Impact of technical standards on results of defect assessment · PDF file4th Pipeline...

4th Pipeline Technology Conference 2009

TÜV Rheinland - Impact of technical standards on results of defect assessment 1

Impact of technical standards on results of defect assessment Author and Presenter: Dipl.-Ing. Wolfgang Schmidt Organization: TÜV Rheinland Industrie Service GmbH Bereich Energiesysteme und Automation Geschäftsfeld Konformität Druckgeräte 51105 Köln Country: Germany Abstract The German technical system of rules for pipelines implements the European guide-lines for the adjustment of the legal system of the member states into national legisla-tion. Extensive detailed regulations have been created which enable an open market for the planning and construction of pipelines. The detailed regulations refer wherever possible to harmonized European standards. The regulations for the operation of long-distance pipelines are however still subject to national regulations. However, unlike the most other national regulations, the German system of rules has no particular standards for the evaluation of defects in pipelines but only determina-tions and requirements for the dimensioning as well as for the inspection of new - faultless - components. These determinations basically apply to the entire operation time of pipelines up to the end of the lifespan. This circumstance can lead to prob-lems e.g. in connection with the evaluation of defects on the basis of the inspection results of intelligent pigs. In this paper we present a procedure developed by TÜV Rheinland, which embodies on the one hand the fundamental requirements of the German system of rules and enables on the other hand a practical method for the evaluation of defects. The evaluation results are compared both with the results obtained by the ANSI/ASME B31 code [7], which is often used as a standard procedure by the in-spection companies using intelligent pigs, and the results according to the detailed RSTRENG procedure [8].



1 German standards and technical rules The German technical system of rules for pipelines implements the European guide-lines for the adjustment of the legal system of the member states into national legisla-tion. Extensive detailed regulations were created which enable the open market for the planning and construction of pipelines. The regulations for the operation of long-distance pipelines are however still subject to national regulations. In Germany, the rules for mineral oil pipelines and high-pressure gas pipelines for the public energy supply are regulated in different laws and decrees. This is historically caused by the fact that the respective supervision authorities are placed on the one hand in the ministry of the environment and on the other hand in the ministry of eco-nomic affairs. The essential regulations on decree level are the „Verordnung über Gashochdrucklei-tungen“ GasHL-VO (decree on high pressure gas pipelines) [1] which applies for gas pipelines of the public energy supply and the „Rohrfernleitungsverordnung“ (decree on long distance pipelines) [2] which applies e.g. for mineral oil pipelines. Technical regulations for high-pressure gas pipelines are settled in the DVGW-rules (Deutscher Verein der Gas- und Wasserfachleute / German society of experts for gas and water) [3]. The corresponding regulations for mineral oil pipelines are settled in the „Tech-nischen Regeln für Fernleitungen“ TRFL (technical regulations for long distance pipe-lines) [4]. The detailed regulations refer wherever possible to harmonized European standards. Important methods of analysis are also specified within the so-called AD2000 system of rules that has been originally created for pressure vessels, but is also widely used for other pressure containing components like e.g. pipelines. The AD2000 rules con-tain detailed regulations for calculation, manufacturing and inspection, materials and special cases like e.g. alternating stresses. 2 Regulations for defect assessment As mentioned above the regulations for the operation of long-distance pipelines are subjected to national regulations. Unlike the most other national regulations, the German system of rules has no particular standards for the evaluation of defects in pipelines but only determinations and requirements for the dimensioning as well as for the inspection of new - faultless - components. These requirements basically ap-ply to the entire operation time of pipelines up to the end of its lifespan. Methods of analysis from abroad like e.g. ANSI/ASME B31G ‘manual for determining the remaining strength of corroded pipelines’ [7], API 579 ‘Fitness for Service’ [9] or BS 7910 ‘guide on methods for assessing the acceptability of flaws in metallic struc-tures’ [10] cannot be applied in Germany without reservation. This circumstance can lead to problems e.g. in connection with the evaluation of defects on the basis of the inspection results of intelligent pigs. In order to be conform to the German system of rules, the calculation approach must be based on an established code or technical rule. Due to its relative complexity, the AD2000-system of rules proves to be a suitable basis.



3 Approaches for defect assessment The following explanation shows a procedure developed by the TÜV Rheinland, which embodies on the one hand the fundamental requirements of the German sys-tem of rules and enables on the other hand a practical method for the evaluation of defects. This method is quoted in the former German code DIN 2413 (1993) [5] and is also described in the DGMK research report 557 (2001) ‘Festigkeitsnachweis von Pipe-lines aufgrund der Messergebnisse von Prüfmolchen’ (proof of pipeline strength based on measurements of inspection pigs) [6]. We compare the evaluation results according to this method both with the results ob-tained by the ANSI/ASME B31 code, which is often used as a standard procedure by the inspection companies using intelligent pigs, and the results according to the de-tailed RSTRENG procedure. The fundamental calculation approach of the TÜV Rheinland method is based on the widely used method of the comparison of areas, which is specified in detail in the German AD2000-code B9 ‘sectors in cylinders, cones and spheres’ [11]. The calcula-tion of the weakening is based on the following universally valid relation:

SK

AA

p p ≤

+⋅

21

σ

(1)

with p = operating pressure [N/mm²] Ap = pressurised area [mm²] Aσ = stressed area [mm²] K = SMYS [N/mm²] S = safety factor [-] This physical principle sets the pressurized cross-sectional areas into balance with the tension supporting cross-sectional areas of the shell. Within the AD2000-code B9 it is used for dimensioning of T-joints or branch pipes. In the present case the defect is considered to be a special case of a branch pipe. Equation (1) is obviously differing from the static equilibrium condition (2) that is used e.g. in the ANSI/ASME B31 Code which reads as follows:

−

−⋅= − ))(/(1

/11

0

0

MAAAA

SS (2)

with S = hoop stress level at failure S = flow stress level of material A = area of defect A0 = defect length x wall thickness M = Folias factor An essential factor for the calculation according to (1) is the so-called ‘range of influ-ence’ or ‘influence length’, which defines according to the AD2000-code B9 the range where the maximum stress at the tapping border declines to the stress level of the



nominal pipe. In fig. 1 we present the sectional view of the tapping region of a T-piece together with the circumferential and longitudinal stresses in the base pipe. Fig. 1: stresses in the taphole area of a T-piece

© Schwaigerer, Festigkeitsberechnung von Bauelementen des Dampfkessel-, Behälter und Rohrleitungsbaus, Springer Verlag This range of influence is calculated for cylindrical pressure vessels according to the following formula according to AD2000 code B9:

b = wtwtDo ⋅− )( (3) with Do = outside diameter wt = wall thickness The comparison of areas according to formula (1) is only performed with the portions which are located within the range of influence that is set up on both sides of the de-fect (value 2 b as shown in fig 2). The implementation of influence range makes the use of a Folias Factor dispensable.



Fig. 2: principle of comparison of areas

We can introduce (see [6]) a dimensionless index number ILD (Index for Longitudinal orientated Defects), which describes the enhancement of the mean stress in the de-fect area in comparison to the nominal stress of a sound pipe

1min

−=alno

defectILDσσ

(4)

Using equation (1) we get two different index numbers on the one hand for internal defects an on the other hand for external defects:

effdefect

effdefectext Awtb

AILD

−⋅⋅=

2 (5a)

wtDD

AwtbA

ILDeffdefect

effdefectext ⋅−

⋅−⋅⋅

=22 0

0 (5b)

The difference between the two formulas results from the fact that for internal defects the defect area Adefect eff becomes part of the pressurised area. We introduce (see [6]) a second dimensionless index number ISL (Index for Static Load), which describes the ratio of the nominal stress of a pipe and the failure stress:

failure

alnoISLσσ min= (6)

In combination with an appropriate failure condition, e.g. a suitable flow stress, we obtain the following failure function (see [6]), which describes the reduction of the bearing capacity of defect pipes in dependence on the defect size given by defect size index ILD:

ILDISL

+=

11 (7)



In fig. 3 we present a diagram, which represents the results of bursting pressure tests [12] together with the ideal failure function (7). Fig. 3: static failure function

When evaluating a bursting pressure test, the load index ISL is calculated using the measured bursting pressure, the unreduced pipe-wall thickness and the effective tensile strength of the pipe material. In order to calculate the defect size index ILD, one needs to know the effective data characterizing the original size of the defect prior to the bursting pressure test. Each pair of indices ISL/ILD determined by a bursting pressure test supplies a measured point. Statistical methods can be used to demonstrate that the failure function almost optimally averages the scatter band of the measured points, which confirms the suitability of the indices and of the failure model. Employing this failure function, we can determine the reduction of the bearing capac-ity of a defect as a function of the defect size. In combination with a suitable safety factor we can determine whether the defect is still fit for operation. If necessary, we can specify a reduced operating pressure at which the defect can still be operated safely. Using the scatter distribution of the test results we can even determine the failure probability of a defect as a function of the defect size and internal pressure loading (see [6]). If detailed information of the defect contour is available, the real defect contour in-stead of the parabolic approach can be used within the defect size index ILD.



The adaptation of the defect contour and the failure condition also enables the as-sessment of other wall thickness reducing defects like sloping laminations and cracks, whereas the assessment of other defect types like dents, ovalities, buckling and ridging is based on fundamentally different calculation approaches. Moreover the TÜV Rheinland method not only enables the determination of the static strength of defects but also the determination of the fatigue strength under dynamic loading. For this purpose we introduce (see [6]) a Wöhler load index IWL, which includes both the dynamic load and the number of load cycles:

)IDLlog(NIWL k⋅= (7) with N = number of load cycles at failure IDL = dynamic load index containing the pulsating stress instead of the static load k = exponent referring to the slope of the Wöhler line Fig. 4: dynamic failure function

This figure is arranged similar to fig. 3 and shows the results of pulsating pressure test of defective pipes together with a dynamic failure function which describes the reduction of the fatigue strength of defects compared with the fatigue strength of sound pipes. The lower line represents a load cycle safety factor of 5, which is in ac-cordance with the requirements of the German system of technical rules.



With this dynamic failure function we can determine the reduction of the fatigue strength of a defect as a function of the defect size. In combination with a given dy-namic load we can determine if the defect is still fit for operation. If necessary we can specify a reduced permissible service life. 4 Comparison of the results of different defect assessment methods In the following the evaluation results obtained by the TÜV Rheinland method are compared both with the results obtained by the ANSI/ASME B31 code, which is widely used as a standard procedure by the inspection companies using intelligent pigs, and the results according to the detailed RSTRENG procedure. The following table shows an exemplary comparison of the assessment results refer-ring to the number of defects being critical and to the recommended interval of a re-peated inspection with an intelligent pig. Tab. 5: comparison of the results of different defect assessment methods

The results come from an inspection of a high-pressure gas pipeline with an intelli-gent MFL-pig, where the inspection results have been evaluated successively with all three methods. The assessment according to the ANSI/ASME B31 manual shows by far the most conservative results and leads to the largest number of defects to be repaired. The consideration of a corrosion rate leads to a non-acceptable number of pipes in need for repair within a period of 6 years. The determination of a reasonable interval for a recurring inspection with an intelligent pig is not possible on this basis. The assessment method according the detailed RSTRENG method leads to a less conservative and realistic number of pipes to be verified. The consideration of a cor-rosion rate leads to a manageable number of defects to be verified within a period of 6 years. This period is also established as a reasonable interval for a recurring in-spection with an intelligent pig, since the number of pipes to be verified increases considerably after that.



The evaluation according to TÜV Rheinland method leads to results, which are basi-cally comparable with those of the detailed RSTRENG method. However, the regard for some particular special features yields even more favourable results. On the one hand it concerns the consideration of the maximum possible in-ternal pressure with respect to the physical limits, which is partially below the permis-sible operation pressure. On the other hand, the defects within corroded areas are assessed separately and an individual corrosion rate is applied for each defect. The results shown above demonstrate that the TÜV Rheinland method is a suitable tool for the evaluation of defects, that is based on the AD2000-code and therefore consistent with the German system of rules and that is diversely applicable and leads to practice-oriented results.



References 1. Verordnung über Gashochdruckleitungen - GasHL-VO

(decree on high pressure gas pipelines) 2. Rohrfernleitungsverordnung

(decree on long distance pipelines) 3. DVGW-Regelwerk

(system of rules of the German society of experts for gas and water) 4. Technische Regeln für Fernleitungen (TRFL)

(technical regulations for long-distance pipelines) 5. DIN 2413 (1993)

’Berechnung der Wanddicke von Rohren gegen Innendruck’ (calculation of the wall thickness of pipes with respect to internal pressure)

6. DGMK research report 557 (2001) ‘Festigkeitsnachweis von Pipelines aufgrund der Messergebnisse von Prüfmol-chen’ (proof of pipeline strength based on measurements of inspection pigs).

7. ANSI/ASME B31G „manual for determining the remaining strength of corroded pipelines“

8. A modified criterion for evaluating the strength of corroded pipe Final report for project PR 3-805 to the Pipeline Supervisory Committee of the American Gas Association J.F. Kiefner, P.H. Vieth Battelle, Ohio; 1989

9. API 579 „Fitness for Service“ 10. BS 7910:1999

Guide on Methods for assessing the acceptability of flaws in metallic structures. Appendix J

11. AD2000-Merkblatt B9 – Ausschnitte in Zylindern, Kegeln und Kugeln 12. [in-house] Tests carried out at TÜV Rheinland for various clients. Data not pub-

lished

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