Rio Oil & Gas Expo and Conference 2020
ISSN 2525-7579
Conference Proceedings homepage: https://biblioteca.ibp.org.br/riooilegas/en/
Technical Paper
Evaluation of the SCC-CO2 susceptibility of
flexible pipe tensile armor wires
Fabio Santos 1
Ingrid Poloponsky 2
Susana Modiano 3
Carlos Ribeiro 4
Eduardo Motta 5.
1. BAKER HUGHES,
2. BAKER HUGHES,
3. BAKER HUGHES,
4. BAKER HUGHES,
5. BAKER HUGHES,
ENGENHARIA , ENGENHARIA DE MATERIAIS. RIO DE JANEIRO - RJ - BRASIL, fabio.santos@bakerhughes.com
ENGENHARIA , ENGENHARIA DE MATERIAIS. RIO DE JANEIRO - RJ - BRASIL, ingrid.poloponsky@bakerhughes.com
ENGENHARIA , ENGENHARIA DE MATERIAIS. RIO DE JANEIRO - RJ - BRASIL, susana.modiano@bakehughes.com
ENGENHARIA , ENGENHARIA DE MATERIAIS. RIO DE JANEIRO - RJ - BRASIL, carlos.ribeiro@bakerhughes.com
ENGENHARIA , ENGENHARIA DE MATERIAIS. RIO DE JANEIRO - RJ - BRASIL, eduardo.motta@bakehughes.com
Abstract
As part of Baker Hughes response to a flexible pipe failure caused by SCC-CO2 disclosed by a safety report issued by
ANP in 2017, an extensive material testing program has been established to assess the susceptibility of the tensile armor
wires to SCC triggered by the presence of CO2.The aim of the test program is to determine both stress and CO2
thresholds where the failure mechanism by SCC-CO2 could be triggered, creating a SCC risk free region to be used for
design purposes.The test program is based on wires taken from manufactured pipes which have experienced the
complete strain history of manufacturing and present representative residual stresses. The small-scale test protocol
includes long term SCC, Corrosion and SRR tests carried out in virgin and pre exposed wires taken from a 12 months full
scale corrosion test.
This paper will present the results of tensile armor wire test campaign, discuss its findings and present the full-scale
validation tests to be executed by Baker Hughes.
Keywords: Flexible Pipe. Corrosion. Stress. SCC-CO2. Tensile Armor
Received: February 27, 2020 | Accepted: Jun 06, 2020 | Available online: Dec 01, 2020
Article Code: 177
Cite as: Rio Oil & Gas Expo and Conference, Rio de Janeiro, RJ, Brazil, 2020 (20)
DOI: https://doi.org/10.48072/2525-7579.rog.2020.177
© Copyright 2020. Brazilian Petroleum, Gas and Biofuels Institute - IBP. This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2020, held between
21 and 24 of September 2020, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the final
paper submitted by the author(s).The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum,
Gas and Biofuels Institute’ opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2020
Proceedings.
Evaluation of the SCC-CO2 susceptibility of flexible pipe tensile armor wires
1. Introduction
Due to recent failures reported in the Pre-salt area by Oil & Gas operators, the corrosion
mechanism identified as Stress Corrosion Cracking (SCC) due to CO2 presence (SCC-CO2) became
a focus of study related to flexible pipes. The SCC-CO2 mechanism occurrence on armour wires of
gas injection flexible pipes was implied in catastrophic failure of pipe in a short time period
considering the service life for which the equipment was designed.
As flexible pipes are widely applied for oil and gas transportation, especially for the ultra deep
water applications of Brazilian Pre-salt, the concern with the integrity of installed pipes and with new
flexible pipes design resulted in a large study around such SCC-CO2 corrosion mechanisms. The
investigations identify the main parameters leading to its occurrence, characterization of cracks, and
replication of the failure mechanisms in a laboratory.
Different test methods have been used to evaluate the susceptibility of tensile armour materials
to the SCC-CO2 corrosion mechanism. This paper presents the main results and observations obtained
for representative material grades, comparing behaviors and delimiting envelopes to ensure safe
operation under flooded annulus combined with service conditions where CO2 is present.
2. SCC-CO2 corrosion mechanism overview
The Stress Corrosion Cracking mechanism is a type of Environmental Assited Cracking which
occurs due to the interaction of three factors: corrosive environment; material susceptibility and total
stress as explained in Hanninnen (2003). SCC cracks nucleation and growth is a results of the balance
between active stress and electrochemical reactions. The stresses required to trigger SCC-CO2
occurrence are tensile in nature and below the material’s yield, both applied load and residual stress
contribute to crack appearance. Crack growth is perpendicular to primary loading direction,
depending on the balance in these driving forces environment/material cracks can be characterized as
intergranular, transgranular or present a mixed mode as observed by Newman (2010). The presence
of multiple cracks and cracks branching are common characteristics of SCC occurrence.
Jones (2017) explains that SCC occurrence is generally associated with an
environment/material combination that results in a corrosion film formation on metals surface leading
to a semi passive state, which reduces uniform corrosion rate but can result in localized corrosion and
a preferential site for SCC cracks nucleation. Thus, an SCC fracture surface presents characteristics
of brittle behavior indicating little or no material loss associated with the failure. Newman (2010)
states that it is not always clear if crack propagation occurs only due to anodic dissolution, per
hydrogen embrittlement or due other mechanism influence.
In a flexible pipe the annulus region is determined by the space between pressure barrier layer
and external outer sheath, both polymeric layers. The pressure and tensile armour layers manufactured
in carbon steel are located in the annulus region as can be seen in Figure 1. During oil and gas
transmission, contaminant gases presented in fluid composition, like CO2 and H2S, permeate through
the inner pressure barrier reaching annulus region. At the same time water ingress into annulus region
may occur due to water vapor permeation also from pipe bore, due to a damage on outer sheath layer
or even as a result of seal failure in an end fitting port. The association of contaminant gases and
water transforms the once dry annulus region into a corrosive environment for the layers of metallic
pressure and tensile armour wires.
The Brazilian Pre-salt reservoirs present high content of CO2, and as a result a new failure mode
for flexible pipe armours was observed in operation. After gas injection pipes tensile armour wires
rupture, the failure analyses carried out verified that wire breaking occurred due to Stress Corrosion
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Fabio Santos, Ingrid Poloponsky, Susana Modiano, Carlos Ribeiro, Eduardo Motta
Cracking due to CO2 presence (SCC-CO2). Figure 2 shows a representative image of SCC-CO2 cracks
initiating from locallised corrosion in laboratory replication experiments.
Figure 1 – Flexible pipe annulus region..
Source: Baker Hughes.
Figura 2 –SCC-CO2 cracks generated in laboratory.
Source: Baker Hughes.
In the specific case of the SCC-CO2 mechanism observed in carbon steel wires, the unique
environment conditions of annulus region results in the formation of a porous and non-protective iron
carbonate film (FeCO3). Ferreira, Klok, Ponte and Farelas (2016) explain that iron carbonate film
precipitation and morphology are directly related to Fe2+ iron íons supersaturation, associated with a
CO2 partial pressure which exceeds the solubility limit of CO32- resulting in a supersaturation of this
specie. Annulus region of a flexible pipe exhibits low confinement ratio, i.e. the relationship between
free volume and steel exposed surface area (ml/cm²), resulting in iron ions supersaturation.
Ferreira et al. (2016) add that parameters like temperature and CO2 partial pressure have an
important role on iron carbonate film formation. CO2 solubility in aqueous solution together with
carbonic acid ionization followed by pH reduction are directly related to the temperature. The CO 2
partial pressure influences other environment parameters like pH and íons concentration in aqueous
solution, which affects the precipitation of corrosion products.
Based on this literature review, it is clear that the SCC-CO2 mechanism is a result of the
interaction of different parameters and that the threshould for each parameter and material shall be
determined to avoid cracking occurrence. In this way, an extensive test campaign including different
tests configuration and environment parameters is being conducted by Baker Hughes aiming to
evaluate tensile armour wires material susceptibility to the SCC-CO2 mechanism and establish a safe
envelope for flexible pipe operation. The material strength, temperature, CO2 pressure, applied load
and residual stress, and iron íons saturation among other parameters are part of this evaluation, to
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Evaluation of the SCC-CO2 susceptibility of flexible pipe tensile armor wires
provide reliable data for a better understanding of the boundaries for the SCC-CO2 corrosion
mechanism occurrence.
3. Materials and methods
Test methodologies for SCC evaluation are intended to determine the susceptibility of a certain
material in a specific environment providing information for a proper materials selection in
accordance with required service. The test conditions shall be representative of the worst case
scenario predicted for service operation. SCC tests can be static or dynamic.
The test campaign presented in this document considers two small scale test methods: constant
displacement in four-point bending configuration (4PB) and slow strain rate testing (SSRT).
Two carbon steel materials commonly used as tensile armour layers for Flexible pipes in
Brazilian pre-salt fields were tested in different temperatures, pressures and strain for SCC-CO2
suscepbility. The test campaign included evaluation of samples from manufacturing process to
consider the influence of residual stress and accumulated inelastic strain history.
3.1. Materials
The two tensile armour wires materials investigated in this study are carbon steel grades with
high strength, being both with UTS higher than 1000MPa and one heat treated. Steel A is used for
sweet applications, i. e., with low H2S content or without H2S. Steel B is a material with lower
mechanical strength but with better corrosion resistance used for H2S content up to partial pressures
of 10mbar in the annulus region. The tensile armour wire full cross section was tested, in this way no
machining was performed in the test specimens. Samples from vendor coils and taken from flexible
pipe were used.
3.2. Constant displacement tests using 4PB devices
There are several configurations for SCC evaluation under static conditions, in this
investigation the test method chosen uses a four point bending device to apply an specific deformation
in tensile armour wire specimens, as presented in Figure 3. The applied strain represents a loading
state equal or higher than material yield at the outer bend surface of the samples.
Various exposure periods were considered for SCC-CO2 evaluation, from 90 days up to 180
days. Compared with common corrosion tests, 180 days is a long exposure period but it is understood
that the SCC-CO2 corrosion mechanism occurs as a function of slower electrochemical processes
when compared with other failure mechanisms such as the ones related to H2S dissolution.
Figure 3 – Four point bending test device
Source: produced by the author.
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Fabio Santos, Ingrid Poloponsky, Susana Modiano, Carlos Ribeiro, Eduardo Motta
A total amount of three samples per each test setup were tested using armour samples retrieved
from an actual flexible pipe to consider the accumulated inelastic manufacturing strain level.
Different temperatures were tested but for the purpose of this paper only the results for 40°C will be
reported considering that this temperature was identified as being critical for SCC-CO2 corrosion
mechanism occurrence. Tables 2 and 3 present the test matrix for Steels A and B, reespectively.
Table 2 – Constant displacement test matrix for Steel A
Material
Internal Setup #
pCO2 (bara)
Temperature
Duration
(°C)
(days)
Samples origin
Steel A
8
30
40
180
From manufacturing (formed)
Steel A
10
10
40
180
From manufacturing (formed)
Steel A
16
30
40
90
From manufacturing (formed)
Steel A
22
10
40
90
From manufacturing (formed)
Steel A
27
10
40
90
From manufacturing (formed)
Steel A
28
10
40
180
From manufacturing (formed)
Steel A
30
5
40
180
From manufacturing (formed)
Steel A
31
5
40
90
From manufacturing (formed)
Steel A
40
5
40
90
From manufacturing (formed)
Steel A
41
10
40
90
From manufacturing (formed)
Steel A
42
10
40
90
From manufacturing (formed)
Steel A
43
62
40
90
From manufacturing (formed)
Steel A
48
10
40
90
From manufacturing (formed)
Steel A
49
10
40
90
From manufacturing (formed)
Steel A
67
10
40
180
From manufacturing (formed)
Steel A
70
30
40
180
From manufacturing (formed)
Source: produced by the author.
After samples loading, the 4PB device is placed in the test vessel; in sequence this system is
filled with test solution and deaerated with nitrogen for 2 hours. Test solution considered on all setups
consists of synthetic sea water in accordance with ASTM D1141. In addition, steel microspheres were
used in order to achieve iron ions supersaturation. Before increasing the pressure, the solution was
bubbled with CO2 for 15 minutes. Subsequently pressure was increased to achieve the required
pressure for each test setup.
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Evaluation of the SCC-CO2 susceptibility of flexible pipe tensile armor wires
Table 3 – Constant displacement test matrix for Steel B
Material
Internal Setup #
pCO2 (bara)
Temperature
Duration
(°C)
(days)
Samples origin
Steel B
26
80
40
180
From manufacturing (formed)
Steel B
33
10
40
180
From manufacturing (formed)
Steel B
34
5
40
180
From manufacturing (formed)
Steel B
35
5
40
90
From manufacturing (formed)
Steel B
37
10
40
90
From manufacturing (formed)
Steel B
38
30
40
180
From manufacturing (formed)
Steel B
39
50
40
180
From manufacturing (formed)
Steel B
55
10
40
180
From manufacturing (formed)
Steel B
56
50
40
180
From manufacturing (formed)
Steel B
57
5
40
180
From manufacturing (formed)
Steel B
58
30
40
180
From manufacturing (formed)
Steel B
59
10
40
180
From manufacturing (formed)
Steel B
61
5
40
180
From manufacturing (formed)
Steel B
62
30
40
180
From manufacturing (formed)
Source: produced by the author.
3.3. Slow strain rate testing (SSRT)
The slow strain rate test (SSRT) is used to explore potential cracking mechanisms by
application of a slow strain rate to failure thus allowing a chance for SCC mechanisms to develop,
Henthorne (2016). Generally, this methodology is widely used to study all alloy systems known to be
susceptible to SCC, providing a method to evaluate the influence of various parameters. In contrast
to fixed displacement/load tests, it may provide a faster way to evaluate the most critical parameters
affecting the SCC-CO2.
Test execution is similar to a standard uniaxial tensile test, except for the adopted strain rate of
1 x 10-6. SSRT shall always be conducted until fracture of test specimen. Figure 4 presents a device
specially manufactured for SSRT fixed in an electromechanical universal test machine .
The SSRT method enables the comparison of several CO2 levels criticality and also the
comparison and ranking of materials in a given environment. A total of four CO2 levels were
considered to evaluate materials behavior together with one control environment, that consists in
reference environment where the material under evaluation is not susceptible to SCC. This work focus
on the results for 40°C considering that this temperature was identified as being critical for SCC-CO2
corrosion mechanism occurrence. The test environment consists of synthetic seawater prepared in
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Fabio Santos, Ingrid Poloponsky, Susana Modiano, Carlos Ribeiro, Eduardo Motta
accordance with ASTM D1141 with CO2 (99,9900% of purity) under different pressures as presented
in Table 4.
Figure 4 – Slow strain rate test device
Source: produced by the author.
Table 4 – Slow strain rate test matrix
Material
Setup
ppCO2 (bara)
Temperature
Samples origin
(°C)
Steel A
7
Control environment
40
vendor coil
Steel A
8
80
40
vendor coil
Steel A
9
10
40
vendor coil
Steel A
10
5
40
vendor coil
Steel A
12
65
40
vendor coil
Steel B
1
Control environment
40
vendor coil
Steel B
2
5
40
vendor coil
Steel B
3
10
40
vendor coil
Steel B
4
80
40
vendor coil
Steel B
6
65
40
vendor coil
Source: produced by the author.
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Evaluation of the SCC-CO2 susceptibility of flexible pipe tensile armor wires
4. Results
After the conclusion of 4PB test exposure period, a series of post-tests were performed to verify
SCC-CO2 cracks occurrence, these being: magnetic particle inspection via wet florescent technique;
stereoscopic optical evaluation was performed after testing before and after specimens cleaning and
microscope evaluation.
In the SSRT, material susceptibility to SCC is primarily evaluated by direct comparison of the
stress on the load-crosshead displacement plot at which the test environment curve deviates from the
control environment curve.
Additionally, SSRT evaluation is also quantified using the ratio between obtained result in test
environment and the results in control environment. When the calculated SSRT ratios decrease in
value from unity material susceptibility to the SCC mechanism is verified. As the closer the SSRT
ratios are from unity, the better is the material resistance to SCC.
4.1. Steel A – 4PB
This section summarizes the main findings of the post-tests evaluations performed for Steel A.
At first, all samples were investigated to verify SCC-CO2 surface cracks using wet fluorescent
magnetic particle technique (MPI). All positive indications positions were marked and observed via
metallographic techniques in order to confirm the presence of SCC-CO2 cracking.
SCC cracking consistent evidences were found on the three samples of setup 22 (10bar/ 40°C/
90days). It shall be highlighted that similar test environment with longer exposure time, setup 10
(10bar/ 40°C/ 180days) has not developed the same pattern of indications. Figure 5(a) presents one
MPI result for setup 22, linear indications along the wire width can be observed. Figure 5(b) presents
the MPI evaluation for Setup 10, showing no indications.
Figure 5 – (a) setup 22 SCC-CO2 indication; (b) setup 10 without indication
(a)
Source: Baker Hughes property.
(b)
After magnetic particle inspections, stereoscopic optical evaluation was performed in the region
between internal rollers (the test-span region) and outside this region before and after cleaning for
removing scale formation; images from one of the samples with SCC-CO2 cracks can be seen in
Figure 6.
Following stereoscopic optical evaluation, samples were cut for defect characterization using a
microscope. Figure 7 presents the images obtained for setup 22 where all the samples presented SCCCO2 cracks with the same pattern. It is important to highlight that these cracks were observed in
samples from manufaturing process and for a loading equivalent to the material’s yield.
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Fabio Santos, Ingrid Poloponsky, Susana Modiano, Carlos Ribeiro, Eduardo Motta
Figure 6 – Setup 22 stereoscope evalution (a) before cleaning ; (b) after cleaning
(a)
Source: Baker Hughes property.
(b)
Figure 7 – Microscope evaluation (a) setup 22 ; (b) setup 30
Source: Baker Hughes property.
4.2. Steel B – 4PB
For Steel B, no SCC-CO2 cracking evidence or fracture was found for any tested setups, unlike
the material Steel A. MPI results showed one indication for test setup 56 and two indications for setup
59, all the other specimens presented no indication of cracks or surface flaws.
In the first instance, samples surface was evaluated before cleaning using a stereoscope, after
that all samples were cleaned and subjected to a second evaluation in a stereoscope to confirm the
presence of SCC-CO2 surface cracks. It was verified that none of the tested samples presented surface
cracks, Figure 8 presents one representative image of these evaluation before and after cleaning.
For this material grade only localized corrosion in the form of pitting was found; for all tested
setups no SCC-CO2 cracking was observed, even considering that the material was subjected to
loading equivalent to the material’s yield.
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Evaluation of the SCC-CO2 susceptibility of flexible pipe tensile armor wires
Figure 8 – Setup 57 stereoscope evalution (a) before cleaning ; (b) after cleaning
Source: Baker Hughes property.
4.3. Materials direct comparison – Slow stress rate test (SSRT)
In general, for both materials it was clear from the stress versus load-crosshead displacement
plot obtained for SSRT that the increase of CO2 pressure implies in a decrease of material properties.
However, this effect is only observed in the plastic region of the curve, i. e. the amount of plastic
deformation withstood by the material is reduced as a function of CO2 level.
Figure 9 shows the time to failure ratios for each CO2 pressure considering both materials
evaluated. It is possible to observe the SSRT ratio decreases from unity due to increase in CO 2
pressure and this effect is more accentuated for Steel A for which the ratio at 80bar of CO 2 reaches
0.78. For Steel B, a lighter reduction was observed.
Together with ratios evaluation, it is important to verify secondary cracking since its occurrence
can be an indicator of susceptibility to SCC. As highlighted per Henthorne (2016), even a high SSRT
ratio might be critical if secondary cracking is confirmed; on the other hand, lack of SCC crack
morphology and absence of secondary crack might imply acceptability of a lower ratio. There are
cases where corrosion mechanisms cause ductility and toughness losses without SCC occurrence.
These investigations are currently being performed in the tested samples, after that it will be possible
to confirm if the ratio reduction observed is due to SCC-CO2. The aim here is to compare both tensile
armour materials behavior and susceptibility, to understand the strength influence.
Figure 9 – Time to failure ratios versus CO2 pressure
Source: produced by the author.
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Fabio Santos, Ingrid Poloponsky, Susana Modiano, Carlos Ribeiro, Eduardo Motta
5. Conclusions
Two tensile armours wires materials susceptibility were investigated adopting complementary
test methodologies. The results presented herein provide information to establish an initial safe region
for flexible pipe operation considering conveyed fluid composition and operational parameters.
Constant displacement tests results show a difference of material susceptibility when comparing
Steel A and Steel B; the material with lower strength presented better results with absence of SCCCO2 cracking for all 14 tested setups even considering conditions as severe as 80bar of CO2 pressure
and straining up to material’s yield. For Steel A, 16 setups were concluded and cracks were found in
one of them, identified as setup 22. The test methodology itself was proved to be acceptable to
investigate the SCC-CO2 corrosion mechanism since one of the setups presented consistent cracking
for all specimens.
It is important to highlight that cracks observed on Steel A on setup 22 were generated for a
stress condition equivalent to material’s yield, this loading state being far more aggressive than any
service scenario even for extreme conditions. Additionally, considering that a total of nine setups
were completed with the same strain conditions and environment parameters, 10bar at 40°C, and only
in one setup SCC-CO2 cracking was triggered, it is assumed that this should be a material
susceptibility boundary. Residual stress and accumulated deformation history might also play a role
on SCC-CO2 cracks triggering, and both are being addressed by measuring samples’ residual stress
and mapping tensile armour layer manufacturing process. These two variables alone may explain why
SCC-CO2 cracks are not observed for all manufacturers.
The results obtained in SSR testing reinforces the main findings of 4PB tests. It is clear that
material identified as Steel A is more affected per environment, i. e. CO2 pressure. SCC-CO2
occurrence in this test should be confirmed after fracture surface observation and secondary cracking
confirmation. However, the direct comparison provided for the materials in this study is considered
enough to support the 4PB test campaign observations. Additionally, the differences in the sample
mechanical properties subjected to different CO2 levels are only perceived after the material’s yield
stress, i.e., in the inelastic domain. It is important to enphasises that the entire Flexible pipe project is
made within the elastic domain, thereby CO2 level appears to do not imply a consistent influence in
the pipe structural integrity capability for the two tensile armour materials discussed herein..
For higher levels of CO2 no cracks were observed and the corrosion morphology was for large
and round pits. Pitting is also associated with the mechanical partial removal of previously formed
scale and chemical dissolution of exposed areas by solutions undersaturated with dissolved Fe2+.
It is understood that there are still uncertainties related to the SCC-CO2 failure mechanism
occurrence. However, the program summarized herein addresses most of them and is being updated
continuously based on the main findings. All the different test methods adopted together with the
wide range of variables considered, should cover and be able to establish a definitive safe area for the
flexible pipe operation.
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