Model tests in Hs = 14.5m waves Courtesy MARIC

Figure 2: Design of advanced backbone

of the ship structure. This is particularly the case concerning slamming induced excitation of whipping and the non-linear excitation driving springing.

To get insight into the impacts on the hull and the structural response, a 10-segmented ship model of a containership was built, mounted on a backbone. This backbone (Figure 2) was used to model the vertical and horizontal bending moments of the containership. The model was tested in moderately low waves of 3 m up to waves with a height of 14.5 m. The tests showed the 2-, 3- and 4-node vertical bending, the 2- and 3-node horizontal bending moment, as well as the torsion moment. The 3-node vertical bending mode is illustrated in Figure 1.

Advanced backbone The shapes of frequencies of the global, flexural vibration modes were provided by MARIC and used to engineer the backbone. This advanced backbone is an aluminium pipe of which the diameter and wall thickness are chosen such that the modal parameters of the model match those of the ship. Because shear forces were to be measured accurately, a backbone from a combination of beam elements and link elements was manufactured. The link elements were made to control the stiffness of the backbone and to measure the forces and bending moments.

Importance of model tests Model tests in 14.5 m waves showed that the highest vertical bending moments are

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about twice the magnitude of that of the horizontal bending moment. The measurements were consistent with the Finite Element computations with respect to the global loads. However, the highest local stresses were underestimated by the computations by some 15%, highlighting the importance of model tests.

The 10-segmented model mounted on the advanced backbone showed very good flexural responses and certainly these tests provide further insight. MARIN is now working on introducing this new way of engineering the backbone to future projects on flexural responses.

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