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

Transcription

1 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

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

3 The Challenge Oil exploration increasingly in harsh environments H 2 S, CO 2 -containig products High pressure-high temperature Temperature ( C) Pressure (MPa) HP/HT Extreme HP/HT Ultra HP/HT 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

4 The Challenge cont d Typical girth weld joints in clad/lined pipeline C-Mn Backing steel DSS weld C-Mn Backing steel CRA inner layer CRA inner layer Automatic Weld Manual Weld Weakest link HISC susceptibility Temperature-induced strength mismatch

5 HISC What is it? Hydrogen-Induced Stress Cracking Susceptible material HISC Stress Hydrogen Cracking due to a combination of load and hydrogen embrittlement (HE) caused by ingress of hydrogen formed at the steel surface due to cathodic polarisation. Time-dependent failure mode.

6 Process (R. Gangloff)

7 Fracture Mechanism f(c H ) R. Gangloff

8 Main Factors 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

9 Existing Guidelines... In general 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

10 Existing Guidelines cont d 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???

11 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

12 Existing Guidelines cont d Non-linear Strain Criteria Surface: ~0.25% Lres strain (assuming residual strain = 0.25%) Embedded: % Lres strain (assuming residual strain = 0.25%) depending on material quality

13 Moreover The design limits in this DNV-RP-F112 are valid only in the absence of sharp cracks in the material. In particular surface breaking cracks are critical. 4 B05 Large-strain scenarios not covered A standard test with clearly defined acceptance criteria to test the susceptibility to HISC of materials exposed to cathodic protection has not yet been established. Qualification testing has to be agreed. 5 C201 A cut off temperature above which HISC does not need to be considered has not yet been established. 3 C802

14 In reality 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.

15 E.g. Scenarios SAFEBUCK III Td = 50 C Pd = 276barg Diameter = 609.6mm Wall thickness = 29.3mm

16 Also Special approach (special calibration blocks and transducers) required for detecting flaws in DSS/SDSS unpredictable fluctuations in attenuation Typical lack of fusion defects in GTAW welded duplex stainless steel Reducing Risk of Hydrogen Induced Stress Cracking (HISC) in Duplex Stainless Steel for Subsea Applications Hannah et al, Shell UK, Duplex 2007.

17 Imperative industry goal Determine the performance of DSS/SDSS (girth) welds subjected to a combination of large strains and high temperature and pressure in seawater under cathodic protection. 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.

18 Benefits 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 CRAclad/lined pipe Improved weldability develop greater understanding of the performance of DSS/SDSS welds in large strain, HT/HP applications optimise design increase utilisation

19 Potential Approach 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

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