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

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

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

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

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)

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?

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 898-1 ”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

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

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

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)

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)

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

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

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

Simple BN example

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

Acknowledgements Ramgo Thodla & Narasi Sridhar