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Stress based design guidelines for hydrogen induced stress cracking (HISC) avoidance in duplex materials

Pedro M. Vargas
Chevron ETC, Texas, USA

Stig Wästberg
Det Norske Veritas (DNV), Høvik, Norway

Paul Woollin
TWI, Cambridge UK

Paper presented at 28th International Conference on Offshore Mechanics and Arctic Engineering (OMAE 2009), Honolulu, Hawaii, 31 May - 5 June 2009.

Abstract

Following the failure of several subsea components made of duplex steel, two JIPs were formed, one by TWI and another by DNV and Sintef to address the failure mechanism and to formulate design guidance for the industry. (TWI: The effects of notches and welds on hydrogen embrittlement stress cracking of duplex stainless steels, Sintef/DNV: HISC) Hydrogen charging from the cathodic protection system in the presence of creep strains embrittles the duplex steel, making the duplex susceptible to cracking (hydrogen-induced-stress-cracking, HISC). Creep effects focused on strain measurements in the test specimens from early work at TWI, favoring a strain based approach in the development of early versions of the design guidance for the industry. This paper summarizes the relevant content from the two JIPs to formulate a stress based design criteria, and provides new FEA assessment of the Foinhaven Hubs to better quantify the effect of residual stresses. The basis for the stress-based design guidelines in DNV-RP-F112 is presented that promises to be easier to apply and equally robust as the strain-based approach.

Introduction

Duplex subsea equipment failures to date have been studied extensively and they can be categorized into two types: 1) Failures in large forgings due to excessive loading, at times combined with unfavorable coarse austenitic spacing, and 2) fillet/socket welds that are under-designed and/or with high ferritic content.

For the forgings, the failure did not typically occur at the weld toe, but instead at a stress concentration location removed but in proximity of a girth weld. The fracture surface showed signs of hydrogen embrittlement in the form of transgranular brittle features. The areas of failure were not coated.

For the fillet/socket welds, due to undersized welds or high loading, overloading was found to be a culprit, and coatings were either not present or damaged.

These HISC failures have initiated from the outer surface which is exposed to cathodic protection.

The experience base suggests that there have been no failures where all the requirements set by the design codes have been fulfilled for the material, fabrication or loading at all times before HISC occured. The reported failures have been in fillet welds not in accordance with code (high ferrite content and/or lack of fusion) and external forces that exceeded design loads.

The level of reliability of these duplex components has been shown to be little less than what industry expects, i.e., most designs can tolerate a little overload/bad design, and still function adequately. Conservatisms built into the codes generally help make this happen. DNV-RP-F112 focuses on providing limits against stress and strain that directly address HISC.

Nomenclature

σm : through wall membrane stress.
σb : through wall bending stress.
σm+b : through wall membrane+bending stress.
R : outside radius
t : wall thickness
γHISC : material factor for coarse grain duplex material
σyield : 0.2% offset yield stress, also called proof stress
SMYS : Specified minimum yield stress, also σyield
Lres : Critical distance for residual stress effects, 2.5σm for the through-wall membrane stress and, σm+b for the through-wall membrane + bending stresses. These structural stresses give more information on the gross creep potential of the wall than the local peak stresses. This has been validated through a significant amount of testing using notched round bars, single edge notched tensile specimens and full scale girth welds.

Figure 2 shows the separation of these stresses from an arbitrary stress gradient. Since the stress based criteria addresses the notch acuity effects separately, and conventional design practices limit any significant effect on the through-wall structural stresses, linear elastic finite element analyses is sufficient to obtain the stresses needed.
σm, ratio of the membrane+bending stresses to the applied nominal membrane stress at the notch cross section, is also reported in Table 1. The average membrane stress is in the plane of the notch, not the nominal remote stresses in the un-notched body of the specimen. These confirm that the SENT specimens are undergoing significant bending: approximately 40% for the V=notch and 50% for the U-notch specimens. The U-notch specimens have more bending due to the larger notch; the applied eccentricity of the loading is higher.

Table 1: Stresses in HISC Specimens



m: σm

U-notch 80% Proof 90% Proof 100% Proof 110% Proof
(MPa) % Proof (MPa) % Proof (MPa) % Proof (MPa) % Proof
Load m 480.122 80.0% 540.288 90.0% 603.32 100.6% 661.028 110.2%
b 276.158 46.0% 308.169 51.4% 334.731 55.8% 358.528 59.8%
m+b 756.28 126.0% 848.457 141.4% 938.051 156.3% 1019.56 169.9%
p 2.51204 0.4% -74.9184 -12.5% -144.919 -24.2% -198.579 -33.1%
  SCF 1.58 1.57 1.55 1.54
Creep m 480.293 80.0% 540.762 90.1% 601.482 100.2% 665.999 111.0%
b 271.498 45.2% 294.2 49.0% 306.393 51.1% 306.202 51.0%
m+b 751.792 125.3% 834.962 139.2% 907.875 151.3% 972.201 162.0%
p -141.719 -23.6% -192.16 -32.0% -226.674 -37.8% -232.687 -38.8%
  SCF 1.57 1.54 1.51 1.46
V-notch 80% Proof 90% Proof 100% Proof 110% Proof
(MPa) % Proof (MPa) % Proof (MPa) % Proof (MPa) % Proof
Load m 480.269 80.0% 540.463 90.1% 600.675 100.1% 661.077 110.2%
b 200 33.3% 223.963 37.3% 246.427 41.1% 264.654 44.1%
m+b 680.269 113.4% 764.426 127.4% 847.102 141.2% 925.731 154.3%
p 169.608 28.3% 108.537 18.1% 55.2359 9.2% 23.3327 3.9%
  SCF 1.42 1.41 1.41 1.40
Creep m 480.435 80.1% 540.788 90.1% 599.755 100.0% 665.014 110.8%
b 197.737 33.0% 216.429 36.1% 227.365 37.9% 236.833 39.5%
m+b 678.172 113.0% 757.217 126.2% 827.12 137.9% 901.847 150.3%
p -16.7974 -2.8% -57.496 -9.6% 55.0239 9.2% -91.9798 -15.3%
  SCF 1.41 1.40 1.38 1.36
, average membrane stress
b: σb, linearized bending stress
m+b: σm+b : σm + σb
p: σp, peak stress such that σm + σb + σp = σnotch

Figure 8 shows the normalized net section stress (again at the notch cross section) as a function of time to HISC failure in hours. Note that the net section average membrane stress limit of 80% is validated, although one V-notch specimen fails at 78% average membrane loading of the proof load.
σb
p: σp, peak stress such that σm + σb + σp = σnotch

The critical location for the Foinhaven hubs is a stress riser (14mm from weld edge) within 2.5σyield = 68%HISCσyield

  • γHISCσyield = 68%HISCσyield

γHISC=0.85 due to the large austenite spacing, and the σm+b <80%σyield is chosen for the stress riser within 2.5√Rt of a girth weld. The stress criteria fits the failure data for the TWI Foinhaven full scale tests.
 
sppawmay09f16.gif
 Fig.16. HISC stress criteria summary

Summary

The stress criteria focuses on providing limits to the through-wall membrane and membrane+bending stresses to avoid loadings that can impose gross membrane-stretching creep or gross through-wall bending creep. Then a penalty is placed: 1) on the presence of stress risers, and 2) on locations that are in the vicinity of girth welds. In addition a material quality factor is applied that reduces the allowable limits for coarse austenite spacing. Figure 16 summarizes the stress-based criteria.

References

  1. Stig Wästberg, Morten Solnørdal, Gustav Heiberg, Tikard Törnqvist and Pedro Vargas, Hydrogen Induced Stess Cracking, (HISC) in duplex stainless steels - DNV-RP-F112, Design of duplex stainless steel subseaequipment exposed to cathodic protection, OMAE2009-79655.
  2. DNV recommended practice, DNV-RP-F112, Design of duplex stainless steel subsea equipment exposed to cathodic protection, October 2008.
  3. T. S. Taylor, T. Pendlington, and R. Bird, Foinhaven super duplex BP materials cracking investigation, OTC May 1999.
  4. P. Woollin and A. Gregori, Avoiding hydrogen embrittlement stress cracking of ferritic austenetic stainless steels under cathodic protection, OMAE2004-51203
  5. FEM analyses of notched tension and bend specimens used in the HISC II and the Ormen Lange HISC projects, DNV technical report no. 2006-3259, revision no. 02, January 10, 2007.
  6. Roy Johnsen, Andre Mikkelsen, Bård Nyhus, Stig Wästberg, Hydrogen Induced Stress Cracking of stainless steels final report for HISC2. SINTEF report STFMKF07029, 2007-09-26.
  7. ABAQUS/Standard User's Manual, Volume III, Version 6.4, Hibbitt, Karlsson & Sorensen, Inc., Pawtucket, RI 12860, www.abaqus.com.