If hydrogen sulfide concentration (H2S) is present in the equipment over the lifecycle in any phase the service shall be considered as sour service and the requirements of SP-2041 and DEP.30.10.02.15 shall applied. Concentration of H2S shall be determined in accordance with DEP 25.80.10.18-Gen.
When assessing materials suitability, the pH and H2S partial pressure shall represent not only normal life cycle exposures but also exposures as can reasonably be expected to occur during an upset or in a stratified flow condition (e.g., normal packer fluid pH versus condensing water pH after tubing to annulus leak, or pH of flowline fluid versus condensing water pH during stratified flow.
For vessels, internal protective coatings are acceptable to protect carbon and low alloy steels against Hydrogen Induced Cracking (HIC) or stepwise cracking, provided that the coating integrity is ensured by means of a suitable coating maintenance program and that a program to detect and monitor HIC formation and growth is in place. (Informative: For practical purposes, this shall only apply to vessels).
Stainless steels
The production stream phase behaviour SHALL [PS] be reviewed to identify if flashing conditions or salt deposition from carryover fluids are present, which conditions concentrate fluid salinity. In the event flashing conditions are present, either a salinity of 250 g/l (expressed as NaCl) or the greater value equivalent to salt saturation in water at operating conditions shall be assumed when selecting and testing the materials. Any testing shall be done in representative water chemistry.
The temperatures given in Table 5.1 shall be used to assess the risk of pitting corrosion, crevice corrosion and chloride stress corrosion cracking of the most common stainless steel type used in Upstream in offshore and onshore salt laden environments (e.g. desert environment). The risk for other stainless steel types shall be referred to the MCI TA2 from Function.
Table 5.1: Typical stainless steel temperature limits.
Stainless steel type
(1)Threshold for pitting corrosion
Threshold for crevice corrosion
Negligible risk of CSCC
Significant risk of CSCC
316L(2) 5oC <0oC <50oC >60oC
6Mo 50oC 30oC <100oC >120oC
22Cr Duplex(3)
40oC 15oC <80oC >100oC
25Cr Super-Duplex(4)
60oC 30oC <100oC >110oC
(1) Table gives requirements for generic stainless steel types. Specific materials and conditions may influence the acceptable temperature
(2) Assumes minimum Molybdenum content of 2%. Higher temperatures may be possible at higher Mo content.
(3) Assumes PREN > 35 (4) Assumes PREN > 40
Contact of zinc with stainless steel items SHALL[PS] be prevented, including zinc coating contamination and contamination by zinc in fire scenarios from other equipment.
Page 32 of 63 Ferritic and Martensitic stainless steels such as those of the 13Cr family are susceptible to both sulphide stress cracking (SSC) and stress corrosion Cracking (SCC) and therefore their application shall be in accordance to DEP.30.10.01.15.
Austenitic stainless steels containing less chromium, nickel and molybdenum than AISI 316, such as AISI 304, shall not be used / applied in PDO production facilities as defined in DEP 39.01.10.12-Gen, Appendix A.
All the austenitic stainless steels wrought, forge and cast products shall be subjected to intergranular corrosion testing in accordance with ASTM A262 Practice E. The materials shall pass required criteria stated in ASTM A262 specification. The intergranular corrosion test shall be performed for each heat in the purchase order.
Austenitic/ferritic stainless steels (duplex stainless steels) can suffer both SCC and SSC, hence hardness requirements from DEP 30.10.01.15 shall apply and strict H2S partial pressure limits shall be followed as given in Part 6 and DEP 30.10.01.15. Both 22 Cr duplex and 25 Cr super-duplex stainless steels are susceptible to CSCC at 80 °C (176 °F) under drop evaporation conditions, and their use at points of salt accumulation shall be avoided.
Applications of duplex stainless steels at Lower Design Temperatures (LDT), for which the design code asks for proof of toughness by impact testing, require an additional specification of the steel being ordered and confirmation of proven toughness on the steel certificate. Welding procedures shall be qualified or re-qualified with impact testing included, when required by the design code for the given LDT. The minimum design temperature of duplex stainless steels is -50°C (-58°F) with maximum thickness of 40 mm (1.6 in) unless otherwise qualified in accordance with DEP 30.10.02.31-Gen section 5.4. Duplex stainless steel shall have PREN > 34, with a nitrogen >
0.14%. The super duplex stainless steel shall contain at least 25%Cr and PREN > 40 and nitrogen >
0.2%. Duplex stainless steel and super duplex stainless steel shall comply with DEP 30.10.02.35-Gen requirements.
All the DSS and SDSS wrought, forge and cast products shall meet following requirements in addition to requirements stated in respective MESC SPEs and relevant DEPs.
Transverse tensile test:
Transverse tensile testing is not required for the pipe nominal diameter ≤ 6”. Diameters 8” and above shall be subjected to transverse tensile test.
Pitting Corrosion testing:
The materials shall be capable of passing the ferric chloride test in accordance with ASTM G 48, Method A, with the following amendments. This corrosion test shall be performed for product qualification only.
The exposure time shall be 24 hours. The test temperature for “22Cr” duplex (ferritic-austenitic) stainless steel shall be 25 °C for parent metal and 22 °C for welds.
The test temperature for “25Cr” superduplex (ferritic-austenitic) stainless steel shall be 40 °C for parent metal and 35 °C for welds.
• The temperature variation shall not exceed ± 0.5 °C.
• The surface finish of the test face shall be ‘as-produced’. Cut faces shall be ground to 1200 grit.
• The evaluation of results shall be via weight loss measurement and macroscopic investigation of the surface. Macrographs obtained by low magnification microscopy shall be provided.
The acceptance criteria shall be a weight loss < 4.0 g/m2 and no initiation of localized corrosion >
0.025 mm (1 mil) at the test face. Note that only corrosion (e.g. pitting) at the test face counts. If the weight loss is > 4.0 g/m2 and it can be positively identified that this is only due to corrosion at the cut faces, the test will be invalid. In this case re-testing shall be carried out on replacement specimens.
Frequency of testing shall be each heat in the purchase order.
Page 33 of 63 Super austenitic stainless steels (>6 % Mo) to ensure corrosion resistance of welds, a nickel alloy filler with increased Mo, such as alloy 625, shall be used. 6Mo materials shall comply with DEP 30.10.02.35-Gen requirements.
All the UNS S31254 wrought, forge and cast products shall be subjected to ferric chloride test in accordance with ASTM G48, Method A. The test temperature shall be 50 °C and the exposure time shall be 24 hours. The test specimens shall be in the as-delivered condition. The test shall expose the external and internal surfaces. No pitting is acceptable at internal or external surfaces at 20 times magnification. The weight loss shall be < 4.0 g/m².
Frequency of testing shall be each heat in the purchase order.
Precipitation hardening stainless steels in Appendix A of DEP 39.01.10.12 Gen, such as UNS S17400 (17-4 PH) and UNS S15500 (15-5PH) shall be prohibited for pressure containment parts in sour environments. Alloy 17-4 shall be limited to a maximum stress of 50% for compressor internal components.
Nickel alloys such as UNS N07718 (Alloy 718) shall meet the requirements of DEP 39.01.10.32-Gen. UNS N07725 (Alloy 725) and UNS N07716 (Alloy 625+ ) shall be specified in accordance with DEP 39.01.10.30-Gen. These materials may suffer from similar issues to those that have been observed with Alloy 718, and, as such, care shall be taken during manufacturing and heat treatment, particularly for critical or highly loaded components
.
Alloy N08825 (Alloy 825) shall be supplied with Ni content greater than 39% and a PREN greater than 30. Quality assurance in supply chain should be closely monitored.
Intergranular corrosion test in accordance with A262 Practice C. Acceptance criteria shall be weight loss < 0.9mm/year and intergranular penetration shall not exceed 30 microns average, with minimum individual maximum 50 microns into the surface that will be exposed to the corrosive environment in the specific application when measures by micrography shall be performed at an appropriate magnification in a minimum of eight separate viewing fields average. The intergranular corrosion test shall be performed for each heat in the purchase order.
For materials cladded with Alloy 825 exposed to post weld heat treatment or other stress relieve treatment during fabrication shall be subject to corrosion test at simulated worst case process conditions to evaluate effect on the materials. Test shall include pitting and crevice assessment.
Alloy UNS N0625
All alloy 625 materials wrought, forge and cast products shall be subjected to integranular corrosion test in accordance with ASTM G28, Method A. The maximum allowed corrosion rate is 0.075mm/month and intergranular penetration shall not exceed 30 microns average, with minimum individual maximum 50 microns into the surface that will be exposed to the corrosive environment in the specific application when measures by micrography at an appropriate magnification in a minimum of eight separate viewing fields average.
Frequency of testing shall be each heat in the purchase order.
Where galling resistance is required, anti-galling compounds, electroplating, or use of different materials should be used for the two parts that come into contact, e.g., N06625 and N07725.
Molybdenum Disulfide SHALL [PS] not be used. An alternative anti-galling approach that may be used is to specify and assure a minimum difference in hardness of 25 HRB of the components.
Glass reinforced plastics. The choice of fibre and resin should be selected after full consideration of the service requirements in accordance with SP-2092 and DEP 30.10.02.13-Gen. GRP pipelines and piping shall be in accordance with SP- 2092, SP-2156 and DEP 31.40.10.19-Gen.
Note: Proprietary materials might be considered upon successful qualification and approval from MCI Corporate Function Discipline Head (CFDH).
All the corrosion tests shall be carried out at PDO and ILAC approved laboratory.
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