Main Technical Requirements for Carbon Steel Pipes and Corrosion-Resistant Alloy Composite Pipes
1. Design and manufacturing standards for submarine pipelines
At present, the design and manufacture of submarine pipelines adopt international standards.
1.1 Design and manufacturing standards of carbon steel pipes
The design and manufacturing standards of carbon steel pipes generally adopt the following two standards:
(1) American Petroleum Institute " Specifications for Steel Pipes"
(2) Submarine Pipeline Systems
1.2 Design and manufacturing standards for corrosion-resistant alloy composite pipes
The design and manufacture standards of base pipes and carbon steel pipe of mechanical composite pipes and metallurgically clad pipes are the same. The design and manufacturing standards of the liner of the mechanical composite pipe generally adopt the American Petroleum Institute's "Corrosion Resistant Alloy Line Pipe". The design and manufacture of mechanical composite pipes and metallurgical composite pipes generally adopt the American Petroleum Institute’s Specification for Lined Corrosion-Resistant Alloy composite Steel Pipes.
2. Main technical requirements of carbon steel pipes and corrosion-resistant alloy composite pipes
The main technical requirements for submarine pipelines include mechanical properties, chemical composition, dimensional tolerances, supplementary requirements for acidic working conditions, supplementary requirements for crack arrest characteristics, and supplementary requirements for plastic deformation. Some special tests are required for corrosion-resistant alloy composite pipes based on these technical requirements, including composite effect inspection, collapse tests (only for mechanical composite pipes), four-point bending tests (only for mechanical composite pipes) and corrosion tests.
2.1 Mechanical properties
Mechanical properties refer to the yield strength, tensile strength, elongation, cold bending properties and impact toughness of steel under standard conditions. The mechanical performance test of carbon steel pipe mainly includes tensile performance tests, Charpy impact tests, hardness tests, flattening tests (only for high-frequency electric resistance welded steel pipes and submerged arc welded steel pipes), guided bending tests (only for high-frequency electric resistance welded steel pipes and submerged arc welded steel pipes). The mechanical performance tests of corrosion-resistant alloy composite pipes include tensile performance tests, Charpy impact tests, hardness tests, flattening tests, and guided bending tests (only for corrosion-resistant alloys composite pipes whose base pipes are high-frequency electric resistance welded steel pipes and submerged arc welded steel pipes). The corrosion-resistant alloy layer should be removed from the samples for the tensile test, Charpy impact test, guided bending test and hardness test of the corrosion-resistant alloy composite pipe. The flattening test shall retain the corrosion-resistant alloy layer.
2.2 Composite effect tests
The composite effect is generally checked by the cohesion test and bond strength test. Use strain gauges to measure the hoop and axial stress changes before and after the corrosion-resistant alloy layer, and the value should be greater than or equal to 20MPa.
The bond strength test includes the base pipe steel pipe pushing method and the liner pushing method. Apply pressure to the sample to separate the outer steel pipe and the inner corrosion-resistant alloy layer of the sample, and the minimum bonding force is required to be greater than or equal to 0.5MPa.
2.3 Collapse tests
Submarine pipelines usually use 3 layers of PE coatings (polyethylene anti-corrosion coatings) and 3 layers of PP coatings (polypropylene anti-corrosion coatings). In the coating process, the pipe needs to be heated to 220°C and kept for about 5 minutes. Then, flush and cool. If there are a lot of impurities and air between the layers of the composite pipe, the interlayer gas will expand due to heat in the heating process. In addition, the wall thickness of the liner is relatively thin, generally only 3mm, which is prone to bulges and wrinkles. The collapse test is required for mechanical composite pipes, but not for metallurgical composite pipes. According to DNV-OS-F101Submarine Pipeline Systems, 1 out of the first 10 mechanically composite pipes shall be selected for the collapse test. According to the actual coating temperature of the submarine pipeline, the recommended temperature for the collapse test is 250°C, and the holding time is 15 minutes. Record the holding curve, cool naturally, and visually inspect the lining of the composite pipe to confirm that there are no defects such as bulges, folds, and bends.
2.4 Four-point bending test of mechanical composite pipes
During the laying, installation and getting into the sea process of the submarine pipeline, it will experience two bendings in different directions, which may cause the separation of the base lining or the wrinkling of the liner pipe. To avoid this problem, the mechanical composite pipe needs to simulate the actual installation conditions to conduct a full-scale four-point bending test to test the bending performance index of the composite pipe, but there is no such requirement for the metallurgical composite pipe.
At present, each standard has not yet proposed the test method and acceptance standard for the four-point bending test, and each manufacturer formulates its test methods and criteria. The test generally includes forward pure bending tests, forward and reverse bending tests, and limit bending tests.
(1) Forward pure bending tests: The composite pipe is subjected to a four-point pure positive loading test, and loaded to the maximum strain value allowed by the material. According to DNV-OS-F101Submarine Pipeline Systems installation dynamic load conditions simplified strain check criteria, the maximum allowable strain of X70 is 0.325%, and the maximum allowable strain of X65 is 0.305%.
(2) Forward and reverse bending tests: the composite pipe is loaded in forward and reverse bending, the maximum load is 87% of the minimum yield strength of the material, and the loading cycle is 30 times.
(3) Ultimate bending tests: the composite pipe is subjected to four-point pure bending and loaded in a positive direction until the lining is wrinkled, and the maximum strain is recorded.
The four-point bending test is used to verify whether the lining of the composite pipe is complete under the bending load, that is, whether there is wrinkling for the lining.
2.5 Corrosion tests
The corrosion test methods of austenitic stainless steel 316L include electrochemical corrosion tests, intergranular corrosion tests and chloride stress corrosion cracking tests in a simulated environment. Simulate electrochemical corrosion tests according to JB/T 7901-2001 "Full Immersion Test Methods of Metallic Materials Laboratory Uniform Corrosion", and simulate field test conditions. The test time is 168 hours. The uniform corrosion rate is required to be less than and equal to 0.025mm/a, without any signs of pitting. Intergranular corrosion tests should be in accordance with Method E ASTM A262 Standard Practices for Detecting Susceptibility of Intergranular Attack in Austenitic Stainless Steel. The test time is 24 hours, and it is required to observe under a microscope 100 times; there should be no intergranular cracks or surface cracks.
The chloride stress corrosion cracking test simulates the field test conditions. The test time is 720 hours, and the four-point bending stress is 100% Rt0.5; the sample is required to be free of cracks. According to the U-shaped bending method of YB/T5362-2006 "Stress Corrosion Test Method for Stainless Steel in Boiling Magnesium Chloride Solution", 42% MgCl2 solution, the temperature is 143±1°C, and the test time are 20 hours. There should be no cracks. Nickel-based alloy 625 surfacing layer adopts pitting corrosion tests, according to ASTM G48 Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution Method A; the test temperature is 40°C. The time is 24 hours. It is required to magnify 20 times to observe and there should be no pitting corrosion. The weight loss should not exceed 4.0g/m2.
At present, the design and manufacture of submarine pipelines adopt international standards.
1.1 Design and manufacturing standards of carbon steel pipes
The design and manufacturing standards of carbon steel pipes generally adopt the following two standards:
(1) American Petroleum Institute " Specifications for Steel Pipes"
(2) Submarine Pipeline Systems
1.2 Design and manufacturing standards for corrosion-resistant alloy composite pipes
The design and manufacture standards of base pipes and carbon steel pipe of mechanical composite pipes and metallurgically clad pipes are the same. The design and manufacturing standards of the liner of the mechanical composite pipe generally adopt the American Petroleum Institute's "Corrosion Resistant Alloy Line Pipe". The design and manufacture of mechanical composite pipes and metallurgical composite pipes generally adopt the American Petroleum Institute’s Specification for Lined Corrosion-Resistant Alloy composite Steel Pipes.
2. Main technical requirements of carbon steel pipes and corrosion-resistant alloy composite pipes
The main technical requirements for submarine pipelines include mechanical properties, chemical composition, dimensional tolerances, supplementary requirements for acidic working conditions, supplementary requirements for crack arrest characteristics, and supplementary requirements for plastic deformation. Some special tests are required for corrosion-resistant alloy composite pipes based on these technical requirements, including composite effect inspection, collapse tests (only for mechanical composite pipes), four-point bending tests (only for mechanical composite pipes) and corrosion tests.
2.1 Mechanical properties
Mechanical properties refer to the yield strength, tensile strength, elongation, cold bending properties and impact toughness of steel under standard conditions. The mechanical performance test of carbon steel pipe mainly includes tensile performance tests, Charpy impact tests, hardness tests, flattening tests (only for high-frequency electric resistance welded steel pipes and submerged arc welded steel pipes), guided bending tests (only for high-frequency electric resistance welded steel pipes and submerged arc welded steel pipes). The mechanical performance tests of corrosion-resistant alloy composite pipes include tensile performance tests, Charpy impact tests, hardness tests, flattening tests, and guided bending tests (only for corrosion-resistant alloys composite pipes whose base pipes are high-frequency electric resistance welded steel pipes and submerged arc welded steel pipes). The corrosion-resistant alloy layer should be removed from the samples for the tensile test, Charpy impact test, guided bending test and hardness test of the corrosion-resistant alloy composite pipe. The flattening test shall retain the corrosion-resistant alloy layer.
2.2 Composite effect tests
The composite effect is generally checked by the cohesion test and bond strength test. Use strain gauges to measure the hoop and axial stress changes before and after the corrosion-resistant alloy layer, and the value should be greater than or equal to 20MPa.
The bond strength test includes the base pipe steel pipe pushing method and the liner pushing method. Apply pressure to the sample to separate the outer steel pipe and the inner corrosion-resistant alloy layer of the sample, and the minimum bonding force is required to be greater than or equal to 0.5MPa.
2.3 Collapse tests
Submarine pipelines usually use 3 layers of PE coatings (polyethylene anti-corrosion coatings) and 3 layers of PP coatings (polypropylene anti-corrosion coatings). In the coating process, the pipe needs to be heated to 220°C and kept for about 5 minutes. Then, flush and cool. If there are a lot of impurities and air between the layers of the composite pipe, the interlayer gas will expand due to heat in the heating process. In addition, the wall thickness of the liner is relatively thin, generally only 3mm, which is prone to bulges and wrinkles. The collapse test is required for mechanical composite pipes, but not for metallurgical composite pipes. According to DNV-OS-F101Submarine Pipeline Systems, 1 out of the first 10 mechanically composite pipes shall be selected for the collapse test. According to the actual coating temperature of the submarine pipeline, the recommended temperature for the collapse test is 250°C, and the holding time is 15 minutes. Record the holding curve, cool naturally, and visually inspect the lining of the composite pipe to confirm that there are no defects such as bulges, folds, and bends.
2.4 Four-point bending test of mechanical composite pipes
During the laying, installation and getting into the sea process of the submarine pipeline, it will experience two bendings in different directions, which may cause the separation of the base lining or the wrinkling of the liner pipe. To avoid this problem, the mechanical composite pipe needs to simulate the actual installation conditions to conduct a full-scale four-point bending test to test the bending performance index of the composite pipe, but there is no such requirement for the metallurgical composite pipe.
At present, each standard has not yet proposed the test method and acceptance standard for the four-point bending test, and each manufacturer formulates its test methods and criteria. The test generally includes forward pure bending tests, forward and reverse bending tests, and limit bending tests.
(1) Forward pure bending tests: The composite pipe is subjected to a four-point pure positive loading test, and loaded to the maximum strain value allowed by the material. According to DNV-OS-F101Submarine Pipeline Systems installation dynamic load conditions simplified strain check criteria, the maximum allowable strain of X70 is 0.325%, and the maximum allowable strain of X65 is 0.305%.
(2) Forward and reverse bending tests: the composite pipe is loaded in forward and reverse bending, the maximum load is 87% of the minimum yield strength of the material, and the loading cycle is 30 times.
(3) Ultimate bending tests: the composite pipe is subjected to four-point pure bending and loaded in a positive direction until the lining is wrinkled, and the maximum strain is recorded.
The four-point bending test is used to verify whether the lining of the composite pipe is complete under the bending load, that is, whether there is wrinkling for the lining.
2.5 Corrosion tests
The corrosion test methods of austenitic stainless steel 316L include electrochemical corrosion tests, intergranular corrosion tests and chloride stress corrosion cracking tests in a simulated environment. Simulate electrochemical corrosion tests according to JB/T 7901-2001 "Full Immersion Test Methods of Metallic Materials Laboratory Uniform Corrosion", and simulate field test conditions. The test time is 168 hours. The uniform corrosion rate is required to be less than and equal to 0.025mm/a, without any signs of pitting. Intergranular corrosion tests should be in accordance with Method E ASTM A262 Standard Practices for Detecting Susceptibility of Intergranular Attack in Austenitic Stainless Steel. The test time is 24 hours, and it is required to observe under a microscope 100 times; there should be no intergranular cracks or surface cracks.
The chloride stress corrosion cracking test simulates the field test conditions. The test time is 720 hours, and the four-point bending stress is 100% Rt0.5; the sample is required to be free of cracks. According to the U-shaped bending method of YB/T5362-2006 "Stress Corrosion Test Method for Stainless Steel in Boiling Magnesium Chloride Solution", 42% MgCl2 solution, the temperature is 143±1°C, and the test time are 20 hours. There should be no cracks. Nickel-based alloy 625 surfacing layer adopts pitting corrosion tests, according to ASTM G48 Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution Method A; the test temperature is 40°C. The time is 24 hours. It is required to magnify 20 times to observe and there should be no pitting corrosion. The weight loss should not exceed 4.0g/m2.
