GUIDELINES_FOR_THE_USE_OF_STRUCTURAL_RELIABILITY_AND_RISK

IGU World Gas Conference, 8 June 2000, Nice, P-405

GUIDELINES FOR THE USE OF STRUCTURAL RELIABILITY AND RISKBASED TECHNIQUES TO JUSTIFY THE OPERATION OF ONSHORE

PIPELINES AT DESIGN FACTORS GREATER THAN 0.72

DIRECTIVES POUR L’UTILISATION DES TECHNIQUES A BASE DERISQUE ET FIABILITE STRUCTURELLE AFIN DE JUSTIFIER

L’EXPLOITATION DE PIPELINES TERRESTRES A DES COEFFICIENTSDE SECURITE SUPERIEURS A 0,72.

A. Francis, R.J. Espiner & A.M. EdwardsBG Technology, United Kingdom

ABSTRACT

Structural reliability and risk based techniques have recently been used to demonstrate safeoperation of high-pressure pipelines at design factors greater than 0.72. In particular several pipelinesare now operating on the Transco National Transmission System, in the UK, at design factors of 0.78owing to the use of the techniques. The overall structural reliability and risk based approach forjustifying safe operation of pipelines at these stress levels is thus now established. However, in orderto establish cost effective and viable widespread use of the technique, formal guidelines arenecessary which will allow analyses to be performed with minimal requirements for regulator liaisonmeetings, external audits and extensive reporting. Structured and comprehensive guidelines, whichhave recently been constructed based on extensive analytical consideration of the underlying issues,are presented in this paper. The guidelines enable a consistent approach to the use of structuralreliability and risk based techniques for the purpose of design and assessment of onshore pipelinesto be adopted. This will result in consistent levels of safety. The guidelines also provide the industrywith a detailed methodology and clear definitions of terminology. This will enable pipeline operatingcompanies to take immediate advantage of the cost saving benefits obtained using the structuralreliability and risk based techniques.

RESUME

Les techniques à base de risque et fiabilité structurelle ont récemment été utilisées pour démontrerl’exploitation en toute sécurité de pipelines haute pression à des coefficients de sécurité supérieurs à0,72. En particulier, plusieurs pipelines sont maintenant en exploitation sur le système detransmission national Transco en Grande-Bretagne, à des coefficients de sécurité de 0,78 grâce àl’utilisation de ces techniques. L’approche générale à base de risque et fiabilité structurelle pourjustifier une exploitation en toute sécurité des pipelines à ces niveaux de contrainte est, de ce fait,maintenant établie. Cependant, des directives sont nécessaires de façon à établir une généralisationrentable et fiable de la technique. Ces directives permettent l’exécution de l’analyse avec desspécifications minimales pour des réunions de liaison avec l’autorité de réglementation, des auditsexternes et des rapports complets. Des directives complètes et structurées, qui ont récemment étéconstruites, basées sur une considération analytique importante des problèmes sous-jacents, ont étéprésentées dans ce document. Ces directives permettent une approche constante de l’utilisation detechniques à base de risque et fiabilité structurelle dans le but de l’adoption d’une conception etévaluation des pipelines terrestres. Ceci aura pour résultat des niveaux constants de sécurité. Lesdirectives offrent également à l’industrie une méthodologie détaillée et des définitions précises determinologie. Ceci permet aux sociétés d’exploitation de pipelines de prendre immédiatementavantage des bénéfices d’économie obtenus en utilisant les techniques à base de risque et fiabilitéstructurelle.



1 INTRODUCTION

The objective of the guidelines given in high-pressure gas transmission pipeline design codesis to ensure that high-pressure gas transmission pipelines are designed, constructed and operatedsafely, reliably and economically.

Both the level of safety and reliability can always be increased by introducing furthermitigation. However, as the level of mitigation increases the cost of operation also increases. In viewof this the fundamental approach of the design codes is to ensure that the risk to society of theharmful events is 'As Low As Reasonably Practicable' (ALARP). This essentially means that steps aretaken to reduce risks provided that the associated cost is reasonable.

The two most basic objectives of internationally recognised design codes, e.g. ASME B31.8[1] and BS8010 [2] are to ensure that the likelihood of an ignited release of gas occurring issufficiently low and to ensure that the consequences of any failure are controlled.

Design codes thus specify a maximum allowable design factor and a minimum allowablebuilding proximity distance to achieve these objectives. Most codes currently allow the operation ofhigh-pressure gas pipelines at design factors up to 0.72 in sparsely populated or unpopulated areas.There is considerable evidence to suggest that the current limits on design factor have contributed tothe safe operation of high-pressure natural gas transmission pipelines. Indeed, there have beenapproximately 500,000 kilometre years of operation of high-pressure gas transmission pipelines in theUK alone without a single ignited release of gas and approximately 2 million kilometre years ofonshore pipeline experience across Europe with very few such incidents [3]. This has led to awidespread view that there is some conservatism in the guidelines given by current design codes.

Consequently, recent amendments to high-pressure pipeline design codes will now allowoperation at design factors greater than 0.72 in sparsely populated or unpopulated areas providedthat it can be demonstrated that it is safe to do so.

Structural Reliability Analysis (SRA) is a methodology that is dedicated to providing such ademonstration. The first successful application of SRA for this purpose was to provide justification foroperating several sections of onshore gas transmission pipelines in the UK at a design factor of 0.78[4, 5, 6]. The UK regulator gave a statement of ‘no objection’ to the proposed operation following adetailed external technical audit of the work and a lengthy consultation period.

It is anticipated that as commercial pressures on designers and operators increase there willbe an increase in the requirement to relax the conservatism present in the design codes withoutsignificantly compromising safety. To this end SRA has recently been used to provide justification forincreasing the maximum allowable design to 0.8 in IGE/TD/1 [7]. Based on this study, a three levelapproach to the justification of safe at a design factor of 0.8 has been proposed.

The first of these is to identify the situations in which safe operation can be inferred from aconsideration of the basic design and operating parameters. i.e. the situations which are currentlyinherently safe at a design factor of 0.8.

The second level identifies situations in which operation at a design factor of 0.8 is safeprovided that some further mitigation is introduced, e.g. slabbing.

The third level requires a complete SRA study to be undertaken. In this case some guidanceon the approach to be used is required if widespread use of the approach is to be adopted.

This paper provides a description of each of the basic steps that must be taken to complete aSRA study. The next section gives an overview of the approach indicating the function of each of theelements and the role they each play in justifying safe operation. Each of the individual elements isthen described in the following sections. The paper concludes with a number of statements on the


current status of the methodology, an identification of the requirements for future developments andan indication of how these should be progressed.

2. STRUCTURAL RELIABILITY ANALYSIS

Structural Reliability Analysis comprises six elements. These are

w Establishment of Limit Statesw Identification of Failure Modesw Formulation of Limit State Functionsw Uncertainty Analysisw Evaluation of Failure Probabilityw Assessment of Results

A brief description of each of these elements and the role they play in the overall analysis is givenbelow.

2.1 Establishment of Limit States

A limit state is defined as the state of a structure when it no longer satisfies a particular designrequirement. The limit states thus determine the conditions that are to be avoided. A leak is a limitstate.

2.2 Identification of Failure Modes

A failure mode is the mechanism that causes the pipeline to reach a limit state. A failure modeis thus always associated with a particular limit state but a failure mode is not a limit state. Corrosiongrowth is a failure mode that can cause a pipeline to leak.

2.3 Formulation of Limit State Functions

A limit state function is a mathematical relationship between the parameters characterising aparticular failure mode that exists when the pipeline has reached a limit state. It is generallyexpressed in the form

021 =)x,...x,x(G n (1)

where x1, x2,…xn denote the n parameters which characterise the failure mode under consideration.Some of the parameters may be dependent on time. In this case the limit state function determinesthe relationship that exists between the parameters at the current time (t = 0, say) that will result in afailure at a later time, futuret .

2.4 Uncertainty Analysis

In any practical engineering circumstance, each of the input values to a limit state function issubject to uncertainty. Uncertainties are accounted for in a structural reliability analysis by describingvariables in statistical terms.

For each limit state function, the variability in the sensitive parameters must be quantified bydata analysis and ultimately the construction of probability density functions. This is achieved byperforming appropriate statistical analyses of the data available from sources including constructionrecords, test certificates and inspection records. The outputs of these calculations are mathematicalfunctions describing the likelihood of occurrence of specified values of particular parameters.


Parameters typically belong to one of four groups: pipeline geometry, material properties,defect dimensions and loads.

2.5 Evaluation of Failure Probability

The probability density functions for each sensitive parameter are used in conjunction with thelimit state functions to determine the probability of failure. For a given limit state and failure mode, thefailure probability is the sum of the likelihoods of occurrence of all combinations of the relevantparameters which cannot coexist in an un-failed state. Integral calculus is used for this purpose sincethe parameters are generally described by a continuum, rather than by discrete values.

2.6 Assessment of Results

The final stage of the SRA is to make a decision based on a consideration of the computedfailure probability.

There are currently several approaches available for this purpose, each having particularmerits. These include comparison of the computed failure probabilities for the case underconsideration with those calculated in association with previous operation, those implicit in designcodes and those based on a consideration of actual failures. However, no formal agreement currentlyexists on which approach is most appropriate. The approach recommended here is one based on theALARP principle and is discussed in brief detail in Section 7.

If the criteria are not satisfied initially then a reduction in failure probability may be explored byanalysing the effects of various mitigating measures.

The overall SRA procedure is shown schematically in Figure 1.

Figure 1 The SRA Methodology

Establish limit states

Identify failure modes

Quantify variation in pipelinegeometry

Calculate probability offailure, pf

Report results

Quantify variation in materialproperties

Quantify variation in defectsize and growth

Identify limit state functions

Is pfacceptable

?

Introduce further mitigatingmeasures

No

Yes

Quantify variation inloads


3. LIMIT STATES

A limit state is defined as the state of a structure at which it no longer satisfies a particulardesign requirement.

For pipelines there are essentially two categories of limit states which require consideration.These are the ultimate and the serviceability limit states.

3.1 Ultimate Limit State

An Ultimate Limit State (ULS) represents the state at which the pipeline cannot contain thefluid it is carrying. This category of limit states has safety implications. Therefore the relevant ultimatelimit states must always be identified and addressed by SRA studies.

3.2 Serviceability Limit State

An Serviceability Limit State (SLS) represents the state at which the pipeline no longer meetsthe full design requirements but is still able to contain the fluid, e.g. can no longer pass sufficient fluidand/or inspection tools. This category of limit states has no direct safety implications but may havefinancial implications. The designer/operator may therefore wish to identify and explore theimplications of the serviceability limit states in addition to those of the Ultimate Limit States.

4. FAILURE MODES

A failure mode is a mechanism that leads to the structure reaching a limit state. It is thusimportant that all of the credible failure modes that can lead to each of the relevant limit states areidentified.

In this context the term ‘credible’ generally means a non-negligible annual probability perkilometre (>10-7 per km-year, say) of the pipeline reaching a limit state due to the failure mode underconsideration. It is often possible to dismiss some recognised failure modes in accordance with thiscriterion without conducting a SRA study. For instance, on some pipelines the occurrence of stresscorrosion cracking (SCC) may be deemed to be incredible based on knowledge that the conditionsthat are required to promote SCC do not exist. However, where there is insufficient evidence todismiss failure modes a priori they must be considered in the SRA study.

The failure modes that most commonly affect onshore pipelines are briefly described below.

4.1.1 External Interference

The occurrence of ground piercing activity, e.g. excavating, in the vicinity of buried onshorepipelines can lead to an inadvertent impact of the pipeline wall, e.g. from the excavator bucket. Thisimpact can lead to one of several types of damage including a dent, a gouge or a dent and a gouge.The implication of the damage depends on the dent depth, the gouge depth and the gouge length.A dent alone does not generally have any safety implications. However, depending on the depth ofthe gouge a breach of the pipeline wall can occur which, depending on the length of the breach, canlead to a rupture running for up to several pipe lengths. This mechanism can lead to an ultimate limitstate and therefore requires due consideration.

4.2 External Corrosion

Coating on the external surface of a pipeline provides a primary protection against externalcorrosion. However, breaches in the coating do occur due to contact with rocks, scrapes fromexcavators or general disbondment. An effective cathodic protection system will usually prevent theoccurrence of corrosion in these situations, but when a breach in the coating occurs and the CPsystem is not operating corrosion can occur. The corrosion process results in a gradual reduction in


the pipeline wall until a breach occurs due to the internal pressure. This mechanism can lead to anultimate limit state and therefore this failure mode requires due consideration.

4.3 Crack-like defects

Other failure modes include processes such as fatigue crack growth of construction defects inwelds. Defects in welds become progressively deeper due to the action of cyclic pressure fluctuationsand will eventually lead to a breach of the pipe wall. This mechanism can lead to an ultimate limitstate, however, in many situations the level and/or frequency of fluctuations are not sufficient tocause growth and this failure mode can be regarded as incredible.

Stress Corrosion Cracking is a process in which cracks are initiated and grow under thecombined effect of stress and environmental conditions. Again this requires a breach of the coatingand the presence of the required environmental conditions. This mechanism can lead to an ultimatelimit state, however it is often an incredible failure mode although the designer/operator must be ableto demonstrate that the required conditions do not exist.

5. LIMIT STATE FUNCTIONS

A limit state function is a mathematical relationship between the parameters characterising aparticular failure mode that exists when the pipeline has reached its limit state. For example, if the limitstate is a leak and the failure mode is external corrosion, the limit state function will define arelationship between diameter, wall thickness, yield strength, ultimate tensile strength and internalpressure which results in a leak due to the failure of the remaining ligament associated with a defectof given length and depth.

A limit state function is required for each credible failure mode and associated limit state. Limitstate functions for the common failure modes of onshore pipelines are well understood and readilyavailable in the literature

6. UNCERTAINTY ANALYSIS

In any practical engineering circumstances, each of the input parameters to a limit statefunction is subject to uncertainty. Uncertainties are accounted for in a structural reliability analysis bydescribing variables in statistical terms.

For some parameters the variability may be slight and/or the limit state function may besufficiently insensitive to the parameter that a single valued estimate of the parameter may beadequate for the study. Best estimates or upper and lower bounds may be used for this purposedepending on the parameter and situation under consideration.

For each limit state function, the variability in the sensitive parameters has to be quantified bydata analysis and ultimately the construction of probability density functions (pdfs). This is achievedby performing appropriate statistical analyses of the data available from sources includingconstruction records, test certificates and inspection records. The outputs of these analyses areprobability density functions, which describe the likelihood of occurrence of specified values of thegiven parameters.

The parameters determining failure can be split into four groups representing geometry,material properties, defect sizes and loads. Each of these is discussed below.


6.1 Quantification of Variation in Pipeline Geometry

The failure behaviour of a pipeline is dependent on the geometry of the pipeline crosssection. The representative parameters are the pipeline outer diameter and wall thickness.

6.1.1 Outer Diameter

The outer diameter of the pipeline is tightly controlled during the manufacturing process andtypically has a very small coefficient of variation. Therefore for the purposes of a structural reliabilityanalysis the outer diameter of a pipeline may be assumed to have a single fixed value equal to thenominal value.

6.1.2 Wall Thickness

The wall thickness, w, of the pipeline is subject to variation due to the nature of themanufacturing process. When the pipe is ordered, a nominal wall thickness, wnom, will be requestedand a minimum allowable value of wall thickness, wmin, will be specified. Produced pipes are subject tosampling procedures to ensure some level of confidence in the minimum value.

A pdf describing the variation in actual values of wall thickness may be found by analysis of asample of measured values from mill inspection records. If no information is available to construct aspecific wall thickness distribution then it may be assumed that the wall thickness is described by ageneric distribution obtained from a large sample of pipeline data, as given in [8] for instance.

6.2 Quantification of Variation in Material Properties

The failure behaviour of a pipeline is dependent on the mechanical properties of the pipelinematerial. The representative parameters are Young’s modulus, yield strength, ultimate tensile strengthand plain strain fracture toughness.

6.2.1 Young’s Modulus

The Young’s modulus of the material is approximately constant and for the purposes of SRAmay be assumed to have a single fixed value.

6.2.2 Yield Strength and Ultimate Tensile Strength

The yield strength and ultimate tensile strength (UTS) of the material are subject to variationdue to the nature of the manufacturing process. Pipe is ordered according to grade, which definesthe specified minimum yield strength (SMYS) and specified minimum tensile strength (SMTS).Produced pipes are subject to sampling procedures to ensure some level of confidence in theseminimum values.

Pdfs describing the variation in actual values of yield strength and UTS may be found byanalysis of a sample of measured values from mill test certificates. If no information is available toconstruct specific distributions then it may be assumed that the yield strength and UTS are describedby generic distributions obtained from a large sample of pipeline data, as given in [8] for instance.


6.2.4 Plain Strain Fracture Toughness

Pipeline toughness is commonly specified in terms of Charpy energy Cv. However, the limitstate functions involving material toughness are formulated in terms of the plain strain fracturetoughness, KIC. Various empirical relationships between KIC and Cv exist.

A probability density function for Charpy Energy may be found by a statistical analysis ofmeasured values of Charpy energy from mill test certificates. The distribution of fracture toughnesscan then be determined by transforming this distribution using the above relationship.

If no information is available to construct a specific fracture toughness distribution then a fixedvalue of fracture toughness corresponding to the specified minimum Charpy energy is recommended.

6.3 Quantification of Variation in Defect Dimensions

6.3.1 Dent Depth

Dents occur due to the application of a force exerted by the ‘indentor’; usually the tooth of anexcavator bucket. The depth of the dent is known to depend on the applied force, the radius, the wallthickness, the yield strength, the ultimate tensile strength and the pressure. A unique pdf is thusrequired to describe the uncertainty in dent depth for each pipeline having a particular combination ofpipeline parameters. The most direct method of constructing these pdfs would be to perform astatistical analysis of the available data describing dent depth variation on each group of pipelineshaving the same characteristics. Unfortunately, due to the broad range of combinations of pipelineparameters in existence and the relatively few occurrences of denting, such data are very sparse. Forthis reason an alternative approach is required.

Although there are very few recorded dents on each group of pipelines having the samecharacteristics, significant data do exist describing the variation in dent depth over the totalpopulation of pipelines. It is thus possible to construct a pdf describing this ‘global’ variation and usethe relationship between dent depth, applied force, radius, wall thickness, yield strength, ultimatetensile strength and pressure to calculate a ‘global’ pdf for the applied indentor force.

This distribution of forces is applicable to all pipelines as it depends only on the nature of theexternal interference. Therefore the distribution of dent depths on a pipeline with a specificcombination of parameters may be obtained from this ‘global’ force distribution.

6.3.2 Gouge Depth

The depth of a gouge in a pipeline is not significantly influenced by any of the pipelineparameters. The depth of gouge is determined by the nature of the external interference. Thevariation in gouge depth can thus be determined by single ‘global’ pdf constructed from the depths ofactual gouges recorded on the total pipeline population.

6.3.3 Gouge Length

The length of a gouge in a pipeline is not significantly influenced by any of the pipelineparameters. The length of the gouge is determined by the nature of the external interference. Thevariation in gouge length can thus be determined by single ‘global’ pdf constructed from the lengthsof actual gouges recorded on the total pipeline population.

6.3.4 Corrosion Defect Depth

The depth of corrosion defects is a time dependent quantity and the variation in thisparameter must thus be described by a time dependent pdf.


It may be assumed that the corrosion depth growth rate is dependent on the instantaneous depthand thus implicitly dependent on time through the dependence of instantaneous depth on time. Thejustification for this assumption is that the corrosion products form a protective barrier as the depthincreases.

This assumption can be used to determine the pdf of corrosion defect depth at any time usinga known pdf at some specific time (e.g. the time of the first inspection) and an estimate of growth rateat this time. The growth rate can be estimated from a consideration of the distributions at the times ofdifferent inspections. If such information is not available for the pipeline under consideration datarecorded on similar pipelines may be used. A detailed description of the application of the aboveapproach is given in [9].

6.4 Quantification of Variation in Loads

6.4.1 Operating Pressure

Although some uncertainty exists in the value of pressure, the effect of this is usually quitesmall compared with the effect of the uncertainty in some of the other parameters. Therefore, it isusually sufficient to represent the pressure by a single deterministic value equal to the maximumoperating pressure (MOP).

7. EVALUATION OF PROBABILITY OF FAILURE

The objective of a structural reliability assessment is to determine the likelihood that thestructure can resist the loads applied to it. Failure occurs if the resistance, R, is lower than the load, S,where R is derived from material and geometric properties, and S is derived from operational loads,fault loads, damage and deterioration. The probability of failure is therefore the probability that R isless than or equal to S, given by:

[ ]0≤−= SRPPf (2)

where P[.] denotes the probability of the event described within the brackets occurring. The equationR – S = 0 defines the limit state function. Denoting the joint pdf of load and resistance by p(R,S,t) theprobability of failing within the time interval (0,t) is given by

∫∫≤−

=0SR

f dRdS)t,S,R(p)t(p (3)

This failure probability is illustrated graphically by the shaded volume in Figure 2.


Figure 2 Failure Probability

In general the load and resistance cannot be separated into two distinct quantities and thelimit state function must be expressed in the form

021 =)x...,x,x(G n (4)

where x1 … xn are the n parameters characterising the failure mode under consideration. In this casethe failure probability is given by

[ ] mn)x...,x,x(G

nf xd...xd)t,x...,x(f)x...,x,x(GPP ...n

110

2121

0 ∫ ∫≤

=≤= (5)

where f(x1, ... xn, t) denotes the joint probability density function at time t. If all of the stochasticvariables are independent of one another, the joint probability density function can be replaced bythe product of the individual probability density functions.

In practice, Equation (5) can rarely be evaluated analytically and recourse is usually made tonumerical integration, approximation techniques, or numerical simulation.


8. ASSESSMENT OF RESULTS

The procedure outlined above can be used to evaluate failure probabilities for anycombination and range of operating conditions. For instance, these can include ranges andcombinations of pressures, inspection frequencies and repair criteria. In design situations ranges ofdesign parameters such as wall thickness and material grade can be included.

The technique thus allows a comprehensive description of the likelihood of failure to beconstructed. The final stage of the process is to form a decision on the fitness-for-purpose based ona consideration of the computed failure probability.

8.1 Previous Safe Operation

One approach is to evaluate the probability that an existing pipeline would have failed atsome time during its previous history of safe operation. This failure probability is implicitly regarded asacceptable and used as the criterion for the assessment of future operation. This approach isparticularly useful for assessing the feasibility of uprating, or reducing inspection frequencies on,existing pipelines. However, it does rely on the pipeline having a considerable history of safeoperation. This approach was used to justify increasing the design factor to 0.78 for existing pipelinesin the UK.

8.2 Code Calibration

Design codes are generally considered to be based on the ALARP principle. Criteria can thusbe constructed by evaluating the failure probabilities for a range of scenarios that are allowed bydesign codes and making the logical claim that adherence to these criteria will result in design andoperating scenarios that are ALARP. This is a valid approach since design codes have generallyresulted in safe operation. However, there is increasing recognition that the codes do lead toinconsistencies. Recent SRA studies have revealed that codes are generally over conservative but insome cases under conservative, i.e. the codes are generally related to the ALARP principle butsometimes they are not.

8.3 ALARP

A current Joint Industry Project [10] is recommending criteria that are formally based on theALARP principle. The proposed criteria are essentially two-dimensional and are expressed as arelationship between risk and the cost per risk averted, see Figure 3. There are three differentregions, negligible risk, tolerable risk and unacceptable risk.

In the negligible risk region, the risk is regarded as being so low that there is no requirementto reduce it even if the cost per unit risk averted was tending to zero.

In the tolerable risk region, the risk is regarded as tolerable provided that it is demonstratedthat that the cost per unit risk averted is above some threshold level. The threshold level increasesmonotonically with increasing risk in this region.

In the unacceptable risk region the risk is regarded as so high that it would not be accepted even ifthe cost per unit risk averted was tending to infinity.


The benefit of this approach is that it will consistently lead to design and operating scenariosfor which the risks are ALARP. It is however, more complex than the code calibration approach sinceit requires some estimates of both harmful consequences and the costs of reducing these.

9. CONCLUSIONS

Guidelines have been presented for assessing the fitness for purpose of onshore pipelinesbased on structural reliability analysis. The approach comprises six elements:

w Establishment of Limit Statesw Identification of Failure Modesw Formulation of Limit State Functionsw Uncertainty Analysisw Evaluation of Failure Probabilityw Assessment of Results

Clear definitions of the key terminology have been given and each of the above six elementshas been described in detail.

Understanding of the first five of the above six elements is well advanced and guidelines forthe use of the techniques have been presented. However industry agreement on the assessment ofthe results of Structural Reliability Analysis is still under development and will be delivered by the JIPdescribed in Section 8.3, which is specifically addressing the issue. This however does not precludeuse of the techniques as is demonstrated by the successful uprating of parts of the Transco NationalTransmission System.

10. REFERENCES

Risk

Acceptance Regime

Riskmax

ALARPcriteria

Figure 3 ALARP Criteria

Unacceptable Risk

Tolerable Risk

Cost Per Risk Averted

Negligible Risk


1. American Society of Mechanical Engineers (1995), Gas Transmission & Distribution PipingSystems, ASME B31.8

2. British Standards Association (1992), Code of Practice for Pipelines, Part 2: Pipelines on Land:Design, Construction & Installation, BS8010

3. Bolt, R. & Owen, R.W. (1999), Recent Trends in Gas Pipeline Incidents (1970 – 1997): A Reportby the European Gas Pipeline Incidents Data Group (EGIG), IMechE Conference on AgeingPipelines, Newcastle upon Tyne, UK

4. Francis, A., Espiner, R.J., Edwards, A.M., Cosham, A. & Lamb, M. (1997), Uprating an In-servicePipeline Using Reliability-based Limit State Methods, 2nd International Conference on Risk based& Limit State Design & Operation of Pipelines, Aberdeen, UK

5. Francis, A., Espiner, R.J., Edwards, A.M. & Senior, G. (1998), The Use of Reliability-based LimitState Methods in Uprating High Pressure Pipelines, International Pipeline Conference, Calgary,Canada

6. Senior, G., Francis, A. & Hopkins, P. (1998), Uprating the Design Pressure of In-service PipelinesUsing Limit State Design and Quantitative Risk Analysis, 2nd International Onshore PipelinesConference, Istanbul, Turkey

7. Francis, A., Edwards, A.M., Espiner, R.J. & Senior, G. (2000), An Assessment Procedure toJustify Operation of Gas Transmission Pipelines at Design Factors up to 0.8, 3rd InternationalPipeline Technology Conference, Brugge, Belgium

8. Jiao, G., Sotberg, T. & Igland, R. (1995), Basic Uncertainty Measures for Reliability Analysis ofOffshore Pipelines, SUPERB Report STF70 F95212

9. Francis, A., Edwards, A.M. & Espiner, R.J. (2000) A Fundamental Consideration of theDeterioration Processes Affecting Offshore Pipelines Using Structural Reliability Analysis, 19th

International Conference on Offshore Mechanics & Arctic Engineering (OMAE 2000), NewOrleans, Louisiana, USA

10. McKinnon, C., Francis, A., Nessim, M., Lamb, M. & Ellinas, C. (1999), Joint Industry Project toDevelop Guidance for Structural Reliability and Risk Based Design and Assessment of OnshorePipelines, 3rd International Onshore Pipelines Conference, Berlin, Germany

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GUIDELINES_FOR_THE_USE_OF_STRUCTURAL_RELIABILITY_AND_RISK

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