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1 Copyright © 2014 by ASME
TUEV NORD CONCEPT LOOP -
LIFETIME OPTIMISATION OF PIPELINES
Ralf Trieglaff TÜV NORD SysTec GmbH & Co. KG
Hamburg, Germany Email: [email protected]
Christian Schrandt TÜV NORD SysTec GmbH & Co. KG
Hamburg, Germany Email: [email protected]
Axel Schulz
TÜV NORD SysTec GmbH & Co. KG Hamburg, Germany
Email: [email protected]
Mayk Schulz IGN GmbH & Co. KG Greifswald, Germany
Email: [email protected]
ABSTRACT
LOOP is a concept to evaluate corroded or damaged
pipelines based on detailed data from UT-pigging. The
procedure of LOOP delivers a 3D-model generated from the
data of a commercial in-line inspection tools (ultrasonic,
magnetic flux). This makes it possible to use the full
functionalities of the relevant finite element software like
evaluation of wall-thinning (LOOP 1) and fracture mechanics
analysis to evaluate cracks in the wall (LOOP 2). In this paper is
given the basic ideas of the LOOP concept, where the main
focus is directed to the LOOP 1 assessment procedure.
Based on a real example of a corroded pipeline is
demonstrated the assessment procedure, which is based on an
elastic-plastic analysis of a real inner contour of the corroded
surface transferred in the finite element geometry model. The
unique element is that the surface data of the UT-pigging is used
directly to generate the geometry model in the FE-software
ANSYS. The assessment procedure is validated by a burst
pressure test of a corroded pipeline. The result of the burst
pressure test is compared with the calculated limit load from an
elastic-plastic analysis based on measured material properties.
Additionally, the assessment procedure is compared with the
results of a limit load analysis based on DIN EN 13445-3 and
with the results of the standard assessment procedure. At the end
the assessment procedure is compared with the procedure given
in API 579-1 standard.
NOMENCLATURE
A Elongation at fracture
Da Outer Diameter
E Elastic modulus
KB Stress characteristic number
KBmax Maximum permissible stress characteristic number
KFL Fault characteristic number
Lf Length of the damage
pi Internal pressure
pmax Maximum allowable internal pressure
RMd Fictive flow stress
Rp Yield strength
Rm Tensile strength
Tf Depth of the damage
sn Nominall wall thickness
sf Minimum wall thickness
v Poisson ratio
σn Nominal stress
Proceedings of the ASME 2014 Pressure Vessels & Piping Conference PVP2014
July 20-24, 2014, Anaheim, California, USA
PVP2014-28755
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Copyright © 2014 by ASME 2
INTRODUCTION
A pipeline is exposed to a number of operational and
environmental conditions during its life cycle. It can experience
various types of damage, such as corrosion on the interior and
exterior surfaces caused by transport medium or a envelope.
Further can occur geometric abnormalities like buckling due to
external influences or cracks due to manufacturing and/or cyclic
loadings (see Figure 1). To determine the state of the pipeline
UT pigging is periodically used for inspection. Then it is
necessary to analyze the data obtained and to evaluate the
findings. The time at which repairs are to be made is a central
question.
With the LOOP-concept the service life of damaged
pipelines can be calculated in advance. LOOP is based on FE-
methods and methods of fracture mechanics, and is particularly
well-suited for lines of various dimensions subject to pressure
and temperature stresses, such as those for natural gas pipelines,
oil pipelines and product pipelines.
In this report, the use of LOOP for determining the
maximum limit load of a defect is presented on the basis of a
real failure in a practical application. In addition, a comparison
of the analysis results with the results of an evaluation method
frequently used in Germany is performed, and a validation of the
method on the basis of a burst pressure test is presented.
Appropriate reserves are indicated in the evaluations and the
consistency of the proposed concept with the requirements of the
API 579-1 standard is presented.
Fig. 1: Possible damage to a pipeline in the life cycle.
PRESENTATION OF THE TÜV NORD LOOP® CONCEPT
In order to reduce the scope of reconstruction of pipelines
and to be able to better estimate the service life, a more precise
evaluation of failure can be performed with LOOP as a
supplement to the standard procedure.
For this purpose, special procedures and calculation methods
based on FE- analysis are used. LOOP consists of two modules
(see Fig. 2), which evaluate reduction in wall thickness
according to an innovative procedure include an evaluation of
crack-like defects with Module 2. With these modules, a detailed
evaluation of nearly all defects occurring in a pipeline is made
possible in order to assure integrity and to make more accurate
service life predictions.
LOOP 1 LOOP 2
Evaluation of reductions in
wall thickness with FEM
Evaluation of cracks caused
by the manufacturing
process and operational
conditions
3D modeling
Modeling of the failure on
the basis of the measurement
data from the UT pigging
+
Plastic collapse analysis
Determination of the safety
against plastic collapse in
the area of the failure
+
Fatigue estimation
Determination of the fatigue
strength under the operating
conditions to be expected
Static verification
Fracture-mechanical
evaluation of cracks according
to API 579-1/ASME FFS-1
+
Crack growth
Calculation of crack growth
under the operating conditions
to be expected
Fig. 2: Structure of the LOOP concept
STANDARD EVALUATION OF A FAILURE
The procedure and the results from the use of the LOOP
concept (LOOP 1) are presented below on the basis of an
example from a practical application.
Large-scale material losses over greater lengths were
detected during the inspection of a product pipeline by means of
an ultrasonic test pigging. In the evaluation of the pigging data it
was determined that, on an abnormality on the interior side,
there was a maximum material loss of 3.4 mm with a measured
wall thickness of 7.2 mm. The abnormality extended on the
interior pipe wall in a range between 300° and 360° with a
maximum length of 2052 mm and a maximum width of 231
mm. This abnormality was classified as critical in the scope of
the preliminary evaluation and had to be evaluated more closely
in regard to its safety against plastic collapse.
For visualization of the described abnormality, the damage
pattern of a similar abnormality from the same pipeline section
is shown in Figure 3.
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Fig. 3: Presentation of a similar failure from the same
pipeline section
Specifications for the failure are listed in Table 1. The
specifications were taken from the ultrasonic scan (UT scan)
shown in Figure 4.
Tab.1: Specifications for the failure from pigging data
Abnormality specifications
Value
Inside/outside location: Inside (material loss)
Length [mm]: 2 052
Width [mm]: 231
Depth [mm]: 3.4
Remaining wall [mm]: 3.8
Wall thickness [mm]: 7.2
Circumferential position start
[degrees]: 300 °
Circumferential position end
[degrees]: 360 °
Fig. 4: UT scan of the analyzed abnormality
Additional important specifications for the inspected
pipeline, which were partly used for the calculation, are listed in
Table 2 below.
Tab.2: Dimensions and operating parameters of the pipeline
Line data
Value
Product conveyed Liquid
Pipe Welded longitudinal seam
Outside diameter 457.0 mm
Wall thickness 7.2 mm
Material St 53.7
Minimum yield strength 360 N/mm²
Minimum tensile strength 510 N/mm²
Design pressure 46.5 bar
Max. operating pressure 42.0 bar
Pigging test UT pigging
STANDARD EVALUATION OF THE FAILURE
For the standard evaluation of critical failures in pipelines,
the evaluation process according to Mackstein and Schmidt [1]
is frequently utilized in Germany. This process is largely
empirical and is based on the results of burst and swelling
pressure tests. Although this process is very simple in use, it
provides very conservative evaluation results, as is shown
below.
Tab.3: Boundary conditions for the calculation
Tensile strength Rm = 510 MPa
Outside diameter of the pipeline Da = 457 mm
Nominal wall thickness sn = 7.2 mm
Minimum wall thickness sf = 3.8 mm
Internal pressure pi = 4.2 MPa
Depth of the damage
(maximum expansion)
Tf = 3.4 mm
Length of the damage Lf = 2 052 mm
For the evaluation of the aforementioned failure, a so-called
fault characteristic number (KFL) and stress characteristic
number (KB) are calculated with this process by means of the
specified formulae. The combination of the two characteristics
provides an evaluation point in the diagram (Fig. 5) with which
a statement about the reliability of the defect can be made
immediately. In the Following the formulas to calculate the fault
characteristic number and stress characteristic number is given
below.
n
pi Da sn 2
(1)
KBn
Rm
(2)
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Copyright © 2014 by ASME 4
f
pi Da sf 2
(3)
KFLf
n
1
(4)
If the calculations are now performed with the indicated
formulae (1) – (4), the results are a value of 0.26 for the stress
characteristic number KB and a value of 0.89 for the fault
characteristic number KFL. When entered into the evaluation
diagram (Fig. 5), it can be determined that this failure is close to
the burst pressure safety line 1.8 and can thus be classified as
critical.
Fig. 5: Loading capacity of pipes with longitudinally-
oriented crack-like defects with static internal pressure (3R
International 34 Mackenstein/Schmidt) [1]
For the calculation of the maximum allowable internal
pressure according to this process, a maximum allowable stress
characteristic number KBmax is taken from the diagram on the
basis of the fault characteristic number KFL. The stress
characteristic number KBmax is arrived at with a consistent fault
characteristic number KFL and on reaching the fracture safety
line 1.8.
pmax
KBmaxsn 2 Rm
Da
(5)
With (5) a maximum allowable internal pressure pmax of 46.6
bar can now be determined. In comparison to the maximum
operating pressure of 42 bar, a safety of only 4.6 bar is provided,
and therefore a decision was made to repair the damaged area of
the pipeline.
EVALUATION OF THE FAILURE WITH LOOP 1
The failure described above is evaluated in the following by
means of the LOOP 1 process. In this connection, an FE model
is created in an initial step by means of realistic fault geometry
based on the measurement data of the UT pigging. Expanding on
this FE model, an analysis of the maximum collapse loads of the
failure is performed by means of two different processes. Then a
fatigue analysis is performed for an estimation of remaining
service life.
3D-MODELLING OF THE FAILURE
In order to perform an analysis with as much detail as
possible, it is essential that the damaged area is factored in as
realistically as possible. This is achieved by means of a realistic
3D modeling of the defect geometry. However, for an exact
modeling, the pipeline must have been inspected by means of
ultrasonic measurement to determine the weakening of the wall
thickness.
The benefits of the UT process are that the wall thickness of
the entire pipeline is recorded in defined measuring grids (e.g.
1.5 mm in the axial direction and 8 mm in the radial direction)
and a highly precise measurement can be achieved. However, an
essential requirement for the modeling is that all the
measurement data can be output in one file. An appropriate
interface for the transfer of the measurement data must be
agreed upon with the pigging company in advance. The
sequence for the model generation is schematically represented
in Fig. 6.
Fig. 6: Sequence for the automatic model generation
However, before the data is processed it must first be filtered
and prepared. Implausible measurements and corresponding
outliers which would falsify the result are thereby eliminated.
Since it is a relatively large defect in this example, there is also a
correspondingly large amount of data available. Around 66,000
data sets (individual measurements) must be processed for the
conversion. For the conversion of the measurements into a
usable 3D model, special software has been developed which
saves a great deal of time and essentially provides a better model
in comparison to previous manual model creation. The generated
3D model, see Fig 7, now serves as a basis for an optimized and
realistic calculation of the maximum capacity for this damage
area, as well as a fatigue analysis.
Fig. 7: 3D model of the failure (left), as well as a detailed cut-
out of 16 mm (right)
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DETERMINATION OF THE COLLAPSE LOAD
For the evaluation of the static safety of the fault in
consideration, two different approaches are addressed below.
Process I is oriented towards the processes frequently used in
Germany, wherein the fault is evaluated on the basis of the
local/global stress analysis according to German AD-Standard
S4, German standard DIN 2413 as well as scrutinizing literature
[2]. Process II is based on the loading capacity analysis
mentioned in DIN EN 13445-3 Annex B [3]. In the Following
we present at first the evaluation process I and subsequently the
evaluation process II.
In order to be able to evaluate a reduction in wall thickness
as permissible, all the following conditions must be met for the
first process:
a. The maximum comparison stress at nominal pressure is
less than the tensile strength of the material.
b. At nominal pressure, only a locally limited plastic
deformation should occur in the area of a magnitude of
the wall thickness.
c. The safety factor against plastic collapse must be at least
1.8.
FEM-code ANSYS [4] is used for the calculations and the
following procedure and boundary conditions are considered for
Process I:
FE model with real abnormality geometry
Elastic-plastic material behaviour (multi-linear material
law)
Geometrically non-linear
In addition, the following material characteristics were taken
as a basis for the creation of the material model. The minimum
specifications indicated in accordance to the material standard
were applied.
Tab. 4: Mechanical-technological specifications for St 53.7
Material St 53.7
Standard values
Yield strength [Rp]: 360 MPa
Tensile strength [Rm]: 510 MPa
Elongation at fracture [A]: 20.0 %
E-module [E]: 200 000 MPa
Poisson ratio [v]: 0.3
Fig. 8: Applied material model for the elastic-plastic analysis
according to Process I
In order to determine the plastic collapse load of the real
modeled failure, in the scope of the calculation the internal
pressure is increased until the solver of the program no longer
achieves any convergence due to the occurring plastic strain and
the calculation is discontinued. A representation of the complete
model can be seen in Fig. 9.
Fig. 9: Representation of the complete model and result of
the elastic-plastic analysis according to Process I
A maximum pressure of 117 bar was determined based on
this process. If an operating pressure of 42 bar is set, there is a
safety of 2.8 against plastic collaps. If a collaps safety factor of
1.8 would be used as a basis for this fault, a permissible
operating pressure of 65 bar would arise through conversion of
the formula (6). Therefore the criterion c) is fulfilled.
(6)
In addition, the plastic deformation must be considered for
the fault area, which should only occur to a locally limited
extent at nominal pressure within the fault area. For this
purpose, the course of the plastic expansion over the pressure is
shown in Fig. 10 for the range of 0 – 2 %. It is recognizable that
no noteworthy expansions occur within the fault at an operating
pressure of 42, in which case the criterion b) would be fulfilled.
For visualization, the 0.2% expansion limit is entered, which
represents the limit between the elastic and plastic deformation.
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Fig. 10: Result of the elastic-plastic analysis according to
Procedure I, course of the plastic expansion from 0 – 2 %
above the internal pressure
In order evaluate the last criterion, a linear-elastic analysis
was performed (Fig. 11). In this figure is shown the equivalent
stress distribution. They must lie below the tensile strength of
the material at operating temperature, in which case the last
criterion is also fulfilled.
Fig. 11: Representation of comparison stresses at 42 bar
internal pressure
The analysis has shown that comparison stresses of a
maximum of 460 N/mm² occur in the area of the failure at a
nominal pressure of 42 bar. Since this is well below the
indicated tensile force of 510 N/mm², the criterion a) is also
fulfilled and the verification of the static safety of the fault
according to Procedure I is provided.
For comparative purposes, a limit load analysis in
accordance with the process given in DIN EN 13445-3 (Annex
B) was performed as Procedure II. In comparison with the
preceding analysis (Procedure I), the following boundary
conditions were considered for the limit load analysis according
to DIN EN:
Linear elastic ideal plastic material law (without
hardening)
Tresca flow criterion (main stress hypothesis)
Fictive flow stress RMd
Maximum plastic expansion limited to 5 %
Partial safety factor of 1.2 on the stress
For the limit load analysis, with a yield strength Rp of 360
MPa, a fictive flow stress RMd of 249.4 MPa arises. This lies
well below the yield strength of the material and is calculated
according to the formula (7).
(7)
In order to determine the plastic collapse load of the real
modeled failure, in the scope of the calculation the internal
pressure is increased until the solver of the program no longer
achieves any convergence due to the occurring plastic
expansions and the calculation is discontinued. In the result of
this analysis, the last convergent load step was reached at 69.5
bar. Since the plastic expansion must be limited to 5 %, a
maximum ultimate pressure of 68 bar and a allowable pressure
of 54 bar arise.
If Procedure I and II are then compared with each other, it
can be seen that a higher maximum allowable operating pressure
is possible than calculated with the standard evaluation. The
static safety of the failure geometry could therefore be verified.
For the Procedure I a considerable reserves of approx. 20 bar
was estimated and for process II approx. 10 bar with Procedure
II could be discovered in comparison with the standard
evaluation.
VALIDATION OF THE CALCULATION RESULTS
The validation of the calculation procedures was performed
by means of a comparison of the calculation results with an
experimentally determined burst pressure. For this purpose,
however, an additional elastic-plastic FE-analysis had to be
performed first. For the calculation and the comparison, it is
essential that true material behavior is taken into consideration
in the FE simulation. In order to determine the influence of the
material, the mechanical-technological characteristics of the
material used here were determined by means of tensile tests.
The tensile tests which were performed in the process provided
the material characteristics mentioned in Tab. 5 below. The
minimum characteristics according to standard are likewise
indicated for the comparison.
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Tab.5: Mechanical-technological characteristics
Material St 53.7
Values determined
from tensile tests
Standard
values
Yield strength [Rp]: 381 MPa 360 MPa
Tensile strength [Rm]: 558 MPa 510 MPa
Elongation at fracture
[A]:
34.3 % 20.0 %
E-module [E]: 200 000 MPa 200 000 MPa
Poisson ratio [v]: 0.3 0.3
On the basis of the determined stress-strain curve from the
tensile test, a material-specific model was then derived for the
calculation of the fracture pressure; see Fig. 12, Measurements
(red curve). The following boundary conditions were taken into
consideration in the calculation with the FEM code ANSYS.
FEM model with real abnormality geometry
Elastic-plastic material behaviour with hardening
Large deformations are permitted
Convergence of the material curve to the material data
from tensile tests
Fig. 12: Derived material model for the analysis of the plastic
collapse load (violet line)
In the result of the elastic-plastic analysis, a fracture pressure
of 132 bar was determined (see Fig. 13).
Fig. 13: Result of the elastic-plastic analysis for
determination of the plastic collapse load, course of the
plastic strain above the internal pressure
In order to investigate the maximum pressure at which the
damaged pipeline actually fails, the removed pipeline section
with a length of approx. 8000 mm was subjected to a pressure
test with water.
The failure of the pipeline, see Fig. 14 to the right, occurred
at an internal pressure of 136 bar.
Plastic collapse load:
132 bar
Burst pressure:
136 bar
Fig. 14: Comparison of the results of the FE-analysis with
the burst pressure test
In a comparative ratio of the experimentally determined
burst pressure to the analyzed plastic collapse pressure, the
result for this validation example is a deviation of 3 %.
FATIGUE ANALYSIS
For lines which are exposed to a pulsating stress, as is
frequently the case with lines carrying liquids, a fatigue analysis
must also be performed in order to evaluate the damage from
cyclical loads. On the basis of a fatigue analysis, the service life
evaluation takes place based on the material fatigue with respect
to the crack formation. For an estimation of the service life,
however, the following boundary conditions must be taken into
consideration:
the load of the pipeline in the past as well as in the
future
growth of the fault in the past as well as in the future
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The stresses also change on the basis of the diminishing
residual wall thickness due to the growth of a fault with
increasing age. For an estimation of the service life, this change
over time should be taken into consideration. This can take place
by means of further calculations, in which, for example, the
erosion due to corrosion in annual stages can be accounted for.
In this example the influences from the changing fault were not
factored (in conservatively).
The cyclical load of the pipeline takes place with a changing
internal pressure. This internal pressure fatigue stress results
from the following loading conditions in this case.
Tab. 6: Loading condition
Loading condition Range
[bar]
Cycle
[/year]
Start and shut down 0 - 42 2
In service fluctuation 42 – 26 – 42 730
With a starting process, the pressure fluctuates in the positive
range between 0 and 42 bar, and/or 0 and y bar with an output
adjustment (pressure fatigue stress, 0<R<1).
For these load conditions, the significant stresses
(circumferential stresses) are calculated in the scope of a linear-
elastic analysis.
Fig. 15: Result of the elastic analysis at 42 bar internal
pressure, representation of the comparison stresses
On the basis of the determined stresses, we have evaluated
this in accordance with the notch stress concept of DIN EN
13445 [3] and determined a permissible number of load cycles
of 300 000. When this number is compared with the actually
occurring and predicted cycles, consideration of the effect of the
changing abnormality becomes superfluous in this case. In
principle, the result of the fatigue analysis can still be assured by
a prediction of the progression of corrosion to be expected. In
the present calculation example, we have dispensed with this.
CLASSIFICATION OF LOOP 1 IN THE EVALUATION PROCESS OF API 579-1 STANDARD
According to API 579 / ASME FFs-1 [5], there is a different
level for the evaluation of abnormalities. In this connection,
Level I represents the lowest evaluation level at which a very
conservative initial evaluation is based on very simplified
assumptions.
Level II, on the other hand, is a detailed evaluation with less
conservativeness and improved results in comparison to level I.
Level III, in turn, is the highest evaluation level. In API 579-
1, 4.4.4 [5], it is shown that an evaluation according to Level III
is usually based of finite element analyses, wherein non-linear
analyses are given priority for determining the ultimate bearing
capacity in accordance with Annex B1. In addition, explicit
reference is made to a direct transfer of the measured wall
thickness profiles to the FE geometry. Therefore, the process
presented here consistently implements the Level III evaluation
procedure. The procedures presented in Annex B1 of API 579-
1[5] for the evaluation of the safety in regard to plastic collapse
in the form of a limit load analysis method and an elastic-plastic
stress analysis method correspond to the procedures I and II
presented in chapter 5.2 for determining the maximum capacity.
Only an adjustment of the safety factors to be used in regard to
Process I has to be considered.
CONCLUSIONS AND SUMMARY
The following was presented in this article:
The Loop concept of the TÜV NORD Group is a very
innovative procedure for the evaluation of wall-
thinning in pipelines and power plants. The concept is
based on a Level 3 verification corresponding to the
requirements of the American regulation API 579 /
ASMIE FFS-1.
In LOOP, an accurately-detailed CAD/FEM model is
generated automatically and affordably from the
measurement data of an ultrasonic test pigging using a
special converter software. The use of the very realistic
FEM model enables the use of the full potential of
inelastic FEM analyses (ultimate bearing capacity) for
the evaluation of wall-thinning. As a result, significant
reserves are revealed in comparison with the simplified
evaluation process according to Level I and II.
The process according to the LOOP concept was
demonstrated using a practical example. In the process, a
computer-permitted operating pressure increase of +35% greater
than Level I/II verification was achieved.
A comparison of the fracture pressure calculations (ultimate
bearing capacity analysis) according to LOOP with the results of
a fracture pressure test revealed a good match of the test and
calculation (deviation: 3%).
Preparation of the LOOP concept for large-scale use is
currently under way.
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REFERENCES
1. Mackenstein, P.; Schmidt, W.: „Beurteilung der Festigkeit
von fehlerhaften Pipelinerohren – Verfahren und
Bewertungskriterien.“, Journal 3R International 34 (1995)
Page 667-673
2. Engbert, F.; Engel, A.; Steiner, M.: „Erfahrungen bei der
Bewertung von Wanddickenminderungen mit der Methode
der Finiten Elemente“, Journal 3R International 40 (2001)
page 642-644
3. DIN EN 13445-3; „Unbefeuerte Druckbehälter – Teil 3:
Konstruktion“; German edition EN 13445-3:2009
4. ANSYS FEM-Software Version 14.5
5. API 579-1/ASME FFS-1, Fitness-For-Service, June 5, 2007
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