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Slam induced bending of high-speed wave-piercing catamarans
thesisposted on 2023-05-27, 09:59 authored by McVicar, JJ
The unique geometry of wave-piercing catamarans causes the slamming process to be complex and challenging to predict. The overall slam generally consists of multiple sequential events (demihull bottom slam, centre bow entry and wet-deck arch slam), which impart energy to the structure on differing spatial and temporal scales. In this thesis, the relationship between the slam impact and the resulting bending response of wave-piercing catamarans was investigated through analytical investigations and numerical simulation. A simplified analytical model of the hull girder being exposed to impact loads was developed and this showed that, compared to conventional craft, the relatively shorter slam duration has the potential to excite higher order bending modes which may contribute significantly to the distribution and magnitude of the hull girder bending moment. One-way and two-way coupled fluid structure interaction simulations accounting for global hull bending, but not local panel flexibility, were established to simulate the slamming behaviour of catamarans. The wet and dry structural systems were identified by input-output system identification experiments conducted on a 2.5m hydroelastic segmented model. A new method for evaluating and accounting for the time variation in hull added mass was developed and implemented in the one-way coupled simulations. The simulations were verified and validated against prior experimental data in regular head seas. The one-way coupled simulations were found to be sufficient to capture the hull response over a wide range of encounter frequencies. Compared to the two-way simulations this significantly reduced the computational resource requirements and computation time. Allowing for the time variation of added mass in the one-way coupled simulations captured temporal variation in whipping frequency but had a relatively small effect on the bending response amplitudes. The effects of global hydroelasticity were thus found to be small, however, increased hull flexibility was found to appreciably reduce the peak slamming forces acting on the model. From the simulation output data, a method for estimating the transient forces acting on the hydroelastic segmented catamaran model from the hull bending and global motion response was developed. It is proposed that using this method would lead to significant weight reduction for future hydroelastic scale models. This would allow experimental slamming characteristics to be considered at reduced displacements or facilitate inclusion of additional instrumentation. The contributions to bending from each of the events within the overall slam were estimated from simulation and it was shown that the centre bow entry and arch slam are the largest contributors to hull bending. Centre bow entry was found to be non-impulsive in relation to the hull girder frequencies, acting to generate a pre-strained hull girder prior to each arch slam. The rapid onset and release of the arch slam was found to cause high frequency excitation; exciting modes with natural frequencies up to four times the fundamental longitudinal bending frequency and having significant implications for the design hull girder bending distribution - particularly for hull sections in the vicinity of the slam location.
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