Download Oliver Moghissi Materials Qualification for Bolting Applications

Materials Qualification for Bolting Applications
NAS Committee on Connector Reliability for Offshore Oil & Gas Operations
Oliver Moghissi
10 April 2017
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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
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Qualification vs Fitness for Service
 Significant # are brittle fracture (primarily H related) & fatigue issues.
 Materials requirements & qualification to address this
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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
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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)
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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?
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Industry Standards
 Current standards specify basic material properties and do not directly address inservice 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
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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
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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?
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*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)
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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)
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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?
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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
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Factors for Bolt Performance have Distributions
 Distributions come from
– Measurement uncertainties
– Non-homogeneous structure and environment
– Stochastic processes
 Examples
– Material properties
– Hydrogen in-service (e.g., CP, coatings)
– Environment (e.g., geometry/chemistry)
– In service load profile (e.g., strain rates)
Hydrogen
– Hydrogen before service
Hardness
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Simple BN example
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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 fitnessfor-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
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Acknowledgements
Ramgo Thodla & Narasi Sridhar
www.dnvgl.com
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