Micrograph of sample above showing the pit at ~ 40x. The tube ID is at the bottom of the photograph.

This alert was originated from a supermajor, ExxonMobil[1].

Subsea umbilical tubes carrying concentrated Low Dose Hydrate Inhibitor (LDHI) leaked as a result of incompatibility with tube metallurgy (super duplex stainless steel). With >3500 kms of chemical injection super duplex tubing installed and the high cost of replacement (more than 50 million dollors), Production Units should not depend on Chemical Vendor's generic recommendations for materials compatibility and should conduct chemical qualification tests, i.e., a modified ASTM G150 to confirm resistance to pitting corrosion prior to introducing chemicals in a production systems. Lessons Learned revisions are recommended to revise both the current Group Practices for materials selection and requirements for chemical qualification.

Background

The cross section of the umbilical that suffered pitting corrosion failure due to LDHI.
SDSS 2507 pits after ASTM G150 test in LDHI.

Five drill centers are tied back to a turret moored floating production storage and offloading (FPSO) facility located off the western equatorial coast of Africa in 475 meters of water. The drill centers are connected to the FPSO by risers and flow lines, and are controlled by steel tubes electrical/hydraulic umbilicals. Subsea chemical injection is also provided by the umbilicals.

Three new subsea umbilicals constructed of Super Duplex Stainless Steel 2507 tubing were installed and feed LDHI as well as other chemicals to the subsea system (see picture to the right). After few months in service unexpected leaks began and progressively worsened. In addition the LDHI feed system (tanks and piping), made of SS316, was found to be severely pitted.

Root Cause Failure Analysis

The following are the conclusions from the Root Cause Failure Analysis (RCFA).

  1. The cause of failure of the umbilical tubes in all three umbilicals is internal pitting corrosion from exposure to a particular type of pure Low Dose Hydrate Inhibitor (LDHI).
  2. Corrosion of SS316 in chemical injection facilities is also caused by this particular LDHI in its pure state and can occur at all service temperatures from below 4°C to all higher temperatures.
  3. The failed SDSS 2507 was correctly manufactured and passed all Global Practices requirements.
  4. An ASTM G150 "Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels" should be added as a required compatibility test when selecting chemicals which will be in contact with austenitic, duplex, and super duplex stainless steels. This test should be modified by using the "specific chemical" for the electrolyte rather than the standard 1 M NaCl solution required in ASTM G150. This should include tests at different concentrations including neat and the "actual concentration" that will be carried in service. Neither SS316 nor SDSS 2507 passed the ASTM G150 test with this particular LDHI. The critical pitting temperature (CPT) was <4°C in LDHI versus 86°C in seawater.
  5. Analysis of fluid samples confirmed that the LDHI, as manufactured, contains high levels of chlorides (> 35K ppm) and develops an acidic pH in the pure state (pH ~ 4) without degradation or contamination.
  6. ASTM G5 "potentiodynamic anodic polarization curve" tests were run at the CPT (4°C). The results of these tests aided understanding the corrosion mechanism of SS316 and SDSS 2507 in pure LDHI. The LDHI disturbs the formation of a protective passivation film, allowing acidified chlorides to break down the film and cause pitting.
  7. The Chemical Product Data Sheet from the chemical supplier for this particular LDHI states that SS316 is a suitable material for use with this chemical. This indicates that conventional immersion corrosion testing may not be sufficient to qualify chemical compatibility.
  8. Other common mechanisms of failure in Austenitic and SDSS were proven to have no contribution to these failures. Such failure mechanisms include stress corrosion cracking, crevice corrosion, microbiological induced corrosion, inadequate mechanical properties, poor welding quality, fatigue, sigma phase and ferrite content.

Potential Business Impact

  1. The associated cost of the umbilicals replacement project was over 50 million dollors.
  2. There are more than 3500 Km of Super Duplex Umbilical tubing installed in this Operator's production units. There are also large amounts of SS316 in chemical injection facilities.

Applicability

  1. All production units when considering a new chemical application or a change in current chemical usage.
  2. New and proposed projects when selecting chemicals to be fed through austenitic, duplex and super duplex stainless steel materials for chemical transportation, storage and injection.

Recommended Actions

  • Production Units Check Current Hydrate Inhibition Program: Confirm that this particular LDHI is not currently in use at any facilities. If it is in use, facility should confirm materials compatibility or change the hydrate injection program to avoid use of this LDHI. Any new chemicals must also be qualified for service environment.
  • Production Units and Project Groups: To qualify a new chemical for use with austenitic, duplex or super duplex stainless steels, consider adding the ASTM G150 modified test (using the "specific chemical" for the electrolyte, instead of seawater) additionally to the tests currently required by GPs. This should include tests at different concentrations including neat and the "actual concentration" of chemicals that will be carried in service. If any findings are shown in the ASTM G150 test, an additional ASTM G5 test can be performed in order to better understand the ASTM G150 results.
  • Production Units and Project Groups: The LDHI Manufacturer Product Data sheet stated that 410 and 316 stainless steels were suitable and compatible with the LDHI and no metals were listed as being unsuitable, indicating that PUs and Project groups can not rely solely on manufacturer’s data for material and chemical compatibility checks.

Potential Group Practice Revisions

  • GP 65-06-01 Electro/Hydraulic Umbilical - Steel Tube – Section 4.1.2b
    • Suggested Addition: The chemical resistance of super duplex shall be qualified for intended service using a "modified ASTM G150 test" in which the specific chemicals should be used for the electrolyte rather than the standard 1 M NaCl solution required in G150. This should require testing of the neat chemical as well as several dilute solutions including the "actual concentration" being carried in service. The new critical pitting temperature (CPT) shall exceed the maximum operating temperature including chemical storage and injection piping.
  • Section 5.3 Materials The chemical resistance of all materials shall be documented. All metal alloys used in the total umbilical system shall be corrosion-resistant in the specified temperature range or equipped with a suitable corrosion protection system, including the platform topside termination assemblies and tube fittings.
    • Suggested Addition: 3) The chemical resistance of all materials shall be "qualified" and documented. Compatibility test using a "modified ASTM G150 test" in which the specific chemicals should be used for the electrolyte rather than the standard 1 M NaCl solution required in G150, shall be used. This should require testing of the neat chemical as well as several dilute solutions including the "actual concentration" being carried in service. The new critical pitting temperature (CPT) shall exceed the maximum operating temperature including chemical storage and injection piping. All metal alloys used in the total umbilical system shall be corrosion-resistant in the specified temperature range or equipped with a suitable corrosion protection system, including the platform topside termination assemblies and tube fittings.
  • GP 29-01-35 Material Selection for Offshore Topside Facilities and Marine Systems – Section 4.4.1
    • Suggested Addition: Chemical resistance shall be qualified and documented for intended service using a "modified ASTM G150 test" in which the specific chemicals should be used for the electrolyte rather than the standard 1 M NaCl solution required in G150. This should require testing of the neat chemical as well as several dilute solutions including the "actual concentration" being carried in service. The new critical pitting temperature (CPT) shall exceed the maximum operating temperature including chemical storage and injection piping.
  • Section 6.1 Table 4 - Acceptable Materials for Utility Systems, recommends to use SS316 for chemical injection.
    • Suggested Change: Delete SS316 as recommended material and add a comment in the table "need material compatibility test".
    • Suggested Change: Note 5: all new chemicals shall be tested for compatibility with materials using a modified ASTM G150 test in which the specific chemicals should be used for the electrolyte rather than the standard 1 M NaCl solution required in G150. This should require testing of the neat chemical as well as several dilute solutions including the "actual concentration" being carried in service. Critical pitting temperature (CPT) obtained from this test shall exceed operating temperature.
  • GP-29-01-03 Material Selection for Onshore Facilities – Section 5.1.3
    • Suggested Addition: Chemical resistance shall be qualified and documented for intended service using a "modified ASTM G150 test" in which the specific chemicals should be used for the electrolyte rather than the standard 1 M NaCl solution required in G150. This should require testing of the neat chemical as well as several dilute solutions including the "actual concentration" being carried in service. The new critical pitting temperature (CPT) shall exceed the maximum operating temperature including chemical storage and injection piping.
  • Section 12.1 Table 4 - Acceptable Materials for Utility Systems, recommends to use SS316 for chemical injection.
    • Suggested Change: Delete SS316 as recommended material and add a comment in the table "need material compatibility test".
    • Suggested Change: Note 5: all new chemicals shall be tested for compatibility with materials using a modified ASTM G150 test in which the specific chemicals should be used for the electrolyte rather than the standard 1 M NaCl solution required in G150. This should require testing of the neat chemical as well as several dilute solutions including the "actual concentration" being carried in service. Critical pitting temperature (CPT) obtained from this test shall exceed operating temperature.
  • TRM 01 Corrosion and Inspection Management Modifications to TRM01 are necessary in agreement to global owner Don Lefevre, here is a preliminary revision. An EMPC team will be put together in order to progress such modifications.
  • Section 2.1 - Introducing New Chemicals into a system.
    • First bullet: Review supporting laboratory test data. For example, compare chemical supplier data against requirements in Section 2.2, "Lab Test Methods," for Selecting Corrosion Inhibitors and Section 2.3, "Biocide Selection and Testing".
    • Suggested Addition: and section 2.6 Chemical qualification tests for material compatibility.
    • Suggested Addition: Add a new section 2.6 Chemical Qualification Test for Material Compatibility
  • Section 2.6.1 IntroductionAn Operator asset had multiples failures in super duplex tubing due to contact with a particular Low Dose Hydrate Inhibitor (LDHI), demonstrating that austenitic and super duplex stainless steel can fail in contact with certain chemicals. It has been demonstrated that a modification of the ASTM G150 test be used to determine chemical compatibility to austenitic, duplex and super duplex stainless steel materials.
  • Section 2.6.2 Material Qualification for Chemical Service: This Section shall be used when selecting a new chemical to be introduced into a system to be in contact with austenitic, duplex or super duplex stainless steel, and shall be used as a reference for laboratory compatibility test.
  • Section 2.6.3 Description of how duplex stainless steel corrosion resistance properties can worsen in presence of an unknown chemical.
  • Section 2.6.4 Laboratory Chemical Compatibility Test All new chemicals should be tested for compatibility with umbilical materials using a modified ASTM G150 test in which the specific chemicals should be used for the electrolyte rather than the standard 1 M NaCl solution required in G150. This should require testing of the neat chemical as well as several dilute solutions including the "actual concentration" being carried in service. If any indication of reduced CPT is observed, polarization studies be performed according to ASTM G5 in order to more fully understand the issues with the chemical and the alloy and whether passivity is maintained or not. CPT shall be above maximum operating and storage temperature.
  • TRM 03 Pipelines and Subsea
    • Section 6.3 Subsea Umbilicals and Distribution Corrosion Resistance
    • Suggested Addition: Chemical compatibility to service materials shall be qualified per TRM01 section 2.6 Chemical Qualification Test for Material Compatibility

Reference

  1. Heider A.Suarez, ExxonMobil Production Company; Robert J.Franco, ExxonMobil Production Company; William E.Bond, ExxonMobil Production Company; Raju S.Pakalapati, ExxonMobil Production Company, "Chemical Induced Pitting Corrosion Of Super Duplex Stainless Steel Umbilical Tubes", CORROSION 2011, March 13 - 17, 2011 , Houston, Texas

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