Materials For Sour Service In Exploration And Production Operations

Document No.

GP 36-25

Applicability

Group

Date

16 March 2009

GP 36-25

Materials for Sour Service in Exploration and Production Operations

Group Practice

BP GROUP ENGINEERING TECHNICAL PRACTICES

16 March 2009

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Foreword This is the first issue of Engineering Technical Practice (ETP) GP 36-25.

Copyright © 2009 BP International Ltd. All rights reserved. This document and any data or information generated from its use are classified, as a minimum, BP Internal. Distribution is intended for BP authorized recipients only. The information contained in this document is subject to the terms and conditions of the agreement or contract under which this document was supplied to the recipient's organization. None of the information contained in this document shall be disclosed outside the recipient's own organization, unless the terms of such agreement or contract expressly allow, or unless disclosure is required by law. In the event of a conflict between this document and a relevant law or regulation, the relevant law or regulation shall be followed. If the document creates a higher obligation, it shall be followed as long as this also achieves full compliance with the law or regulation.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Table of Contents Page Foreword ........................................................................................................................................ 2 Introduction ..................................................................................................................................... 6 1.

Scope .................................................................................................................................... 7

2.

Normative references............................................................................................................. 7

3.

Terms and definitions............................................................................................................. 9

4.

Symbols and abbreviations .................................................................................................. 10

5.

General outline of a sour service plan.................................................................................. 11

6.

Factors that affect cracking .................................................................................................. 11 6.1. General materials factors .......................................................................................... 11 6.2. Maximum allowable hardness ................................................................................... 11

7.

Materials selection for sour service conditions in exploration and production operations ..... 11 7.1. Definition of sour environment severity ..................................................................... 11 7.2. Metallic materials ...................................................................................................... 12 7.3. Nonmetallic materials................................................................................................ 12

8.

Application of NACE MR0175/ISO 15156 to material selection for exploration and production operations............................................................................................................................ 13 8.1. Qualification of cracking resistant carbon and low alloy steels and cast irons (for SSC, SZC, HIC, and SOHIC resistance) (NACE MR0175/ISO 15156 - Part 2 and Part 1) . 13 8.2. Qualification of cracking-resistant CRAs and other alloys for SSC, SCC, and GHSC (NACE MR0175/ISO 15156 - Part 3 and Part 1) ....................................................... 16

9.

Equipment specific requirements ......................................................................................... 19 9.1. Pressure vessels....................................................................................................... 19 9.2. Heat exchanger tube bundles ................................................................................... 20 9.3. Piping........................................................................................................................ 20 9.4. Pipelines ................................................................................................................... 21 9.5. Downhole tubulars and equipment............................................................................ 22 9.6. Valves....................................................................................................................... 22 9.7. Rotating machinery ................................................................................................... 23 9.8. Instrumentation ......................................................................................................... 27 9.9. Bolting....................................................................................................................... 27 9.10. Bellows ..................................................................................................................... 28 9.11. Low temperature plant .............................................................................................. 28 9.12. Metallic overlays - Explosively clad, roll bonded, and fusion bonded corrosion resistant overlays .................................................................................................................... 28

10.

Fabrication........................................................................................................................... 28 10.1. Weld procedure qualification..................................................................................... 28 10.2. Heat treatment general requirements........................................................................ 29 10.3. PWHT for carbon steel pipework............................................................................... 30

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

10.4. Removal of fabrication defects.................................................................................. 30 10.5. NDE .......................................................................................................................... 30 10.6. PWHT for carbon steel plate ..................................................................................... 30 11.

Special duties ...................................................................................................................... 30 11.1. General..................................................................................................................... 30 11.2. H2S containing alkaline services ............................................................................... 30 11.3. Sour streams containing carbonates ......................................................................... 30 11.4. Sour streams containing amines ............................................................................... 31

12.

Identification, stamping, and marking................................................................................... 31 12.1. Hard stamps ............................................................................................................. 31 12.2. Marking paints and crayons ...................................................................................... 31

13.

Inspection ............................................................................................................................ 31 13.1. BP approval of procedures........................................................................................ 31 13.2. Documentation and inspection .................................................................................. 31 13.3. Hardness checks ...................................................................................................... 31 13.4. Heat treatment .......................................................................................................... 32

Annex A (Normative) Specification for Z quality steel plate ........................................................... 33 A.1. Scope .................................................................................................................................. 33 A.2. Definition.............................................................................................................................. 33 A.3. Material................................................................................................................................ 33 A.3.1. Standards ................................................................................................................. 33 A.3.2. Steel making process................................................................................................ 33 A.3.3. Chemical composition............................................................................................... 33 A.4. Inspection requirements for plate ......................................................................................... 33 A.4.1. Ultrasonic examination.............................................................................................. 33 A.4.2. Through thickness tensile test................................................................................... 33 A.5. Weld repair of plate.............................................................................................................. 34 Annex B (Normative) Specification for HIC resistant steel plate .................................................... 35 B.1. Scope .................................................................................................................................. 35 B.2. Manufacturing ...................................................................................................................... 35 B.2.1. Standards ................................................................................................................. 35 B.2.2. Process..................................................................................................................... 35 B.2.3. Heat treatment .......................................................................................................... 35 B.2.4. Chemical composition............................................................................................... 35 B.3. Testing................................................................................................................................. 36 B.3.1. Ultrasonic examination.............................................................................................. 36 B.3.2. HIC/SWC test ........................................................................................................... 36 B.4. Weld repair of plate.............................................................................................................. 36 Annex C (Informative) Background on factors that affect cracking in H2S containing (sour) environments ....................................................................................................................... 37 C.1. General material factors....................................................................................................... 37 C.2. General environmental factors ............................................................................................. 37 Page 4 of 46

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

C.3. Factors affecting SSC .......................................................................................................... 37 C.4. Factors affecting SZC .......................................................................................................... 38 C.5. Factors affecting GHSC ....................................................................................................... 39 C.6. Factors affecting HIC/SWC and blistering ............................................................................ 39 C.7. Factors affecting SOHIC ...................................................................................................... 39 C.8. Factors affecting SCC.......................................................................................................... 40 Annex D (Informative) Example of domain diagrams .................................................................... 41 Annex E (Informative) In service issues ........................................................................................ 42 E.1. General................................................................................................................................ 42 E.2. Inspection ............................................................................................................................ 42 E.3. Hydrogen flux monitoring ..................................................................................................... 42 E.4. Mitigating methods............................................................................................................... 43 E.5. References .......................................................................................................................... 43 Annex F (Informative) Examples of previous in service H2S related failures.................................. 44 F.1. Oil flow line HIC failure of carbon steel pipe (BP) - 1972...................................................... 44 F.2. Downhole tubular string SSC failure of alloyed 13% Cr steel (BP) - 2001 ............................ 44 F.3. Failure by SOHIC of carbon steel spiral welded pipe (Saudi Aramco) - 1998 ....................... 44 F.4. Failure by SSC/SOHIC of carbon steel pressure vessel (Unocal) - 1984 ............................. 45 Bibliography .................................................................................................................................. 46

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Introduction Numerous types of material are used in sour service, including carbon steel, low alloy steels, corrosion resistant alloys, and other alloys. This has resulted in an extensive amount of satisfactory service experience. Materials need to be chosen with care, however, since they are potentially susceptible to various cracking mechanisms, of which the particular type depends on specific material. For satisfactory service, therefore, it is necessary to understand the environmental conditions and the metallurgical condition of the material that gives rise to the maximum resistance to cracking. Despite much satisfactory service experience, the oil industry, including BP, has seen a number of failures caused by exposure to wet H2S environments, affecting different equipment and spanning many years. Such failures can potentially be very costly and have severe health, safety, and environmental consequences. Some examples of previous failures are given in Annex F. Responsibility is now placed on the equipment user by NACE MR0175/ISO 15156 to ensure that materials used in sour service are fit for purpose. Important issues discussed in this GP are:         

Environmental conditions. Sulphide stress cracking (SSC) resistance of materials. Stress corrosion cracking (SCC) resistance of materials. Hydrogen induced cracking (HIC)/stepwise cracking (SWC) resistance of materials. Stress oriented hydrogen induced cracking (SOHIC) and soft zone cracking (SZC) resistance of materials. Equipment specific requirements. Fabrication. Related services (e.g., carbonate/H2S environments). In service issues.

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1.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Scope a.

This Group Practice (GP) provides requirements for selection of materials used in exploration and production operations. As such, this GP is based on NACE MR0175/ISO 15156. Refining and marketing operations are covered in separate GP 06-20 and associated NACE MR0103. The different “exploration and production” and “refining and marketing” referenced standards reflect different operating conditions, with higher pressures and more acid conditions (lower pH) being present in oil and gas production plant, which often make the conditions significantly more onerous. Changes were made to NACE MR0175 (now NACE MR0175/ISO 15156) that related the materials performance more closely to oil and gas environments, particularly for SCC. Resulting guidelines became inappropriate for the more alkaline (and hence, generally less aggressive) conditions associated with refining. Hence, a refining specific document (NACE MR0103) was developed.

b.

c.

2.

This GP is supplementary to NACE MR0175/ISO 15156 and includes: 1.

Requirements to avoid environmental cracking of materials used for equipment in sour service (i.e., fluids containing water and hydrogen sulphide (H2S).

2.

Additional BP specific requirements, including other aspects related to cracking not included in NACE MR0175/ISO 15156.

General corrosion is not covered in this GP and is therefore a separate consideration.

Normative references The following referenced documents may, to the extent specified in subsequent clauses and normative annexes, be required for full compliance with this GP: •

For dated references, only the edition cited applies.

•

For undated references, the latest edition (including any amendments) applies.

BP GIS 18-013 GIS 18-014 GIS 36-102 GIS 42-101 GIS 42-102

GIS 46-010 GN 36-004 GN 36-013

Integral Cladding, Weld Overlay, and Limited Loose Lining of Pressure Vessels and Components. Heat Exchanger Tube End Fixing. Hardness Testing, Post Weld Heat Treatment, Stress Relief, and Pickling for Pressure Vessels, Piping, and Other Components. Fabrication, Assembly, Erection, and Inspection of Carbon, Carbon Manganese, and Low Alloy Steel Pipework (ASME B31.3). Fabrication, Assembly, Erection, and Inspection of Austenitic and Duplex Stainless Steel, Cupro-Nickel, Nickel Base Alloy, Titanium, and Zirconium Pipework (ASME B31.3). New Pressure Vessels. Best Practice for Selection of Materials for Downhole Equipment (Revision 2.1). Guidelines for Selecting Downhole Tubing and Casing Materials for Oil and Gas Production Wells (2006 Edition).

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GN 36-017 GP 06-20 GP 36-10 GP 36-12

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Domain Diagrams for 316L Stainless Steel in Oil and Gas Production Environments. Materials for Sour Service. Metallic Materials Selection. Amine Services.

American Petroleum Institute (API) API RP 579 API RP 945 API Spec 5CT

Fitness-for-Service. Avoiding Environmental Cracking in Amine Units. Specification for Casing and Tubing.

American Society for Non-Destructive Testing (ASNT) ASNT TC-1A

Personnel Qualification and Certification in Nondestructive Testing.

American Society for Testing and Materials (ASTM) ASTM A193/A193M ASTM A194/A194M ASTM A264 ASTM A265 ASTM A395/A395M ASTM A516/A516M ASTM A841/A841M

Alloy-Steel and Stainless Steel Bolting Materials for High Temperature or High Pressure Service and Other Special Purpose Applications. Carbon and Alloy Steel Nuts for Bolts for High Pressure or High Temperature Service or Both. Stainless Chromium-Nickel Steel-Clad Plate. Nickel and Nickel-Base Alloy-Clad Steel Plate. Ferritic Ductile Iron Pressure-Retaining Castings for Use at Elevated Temperatures. Pressure Vessel Plates, Carbon Steel, for Moderate- and LowerTemperature Service. Steel Plates for Pressure Vessels, Produced by Thermo-Mechanical Control Process (TMCP).

British Standards Institute (BSI) BS EN 10028 BS EN 10028-1 BS EN 10028-2 BS EN 10028-3 BS EN 10028-4 BS EN 10028-5 BS EN 10028-6 BS EN 10028-7 BS EN 10160 BS EN 10164

Flat products made of steel for pressure purposes. Flat products made of steels for pressure purposes. General requirements. Flat products made of steel for pressure purposes. Non-alloy and alloy steels with specified elevated temperature properties. Flat products made of steel for pressure purposes. Weldable fine grain steels, normalized. Flat products made of steels for pressure purposes. Nickel alloy steels with specified low temperature properties. Flat products made of steels for pressure purposes. Weldable fine grain steels, thermomechanically rolled. Flat products made of steel for pressure purposes. Weldable fine grain steels, quenched and tempered. Flat products made of steels for pressure purposes. Stainless steels. Ultrasonic testing of steel flat product of thickness equal or greater than 6 mm (reflection method). Steel products with improved deformation properties perpendicular to the surface of the product. Technical delivery conditions.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

International Organization for Standardization (ISO) ISO 6508-1 ISO 11960 ISO 15156-1

ISO 15156-2

ISO 15156-3

Metallic materials - Rockwell hardness test - Part 1: Test method (scales A, B, C, D, E, F, G, H, K, N, T). Petroleum and natural gas industries - Steel pipes for use as casing or tubing for wells. Petroleum and natural gas industries - Materials for use in H2S-containing environments in oil and gas production - Part 1: General principles for selection of cracking-resistant materials. Petroleum and natural gas industries - Materials for use in H2S-containing environments in oil and gas production - Part 2: Cracking-resistant carbon and low alloy steels, and the use of cast irons. Petroleum and natural gas industries - Materials for use in H2S-containing environments in oil and gas production - Part 3: Cracking-resistant CRAs (corrosion-resistant alloys) and other alloys.

NACE International (NACE) NACE MR0103 NACE MR0175 NACE RP0472 NACE TM0284

3.

Standard Material Requirements - Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments. Petroleum and natural gas industries - Materials for use in H2S-containing environments in oil and gas production. Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments. Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking.

Terms and definitions For the purposes of this GP, the following terms and definitions apply: Hydrogen flux A flow of hydrogen per unit area that is typically measured in picolitre (pl) of gaseous hydrogen at 20°C (68°F) per cm2 steel surface per second (i.e., pl/cm2/s). Hydrogen flux is affected by pH and pH2S and other factors, such as the presence of cyanides and arsenic. Nascent hydrogen Hydrogen atoms that are produced by corrosion may diffuse into the steel and result in various damage mechanisms. This species is often referred to as nascent hydrogen and/or H0. pH (measure of acidity/alkalinity) Minus log of hydrogen ion concentration. pH2S (partial pressure of H2S) Pressure of H2S that would be exerted if present alone, at the same temperature, in the total volume occupied by the mixture. Pressure If the NACE standards refer to pressure, this shall be taken as design pressure. Sour service Exposure to environments that contain H2S and liquid water (H2O) and can cause cracking of materials by the mechanisms addressed in this GP.

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4.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Symbols and abbreviations For the purpose of this GP, the following symbols and abbreviations apply (see NACE MR0175/ISO 15156 Parts 1, 2, and 3 - Section 3 for full details): BOF

Basic oxygen furnace.

CE

Carbon equivalent.

CLR

Crack length ratio.

CRA

Corrosion resistant alloy.

CSR

Crack sensitivity ratio.

CTR

Crack thickness ratio.

ERW

Electric resistance welded.

FFKM

Perfluoroelastomer.

FKM

Fluoroelastomer.

FPB

Four point bend.

GHSC

Galvanically induced hydrogen stress cracking.

HAZ

Heat affected zone.

HIC

Hydrogen induced cracking.

HNBR

Hydrogenated nitrile rubber.

HP/HT

High pressure/high temperature.

HRC

Rockwell hardness.

HSC

Hydrogen stress cracking.

NBR

Nitrile rubber.

NDE

Nondestructive examination.

OD

Outer diameter.

PACR

Predicted average cracking ratio.

pl

picolitre.

ppm

Parts per million.

PTFE

Polytetrafluoroethylene.

PWHT

Post weld heat treatment.

Q+T

Quenched and tempered (steel).

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5.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

RBI

Risk based inspection.

SAW

Submerged arc welded.

SCC

Stress corrosion cracking.

SOHIC

Stress oriented, hydrogen induced cracking.

SSC

Sulphide stress cracking.

SWC

Stepwise cracking.

SZC

Soft zone cracking.

TFEP

Tetrafluoroethylene polymer.

TMCP

Thermo mechanical control process.

UT

Uniaxial tension.

WFMT

Wet fluorescent magnetic particle testing.

General outline of a sour service plan In view of the propensity to failure of equipment in wet H2S environments, if not properly controlled, a sour service plan shall be developed that covers the following:

6. 6.1.

a.

Initial materials selection that complies with NACE MR0175/ISO 15156.

b.

Detailed operational aspects, including environmental conditions and inspection requirements, to confirm continued integrity (see Annex E).

c.

Additional requirements set by appropriate BP ETP specifications.

Factors that affect cracking General materials factors Background on cracking mechanisms from exposure to H2S containing (sour) environments is given in Annex C.

6.2.

Maximum allowable hardness Unless stated otherwise in this GP, the maximum acceptable hardness levels to minimise the potential for SSC cracking shall be as specified in NACE MR0175/ISO 15156.

7. 7.1.

Materials selection for sour service conditions in exploration and production operations Definition of sour environment severity a.

H2S 1.

In accordance with the definition of NACE MR0175/ISO 15156 and this GP, the presence of any measurable levels of H2S can result in a sour environment, depending on the cracking mechanism being considered.

2.

If H2S conditions vary, peak transient conditions shall be used in design.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Higher H2S contents increase the severity of the environment. b.

Acidity shall be defined. Acidity generally characterises upstream environments due to the presence of CO2, organic acids (e.g., acetic acids), and even to a lesser extent, H2S itself.

c.

Chloride content shall be defined. Generally, higher chloride contents increase the severity of the environment.

d.

Contaminants, such as cyanides, arsenic, and ammonia, shall be reported and considered in material selection. These contaminants are primarily present in downstream process environments, but their presence in upstream operations would affect environment severity.

e.

Defined environment - Conditions shall be clearly defined prior to establishing the suitability of a material. Materials may behave differently in different conditions (e.g., duplex SS is more resistant to cracking if the environment is not very acidic and chloride levels are low).

7.2.

Metallic materials a.

Metallic materials selection shall comply with NACE MR0175/ISO 15156, except as modified by this GP.

b.

The following is an overview of NACE MR0175/ISO 15156 requirements: 1.

Mechanisms - SCC, HIC/SWC, SOHIC, SZC, SSC, and GHSC are included.

2.

Selection of metallic materials for sour service shall involve the following, as a minimum: a)

Qualifying the suitability of materials by any of the following: 1) Selecting from a prequalified list in NACE MR0175/ISO 15156. 2) Laboratory testing. 3) Documented field experience.

7.3.

b)

Defining the service environment in terms of the aggressive species.

c)

Documenting material properties that affect cracking and confirming that they meet sour service requirements (e.g., hardness values).

d)

Documenting quality control of the material.

e)

Evaluating materials for resistance to cracking by the various mechanisms.

Nonmetallic materials a.

General 1.

Nonmetallic seal materials and seal geometry shall be subject to BP approval. Due to the large number of factors that need to be taken into account, this approval needs to be considered on a case by case basis. Subject matter experts should be consulted in making this approval.

2. b.

Elastomer selection shall take into account the complete operating conditions to be encountered (i.e., temperature, pressure, and environment).

Low H2S levels

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8.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

1.

NBR shall have a maximum long term limit of exposure between 10 ppm and 100 ppm H2S, depending on seal geometry and temperature.

2.

HNBR shall have a maximum long term limit of exposure between 100 ppm and 1 000 ppm H2S, depending on seal geometry and temperature.

c.

For H2S levels less than or equal to 5%, either FKM “Viton” type fluoroelastomers or TFEP “Aflas” shall be used.

d.

H2S levels greater than 5% 1.

FFKM perfluoroelastomers (Chemraz or Kalrez) shall be used.

2.

PTFE may also be selected, but it requires a different type of seal configuration compared with that for elastomers.

Application of NACE MR0175/ISO 15156 to material selection for exploration and production operations

8.1.

Qualification of cracking resistant carbon and low alloy steels and cast irons (for SSC, SZC, HIC, and SOHIC resistance) (NACE MR0175/ISO 15156 - Part 2 and Part 1)

8.1.1.

NACE MR0175/ISO 15156 requirements

The following cracking mechanisms shall be considered: a.

SSC resistance.

b.

SZC resistance.

c.

HIC/SWC resistance.

d.

SOHIC resistance.

8.1.2.

SSC resistance

8.1.2.1.

Selection from the prequalified list of SSC resistant materials

a.

b. 8.1.2.2.

Definition of sour service environment for SSC resistance (Part 2 of NACE MR0175/ISO 15156) as follows: 1.

pH2S greater than or equal to 0,3 kPa (0,003 bar [0,05 psi]).

2.

SSC resistant steel is selected.

Prequalified list is in NACE MR0175/ISO 15156, Part 2 - Annex A, Carbon and low alloy steels hardness requirements - Section A.2.

Selection based on laboratory testing for specific conditions NACE MR0175/ISO 15156, Part 2 - Section 7 and Annex B

a.

NACE MR0175/ISO 15156 allows for a material that is not included in the prequalified list, or is outside the prequalified environmental limits, to be qualified by testing under the conditions (i.e., principally the in situ pH and pH2S) specific to, or more severe than, the service environment. A pH2S versus in situ pH diagram (domain diagram) indicates regions of different environmental severity for SSC resistance.

b.

BP specific requirements for SSC resistance 1.

A number of grades are considered fully sour resistant such that qualification testing and use of domain diagrams are not considered necessary. These include API Spec 5CT/ISO 11960 grades K55, J55, L80, C90, and T95. Page 13 of 46

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

2.

BP has developed individual material domain diagrams for SSC resistance that indicate domains in which the “not fully sour resistant” material may be used. An example is given in Annex D. Downhole tubing and casing materials for carbon and low alloy steels for which domain diagrams have been produced include:   

8.1.2.3.

API Spec 5CT/ISO 11960, Grade N80 carbon steel. API Spec 5CT/ISO 11960, Grade P110 carbon steel. Grade C110 “sour resistant” low alloy steel.

Qualification based on field experience

Qualification based on field experience shall be allowed if the following procedure is followed: a.

Materials properties are defined and documented.

b.

Service environment is defined and documented.

c.

Service experience is at least 2 yr to give sufficient confidence.

d.

A full inspection of the equipment has been performed after at least 2 yr of service. So far, BP has not made use of this approach, due to the difficulty in obtaining and documenting the necessary service history. Instead, prequalification by laboratory testing has been preferred. Nevertheless, it remains an acceptable approach if the conditions in a. through d. can be fulfilled.

8.1.3.

Requirements for HIC/SWC resistance (NACE MR0175/ISO 15156, Part 2 - Annex B, B5)

8.1.3.1.

General

a.

NACE MR0175/ISO 15156 requires the user to consider HIC/SWC if there is even a trace amount of H2S present. This should be interpreted as the presence of a measurable amount of H2S (normally around 1 ppm by volume in the associated gas phase) if evaluating flat rolled carbon steel products for sour service (NACE MR0175/ISO 15156 part 2 - clause 8).

b.

Test procedures are given in NACE MR0175/ISO 15156, Annex B, clause B5, and shall comply with NACE TM0284, with the exception that alternative environments may be used (NACE MR0175/ISO 15156, Table B.3).

c.

Other products (e.g., forgings, castings, and seamless products) shall simply restrict the sulphur content in the steel in accordance with NACE MR0175/ISO 15156. These products are not normally considered prone to HIC/SWC.

8.1.3.2.

BP specific requirements for flat rolled products (e.g., plate and SAW line pipe)

a.

Material selection 1.

If HIC/SWC resistance is required, one of the following three approaches shall be used for material selection: a)

Z-quality steel (used in vessel manufacture, etc.) detailed in Annex A.

This material is not HIC tested but has been shown to be effective in some environments. b)

HIC resistant steel as detailed in Annex B, which also refers to improvement in SOHIC resistance.

Improvements in steel quality and reduced sulphur and phosphorus content have lead to improved HIC resistance. c)

Carbon steel that is internally fully clad with stainless steel/nickel based alloy.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Typical standards include:   b.

ASTM A264. ASTM A265.

BP special HIC test requirements (relevant to Annex A.2 of NACE MR0175/ISO 15156) 1.

For H2S less than 1 ppm by volume in the gas phase, no consideration of HIC/SWC is required unless pH2S is greater than or equal to 0,3 kPa (0,05 psi):

2.

For pH2S greater than or equal to 0,3 kPa (0,05 psi):

3.

a)

HIC test shall be in accordance with NACE TM0284 in the following solution (test solution A): 5% NaCl + 0,5% CH3COOH + pH2S 100 kPa (15 psi).

b)

BP acceptance criteria shall be, unless agreed otherwise with BP,: CLR less than 10%, CTR less than 3%, CSR less than 1%.

For intermediate H2S that is greater than 1 ppm but less than 0,3 kPa (0,05 psi), either of the following courses of action shall be taken: a)

Use mills/manufacturing routes with demonstrated capability of producing fully HIC resistant steel.

b)

Treat as for the pH2S greater than or equal to 0,3 kPa (0,05 psi) case.

Note that recent experience within BP has shown that some HIC of materials that are not designed to be fully HIC resistant can occur, even under very mildly sour conditions. Therefore, care is recommended and should be taken if opting for application specific HIC testing under “milder environment conditions” rather than the standard NACE test, Solution A, until and unless suitable acceptance criteria to replace the “no cracking” criteria in NACE MR0175/ISO 15156 are established. c.

Exceptions 1.

There is evidence from experience within BP and elsewhere that, under some conditions of very high H2S levels, HIC has not been a problem. Therefore, special precautions may not be necessary in these cases. This appears to be caused by a highly protective sulphide scale that stifles corrosion such that the level of atomic hydrogen is very low.

2.

8.1.4.

Due regard, therefore, may be taken of local experience if specifying special requirements to prevent HIC as follows: a)

If measures are not taken, details of the conditions and satisfactory experience shall be fully documented within the business unit/facility in which it is being applied.

b)

Every situation shall be considered individually on the basis of the merits of long term experience and consequences.

Potential requirements for SOHIC and SZC testing (Part 2 Annex B, B4 of NACE MR0175/ISO 15156)

a.

NACE MR0175/ISO 15156 states that SOHIC and SZC testing should be considered for plate steels and products made from welded plate (Part 2 - clause 7.2.2). This may involve either small scale tests (clause B.4.2) or a full pipe ring test (clause B.4.3).

b.

Tests for SOHIC and SZC are not yet standardised, but evaluation can be performed in accordance with Annex B, clause B.4, either by: 1.

Sectioning unfailed UT or FPB SSC test samples to check that there are no ladderlike cracks greater than 0,5 mm (0,020 in) in the through thickness direction.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

2. c.

Testing remaining tensile strength (after hydrogen degassing) to confirm it is greater than or equal to 80% of original actual tensile strength of material.

BP specific requirements 1.

For cases in which SOHIC/SZC is considered a significant risk, BP may specify steel that has increased resistance to these cracking mechanisms.

2.

Annex B indicates the benefits of TMCP and Q+T steels, in this respect.

8.2.

Qualification of cracking-resistant CRAs and other alloys for SSC, SCC, and GHSC (NACE MR0175/ISO 15156 - Part 3 and Part 1)

8.2.1.

Overview

a.

Material shall be qualified for the following conditions: 1.

2.

SSC and SCC resistance, which is achieved by either: a)

Selection from prequalified material.

b)

Laboratory testing using the intended or more severe service conditions.

c)

Based on satisfactory documented service experience.

GHSC resistance, which should be considered only if a specific problem is anticipated. Previous BP experience has shown that this is not often a major issue. As such, it has only rarely been considered.

b.

8.2.2.

Latest details of the capability of specific alloys should be checked, as: 1.

Changes have been made to the pH2S/pH limits compared with earlier NACE MR0175 revisions.

2.

Some alloys have been withdrawn or their use restricted.

Prequalified CRA/other alloys, Part 3 - Annex A of NACE MR0175/ISO 15156

Part 3, Annex D, lists the chemical composition of CRA/other alloy materials. Part 3, Annex A, lists materials capability in sour service of CRA/other alloy materials, arranged by material groups, and providing:  

a.

Acceptable metallurgical conditions. Acceptable environmental limits defined by: -

pH2S.

-

Temperature.

-

Cl– concentration.

-

Elemental S.

In addition to NACE MR0175/ISO 15156, Part 3, relevant BP material diagrams for SSC resistance (refer to 8.2.3) shall be consulted. BP has some limits that are different from NACE MR0175/ISO 15156 (generally less onerous but, in some cases, more onerous). In some cases, a BP specific interpretation of NACE MR0175/ISO 15156 has been required (e.g., for 17/4PH precipitation hardening steel [UNS S17400], the BP limits are more onerous than those in NACE MR0175/ISO 15156).

b.

BP specific requirements for 300 series austenitic SS 1.

Heat treatment as follows (refer to GIS 36-102 for additional guidelines):

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

a)

Parent material/cold formed items shall be subjected to either: 1) Solution annealing at 1 050°C (1 920°F). 2) Thermal stabilisation at 900/950°C (1 650/1 740°F).

b)

Welded equipment, stabilised, or low carbon grades shall be used and shall be heat treated using one of the conditions in a), as specified by BP.

c)

For pipework in low temperature service or if the piping code includes notch toughness requirements, welding procedure tests should be performed to ensure that PWHT does not result in unacceptable embrittlement.

Stress relieving heat treatments in the range of 900°C/950°C (1 650°F/1 740°F) can result in a reduction in notch toughness of ferrite containing weldments. 2.

Austenitic stainless steels shall not be used for bellows or other components in the cold worked or highly stressed condition. UNS N08825, among others, may be used for this duty.

3.

Socket welded fittings, threaded couplings, or any other weld detail that could result in a crevice on the process side shall not be allowed. This is because of significantly greater risk of crevice corrosion in sour/chloride service.

4.

8.2.3.

Fusion bonded overlays of Stellite or Colmonoy may be applied to stabilised or low carbon grades of austenitic stainless steels, provided that the component is subsequently heated rapidly to 900°C/950°C (1 650°F/1 740°F), held for 1 hr per 25 mm (1 in) of thickness, and cooled in still air.

Qualification of CRAs by laboratory testing

Qualification is covered in NACE MR0175/ISO 15156, Part 3, Annex B. Cracking mechanisms for different materials are given in NACE MR0175/ISO 15156, Table B.1. a.

b.

Appropriate individual BP material diagrams for SSC resistance, that indicate domains in which the material may be used, shall be consulted. These material diagrams are available in: 1.

GN 36-004 (downhole equipment materials).

2.

GN 36-013 (downhole tubular materials).

3.

GN 36-017 (for 316L SS solid and clad, parent and weld).

In addition, “application specific” laboratory qualification testing can be considered. Downhole tubing and casing CRA materials for which domain diagrams have been produced include:     

API Spec 5CT/ISO 11960 L80 13Cr steel. Super 13Cr 95 ksi Alloys. Annex D of this GP gives examples of domain diagrams for 1 000 ppm and 120 000 ppm chloride. Super 13Cr 110 ksi alloys. 22%Cr Duplex SS. 25%Cr Super Duplex SS.

Forgings and bar stock CRA materials commonly used in the manufacture of downhole equipment for which domain diagrams have been produced include: 

13Cr Steel (AISI 410 and AISI 420 modified).

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

   

Super 13Cr 95 ksi alloys. Super 13Cr 110 ksi alloys. 17/4PH precipitation hardening SS. Alloy 450 precipitation hardening SS.

8.2.4.

Qualification of CRAs by field experience

8.2.4.1.

General

8.2.4.2.

a.

Materials properties shall be defined and documented.

b.

Service environment shall be defined and documented.

c.

Service experience shall be at least 2 yr to provide sufficient confidence.

d.

Equipment shall receive a full inspection after at least 2 yr of service.

UNS S17400 martensitic SS for equipment

Background 

 





a.

NACE MR0175/ISO 15156 places severe restrictions on the use of UNS S17400 as a result of a number of service failures. Previously, the material was considered acceptable for any application. The document restricts the material to nonpressure containing/low load bearing service or very mildly sour conditions. BP has generated data to demonstrate that even the more restricted application limits in NACE MR0175/ISO 15156 may be unacceptable for many applications (refer to 7.2.2 and 7.2.3). BP has not recommended UNS S17400 for tubing hangers for some time due to concerns about SSC/hydrogen cracking. Because of this, new restrictions are not a problem in this area. BP does have numerous valves in service with UNS S17400 stems, as this has been a very popular material used by the industry because of its high strength. BP experience is that many stems have provided good, long term service. In light of this and the numbers involved, the need to immediately change out the stems is not seen as necessary in this particular application, subject to the following information in this clause.

BP current policy on UNS S17400 valve stems for sour service follows: 1.

New procurements shall fully comply with NACE MR0175/ISO 15156 and the additional environmental limits in the UNS S17400 BP domain diagrams in GN 36-004. This probably results in elimination of S17400 and the use of Alloy 718 valve stems for many sour service applications.

2.

For existing/replacement valves from stock, continued use of UNS S17400 may be justified if both the following apply: a)

There have been no service failures.

b)

Operating conditions have not changed for the worse (particularly higher H2S levels and/or lower in situ pH).

Many BP applications have been on ball valves that have mostly experienced low stress for much of their lives, except for the brief period when the valve has operated.

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b.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

For every valve replacement, continued use of UNS S17400 is acceptable, if it can be confirmed that: 1.

H2S levels will be maintained within the original specification.

2.

The in situ pH has not reduced.

3.

No failure has occurred as a result of SSC, SCC, or other H2S related mechanisms.

4.

No change of duty is intended (i.e., original design basis has not changed). An approach for valve replacement could be for the asset to conduct a review of its current and anticipated H2S, pH, failure history, etc. Afterward, BP could then issue a blanket statement, valid for a limited period (e.g., 2 yr or 3 yr) that the four points detailed in b. apply and the continued use of UNS S17400 is considered acceptable. This is attractive, since it involves a once only effort rather than a review for every valve replacement.

8.2.4.3.

UNS N07090 (Nimonic alloy 90)

Material is included in NACE MR0175/ISO 15156, Table A.36. Hardness shall be limited to:

9.

a.

HRC 35 (solution annealed + age hardened).

b.

HRC 50 (cold worked + age hardened) for springs.

Equipment specific requirements

9.1.

Pressure vessels

9.1.1.

Carbon and low alloy steels

a.

Carbon steel composition 1.

Carbon content for plate shall be a maximum of 0,23%.

2.

CE shall be 0,43% maximum, based on product analysis and calculated in accordance with the CE International Institute of Welding (IIW) formula as follows:

CE  C 

Mn Cr  Mo  V Ni  Cu   6 5 15

b.

Service conditions - If the severity of sour service conditions varies within equipment, the whole vessel shall be designed to be suitable for the most stringent conditions.

c.

SSC resistance - Material hardness requirements shall comply with NACE MR0175/ISO 15156.

d.

HIC/SWC and SOHIC resistance 1.

Material shall be selected to take account of HIC/SWC cracking.

2.

Consideration should be given to the potential need for measures to avoid SOHIC.

e.

Internal bolting - Internal bolting shall be resistant to SSC.

f.

PWHT and/or stress relief - Pressure vessels, including heat exchanger shells, channels, and floating heads, should be stress relieved. Stress relieving heat treatment procedures shall comply with GIS 46-010, GP 36-10, and GIS 36-102.

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9.1.2.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Austenitic stainless steel, duplex stainless steel, nickel base alloys, and internally clad carbon steel

Use of austenitic stainless steel, duplex stainless steel, nickel base alloys, and internally clad carbon steel shall be subject to BP approval. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation. 9.2.

Heat exchanger tube bundles

9.2.1.

Carbon and low alloy steels

a.

The following procedures shall be implemented for tube end welding: 1.

Carbon steel - If satisfactory hardness cannot be obtained in the “as welded” condition, either PWHT shall be applied or the tubesheet shall be faced with low carbon weld metal and stress relieved prior to tube end welding. PWHT limits hardness in the weld and heat affected zones.

2.

Low alloy steel - Either PWHT of tube end welds shall be applied, or the tubesheet shall be overlaid with a nickel base alloy, such as alloy 625 (UNS N06625). The PWHT required to achieve low hardness and adequate resistance to SSC is difficult. Overlaying may prove an attractive alternative.

b. 9.2.2.

Welding procedure qualification - The welding procedure qualification tests shall comply with this GP and GIS 18-014.

Austenitic, duplex, nickel alloys, and internally clad carbon steel

Welding procedure qualification tests for these materials shall comply with GIS 18-014. 9.3.

Piping

9.3.1.

Carbon and low alloy steels

a.

b.

c.

Carbon steel composition 1.

The carbon content for piping made from plate, seamless pipe, and forged fittings shall be a maximum of 0,23%.

2.

The CE shall be 0,43% maximum, based on product analysis and calculated in accordance with CE (IIW) formula in accordance with 9.1.1.a.2.

HIC/SWC and SOHIC/SZC - For pipe made from plate in which hydrogen blistering, HIC/SWC, or SOHIC may occur, material shall be selected to take into account: 1.

HIC/SWC cracking.

2.

Potential measures considered for avoidance of SOHIC/SZC cracking.

ERW pipe - ERW pipe shall not be used for sour service, unless there is specific BP approval. For the BP approval process the following information would be required:

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation. d.

9.3.2.

PWHT 1.

Carbon steel piping shall be welded with the objective of meeting NACE MR0175/ISO 15156 hardness criteria in as welded conditions.

2.

If the hardness requirement cannot be met in the as welded condition or the applicable code requires PWHT, requirements of GIS 36-102 shall be taken into account.

3.

Low alloy steel piping shall be subject to PWHT, irrespective of pipe size or wall thickness.

Austenitic, duplex, and nickel alloys

For fabricated piping, hardness levels and any associated heat treatment shall comply with NACE MR0175/ISO 15156. 9.4.

Pipelines a.

Material specifications shall be subject to BP approval. ERW pipe is not allowed for sour service use without specific BP approval. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

b.

HIC/SWC testing 1.

Production control tests in accordance with NACE TM0284 shall be performed on pipeline steels, specified by BP, to confirm that they are resistant to HIC as detailed elsewhere in this GP.

2.

Other tests may be specified by BP for manufacturing procedure qualification. Acceptance criteria for these tests shall be subject to BP approval. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

c.

SOHIC/SZC testing should be considered as detailed elsewhere in this GP.

d.

Full diameter pipe HIC/SWC/SOHIC testing may be required. If so, details will be specified by BP.

e.

Mill and site welding procedures shall be performed as specified in 10.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

f.

When assessing the results of hardness test surveys, the specific service environment to which the pipeline will be subjected shall be taken into account.

g.

PWHT of field girth welds of pipelines is not required, unless specified by BP, in which case the details shall be subject to BP approval. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

9.5.

Downhole tubulars and equipment Specific requirements for the selection of downhole tubular steels are described in 8. and shall take into account the following key variables: a.

In situ pH.

b.

Concentration of H2S.

c.

Concentration of chlorides.

d.

Temperature. Further advice on materials selection, including that for sour service, is available in GN 36-004 for downhole equipment and GN 36-013 for downhole tubulars.

9.6.

Valves Standards for material selection and fabrication procedures shall comply with NACE MR0175/ISO 15156 in regards to the following: a.

b.

Contact with the environment 1.

Valve parts that can come into contact with the sour environment shall comply with this GP.

2.

Items that are not freely vented to atmosphere (e.g., insulated and buried equipment and bolts inside flange protectors) and where leakage of the process stream could subject the equipment to a sour environment shall comply with this GP.

3.

Valve parts not subjected to sour environment need not comply with this GP.

Plated components 1.

Material on which plating is deposited shall comply with this GP.

2.

The plating material shall be resistant to sour service.

3.

Cadmium and zinc plating shall not be used for this service Cadmium and zinc plating corrode and can also result in GHSC.

c.

Welding and repairs - PWHT 1.

Structural welding and weld repairs of any cast component in contact with sour service shall be followed by PWHT for low alloy steels, martensitic stainless steels, and other materials in which welding can produce unacceptable microstructures and/or hardness.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

2.

Repairs to valve balls, gates, discs, plugs, etc., shall be subject to PWHT for low alloy steels, martensitic stainless steels, and other materials in which welding can produce unacceptable microstructures and/or hardness. It is worth considering having the valve manufacturer weld pup pieces on each end of a welded valve to avoid field PWHT issues.

d.

e.

Valve springs resistant to SSC 1.

Valve springs shall be made from materials resistant to SSC.

2.

The requirement in 1. shall also apply to bellows sealed safety or relief valves that discharge into a common header or piping system.

Protected valve springs 1.

Suitably protected nonresistant materials may be proposed for BP approval for large springs if the cost of resistant materials is high and if failure is not critical. Typical protection methods include: For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

2.

a)

Flexible two pack epoxy paint formulation for service up to 100°C (212°F).

b)

Aluminium metal spray plus sealer.

c)

Fluorinated polymer coatings applied by the fluidised bed process.

If coatings have been accepted: a)

The complete spring shall be protected.

b)

Special attention shall be given to the half coils at the spring ends.

f.

Internal bolting in valves shall be resistant to SSC.

g.

Packing 1.

The packing selected shall be compatible with the stem material.

2.

Uninhibited graphite or carbon type packing shall not be used.

9.7.

Rotating machinery

9.7.1.

General

a.

b.

c.

Sour service SSC resistance 1.

If H2S is present, rotating machinery shall comply with NACE MR0175/ISO 15156.

2.

For reciprocating compressors, the presence of any level of H2S shall result in the specification and use of suitable SSC resistant materials.

General material requirements 1.

Materials selected, fabrication procedures, and associated equipment shall comply with NACE MR0175/ISO 15156.

2.

Items made from 11%/13% Cr shall be double tempered.

Cast irons

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

NACE MR0175/ISO 15156 does not allow for the use of grey, austenitic, or white cast irons for pressure retaining parts. 1.

Cast irons may be used for internal components if allowed by the equipment standard and shall be subject to BP approval. NACE MR0175/ISO 15156 states that the use of ferritic ductile (nodular) iron (e.g., ASTM A395/A395M) is acceptable for equipment, unless specified otherwise by the equipment standard. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

2.

None of these materials shall be weld repaired.

d.

Carbon steel for fabricated casings exposed to wet H2S - Plate shall be HIC resistant and shall comply with Annex A or Annex B.

e.

Shafts and piston rods 1.

Shafts and piston rods in plain carbon, low alloy, medium alloy, and 11%-13% chromium steels shall: a)

Be heat treated to minimise residual stresses.

b)

Have a hardness not exceeding 248 HV10 (HRC22) and a yield stress not exceeding 620 N/mm2 (90 000 lbf/in2).

The NACE MR0175/ISO 15156 limiting condition for this material with pH 3,5 is 1,5 psi H2S. 2.

Shafts in austenitic and duplex stainless steel shall be in the solution annealed condition.

3.

Precipitation hardened nickel alloy shafts shall comply with NACE MR0175/ISO 15156.

4.

17Cr 4Ni precipitation hardening stainless steel (UNS S17400) shall comply with 8.2.4.2.

5.

Straightening of shafts after completion of machining shall not be commenced without prior BP approval. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication and installation.

6.

Machining errors may not be rectified by metal coatings without specific BP approval. Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156).

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation. 7.

Ceramic coatings, metallic coatings, or overlays may not be used without specific BP approval. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication and installation.

f.

Welds - Fabrication welds and repair welds shall be heat treated as follows: 1.

Carbon steel - PWHT in accordance with GIS 36-102.

2.

Low alloy steels - PWHT details shall be subject to BP approval.

3.

Martensitic stainless steel (11%-13% chromium steel) shall be reheat treated completely, including double temper.

4.

Austenitic stainless steels - Do not normally require PWHT. Any thermal treatment shall be subject to BP review and approval.

5.

Duplex stainless steels - Subject to BP approval. In 2, 4 and 5, for the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

9.7.2.

Centrifugal pumps

a.

Standards - Centrifugal pumps shall comply with NACE MR0175/ISO 15156. Austenitic nodular irons - Use of this material for pressure retaining parts for sour service is excluded by NACE MR0175/ISO 15156.

b.

9.7.3.

Shafts 1.

Shafts in plain carbon and low alloy steels shall be protected from the process stream by corrosion resistant sleeves, cap nuts (if applicable), etc.

2.

Unprotected 11%-13% chromium steel may be used only if it has adequate corrosion resistance to the process fluid and shall comply with 9.7.1.

Reciprocating compressors

a.

Sour service SSC resistance - For reciprocating compressors, the service shall be regarded as sour if the gas contains any level of H2S.

b.

Standards - Materials and fabrication procedures shall comply with NACE MR0175/ISO 15156.

c.

Piston rods

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d.

e.

f.

g.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

1.

Piston rods shall be either 11%-13% chromium steel or an alternative material approved by BP and shall comply with 9.7.1.

2.

Rods may be hardened in the region of the packing by surface induction hardening.

Liners 1.

Liners shall be resistant to the corrosive environment.

2.

If cast iron would corrode, a suitable grade of austenitic cast iron may be proposed for BP approval.

Miscellaneous components 1.

Valve plates, rings, channels, seats, and stops shall be made from 11%-13% chromium steel, unless approved otherwise by BP.

2.

Items made from 11%-13% Cr shall be double tempered.

Valves involving flexing plates 1.

Valves involving flexing plates are not normally allowed.

2.

Potential exceptions, only after BP approval, could be as follows: a)

Proof is available of satisfactory service.

b)

Valve plate stresses for the application proposed are low enough to render SSC unlikely.

Valve springs 1.

Valve springs shall comply with NACE MR0175/ISO 15156.

2.

The design stress shall not exceed 276 N/mm2 (40 000 lbf/in2). In c, d and e, for the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

9.7.4.

Centrifugal compressors and expanders

a.

Standards - Centrifugal compressors and expanders shall comply with NACE MR0175/ISO 15156.

b.

Fabrication - Fabrication processes that result in cold worked material (e.g., riveting of impellers) shall not be used, unless prior BP approval has been obtained. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation

9.7.5.

Rotary type positive displacement compressors

Rotary type positive displacement compressors shall comply with NACE MR0175/ISO 15156.

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9.7.6.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Lubrication, shaft sealing, and control oil system

Extent of sour service - Equipment in contact with either seal oil or gas that is sour, as defined in this GP (e.g., vessels, pumps, piping, valves), shall comply with this GP in regards to material selection and fabrication procedures. 9.8.

Instrumentation a.

Process line specification - Instrument piping shall comply with the associated process line specification.

b.

Items that cannot be heat treated 1.

Bellows, diaphragms, bourdon tubes, items that cannot be heat treated after welding, and components that cannot function in the softened condition shall be fabricated from materials resistant to cracking in the hardened and/or cold worked or nonheat treated conditions as defined in NACE MR0175/ISO 15156.

2.

UNS N08825 and UNS N04400 may be proposed for BP approval. UNS N08825 and UNS N04400 have given satisfactory service in certain environments. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

c.

9.9.

Compression fittings 1.

Compression fittings in type 316L SS may be used in accordance with NACE MR0175/ISO 15156.

2.

For relatively high temperature and high chloride service, BP may require the use of more corrosion resistant alloys.

Bolting a.

Sour service SSC resistance - Bolting shall comply with NACE MR0175/ISO 15156 if in contact with any concentration of wet H2S, either directly or indirectly (i.e., bolting in items that are not freely vented to atmosphere, such as insulated and buried equipment and bolts inside flange protectors, for which leakage of the process stream could subject the equipment to a sour environment).

b.

Austenitic stainless steels

c.

1.

Austenitic stainless steels shall comply with NACE MR0175.

2.

Austenitic stainless steel bolts and nuts, if required, shall be free from cold work and shall be solution treated after thread forming as follows: a)

Bolts shall be Class 1A of ASTM A193/A193M (e.g., B8MA [Type 316] bolts solution treated after all cold work including thread forming).

b)

Nuts shall be of the “A” suffix variety of ASTM A194/A194M (e.g., Grade 8MA [Type 316] solution treated after all hot or cold working).

High strength steels - High strength steels for internal bolting and springs, bellows, and parts of reciprocating compressors shall comply with NACE MR0175/ISO 15156, if in contact with any concentration of wet H2S.

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9.10.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Bellows a.

Sour service - Materials for bellows shall comply with NACE MR0175/ISO 15156, if in contact with any concentration of wet H2S.

b.

Materials 1.

2.

Austenitic stainless steels shall not be used for bellows, but material selection shall be made from the following, depending on process conditions: a)

Alloy 825 (UNS N08825).

b)

Alloy 625 (UNS N06625).

c)

Alloy 400 (UNS N04400).

Other materials may be proposed for BP approval. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

9.11.

Low temperature plant a.

Carbon and low alloy ferritic steels and weld metals containing more than 1% nickel shall not be used for sour service, except as allowed in b.

b.

If low temperatures are encountered, steels and weld metals containing more than 1% nickel. may be used, subject to BP approval, and provided that formation of liquid water can be prevented at all times. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

9.12.

Metallic overlays - Explosively clad, roll bonded, and fusion bonded corrosion resistant overlays a.

Overlays of these types in austenitic stainless steel and nickel base alloys may be used as effective barriers in wet H2S environments, provided that the material is suitable for the specific process conditions. Refer to GIS 18-013 for guidelines and requirements.

b.

If such overlays are used and will remain intact over the service life (e.g., it can be demonstrated that they will not erode away), the backing material need not comply with this GP.

10. Fabrication 10.1.

Weld procedure qualification a.

Weld procedure details

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

1.

Procedure qualification records should identify the heat details and manufacturer of the steel. It is important to have a qualified weld procedure that ensures a satisfactory hardness is achieved.

2. b.

c.

d.

Steel used in fabrication shall not exceed that used for qualification by more than 0,02 CE.

BP approval of qualification procedures 1.

Proposals for qualifying the welding procedures, particularly related to the control of hardness, shall be subject to BP approval before commencement of work.

2.

For “as welded” equipment, the proposed method of meeting hardness limits shall be subject to BP approval.

3.

Qualification procedures shall be subject to a guarantee by fabricator or vendor.

Vendor hardness survey after welding 1.

The hardness surveys shall be performed on agreed samples that are cut from approved procedure test coupons.

2.

Test coupons shall be ground smooth and macro etched to reveal various zones of the weld.

3.

Hardness traverses shall be made, as required by NACE MR0175/ISO 15156.

Hardness results 1.

Results shall comply with NACE MR0175/ISO 15156 and GIS 36-102.

2.

Only the Vickers hardness test (5 kg or 10 kg load) shall be used for hardness traverse measurements of welds, unless agreed otherwise by BP. Hardness surveys using the Vickers testing method produce a more detailed picture of weld hardness and its variations. Hardness surveys using the HRC testing method might not detect small zones in welds or HAZs where hardness exceeds acceptance criteria for the Vickers method, and SCC cracks may initiate.

e.

BP approval of results - The results of the qualification tests plus welder qualifications shall be subject to BP approval.

f.

Quality assurance/control - Attention shall be given to quality assurance/control to ensure that the fabrication welding is performed in accordance with the qualified procedure.

10.2.

Heat treatment general requirements

10.2.1.

Fabrication/fabrication repairs/castings

10.2.2.

a.

Heat treatment shall be performed after fabrication, fabrication repairs, and for casting of low alloy steels, martensitic stainless steels, and other materials in which unacceptable microstructures and hardness values may occur.

b.

PWHT shall comply with GIS 36-102.

Approval of heat treatment procedure

For equipment that is to be post weld heat treated, the following information shall be subject to BP approval: a.

Details of the method of heating and cooling. Refer to GIS 36-102 for requirements for heat treatment.

b.

Details of the position of thermocouples.

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10.3.

10.4.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

c.

Special requirements for heat treatment.

d.

Copies of relevant heat treatment charts on completion.

PWHT for carbon steel pipework a.

In many cases, PWHT of pipework is not required. PWHT, if required, shall comply with GIS 42-101, GIS 42-102, and GIS 36-102.

b.

In caustic or other services in which environmental cracking may occur, PWHT shall be performed in accordance with GIS 36-102.

Removal of fabrication defects a.

No fabrication defects (such as arc burns) shall be allowed to remain on new equipment.

b.

These defects and their repairs, if allowed, shall be reported to BP and subject to BP approval.

c.

Documented repair procedures shall be provided to BP. This is to avoid confusion as to the origin of any defects found if equipment is inspected after it has been in wet H2S service.

10.5.

NDE Internal welds of new vessels shall be 100% WFMT.

10.6.

PWHT for carbon steel plate a.

PWHT should be performed on equipment that is made from plate and used for service in sour environments. PWHT reduces hardness and also reduces residual stresses, which is beneficial if there is susceptibility to SOHIC.

b.

Consideration shall be given as to whether it is necessary to heat treat at higher temperatures than code minimum values. Refer to GIS 36-102.

11. Special duties 11.1.

General Conditions and considerations associated with the special duties in 11.2 through 11.4 are not generally covered by NACE MR0175/ISO 15156.

11.2.

11.3.

H2S containing alkaline services a.

This duty is rarely, if ever, encountered in exploration and production operations.

b.

If required, refer to GP 06-20.

Sour streams containing carbonates a.

Welds and bends shall be stress relieved in sweetening plants using a potassium carbonate solution that is normally between 20% and 30%.

b.

Refer to GIS 36-102. This is because this type of solution can cause SCC of welded carbon steel equipment.

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11.4.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Sour streams containing amines Avoidance of environmental cracking in amine environments shall be achieved in accordance with GP 36-12 and API RP 945.

12. Identification, stamping, and marking 12.1.

12.2.

Hard stamps a.

Conventional sharp “V” stamping shall be acceptable only on the outer circumferences of flanges.

b.

Round “V” stamps may be used elsewhere, provided that identities are placed on the external surfaces of low stress areas.

c.

If stamps have been inadvertently applied to high stress areas, subsequent heat treatment shall be required.

Marking paints and crayons a.

Conventional paints, crayons, and adhesive tapes frequently used for temporary marking during fabrication, etc., may contain significant amounts of chloride and heavy metals.

b.

Unless approved by BP, these marking materials shall not be used on any stainless steel. If used on carbon or low alloy steels, they shall be removed before heat treatment (if applied) and before shipment (if heat treatment is not required). For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

13. Inspection 13.1.

BP approval of procedures Prior to commencement of work, the fabricator or vendor shall submit proposals for qualifying welding procedures to BP, particularly those related to the control of hardness.

13.2.

Documentation and inspection Documentation and inspection shall be provided to: a.

Prove the identities of materials of construction.

b.

Establish that the correct heat treatment has been applied, such that the finished product fully complies with this GP.

13.3.

Hardness checks

13.3.1.

Types of hardness tests

a.

The type of hardness tests that can be performed vary, depending on whether the item can be sectioned and tested in the laboratory or can only be tested as a finished product. Sectioning and testing in the laboratory is the preferred methodology and shall be used, if possible.

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b.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Testing of sectioned samples allows determination of the HAZ hardness, as well as the parent metal and weld metal. This is the case for: 1.

Welding procedure qualification tests.

2.

Production test plates, if these are required by the fabrication specification.

3.

For small items (e.g., small springs and pins that cannot be hardness tested individually), the manufacturer shall conduct tests on a random basis by selecting components and sectioning them for laboratory hardness testing to ensure that the product fully complies with this GP.

4.

Procedures for tests described in 3. shall be subject to BP approval. This does not apply to austenitic stainless steels in the solution annealed condition. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

c.

13.3.2.

13.4.

Testing of welded items that cannot be sectioned - If the hardness can be checked without damaging the component, the manufacturer shall conduct hardness tests on parent metal and weld metal to ensure that hardness requirements of this GP are met.

Hardness testing procedure/frequency

a.

Hardness testing procedure/frequency shall comply with NACE MR0175/ISO 15156 and GIS 36-102.

b.

Vickers hardness test (5 kg or 10 kg) shall be used for determining hardness of weldments that can be sectioned. The Rockwell (ISO 6508-1) using 15 N scale can be used as an alternative, by agreement only.

c.

Other hardness tests (e.g., Rockwell C) are considered unsuitable.

d.

For weld items that cannot be sectioned, the hardness test method shall be agreed upon with BP. Brinell hardness testing is frequently used for bulk components.

Heat treatment a.

For corrosion resistant alloys, it shall be proved to the satisfaction of the inspector that the specified heat treatment has been performed correctly.

b.

The requirements of GIS 36-102 shall be met.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Annex A (Normative)

Specification for Z quality steel plate A.1.

A.2.

Scope a.

This annex defines requirements to achieve enhanced resistance to wet H2S service.

b.

Experience has proven that steel made with enhanced through thickness properties has improved resistance to HIC. This type of steel is known as “Z” quality steel.

Definition Z quality steel plate shall be designated as BS EN 10028-3 P275 + BS EN 10164, Z35.

A.3.

Material

A.3.1.

Standards The plate furnished shall comply with BS EN 10028-3 or equivalent steel grades.

A.3.2.

A.3.3.

Steel making process a.

Steels shall be made by a low sulphur and low phosphorus refining process (e.g., in an electric furnace with double deslagging or in the BOF).

b.

Steel shall be vacuum degassed while molten, using a BP approved process.

Chemical composition a.

The following supplementary requirements shall apply: Check analysis % Carbon Sulphur Phosphorus CE

0,20 max. 0,008 max. 0,025 max. 0,43 max. (IIW)

b.

The CE values used in the table in a. shall be calculated by means of the IIW CE formula in accordance with 9.1.1.a.2.

c.

Rare earth metals shall not be used.

A.4.

Inspection requirements for plate

A.4.1.

Ultrasonic examination Material shall be ultrasonically tested in accordance with BS EN 10160 quality class S1/E1 or a BP approved equivalent.

A.4.2.

Through thickness tensile test a.

Each plate shall comply with acceptance class Z35 of BS EN 10164 or equivalent.

b.

BP may agree to a retest after consideration of information supplied. The through thickness test shall be made after the completion of all heat treatments.

c.

In addition to a. and b., tests required by the relevant material specification shall be performed.

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A.5.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Weld repair of plate Unless approved by BP, plate surface defects shall not be repaired by weld and shall be subject to an agreed repair procedure before the work is performed. For the BP approval process the following information would be required: Data demonstrating that the option offered provides adequate sour resistance, in the form of appropriate test data and/or documented service experience (in line with the requirements of NACE MR0175/ISO 15156). Specifications demonstrating that the required degree of quality control/assurance to ensure successful deployment will be provided during manufacture, fabrication, and installation.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Annex B (Normative)

Specification for HIC resistant steel plate B.1.

Scope a.

This annex defines requirements for the manufacture and testing of steel plates with maximum resistance to HIC/SWC for pressure vessels.

b.

For some less severe environments, as advised by BP, “Z” quality plate may be specified, in which case Annex A shall apply.

B.2.

Manufacturing

B.2.1.

Standards Plates furnished shall comply with either BS EN 10028 Part 3, ASTM A516/A516M (normalised), BS EN 10028 Part 6 (Q+T), or ASTM A841/A841M (TMCP) or equivalent standards.

B.2.2.

B.2.3.

Process a.

Steels shall be made by a low sulphur and low phosphorus refining process (e.g., in an electric furnace with double deslagging or in the BOF).

b.

Steels shall be vacuum degassed while molten.

Heat treatment a.

Plates for vessels shall be in the normalised, Q+T, or TMCP condition.

b.

In severe environments, maximum resistance to SOHIC shall be obtained by using Q+T or TMCP steel. This enables the use of lower carbon content and lower CE steel. Lower oxygen of less than or equal to 15 ppm is also beneficial.

B.2.4.

Chemical composition a.

The following requirements shall apply: Check analysis % Carbon Sulphur Phosphorus Oxygen Niobium Vanadium Titanium Boron CE

0,20 max 0,002 max 0,008 max 0,003 max (1) 0,01 max (2,3) 0,01 max (2,3) 0,01 max (2) 0,000 5 max (2) 0,43 max (IIW)

Notes: 1. Oxygen analysis can be waived if steel is vacuum degassed. 2. There should be no deliberate additions of these microalloy elements. Microalloy elements are considered to be deliberately added above the indicated limits. 3. Nb + V max. should additionally be 0,015 wt %.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

b.

IIW CE formula is given in 9.1.1.a.2.

c.

The specific chemical composition offered shall be subject to BP approval.

B.3.

Testing

B.3.1.

Ultrasonic examination Material shall comply with BS EN 10160 quality class S1/E1, ASME SA-578 Level A, including supplementary requirement S1 or an equivalent that is BP approved.

B.3.2.

B.4.

HIC/SWC test a.

Tests shall be made in accordance with NACE TM0284, in the NACE TM0284 test solution A (i.e., 5 wt. % NaCl + 0,5 wt. % glacial acetic acid).

b.

One set of 3 specimens shall be tested from each thickness of plate from each heat.

c.

BP acceptance values shall be as follows, unless agreed otherwise: 1.

Less than or equal to 10% CLR.

2.

Less than or equal to 3% CTR

3.

Less than or equal to 1% CSR.

d.

Following exposure, test coupons shall be ultrasonically tested for evidence of HIC/SWC in accordance with BS EN 10160, prior to sectioning.

e.

Additional sections for microscopic examination shall be prepared through any suspect locations, subject to BP approval.

f.

Note that some vendors are able to meet the following criteria: 1.

Less than or equal to 5% CLR.

2.

Less than or equal to 1,5% CTR.

3.

Less than or equal to 0,5% CSR.

Weld repair of plate Plate surface defects shall not be repaired by welding.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Annex C (Informative)

Background on factors that affect cracking in H2S containing (sour) environments C.1.

C.2.

General material factors a.

Chemical composition, including CE.

b.

Thermal history.

c.

Strength.

d.

Hardness.

e.

Microstructure and homogeneity.

f.

Presence and type of nonmetallic inclusions.

g.

Stress level (applied and residual, including thermally and mechanically induced).

h.

Effect of welding, repairs, and PWHT on the factors in a. through g.

General environmental factors a.

C.3.

Wet H2S → Hº → H2 production 1.

Nascent hydrogen is produced on the material surface by wet H2S corrosion reaction, as well as other corrosion mechanisms (e.g., CO2 corrosion).

2.

In many environments, two nascent hydrogen atoms combine to form molecular hydrogen and bubble away from the surface as 2H0 → H2 (↑).

3.

However, environments containing dissolved hydrogen sulphide can inhibit this reaction and allow the Hº to build up on the surface and then diffuse into the metal.

4.

Inside the metal, atomic hydrogen can result in SSC, as well as HIC and its related mechanisms, if H0 recombines to form the more stable molecular H2, which is considerably larger and becomes trapped in the metal.

b.

pH - Hydrogen generation is low in neutral solutions (pH 7) but increases at both lower and higher levels of pH.

c.

Other contaminants - Some contaminants, such as As and CN–, increase the concentration of H0 on the surface, likely by inhibiting the formation of H2 and thus increasing hydrogen entry and flux.

d.

Velocity - Protective sulphide films can be damaged by erosion under high velocity conditions and also by cleaning for inspection.

e.

Temperature - Increasing temperature increases the amount of hydrogen by increasing the corrosion reaction rate but can also reduce the damaging effects of absorbed hydrogen, in some cases, by increasing the mobility/diffusion rate through the material (e.g., carbon and low alloy steels).

Factors affecting SSC a.

Mechanism 1.

SSC occurs under the combined action of tensile stress and corrosion in the presence of water and H2S. This is a form of HSC.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

2.

b.

c.

d.

C.4.

Atomic hydrogen produced by corrosion diffuses into the steel and results in hydrogen embrittlement at high hardness/stress locations. The presence/level of H2S and presence of sulphide scales increase the amount of atomic hydrogen available for diffusion by “poisoning” the hydrogen recombination reaction, making HSC more likely.

Strength and hardness 1.

High strength steels and areas of high hardness (e.g., a weld HAZ or areas of high residual stresses from cold work or thermal history) are particularly susceptible to SSC.

2.

Steels containing elements that increase hardenability (e.g., C, Cr, Mo, Nb, and V) lead to higher strength and susceptibility.

3.

Restriction of carbon content and CE is beneficial in restricting strength and hardness. NACE MR0175 indicates that nickel should be less than 1% in steels for sour service. There is, however, evidence that this can be successfully exceeded under some conditions.

4.

High strength weld filler metal has suffered SSC in the past, which led to the development of NACE RP0472.

5.

Compliance with specified hardness levels for weldments is achieved in practice by control of weld procedures, including the selection of consumable composition, strength, and/or PWHT.

Stress 1.

Cracking is promoted by high stress levels.

2.

Applied stresses and residual stresses (e.g., resulting from welding, thermal history, or cold work) need to be taken into account.

Temperature 1.

Maximum SSC susceptibility often occurs at a low or intermediate temperature.

2.

For carbon and low alloy steel, plus some CRAs (e.g., 13Cr martensitic SS), the “maximum susceptibility” temperature can be taken at approximate ambient conditions. In some circumstances, if lower temperatures can be experienced (e.g., low seabed temperatures), susceptibility to SSC can be increased.

3.

For a number of CRAs, maximum susceptibility occurs at an intermediate temperature. For example, in the case of duplex stainless steels, maximum susceptibility occurs at a temperature in the range 80°C to 100°C (176°F to 212°F) due to a “mixed mode” (SSC and SCC) failure mechanism.

Factors affecting SZC a.

b.

Mechanism 1.

SZC is a form of SSC that occurs if the steel contains a local “soft zone” of low yield strength.

2.

Soft zones with low yield strength a)

Under service stresses, such soft zones may yield and produce local plastic strain and thereby increase the susceptibility to cracking of an otherwise SSC resistant material.

b)

Such soft zones are typically associated with the HAZ of welds in carbon steel.

Key factors affecting SZC are hardness differential and localised stress level.

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C.5.

Factors affecting GHSC a.

b.

C.6.

Mechanism 1.

HSC is cracking that results from presence of hydrogen in a metal and tensile stress that may be residual and/or applied.

2.

GHSC is cracking in metal/service condition combinations that are not necessarily sensitive to SSC but the metal may be embrittled by hydrogen if galvanically coupled as the cathode to another metal subject to active corrosion (i.e., the anode) in the same H2S containing service conditions.

Key factors in GHSC are potential difference and anode/cathode area relationship.

Factors affecting HIC/SWC and blistering a.

b.

c.

d.

C.7.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Mechanism 1.

Hydrogen atoms diffuse into steel and can collect at inclusions and other discontinuities and combine at such locations to form stable molecular hydrogen.

2.

The H2 molecule is considerably larger than H0 and is too large to diffuse through the metal. Thus, H2 is trapped, builds up, and a pressure is then exerted at the location within the metal.

3.

The process in 2. may lead to laminar cracking in the plane of nonmetallic inclusions that result in HIC or blistering.

4.

Transverse cracks can occur between laminar cracks on different planes. Such cracking is known as SWC.

5.

HIC/SWC cracking does not require application of an external stress, because an internal stress is built up by the accumulated hydrogen pressure.

Steel cleanliness 1.

Historically, conventional steels contained a lot of sulphur and phosphorus that contributed to forming nonmetallic inclusion. These sites are traps for H0 to H2 combination.

2.

In some cases, blistering tends to form if hydrogen trap sites are near the surface. Newer clean steels (ultra low S and P) are now available with significantly better resistance to HIC and blistering.

Microstructure 1.

Steels with banded microstructures and containing anomalous structures are more vulnerable to HIC.

2.

Improvements in resistance to HIC have been obtained by normalising, quench and tempering, and manufacturing with the TMCP process.

Product form 1.

Plate steel is most susceptible to damage due to the elongated nature of the inclusions.

2.

In contrast to plate steel, seamless pipe and forgings have more rounded inclusions and do not have significant sensitivity to HIC/SWC and blistering.

Factors affecting SOHIC a.

Mechanism

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16 March 2009

b.

c.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

1.

SOHIC consists of a stacked array of small laminar cracks/blisters that are joined by HIC and aligned in the through thickness direction of the steel, as a result of high localised tensile stresses.

2.

SOHIC has similarities to HIC/SWC but also is related to SSC and typically found in the weld HAZ, though not exclusively so.

Stress effects 1.

Residual and applied stresses play an important role in the production of SOHIC, which differs from the case with HIC.

2.

High stresses promote cracking, as does the presence of stress raising notch like defects and SSC cracks.

Material factors affecting SOHIC 1.

Some of the newer, clean HIC resistant steels are believed to be more susceptible to SOHIC. This contention is supported by their lower threshold stress in the SSC test. Others, however, have a very high threshold stress, indicative of good SOHIC resistance.

2.

A Materials Properties Council (MPC) study showed that microstructural banding, calcium treatment, and carbide microhardness had a deleterious effect, and a PACR parameter was defined. This approach is useful, but there are still exceptions to the rule.

3.

PACR parameter PACR = 35FcaBI + 0,05(VHN12,5 kg - 330). PACR - Predicted average cracking ratio. Ferrite/pearlite banding index - (BI = ASTM E1268 banding index). Carbide hardness - (VHN12,5 kg = Vickers hardness using 12,5 kg load). Ca treatment - (Fca = calcium treatment factor, yes = 1, no = 0,1).

d.

C.8.

PWHT - There is a beneficial effect of PWHT in reducing residual stress and tempering any hard zones that might otherwise lead to SSC starter cracks.

Factors affecting SCC a.

Mechanism - Cracking of metals involving anodic processes of localised corrosion and tensile stress (residual and/or applied) in the presence of water and H2S.

b.

Key factors - Higher levels of stress, chlorides, presence of oxidants, and elevated temperature increase the susceptibility to SCC attack.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Annex D (Informative)

Example of domain diagrams H2S Limits for 95 ksi Super 13Cr Alloys -

(High chloride waters - 120 000 ppm Cl ) ACCEPTABLE

5,5 0,03 bara pH 4,5 FURTHER ASSESSMENT REQUIRED

3,5 UNACCEPTABLE

0,001

0,01

0,1

1,0

pH2S (bara) Domain Diagram for the sulphide stress cracking limits of 95 ksi super 13Cr alloys in high chloride (120 000 ppm Cl ) waters.

–

(Lower chloride waters – 1 000 ppm Cl )

ACCEPTABLE

5,5

pH

4,5 FURTHER ASSESSMENT REQUIRED

3,5 UNACCEPTABLE

0,001

0,01

0,1

1,0

pH2S (bara) Domain Diagram for the sulphide stress cracking limits of 95 ksi super 13Cr alloys in low chloride (1 000 ppm Cl ) waters.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Annex E (Informative)

In service issues E.1.

E.2.

E.3.

General a.

It is beyond the scope of this document to give detailed guidance on “in service issues” for material operating in sour service.

b.

Some key issues that need to be considered follow in this annex.

Inspection a.

There is a need to perform inspection, if possible, to monitor the performance of sour service materials.

b.

Clearly, some types of equipment lend themselves more readily to inspection than others. The following are issues for consideration: 1.

External inspection - Straight beam and angle beam ultrasonic testing and automated ultrasonic testing are frequently used to inspect for cracking.

2.

Global external inspection - Acoustic emission can be used for external global inspection to locate areas of active cracking for further characterisation by ultrasonic testing.

3.

Internal inspection - Visual examination of the inside surfaces and weld areas is used to identify the presence of any cracking. Other more sensitive techniques include wet fluorescent magnetic particle inspection and alternating current field method.

4.

NDE operators - Operators require appropriate training, such as ASNT SNT-TC-1A or equivalent, to Level 1 and Level 2 for interpretation of data.

5.

RBI - RBI techniques can be used to prioritise inspection. This takes account of both probability and consequence of failure.

6.

Defects - Any defects that are found should be evaluated for fitness for service using techniques, such as those given in API 579. The potential for continued crack growth shall be considered. Remedial action may involve local grinding and weld repairs (after hydrogen outgassing).

Hydrogen flux monitoring a.

The amount of hydrogen generated by corrosion and thereby available as diffusible atoms is important in assessing severity of service.

b.

Monitoring of hydrogen flux is therefore beneficial, and the following techniques are available: 1.

Intrusive finger pressure probes measure the buildup of pressure inside a tube inserted in the environment, following diffusion through a membrane.

2.

Patch probes that measure hydrogen atoms diffusing through the equipment by the buildup of pressure in the cavity between a patch and the equipment.

3.

An electrochemical cell involves diffusion of hydrogen through a metallic palladium membrane, which forms the partition of a two compartment cell. Emergent hydrogen atoms are oxidised and measured by a current flow in the cell.

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E.4.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

4.

Beta foil is a thin foil glued to the outside of the equipment, and a vacuum is drawn between the foil and the equipment. As hydrogen diffuses into this zone, the pressure increases and is monitored.

5.

The Hydrosteel 6000 uses a flexible metallic plate collector with a spiral pathway on the surface to sweep up any hydrogen diffusing out of the vessel/pipe of interest, in a flow of air, into the measuring equipment.

Mitigating methods Several different remedial measures have been used in situations where hydrogen induced cracking has been a problem. These include:

E.5.

a.

Improved materials have been applied with greater resistance to hydrogen induced cracking.

b.

Metal lining using a stainless steel type of liner has been used, produced either by roll bonding or by weld overlaying.

c.

Coatings have also been used, which minimises corrosion and hydrogen flux. However, attention needs to be given to the quality of the base material, in case of breakdown of the coating at some stage.

d.

Surface preparation that is used before performing internal inspection leaves the metal surface in a condition in which a high hydrogen flux can occur on recommissioning. Some temporary protection for the surface is desirable to minimise the hydrogen flux until a protective sulphide film can form.

References Refer to API RP 579 and ASNT SNT-TC-1A for further information on fitness for service and recommended practices for SNT-TC-1A.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Annex F (Informative)

Examples of previous in service H2S related failures F.1.

F.2.

F.3.

Oil flow line HIC failure of carbon steel pipe (BP) - 1972 a.

HIC failure of carbon steel pipe occurred in subsea sour oil flowline located offshore in the Middle East.

b.

The pipe was NPS 30, X60 which was sub-arc seam welded.

c.

Pipe was formed from fully killed, controlled rolled mild steel plate in which the microstructure exhibited long, thin manganese sulphide inclusions.

d.

HIC cracking occurred between manganese sulphide inclusions.

e.

Cracks linked through the plate thickness, resulting in 90 mm (3.5 in) long longitudinal crack.

Downhole tubular string SSC failure of alloyed 13% Cr steel (BP) - 2001 a.

SSC failure of alloyed 13% Cr steel occurred on gas production downhole tubulars in an onshore gas field in Louisiana, U.S.

b.

Wells were HP/HT gas producers yielding 45 mmscf/d, containing 50 ppm H2S and 8,7 mol % CO2 (0,03 bar H2S [0,5 psi H2S] pH 3,5).

c.

Well completion was with an alloyed martensitic 13Cr steel, (13Cr/4Ni/1Mo) 95 ksi strength tubing.

d.

Packerless completion allowed produced fluid to contact the tubing OD.

e.

Tubing failed by SSC at localised sites of cold work on the OD (at locations with slip and tong marks).

f.

Gas, water, and sand flowed from the tubing bore to the annulus, resulting in significant erosion of both the tubing and casing.

g.

Tubing was replaced with 95ksi super martensitic SS (13Cr/5Ni/2Mo) that has increased H2S resistance over the previously used 13Cr/4Ni/1Mo alloy. Care was taken to minimise slip/tong damage.

h.

Casing was repaired with an expandable solid casing patch.

Failure by SOHIC of carbon steel spiral welded pipe (Saudi Aramco) - 1998 a.

Spiral welded pipe can be particularly susceptible to SOHIC.

b.

SOHIC failures on spiral welded pipe occurred in the Middle East, and SOHIC cracking was found on 3 of 39 joints.

c.

Residual stresses are very high in spiral welded pipe that was subject to a high hydrogen charging rate.

d.

Material that suffered SOHIC had a threshold stress of 1,2 MPa (17 ksi), while nonfailed material had a higher threshold stress of 2,1 MPa to 2,4 MPa (30 ksi to 35 ksi).

e.

Conventional longitudinal welded pipes were present in the line and exhibited HIC but not SOHIC.

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F.4.

GP 36-25 Materials for Sour Service in Exploration and Production Operations

Failure by SSC/SOHIC of carbon steel pressure vessel (Unocal) - 1984 a.

Failure by SSC/SOHIC of a carbon steel pressure vessel occurred at a Unocal refinery in Lemont, Illinois, U.S., with several fatalities.

b.

The vessel was in amine absorber service.

c.

Failure occurred by SOHIC emanating from an SSC crack.

d.

Cracking initiated at the location of a repair to a stress relieved vessel was performed without PWHT.

e.

Other features were a mismatched joint and some internal cladding with Monel alloy 400 nearby.

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GP 36-25 Materials for Sour Service in Exploration and Production Operations

Bibliography British Standards Institute (BSI) [1]

BS EN 10204, Metallic materials. Types of inspection documents.

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