Steel Plate Coiled Pipe Testing

Steel plate coiled pipe inspection mainly conducts scientific testing and analysis on the geometric dimension and shape inspection, weld quality inspection, material chemical composition analysis, mechanical property testing, non-destructive testing and pressure resistance and sealing testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Geometric dimension and shape inspection: Mainly measures the diameter, wall thickness, ovality, straightness and length of the finished steel pipe. Precisely controlling these parameters is the basis for ensuring that the pipe meets the design drawings, connection matching and structural stability. For example, the excessive ovality will affect the subsequent welding quality, while the straightness deviations may cause the installation difficulties or stress concentrations.

Weld quality inspection: For the steel plate coiled pipes welded with the spiral seams or straight seams, the weld is the core area of the inspection. It is necessary to check the surface defects such as the appearance forming, width, reinforcement, undercut and misalignment of the weld, and further explore whether there are the internal defects such as the blowholes, slag inclusions, lack of fusion, incomplete penetration and cracks inside the weld. This is the key to evaluating the structural integrity of the pipe.

Material chemical composition analysis: Through the methods such as the spectrometry analysis and carbon-sulphur analysis, it is verified whether the chemical composition of the coiled steel plate meets the requirements of the relevant material standards (such as Q235B, Q345R and L245). The accurate contents of the elements directly affect the mechanical properties, welding performance and corrosion resistance of the steel pipe, and are the primary verification item of the material conformity.

Mechanical property testing: The samples are taken from the base material and weld areas of the steel plate, and the tensile testing, bending testing, impact testing (at the room temperature and low temperatures) and hardness testing are carried out. These tests aim to evaluate whether the strength, plasticity, toughness and hardness of the steel pipe meet the design and use requirements, especially crucial for the pipes bearing the high pressures or used in the low-temperature environments.

Non-destructive testing (NDT): Comprehensively evaluates the internal and external quality of the steel pipe without damaging it. The common methods include the radiographic testing (RT) or ultrasonic testing (UT) for exploring the internal defects, and the magnetic particle testing (MT) or penetrant testing (PT) for checking the surface and near-surface defects. The non-destructive testing is the core means of finding the hidden defects and ensuring the safe operation of the pipe.

Pressure resistance and sealing testing: Usually carried out through the hydrostatic testing or pneumatic testing. Under the specified pressure, the pressure is maintained for a certain period, and it is checked whether there are the leakages, abnormal deformations or pressure drops in the pipe body and welds. This is the final comprehensive inspection for verifying the pressure-bearing capacity and overall sealing performance of the steel pipe under the limit pressures.

Surface quality and anti-corrosion layer testing: Checks whether there are the defects such as the scratches, dents and corrosion on the outer surface of the steel pipe. For the steel pipes with the anti-corrosion requirements (such as the epoxy powder coating and 3PE), the thickness, adhesion, continuity and insulation performance of the anti-corrosion layers also need to be detected, to ensure their long-term corrosion resistance.

Testing Scope

Incoming inspection of the raw materials: The source control of the steel plates used for the pipe coiling is carried out, including the verification of the quality certificates, inspection of the steel plate dimensions and surface conditions, and the necessary re-inspection of the chemical composition and mechanical properties. Ensuring the qualified quality of the raw materials put into production is the first line of defence for guaranteeing the final product quality.

Production process inspection: The real-time or periodic inspections are carried out in the key processes such as the steel plate pre-treatment, coiling forming, welding, straightening and sizing. The forming parameters, welding process parameters (current, voltage and speed) and bevel quality are emphatically monitored, realizing the process quality control and timely correcting the production deviations.

Finished product ex-factory inspection: The comprehensive inspection carried out after the completion of the steel pipe production, according to the product standards (such as GB/T 9711 and API 5L) and the customer technical requirements. It covers all the key testing items, and only the steel pipes that pass all the tests can be issued the quality certificates and allowed to leave the factory.

Arrival acceptance inspection: The re-inspection carried out after the steel pipes are transported to the use sites or customer warehouses. It usually includes the document verification, quantity counting, spot check of the geometric dimensions, appearance and anti-corrosion layer quality, and the non-destructive testing sampling when necessary, to confirm that there is no damage during the transportation and the state is consistent with the ex-factory state.

Periodic inspection of the in-service pipelines: For the steel plate coiled pipelines already in service, according to the pressure pipeline safety technical specifications, the periodic online or offline inspections are carried out. The corrosion thinning, crack propagation and stress concentrations are mainly detected, and their remaining lives and operational safety are evaluated. It is the key link of the preventive maintenance.

Special testing for the specific application fields: According to the final uses of the steel pipe, the special testing items may be added. For example, the stricter Charpy impact testing is required for the submarine pipelines; the intergranular corrosion testing is added for the chemical industry; the cleanliness and the migration amounts of the specific metal ions are detected for the food industry.

Testing Methods

Dimension measurement method: The tools such as the outer diameter micrometers, wall thickness gauges, π tapes, large callipers and laser range finders are used for the direct measurement. For the ovality, the maximum and minimum outer diameters of the same cross-section are usually measured; for the straightness, the pull-line method or optical collimator is adopted for the detection.

Visual testing (VT): The inspectors systematically observe the inner and outer surfaces and welds of the steel pipe with the aid of the tools such as the endoscopes, reflecting mirrors and magnifying glasses under the sufficient lighting. This is the most basic and direct detection method, used to find the obvious surface defects and irregularities.

Radiographic testing (RT): The X-rays or γ-rays penetrate the steel pipe and form the images on the films or digital detectors. By analysing the density differences in the images, the shapes, sizes and positions of the internal defects such as the blowholes, slag inclusions and incomplete penetration in the welds can be intuitively judged, and the testing results can be stored for a long time.

Ultrasonic testing (UT): Utilizes the principle that the high-frequency sound waves reflect when encountering the defects during the propagation in the material. Through the probes receiving the reflected waves and displaying the waveforms on the instrument screen, the depths, equivalent sizes and positions of the internal defects can be precisely determined. It is particularly suitable for detecting the planar defects such as the cracks and lack of fusion in the welds, and has no radiation hazards to the human body.

Magnetic particle testing (MT): The steel pipes of the ferromagnetic materials (such as the carbon steel) are magnetized, and the defects on the surface or near-surface will produce the leakage magnetic fields, attracting the applied magnetic particles to form the magnetic indications. This method is extremely sensitive to the linear defects such as the cracks and folds, with the relatively simple and fast operation, but is only applicable to the surface and near-surface detection of the ferromagnetic materials.

Penetrant testing (PT): The penetrant containing the dye is coated on the steel pipe surface to make it penetrate into the surface open defects, and after the cleaning, the developer is applied, and the penetrant in the defects is sucked out and displays the indications. It is applicable to the detection of the surface open defects of all the non-porous metal materials, especially the non-ferromagnetic materials.

Pressure testing method: The hydrostatic testing is the most commonly used method. The pipe is filled with water, the air is drained, and then the pressure is slowly increased to the test pressure (usually 1.5 times the design pressure), and the pressure is maintained for the inspection. The pneumatic testing has the higher risks and requires the strict safety measures, usually used for the occasions where the hydrostatic testing cannot be carried out due to the structural or use reasons.

Testing Standards

Domestic Standards:

1. GB/T 9711, Petroleum and natural gas industries — Steel pipe for pipelines — Technical delivery conditions

2. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

4. GB/T 232-2010, Metallic materials — Bend test method

5. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

6. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

International Standards:

1. API 5L, Specification for line pipe

2. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Geometric dimension measuring equipment: Including the digital ultrasonic thickness gauges (for the precise wall thickness measurement), π tapes or large tape measures (for measuring the pipe diameters and circumferences), laser profile scanners (for the fast acquisition of the overall shapes and ovality data), straightness detectors (such as the laser collimators), and the large length measuring machines or laser range finders for the length measurement.

Non-destructive testing equipment: With the various types, such as the X-ray flaw detectors or γ-ray sources (with the films or digital imaging plates) for the RT; digital ultrasonic flaw detectors (with the probes of the multiple angles) for the UT; magnetic particle flaw detectors (including the mobile yokes and fluorescent magnetic particles) for the MT; coloured penetrant testing reagent kits for the PT. The phased array ultrasonic (PAUT) and TOFD detectors are the advanced equipment used for the weld detection in recent years.

Mechanical property testing machine: Mainly including the universal material testing machines (for the tensile and bending testing), pendulum impact testing machines (for the Charpy V-notch impact testing, evaluating the material toughness), as well as the portable or bench-type hardness testers (such as the Brinell, Rockwell and Leeb hardness testers), used to test the hardness values of the steel pipe base material and welding heat-affected zones.

Chemical composition analyser: The direct-reading optical emission spectrometer is the preferred equipment for the rapid on-site analysis of the metal element contents, with the high precision and fast speed. In addition, the carbon-sulphur analyser is specially used for the precise determination of the carbon and sulphur contents. For the more precise analysis in the laboratory, the equipment such as the ICP spectrometers is also used.

Pressure testing device: Mainly composed of the high-pressure pumps (electric or manual), pressure gauges (or pressure sensors), piping systems, sealing plugs and safety protection facilities. The hydrostatic testing of the large pipes requires the dedicated testing pump stations and real-time pressure monitoring and recording systems, ensuring the safe and controllable testing process.

Surface and coating testing instruments: Including the coating thickness gauges (magnetic method or eddy current method, used to measure the anti-corrosion layer thickness), electric spark leak detectors (used to detect the pinholes and leak points of the anti-corrosion layers), and adhesion testers (such as the cross-cut method or pull-off method instruments). In addition, the high-brightness cold light sources and endoscopes are the auxiliary tools of the surface visual inspection.

Data recording and management system: The modern testing is inseparable from the digital tools, such as the intelligent thickness gauges and digital ultrasonic flaw detectors with the data storage and transmission functions, as well as the laboratory information management systems (LIMS) or dedicated quality management systems for managing all the testing data, reports, pictures and process parameters, realizing the traceability of the testing process.