Proceedings of the 2022 14th International Pipeline Conference
IPC2022
September 26-30, 2022, Calgary, Alberta, Canada
TMCP PIPE FOR SOUR SERVICE: A NEW QUALIFICATION APPROACH
Brian Newbury1, Volker Schwinn2, Jens Schroeder3, Graham Alderton4, Andrew Prescott1, and
Andrew Wasson1
1ExxonMobil Technology and Engineering Company
Aktien-Gesellschaft der Dillinger Hüttenwerke (DILLINGER)
3EUROPIPE GmbH
4Liberty Steel
2
ABSTRACT
Historically, C-Mn steels have been extensively used in line
pipe applications for sour service oil and gas environments, i.e.
in the presence of hydrogen sulfide (H2S). In the past few
decades, the emergence of the Thermo-Mechanically Control
Process (TMCP) manufacturing method has further optimized
the benefits of increased fabricability, weldability, and large cost
benefits over alternate materials for large diameter pipe
manufacturing techniques. However, implementation of C-Mn
steels in sour service does require increased focus on steel
cleanliness and hardness control to avoid susceptibility to
hydrogen damage mechanisms such as hydrogen induced
cracking (HIC) and sulfide stress cracking (SSC). These
additional requirements have been addressed in industry
standards such as NACE MR-0175/ISO 15156. Despite these
industry practices, recent pipeline projects have experienced
failures related to SSC and have found increased surface
hardness due to Local Hard Zones (LHZs) in delivered and
installed pipe. The LHZ phenomena appears to be a relatively
new concern to the industry, and has led to concern over use of
TMCP steels after the Kashagan pipeline failure in 2013.
ExxonMobil has developed a qualification program based on the
requirements of NACE MR-0175/ISO 15156 and new insights
regarding the root cause of LHZs. This work reports details on
this qualification approach implemented for X65 grade line pipe
from recent qualification activities with TMCP plate and line
pipe suppliers.
LCZ
LHZ
NDT
POD
QA/QC
SSC
TM
Local Cold Zones
Local Hard Zones
None Destructive Testing
Probability of Detection
Quality assurance/control
Sulfide Stress Cracking
Thermo-Mechanical rolling process
1. INTRODUCTION
Oil and gas resources that contain hydrogen sulfide (H2S),
often called sour service, are an important part of the world’s
energy portfolio. However, the presence of H2S can lead to
hydrogen ingress into C-Mn steels as a result of corrosion
reactions generating monoatomic hydrogen on the steel surface.
In these environments it is important to ensure that the pipeline
materials are resistant to hydrogen cracking and related damage
mechanisms. Industry practice has been documented in the
NACE MR-0175/ISO15156 standard, with common practice to
limit the hardness of C-Mn steels to below 248 HV10 for Region
3 of Figure 1. While considered the most severe region of the
diagram, it can be seen that Region 3 covers a large range of pH
and H2S concentrations. Despite this environmental variance
industry practice generally does not discriminate within the
regions of Figure 1; and once a material is qualified for a higher
region (e.g., Region 3) it is often accepted as qualified for lower
regions.
In September of 2013, the Kashagan project located in the
Caspian Sea experienced the failure of two 28-inch x 95 km long
pipelines [2-5]. While the pipelines in the Kashagan project
were replaced with corrosion resistant alloy (CRA) clad material,
ExxonMobil studied the failure with a longer term focus of reenabling use of C-Mn line pipe material in future company
severe sour service applications.
To service this goal,
ExxonMobil published a detailed discussion of a hypothesized
root cause for the Kashagan failures in Fairchild et al [6], and a
discussion of a C-Mn line pipe qualification procedure in
Newbury et al [7]. Subsequent to these papers, ExxonMobil has
implemented this qualification program with a number of plate
Keywords: C-Mn steel, sour service, sulfide stress cracking,
local hard zones, TMCP, materials qualification
NOMENCLATURE
4PB
Four Point Bend
ACC
Accelerated Cooling
AYS
Actual Yield Stress
FRT
Full Ring Ovalisation Test
HACC
Heavy Accelerated Cooling
HIC
Hydrogen Induced Cracking
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IPC2022-87146
FIGURE 1: NACE DIAGRAM INDICATING SOUR
REGIONS FROM LOWEST (0) TO HIGHEST (3) SEVERITY.
One key learning highlighted in the Kashagan root cause
study reported by Fairchild et al [6] was the identification of
local hard zones [LHZs] and their role in SSC crack initiation in
material nominally qualified for sour service to industry practice.
LHZs were found to be very thin (<500µm deep), very
infrequent, and randomly distributed on the pipe surface. The
authors’ experience in Fairchild et al has shown that LHZs may
only affect a few percent of steel plate contained in a
transmission pipeline steel order, and of that affected plate LHZs
may make up a very small fraction (1-5%) of the final pipe’s
surface area. It was hypothesized that even a very extensive
random sampling program would have a small chance of success
in locating LHZs in qualification or production, but that a
transmission pipeline could have an unacceptable level of risk of
containing LHZs in the constructed line. This realization led to
the development of the qualification summarized in Newbury et
al [7].
2. QUALIFICATION PROGRAM
Detailed discussion of the qualification program can be
found in Newbury et al [7]. A few key considerations formed
the basis of the program, which aims to identify process upsets
in the plate manufacturing process which can create conditions
that allow LHZs to form. This work details execution of the
qualification program for X65 grade line pipe intended to be used
on two different severe sour service pipeline projects.
The first is that the root cause of LHZs lies in the steel- or
plate making process as discussed in Fairchild et al [6], however
the customer interface is often only with the pipe manufacturer.
Due to the multitude of techniques to manufacture an API or
DNV grade line pipe, the qualification program had to involve
all aspects of materials manufacturing from steelmaking to final
coating effects on the line pipe. It also meant that qualification
2
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activities were initially considered specific to the production
route and alloy chemistry.
A second consideration was to manufacture multiple
commercial-sized heats of steel plate while attempting to
manufacture realistic bounds in plate chemistry and other
manufacturing parameters. In addition, some plates were
manufactured explicitly outside of commercial practice in an
attempt to generate LHZs and ultimately SSC test failures. The
combined body of data generated from this production would
help define future production process boundaries, based on the
boundary of passing and failing SSC tests. The commercial scale
materials trials were thought to eliminate questions about lab-toproduction scale changes in manufacturing process, while also
providing the best chance to generate actual LHZs based on the
statistics challenge mentioned in the introduction.
A third consideration was to ensure compliance with NACE
MR-0175/ISO 15156 requirements when qualifying materials to
project-specific test environments. The infrequency of LHZs
raised concern amongst the authors in Newbury et al [7] that
samples taken from pre-specified test coupon locations would
not meet the requirements for NACE qualifications based on
laboratory testing. Within NACE MR0175/ISO 15156
requirements in section 8.3.2 state it is up to the end user to verify
that the test samples are metallurgically representative of the
final product as well as represent the metallurgical condition of
greatest susceptibility to sour cracking for the final product. The
LHZ phenomena creates a scenario where it is easy to
unknowingly test materials that are not representative of the
greatest SSC susceptibility (i.e., containing potential LHZs) in a
qualification program.
To address this third consideration required a fundamental
shift in QA/QC practice in the plate manufacturing process. In
addition to standard temperature monitoring processes for
process control, additional thermal monitoring that would
characterize the temperature of 100% of the plate surface
eventually exposed to the sour service environment was
required. This requirement is also in effect for future production
to ensure all plate surface is in compliance with the qualification.
Monitoring was required just prior and after the accelerated
cooling steps of TMCP plate manufacturing. This ensured that
no phase transformations occurred prior to entering accelerated
cooling due to temperature drop below process target, as well as
identifying the coldest locations after accelerated cooling. Both
of these temperatures are critical to controlling LHZ formation
mechanisms as discussed in Fairchild et al [6].
The coldest regions of plate in both material heats post
accelerated cooling, as identified by thermal monitoring, were
deemed “regions of interest” (ROIs) to be labelled and traced
through remaining plate processing, transportation, and pipe
forming processes. These regions were then exposed to either
full ring ovalization SSC testing (FRT) according to BS 8701, or
small scale four point bending (4PB) testing according to NACE
TM0316 [8, 9]. Because of the thin nature of LHZs, the tensile
surface of the samples was left as-received for 4PB and very light
sand blasting was applied on full ring tests. Prior to testing the
mills, pipe mills, and fully integrated line pipe mills. This work
will highlight some of these qualification efforts and discuss key
learnings generated during qualification and materials
production.
FIGURE 2: INTEGRATED OVERALL PRODUCTION
CONCEPT AT DILLINGER.
The occurrence of damage by HIC or SSC depends on the
fraction and morphology of material imperfections, material
fracture toughness and the geometrical features of potential
cracking sites. The fundamental requirements for appropriate
steels can be defined in terms of cleanness, i.e. inclusion
content and shape, plus internal homogeneity and toughness
of the material, i.e. the microstructure design. Interfaces or
imperfections where hydrogen can recombine should be
prevented or reduced to a minimum amount. Remaining
inclusions should be homogeneously distributed and clusters
must be avoided. The general strategy of the integrated overall
production concept takes into account all these aspects. Table
1 documents the key conditions of this concept.
3. PLATE PRODUCTION - CONCEPT
For the qualification efforts plates were produced following
the approach of the integrated overall production concept as
explained in more detail below. Those plates represent the actual
best practice condition and are designated below as “Design
Condition”.
In addition plates were produced with “Off-Design
Condition” to identify the limits of the “Design Condition” and
to elaborate the key parameters and the boundary conditions that
could produce material with increased HIC or SSC susceptibility.
Particular focus was placed on the ACC process, e.g. the
influence of local cold zones (LCZ) created during ACC that
may impair SSC [6, 7].
TABLE 1: KEY CONDTIONS OF THE PRODUCTION
CONCEPT.
3.1 Integrated overall production concept
In order to provide consistent properties, the production of
sour service steels relies on the utilization of an integrated overall
production concept (Figure 2). This involves the steel- and platemaking as well as the application of a specific quality assurance
system and NDT testing to ensure delivery of plates free from
LHZ.
3.2 Steelmaking
For the steelmaking a specific production route is applied. A
key point is the utilization of a vertical type caster with soft
reduction (Figure 3). The cleanliness is improved by this caster,
since inclusions are able to rise up to the casting powder and the
remaining inclusions are distributed homogeneously across the
slab thickness. Manganese is known to segregate and as a
consequence could deteriorate HIC properties, a proper
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sand blasting procedure was qualified to show any LHZs, if
present, would not be removed.
The ROIs were tested under NACE TM-0177 “solution A”
as well as two project-specific environments within NACE
Region 3 of Figure 1. This allowed direct comparison to both
historical qualification efforts (to the NACE solution A
environment) as well as addressing concerns regarding potential
materials performance variability across Region 3. Upwards of
30 ROIs were selected to cover the SSC testing campaign over
the three testing environments per pipe mill. Each ROI was
selected to have a consistent temperature over an area of
approximately 100 x 150 mm. If a ROI was designated for 4PB
testing, three coupons were cut for testing in triplicate. If the
ROI was selected for FRT SSC testing, it was left as-is in the
required ring coupon.
After execution of the extensive SSC testing campaign, the
data was analyzed to identify manufacturing conditions which
generate a concern for SSC. A qualification window was then
designated for the manufacturing route customized to each line
pipe manufacturer (plate chemistry, manufacturing parameters,
and pipe forming process).
application of a soft reduction during casting is essential to
reduce the amount of macro-segregations (Figure 4). As a result,
the limitation of the manganese content does not need to be
overly restrictive.
b.)
FIGURE 5: TYPICAL MICROSTRUCTURE OF THE PLATES
PRODUCED WITH THE “DESIGN CONDITION.”
3.4 QA/QC system - updated
During design and qualification stage, all the relevant
production and process parameters are defined with target values
and tolerances to meet the complete properties profile. During
production certain deviations or incidents can occur that have a
potential influence on HIC or SSC properties. For that reason a
special adapted QA/QC system was installed that is able to detect
a deviation of the actual values and to decide if such slabs or
plates must be prohibited from release and delivery (because of
a major deviation) or additionally tested to prove conformity
with the specification (because of minor deviation).
The plates produced for the qualification with “Design
Condition” had no deviations. As mentioned above the plates
produced with “Off-Design Condition” by intention deviate from
the target values of the “Design condition”.
The previous mill practice relied on the temperature
measurement by optical pyrometer in the center region of the
plate as it passes under the pyrometer. As mentioned above,
special attention was paid on the ACC process and the
parameters that may create LHZs or may impair SSC resistance.
As a consequence the new approach of the qualification program
requires 100% plate surface temperature monitoring by a thermal
camera at the entrance and exit of the ACC process (Figure 6).
This temperature measurement of the thermal scanner is
performed on the rolled plate (so called mother plate) and
includes colder areas at the edges and ends. These areas are
trimmed off at a later stage. Sample plate surface temperature
scans prior to edge and end trimming can be found in Newbury
et al. [7] A mother plate is divided typically in two or even three
final plates (so called daughter plates). Therefore a specific
program was developed and implemented in an upgraded
QA/QC system that can clearly assign the temperature
measurement of the thermal cameras to the final plate and
provides additional features for the evaluation and traceability of
the temperatures in local areas. This system was used to select
the ROIs discussed in section 2, and is essential to correlate the
ROI test results of the qualification program which define lower
bound qualified values with the local temperatures measured
during plate production.
FIGURE 3: COMPARISON OF VERTICAL AND CURVED
TYPE CASTERS.
FIGURE 4: INFLUENCE OF SOFT REDUCTION DURING
CASTING.
3.3 Plate making
The metallurgical concept relies on a TM+HACC rolling
and cooling process to achieve a homogeneous and fine granular
bainite microstructure without banding (Figure 5). This implies
finish rolling and start accelerated cooling above Ar3. However,
for an optimized balance of HIC and SSC resistance the cooling
rate is restricted and the finish cooling temperature is maintained
high enough to limit macro- and micro-hardness.
Another important point of the concept is the use of slabs
with a sufficiently high thickness. For the qualification program
slabs with a nominal thickness of 400 mm were used. Thus
enhanced total deformation ratios during roughing and finish
rolling are feasible and the metallurgical concept becomes more
robust.
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a.)
FIGURE 6: SCHEMATIC POSITIONS OF THE THERMAL
CAMERAS AT THE ENTRANCE AND EXIT OF ACC.
3.5 NDT for LHZ
During steel and plate making different possible causes for
LHZ exist. Due to the infrequent occurrence and small size, LHZ
are difficult to detect using the standard hardness testing
included in most specifications. Standard surface hardness
testing could only work reliably if it was applied on a very fine
grid scale involving thousands of hardness measurements.
Therefore, large scale hardness testing for the detection of LHZ
is inappropriate for an industrial plate production process. To
overcome this problem, an Eddy Current testing technique was
developed which allows 100 % plate surface inspection as an
integral part of the industrial plate production process which
reliably detects all areas with increased hardness and size
resolution of 20mm [10].
In a first step and applied for this qualification program a
manual NDT system for QA/QC was implemented and utilized
with testing on both sides covering 100 % of the surface area.
Since it was the most conclusive method, it was also previously
used for the root cause analysis for LHZ and thus the parameters
causing LHZ could be identified and optimized subsequently.
However, it is worth to mention that although the amount of
LHZs were clearly reduced, it is not possible to eliminate them
completely. Within this qualification program two LHZs were
detected by the NDT system and verified by hardness testing and
metallographic investigations. These plates were not further
processed to pipes.
In order to increase the capacity of plate material inspected
for LHZ during production, an inline automatic NDT device has
been installed. The inline system allows inspection of all plates
on both sides simultaneously without any impact on production
rates. The commissioning and qualification including the POD
(probability of detection) of this device was finalized after this
qualification program.
FIGURE 7: ROI SHOWING TOOLING WEAR MARKING
(INSIDE DASHED YELLOW BOXES) ON THE OUTSIDE
OF THE PIPE FROM PIPE FORMING. NOTE SOME WEAR
MARKS APPEAR DARK OR BRIGHT DUE TO SHOP
LIGHTING. THIS ROI WAS RE-LABELED FOR EASE OF
VISUAL IDENTIFICATION
DURING
REMAINING
PROCESSING.
Mechanical properties were required in as-manufactured
and aged condition and sour service testing was conducted in the
aged condition. To accommodate all test conditions test rings
were removed prior to ageing, the full pipe was then sent for
4. PIPE PRODUCTION AND TESTING
In this work, TMCP plates were supplied to two UOE
SAWL pipe mills. Each pipe mill received approximately twenty
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plates for forming into pipe and testing. Prior to receiving the
plates, it had already been identified that the ROI marking on the
plate surface could not be solely relied upon as method for later
identification at the pipe mill therefore the X-Y Co-ordinates of
each ROI were recorded at the plate mill. Plate shipment can
involve, rail, sea, and truck and therefore plates can be handled
several times. Although these transport routes are well
established and have shown not to be an issue with the standard
plate marking, testing these specific ROI was a fundamental
aspect of this qualification and warranted the backup traceability
method.
Pipe manufacture involves press forming of the plates and
plate movement by roller tables so there was concern this could
remove the ROI marking. As a precaution the ROI marking was
transposed to the eventual outside pipe face of the plate by the
plate mill. Loss of marking concern was mainly during the O
pressing where the dies contact the top plate surface. The surface
sees considerable frictional forces, and to reduce these a
lubricant is applied, combined this gives the ideal condition for
marking removal. After pressing and immediately before further
processing plates were re-inspected, cleaned, and the marking
reapplied. As a precaution this inspection and reapplication was
conducted as necessary at every stage of the process. A ROI label
to be re-marked is shown in Figure 7. To do this the production
line must be stopped, and thus only reserved for major
qualifications and not performed for commercial production.
4.1 Sour and Mechanical Testing
The sour test program involved testing the ROIs plus the
seam weld. These ROIs were spread across plate material
manufactured in the design condition as well as the off-design
condition. Across the two pipe mills, the evaluation consisted of
four different steel production heats and involved 144 HIC
specimens, 108 4PB SSC specimens and 48 full ring ovalisation
SSC tests. HIC testing was conducted in accordance with NACE
TM0284-2011 solution A and involved sets of three full wall
thickness specimens at the standard NACE locations of seam
weld, 90 degrees, and 180 degrees around the pipe
circumference.
SSC samples were extracted from the aged pipes and 4PB
testing to NACE TM0316 for 720 hours was performed with an
applied load of 90 % actual yield stress (AYS). The AYS was
determined from tensile tests conducted in the aged condition.
Longitudinal base metal SSC samples taken from within the ROI
and had the inside surface left intact meaning without surface
machining. This allowed potential LHZ cracking tendency to be
quantified. Samples were loaded per the NACE standard as
shown in Figure 10.
There was a consideration to use a non-machined transverse
weld specimen, but this was shown to overly load the weld toe
as a direct result of the thicker and stronger weld metal as seen
in Figure 11. Strain gauges were applied at the weld toe and
within the base material and the sample loaded, whilst the weld
toe reached loads exceeding AYS, the base material was still less
than 50 % AYS.
FIGURE 8: SAMPLE CUT SCHEMATIC SHOWING BULT
RINGS TO BE REMOVED FROM THE FULL LENGTH LINE
PIPE (TOP) AND THEN PER-RING TEST BLANK CUT
PLANS TO ACCOUNT FOR SOME OF THE PROGRAM’S
MECHANICAL TESTING.
Pipe specifications often specify mechanical testing in the
aged condition, but this is normally done on samples extracted
from aged coupons or the samples themselves aged prior to
testing and this can be managed within the test house. For this
qualification it was a requirement for the full length to be aged.
Thermocouples were attached to the pipe outer surface at
positions away from any of the ROI. Multiple pipes were loaded
into the furnace and heated to 250 °C, reaching temperature took
more than 5 hours illustrated in Figure 9. Once the temperature
was attained, it was held for a minimum of 60 minutes before
allowing to cool in air. The ROI marking was then be reinspected and reapplied, as necessary.
FIGURE 10: 4PB SSC TEST LOADING INDICATING NONMACHINED AS-RECEIVED PIPE INNER SURFACE.
FIGURE 9: FURNACE SCHEMATIC FOR ARTIFICAL
AGING FOUR LINE PIPES (RIGHT) AND THE RESULTANT
THERMOCOUPLE DOCUMENTATION OF THE AGING
HEAT TREATMENT (LEFT).
UOE pipe forming is at ambient temperature and introduces
cold deformation. One aspect of the qualification was to quantify
the increase in hardness resulting from pipe forming. Direct
testing on the pipe surface with a portable hardness tester was
attempted but the curved surface of the pipe makes the result
overly sensitive to the alignment of the probe and the surface
preparation, and the outcome is often an excessive scatter in the
measured values. Hardness testing on macro samples at 1 mm
below the surface and micro hardness at 0.25 mm below the
surface were conducted.
FIGURE 11: NON-MACHINED SEAM WELD SSC
COUPON ILLUSTRATING WELD REINFORCEMENT AND
STRAIN GAUGE LOCATIONS.
To accurately characterize the SSC performance of the seam
weldment, fully machined coupons were required to address the
applied stress discrepancies measured by the strain gauges. It
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ageing, and then further test rings cut after the ageing. This
involved a significant amount of planning and required a
bespoke cutting instruction per pipe as shown in Figure 8.
Across the two pipe mills over 180 rings were cut to account for
the SSC and mechanical testing requirements.
should be noted that LHZs, if present, would be erased by the
heat input from seam welding and thus the fully machined
samples are the most accurate way to characterize weldment
microstructure SSC resistance.
FIGURE 13: SCHEMATIC OF INTERNAL LOADING AND
STRAIN GAUGE EQUIPMENT FOR FRT SSC TESTING.
[12]
C2
In this program, it was found that available internal loading
equipment was not able to apply and hold constant the required
applied stress. An applied load target of 80% AYS was chosen
on recommendation of testing SMEs from the contracted test
house executing the SSC program. This was chosen to account
for residual stress remaining in the FRT pup pieces to
approximate the 90% AYS applied in the small scale 4PB
specimens. Residual stresses would be relieved in the small
scale 4PB specimens. The reference yield stress was measured
from aged tensile specimens to conservatively mimic the asinstalled pipe after coating. External loading as shown in Figure
14 below is an alternative method given in BS8701 and allows
for higher loads to be applied.
C3
C1
FIGURE 12: NACE DIAGRAM WITH TEST CONDITIONS.
4.2 Full Ring Ovalisation SSC Testing
Both pipe mills conducted eight full ring ovalisation tests
(FRT) in each of the same three conditions as used for the 4PB
SSC scope. The rings were taken with the ROI in the centre of
the ring and aligned as the target area for maximum applied
tensile stress. Testing was conducted in accordance with BS 8701
[8].
Conducting full rings tests is not an easy task and requires
specialist experience and therefore limited to laboratories with
proven expertise in this area. The type of rig design has a direct
impact on how the rings are loaded and the consistency of
applied stress.
Internal loading can be used by means of a turnbuckle with
the maximum stress perpendicular to the applied load as
indicated in Figure 13. The advantage of this design is that it
allows UT examination during the exposure period, however,
this method was designed for loading at 72 % SMYS and the
threads on the turnbuckle may limit the maximum amount of
load that can be applied.
FIGURE 14: SCHEMATIC OF THE ALTERNATE
EXTERNAL LOADING CLAMP FRT SETUP UTILIZED IN
THIS TESTING PROGRAM. [12]
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SSC testing was conducted in three different environmental
conditions. They are listed in Figure 12 as C1, C2, and C3:
 Condition one: 1 bar pH2S, Solution A NACE TM017
pH3 (2500 ‐ 2800 ppm H2S)
 Condition two: A project-specific NACE region 3
condition with a mixture of H2S and CO2 totaling 14.5
psi
 Condition three: A project-specific NACE region 3
condition with a mixture of H2S and CO2 at positive
pressure
Strain gauges were placed in the region of interest and the
bolts tightened until the target stress was achieved (Figure 15).
Within a small distance from the location of maximum tensile
stress there were noticeable changes in the strain gauge values.
Additional gauges were added for the first tests and the
procedure refined on site to maximize applied stress consistency.
It was a challenge to achieve the 80% AYS loads at the ROI
whilst also not overstressing areas outside of the ROI. Small
clamp misalignments or minor geometric changes in the sample
pipe, well within acceptable manufacturing geometrical
tolerances specified in all line pipe standards, could lead to
overstressed regions of the sample. If these are not well
understood, it can lead to misinterpretation of test results. Strain
gauges were removed after the applied load was proven to be
stable.
Pipe Transverse Yield Strength (Round Bar)
600
580
520
500
480
460
450
440
Codition
As welded
mill
Aged
As welded
A
Aged
B
Pipe Transverse Tensile Strength (Round Bar)
630
Rm [MPa]
610
590
570
550
535
530
Codition
mill
As welded
Aged
A
As welded
Aged
B
FIGURE 16: SAMPLE LINE PIPE YIELD AND TENSILE
STRENGTH VALUES FOR BOTH PIPE MILLS IN THE ASFORMED AND AGED CONDITIONS.
In terms of hardness, there was a 10 to 15 HV10 increase
attributed to pipe forming and then a further smaller increase
after ageing as shown in Figure 17 below. The average in pipe
for X65 remained low, even in the aged condition. Over a larger
production run the hardness range would widen therefore
consideration is needed when setting any specification limit. For
pipe a maximum hardness of 230HV10 is an appropriate
maximum value for the base material, this gives allowance for
potential hardness increase in the weldment so the installation
contractor can remain below the NACE limit during girth
welding.
FIGURE 15: VALIDATION OF APPLIED STRESS AT ROI
VIA STRAIN GAUGE MONITORING. [12]
5. RESULTS
For both pipe mills the mechanical properties of DNVGL STF101 L450 SFD were confirmed with Charpy toughness at -30 °C
and BDWTT and CTOD confirmed at -10 °C. The ageing
coating simulation of the pipes made the yield strength increase
by 20–40 MPa and the tensile strength by 10–15 MPa as shown
in Figure 16. SSC testing was based on the AYS therefore this
increase was seen as conservative to applied loads based on
SMYS.
FIGURE 17: HARDNESS CHANGE FROM PLATE TO ASFORMED PIPE TO AGED PIPE.
Micro hardness was conducted 0.25 mm below the pipe surface
and the values were measured to be approximately twenty points
higher than 10 kg load values collected 1-2 mm below the
surface. This level increase was expected because of the smaller
microhardness sampling volume and ability to characterize
surface-cooled microstructures. The near-surface microhardness
8
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Rt0.5 [MPa]
560
540
FIGURE 18: PITTING CORROSION ON THE ID SURFACE
OF AN FRT SAMPLE TESTED IN CONDITION TWO.
The 4PB samples tested in conditions 2 and 3 would often
show stress grooving as result of the severe environmental
conditions. Stress grooves manifested as blunt features due to
local corrosion at the surface aligned with prevalent stress fields
in the 4PB samples. Figure 19 illustrates multiple stress grooves.
These features are not sharp and not to be mistakenly reported as
SSC. Post-test micro hardness measurements confirmed the
hardness around and at the tip of these groves to be below the
limits at which SSC would occur. Additional work within
ExxonMobil showed that these features were exacerbated by the
high applied loads (90 %AYS of aged tensile specimens).
Decreasing the applied loads minimized and ultimately
eliminated the stress grooves.
FIGURE 20: SSC CRACK IN AN OFF-DESIGN SAMPLE
INDICATING PLATE PRODUCTION OUTSIDE OF
PARAMETERS SUITABLE FOR SSC-RESISTANT LINE
PIPE.
This qualification work has shown that SSC susceptibility is
increased with increasing fraction of lath bainite, attributed to
high cooling rates during plate quenching or low stop cooling
temperatures. The critical amount of lath bainite to cause SSC
susceptibility is dependent on the plate manufacturing process,
and thus different for all plate suppliers. Critical parameters such
FIGURE 19: STRESS GROOVING SEEN IN TESTING
UNDER CONDITIONS TWO AND THREE.
For the off-design condition, some of the samples
representing the most aggressive localized cooling during plate
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manufacture and tested in all three environmental conditions
produced failures. This was as per program design as it identified
plate manufacturing conditions unsuitable for SSC resistance.
An example SSC crack in one of these off-design test samples is
seen in Figure 20. Through careful evaluation of each of the
small and large scale SSC samples, including microhardness on
the post-test sections and correlation to the specific plate
manufacturing parameters, a safe boundary for the relevant plate
manufacturing processes could be defined for production
QA/QC oversight.
For all SSC test evaluations, the test acceptance criteria
reported in Anderson et al [11] was used. This acceptance
criteria were developed to aide in assessing SSC test coupons
that show evidence of significant localized corrosion attack as
seen in conditions two and three, which resulted from high
amounts of CO2 present in the test environment.
Both the large scale FRT and small scale 4PB SSC coupons
were found to reliably characterize SSC performance of the
tested line pipe. However, specialist knowledge regarding
loading effects, test environment selection, and final test result
interpretation was required for both types of tests. FRT testing
showed significant sensitivity to loading clamp alignment and
geometrical variation within the pipe samples (well within
acceptable limits by common line pipe codes). This sensitivity
could lead to over or underloading test samples if not fully
understood. 4PB SSC samples showed tendency to develop
stress grooves in certain test environments as a result of the test
setup. Both scenarios required specialist knowledge to address
and interpret test results appropriately.
locations were required to accurately quantify presence of
LHZs.For the design rolling condition, both pipe mills passed the
HIC, the 4PB SSC, and the FRT SSC tests in all of three
environmental conditions. Pitting corrosion was observed in the
FRT samples tested in condition two as seen in Figure 18. An
example of this is shown below. This occurrence of pitting was
attributed to the increased level of CO2.
6. CONCLUSION
A new qualification approach for C-Mn line pipe steels in
severe sour service has been developed and implemented with a
number of steel plate and line pipe manufacturers. A review of
the experience implementing this qualification program with one
steel plate manufacturer and two line pipe manufacturers has
been provided. The program did not require a fundamental
change in the metallurgical design of sour service plate, but did
require a shift in the monitoring practice of critical plate
manufacturing processes. This change involved measuring the
surface temperature of 100% of the steel plates, and selecting
regions of interest, representing the metallurgical condition most
susceptible to SSC cracking, for testing after final manufacture
into line pipe. This process was then implemented on the
manufacturing line of both the plate mill and line pipe mills,
ensuring representative product to a production order. Material
was manufactured in “design condition” and “off-design”
parameters to ensure a verified production window, with
demonstrated SSC resistance, was identified. This process was
executed successfully at the two line pipe mills.
All sour tests were found to pass for the design condition
line pipes at both mills. SSC test failures in the off-design
condition material confirmed limits to plate manufacturing
parameters that govern future the production of any commercial
order. Full ring ovalisation tests for SSC require significant
understanding of the load distribution to ensure valid tests and to
prevent misinterpretation of results.
The results of the qualification process have led to an
updated QA/QC system in which the derived limits have been
implemented. An additional component of this updated QA/QC
system is the added in-line NDT inspection based on eddy
current. The use of thermal cameras and in-line eddy current
inspection has been found to be effective at identifying LHZs
from both overcooling and carbon enrichment. The shift in
QA/QC process and new in-line NDT inspection for LHZ
detection did not inhibit production rates in the plate mill as
demonstrated in a recent large sour service production order.
ACKNOWLEDGEMENTS
The authors would like to acknowledge Dr. Douglas
Fairchild’s significant contributions and assistance during
execution of the qualification program.
REFERENCES
[1] ANSI/NACE MR0175/ISO 15156-2015: Petroleum, petrochemical, and natural gas industries - Materials for use in H2Scontaining environments in oil and gas production
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[2]
https://www.ncoc.kz/Documents/Sustainability_report_2015_
en.pdf
[3] D. Baxter, E. Ostby, S. Chong, A. Venas, “Use of C-Mn
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17-22, 2018, Madrid, Spain.
[4] https://www.reuters.com/article/oil-kashaganidUSL6N0S52P420141010
[5]
https://www.offshoreenergytoday.com/kashagan-notgetting-back-on-line-in-2014/
[6] Fairchild D.P. Newbury B.D., Anderson T.D., Thirumalai
N.S., “Local Hard Zones in Sour Service Steels”, 38th Int. Ocean,
Offshore and Arctic Eng. Conference, OMAE, Glasgow,
Scotland, 2019
[7] Newbury B.D., Fairchild D.P., Prescott C.A., Anderson T.D.,
Wasson A.J., “Qualification of TMCP Pipe for Severe Sour
Service: Mitigation of Local Hard Zones”, 38th Int. Ocean,
Offshore and Arctic Eng. Conference, OMAE, Glasgow,
Scotland, 2019
[8] BS8701-2016: Full ring ovalization test for determining the
susceptibility to cracking of linepipe steels in sour service
[9] NACE TM0316-2016: Standard Test Method Four-Point Bend
Testing of Materials for Oil and Gas Applications.
[10] Schneibel G., König C., Gopalan A., Dussaulx J.M.,
“Development of an Eddy Current based Inspection Technique
for the Detection of Hard Spots on Heavy Plates”, 19th World
Conference on Non-Destructive Testing, Munich, Germany,
2016
[11] T.D. Anderson, D.P. Fairchild, W. Huang, N. Thirumulai, G.
Wadsworth, A. Ozekcin, H. Jin, “Micrographic Acceptance
Criteria for SSC Testing”. Corrosion Conference & Expo,
(Houston, TX: NACE 2020).
[12] P. Dent, Element Materials Technology, NACE
International Webinar “Mitigation of SSC Failure from TMCP
Local Hard Zones”
as plate chemistry, rolling schedule, descaling practice, finish
rolling/ACC start/end quench temperatures all play a roll in the
final microstructure and its SSC susceptibility.