|Ship repair involves, to a very large extent, the replacement of old worn pipes. Central to this is the need to measure accurately flanged pipes in 3D co-ordinates, including straight sections, toroidal bends and reducers, and the relative positions of the end flanges with respect to the centreline orientation of the pipes. From these measurements, new pipes are fabricated to replace the worn pipes. The new fabricated pipes must match dimensionally the worn pipes to a sufficient degree of accuracy, typically in the order of 1-2 mm.Typical diameters of such pipes range from around 100mm to more than 350mm with lengths of up to 6m. The weights of such pipes can be up to almost a ton.|
|Figure 2: Computer-controlled pipe fixturing system
A research project, funded jointly by both NSTB and local shipyards, is now being undertaken in the Department of Mechanical & Production Engineering which aims at the development of an automated Intelligent Pipe Measurement System (IPMS).In the IPMS system, a computer-controlled robotic fixturing system is used to hold, and at the same time, measure the relative coordinates of the end flanges of the old pipes. This is illustrated in Figure 2.
|A laser ranging system, together with a machine vision system, mounted on an instrumented two-degree-of-freedom gimbal system (see Figure 3) is then used to measure the co-ordinates of a specified number of points on the surface of the pipe. The vision system is first used to determine the outlines of the pipe and, from this, the approximate locations of the points to measure. The computer-controlled laser ranger is then moved to target these points one at a time and their co-ordinates in 3D space measured.From these measurements, the diameter of the pipe, and the 3D cordinates of the centreline of the pipe is determined. The robotic fixturing system is also instrumented with sensors which allows the computer to determine the type of flanges used.||
Figure 3: Two-axis gimbal for camera and laser
The robotic fixturing system is subsequently used in the fabrication process. Here, from the identification number of the old pipe measured, the robotic fixture is positioned to the positions where the end flange should be and the flanges of the new pipe is welded with the assistance of this fixture. The geometry of the newly fabricated sections of pipes can also be measured on-line by the system and checked with its stored geometry for accuracy and adjustments can be made, where necessary. In this way the dimensional accuracy of the newly fabricated pipe is controlled to within the desired tolerances.
This project is carried out in collaboration with Assoc Prof Marcelo Ang Jr, Dr Senthil Kumar, MrYang Peirong, Mr Xi Weiya, Mr HC Mok and Mr AC Lim.
Poo Aun Neow, Assoc
Prof Marcelo Ang Jr
Oct. 2000, by E. Burdet