1. 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

2. 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 ft.)

3. Deepwater Drilling Complexity

4. Deepwater Drilling Complexity - Large structures with multiple connections

5. Deepwater Production Subsea System Complexity
EXPORT LINES
EXPORT LINES
EXPORT LINES

6. Subsea Trees & Jumpers

7. 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.

8. Examples of Subsea Flanged Connection
Riser Flange
Flexible flowlines
BOP Configuration
Wellhead components

9. Example of the number of fasteners in use in various systems
CP Anodes

10. Common Application of High Strength Fasteners
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.

11. 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

12. Challenges Loading conditions Environmental 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

13. Materials Options for Subsea Fasteners

14. Materials Options for Subsea Fasteners

15. 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.

16. KISCC as a Function of Yield Strength for 4340 Alloy Steel
Atlas of Stress Corrosion Cracking data, ASM International, 1984

17. 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

18. 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
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 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

19. 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.

20. Materials Susceptibility to HE
Alloy / Condition
HE Susceptibility
Comments
AISI 4340 and 4130 > 120 ksi Sys
Susceptible for hardness > 34 HRC at potential of mV or more negative
Based on lab testing and field experience
Grade L7 ASTM A-320
Susceptible for hardness > 34 HRC at potential of mVSCE or more negative
SAF 2507
Slight or some effect of cathodic protection
Crack growth tests
254 SMO
No or very little effect of cathodic protection
Ferralium 255
Susceptible at potential of mVSCE
Based on slow strain rate 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 mV or more negative
Based on field experience and slow strain rate tests
Marinel
No or very little effect of CP
Beryllium Copper

21. Materials Susceptibility to HE
Alloy / Condition
HE Susceptibility
Comments
Alloy X-750
Susceptible at potential of mV or more negative
Based on field experience and slow strain rate tests
Alloy 925 (solution treated &dual aged)
Susceptible at potential of mV
Based on slow strain rate tests
Alloy 625
Slight or some effect of cathodic protection
Crack growth tests
Alloy 718 (solution treated &dual aged)
Susceptible at potential of mV or more negative
Resistant
Based on CT tests
Alloy 725 (solution treated and dual aged)
Alloy 945
Susceptible at 5 mA/cm2 CP current
Based on notch tensile tests
Alloy 946
Ti-6Al-4V (solution treated and annealed)
Variable susceptibility
Based on slow strain rate tests, notched bars, U-bends.
Ti-5111 (as forged)
Based on slow strain rate tests and fracture toughness

22. 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 for 105 – 125 ksi YS
Titanium alloys (Ti-6-4 ELI used in Heidrun drilling riser)

23. 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 mV in some areas near anodes or systems where both impressed current and anodes could co-exist or possible stray current.

24. Fasteners Coating Electroless nickel 1600 ◦F
Electroplating Temperature Limit
Cadmium electroplating ◦F
Zinc and Zinc Nickel electroplating ◦F
Hot dip Coating
Hot dip galvanizing ◦F
Hot Dip-spun galvanizing ◦F
Mechanical plating
Zinc and aluminum plating (+ phosphating) 800 ◦F
Phosphating
Zinc Phosphate Ambient
Organic Coating
Xylan (PTFE) ◦F
Xylar ◦F
Teflon or PTFE ◦F
Electroless nickel ◦F

25. Most Widely Used Coatings for Subsea Applications
Zinc Electrplating
Dip galvanizing
Zylan
Zinc Phosphate

26. 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

27. 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.)

28. 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.

29. Materials Options
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
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

30. 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.