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Corrosion Fatigue Behavior of Flexible Pipe Tensile Armor Wires in a CO2 Environment
Fabio Santos1,a, Fabio Pires1,b, Richard Clements1,c, Judimar Clevelario1,d, Terry Sheldrake1,e, Luís Felipe Guimarães Souza2,f , Paulo Pedro Kenedi2,g
1 GE Oil & Gas, Rua Paulo Emídio Barbosa, 485 - Quadra 6.1(parte), Módulo 10, Parque tecnológico da UFRJ, Cidade Universitária, Ilha do Fundão, Rio de Janeiro, RJ, 21941-615, Brazil
2 PPEMM - Programa de Pós-Graduação em Engenharia Mecânica e Tecnologia de Materiais, CEFET/RJ, Av. Maracanã, 229 - Maracanã, RJ, 20271-110, Brazil
[email protected], [email protected], [email protected],
[email protected], [email protected], [email protected], [email protected]
Keywords: CO2, Flexible Pipe, Fatigue
Abstract. The new offshore areas being explored in Brazil presents higher concentration of CO2
compared with most existing offshore fields. The presence of these more aggressive environments
has led to the development of new technologies. Due to the construction characteristics of flexible
pipes, any increase in CO2 concentration in the conveyed fluid will, in turn, increase the CO2
concentration in the pipe annulus, subjecting the metallic armor layers to a more aggressive
environment. Evaluation of the CO2 effects of corrosion fatigue behavior in tensile armor wires is
therefore of vital importance. A comprehensive corrosion fatigue experiment for tensile armor wires
in environments up to 10 bar of CO2, has been established and the experimental results have shown
a fatigue life reduction in the tensile amour wires due to higher levels of CO2.
Introduction
Unbonded dynamic flexible risers are used in offshore developments connecting subsea
structures with surface floating production units. These flexible pipes have a composite structure
comprising helically-wound metallic wires and tapes, and extruded thermoplastics. Fig. 1 presents a
typical rough-bore (carcass containing) flexible pipe structure.
Figure 1. Typical flexible pipe structure
It is well known that due inherent materials characteristics, flexible pipes are susceptible to gas
permeation through their polymeric pressure barrier into the pipe annulus (defined as the space
between the pressure barrier and the outer sheath).
Tensile Armor Wires
Anti Friction Layers
Outher
Sheath
Pressure Armor
Pressure Barrier
Carcass
Materials Science Forum Vol. 758 (2013) pp 77-82Online available since 2013/Jun/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.758.77
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 130.207.50.37, Georgia Tech Library, Atlanta, USA-14/11/14,17:48:36)
When this permeated CO2 is associated with condensed water, or seawater, which ingresses into
the pipe annulus due to an outer sheath breach, a corrosive environment is formed diminishing the
fatigue strength of the metallic armors inside the pipe annulus.
The CO2 concentrations normally experienced by the industry are around 2%, leading to a
maximum partial pressure of 2 bar CO2 in the pipe annulus. However, there is an expectation that
CO2 concentrations will increase in the produced fluids from deeper reservoirs. The high increase in
the CO2 partial pressure expected in the pipe annulus will lead to more severe environments,
impacting on the fatigue service life of flexible risers. A research program has been established to
improve understanding of the deleterious effects of CO2 on the fatigue resistance of the tensile
armor wire of flexible risers.
Experimental procedure
The experimental procedure consists of performing fatigue tests to obtain SN curves for the
steel armor wires. A test program has been established to evaluate the material’s fatigue behavior
under different test environments as in Santos [1] and Santos et al [2]. To implement testing at high
CO2 pressures, a test fixture and a fatigue test pressure vessel, shown in Figure 2, were developed.
The test fixture was developed to accommodate four flexible pipe tensile armor wire test specimens.
Except for cleaning and degreasing, no other surface preparation was performed on the test
specimens. Since there is no observation window in the pressure vessel to assess the failure of any
specimen, a load cell was positioned in series with the load actuator to indicate the failure of each
specimen by a step-like reduction of load.
The experimental apparatus consisted of an Instron servohydraulic materials test machine, a
data acquisition system, Kyowa MCG-6, and a microcomputer with LabView software. The CO2
pressure was monitored to assure the maintenance of the correct test environment. Additionally, the
test program was accomplished in a controlled humidity and temperature environment.
To establish a reference for the test program, a preliminary set of tests was performed using a
test environment with aerated seawater. Further test environments consisted of de-aerated seawater
associated with CO2 at different partial pressures, as shown in Table 1. In order to reduce the test
scatter resulting from material variability, the test specimens were taken from a single material
batch (yield strength of 1230 MPa and ultimate strength of 1420 MPa), with 9 x 3 mm cross-
section; the chloride content was maintained at 115,000 ppm.
Table 1. Fatigue tests parameters
The selection of the fatigue test loading frequency plays an important role in the correct
assessment of the corrosion-fatigue SN curves. Experimental results for the assessment of the
corrosion-fatigue behavior of flexible pipe tensile armor wires, published by Berge et. al. [3],
indicates that corrosion fatigue test loading frequencies of between 0.5 and 2.0 Hz, provide
equivalent and realistic results. A frequency of 1 Hz was therefore selected for this research.
While much has been contributed to the research of corrosion fatigue areas, Berge et. al. [4] paid
particular attention to the effects of corrosive environments in the pipe annulus. Woolin et al. [5]
presented a good description of various types of fatigue tests using offshore steels. Horstmann et al.
[6] measured fatigue growth rates for offshore steels for freely corroding and cathodically polarized
environments. The influence of testing frequency and specimen thickness were also accessed.
As can be seen in Fig. 2.a, the loading method adopted in this program has been based on a
standard four-point bending displacement controlled test method, conventionally used by the
industry for fatigue tests of tensile armor wires. The test fixture was designed in such a way that any
Curve Test Media
CO2
Partial
Pressure
(bara)
Iron ion
saturation Status
A Aerated seawater ------- NO Complete
B Deaerated seawater 3 NO Complete
C Deaerated seawater 10 NO Complete
78 Functional and Structural Materials II
electrical contact between the test specimen and the test fixture was avoided and simultaneous
testing of four specimens was made possible. Fig. 2.b shows the corrosion fatigue pressure vessel,
used in the test program, mounted in the test machine.
(a) (b)
Figure 2. (a) Four point bend test fixture and (b) Fatigue test apparatus
Five different stress ranges were selected for this research program. The stress ranges were
defined to allow accurate definition of the SN curve’s slopes. In order to achieve an even
distribution in the construction of the SN curves, the stress ranges were selected between 30% and
90% of the actual yield strength.
The tests were conducted within the parameters presented in the experimental procedure section,
and performed until the failure of all four specimens assembled in the test fixture, or until a
maximum number of cycles had been reached. After 107cycles the test would be stopped, the setup
disassembled and the samples inspected for cracks.
Fig. 3 presents a typical aspect of the test specimens after the end of the test.
(a)
(b)
Figure 3. Typical specimen aspect after test (lateral and bottom views) for: (a) Deaerated Seawater
+ 3 bar CO2 and (b) Deaerated Seawater + 10 bar CO
Materials Science Forum Vol. 758 79
Fatigue Analysis
The SN curve can be expressed by a logarithmic form of the stress range (∆S) and the fatigue
life (N) presented in (1), where ’a’ and ’b’ are constants, experimentally determined for each test
environment. The guidelines established in ASTM Standard E739 [7], have been followed to
generate the linear logarithmic SN curve, as expressed in Eq.1 from the test data.
( ) ( )S
baN ∆⋅+= loglog (1)
The derivation of the SN curve was based on the effective stress range experienced by the test
specimens during the fatigue test. The effective stress range is obtained through mean stress
correction of the applied test stress ranges; the Goodman equation was adopted to perform this
mean stress correction. The evaluation of the experimental test data was taken with a confidence
level of 95.4%. Veers [8] proposed an analysis method where intercept parameter ’a’ is considered
as being a random variable in linear proportion to (∆S), which behaves as per normal distribution. It
is possible to determine the recommended confidence level 95.4% (a95%) using Eq. 2. The lower and
upper limits of the confidence intervals are defined as two parallel lines to the assessed mean SN
curve with a difference in its intercept value (a95%).
STEaa ⋅±= 2%95 (1)
Where STE is the standard deviation of the dependent life, the SN design curve is:
(3)
Results
With the assessment of the SN curves for the test environments shown in Table 1 and the
fatigue analysis discussed above, it was possible to experimentally determine the two standard
deviation off-set design SN curves for each of the three test environments, as shown in Fig. 4.
(a) (b)
(c)
Figure 4. SN curves assessed during the test program: (a) Aerated Seawater SN Curve, (b) De-
aerated Seawater SN Curve + 3 bar CO2 and (c) De-aerated Seawater SN Curve + 10 bar CO2
80 Functional and Structural Materials II
When both SN curves were assessed in de-aerated seawater with 3 bar and 10 bar CO2,
respectively, curves ’b’ and ’c’ had a slighter difference in the curves slope, indicating that the
increase in the CO2 partial pressure in the test environment intensified the severity of the media,
reducing the fatigue resistance of the tensile armor wires. Despite being discrete, the slope change
in the SN curve affects the overall performance of the flexible pipe. It is therefore recommended
that service conditions be carefully evaluated by means of a sensitivity analysis using the
experimental data to assess the resulting service life for each environment.
Impact on the Service Life of Flexible Risers
In order to evaluate the impact of slope change of the design SN Curves assessed in both 3 and
10 bar CO2 partial pressure, a series of global and local fatigue analyses were performed, resulting
in reduction of the overall fatigue service life as a function of the CO2 partial pressure variation.
The design SN Curve, assessed for the aerated seawater environment (curve ’A’), was used as
reference. The value of 100% of the service life was attributed to the results of fatigue analysis
performed for this pipe.
Fig. 5 presents the results of the comparative analysis performed for the selected flexible pipe,
considering the aerated seawater SN curve (A) as reference, the 3 bar SN curve (B) and the 10 bar
SN Curve (C). Service life for curve ’B’ and curve ’C’ have decreased in comparison to reference
curve ’A’. Curve ’B’ has a decrease of 21% in service life, whereas curve ’C’ has a decrease of
29% in service life, confirming that increasing pressures of CO2 can be detrimental to the service
life of the pipe.
79%
71%
10
0%
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
A B C
SN Curve
Serv
ice L
ife
Figure 5. Service life as a function of the selected SN curve
Conclusions
The proposed test methodology was used to detect the influence of CO2 partial pressure on the
fatigue resistance of tensile armor wires and was seen as a useful tool to generate design data for
pipes designed to operate with fluids presenting high concentrations of CO2.The reduction in
service life, calculated as a result of CO2 partial pressure, should not be disregarded for design
purposes and the incorporation of experimental data in design tools enhances the reliability of the
riser systems. The test program shall be extended to confirm the detrimental tendencies of higher
CO2 partial pressures on the overall service life of flexible pipes.
Acknowledgements
The authors would like to thank GE Oil & Gas and CAPES for their continuous support of this
program and the Brazilian National Institute of Technology (INT), which was responsible for the
execution of the experiments.
Materials Science Forum Vol. 758 81
References
[1] F. P. Santos: Avaliação dos Efeitos da Pressão Parcial de CO2 no Processo de Corrosão-Fadiga
em Armaduras de Tração de Dutos Flexíveis, Master Degree (in Portuguese), Centro Federal de
Educação Tecnológica Celso Suckow da Fonseca (2011).
[2] F. P. Santos, F. Pires, R. Clements, ; J. Clevelario, T. Sheldrake, L. F. G. Souza, P. P. Kenedi:
Evaluation of the Effects of CO2 Partial Pressure on the Corrosion Fatigue Behavior of Flexible
Pipe Tensile Armor Wire, Offshore Technology Conference - OTC 2011, Houston – Texas
(2011).
[3] S. Berge, N. K. Langhelle and T. G. Eggen: Environmental Effects on Fatigue Strength of
Armor Wire for Flexible Risers, OMAE2008-57132, Portugal (2008).
[4] S. Berge, E. Bendiksen, J. Gudme, K. Langhelle and R. Clements, in: Corrosion Fatigue Testing
of Flexible Riser Armor Procedures for Testing and Assessment of Design Criteria,
OMAE2003-37327, Mexico (2003).
[5] P. Woollin and R. Clements: Fatigue Crack Propagation in C-Mn Steel Haz Microstructures
Tested in Air and Seawater, OMAE1998-2601, Portugal (1998).
[6] M. Horstmann, J. K. Gregory and K. H. Schwalbe: Int. J. Fatigue Vol. 17 n. 4 (1995), pp. 293-
299.
[7] ASTM Standard E739, in: Standard Practice for Statistical Analysis of Linear or Linearized
Stress-Life (S-N) and Strain – Life (ε-N) Fatigue Data (2004).
[8] P. S. Veers, in: Fatigue Strength Prediction and Analysis, ASM Handbook, Fatigue and
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ASM International (1996).
82 Functional and Structural Materials II
Functional and Structural Materials II 10.4028/www.scientific.net/MSF.758 Corrosion Fatigue Behavior of Flexible Pipe Tensile Armor Wires in a CO2 Environment 10.4028/www.scientific.net/MSF.758.77
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