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SAFER, SMARTER, GREENER DNV GL © 2015 17 th October 2018 Materials for HPHT Applications 1 Current Issues & Future Solutions Narasi Sridhar and Ramgopal Thodla
DNV GL © 2015 17 August 2017 SAFER, SMARTER, GREENERDNV GL © 2015
17th October 2018
Materials for HPHT Applications
1
Current Issues & Future Solutions
Narasi Sridhar and Ramgopal Thodla
DNV GL © 2015 17 August 2017
2
>10 km
DNV GL © 2015 17 August 2017
API 17TR8: HPHT Design Guidelines for Subsea Applications
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DNV GL © 2015 17 August 2017
API 17TR8: HPHT Design for Subsea Applications
▪ HPHT design is a significant new challenge
facing the subsea sector, particularly in the
Gulf of Mexico
▪ API 17TR8 provides HPHT Design
Guidelines, specifically for subsea
applications
▪ First Edition issued February 2015
▪ Second Edition released in 2018
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DNV GL © 2015 17 August 2017
API 17TR8: HPHT Design for Subsea Applications
5
▪ Subsea HPHT design challenges
– T > 350°F
– P > 15ksi
– Harsh environmental conditions
– Sour production
– High H2S/Elemental S
– High Cl-
– Seawater with CP
– Low T (40°F)
– Also elevated T?
– Design approach
– Stress based vs. Fracture mechanics
– Failure modes
– Fracture
– Fatigue/EAC
DNV GL © 2015 17 August 2017
Materials Focus
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▪ Subsea materials typically high strength steels
▪ But elevated T and P generally requires use of high strength nickel based alloys
and/or clad material
▪ Modification of design philosophy (Fracture mechanics vs. Stress based)
– Environmentally assisted fracture & fatigue become critical in design
▪ Testing required to characterize environmentally assisted cracking behavior
– SSR
– Fracture toughness
– Fatigue (FCGR / S-N)
▪ Operating conditions
– HPHT
▪ Shut in conditions
– Seawater + CP at low T
DNV GL © 2015 17 August 2017
API 17TR8: HPHT Design for Subsea Applications
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▪ Annex D: Material Characterization Protocols
– New in Second Edition
– Guidelines for use of metallic materials (low
alloy steels and CRAs) for HPHT applications
– Generating material properties suitable for
the application of fracture mechanics based
approaches to the design of subsea
equipment
DNV GL © 2015 17 August 2017
ISO 15156/MR-01-75 Limits
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How to specify limit regions of application?
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Overall Framework to Understand Environmentally Assisted Cracking
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Environmentally Assisted Cracking Regimes of CRA’s
pH
Pote
ntial
221 HeH
OH4e4OH2O 22
DepassivationpH
SSC
SSC requires H generated by corrosion – for CRA this will happen only below depassivation pH
HSCHSC requires galvanic coupling with steel and certain microstructural conditions (e.g., aging, strength, etc.)
SCC
SCC controlled by local anodic dissolution, stress, microstructure.Focus of JIP
SCC
SCC controlled by local anodic dissolution, stress, microstructure
DNV GL © 2015 17 August 2017
A concept for conditions leading to SCC
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Time (Environmental Variables)
Para
mete
rs Corrosion Potential
Strain Rate
Critical Potential
Onset of SCC
Onset of Pitting/Crevice
Just three parameters drive the SCC of CRA’s
Uncertainties in these parameters lead to risk
Critical
strain
rate
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Loading Effects
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MTD Fatigue and Fracture Capabilities
Servo Electric Frames 2250 – 4500 kgf (5,000 – 20,000 lbf capacity)
− FCGR, Smooth SN, and SSR 0.1 – 35 MPa (14.7 – 5,000 psia) @ -
40 to 220°C
Servo Hydraulic Frames 5,000 – 50,000 kgf (11,000 – 110,000 lbf
capacity)
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DNV GL © 2015 17 August 2017
Typical Operating Conditions
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Pressu
re
Tem
peratu
re
5ksi/10ksi/20ksi
Ambient
100°F/250°F/400F
Time
20 – 50 days
~a few days
~10’s min to hours
– Flow lines
– Fatigue loading from pressure/thermal transients (~hours)
– Static loading associated with long steady operations
(~months)
– Risers
– Wave motion & Vortex Induced Vibration (VIV)
DNV GL © 2015 17 August 2017
Design Considerations
15
Time
Load
Ambient/Seabed Temperatures
20 – 50 days
Design PhilosophyNo CGR K<Kth
FCGR- Low frequency
- Wave frequency
Exposure - Sour Environments (H2S/CO2 containing)
- Seawater + CP
~a few
days
~10’s min to hours
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Effect of frequency and R-ratio (718)
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SCC of CRA in Sour Service
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DNV GL © 2015 17 August 2017
CRA JIP - Major Results of Phase 1
▪ The concept that SCC occurs only above the repassivation potential for
localized corrosion was validated for CRA’s
▪ Crack growth rate tests were shown to be more reliable than SSRT and
C-ring tests
▪ The localized corrosion susceptibility of CRA’s in sour environments was
successfully modelled
▪ A predictive model for CRA’’s in sour environments was developed in
collaboration with OLI Systems
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DNV GL © 2015 17 August 2017
CRA JIP - Phase II Ongoing
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▪ a
• Validation of Erp Prediction and its Correlation with SCC Task1
• Rapid Performance Evaluation with Rising Displacement TestsTask 2
• CGR Tests to Explore Environmental Limits Task 3
• Guidelines for Performing Rising Displacement Tests to Evaluate CRA’s in Sour Environment
Task 4
• OLI Model CompletionTask 5
DNV GL © 2015 17 August 2017
Status of Phase II Results
▪ The results from rising displacement tests clearly showed the difference
in the material performance as a function of H2S partial pressure
– The impact of other environmental variables can be explored
– The benefit of the rising displacement tests is that the tests can be
done fairly quickly compared to the CGR tests
– However, we should be cautious in using the da/dt results to judge the
service life of the material
▪ Changing conditions in the CGR can result crack stalling without careful
management of the crack growth
– It seems that dynamic strain is necessary to sustain the crack growth
for S13Cr
– More efforts are being carried to established a sound way to change
conditions in the CGR test to evaluate the CGR as a function of the
environmental variables
DNV GL © 2015 17 August 2017
Effect of Metallurgy - 718
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Metallurgy of 718
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Carbides: (Nb,Ti)C - Cubic
’: Ni3(Al,Ti) – Ordered FCC
”: Ni3(Nb,Al,Ti) – Ordered BCT
: Ni3(Nb8Ti2) - Orthorhombic
API – 120K
Solutionize
API – 150K
DNV GL Log ID Solutionizing Aging
2276 1030C/1.5h/WQ 780C/7h/AC
2624 1030C/1.5h/WQ 718C/8h + 621C/8h/AC
Heat Treatment
DNV GL Log ID Ni Fe Cr Nb+Ta Mo Ti Al Co Cu C
2276 (120K) 53 18.72 18.4 4.91 2.88 0.98 0.44 0.21 0.07 0.022
2624 (150K) 53.1 18.2 18.5 5.08 3.04 0.95 0.55 0.31 0.04 0.016
Chemical Analysis (wt%)
A range of HT conditions in the API Spec can produce varying
strength/microstructure
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Crack Propagation
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120 ksi 150 ksi
Crack morphology is IG in 120K and TG in the 150K
DNV GL © 2015 17 August 2017
Microstructure
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150 ksi
120 ksi
120 ksi
150 ksi
phase is evident in 120K along the
boundaries, no significant along the
150K boundaries
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Nature of ” Precipitates
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120 ksi 150 ksi
Is size the fundamental parameter of importance in H
embrittlement?
DNV GL © 2015 17 August 2017
Effect of Applied Potential - 718
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▪ CGR is a strong function
of applied potential
▪ No significant difference
between 120K vs 150K
under constant K
conditions.
▪ Low stable CGR is
observed at applied
potentials in the range of
-950mV to -850mV SCE.
-1200 -1100 -1000 -900 -800
10-8
10-7
10-6
10-5 718 - 150ksi (2624)
718 - 120ksi (2276)
718 - 120ksi (2276)
718 - 120ksi (2948)
718 - 140ksi - AM (2988)
CG
R (
mm
/s)
Potential (mV SCE)
3.5wt% NaCl
pH = 8.2
40F
K = 90ksiin
DNV GL © 2015 17 August 2017
Is this behavior unique to 718?
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Effect of K on CGR – Characteristics Needed for Design
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CGR behavior of different alloys systems is significant different –
718 appears to be the most resistant 725 is the most susceptible
DNV GL © 2015 17 August 2017
Key Questions to Address
▪Does the specimen size have an effect on the
measured CGR?
– i.e. at the same nominal K value does the effect of specimen
size effect the measured CGR?
▪What is the effect of specimen size on the
measured Kth?
– Are rising displacement tests affected by specimen size?
–Does the Kth decrease with decreasing specimen size due to
increased plasticity which might promote H embrittlement?
– If so, what is the right specimen size to use?
DNV GL © 2015 17 August 2017
Performance Assessment of Sub-sea Bolts
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DNV GL © 2015 17 August 2017
Background
▪ Low Alloy Ferritic Steels are commonly used for bolting applications
▪ Currently the steels are specified based on hardness limits (34HRC) for H
embrittlement resistance.
– This limits the YS to about 105ksi
– Different standards have different limitations on the HRC and YS requirements
▪ Low alloy steels in general are susceptible to H embrittlement.
– Function of YS
– Microstructure
– H concentration
▪ Hydrogen in the steels could be from multiple sources
– Plating processes in particular electrolytic processes
– Hydrogen from seawater + CP
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DNV GL © 2015 17 August 2017
Issues involved
▪ Uncertainties in field conditions
▪ Uncertainties in lab test data
– Effect of test methodology on measured results
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DNV GL © 2015 17 August 2017
Analysis of field failures (BSEE, others)
▪ Failure probabilities are skewed
– Not all failures and near failures are historically reported
– Failure analyses focus mostly on failed bolts and not on intact bolts
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Hardness Failure fraction
28-32 0.29
33-38 0.44
39-42 0.92
>43 1.0
DNV GL © 2015 17 August 2017
Test Methods to Evaluate Cracking Susceptibility
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Rising Displacement Tests
• Sensitive to rate typically take 4-10days
• Increasing K
• Identify crack initiation
• Measure CGR
Constant Load Tests
• Duration (1month to 1 year)
• Identify cracking/no cracking
(Constant K)
• Can measure CGR
DCB/WOL
• Duration (1month to 1 year)
• Identify crack arrest (Decreasing K)
• Can measure CGRa K
t
DNV GL © 2015 17 August 2017
10-3 10-2 10-1 100
1
10
K = 20ksiin (692Nmm-3/2)
R = 0.5
RT
Production Environment
NaCl:74.196g/L
NaHCO3: 0.13g/L
pH2S = 0.21psia
Inh: 40ppm (EC1304A)
RT
Production Environment
PP As Fabricated
PP Intrados
PP Intrados - No H2S
FC
GR
en
v/F
CG
Rin
-air
f (Hz)
FCGR Behavior
FCGR is a strong function of frequency in sour environments.
Yu, Weiwei, et al.CORROSION 2017,
2017. NACE International.
DNV GL © 2015 17 August 2017
Fracture Toughness in Environment
▪ Decreasing K-rate results in decreasing toughness.
▪ No significant difference in initiation toughness lower than a K-rate of 0.16Nmm-
3/2/s however, the slope of the R-curve continues to decrease at the lower rate
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0.0 0.5 1.0 1.5 2.0 2.5 3.00
200
400
600
800
K-rate - 1.6Nmm-3/2
/s
K-rate - 0.16Nmm-3/2
/s
K-rate - 0.05Nmm-3/2
/s
K-rate - 0.05Nmm-3/2
/s
K-rate - 0.05Nmm-3/2
/s
J (
N/m
m)
a (mm)
5wt% NaCl
pH = 5
pH2S
= 0.46psia
DNV GL © 2015 17 August 2017
Variability in embrittlement susceptibility (laboratory data)
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Depending on the test technique, significant variability in lab test results
versus hardness
DNV GL © 2015 17 August 2017
Preliminary Data – Effect of Applied Potential
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Cracking of LAS is a strong function
of H concentration
• Measurement of quantitative
parameters to characterize
differences due to applied potential?
• CGR is proposed as a quantitative
measure for susceptibility
The H concentration in LAS
is a strong function of
microstructure and applied
cathodic potential
DNV GL © 2015 17 August 2017
Role of Applied Potential – Cyclic load vs. Static Loading (LAS)
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Effect of material strength
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Uncertainties in lab data - 718
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DNV GL © 2015 17 August 2017
Testing methodology issues
▪ Rising displacement test provides quick answers but is too conservative for
design. It also does not provide a reasonable measure of crack growth rate (CGR)
▪ Pseudo-static tests (low frequencies with long hold times but R=0.6) provide
more realistic CGR but take a lot of time to generate sufficient data
– Tests with higher R ratios can stall and create uncertainty in prediction
▪ Cyclic loading test at moderate R-ratios (R = 0.2 to 0.6) and 0.1mHz provide
slightly higher CGR, but take shorter time
– There is insufficient data to extrapolate from low frequency cyclic tests to
pseudo-static tests
– Materials such as 718 may be ok due to planar deformation mode; Low alloy
steels may be more sensitive
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DNV GL © 2015 17 August 2017
Probabilistic prediction
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DNV GL © 2015 17 August 2017
Probabilistic approach based on analytical model (Gangloff model)
▪ Assume an analytical expression for CGR as a function of K and fit experimental data at
each applied potential to derive model parameters. Use Monte Carlo method to derive
distribution
▪ Calculate effective K for bolts under remote loading and assumed initial flaw size and
pre-load distributions
▪ Assume external CP is the same as at the thread roots
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Gangloff, Corrosion, 72(7), 2016
DNV GL © 2015 17 August 2017
Example calculation using Gangloff ScCrack Model
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• CGR data taken from
DNVGL data (145 ksi
steel)
• Initial crack size
randomized around 1
mm
• Applied CP: -1050 to -
900 mV SCE
DNV GL © 2015 17 August 2017
Alloy 718 CGR probability calculation (using DNVGL data)
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• Sensitivity analysis showing pre-load has
the biggest effect (among modelled
variables)
• Probability distribution of CGR
DNV GL © 2015 17 August 2017
Potential distribution within crevices
▪ Turnbull and May (1983) did measurements of crevice potentials in 3.5% NaCl and artificial
seawater on steels. They did not see a major difference in CP levels in crevice vs. outside down
to -1050 mV vs. SCE.
– As long as seawater can penetrate, CP will penetrate
– Calcareous deposits may influence CP penetration depending on porosity and tortuosity (but it
is likely to influence only how fast CP propagates)
▪ Modeling can be done in various crevices using commercial finite element codes
– Beasy does steady-state model assuming constant conductivity throughout crevice area and
unchanging with time
– COMSOL does transient model and can vary conductivity within crevices and with time
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DNV GL © 2015 17 August 2017
Discussion points
▪ Laboratory CGR data can be used to generate probability curves for crack growth
– Need more data to improve fit equations
– Faster CGR data generation methods should be used to correlate to SCGR data
to increase data throughput
▪ Probabilistic modelling can be used identify most sensitive parameters and
prioritize data needs
▪ Modeling of CP distribution inside tight crevices in bolts can be performed using
commercial FEA codes
– Assumptions about conductivity can be made
– Conductivity can be calculated to include calcareous deposits
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DNV GL © 2015 17 August 2017
Summary
▪ A framework is available to predict the performance of CRA in HPHT applications
– Significant data needs exist, but are being developed
– Understanding test method effects and being able to extrapolate to long-term
performance are needed
▪ Strength/hardness is NOT the sole factor in determining environmentally assisted
cracking
– Microstructural characteristics are important
– As new alloys/manufacturing methods are developed, existing hardness specs
may not be adequate
▪ Probabilistic framework helps in assembling diverse dataset and making decisions
under uncertainty
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DNV GL © 2015 17 August 2017
SAFER, SMARTER, GREENER
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Thank you
Questions?
51
Ramgopal Thodla
