SY/T 10005-1996 Recommended practice for ultrasonic inspection of offshore structures and examination guide for ultrasonic technician qualification
Some standard content:
SY/T 10005—1996 Offshore Oil and Gas Industry Standard of the People's Republic of China Recommended Practice for Ultrasonic Examination of Offshore Strucral Fabrication and Guidelines for Qualification ofUltrasonicTechnicians
Issued on 1996-08-19
China National Offshore Oil Corporation
Implemented on 1996-08-19
Policy Statement
Chapter 1
Chapter 2
Chapter 3
Chapter 4
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Personnel Qualification Assessment
Technical Recommendations·
Terms of Ultrasonic Inspection
Examples of Written Test Questions
Manufacture and Assessment of Simulated Components
Examples of Scoring for Technician Operation Assessment
Acceptance Criteria
References
Notice to User: This publication has been partially changed compared with the previous version. According to the convention of P1 standards, the corresponding changes should be marked with a black thick line on the left margin. This edition has been substantially modified compared with the previous version, so the margin mark has been cancelled. Note: This edition replaces the first edition issued in March 1980, including the modified parts that were formally approved at the 1987 standardization meeting.
Any reprint or translation of this publication in whole or in part requires permission from the Director of Development, American Petroleum Institute, 2535 Dallas, TX 75202.
(45)
In order to meet the needs of my country's development of offshore oil and gas resources, our company may adopt the American Petroleum Institute's Recommended Practice for Ultrasonic Examination of Offshore Structures and Guidelines for Qualification of Ultrasonic Terhnirians (1988), i.e., API RP2X "Recommended Practice for Ultrasonic Examination of Offshore Structures and Guidelines for Qualification of Ultrasonic Terhnirians" (1988), as the recommended corporate standard of China National Offshore Oil Corporation. If there is any objection to the translation of this standard, the original standard used shall prevail! In the design, construction and use of offshore oil and gas development projects involving the laws, regulations and provisions of the government of the country where the source standard is located or other competent authorities, the corresponding laws, regulations and provisions promulgated by the government of the People's Republic of China or the competent government departments shall be followed. The data or quantitative calculation methods of environmental conditions such as wind, waves, currents, ice, temperature, earthquakes, etc. in the original standard can be used as a reference if they are suitable for my country's actual conditions. Otherwise, the data and quantitative calculation methods that meet the actual environmental conditions in my country should be used. The basic units of measurement are mainly based on legal measurement units, that is, the legal measurement unit value is in front, and the corresponding value of the imperial unit is marked in brackets afterwards. The formulas, curve shape characteristics, constants and coefficients in the original standard are not changed. Where imperial units are used, they are still used. China National Offshore Oil Corporation
China National Offshore Oil Standardization Technical Committee 1993.11.15
Policy Statement
1. API publications are necessarily for general issues. When it comes to specific issues, local, state and federal laws and regulations should be consulted.
2. API does not assume the obligation of employers, manufacturers or suppliers to inform, train or maintain the integrity and safety of their employees for safety risks and preventive measures. Nor does it assume their responsibilities under local, state or federal laws. 3. The contents of any APIH publication shall not be construed, by implication or otherwise, as granting any right to make, sell or use any method, apparatus or product involved in the patent. Nor shall any content of this publication be construed as absolving anyone of liability for infringement of patent rights.
1. Normally, the API standard is reviewed at least once every five years and revised, re-identified or revoked. Sometimes, this review cycle can be extended by 10 times, up to 20 years. As the current API standard, this publication shall be valid for no more than five years from the date of publication, unless it is authorized to be reprinted, and its validity period is extended. The status of publications can be checked from the API editorial department (Tel. 202-682-8000). API (122>LST, NW, Washington D: 20005) publishes a daily list of publications and materials every year and updates it quarterly. Scope
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Chapter General
a. This document contains recommendations for the qualification of ultrasonic technicians for pulse reflection ultrasonic testing of structures constructed on the seams. It also provides further recommendations for the control of ultrasonic testing and for the integration of ultrasonic testing methods into an overall quality control program. It also discusses the relationship between joint design, the meaning of defects in welds and the ability of ultrasonic technicians to detect critical sizes of defects. b. The operator should not use this document as a standard or specification for design. 1.2 Design
These recommendations are intended to be a guide for the development of an ultrasonic testing control program for the construction and installation of offshore fixed facilities. They also cover the comprehensive control of fractures on offshore structures, including basic aspects of design, material selection and non-destructive testing. It is intended that the operator's ultrasonic expert develop a detailed ultrasonic testing program. In this program, a special calibration reference reflector is established to meet the requirements of the non-destructive testing program. b. These recommendations establish that the design of the structure complies with the latest version of API RP2A "Recommended Practice for Planning, Design and Construction of Offshore Retaining Units". Operators and designers should be aware that even structures that have been inspected by qualified technicians may still have undetected defects. When making requirements for ultrasonic inspection, consideration should be given to the ease of connection of specific components, the possibility of using other inspection methods, the feasibility of repairs, and the quality of the structure. The severity of the overall damage to the structure. These issues are closely related to the inspection scope and acceptance criteria. Some typical applications of ultrasonic testing know that the inspection scope of some special components is simplified. These simplifications are based on industrial practice, especially the jacket structure of the vessel. Some of the components listed above are also inspected by other methods, such as radiography. In areas where other methods (i.e. radiography) have greater confidence, in addition to considering the advantages of this method, the convenience of ultrasonic testing should also be carefully weighed. Other methods are similar to ultrasonic testing, and generally require more information to obtain proportional information. The time and cost are long. Retrograde inspection using multiple methods is often more reliable than using only one method, and the results are better. Ultrasonic inspection technology is used in offshore engineering because other inspection methods cannot evaluate the entire cross-section of the welds of some components (especially the T, K, and Y nodes of tubular trusses). Operators and designers should be aware of the limitations of ultrasonic inspection technology for weld inspection. (Some more important limitations will be discussed in detail in Chapter 3.) It is also recognized that the failure to detect certain defects or false reporting of defects is a "weakness" of ultrasonic inspection. Therefore, the following quality assurance measures are proposed as a supplement to the entire quality control plan: 1. Qualify all welders participating in the construction of structures (especially tubular nodes) and qualify the welding procedures used:
2. Before welding, perform group inspections on designated important joints, including inspection of the shape of the joints: 3. In order to meet the requirements of the procedure, the entire welding process should be retrogradely monitored: 4. Perform visual inspection after welding to check the defects of the welded joints and the shape of the welds. e. In the previous article, it was recommended that designers and operators use steel grades with a certain fracture toughness for all critical parts and control the diameter to ensure the final toughness of the manufactured structure. Structures that bear the main loads are subject to multiple loads. In order to improve their reliability, most (not necessarily all) construction defects can be eliminated through inspection and repair to improve the overall quality level. Operators should consider the structural design and potential failure mechanisms and determine an acceptance standard that is appropriate to the structural design point. 1.3 Definitions
a. General || The explosion-proof terms cited in this article are from the AWS published by the American Society of Mechanical Engineers1)3.The definition of ultrasonic testing is given in the "Welding Terms and Definitions". The definition of ultrasonic testing terms is given in the first chapter of the terminology section of this article. Other relevant terms are briefly described as follows: *: The original text has three references to the word defect: discontinuity (discontinuity), flaw (defect) and defect (defect beyond the standard). For ease of reading and understanding, we translate discontinuity and ftaw as "defect" without distinction and translate clefert as "exceeding the standard defect". The exact meanings of these three words are consistent with the terminology of the first chapter.
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Acceptance criteria - restrictions on the shape, size, type and location of defects within the specified design requirements. Agency personnel - technicians employed and designated by an independent agency who provide manufacturing inspection services to operators according to a contract.
Designer - an individual, entity, company or other organization employed by the owner or operator. They design, detail and develop specifications for a project.
Fracture control plan - a type of project plan. It is a comprehensive project plan that considers design options, material selection, process controls and inspection procedures.
Inspector - an independent person representing the owner whose responsibility is to ensure that the construction of the project is carried out in accordance with the control specifications and technical specifications. Operator - an individual, entity, company or other organization hired by the owner to supervise the construction of the project or the operation of the facility. Qualified technician - an inspector who has been qualified and deemed competent to perform the specified work. Ultrasonic testing - an inspection method that uses electronic technology to determine the location and size of defects in materials or welds, and reports whether they meet acceptance criteria in order to evaluate materials and construction. Ultrasonic testing procedure - a written detailed technical document of the inspection method and acceptance criteria. Ultrasonic specialist - a person who has extensive experience in the development and implementation of ultrasonic inspection procedures (an individual with a certificate recognized by the American Society for the Detection of Specialty Steel Structures or an engineer with expertise in ultrasonic inspection). 1. 4 References
a. The documents referenced in this article that are not API standards are as follows. Unless otherwise specified, only the latest versions of these standards shall apply. 1. ASTM Standards
A435/A435M-82 "Longitudinal Ultrasonic Testing of Steel Plates" A578/A578M-85 "Longitudinal Ultrasonic Testing of Plain and Composite Steel Plates for Special Purposes" 2. AWS Standards
A3.0)-85 Standard Welding Terms and Definitions" D1.1-1988 Specification for Welding of Steel Structures
Chapter 2 Personnel Qualification Assessment
2. 1 Overview
. The following paragraphs provide guidelines for the qualification of employees to perform ultrasonic inspections of materials and structures during the construction of fixed offshore platforms. These guidelines attempt to guide operators in determining the proficiency of technicians in the inspection of joints and special fabricated components during the construction of offshore platforms because the inspection methods used for other structural construction do not work well in this case. b. Proposed by industry personnel. The evaluation of the proficiency of technicians shows that the reliability of the technician's operation depends to a large extent on the technician's training experience and proficiency in industrial construction practices. These guidelines will help operators evaluate the above factors. c. The purpose of these recommendations is to confirm the qualification level of the technician only. When performing ultrasonic inspection, the soil and pipe technician shall meet the minimum qualifications of this regulation. Specially trained personnel may perform the inspection as an assistant or under the constant guidance and supervision of the qualified technician. However, the qualified technician shall bear full responsibility for the inspection performed and the inspection results. d. The technician responsible for the inspection of tubular welded structures shall be fully familiar with the pulse reflection shear wave ultrasonic flaw detection equipment and the technology that can be used to determine the flaws from one side or one side on the curved surface. e. The technician shall be specially trained to be able to use trigonometric functions to accurately locate the ultrasonic reflector and to use the amplitude and beam boundary method to assess the size of the flaw. The technician shall be trained and have some experience in measuring effective beam angle, beam spread, transfer compensation and distance amplitude correction.
f. The operator shall require the technician to demonstrate his proficiency in satisfactory operation in the pre-employment card review. The review shall consist of two parts: written examination and practical operation, and shall be conducted by the operating ultrasonic expert and an organization recognized by the operator. The card review shall include the specific requirements of the ultrasonic inspection procedure and the special acceptance criteria applicable to a specified structure. g. The ability to solve inspection problems shall be measured by a theoretical knowledge test of acoustics or electronics and related knowledge. —2
2.2 Prerequisites for assessment
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a. Many technicians seeking a career are seriously lacking in training and experience. In order to reduce excessive costs in the personnel screening process, it is recommended that applicants have at least the following qualifications:
1. Prove that they are proficient in ultrasonic testing in accordance with the provisions of the nationally recognized supermarket technician qualification outline. 2. Have a cumulative experience of 400 hours in ultrasonic testing of tubular components, piping or pressure vessels within six months before the examination. 2.3 Qualification Assessment
. Both written examination and practical operation examination should be conducted to ensure that the candidate understands the principles of ultrasonic testing and demonstrates his ability to find and assess defects in typical welded components. b. Written Examination The written examination should assess the technician's knowledge of basic principles and the ability to apply relevant theories to field operations. The scope involved should include equipment calibration, probe selection, attenuation theory, defect theory, defect location, defect order and acceptance criteria for a specified structure. The mathematical calculation part of the examination should meet the requirements of field calculations. The test questions should not only include some random answer questions, multiple choice questions or true and false questions, but also defects Positioning and quantification questions. See Appendix A for examples of interview questions (Note: These questions are only for guidance and should not be directly taken from Appendix A for examination). Written interview questions should include questions on ultrasonic inspection and welding, as well as additional questions on practical operation and inspection procedures. The purpose of the written examination is to further screen applicants for the actual examination. The minimum passing score for the written examination should be determined based on the proportion of random questions, multiple-choice questions or non-multiple-choice questions and the accuracy of each question. Candidates for the vision test should undergo an eye examination or Provide a recent medical examination certificate. Prove that his natural or corrected near vision can clearly see one letter on the Jacger standard test chart at a distance of not less than 300mm (12in>, and that his natural or corrected vision is not less than 20/43.
All technicians who are hired shall have their vision re-examined at least once a year, and the corrected vision shall also be kept within the above limits. d. Practical Examination The practical examination is to determine the technician's ability to detect and evaluate welding defects. Demonstrating this ability is the purpose of the qualification examination. This is more important than any other requirement. The operator is responsible for preparing the test pieces. The number of test pieces should be representative of the actual manufacturing structure and can fully simulate tubular nodes or flat joints of similar typical cross-sections. Two or four test pieces 450-600mm (18-24in) long. Only typical joint shapes contain: - one or more "defects". Such test pieces are sufficient to test the technician's ability. The joint design used in the past scoring scheme is shown in Figure 2.3a.
The test piece may contain natural defects or artificial reflectors, such as non-metallic inclusions in the weld deposit, grooves or holes machined in the weld, or a thin iron sheet welded on the groove surface to simulate the unmelted surface, etc., see Figure 2.3b. The simulated component should generally contain enough natural defects to test the technician's ability; however, it may be more ideal to replace artificial reflectors in key positions to evaluate the technician's technical level. For the manufacture and evaluation of simulated components, see Appendix B.
Test pieces used for multiple tests should be manufactured with some representative reflectors to minimize natural defects. These test pieces are inspected by the ultrasonic expert to confirm the detectability of the implanted defects and to confirm that there are no unrepresentative reflectors in the test piece. The ultrasonic expert should determine the characteristics of each reflector in the test piece and indicate which type it belongs to as defined in 3.8.The characteristics, size and location of the reflectors can be discussed with the technician after the test. If the test pieces are to be used for future tests, the examiner should be careful to keep the specific details of each test piece confidential to avoid any disclosure that may affect the test results.
All materials used to make the test pieces should be tested using the longitudinal wave method to ensure that there are no interlayers or inclusions that may affect the test. Conversely, defective materials can be deliberately selected as test pieces to assess the technician's ability to judge these defects. Welded test pieces of intersecting plates or pipes should be made of steel with thickness-square characteristics to reduce lamellar tearing in the test pieces. Since the reflectivity of natural and artificial planar defects is affected by residual compressive stresses in the weld, they need to be thermally stress relieved or fully normalized to ensure that the reflective surface represents the actual defect size. Smooth planar reflectors subjected to sufficient compressive stress cannot be detected by ultrasonic testing, but can be easily found by sectioning or notch fracture testing. 13
*T\ joint mouth shape
*Y\ joint groove shape
13/16\
Outer diameter of the joint is 400 mm (16 in.)
Frequency carrying effect shape
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\Y\ joint groove shape
The plate width used for sweeping should be ≥200mm (8in.) trend
Simulated pull splicing die mouth shape
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“Defect” is set on the effective mouth surface,
The gas shielded spot welding
filling weld with low welding process
hit And check the welding symptoms
slowly return to the standard with blood electrode
gas shielded welding is recommended,
other safety aspects can be found in the paper "Evaluation of Strictness of Welding Failures" published in November 1978, Figure 2.36, the method of measuring the lack of fusion defect on the groove surface is set. The size of the reflector in the test piece or the size of the defect in the test piece should meet the acceptance standard range specified in the structural specification. , the technician shall submit a written report of the inspection test piece, including the type (spherical, round or flat), size (length and width), position along the seam and the position of the defect in the weld section of all defects, and the report shall be used as the basis for scoring: The technician's score is given by the following formula:
Where. P is the percentage of the ratio of the correct detection length of the reflector to the actual length; R is the total score including the false detection defect, 0-100La is the actual length of the reflector on the test piece: (2)
L--Correctly located and quantified defect indication length (the smaller of the reflector report length or the actual reflector length is considered reliable)
1.-The total length reported by the technician (including correct and incorrect) Lr is the length of the false defect.
Evaluation of the technician's operation shall be made independently for each inch of the weld length on the weld test piece. When the determined reflector position and size are accurate enough to meet the defect tolerance specified in the acceptance standard, the identification of the defect shall be considered correct. When the indicated dimension is one-half or two times the actual dimension, its accuracy should be considered within the limits of the inspection method. Formula (1) represents the technician's ability to locate and quantify defects in the test piece. The technician should score 70 points or more according to formula (1) to be considered for employment as an ultrasonic inspection technician for inspecting tubular structure welds. Formula (2) represents the technician's ability to confirm the defect-free weld area in the test piece. A technician who scores low according to this formula may require a large amount of unnecessary repair in the inspection of the actual structure. When evaluating the technician's work ability, the operator should consider the following: The consequences of necessary repair, that is, the weld repair is carried out under conditions worse than the original welding conditions, and the possibility of defects in the repaired weld is increased. Therefore, according to formula (2), the technician should score at least 50 points. Examples of test pieces, the format of the test report, the results of the test pieces, and the evaluation of the test piece grades are included in Appendix C. 5
2.4 Re-evaluation
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a. Qualified technicians who have not performed ultrasonic inspection of tubular structures for six months or more or whose work ability is questioned for some special reason should be re-evaluated. Chapter 3 Technical Recommendations
3.1 Applicability of Ultrasonic Inspection to Offshore Structuresa. Offshore structures require a large number of T, K, and Y-shaped nodes, pipes with ring plates and reinforcing partitions, and profiles that intersect with reinforcing ribs and auxiliary components in a T-shaped manner. In the non-destructive inspection of the above-mentioned welded joints, other inspection methods have limitations, and currently only ultrasonic inspection is the most suitable method.
b. This section will This paper briefly describes the types of structures that are suitable for ultrasonic testing, and puts forward some suggestions on the scope of inspection during the structural manufacturing and installation stages. 3.1.1 Inspection of structural plates
a. Chord plates at tube joints. If they contain interlayers and slag, they often peel off in the longitudinal direction of the plates under the action of welding stress and deformation. The peeling phenomenon caused by the extension of cracks between adjacent slag inclusions is called lamellar tearing. In plate manufacturing, if shrinkage deformation occurs after welding at the joints between the web and ribs, damage similar to lamellar tearing may also occur. Ultrasonic inspection technology can detect potential interlayers and major slags - these may be the cause of the above-mentioned problems during the structural manufacturing process. h. For the cough bar sheet materials (thickened sections of Lang nodes) at the tube joints, and the flange plates of the plate beams that need to be strictly strengthened, the steel manufacturer may conduct ultrasonic inspections on them according to the A435 or A5781 grades in the ASTM standard before the steel leaves the factory. The contents of these two specifications are basically the same, and both stipulate that the plate will be considered unqualified only when the main defects of the plate cause the sound energy to be completely lost (that is, when the plate contains interlayer defects). ASTMA5781 grade can be used to determine whether the ultrasonic display results of some important parts of the plate are correct. In addition, ASTMA578T grade also requires that some plates less than 76mm (3in) Long slag inclusion defects are reported separately. Grade A5781 requires that the central slag defects be reported separately, but may also require the steel mill to perform special treatment on the steel plate, the cost of which is added to the steel cost. c. For plates in the node design area, it is recommended to conduct on-site inspection. For plates that have been ultrasonically inspected before leaving the factory, re-inspections should also be carried out in the above-mentioned areas to further confirm the quality of these plates. In the node area, 100% ultrasonic inspection should be carried out on the end or back surface along the designed intersection line. During the inspection of plate or pipe components, if there is an abnormality in the ultrasonic display, the position of the component in the structure should be changed or adjusted immediately to avoid the defects from concentrating in the weld area. It is best not to have any abnormal ultrasonic display within a strip of at least 150mm6in. Any defects that cannot be avoided in the weld area should be carefully measured as a basis for re-evaluation after welding. Although ultrasonic testing is very accurate, it is still powerless to detect tiny slag inclusion groups that cause lamellar tearing. Therefore, when the plate thickness increases to 25m (1in) or more, or the node design is more complicated, it is recommended to use steel plates specially treated by the steel mill to avoid lamellar tearing problems. 3.1.2 Inspection at the onshore manufacturing stage
%. Inspection, repair and re-inspection should be completed as early as possible in the initial manufacturing stage and before the overall assembly of the structure. Inspections that can be completed at the manufacturing site should not be delayed to the offshore installation stage. b. Recommendations on the scope of inspection for the manufacture of tie bars, chords, and steel piles 1. Generally, structural steel pipes used for conductor frame legs, tie bars, and steel piles have been subjected to "body inspection" during the manufacturing process of the steel plant. The scope of inspection is usually carried out in accordance with the specifications of API SPEC 23 "Manufacture of Structural Steel Pipes". If ultrasonic inspection is required instead of radiographic inspection, the scope of inspection should be equivalent to the inspection requirements of API 2B standard, that is, 100% of the length of each circumferential weld and 10% of the length of each longitudinal seam should be inspected. 2. Except for the incandescent seams with known defects, when conducting local inspections on longitudinal seams, at least three sections in each longitudinal seam should be randomly selected for inspection. In the initial stage of manufacturing, it is recommended that the scope of local sampling inspections should include spot welding areas, although these spot welds will be melted or covered in subsequent buried arc welding.
3. It is recommended to inspect the longitudinal seams of chords or The longitudinal seams of pipes used for pressure vessels storing liquids are subject to 100% inspection. 4. It is recommended to conduct 100% inspection on all other butt ring welds. c. Recommendations on the scope of inspection during the assembly stage of conductor pipes and towers 1. Due to the unique geometric shape and manufacturing process of tubular structure nodes, ultrasonic inspection technology must be used. 2. For the node welds of important reinforcements that intersect with the chord, a 100% length inspection should be carried out. For over-static structures, some important welds can be selected at random for 100% inspection to ensure that the minimum quality requirements are met. 3. For the node welds between important reinforcements (such as lap nodes and K-type nodes), a 100% inspection should be carried out. For over-static structures, the inspection should be carried out in accordance with the standards of the second item above.
4.The inspection scope of the connection welds of the minor tie bars in the truss structure shall depend on the stress level and the severity of the working conditions. The minor tie bars in the mud surface, splash zone, collision zone (such as ship-side parts) shall be 100% inspected. For welds in other locations, the welds to be inspected shall be determined by the operator.
5. For minor tie bars and accessories (such as wellhead guides), sampling inspection shall be used to determine and control the overall weld quality. d. Recommendations on the inspection scope of deck components
1. If the deck structure is a tubular truss design, the inspection scope shall be the same as that specified in 3.1.2.c above. 2. If the deck is a plate-beam structure, radiographic or ultrasonic inspection shall be carried out according to the following provisions according to the specific circumstances: All flange joints and flange joint welds
Deck leg column and main beam connection weld chain bzxz.net
Deck leg column and truss tie bar weld
Deck leg column and ring plate or bulkhead weld (except fillet weld) 100% inspection;
100% inspection,
100% inspection:
100% inspection.
In addition, all web joints and web and flange connection seams (except fillet weld) shall be sampled for inspection. 3.1.3 Inspection during offshore installation stage
a. Due to the poor conditions and difficulty of offshore construction, it is necessary to increase (rather than reduce) the scope of non-destructive inspection. During the offshore installation stage, the inspection of large sections has increased the scope of application of ultrasonic inspection, which greatly reduces the radiation hazard in the limited working space. This is also where ultrasonic testing is superior to radiographic testing.
b. Recommendations on the scope of inspection during the offshore installation phase 1. The scope of inspection of offshore pile joint welds ranges from random sampling inspection to full 100% inspection, and the requirements vary depending on the situation. For joints that are subjected to high shear stress and high tensile stress during the piling process and the platform use stage, all joints should be 100% inspected. 2. The joints between deck legs and steel piles should be 100% inspected. 3. The joints of deck main beams and trusses should be 100% inspected. 4. There are special restrictions on the inspection of fillet welds of the gap filler plate between piles and ducts. For these welds, whether ultrasonic testing or radiographic testing is used, the results are unreliable. If cement slurry is not poured into the annular space between the steel pile and the duct, in this case, it is recommended to use the magnetic particle inspection method for the first and last few fillet welds. Whether to use the ultrasonic method should be decided by the operator and the contractor in advance. 3.2 Characteristics and limitations of ultrasonic testing of welds 2. Ultrasonic testing, like other testing methods, has many advantages and some limitations. Among them, the most prominent problem is that ultrasonic testing cannot record the test results permanently like radiographic testing, and the accuracy of ultrasonic testing depends largely on the level of technicians and their usual training. Understanding these limitations of ultrasonic testing and clarifying the reasons for accidental errors in technicians' testing are very necessary for formulating a comprehensive testing procedure. This section will explain the main characteristics and limitations of ultrasonic testing technology. 3.2. I Comparison of ultrasonic and radiographic testing
a. Although ultrasonic and radiographic methods are very different in terms of the types of defects detected and the ability of technicians to evaluate defects, people always compare the two methods. In order to evaluate the characteristics and limitations of the two methods as reasonably as possible, in the following comparisons, we will discuss the two parts of detection and evaluation separately. b. Detection
Radiographic methods are very sensitive to three-dimensional defects such as incomplete penetration, slag inclusions and pores. However, they are less sensitive to other types of two-dimensional defects such as cracks and lack of fusion. This is especially true when the direction of the ray is oblique to the defect. In order to make the defect image on the film recognizable, the thickness of the defect in the direction parallel to the ray must be equivalent to two-tenths of the weld thickness. As the thickness of the weld increases, the quality of the defect imaging decreases due to the increase of scattered rays in the weld.
Unlike the ray method, the ultrasonic method is highly sensitive to two-dimensional defects, but poor for three-dimensional defects. Ultrasonics are also difficult to detect tightly closed cracks and unfused defects. However, compared with ray, the threshold limit of ultrasonic testing is small. For cracks and similar defects in areas with high residual compressive stress after welding, the ultrasonic method is often unable to detect them due to the tight closure of the defects. This limitation of ultrasound has little effect in most cases, because ordinary fracture failures mostly occur in the tensile zone, not in the compressive zone. However, welds that need to be stress relieved should be tested in the 7-
processing period before they can be put into use. Q/IIS 7007—93
The main limitation of shear wave ultrasonic testing is that relatively large planar defects will reflect the acoustic beam and the probe will not receive the defect reflected wave, especially in single-angle or single-probe testing. On the contrary, smaller defects will cause the reflected wave to diverge, increasing the probability of error detection.
The directionality of the ultrasonic beam will also make it difficult to detect certain types of defects. For example, if the sensitivity of detecting planar defects is used to detect single pores and randomly distributed pores, it will be difficult to distinguish them. This is because the area of spherical defects perpendicular to the sound beam only occupies a small part, so the reflected sound energy will be disproportionate to the actual size of the defect (the situation of cylindrical defects is better). C. Defect assessment
Defect assessment refers to the identification of detected defects and the determination of their size. Although in most fracture control, the influence of some defects, pores and isolated defects is not very large, any abnormal display in the inspection should be found out to find out its source and characteristics. Doing so is not only conducive to removing excessive defects but also to taking corrective measures accordingly. Among the two inspection methods, radiographic inspection is a better identification method. On the contrary, the information that can be detected by the ultrasonic method is not enough to complete the defect identification work. In order to make a reasonable interpretation of the displayed signal, the ultrasonic technician must be fully familiar with the welding method and the pre-welding assembly, and accurately measure the position of the reflector in the cross section of the weld. In addition, combined with the control of the probe angle and beam, the ultrasonic technician can and can only identify the two basic geometric shapes of defects, namely spherical, cylindrical and flat. The characteristics of ultrasonic inspection itself determine that it cannot distinguish between root fusion and incomplete penetration. What is more serious is that if this fusion defect exists at the same time as radial cracks, ultrasonic inspection is powerless. Planar defects are the main factor causing fracture damage. Therefore, if ultrasonic technicians can accurately distinguish one-dimensional half-surface defects from other defects, it is unnecessary to further subdivide the nature of defects for most construction projects. Defect identification without quantitative analysis is of little significance to engineering fracture control. When using the ray method to determine the size of the defect, it is limited to determining the projection length of the defect. This does not mean that the ray method cannot determine the other dimensions, location and orientation of the defect in the weld section, but because these contents are not suitable for on-site inspection. Before discussing the characteristics of ultrasonic determination of defect size, it is necessary to analyze the scope of application and limitations of different measurement methods. There are two commonly used measurement methods, each with different characteristics. The first is the amplitude comparison method. This method is relatively simple and is particularly suitable for detecting small-sized defects that can be within the beam cross section. However, the inspection accuracy will decrease rapidly as the defect increases. When the defect exceeds 6mim (1/4in), it will exceed its accuracy range. The main disadvantage of this method is that since the total reflection area controls the size of the wave height, the length and width of the reflector cannot be distinguished during the inspection.
The second method is the beam boundary method. This method has a great advantage in determining the length and width of most important defects. Theoretically, this method is very accurate, but due to some other unavoidable factors, such as probe characteristics, instrument calibration and technician level, the actual inspection accuracy will be slightly reduced. As the thickness of the weld decreases, the inspection accuracy of this method will be greater. No matter how hard we try, the accuracy error in the width direction is difficult to be less than ±2.5mm (+3/32in). 3.2.2 The influence of different types of welds on ultrasonic inspection a. For full penetration butt welds of plates or pipes with a thickness or thickness greater than 12.5mm (it), the results will be more accurate when using ultrasonic inspection methods. Although the geometric shapes of T, K, and Y-type node welds are complex, they have little effect on the effective inspection ability of ultrasonic waves. The weld inspection effect is reduced in the following order: welds flush with the parent material after grinding, double-sided welds and single-sided welds. b. Partially penetrated welds are another difficulty faced by ultrasonic inspection. The inherent inaccuracies in detecting the root fusion boundary are the same as those in detecting any other defect, and more importantly, the ultrasonic method cannot distinguish between cracks existing in the first layer of the joint and intentionally left or allowed boundary failure to fusion.
c.Ultrasonic testing is generally not very reliable for fillet welds, especially for welds with thin or medium thickness. During inspection, large fillet welds that can accommodate the entire beam are rare, and the beam can detect potential defects in fillet welds at the best angle of incidence. Generally speaking, for fillet welds in important structural positions, other inspection methods are usually used or supplemented during inspection. 3.2.3 Effect of section thickness on ultrasonic inspection. The effect of section thickness on ultrasonic and radiographic inspection is exactly the opposite. For thinner sections, radiographic inspection (using appropriate radiation sources) is more accurate and less expensive. In contrast, ultrasonic inspection is more difficult for thinner section welds, and it is difficult to distinguish the reflected waves of harmful defects in the weld from the reflections at the root and protrusion of the weld. When measuring small defects, it is generally limited to amplitude measurement. For fully welded butt welds with a depth of less than 25mm (1in), the radiographic method is better in terms of effectiveness and cost of use. As the thickness increases by 8
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