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Underwater ultrasonic nondestructive testing technology

Jul 27, 2018

Why study underwater ultrasonic testing

Along with the development of modern industry and science and technology, nondestructive testing technology in equipment and equipment operation, the guarantee of product quality, increase productivity, reduce costs, and other fields play a greater role, nondestructive testing has already developed into is an independent comprehensive discipline, and ultrasonic testing technology in nondestructive testing field occupies an extremely important position, have got extensive application in many fields.

However, the modern petroleum industry has gradually developed to the sea, and there is an increasing demand for underwater ultrasonic inspection technology. Therefore, underwater ultrasonic inspection technology is favored by various offshore oil companies.  

Underwater ultrasonic testing (UWUT) is one of the most important underwater nondestructive testing methods.

The underwater installation or repair of Marine engineering structures requires a lot of underwater welding, and the quality of the weld is often much worse than that of the land.

UWUT must be used to detect the quality of the weld. During the operation of platform, submarine pipeline and other structural parts, it is necessary to conduct UWUT as well as to master the expansion of internal defects.

【Ultrasonic testing has the following advantages】

1. It is applicable to nondestructive testing of metal, non-metal and composite materials and other components;

2. With strong penetration ability, the internal defects of specimens within a large thickness range can be detected. For gold materials, thin wall pipe and plate with thickness of 1~2mm can be tested, and steel forgings with length of several meters can also be tested.

3. Relatively accurate defect positioning;

4. High detection rate of area type defects;

5. High sensitivity, which can detect defects with small internal dimensions of the specimen;

6. Low cost, fast speed, portable equipment, harmless to human body and environment, and convenient for on-site use.

As China's oil industry gradually moves into the sea, the importance of underwater technology becomes increasingly prominent. Regular inspection is required for underwater pipelines, underwater operating machinery and equipment, etc., which rely heavily on underwater ultrasonic inspection technology. The development of underwater ultrasonic inspection technology will directly affect the development of the Marine industry in the future.

Therefore, it is very necessary to study underwater ultrasonic testing technology.


Development status of ultrasonic nondestructive testing in China

In recent years, China has made great progress and development in ultrasonic nondestructive testing. Ultrasonic nondestructive testing (NDT) has been applied to almost all industrial sectors and is increasingly being used.

The basic research on the theory, method and application of ultrasonic testing technology is deepening gradually, and many advanced international achievements have been achieved. Many different applications of microcomputer control automatic ultrasonic testing system have been applied in practical production.

The main research of our country in this respect is: computerized ultrasonic equipment; User friendly interface operating system software; Ultrasonic digital signal processing, including artificial intelligence, neural network, pattern recognition, phase compensation, etc. High frequency ultrasonic nondestructive testing technology: various scanning imaging technology; Multi-coordinate, multi-channel automatic ultrasonic detection system; Ultrasonic robot detection system: automatic scanning ultrasonic imaging detection of complex components (such as the development and research of auxiliary equipment such as multi-dimensional probe adjustment structure with over 5 dimensions).

Many of these achievements have reached the international advanced level, and these researches are the guarantee for the continuous development of ultrasonic nondestructive testing technology in China.

However, China's ultrasonic nondestructive testing industry from the overall level, and developed countries have a large gap in quality inspection. Specific performance in the following aspects:

1. The proportion of senior technical staff and senior operation staff in the inspection professional team is small, which greatly hinders the progress of automation, intelligence and imaging of ultrasonic nondestructive testing technology.

The older generation of experienced flaw detection workers lack the theoretical summary of translating practical experience into theoretical summary, while the younger inspection workers have rich computer and other modern technology but lack practical experience. This may lead to the existing ultrasonic testing software system to varying degrees of defects and reduce the reliability of testing.

Especially like expert system software, to and automatically judge injuries. The writing of software that automatically evaluates the level of defects should be given enough attention.

To organize certain manpower and talent to collect and summarize the field experience of ultrasonic nondestructive testing, constantly enrich the testing theory and standard, and combine the non-destructive testing technical staff and computer technical staff, so as to develop practical nondestructive testing software.

In addition, a correct concept should be set up for all kinds of non-destructive testing software. Any software relies on correct detection methods, testing status and results obtained within certain applicable limits.

2. Relatively few professional non-destructive testing staff and low utilization rate of existing non-destructive testing equipment. After 40 years of development, China's nondestructive testing technology has been applied in nearly 30 system fields, and nearly 300,000 people are directly engaged in non-destructive testing technology.

According to the investigation and survey of large machinery manufacturing enterprises in China, they all have many non-destructive testing equipment. However, due to the lack of understanding of the importance of non-destructive testing and the lack of staff specialized in non-destructive testing, non-destructive testing has not played its due role in production.

The lack of books on nondestructive testing is not conducive to the training of non-destructive testing successors.

3. Lack of attention to the application of information technology in the field of nondestructive testing technology. When information technology is combined with NDT, a large amount of information can be obtained from the detection process to the maximum extent. The construction of nondestructive testing information technology in China is still in a rather weak stage.

In a word, the problems that need to be solved urgently at present are those related to and to be solved in practical engineering application, such as the standardization of detection methods, the standardization of defect detection, the formulation of testing and acceptance standards, the procedural operation procedures, and the informatization of detection technology.

In addition, attention should be paid to the national unified training, appraisal and assessment of NDT qualification, so as to make the training of NDT personnel in line with international standards.


Principle of underwater ultrasonic testing technology

The UWUT is not simply moving the uut method from the ground to the complex underwater environment. Some seemingly simple problems on land must be properly addressed in the UWUT. For example, the selection, testing, adjustment and sensitivity setting of probe instrument, how to observe, record and judge underwater, etc.

The basic principle of UWUT is the same as that of conventional UT method on land. Pulse reflection method and resonance method are the main methods. When this method is used for detection, it is necessary to establish contact between divers and onshore technicians from the perspective of ultrasonic echo display, so as to help UWUT workers to make correct judgment. Therefore, UWUT usually adopts land and water synchronous ultrasonic detection system.

【Underwater ultrasonic testing instrument and probe】

Like all UWNDT devices, UWUT devices must meet the requirements of relevant specifications and standards (such as asme73-wa/oct-18, BS4994, etc.). Usually the instrument manufacturer or owner shall be approved by the relevant classification society or by a single instrument/device.

(1) water tightness and water pressure resistance, especially the main control display immersed in the water, are like a pressure vessel, whose pressure-resistant shell and polypropylene window should be able to withstand the external pressure at a depth of tens of meters to prevent the working depth from being crushed and destroyed, and to withstand the internal overpressure in the water environment.

(2) the staff shall ensure that all submersible equipment and devices are safe, so that the diver shall avoid electric shock as much as possible; Generally speaking, the fault current passing through the diver's body shall not exceed the "sensitivity" of 0.5mA (ac) or 2mA (dc).

(3) technical performance usually, the underwater ultrasonic instrument must be able to apply pulse reflection technology and double probe technology, covering a minimum frequency range of 2-6mhz. Under test conditions, the echo of 5% of the total amplitude of the screen can be clearly detected. At the same time, can be seen from the front directly depict the benchmark curve flat screen, apply to the longitudinal wave sensors, sensor T/R, 45 °, 60 °, 70 ° and 80 ° Angle probe.

(4) the environmental adaptability system and components must adapt to its working environment.

(5) operability bar is usually designed to be less buttons and easy to identify, easy to operate and calibrate. Each UWUT device should have a corresponding operation manual (including equipment/instrument, application limits, and specification of design criteria that affect the application of the equipment) and calibration procedures. The DAC100% curve positioning reference sensitivity was obtained by adjusting the maximum echo of 1.6 mx1 and 6mmX38mm square groove reflection in the APIRPZXA test block. The sensitivity compensation is usually determined by considering materials, curvature, coupling and other factors, generally 12dB.

(6) before and after each inspection, the incident point, refractive index Angle and DAC curve of the probe must be checked.

(7) coupling agent underwater detection, seawater is a natural coupling agent. Therefore, all adjustment and testing on land shall adopt seawater coupling.

In conclusion, it can be concluded that NDT technology has a wide range of applications under water and can play a very important role under water, especially for the detection of such close instruments as water pressure vessels.

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