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Contents 1. Non Destructive Testing Requirements 2. Non Destructive Testing Methods Magnetic Particle Examination 3. Non Destructive Testing Methods Dye Penetrant Examination 4. Non Destructive Testing Methods Ultrasonic Examination 5. Non Destructive Testing Methods Radiographic Examination 6. Pressure Testing Procedure 2
1. NON DESTRUCTIVE TESTING REQUIREMENTS As it has already been mentioned welding requirements including NDT extent and welding defects acceptance limits are covered in EN 12732 Gas Supply Systems Welding Steel Pipework Functional Requirements Initially the standard allocates quality requirements categories: 3
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The pipeline operator shall specify the acceptance criteria. The acceptance criteria given in Table G.1 for radiographic findings and in Table G.3 for ultrasonic findings are based on workmanship criteria. The pipeline operator may use EPRG guidelines (Tables G.2) or alternative acceptance criteria based on fracture mechanics analysis and fitness-for-purpose criteria, provided they are fully documented and based on a proven safe approach. Weld defects detected by means of ultrasonic examination shall be evaluated in accordance with the requirements of Table G.3. The acceptance requirements for alternative examination methods shall be determined from case to case. 8
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The EPRG (European Pipeline Research Group) guidelines on the assessment of defects in transmission pipeline girth welds are based on literature reviews, an extensive laboratory test program, published experimental data and accepted fitness for purpose methodologies. The guidelines are structured in three tiers and specify defect acceptance levels intier 1 (good workmanship) and defect limits in Tiers 2 and 3 (fitness-for-purpose). The application of current welding standards can lead to quite different defect limits. but the EPRG guidelines provide uniform acceptance levels and defect limits, with a comprehensive technical justification 12
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2. NON DESTRUCTIVE TESTING METHODS MAGNETIC PARTICLE EXAMINATION 21
3. NON DESTRUCTIVE TESTING METHODS DYE PENETRANT EXAMINATION 22
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4. NON DESTRUCTIVE TESTING METHODS ULTRASONIC EXAMINATION 24
During the initial creation and transmission of ultrasounds a near field area is created which makes difficult the detection of defects 25
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In the last years automatic ultrasonic test systems have been developed which make use of phased array Ultrasonic Heads and Time of Flight Diffraction (TOFD) techniques for improved detection and sizing of the indications. Such systems provide very fast examination of a production field weld without having also the safety risks associated with radiographic testing. As a drawback automatic UT systems requires a dedicated calibration block for each weld configuration with the same diameter, thickness, and material as the pipes to be used on site. Additionally defect criteria and acceptance levels cannot be taken from existing standards and need to be set by the system manufacturer. 27
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5. NON DESTRUCTIVE TESTING METHODS RADIOGRAPHIC EXAMINATION 29
Factors affecting the radiographic technique: Material thickness Geometric Unsharpness Ug = s*b/a where: S = source size b = material thickness a = distance of the source from the material surface Sensitivity (diameter of the smaller visible wire / material thickness) x 100 %. 30
MISALIGNMENT 31
EXCESSIVE ROOT PENETRATION 32
UNDERCUT 33
ROOT CONCAVITY 34
INCOMPLETE PENETRATION 35
CLUSTER POROSITY 36
TRANSVERSE CRACKS 37
TUNGSTEN INCLUSIONS 38
6. PRESSURE TESTING PROCEDURE Basic pressure testing requirements are described in EN 1594 Gas supply systems pipelines for maximum operating pressure over 16 bar functional requirements. Based on these requirements a pipeline pressure test comprises of the following stages: - Definition of pipeline test sections and test planning - Pipeline cleaning and gauging - Water filling and test preparation - Pressure test (including strength and tightness test) - Dewatering - Final pigging and acceptance 39
TEST PLANNING Initially the pipeline section needs to be broken down to sections which shall be agreed with the Owner based on the criteria given by EN 1594 and other project requirements i.e. level between lowest and highest point in each section to be less than 300m for mountainous areas and 60m for areas with low elevations. Prior to the initiation of the pressure test the following need to submitted to the Owner for approval: - Detailed pressure test program and method statements for all scheduled activities - Approvals from local authorities according to regulatory requirements - Submission to the Owner for review of all relevant technical documentation including detailed pipeline section profile, Weld log book covering all weld seams within the test section, layout plans for the designated launching and receiving stations and water analysis report. 40
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CLEANING AND GAUGING Prior to water filling cleaning and gauging of the pipeline shall be conducted for the whole pipeline test section. The purpose of cleaning is to remove any debris existing in the pipeline from the construction period. Gauging verifies the internal roundness of the pipeline and the absence of any unacceptable deformation. Cleaning is performed using a train of high seal bidirectional cleaning pigs which travel through the test section by the application of a suitable back pressure using water or air. Wire brushing pigs may be used but when the pipeline is internally lined the use of such pigs may damage the internal coating of the pipeline. Passage of the cleaning pig train shall be performed as many times as necessary for all debris to be removed from the pipeline. Line valves hall not be installed during cleaning operations to avoid valve damages caused by debris and other foreign bodies. 42
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CLEANING AND GAUGING Following successful cleaning of the pipeline an aluminum gauging plate is attached to a gauging pig and a gauging run is performed to verify the internal diameter of the pipeline and the absence of unacceptable deformations. The gauging plate diameter is 95% - 10 mm of the pipe internal diameter The gauging run is considered acceptable if the aluminum plate shows no signs of damage to the outer edges thereby revealing that contact was made to the internal pipe wall during passing through the section. 44
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WATER FILLING AND EQUALIZATION Suitable properties for the water to be used during pressure testing include a ph of 6-8 and a low salt content. In case the water does not fulfill the chemical requirements or if it is going to stay in the pipeline for a period of over 1 month then corrosion inhibitors and/or biocides need to be added. Selection of the suitable additives must be agreed with the Owner and all environmental effects of these chemicals must be taken into consideration prior to disposal. During water filling it is critical that large quantities of air are not trapped into the pipeline. To achieve this, multiple sealing pigs suitable for expulsion of air from the pipeline are used in front of the main water volume. During water filling the time of departure and arrival of pigs, the volume of water used and its temperature are recorded in the relevant form. 46
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WATER FILLING AND EQUALIZATION On completion of the water filling procedure the equalization period begins. The pressure during this period is held between 1/8 and 1/4 of the strength test pressure. During the equalization period, the pressure is continuously monitored, while also the pipe temperature is measured at least every six hours by suitably located thermometers. At least three measuring points shall be used. Thermometers must be placed o.5 km from each end of the test section and not further than 2.5 km from each other. The equalization is considered complete when the temperature difference of the thermometers does not change more than 0.5 C between two consecutive measurements. 48
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STRENGTH TEST The strength test pressure (Ps) is calculated based on the requirements of EN 1594. Typical strength test stages include the following: - Pressurizing of the test section at a maximum rate of 2 bars/min - When Ps is reached the pressure is maintained for 6o min (1 st hold time) - Air inclusion test is performed by dropping the pressure 1 bar and comparing the water volume needed for this drop to the one theoretically calculated. - After inclusion test the pressure is reduced at a minimum of 2 bar at the highest pipeline point and hold for 30 min (2 nd hold time) - Pressure is increased at Ps and maintained there for 10 min (3 rd hold time) - Pressure is reduced at the tightness test ressure (Pt) 50
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TIGHTNESS TEST The tightness test pressure (Pt) is defined prior to the pressure test and shall not be less than the pipeline design pressure (DP). Tightness tests have a duration of at least 8 hours but a duration of 24 hours is the usual practice. During the whole tightness test duration corresponding values of time, pressure and pipeline temperature are recorded every hour. Evaluation of the tightness test is performed through specific calculations which verify any observed change in the Pt. 52
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DEWATERING On completion of the test the pipeline shall be emptied of water using swabbing pigs driven by compressed air. The disposal of water shall be done under an approved and in areas where no environmental problems will be created. Special care shall be taken when chemical additives have been used in the water. 58