HISC Performance Validation of DSS Welds for Large-Strain, HP/HT Subsea Applications

Requirements for an Improved Assessment Approach

Alexander Amadioha & Andrew Low

AOG 2015 Conference, Perth, Australia, 12 March 2015

Agenda

� Introduction/Background�Existing Guidelines�Associated Risks and Qualification Gaps�A New Approach�Summary

� Oil exploration increasingly in harsh environments� H2S, CO2-containig products� High pressure-High temperature

� Consequently, increasing use of CRA materials� Solid/bulk, clad, lined linepipes

� Limited choice of welding consumables� Mainly DSS/SDSS; Ni Alloys (625, 686, 718, 725, etc.)

� Advantage: High strength retention and excellent corrosion resistance at elevated temperature

The Challenge

Temperature (°C) Pressure (MPa)HP/HT 150 - 175 69 - 103

Extreme HP/HT 175 - 200 103 - 138Ultra HP/HT ≥ 200 ≥ 138

The Challenge…cont’d

Automatic Weld Manual Weld

C-MnBacking

steel

CRA inner layer CRA inner layer

DSSweld C-Mn

Backing steel

Weakest link� HISC susceptibility� Temperature-induced strength

mismatch

Typical girth weld joints in clad/lined pipeline

HISC… What is it?

Susceptible material

HydrogenStress

HISC

• “Cracking due to a combination of load and hydrogen embrittlement (HE) caused byingress of hydrogen formed at the steel surface due to cathodic polarisation”.

• Time-dependent failure mode.

Hydrogen-Induced Stress Cracking

Process (R. Gangloff)

Fracture Mechanism f(CH)

R. Gangloff

� Permeability and solubility of hydrogen in the materials� Surface Characteristics (FJC Performance, etc.) � Microstructure

� Prior load history (cyclic and monotonic)

� Loading mode� Cyclic loading and ratcheting, static

� Weld joint strength mismatch

� Temperature� Pressure� CP potential� Residual stress

Main Factors

DNV-RP-F112; EEMUA 194…

� limit design stresses/strain� Within elastic limit or 0.5% strain

� apply effective barrier (coating)� 100% holiday-free coating cannot be guaranteed over the life of

field.� Cathodic disbondment, mechanical damage, etc.

� limit CP potential (less negative)� Not always practical

Existing Guidelines... In general

� Main design guidelines in DNV-RP-F112:2008

� Two criteria specified� Stress criteria (linear elastic)� Strain criteria (non-linear)

� §2 B101, Guidance Note: “it is the opinion of DNV that the probability of HISC failure is negligible when stress and strain are below the limits set forth . The probability of HISC failure for stress and strain above these limits is not known”.

� what are these limits???

Existing Guidelines… cont’d

� (Linear Elastic) Stress Criteria:� A design where the stress everywhere in the component is below 80%

of γHISC · SMYS is acceptable, i.e. <0.68SMYS for materials with coarse austenite spacing and < 0.85SMYS for materials with fine austenite spacing

Existing Guidelines…cont’d

� Non-linear Strain Criteria

Existing Guidelines…cont’d

� Surface: ~0.25% Lres strain (assuming residual strain = 0.25%)� Embedded: 0.35 – 0.75% Lres strain (assuming residual strain = 0.25%)

depending on material quality

� “The design limits in this DNV-RP-F112 are valid only inthe absence of sharp cracks in the material. In particularsurface breaking cracks are critical”. §4 B05

� Large-strain scenarios not covered

� “A standard test with clearly defined acceptance criteriato test the susceptibility to HISC of materials exposed tocathodic protection has not yet been established.Qualification testing has to be agreed”. §5 C201

� “A cut off temperature above which HISC does not needto be considered has not yet been established”. §3 C802

Moreover…

� Girth welds in solid/CRA-clad/lined pipes made with full penetration DSS/SDSS consumables inherently contain defects - surface-breaking and embedded - of “tolerable” sizes based on ECA-determined criteria.

� pipelines in service subsea subjected to significant levels of plastic deformation (>0.5 to 2% in some cases), e.g.� seismic events� ground movement (e.g. subsidence, landslide, etc.)� lateral and upheaval buckling and � restrained thermal expansion.

In reality…

E.g. Scenarios

Td = 50°CPd = 276bargDiameter = 609.6mmWall thickness = 29.3mm

SAFEBUCK III

� Special approach (special calibration blocks and transducers) required for detecting flaws in DSS/SDSS� unpredictable fluctuations in attenuation

Also…

“Reducing Risk of Hydrogen Induced Stress Cracking (HISC) in Duplex Stainless Steel for Subsea Applications”

Hannah et al, Shell UK, Duplex 2007.

Typical lack of fusion defects in GTAW welded duplex stainless steel

� Determine the performance of DSS/SDSS (girth) welds subjected to a combination of large strains and high temperature and pressure in seawater under cathodicprotection.

� Develop appropriate qualification route for DSS/SDSS girth welds for industry now

� Develop a standard test with clearly defined acceptance criteria to test the susceptibility to HISC of materials exposed to cathodic protection.

Imperative industry goal…

� enhanced use of DSS/SDSS girth welds for offshore projects where� schedule reduction� reduction of complex operation ECAs where temperature effects

impact weld strength performance� for linepipes, improvement in reeling technology for CRA-

clad/lined pipe � Improved weldability

� develop greater understanding of the performance of DSS/SDSS welds in large strain, HT/HP applications� optimise design � increase utilisation

Benefits

� Establish a failure parameter/criterion, independent of geometry, capable of predicting the stress state at critical locations.

� Global, fracture mechanics-based approach� Fracture toughness testing of pre-cracked DSS specimens exposed

to CP consistently show CTOD values below 0.05mm. §3 D101

� Local, micro-mechanical damage mechanics approach� Enhanced transferability of damage parameters

� development of a damage-tolerant approach for HISC integrity assessment

Potential Approach

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This presentation contains the professional and personal opinions of the presenter, which are given in good faith. As such, opinions presented herein may not always necessarily reflect the position of INTECSEA as a whole, its officers or executive.

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