1. Materials Qualification for Bolting Applications
NAS Committee on Connector Reliability for Offshore Oil & Gas Operations

2. Bolting Background
Subsea bolted connections can be critical to system integrity
Wellheads, Xmas trees, Flanges, Structural connections, etc.
Specs & standards
Usually adequate but not uniform
Non-conformance
Bolting material performance
Strength, Corrosion, Galling, Hydrogen susceptibility
Bolting manufacture & quality management
Varied & not fully known because of traceability
Few identified failures
Is it just statistics of crack distribution?
Overlapping/Aggregate uncertainties
Significant consequence
Weak reporting of incidents (failure or not) when not mandated

3. Qualification vs Fitness for Service
Significant # are brittle fracture (primarily H related) & fatigue issues.
Materials requirements & qualification to address this

4. Commonly Used Materials
Low Alloy Steels both ferritic and martensitic (up to 125ksi)
Typically limited to 105ksi subsea
Monel (e.g., K500)
Used both topside and subsea both by O&G and Navy (with known failures)
Stainless Steels (ASTM A286)
300/400 series (susceptible to localized corrosion & HE)
Duplex
2507 (HE susceptible)
Nickel Based Alloys
718/725 (HE susceptible)
C276/625/686
‘Alloying up’ is not always the answer

5. What Are We Testing? What are technical causes for failures?
Material properties
e.g., Hardness criteria (e.g., >34 HRC); non-homogeneous in bolt
Excess hydrogen
Before service (i.e., and not baked out)
In-service (e.g., CP, coatings)
Environment
e.g., crevice, biofilm, contact with internal fluids
Load & design characteristics
Bolt installation
In service load profile (e.g., strain rates)

6. Why are We Testing?
Ultimately, we are assessing a material’s fitness for service
Material qualification is normally testing according to a standard and comparing to a pass/fail criterion
A single criterion represents many service conditions (usually hardness)
Prescriptive over Performance
A safety-factor or other conservatism is usually built in
In most cases, it is costly
In some cases, it compromises safety
Leads to exception requests
Can failures occur despite meeting every existing materials spec?

7. Industry Standards
Current standards specify basic material properties and do not directly address in- service performance
ASTM A 193 ”Alloy steel and stainless steel bolting materials for high temperature service” (ferritic steels Grade B7, B7M)
ASTM A 320 ”Alloy steel bolting materials for low temperature service” (ferritic steels Grade L7, L7M)
ASTM A 354 ”Quenched and tempered alloy steel bolts, studs and other externally threaded fasteners”
ISO ”Mechanical properties of fasteners made of carbon steel and alloy steel – Part 1: Bolts, screws and studs
API 20E - Alloy and Carbon Steel Bolting for Use in the Petroleum and Natural Gas Industries
API 20F – Corrosion Resistant Bolting for Use in the Petroleum and Natural Gas Industries

8. Example – Low Alloy Steel Microstructures in API 20E
Requirements not explicitly tied to in-service damage modes
Requirements include
Processing (e.g., cast, forged, continuous Cast (not allowed for BSL-3))
Limits on banding, porosity, segregation
Wrought Microstructure is desired

9. Qualification Testing – Hardness
Hardness measurements used as proxy for hydrogen susceptibility
Requirements for steels vary from 34 – 38HRC among standards and company specs for subsea service
Hardness measurements required on center of bolt
Properties vary across the bolt (especially for rolled threads)
Are acceptance criteria already conservative to account for this?
Or do we measure highest hardness?
Do we have different specs for different bolts?
*B. Craig

10. Qualification for Hydrogen Content
H content measured using ASTM F1113
H ingress during manufacture (e.g., plating) and typically baked out
Hydrogen ingress can also occur in-service (e.g., CP)

11. Standard ASTM Qualification for H Embrittlement
ASTM F1624 ‘Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique1
Step loaded method to identify threshold stresses for fastener crack initiation in environment (e.g., primarily seawater + CP)

12. Load Rate and Profile affects Fracture Toughness Measurement
Slow continuous rising displacement test (modified ASTM E1820) shows low toughness
Step loading (ASTM 1624) is sensitive to hold time, even at same effective loading rate
Which represents service conditions?
Which is conservative or non-conservative?
K-rate loading 10x lower than API 17TR8

13. Simulating Service Conditions – HPHT Example
Typical service conditions involve some load changes and long periods of constant loads
Test methods must simulate loading profiles to represent service conditions
Load magnitude and Load Profile are both important

14. Factors for Bolt Performance have Distributions
Distributions come from
Measurement uncertainties
Non-homogeneous structure and environment
Stochastic processes
Examples
Material properties
Hydrogen before service
Hydrogen in-service (e.g., CP, coatings)
Environment (e.g., geometry/chemistry)
In service load profile (e.g., strain rates)
Hardness
Hydrogen

15. Simple BN example

16. Conclusions Current state Majority of fasteners perform as intended
Exceptional failures need to be addressed
True failure rates not known
Currently being addressed
Existing specs & standards for materials qualification adequate most of the time
Lack uniformity
Conservative most of the time and non-conservative some of the time
Non-conformance is possibly deficiency
Future
Step-change improvement realized by performing materials qualification through fitness- for-service lens
Improved understanding of failures will require better failure analysis and reporting
Probabilistic component of performance requires understanding of distributions and how they aggregate

17. Acknowledgements Ramgo Thodla & Narasi Sridhar