Construction of the Uncertainty Budget for a Large-Volume Dimensional Metrology Instrument

The construction of cruise-ship hulls involves a series of operations on parts of considerable size (i.e., several tens of metres) and mass (i.e., several tens of tons). Various factors can cause deviations from the nominal dimensional specifications, such as self-weight deflection, thermal expansion, and welding-induced deformations. These factors may introduce deviations of a few millimetres, making accurate dimensional verification essential to avoid costly and time-consuming repairs during final hull erection. Large-volume metrology (LVM) instruments are therefore required for these verifications. This chapter develops an uncertainty budget for dimensional verifications carried out on the panel line of a shipyard, where metal plates are assembled and welded to build larger hull parts. Inspections concern reference distances on the panels, usually located in areas that will later be involved in manufacturing or assembly. Measurements are collected using a total station equipped with a contact probe, under the relatively severe environmental conditions of the shipyard, which is characterised by significant temperature fluctuations, variable lighting, vibrations, etc. In accordance with the Guide to the Expression of Uncertainty in Measurement (GUM), the relevant sources of uncertainty are identified and classified as Type A or Type B. The individual contributions are then combined to calculate the expanded uncertainty, which is found to be around 1.5 mm for distances of about ten metres. With the appropriate adaptations, the proposed methodological approach can be extended to other LVM instruments used in the shipbuilding context.