Development of Curvature Gap Estimation System for Deciding Thermal Forming Instructions of Ship Curved Shell Plates Using Laser Scanner

Abstract

A thick steel plate with a unique curvature was employed to make the outer shell of a ship. This curved shell plate is shaped one at a time by craftsmen carrying out plastic deformation using gas heating. The process includes evaluation of forming accuracy and selection of thermal forming instructions. Both are done using fitting molds called “wooden templates” in a manner that is qualitative but dependent on individual skills. Thus, there is a problem of variation in quality. To solve the problem, research and development have been promoted on a manufacturing process assisted by a laser scanner that is a highly accurate three-dimensional measuring device. An evaluation method for forming accuracy has been established and has reached a satisfactory level for operation on site. However, the method of automatic selection of thermal forming instructions is still immature. Focusing on the curvature of frame lines on the outer plate that acts as an index when instructions for thermal forming are decided upon, a curvature gap estimation system was developed for outer plate frame lines using a laser scanner. Here, a frame line refers to the standard to be compared with a design shape to evaluate the forming accuracy of the members. The system extracts from measured data a point cloud that makes up each frame line, calculates curvature at a given point on the frame line, and visualizes it with a graph and a color map. This system uses an evaluation method whose curvature calculation has sufficiently appropriate accuracy and that is feasible and useful on site. First, the sufficiently appropriate accuracy of the curvature calculation was confirmed using a measurement form of a cylindrical model that simulated a gap between the distance direction generated by measurement with the laser scanner and the direction of laser irradiation. Next, the feasibility and usefulness on site were confirmed by applying the measurement method to the processing data of the ship shell outer plate shape that was obtained through the curving process in the shipyard, and then by comparing the record of regions thermally formed by the worker with index calculation results made by the system.