20 Modal Test Results of a Ship Under Operational Conditions 201 3,4 5 3 4 8 19 1 2 6 7 8 9 10 11 12 13 14 15 1,2 26 25 24 23 22 21 20 19 18 17 16 Weather deck Main deck y x z y z x Main deck Cross views Weather deck Main deck Side view Fig. 20.2 Overview drawing of the vessel, including the measurement grid used. All measurement DOF’s are marked with filled circles and a number The main data of the vessel is as follows: – Shipyard: Flensburger Shiffbau-Gessellschaft. – Total length: 210 m. – Width (mid main deck): 29.60 m. – Height to main deck: 10.90 m. – Draught (design): 8.45 m. – Deadweight: 19,200 tons. – Design speed: 20 knots. 20.2.2 Measurement Equipment and Setup The following Data Acquisition (DAQ) and sensor equipment was used: – 3x National Instrument 4,497, 16 channel, 24 bit analog inputs cards. – 45x Dytran 3097A3 accelerometers, 500 mV/g, IEPE. – Up to 300 m coaxial cables, in total more than 5 km of coaxial cable. – In-house software for DAQ control, based on MATLAB DAQ-toolbox. The measurement points (26 in total) were spread out on the main deck of the vessel, two points on the deck house (fore), and two points on the flume tank (aft) as shown in Fig. 20.2. The horizontal (x–y) plane was investigated for torsion modes and so all points were measured for vertical vibration (z-direction). The longitudinal vertical (x–z) plane is only investigated for bending modes, hence only points on one side of the deck (point 5–15) were measured for horizontal vibrations (y-direction). Longitudinal vibrations (x-direction) are not considered in the present work, but at the four points at the deck house and flume tank, where all three transversal directions were measured for further investigations of local phenomena, not included in the present work. At measurement point 5–15 where two directions were measured, small aluminum cubes were used for mounting the accelerometers, see Fig. 20.3.
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