Increasing ULCS structural response knowledge through 3DFEM and a comprehensive full-scale measurement system

It is now commonly admitted that due to the increased size of container vessels together with their operation conditions it is necessary to take the dynamic response of their structure into account in the structural assessment. The incorporation of these springing and whipping dynamic effects in the design and verification procedures requires that these phenomena are correctly known and modelled. Full scale measurement is an essential part of this process. In 2010, Bureau Veritas together with CMA-CGM and DSME installed a comprehensive instrumentation on-board one of the latest ULCS 13300TEU type to be launched. On one side the excitation is assessed through a dedicated sea state measurements system and on the other side, a large number of sensors are deployed to characterize the overall structural response: Twelve three-axis accelerometers and eighteen Long Base Strain Gages (LBSG). However, the measured signals further need to be converted to global load parameters such as the vertical bending moment. Among the different methods one is to use a Finite Element Method (FEM) model of the ship structure to analyse the LBSG strains. The method proposed in this paper considers the natural vibration modes of this 3DFEM model as a suitable modal base on which the hull girder bending and torsion can be decomposed, giving access to the entire distribution of internal loads along the ship. Conversion matrices from the measured strains to the modal coordinates and to the hull girder internal loads are determined. A key point in this process is the correct evaluation of the LBSG strain in the FEM model, which is not as straight forward as one can think. In a first step, two sensitivity studies are conducted on the LBSG strains associated with the FEM results: The first one focuses on the modelling aspect through a refined mesh study and the second one compares the full response including all the 3D FEM degrees of freedom to a truncated modal approach. Recommended practices are determined. In a second step, a hydro-elastic numerical simulation of the ship structural response is performed, and the conversion matrix intended for full scale measurements is applied to the numerically determined LBSG strains. The resulting internal loads values are compared with the exact ones, obtained by the integration of external loads, for the verification of the accuracy of the chosen conversion matrix.