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

GUIDELINES FOR THE USE OF STRUCTURAL RELIABILITY AND RISK BASED TECHNIQUES TO JUSTIFY THE OPERATION OF ONSHORE PIPELINES AT DESIGN FACTORS GREATER THAN 0.72 DIRECTIVES POUR L’UTILISATION DES TECHNIQUES A BASE DE RISQUE ET FIABILITE STRUCTURELLE AFIN DE JUSTIFIER L’EXPLOITATION DE PIPELINES TERRESTRES A DES COEFFICIENTS DE SECURITE SUPERIEURS A 0,72. A. Francis, R.J. Espiner & A.M. Edwards BG Technology, United Kingdom

ABSTRACT Structural reliability and risk based techniques have recently been used to demonstrate safe operation of high-pressure pipelines at design factors greater than 0.72. In particular several pipelines are now operating on the Transco National Transmission System, in the UK, at design factors of 0.78 owing to the use of the techniques. The overall structural reliability and risk based approach for justifying safe operation of pipelines at these stress levels is thus now established. However, in order to establish cost effective and viable widespread use of the technique, formal guidelines are necessary which will allow analyses to be performed with minimal requirements for regulator liaison meetings, external audits and extensive reporting. Structured and comprehensive guidelines, which have 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 structural reliability and risk based techniques for the purpose of design and assessment of onshore pipelines to be adopted. This will result in consistent levels of safety. The guidelines also provide the industry with a detailed methodology and clear definitions of terminology. This will enable pipeline operating companies to take immediate advantage of the cost saving benefits obtained using the structural reliability and risk based techniques.

RESUME Les techniques à base de risque et fiabilité structurelle ont récemment été utilisées pour démontrer l’exploitation en toute sécurité de pipelines haute pression à des coefficients de sécurité supérieurs à 0,72. En particulier, plusieurs pipelines sont maintenant en exploitation sur le système de transmission national Transco en Grande-Bretagne, à des coefficients de sécurité de 0,78 grâce à l’utilisation de ces techniques. L’approche générale à base de risque et fiabilité structurelle pour justifier 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 généralisation rentable et fiable de la technique. Ces directives permettent l’exécution de l’analyse avec des spécifications minimales pour des réunions de liaison avec l’autorité de réglementation, des audits externes et des rapports complets. Des directives complètes et structurées, qui ont récemment été construites, basées sur une considération analytique importante des problèmes sous-jacents, ont été présentées dans ce document. Ces directives permettent une approche constante de l’utilisation de techniques à base de risque et fiabilité structurelle dans le but de l’adoption d’une conception et évaluation des pipelines terrestres. Ceci aura pour résultat des niveaux constants de sécurité. Les directives offrent également à l’industrie une méthodologie détaillée et des définitions précises de terminologie. Ceci permet aux sociétés d’exploitation de pipelines de prendre immédiatement avantage des bénéfices d’économie obtenus en utilisant les techniques à base de risque et fiabilité structurelle.

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

1

INTRODUCTION

The objective of the guidelines given in high-pressure gas transmission pipeline design codes is to ensure that high-pressure gas transmission pipelines are designed, constructed and operated safely, reliably and economically. Both the level of safety and reliability can always be increased by introducing further mitigation. However, as the level of mitigation increases the cost of operation also increases. In view of this the fundamental approach of the design codes is to ensure that the risk to society of the harmful events is 'As Low As Reasonably Practicable' (ALARP). This essentially means that steps are taken 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 is sufficiently low and to ensure that the consequences of any failure are controlled. Design codes thus specify a maximum allowable design factor and a minimum allowable building proximity distance to achieve these objectives. Most codes currently allow the operation of high-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 to the safe operation of high-pressure natural gas transmission pipelines. Indeed, there have been approximately 500,000 kilometre years of operation of high-pressure gas transmission pipelines in the UK alone without a single ignited release of gas and approximately 2 million kilometre years of onshore pipeline experience across Europe with very few such incidents [3]. This has led to a widespread 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 allow operation at design factors greater than 0.72 in sparsely populated or unpopulated areas provided that it can be demonstrated that it is safe to do so. Structural Reliability Analysis (SRA) is a methodology that is dedicated to providing such a demonstration. The first successful application of SRA for this purpose was to provide justification for operating 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 â€˜no objectionâ€™ to the proposed operation following a detailed external technical audit of the work and a lengthy consultation period. It is anticipated that as commercial pressures on designers and operators increase there will be an increase in the requirement to relax the conservatism present in the design codes without significantly compromising safety. To this end SRA has recently been used to provide justification for increasing the maximum allowable design to 0.8 in IGE/TD/1 [7]. Based on this study, a three level approach 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 a consideration of the basic design and operating parameters. i.e. the situations which are currently inherently safe at a design factor of 0.8. The second level identifies situations in which operation at a design factor of 0.8 is safe provided that some further mitigation is introduced, e.g. slabbing. The third level requires a complete SRA study to be undertaken. In this case some guidance on 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 a SRA study. The next section gives an overview of the approach indicating the function of each of the elements and the role they each play in justifying safe operation. Each of the individual elements is then described in the following sections. The paper concludes with a number of statements on the current

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

2.

STRUCTURAL RELIABILITY ANALYSIS Structural Reliability Analysis comprises six elements. These are

w w w w w w

Establishment of Limit States Identification of Failure Modes Formulation of Limit State Functions Uncertainty Analysis Evaluation of Failure Probability Assessment of Results

A brief description of each of these elements and the role they play in the overall analysis is given below. 2.1

Establishment of Limit States

A limit state is defined as the state of a structure when it no longer satisfies a particular design requirement. The limit states thus determine the conditions that are to be avoided. A leak is a limit state. 2.2

Identification of Failure Modes

A failure mode is the mechanism that causes the pipeline to reach a limit state. A failure mode is thus always associated with a particular limit state but a failure mode is not a limit state. Corrosion growth 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 a particular failure mode that exists when the pipeline has reached a limit state. It is generally expressed in the form

G( x1 , x 2 ,...x n ) = 0

(1)

where x1, x2,â€Śxn 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 determines the relationship that exists between the parameters at the current time (t = 0, say) that will result in a failure at a later time, tfuture . 2.4

Uncertainty Analysis

In any practical engineering circumstance, each of the input values to a limit state function is subject to uncertainty. Uncertainties are accounted for in a structural reliability analysis by describing variables in statistical terms. For each limit state function, the variability in the sensitive parameters must be quantified by data analysis and ultimately the construction of probability density functions. This is achieved by performing appropriate statistical analyses of the data available from sources including construction records, test certificates and inspection records. The outputs of these calculations are mathematical functions describing the likelihood of occurrence of specified values of particular parameters.

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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 the limit state functions to determine the probability of failure. For a given limit state and failure mode, the failure probability is the sum of the likelihoods of occurrence of all combinations of the relevant parameters which cannot coexist in an un-failed state. Integral calculus is used for this purpose since the 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 computed failure probability. There are currently several approaches available for this purpose, each having particular merits. These include comparison of the computed failure probabilities for the case under consideration with those calculated in association with previous operation, those implicit in design codes and those based on a consideration of actual failures. However, no formal agreement currently exists on which approach is most appropriate. The approach recommended here is one based on the ALARP 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 by analysing the effects of various mitigating measures. The overall SRA procedure is shown schematically in Figure 1. Establish limit states

Identify failure modes

Identify limit state functions

Quantify variation in pipeline geometry

Quantify variation in material properties

Quantify variation in defect size and growth

Calculate probability of failure, pf

Is pf acceptabl e? Yes Report results

Figure 1 The SRA Methodology

No

Introduce further mitigating measures

Quantify variation in loads

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

3.

LIMIT STATES

A limit state is defined as the state of a structure at which it no longer satisfies a particular design 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 the fluid it is carrying. This category of limit states has safety implications. Therefore the relevant ultimate limit 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 meets the full design requirements but is still able to contain the fluid, e.g. can no longer pass sufficient fluid and/or inspection tools. This category of limit states has no direct safety implications but may have financial implications. The designer/operator may therefore wish to identify and explore the implications 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 thus important that all of the credible failure modes that can lead to each of the relevant limit states are identified. In this context the term â€˜credibleâ€™ generally means a non-negligible annual probability per kilometre (>10-7 per km-year, say) of the pipeline reaching a limit state due to the failure mode under consideration. It is often possible to dismiss some recognised failure modes in accordance with this criterion without conducting a SRA study. For instance, on some pipelines the occurrence of stress corrosion cracking (SCC) may be deemed to be incredible based on knowledge that the conditions that are required to promote SCC do not exist. However, where there is insufficient evidence to dismiss 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 onshore pipelines can lead to an inadvertent impact of the pipeline wall, e.g. from the excavator bucket. This impact 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 of the gouge a breach of the pipeline wall can occur which, depending on the length of the breach, can lead to a rupture running for up to several pipe lengths. This mechanism can lead to an ultimate limit state and therefore requires due consideration. 4.2

External Corrosion

Coating on the external surface of a pipeline provides a primary protection against external corrosion. However, breaches in the coating do occur due to contact with rocks, scrapes from excavators or general disbondment. An effective cathodic protection system will usually prevent the occurrence of corrosion in these situations, but when a breach in the coating occurs and the CP system is not operating

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

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 an ultimate 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 in welds. Defects in welds become progressively deeper due to the action of cyclic pressure fluctuations and will eventually lead to a breach of the pipe wall. This mechanism can lead to an ultimate limit state, however, in many situations the level and/or frequency of fluctuations are not sufficient to cause growth and this failure mode can be regarded as incredible. Stress Corrosion Cracking is a process in which cracks are initiated and grow under the combined effect of stress and environmental conditions. Again this requires a breach of the coating and the presence of the required environmental conditions. This mechanism can lead to an ultimate limit state, however it is often an incredible failure mode although the designer/operator must be able to demonstrate that the required conditions do not exist.

5.

LIMIT STATE FUNCTIONS

A limit state function is a mathematical relationship between the parameters characterising a particular failure mode that exists when the pipeline has reached its limit state. For example, if the limit state is a leak and the failure mode is external corrosion, the limit state function will define a relationship between diameter, wall thickness, yield strength, ultimate tensile strength and internal pressure which results in a leak due to the failure of the remaining ligament associated with a defect of given length and depth. A limit state function is required for each credible failure mode and associated limit state. Limit state functions for the common failure modes of onshore pipelines are well understood and readily available in the literature

6.

UNCERTAINTY ANALYSIS

In any practical engineering circumstances, each of the input parameters to a limit state function is subject to uncertainty. Uncertainties are accounted for in a structural reliability analysis by describing variables in statistical terms. For some parameters the variability may be slight and/or the limit state function may be sufficiently insensitive to the parameter that a single valued estimate of the parameter may be adequate for the study. Best estimates or upper and lower bounds may be used for this purpose depending on the parameter and situation under consideration. For each limit state function, the variability in the sensitive parameters has to be quantified by data analysis and ultimately the construction of probability density functions (pdfs). This is achieved by performing appropriate statistical analyses of the data available from sources including construction records, test certificates and inspection records. The outputs of these analyses are probability density functions, which describe the likelihood of occurrence of specified values of the given 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.

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6.1

Quantification of Variation in Pipeline Geometry

The failure behaviour of a pipeline is dependent on the geometry of the pipeline cross section. 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 and typically has a very small coefficient of variation. Therefore for the purposes of a structural reliability analysis the outer diameter of a pipeline may be assumed to have a single fixed value equal to the nominal value. 6.1.2

Wall Thickness

The wall thickness, w, of the pipeline is subject to variation due to the nature of the manufacturing process. When the pipe is ordered, a nominal wall thickness, wnom, will be requested and a minimum allowable value of wall thickness, wmin, will be specified. Produced pipes are subject to sampling 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 a sample of measured values from mill inspection records. If no information is available to construct a specific wall thickness distribution then it may be assumed that the wall thickness is described by a generic 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 pipeline material. The representative parameters are Youngâ€™s modulus, yield strength, ultimate tensile strength and plain strain fracture toughness. 6.2.1

Youngâ€™s Modulus

The Youngâ€™s modulus of the material is approximately constant and for the purposes of SRA may 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 variation due to the nature of the manufacturing process. Pipe is ordered according to grade, which defines the 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 these minimum values. Pdfs describing the variation in actual values of yield strength and UTS may be found by analysis of a sample of measured values from mill test certificates. If no information is available to construct specific distributions then it may be assumed that the yield strength and UTS are described by generic distributions obtained from a large sample of pipeline data, as given in [8] for instance.

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6.2.4

Plain Strain Fracture Toughness

Pipeline toughness is commonly specified in terms of Charpy energy Cv. However, the limit state functions involving material toughness are formulated in terms of the plain strain fracture toughness, KIC. Various empirical relationships between KIC and Cv exist. A probability density function for Charpy Energy may be found by a statistical analysis of measured values of Charpy energy from mill test certificates. The distribution of fracture toughness can 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 fixed value 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 an excavator bucket. The depth of the dent is known to depend on the applied force, the radius, the wall thickness, the yield strength, the ultimate tensile strength and the pressure. A unique pdf is thus required to describe the uncertainty in dent depth for each pipeline having a particular combination of pipeline parameters. The most direct method of constructing these pdfs would be to perform a statistical analysis of the available data describing dent depth variation on each group of pipelines having the same characteristics. Unfortunately, due to the broad range of combinations of pipeline parameters in existence and the relatively few occurrences of denting, such data are very sparse. For this reason an alternative approach is required. Although there are very few recorded dents on each group of pipelines having the same characteristics, significant data do exist describing the variation in dent depth over the total population of pipelines. It is thus possible to construct a pdf describing this ‘global’ variation and use the relationship between dent depth, applied force, radius, wall thickness, yield strength, ultimate tensile 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 the external interference. Therefore the distribution of dent depths on a pipeline with a specific combination 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 pipeline parameters. The depth of gouge is determined by the nature of the external interference. The variation in gouge depth can thus be determined by single ‘global’ pdf constructed from the depths of actual 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 pipeline parameters. The length of the gouge is determined by the nature of the external interference. The variation in gouge length can thus be determined by single ‘global’ pdf constructed from the lengths of 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 this parameter must thus be described by a time dependent pdf.

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It may be assumed that the corrosion depth growth rate is dependent on the instantaneous depth and thus implicitly dependent on time through the dependence of instantaneous depth on time. The justification for this assumption is that the corrosion products form a protective barrier as the depth increases. This assumption can be used to determine the pdf of corrosion defect depth at any time using a known pdf at some specific time (e.g. the time of the first inspection) and an estimate of growth rate at this time. The growth rate can be estimated from a consideration of the distributions at the times of different inspections. If such information is not available for the pipeline under consideration data recorded on similar pipelines may be used. A detailed description of the application of the above approach 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 quite small compared with the effect of the uncertainty in some of the other parameters. Therefore, it is usually sufficient to represent the pressure by a single deterministic value equal to the maximum operating pressure (MOP).

7.

EVALUATION OF PROBABILITY OF FAILURE

The objective of a structural reliability assessment is to determine the likelihood that the structure 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 is less than or equal to S, given by:

Pf = P [ R − S ≤ 0]

(2)

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

pf ( t ) =

∫∫ p( R , S , t )dRdS

R −S ≤0

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

(3)

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Figure 2 Failure Probability

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

G( x1 , x 2 , ... x n ) = 0

(4)

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

Pf = P [G( x1 , x 2 , ... x n ) ≤ 0] =

∫ ... ∫

f ( x1 , ... x n , t ) dx1 ... dx m

G( x1 , x 2 ,... x n ) ≤ 0

(5) where f(x1, ... xn, t) denotes the joint probability density function at time t. If all of the stochastic variables are independent of one another, the joint probability density function can be replaced by the product of the individual probability density functions. In practice, Equation (5) can rarely be evaluated analytically and recourse is usually made to numerical integration, approximation techniques, or numerical simulation.

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8.

ASSESSMENT OF RESULTS

The procedure outlined above can be used to evaluate failure probabilities for any combination and range of operating conditions. For instance, these can include ranges and combinations of pressures, inspection frequencies and repair criteria. In design situations ranges of design parameters such as wall thickness and material grade can be included. The technique thus allows a comprehensive description of the likelihood of failure to be constructed. The final stage of the process is to form a decision on the fitness-for-purpose based on a 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 at some time during its previous history of safe operation. This failure probability is implicitly regarded as acceptable and used as the criterion for the assessment of future operation. This approach is particularly 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 safe operation. This approach was used to justify increasing the design factor to 0.78 for existing pipelines in the UK. 8.2

Code Calibration

Design codes are generally considered to be based on the ALARP principle. Criteria can thus be constructed by evaluating the failure probabilities for a range of scenarios that are allowed by design codes and making the logical claim that adherence to these criteria will result in design and operating scenarios that are ALARP. This is a valid approach since design codes have generally resulted in safe operation. However, there is increasing recognition that the codes do lead to inconsistencies. Recent SRA studies have revealed that codes are generally over conservative but in some cases under conservative, i.e. the codes are generally related to the ALARP principle but sometimes they are not. 8.3

ALARP

A current Joint Industry Project [10] is recommending criteria that are formally based on the ALARP principle. The proposed criteria are essentially two-dimensional and are expressed as a relationship between risk and the cost per risk averted, see Figure 3. There are three different regions, negligible risk, tolerable risk and unacceptable risk. In the negligible risk region, the risk is regarded as being so low that there is no requirement to 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 demonstrated that that the cost per unit risk averted is above some threshold level. The threshold level increases monotonically 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 if the cost per unit risk averted was tending to infinity.

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Cost Per Risk Averted

Acceptance Regime Tolerable Risk Negligible Risk

Unacceptable Risk ALARP criteria

Riskmax

Risk

Figure 3 ALARP Criteria

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

9.

CONCLUSIONS

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

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

w w w w w w

Establishment of Limit States Identification of Failure Modes Formulation of Limit State Functions Uncertainty Analysis Evaluation of Failure Probability Assessment of Results

Clear definitions of the key terminology have been given and each of the above six elements has been described in detail. Understanding of the first five of the above six elements is well advanced and guidelines for the use of the techniques have been presented. However industry agreement on the assessment of the results of Structural Reliability Analysis is still under development and will be delivered by the JIP described in Section 8.3, which is specifically addressing the issue. This however does not preclude use of the techniques as is demonstrated by the successful uprating of parts of the Transco National Transmission System.

10.

REFERENCES

1. American Society of Mechanical Engineers (1995), Gas Transmission & Distribution Piping Systems, 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 â€“ 1997): A Report by the European Gas Pipeline Incidents Data Group (EGIG), IMechE Conference on Ageing Pipelines, Newcastle upon Tyne, UK 4. Francis, A., Espiner, R.J., Edwards, A.M., Cosham, A. & Lamb, M. (1997), Uprating an In-service Pipeline Using Reliability-based Limit State Methods, 2 nd 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 Limit State 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 Pipelines Using Limit State Design and Quantitative Risk Analysis, 2 nd International Onshore Pipelines Conference, Istanbul, Turkey 7. Francis, A., Edwards, A.M., Espiner, R.J. & Senior, G. (2000), An Assessment Procedure to Justify Operation of Gas Transmission Pipelines at Design Factors up to 0.8, 3 rd International Pipeline Technology Conference, Brugge, Belgium 8. Jiao, G., Sotberg, T. & Igland, R. (1995), Basic Uncertainty Measures for Reliability Analysis of Offshore Pipelines, SUPERB Report STF70 F95212 9. Francis, A., Edwards, A.M. & Espiner, R.J. (2000) A Fundamental Consideration of the Deterioration Processes Affecting Offshore Pipelines Using Structural Reliability Analysis, 19 th International Conference on Offshore Mechanics & Arctic Engineering (OMAE 2000), New Orleans, Louisiana, USA 10. McKinnon, C., Francis, A., Nessim, M., Lamb, M. & Ellinas, C. (1999), Joint Industry Project to Develop Guidance for Structural Reliability and Risk Based Design and Assessment of Onshore Pipelines, 3rd International Onshore Pipelines Conference, Berlin, Germany