predicting the probability of failure of gas pipelines including inspection and repair procedures

Preparing for the Hydrogen Economy by Using the Existing Natural Gas System as a Catalyst // Project Contract No.: SES6/CT/2004/502661

NATURALHY is an Integrated Project funded by the European Commission’s Sixth Framework Programme (2002-2006) for research, technological development and demonstration (RTD)

Predicting the Probability Of Predicting the Probability Of Failure of Gas Pipelines Failure of Gas Pipelines Including Inspection and Including Inspection and

Repair ProceduresRepair ProceduresL.Zhang, R.Adey

Computational Mechanics InternationalSan Sebastian 12.09.2007



ContentsContents

1. Why Probability of Failure (POF) analysis

2. An introduction to the POF calculation method

3. Overview of POF calculation tool- PipeSafety developed for NATURALHY project

4. A sample problem 5. Summary



Probability Of FailureProbability Of Failure

What is Probability of Failure? A number which indicates the probability that a

pipeline could fail within a given timeframe Failure criterion: leakage including breakage

(zero tolerance to gas/hydrogen leakage) The Probability of Failure can be used to

assess the impact on the safety of a pipeline of changes in the design and management and the type of gas carried by the pipeline

The POF also provides a link to economics type models



The POF can include the impact on the pipeline safety of: The type of gas being carried and its

impact on the materials or defects present in the pipeline

The operational loads on the pipeline The inspection strategy and the

performance of the inspection tools The repair strategy The quality of the pipeline

• e.g. The number and type of defects present




HydrogenHydrogen

Hydrogen has been shown to change the behaviour of materials subject to low cycle fatigue loads

These changes are associated with plastic deformation of the material

Hydrogen is therefore expected to change the behaviour of sharp crack like defects

The POF calculation is therefore focused on crack-like defects




A probabilistic fracture mechanics approach has been used to determine the behaviour of crack like defects

There are three main ways of obtaining the POF Analytical solution First/Second Order Reliability Method

(FORM) Monte-Carlo simulation with/without

variance reduction technique



POF Calculation MethodPOF Calculation Method

The current failure criteria used in the model is based on BS7910 level 2 FAD

Other failure models could be added to the POF tool




Monte-Carlo simulation

1

where is number of failures and is the total number of samples

with Stratified Monte-Carlo simulation low POF can be predicted by

( )

where ( ) is the probability that a crack

ff

f

nfi

f ii i

i

MP

MM M

MP P D

M

P D

lies in cell

and n is the number of cellsiD




All the inputs into the model can be expressed probabilistically Defect distribution, material behaviour etc.

Common probability density functions Lognormal distribution

Weibull distribution

Exponential distribution, normal distribution…etc.

2

ln

lnln

ln1 1exp

22

where x can represent crack depth or length

x

xx

xPDF x

x

1

nx

xPDF x e




The POF of a whole pipeline can be calculated as follows:

single1 (1 )nfP p



POF Calculation Tool POF Calculation Tool PipeSafetyPipeSafety

1) Main page



2) Pipe geometry




3) Defect definitions




4) Loads definitions




5) Residual stresses




6) Material properties


Data is being obtained by

experimental testing as part of the

NATURALHY project



7) Inspection tool characteristics




8) Repair procedure




9) Analysis and other options




Sample ApplicationSample Application

Basic input data



Basic unit (mm,MPa)

Cracks in Natural Gas

Cracks in 100% hydrogen

c 5.2e-13 2.4e-14

m 3 3.8

Fatigue crack growth data from TNO, Netherlands for natural gas and 100% hydrogen

Sample ApplicationSample Application



X52 Base Material In 100% Natural X52 Base Material In 100% Natural GasGas

Repair criteria Pre-inspected and then inspected at every 10 years If a detected crack will lead to failure in 12 years it

is repaired A pipeline with 10 cracks/km

Fracture and fatigue properties c=5.2e-13 m=3 KIC=4743.416MPa*mm1/2

Result Cumulative POF of a single defect (60 years): 3.0e-5 Total POF: 3.0e-4 per km



POF Of A DefectPOF Of A DefectBase MetalBase Metal

0.00E+00

1.00E-03

2.00E-03

3.00E-03

4.00E-03

5.00E-03

6.00E-03

7.00E-03

Natural Gas 100% Hydrogen



POF Of A DefectPOF Of A Defectin weldsin welds

0.00E+00

1.00E-01

2.00E-01

3.00E-01

4.00E-01

5.00E-01

6.00E-01

7.00E-01

8.00E-01

Natural Gas andwelds

100% Hydrogenand welds



Improve InspectionImprove Inspection

What happens if we improve the

sensitivity of the inspection tool?

New inspection tool Probability of Detection Curve



POF Of A DefectPOF Of A DefectBase MetalBase Metal

0.00E+00

1.00E-03

2.00E-03

3.00E-03

4.00E-03

5.00E-03

6.00E-03

7.00E-03

Hydrogen (POD=95% at4mm)

Hydrogen(POD=98% at4mm)

Higher sensitivity of inspection tool reduces

the POF



SummarySummary

Probability of Failure analysis enables the impact of hydrogen on the safety of pipelines to be assessed

The analysis does not only consider the impact of hydrogen on the micro mechanical behaviour of the materials but also the way the pipeline is managed Inspection strategy Repair strategy Operational loads



SummarySummary

It can be used as a sensitivity tool to assess management options and can link to more economic models

The initial crack size distribution is an important input parameter and further work is required to validate this and other data

Material properties such as crack growth threshold value, fatigue properties and toughness are key parameters



The EndThe End

Question time

predicting the probability of failure of gas pipelines including inspection and repair procedures

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