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Birger Etterdal
2011-11-14
In-line inspection (intelligent pigging) of offshore pipeline
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Content
Type of defects
Different inspection tools
Objective of any inspection
Focus on corrosion mapping for submarine pipelines
Selection of inspection technology for corrosion mapping
Deliverables from in-line inspections
Awareness of defect sizing uncertainties
Working with detailed data from UT-based in-line inspections
Are there any alternatives to in-line inspections?
2
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Type of “defects” that we normally look for in offshore pipelines
Metal loss defects (corrosion)
Cracks
Cross section imperfections (dents,
ovalisation, etc.)
Pipeline curvature
3
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Defect types and in-line tools
4
MFL- Pig UT- Pig
Caliper pig
Geo-pig
In some cases these
are combined into
one assembly
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Objective of any inspection
5
Purpose: establish a basis for condition/integrity evaluations
In too many cases, critical anomalies could have been “detected” at an earlier
stage if the inspection results had been properly reviewed (and properly reported)
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
“Typical failure statistics” and use of in-line inspections (ILI)
6
Anchor
18 %
Impact
24 %
Corrosion
27 %
Structural
5 %
Material
10 %
Other
11 %
Nat. Hazard
5 %
Degradation over time
ILI with metal loss detection tool
Weld defects?
Early phase problem
In some cases:
ILI w/crack detection
Event based
External inspection,
and in some cases
ILI with caliper / geo pigs More relevant for materials
in environment where forms of stress corrosion cracking
can occur
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Selecting the right tool for corrosion mapping
Gas (MFL) or liquid line (MFL or UT)
Stainless steel (UT)
Very thick wall (UT); “very thick wall” is diameter dependent
Difficult or inadequate cleaning (MFL)
Type of corrosion
- Small pits (MFL)
- Defect at girth welds (MFL)
- Long axial grooves (UT)
- Uniform corrosion (UT)
- Severe corrosion with respect to pipeline integrity (UT)
7
This pipeline was inspected just some few months
before it bursted
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
slide 8
MFL versus UT
Advantages
- Can be used for both gas and
liquid pipelines
- Can be used for very long
pipelines
- Large number of vendors
Advantages
- Direct measurement of the
wall thickness
- High accuracy
- Provides shape of the defects
Disadvantages
- Can not measure the absolute
wall thickness
- Known to be “less” accurate
- May not detect long axial
grooves
- Can not be used in case of
very thick pipelines
Disadvantages
- Can normally only be used in
liquid pipeline
- Normally more costly
- Sensitive to “pipeline cleaning”
MFL Tools UT Tools
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Deliverables from In-Line Inspections
Inspection report – In most cases in accordance with the POF-specification
(POF = Pipeline Operators Forum)
Pipe book (or pipetally listing)
Feature listing – list of e.g. corrosion defects with position information and size
information
On demand – detailed inspection results (e.g. grid of remaining wall thickness from
UT-based inspection)
9
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
slide 10
Pipe book versus system drawing
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
slide 11
Typical Metal Loss Feature List (MFL)
Note that the defect depth is given as
percentage of nominal (reference) wall thickness
This value is NOT measured
(in most cases not given in the feature list)
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
slide 12
Detailed inspection results
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14 slide 13
Uncertainties in defect sizing
Wall thickness
For onshore pipeline, “verifications digs” are usually a part of the in-line
inspection scope, i.e. the worst detected defects are excavated and the wall
thickness measured by e.g. UT. This is not practical for offshore pipelines
(although done in some cases).
The sizing accuracy is dependent on the metal loss shape, e.g. depth sizing of
a pin-hole defect is more difficult compared to corrosion defects “larger extent”
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
slide 14
MFL – AUT inspection results (onshore pipeline)
Comparion between in-line inspection (MFL) and external AUT
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100
Defect Depth - MFL [%]
De
fect D
ep
th -
AU
T [%
]
Defects
+/- 20% accuracy
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
slide 15
MFL – AUT inspection results (onshore pipeline)
Comparion between in-line inspection (MFL) and external AUT
0
10
20
30
40
50
60
70
80
90
100
0 20 40 60 80 100
Defect Depth - MFL [%]
De
fect D
ep
th -
AU
T [%
]
Defects
+/- 10% accuracy
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Echo loss – the largest “problem” with UT-based ILI
Corrosion ›› Irregular inner (or outer) surface
Inadequate cleaning (it is not uncommon to send 20-30 cleaning pigs prior to an UT
in-line-inspection). The cleaning is also affected by corrosion.
Presence of “hard scale”
Pig velocity
…
…
16
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Use of “stand-off” data to fill in lacking WT-data
17
WT = WTSO – SO
WTSO
signal
1st reflection (SO)
2nd reflection
SO
WT
WT = Wall thickness
SO = Stand-off (distance between UT-probe and inner wall)
SOWT = WT + SO in un-corroded area ≈ constant over a moderate axial length
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Example – Use of “stand-off” data to fill in lacking WT-data
18
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
UT based in-line inspections can also be very good and
consistent from inspection to inspection!!
19
2010 inspection 2011 inspection
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Specification for assessment of corroded pipes based on
detailed data from UT-based in-line inspections
DNV has together with Statoil and DONG Energy
developed a specification for how to assess pipelines
with axially long corrosion defects (hereunder procedure
for “cleaning” UT-inspection data). NDT Systems &
Services, Germany has also participated in this work.
The above will be included in next update of DNV’s
recommended practice DNV-RP-F101 – Corroded
Pipelines (to be reissued 2013 - JIP project is launched)
Validation (on-going):
- A 45m section has been retrieved from a severe corroded
flowline
- This flowline has been internally inspected several times and
the last one just before retrieval
- This 45m section will be scanned with AUT (FORCE Technology)
- Compare ILI data to AUT data
- 4 segments will be burst tested
20
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
Are there any alternatives to In-Line Inspections?
No, but
- an adequate corrosion management system is the
most important “tool”. In-line inspection should be
used to verify that the corrosion management
system is working as intended!
- spot checks topside (or in some cased subsea) give
some indications – however no guarantee that the
pipeline is free for severe corrosion defects.
- note that the worst incidences caused by corrosion
relate to topside/riser above still water level where
other inspection methods are applicable (and in
some cases better)
21
No corrosion Severe corrosion
14 km long 16” water injection flowline
© Det Norske Veritas AS. All rights reserved.
IN-LINE INSPECTION (INTELLIGENT PIGGING) OF OFFSHORE PIPELINE
2011-11-14
22
Safeguarding life, property
and the environment
www.dnv.com