difference in distance
between the inside and
outside radius of the
swept bends from the
ring position. Slide
24
The scan shows clean pipe with no
significant corrosion present. The double
echos at positions 3 and 4 are due to the
difference in distance between the inside
and outside radius of the swept bends from
the ring position.
Slide
25 6" riser location This slide shows the ring location and the
vertical section of the riser.
Slide
23 8" riser details
* This riser was 8" diameter by 20mm thick. It
was located on an offshore installation in
shallow water.
*The coating consisted of EPDM wrap.
* No significant corrosion was detected within a
14m test range This section is included as a comparison
between a riser in good condition compared
to the following 1 in poor condition.
Slide
26 6" riser details
* This riser was 6" diameter by 18mm thick. It was
located on an offshore installation in shallow water.
* The bitumen type coating was severely broken
down in the splash zone area. External corrosion
could be seen visually on this riser but due to the
heavy corrosion products the remaining thickness
could not be assessed.
* Corrosion was detected up to 50% wall loss within
a 10 metre test range. This test range was limited
by the extent and severity of the corrosion present. This riser was 6" diameter by 18mm thick.
It was located on an offshore installation.
The bitumen type coating was severely
broken down in the splash zone area.
Corrosion was detected up to 50% wall loss
within a 10 metre test range. This test range
was limited by the extent and severity of the
corrosion present.
This scan shows the
extent of the
corrosion. There is
lots of red and black
together. It shows a
lot of general
corrosion, This was
backed up with
manual UT where
possible. Slide
28 6" riser results
This scan shows the extent of the corrosion.
There is lots of red and black together. It
shows a lot of general corrosion, which was
backed up with manual UT carried out by a
diver. A good correlation was achieved
between the results of the LRUT and the
manual scanning.
Slide
29 30" riser location This slide shows another 30" riser location.
We are shooting towards the ground from
this ring location. It is typical of the
working conditions in this area and is
located at a river crossing and had a
combination of coatings, Bitumen where it
entered the ground and EPDM/Concrete
where it entered the water.
Slide
27
* .
! !
6" riser drawing
!
!
!
!
!
!
!
! Shows a drawing of the layout of the riser
with corrosion detected beyond the lower
riser bend.
Slide
30 Conclusion
Long range UT has proven to be a useful tool
in the inspection of offshore risers, enabling
the pipeline reliability engineers to predict
remaining life span and direct maintenance
costs, where appropriate, without the
requirement for costly pipeline shutdown and
pigging operations. Long range UT has proven to be a useful
tool in the inspection of offshore risers
enabling the pipeline reliability engineers to
predict remaining lifespan and direct
maintenance costs, where appropriate,
without the requirement for costly shutdown
or pigging operations.
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