In-line inspection (intelligent pigging) of offshore pipelinendt.sitegen.no/customers/ndt/files/PIG on pipeline.pdf · in-line inspection (intelligent pigging) of offshore ... in-line

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?

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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

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Defect types and in-line tools

4

MFL- Pig UT- Pig

Caliper pig

Geo-pig

In some cases these

are combined into

one assembly



2011-11-14

Objective of any inspection

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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)



2011-11-14

“Typical failure statistics” and use of in-line inspections (ILI)

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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



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



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



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)

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slide 10

Pipe book versus system drawing



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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)



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slide 12

Detailed inspection results



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”



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



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



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

…

…

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Use of “stand-off” data to fill in lacking WT-data

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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



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Example – Use of “stand-off” data to fill in lacking WT-data

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2011-11-14

UT based in-line inspections can also be very good and

consistent from inspection to inspection!!

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2010 inspection 2011 inspection



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

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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)

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No corrosion Severe corrosion

14 km long 16” water injection flowline



2011-11-14

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Safeguarding life, property

and the environment

www.dnv.com

Documents

In-line inspection (intelligent pigging) of offshore pipelinendt.sitegen.no/customers/ndt/files/PIG on pipeline.pdf · in-line inspection (intelligent pigging) of offshore ... in-line