National Academies of Sciences Workshop on
Subsea Bolt Performance
Session I - SUBSEA FASTENER DESIGN
REGULATIONS
Part 1 – Fastener Systems in Use in Critical Equipment
and the Diverse Environments in Subsea Oil and Gas
Drilling and Production
Khlefa A. Esaklul
Corrosion and Materials Advisor
Asset Integrity – Worldwide Engineering & Operation
Occidental Oil and Gas Corporation
Chair of NACE International Technical Coordination
Committee
Introduction
• Deepwater and subsea operation continue to be the future of the oil
and gas production to meet the growing energy needs
• As the demand for oil and gas increases, exploration in deepwater
increased and extended to higher water depths, higher pressure,
higher temperature and more aggressive environments
• This resulted in more complex operation and demand for higher
thickness, higher strength and cracking resistant materials for the
various components in these operations
• A decade ago the depth was < 7000 ft., today it is exceeding 10,000 ft.
of water depth (Pacific Santa Ana vessel can operate at 12000 ft.)
Deepwater Drilling Complexity
Deepwater Drilling Complexity - Large structures with
multiple connections
Deepwater Production Subsea System Complexity
EXPORT LINES
Subsea Trees & Jumpers
Background
• Flanged connections are still an integral part of any offshore
developments with fasteners being one of the primary means for
assembly.
• Development of deepwater reservoirs with higher reservoir pressure
and temperature requires a class of materials with optimum
combined properties that exceed the commonly used subsea
materials.
• Costly intervention and the demand for higher safety and
environmental protection increased the need for inherent design
reliability and highly reliable and proven performance parts.
• Fasteners of all types and sizes are integral part of these
components
• Fasteners with diameters that exceed 2.5 inch (100 mm) are
increasingly becoming more common.
Examples of Subsea Flanged Connection
Riser Flange
Flexible flowlines
BOP Configuration
Wellhead components
Example of the number of fasteners in use in various systems
Common Application of High Strength Fasteners
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US Bolt Website
Drilling risers
Connectors
Blowout preventers (BOP)
Subsea assemblies
Trees and wellheads
Risers, flowlines and pipelines tie point flanges
Internal assembly bolts for valves, connectors,
etc.
Fasteners Materials Selection Criteria
• Mechanical Properties
- Strength
- Toughness
• Corrosion Resistance
- General Corrosion
- Galvanic Corrosion
- Localized Corrosion (Pitting, Crevice, etc.)
• Resistance to Environmental Assisted Cracking
- Stress Corrosion Cracking
- Hydrogen Embrittlement
- Sustained Load Cracking
Challenges
• Loading conditions
– Static (weight, fluids column, pressure, etc.)
– Dynamic (ocean current, wave action, Vortex Induced Vibration, etc.)
• Environmental conditions
– External (salt water, temperature, CP interaction, stray current etc.)
– Internal (drilling fluids, produced fluids, etc.)
• Limited or difficult monitoring
• Inaccessibility
– For inspection
– For maintenance
Materials Options for Subsea Fasteners
Alloy
Group
Steels
Stainless
Steels
Ni
alloys
Material
A193-B7
AISI 4130
AISI 4340
SAF 2507
A286
Custom 455
PH 13-8 Mo
S17400
S17700
S15700
Ferralium 255
K-400
K-500
Alloy 718
Alloy 725
Alloy 925
Alloy 945
Alloy 946
Alloy 59
Alloy X-750
Alloy 625
Rene 41
0.2 %
Yield Strength (ksi)
75-105
120
145
80-95
95-119
135-231
190-205
145 – 170
140 – 230
170 - 230
110
30
86
120-145
120
110
140
155
128 -168
92
120
157
Tensile Strength
(ksi)
100-125
135
160
116-145
214-260
160-260
220-235
200
130
75-100
105-150
150-180
165
168
170
185
149 - 170
162
165
206
Materials Options for Subsea Fasteners
Alloy
Group
Co-Ni
Alloys
Be-Cu
Alloys
Ti
Alloys
Material
MP35N
MP159
25C17200
M25C17300
165C17000
Ti-6Al-4V ELI
Ti 38-6-44
Ti –5-1-1-1
Ti-16-2.5-3
Ti-6-6-2
Ti-6-2-4-6
Ti-7-4
Corona-5
Ti-4.5-5-1.5
Ti-10-2-3
Ti-8-8-2-3
Ti-11.5-6-4.5
Ti-13-11-3
Ti-15-3
Ti Beta 21S
0.2 %
Yield Strength (ksi)
230
250
145 – 190
145 – 190
125 – 185
128
115
111
120
145 – 160
150 – 160
150 – 160
120 – 153
150
170
175
160
190
205
Tensile Strength
(ksi)
280
260
200
120
126
130
SCC and HE Resistance
• High strength steel are susceptible to SCC and HE when cathodically
protected and their susceptibility increases with increasing YS.
• Steels with YS < 120 ksi are generally resistant to SCC and HE.
• Steels with YS > 120 ksi, the resistance decreases with increase in
strength. Typical KISCC is 50 – 75 ksi-in for steels with YS = 145 ksi.
Typical KIC for this steel is 200 ksi.in .
• NASA showed that in the absence of CP, AISI 4340 is resistant to
SCC up to tensile strength of 180 ksi (40 HRC) ~ 155 ksi YS.
KISCC as a Function of Yield Strength for 4340 Alloy Steel
Atlas of Stress Corrosion Cracking data, ASM International, 1984
KISCC as a Function of Yield Strength
Ref 17 in An Introduction to the Design and Behavior of Bolted Joints by John Bickford
Y. Chung Threshold preload levels for avoiding stress corrosion cracking in high strength bolts Tech Report 1984
Strength Limit with CP
• Historically alloy steel fasteners were limited to ASTM A320 L7M and
ASTM A193 B7M grades with a maximum hardness of 22 HRC, i.e.,
the specified limit for sour service applications per NACE
MR0175/ISO 15156.
• Studies have shown that limiting sub-sea steel fasteners to the sour
service requirements (22 HRC) is overly conservative.
• Instead subsea steel fasteners exposed to CP can be used to a
maximum hardness of 34 HRC per ISO/DIS 13628-1 recommended
practice.
• API 17D and Norsok Standard limit the hardness to HRC 35 for steel.
• For Corrosion Resistant Alloys (CRAs) materials, limits are per NACE
MR0175/ISO 15156
Qualification Testing
• In view of the limited data, selection of subsea fasteners applications
still relies on qualification testing for the specific application.
• HE testing can be by slow strain rate tests, C-ring, U-bends, notched
bars or fracture mechanics tests.
• Most of the experimental data suggest that many materials are
prone to hydrogen embrittlement based on accelerated testing but
until recently there have been limited reported failures in the field.
• The challenge has been in how to establish reliable test methods for
materials qualification and asses the risk to HE.
Materials Susceptibility to HE
Alloy / Condition
HE Susceptibility
Comments
AISI 4340 and 4130 >
120 ksi Sys
Susceptible for hardness > 34 HRC at
potential of - 950 mV or more negative
Based on lab testing and field
experience
Grade L7 ASTM A320
Susceptible for hardness > 34 HRC at
potential of - 950 mVSCE or more
negative
Based on lab testing and field
experience
SAF 2507
Slight or some effect of cathodic
protection
No or very little effect of cathodic
protection
Susceptible at potential of -1000 mVSCE
Crack growth tests
Alloy 286 (solution
treated and aged)
Resistant
Based on SST, C-ring, tensile and
fracture mechanics tests
K-500 All conditions
Susceptible at potential of - 850 mV or
more negative
No or very little effect of CP
Resistant
Based on field experience and
slow strain rate tests
Crack growth tests
Based on slow strain rate tests
254 SMO
Ferralium 255
Marinel
Beryllium Copper
Crack growth tests
Based on slow strain rate tests
Materials Susceptibility to HE
Alloy / Condition
Alloy X-750
HE Susceptibility
Comments
Susceptible at potential of - 1000 mV
or more negative
Susceptible at potential of - 1250 mV
Based on field experience and slow
strain rate tests
Based on slow strain rate tests
Crack growth tests
Alloy 725 (solution
treated and dual
aged)
Slight or some effect of cathodic
protection

Susceptible at potential of - 850
mV or more negative
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Resistant
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Susceptible at potential of - 850
mV or more negative

Resistant
Alloy 945
Susceptible at 5 mA/cm2 CP current
Based on notch tensile tests
Alloy 946
Susceptible at 5 mA/cm2 CP current
Based on notch tensile tests
Ti-6Al-4V (solution
treated and annealed)
Variable susceptibility
Based on slow strain rate tests,
notched bars, U-bends.
Ti-5111 (as forged)
Resistant
Based on slow strain rate tests and
fracture toughness
Alloy 925 (solution
treated &dual aged)
Alloy 625
Alloy 718 (solution
treated &dual aged)
Based on slow strain rate tests
Based on CT tests
Based on slow strain rate tests
Based on slow strain rate tests
Subsea High Strength Fasteners in Use
• High strength steels
– AISI 4140 and 4340 for 125 ksi YS
– AISI 4340 for >135 ksi YS
• PH Nickel alloys
– 718, 725, 945HS, 946 and 625HS for > 135 ksi YS
– 718, 725, 925, 945, 625HS for 125 ksi YS
• PH Stainless steel
– A286 UNS 06660 for 105 – 125 ksi YS
• Titanium alloys (Ti-6-4 ELI used in Heidrun drilling riser)
Corrosion Control
• For above water and in the splash zone, coating and
encapsulation are used with good success.
• For subsea, cathodic protection works well such that no
coating is required. Coating is used to protect fasteners
prior to installation.
• Cathodic protection potential can vary and could exceed
-1100 mV in some areas near anodes or systems where
both impressed current and anodes could co-exist or
possible stray current.
Fasteners Coating
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Electroplating
– Cadmium electroplating
– Zinc and Zinc Nickel electroplating
Hot dip Coating
– Hot dip galvanizing
– Hot Dip-spun galvanizing
Mechanical plating
– Zinc and aluminum plating (+ phosphating)
Phosphating
– Zinc Phosphate
Organic Coating
– Xylan (PTFE)
– Xylar
– Teflon or PTFE
• Electroless nickel
Temperature Limit
608 ◦F
788 ◦F
800 ◦F
800 ◦F
800 ◦F
Ambient
450 ◦F
1000 ◦F
450 ◦F
1600 ◦F
Most Widely Used Coatings for Subsea Applications
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Zinc Electrplating
Dip galvanizing
Zylan
Zinc Phosphate
Why there are Less failures than Predicted?
• Low operating stress - Design stresses are at 66% YS.
• Design based on worst-case conditions that include too extreme
loading conditions (100 Year Storm) applied stress ~ 70 – 80% UTS.
• Fasteners are shielded from CP system via Isolation and grease
packing.
• The severity of the tests used to qualify the materials for these
applications.
• Recent failures suggest these conditions may have changed and/or
co-existed as drilling extended to higher water depths
Why did the recent failures occur?
• 3 ¼ inch diameter bolts with a range of hardness measured in one lot
used for drilling riser applications over a decade ago exceeded the 34
HRC limit with no failures.
• Is it the load?, the material? or the environmental conditions?
• Data appears to suggest that a combination of the above are the likely
cause
– Loading conditions could be under estimated (wave action, ocean current, depth, etc.)
– HE susceptibility increases with stress level particularly at stresses approaching yield
strength for most if not all materials
– Quality assurance may not be sufficient to control strength, microstructure, hydrogen
ingress, etc.
– CP overprotection may have increased either through over design or unaccounted for
conditions (Zn coating, anode and impressed current, low temperature, etc.)
Materials Specifications and Quality Assurance

High strength alloys for subsea applications must adhere to
specifications mainly heat treatment, degree of cold work and
maximum hardness to ensure sufficient resistance to EAC.

Tighter specifications are needed to ensure adequate resistance to
HE.
Materials Options
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For large diameter (> 2 ½ inch) fasteners with yield strength of 150 ksi, the
options are limited to the following alloys:
- Alloy steels (AISI 4340)
- Alloy 718
- Alloy 725
- Alloy 945
- Alloy 946 HS
- MP 159
- Ti Alloys
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MP 35N, 17-4PH H1100, Alloy A286, Alloy 925, Rene 41, Alloy 625, Alloy
686 and Be-Cu alloys do not meet the strength / size requirements
Summary
• Several materials options are available to meet the needs of the
industry in high strength fasteners.
• These materials require more characterization of their limits to HE
and effect of sacrificial coatings where used
• There is a need for better monitoring systems to measure the level
of CP in subsea systems and any potential interference.
• Tighter specifications and quality control are needed to ensure
materials are within specified limits (e.g. hardness) to ensure
adequate resistance to HE.
• Effect of thread cutting rolling vs. machining is still unresolved
• On line monitoring of loads in drilling risers are needed to determine
the effect of ocean currents, VIV, etc.