Non-obstructive measurement technique for inner wall defects in small-diameter pipes

The high-pressure flexible hoses used in oil and gas drilling have small diameters and long dimensions. When the structured light method is used to detect the defects on their inner walls, there are problems such as the obstruction of the supporting structure and the inability to scan and measure the complete inner wall. For this purpose, a conical structure light unobstructed scanning measurement system based on the arrangement of dual cameras and lasers on the same side has been developed. Based on the pinhole imaging model and the properties of spatial conical surfaces, it is analyzed that in the lateral placement architecture where the camera and the conical structure light exciter are suitable for scanning measurement, the spatial conical light overlapped on the image plane, making it impossible to achieve a one-to-one correspondence between pixel coordinates and world coordinates. By adding a measurement camera in the vertical direction, the overlapping area of the horizontal camera's imaging is supplemented to achieve blind-spot-free scanning and detection of the inner wall of the pipeline. Based on the system calibration of the conversion between pixel distance and three-dimensional spatial distance, tests were conducted on pipes with inner diameters of 51 and 64 mm containing defects of known sizes. The test results indicated that the root mean square error of the defect size at all positions was ≤0.076mm, and its good robustness within a certain pipe bending range was verified. Meanwhile, the tilt error and the irregular error of the laser strip edge existing in the measurement system were analyzed. The results show that this system has high application value in the measurement of inner wall defects of small-diameter and long-sized pipelines and can meet the actual engineering requirements.