Materialanforderungen und Auswahl von Unterwasserventilen

Deep-sea exploitation of oil and gas is the future development trend of resource development; underwater valves are the key components of underwater oil extraction trees and pipeline equipment, and material selection is an important aspect of underwater valve design. The basic requirements for material selection of underwater Ventile and the characteristics of various types of materials are introduced, and the factors to be considered when selecting materials for underwater valves are summarized according to the provisions of the relevant design specifications and suggestions for material selection of underwater valves are also put forward. The conclusions obtained can provide a basis for designing underwater valves and have a certain guiding effect on the independent research and development of domestic underwater oil and gas extraction equipment.

0. Introduction

China’s research and development of marine underwater equipment started relatively late; no underwater wellhead device oil recovery tree or other complete sets of equipment scale production capacity exists. China’s marine underwater oil and gas extraction equipment is mainly imported from Cameron FMC and other foreign companies. Independent research and development and manufacturing of underwater wellhead devices and oil tree equipment is China’s offshore oil and gas industry development must go through.
Underwater valves (mainly gate valves und Kugelhähne) are the key components of underwater tree and pipeline manifolds and play an important role in underwater oil recovery equipment. With the development of oil recovery trees to the deep sea, the material and performance of underwater valves put forward very high requirements. The author, according to the actual underwater valve research and development, the underwater valve design and development process of material requirements and selection is introduced.

1. Selection of underwater valve materials needs to consider factors

The marine and underwater oil and gas exploration equipment with valve material selection is based on the following standards.

• (1) API 17D Design and Operation of Subsea Production Systems Part 4: Subsea Wellhead Units and Production Tree Equipment.
• (2) API 6A Wellhead Installation and Treeing Equipment;
• (3) M-001 Material Selection (NORSOK Specification).
• (4) NACE MR0175 Oil and Gas Industry – Materials Used in Hydrogen Sulfide Containing Environments in Oil and Gas Production.
• API 6A and API 17D specifications for underwater valves with specific requirements, such as strength, impact, testing, and quality control requirements. In specific applications, a need to determine the strength of the material according to the pressure level of the equipment; for example, for Flansche and end connections, the pressure level of 70MPa equipment should be used for the minimum yield strength of 420MPa material, pressure level of more than 70MPa equipment should be used for the minimum yield strength of 525MPa material. Nace MR0175 specification for the environment containing hydrogen sulfide The use of anti-sulfide stress corrosion (SSC) materials and low alloy materials exposed to acidic environments to make specific provisions, such as the provisions of the maximum hardness of low alloy materials cannot exceed 22HRC; nickel mass fraction maximum cannot exceed 1%; at the same time, all types of materials heat treatment (including normalizing, normalizing + tempering, quenching + tempering), such as the specific requirements put forward.

NORSORK M-001 specification based on the PED directive (Pressurized Equipment Directive) risk control guidelines and PRE (Pitting Resistance Equivalent), CPT (Critical Pitting Temperature), and CCT (Critical Crevice Corrosion Temperature) and other parameters of the underwater structure of the material selection put forward specific requirements.
The materials used in subsea oil production equipment face three major challenges: H₂S corrosion, CO₂ corrosion, and Cl^- corrosion. With the development of deepwater drilling technology, underwater oil recovery equipment also needs to be able to meet the well waxing, anti-asphalt, anti-hydrate, and anti-scaling process and add a variety of chemical reagents caused by corrosion; these chemical reagents will bring a variety of negative impacts on the metal and non-metallic materials, a combination of various factors together, the requirements of the underwater oil recovery equipment with materials that can simultaneously meet the production of fluids, annular media and injected chemical reagents. Comprehensive impact, at the same time, cathodic protection caused by hydrogen embrittlement should also be considered. Based on the above aspects, underwater valve material selection faces special challenges; in selecting underwater valve materials, the following factors should be considered.

• (1) The chemical composition of the production fluid in contact with the inner parts of the valve.
• (2) Working pressure and temperature range.
• (3) The role of the current caused by the contact of different metal materials.
• (4) Crevice corrosion caused between the sealing surface and flange surface.
• (5) Applicable temperature and chemical resistance of non-metallic materials.
• (6) Cathodic protection of materials;
• (7) Surface coatings;
• (8) Weldability and overlay properties of materials;
• (9) Compatibility of the material with the injection medium;
• (10) Material availability and cost;
• (11) The mutual movement of parts that should be considered material wear resistance.

2. Various types of material properties

According to the use of underwater valve requirements and various types of material properties, underwater valves mainly use the following types of materials: carbon steel and low alloy steel, martensitic stainless steel, austenitic stainless steel, duplex stainless steel, nickel-based alloys, and non-metallic seals.
2.1 Carbon steel
Carbon steel is very susceptible to corrosion by internal and external fluid media; in free water, CO₂ can quickly corrode carbon steel. The feasibility of using carbon steel as a material for subsea equipment is based on strict control of the CO₂ dew point and the ability of the material to resist deformation at lower design temperatures.
The heat-affected zone of a carbon steel weld has a risky effect on material properties (e.g., hardness and strength), and the heat-affected zone around a carbon steel weld may have a lower resistance to CO₂ corrosion, so carbon steel cannot be used in underwater equipment where CO₂ corrosion is present.
2.2 Low alloy steel
Used for underwater valves of low-alloy steel materials are mainly AISI 4130, AISI 8630, F22 (UNS K21590), or F22V (UNS K31835); these materials can be tempered by heat treatment to obtain a high level of comprehensive mechanical properties, generally reach the API 6A specification of the 60K material performance requirements and has a good low-temperature impact performance. In the lower CO₂ environment (CO₂ partial pressure ≤ 0.21MPa), materials have a certain degree of corrosion resistance. Due to the carbon mass fraction is not high (generally 0.35% or less), so can be used as the base material of the pressure parts. The hot wire TIG welding process in the base material is surfacing corrosion-resistant alloy materials used to manufacture underwater valves suitable for high H₂S corrosion, CO₂ corrosion, and Cl^-corrosive environments.
2.3 Rostfreier Stahl
Stainless steel materials used for underwater valves are mainly martensitic stainless steel (such as 410SS, F6NM), 300 series austenitic stainless steel (such as 304SS, 316SS), precipitation hardening stainless steel (such as 17-4PH), and duplex stainless steel (such as 2205, 2507) and so on. Various types of stainless-steel material properties are as follows.
410 stainless steel in the CO₂ corrosive environment has good corrosion resistance below the lower toughness in the -18 ℃ temperature environment to meet the API 6A specification -18 ℃ low-temperature impact and PSL-3 level equipment performance requirements. When there are strict requirements for the sealing surface, it is necessary to overlay weld Inconel-625 alloy on the surface.
F6NM and 410SS compared to a higher resistance to corrosion and pitting ability, with good weldability, low-temperature impact resistance, and high hardenability, to meet the API 6A specification -46 ℃ low-temperature impact requirements. The disadvantage is that the forging process is not good; there is a high deformation resistance when forging, easy to forge cracks. When there are strict requirements for the sealing surface, it is necessary to overlay weld Inconel-625 alloy on the surface.
17-4PH is composed of copper, niobium/columbium precipitation hardening martensitic stainless steel, with high strength, hardness corrosion resistance, and other characteristics; after solution aging heat treatment, the mechanical properties of the product more perfect, can reach 724MPa more than the yield strength to meet the API 6A specification of -60 ℃ low-temperature impact requirements, and its corrosion resistance up to the level of 304 stainless steel. According to the provisions of NACE MR0175, 17-4PH is used for valve stems, and suspension materials can only be used for H₂S partial pressure ≤ 3.5kPa use of the environment.
Duplex stainless steel is used for underwater equipment duplex stainless steel, mainly UNS 31803 (2205) and UNS 32750 (2507); these two kinds of stainless steel have good corrosion resistance, can be used for most of the production of fluids, and more than 80 ℃ chloride stress corrosion environment. UNS 31803 and UNS 32750 duplex stainless steel has the resistance to hydrogen-induced cracking (HIC) ability, UNS 31803 and UNS 32750 duplex stainless steel has the resistance to hydrogen-induced cracking (HIC) UNS 31803 and UNS 32750 duplex stainless steels can resist hydrogen-induced cracking (HIC), and the lowest design temperature of the material can reach -50 ℃. UNS 31803 stainless steels have good welding performance, but the weldability of UNS 32750 is poor. Large cross-section duplex stainless steel parts should be the correct solution for aging heat treatment to obtain the required ferrite-austenite equilibrium phase. Duplex stainless steel pitting resistance equivalent (PRE) should generally exceed 40. Otherwise, it can only be used for the manufacture of thin-walled parts.
2.4 Nickel-based alloys
When the use of underwater valves is more than the environment of carbon steel, 13Cr stainless steel and duplex stainless steel corrosion resistance should be used in various types of nickel-based alloys as valves and fluid-media contact parts of the material. Commonly used in underwater valves, nickel-based alloy materials are Inconel-625, Inconel-718, Inconel-725, Inconel-925, and Monel K-500 alloy. Nickel-based alloys have excellent corrosion resistance and can resist various corrosive fluid media; the disadvantage of machining difficulties, expensive, in the aging state of the welding performance could be better. When sealing surface materials, do not need to weld other corrosion-resistant alloys. In general, nickel-based alloys are mainly used to manufacture API 6A and API 17D specifications in line with the HH level wellhead devices and oil tree parts.

3. Provisions of various codes on the selection of materials

To solve the environmental constraints on the material, stainless steel is divided into two levels:

• ① stainless steel for the environment of the H₂S partial pressure less than 3.5kPa, the water phase pH ≥ 4.5; 
• ② stainless steel for the environment of the H₂S partial pressure greater than or equal to 3.5kPa and less than 10.5kPa, the water phase pH ≥ 3.5.

3.1 API 6A and API 17D specification of the valve material requirements

API 6A and API 17D specification, underwater valve materials are mainly based on H₂S partial pressure, CO₂ partial pressure, medium acidity and alkalinity and corrosivity, and other factors, underwater valve pressure parts, pressure control parts and valve stem and other parts of the material is graded in detail, the requirements of the materials at all levels are described below:

• AA class material, its H₂S partial pressure <0.35kPa, CO₂ partial pressure <49kPa, non-acidic environment, no corrosion conditions, pressure parts materials for carbon steel and low alloy steel, pressure control parts materials for carbon steel and low alloy steel, valve stem materials for carbon steel and low alloy steel.
• BB-class materials, its H₂S partial pressure <0.35kPa, CO₂ partial pressure 49-210kPa, non-acidic environment, slightly corrosive conditions, the material of the pressure-bearing parts of stainless steel, the material of the pressure-control parts of stainless steel, the material of the valve stem is stainless steel.
• CC class material, its H₂S partial pressure <0.35kPa, CO₂ partial pressure 210-1400kPa, non-acidic environment, medium to highly corrosive conditions, the material of the pressure bearing parts is stainless steel, the material of the pressure control parts is stainless steel, the material of the valve stem is stainless steel.
• DD-0.5 class materials, H₂S partial pressure <3.5kPa, CO₂ partial pressure <49kPa, acidic environment, no corrosive conditions, the materials of pressure-bearing parts are carbon steel and low alloy steel, the materials of pressure-control parts are carbon steel and low alloy steel, and the materials of valve stems are ① type stainless steel.
• DD-1.5 class material, its H₂S partial pressure <10.5kPa, CO₂ partial pressure <49kPa, acidic environment, no corrosion conditions, the material of the pressure-bearing parts is carbon steel and low alloy steel, the material of the pressure-control parts is carbon steel and low alloy steel, the material of the valve stem is ② type stainless steel.
• DD-NL class material, its H₂S partial pressure>10.5kPa, CO₂ partial pressure<49kPa, acidic environment, no corrosion conditions, the material of the pressure-bearing parts is carbon steel and low alloy steel, the material of the pressure-control parts is carbon steel and low alloy steel, the material of the valve stem is carbon steel and low alloy steel.
• EE-0.5 grade material, its H₂S partial pressure <3.5kPa, CO₂ partial pressure 49-210kPa, acidic environment, slightly corrosive conditions, the material of the pressure-bearing parts is carbon steel and low alloy steel, the material of the pressure-control parts is ① stainless steel, the material of the valve stem is ① stainless steel.
• EE-1.5 class material, its H₂S partial pressure <10.5kPa, CO₂ partial pressure 49-210kPa, acidic environment, slightly corrosive conditions, the material of the pressure-bearing parts is carbon steel and low alloy steel, the material of the pressure-control parts is ② type stainless steel, the material of the valve stem is ② type stainless steel.
• EE-NL class material, its H₂S partial pressure>10.5kPa, CO₂ partial pressure 49-210kPa, acidic environment, slightly corrosive conditions, the material of the pressure bearing parts is carbon steel and low alloy steel, the material of the pressure control parts is ② stainless steel, the material of the stem is CRA (corrosion-resistant alloy).
• FF-0.5 class material, its H₂S partial pressure <3.5kPa, CO₂ partial pressure 210-1400kPa, acidic environment, medium to highly corrosive conditions, the material of the pressure-bearing parts is ② stainless steel, the material of the pressure-control parts is ② stainless steel, the material of the valve stem is ① stainless steel.
• FF-1.5 class materials, H₂S partial pressure <10.5kPa, CO₂ partial pressure 210-1400kPa, acidic environment, medium to highly corrosive working conditions, the material of the pressure-bearing parts ② stainless steel, pressure control parts ② stainless steel, valve stem material ② stainless steel.
• FF-NL class material, its H₂S partial pressure>10.5kPa, CO₂ partial pressure 210-1400kPa, acidic environment, medium to highly corrosive working conditions, the material of pressure-bearing parts is ② type stainless steel, the material of pressure-control parts is ② type stainless steel, the material of valve stem is CRA.
• For HH class materials, the partial pressure of H₂S >10.5kPa, the partial pressure of CO₂ >1400kPa, acidic environment, highly corrosive working conditions, the materials of pressure-carrying parts, pressure-control parts, and valve stem are CRA.

3.2 NACE MR0175 specification for the valve material requirements

According to NACE MR0175 specification, the material selection of valves should be by the following requirements:

• Low alloy steel used in H₂S partial pressure, Cl^- and pH value of the environment is not limited, can be used to do any part of the valve, the maximum temperature of 177 ℃, the yield strength of the material is 420-490MPa.
• The austenitic stainless steel used in H₂S partial pressure <105kPa, Cl^- and pH value of the environment without limitations (or H₂S partial pressure <350kPa, Cl^- mass concentration <50mg/L and pH value of the environment without limitations) can be used for any part of the valve, the maximum temperature of 60 °C, the yield strength of the material 210-245MPa. The martensitic stainless steel used in H₂S partial pressure, Cl^- without limitations (or H₂S partial pressure <350kPa, Cl^- mass concentration <50mg/L and pH value of the environment without limitations), can be used to make any part of the valve, maximum temperature of 60 °C, material yield strength of 210-245MPa. Partial pressure, Cl^-unrestricted and pH ≥ 3.5 environments, can be used to do pressure parts and pressure control parts (or H₂S partial pressure <10.5kPa, Cl^-unrestricted, and pH ≥ 3.5 environments, can be used to do the valve stem), the maximum temperature of 177 °C, the yield strength of the material is 420-525MPa.
• 17-4PH stainless steel used in H₂S partial pressure <3.5kPa, Cl^-unrestricted, and pH ≥ 4.5 environments can be used as a valve stem and pressure control parts, the maximum temperature of 177 ℃, the material yield strength of 630-735MPa.
• The duplex stainless steel used in H₂S partial pressure <10.5kPa, Cl^- and pH value unlimited environment can be used as any part of the valve, with a maximum temperature of 232 ℃, yield strength of 420-455MPa.
• The super duplex stainless steel used in H₂S partial pressure <21kPa, Cl^- and pH value of the unlimited environment, can be used as any part of the valve, with a maximum temperature of 232 ℃, yield strength of 525-595MPa.
• A286 is used in the environment where the partial pressure of H₂S is <105kPa, Cl^- and pH values are not limited, it can be used for any part of the valve, the maximum temperature is 65℃, and the yield strength of the material is 735MPa.
• Inconel-718 and Inconel-925 are used in environments where the partial pressure of H₂S is <4.2MPa, and where there is no limitation of Cl^- and pH values, and can be used to make any parts for valves, with a maximum temperature of 232℃ and a material yield strength of 630-945MPa.
• Inconel-725 is used in environments where the partial pressure of H₂S is <4.2MPa, and where there is no limitation of Cl^- and pH values, and can be used to make any parts of valves, with a maximum temperature of 232°C and a yield strength of 455-525MPa.
• Inconel-625 and Inconel-825, used in H₂S partial pressure, Cl^- and pH value of the environment without limitations, can be used to do any part of the valve, the maximum temperature of 232 ℃, the yield strength of the material is 245-525MPa.
• Cobalt-based alloys used in H₂S partial pressure, Cl^- and pH value of the environment without limitations, can be used to do any part of the valve, the maximum temperature of 177 ℃, and the yield strength of the material is 350-525MPa.

4. Selection of materials for underwater valves

Underwater valve material selection, in addition to the characteristics of various types of materials and API 6A, 17D, and NACE MR0175 specification, in the actual engineering design, should also take into account the tensile strength of the material, yield strength, elongation, sectional shrinkage, hardness, impact toughness, and other special properties, such as friction resistance, resistance to abrasion and corrosion resistance.
According to API 6A and API 17D specifications of the material properties and influencing factors, the author combined practical design experience of several typical levels of materials in the design of underwater valves to do the test analysis and give detailed processing requirements.
4.1 DD-NL level materials
Valve body and bonnet material for AISI 4130, sealing surface surfacing Inconel-625; stem material AISI 4130/AISI 4140, surface nitriding; gate and seat material AISI 4130, surface welding WC-Co-Cr alloy, the material yield strength of 525MPa; metal sealing material Inconel The material of metal seal adopts Inconel-825, and the yield strength of the material is 245MPa.
4.2 EE-NL grade materials
The material of the valve body and bonnet is AISI 4130; the sealing surface is overlay welded with Inconel-625; the yield strength of the material is 525MPa; the material of the valve stem is AISI 4130/AISI 4140; the surface is nitrided, the yield strength of material is 525MPa; the material of valve gate and seat is AISI 4130, the surface is surface welded with WC-Co-Cr alloy, the yield strength of material is 525MPa. Yield strength of 525MPa; metal seals are made of Inconel-825, with a yield strength of 245MPa.
4.3 FF-NL grade materials
The material of the valve body and bonnet is 410SS/F6NM, the sealing surface is overlay welded with Inconel-625, and the material yield strength is 525MPa; the material of the valve stem is Inconel-718, and the material yield strength is 840MPa; the material of valve plate and the seat is 410SS, and the surface of the valve plate and the seat is spray welded with WC-Co-Cr alloy, and the material yield strength is 525MPa; the material of metal seals is Inconel-825, and the material yield strength is 245MPa. The material of metal seals adopts Inconel-825; the material yield strength is 245MPa.
4.4 HH grade material
The material of valve body and bonnet is AISI 4130/8630, the inner cavity and sealing surface are overlay welded with Inconel-625, and the yield strength of the material is 560MPa; the material of valve stem is Inconel-718, and the yield strength of the material is 840MPa; the material of valve plate and seat is Inconel-718, and the surface of valve plate and seat is spray-welded with WC-Co-Cr alloy, and the yield strength of the material is 840MPa. The material of the valve plate and seat is Inconel-718 with WC-Co-Cr alloy sprayed on the surface, and the material yield strength is 840MPa; the material of the metal seal is Inconel-825, and the material yield strength is 245MPa.

5. Conclusion

• (1) According to the provisions of the relevant norms, the valve design factors affected more due to the underwater valves located in harsh environments, corrosive and serious; it is necessary to fully consider the corrosion of H₂S, CO₂, and Cl^- and other factors on the valve material.
• (2) According to the relevant norms, systematically analyze the adaptability and processability of commonly used materials for underwater valves, and according to different application environments and influencing factors, summarize the precautions for different materials in the use process.
• (3) For the different working conditions of underwater valves, a variety of different working conditions to determine the material selection program through the various types of material properties and design specifications on the material selection analysis, the material selection program identified in line with API 17D, API 6A, and NACE MR0175 specifications, the conclusions obtained can be used as a basis for the design of underwater valves.

Author: Li Shulin