In offshore construction, there is a demand for vessels with ample deck space. Large catamarans could provide larger deck space for the same tonnage. However, one operational concern for catamarans is wet deck slamming. Wave impact with the wet deck can excite the global flexible vibration modes of the vessel. While extensively studied for high-speed catamarans, the dynamic response induced by slamming in large, slow-moving, heavy-lift catamarans remains poorly understood.

This thesis presents a fast numerical method for predicting the dynamic response of a heavy-lift catamaran subjected to wet deck slamming in head seas. The method combines Capytaine, a BEM potential flow solver, with ANSYS Mechanical, a finite element software package. ANSYS calculates the dry modal properties, and Capytaine includes the water effects to obtain the wet modal properties. Capytaine also finds the rigid body motions resulting from wave loading. A simplified slamming pressure formulation based on immersion velocity and wetted length is applied. Both monochromatic and bichromatic wave conditions are investigated to assess their influence on the immersion behaviour.

It was found that monochromatic waves produce immersion of long duration. The proposed method cannot accurately determine forces and responses due to the varying properties during a slamming event. Additionally, the limited impulse can not effectively excite the eigenmodes due to the long wetting time compared to the natural periods. In contrast, bichromatic wave conditions lead to shorter and more severe immersion events. For the bichromatic wave loading, the modal dynamic oscillatory response was found as a result of wet deck immersion.

To assess the accuracy of the proposed numerical method, a validation against experimental measurements is performed. The comparison focused on accelerations resulting from slamming events and the intervals between such events. The resulting accelerations of the proposed numerical method slightly underestimated the measured accelerations. One problematic finding was that significant slamming events could disrupt the vessel's oscillatory motions and that resolving the disruption cannot be achieved in the frequency domain. The proposed numerical method can be used to identify conditions where slamming can be problematic quickly. However, to obtain accurate displacements, accelerations, and stresses, further analysis by another method is required.

The thesis provided insight into the dynamic behaviour of a heavy-lift catamaran induced by slamming. An indication of slamming effects can be calculated in the early design stages. More importantly, identifying problematic wave loading cases can be done at a relatively low numerical cost. The exact magnitude of the slamming effects requires a more detailed time-domain analysis.

Catamarans are popular in the offshore sector as they combine good transverse stability and ample deck space with low wave resistance. However, their slender hull shape results in low restoring qualities in heave and pitch motions. The large motions in rough weather can often result in water impacting the underside of the deck connecting the two hulls, a phenomenon called cross-deck slamming. The impulse excitation from cross-deck slamming can then produce a transient hydroelastic response of the structure called whipping. Whipping excites mode shapes that would not normally be present in the response, as their natural frequencies are significantly higher than the wave encounter frequency. This results in detrimental contributions to fatigue life through high-amplitude cyclical bending moments. Both the calculation of slamming loads and the prediction of resulting structural responses have been a challenge for several decades. The highly nonlinear and three-dimensional character of the phenomenon, combined with the strongly coupled fluid-structure interaction means that it is unpredictable, and even the definition of slamming events has been a matter of disagreement among researchers.\\

Experiments are still a vital part of these investigations, for validating ever-improving numerical techniques. An essential issue with experiments is the extent to which mode shapes and natural frequencies can be emulated in model scale. Traditional hydroelastic models are segmented and use either a flexible backbone or flexible joints to introduce stiffness. This often results in an excellent description of the 2-node bending mode, but an increasing error for higher modes leads to stress inaccuracies. In this investigation, a fully elastic model of a catamaran is designed and produced for hydroelastic experiments. The advantages and limitations of the concept are identified, and the verification against structural models is presented.

The common practice of designing a ship is to look for ships with similar specifications and alter it to the client's needs. This often leads to structurally redundant and therefore overdimensioned ship designs. Hence, much improvement could be expected from ship structures where optimization algorithms help advise in the design process. In this research, the midsection of a Trailing Suction Hopper Dredger (TSHD) is optimized. A TSHD midsection generally consists of longitudinal stiffened panels and transverse web frames. The typical web frame and longitudinal stiffener layout is optimized, as a weight improvement of this section can have a large effect on the total weight of the ship since it is repeatedly reoccurring.

This research has optimized the midsection of a reference TSHD in two ways; the first step was to perform a shape optimization for the longitudinal stiffener arrangement, which was followed by a topology optimization for the transverse web frame. Both optimization objectives were to minimize mass. The order of optimization follows the hierarchy in which stresses are introduced into the structure; from the plates that make up the hull toward the stiffeners and eventually the web frames. The complete optimization was performed a total of seven times, for seven different web frame spacings ranging from 25% to 175% of the web frame spacing of the reference TSHD.

For the shape optimization, a Simulated Annealing algorithm was used. The reference ship was simplified to be able to parameterize the geometry into eleven panels with T-stiffeners. Each panel has a set of variables that describe its geometry; the plate thickness, number of stiffeners, stiffener web height, flange width and web and flange thicknesses. Although feasible results came out of the optimization, no clear parallel was found when comparing the plates of different web frame spacings. This is due to the fact that it is a high dimensional problem. Although no clear parallels were found, the results were able to cope with all the loads.

The topology optimization was performed with a modified Bi-directional Evolutionary Structural Optimization (MBESO) method. The applied modifications ensure a fast convergence for large topology optimization problems. The new method was first verified by comparing results to two benchmark cases from the original BESO method, which was followed by three examples of common topology optimization benchmarks. Once established that the modified method was capable of reproducing test cases, an aspect ratio analysis was performed to better understand the transmission of stress. After that, the full geometry of the midsection was divided into smaller basic models and the same optimization was carried out in order to help interpret underlying physics of the final results. Finally, the topology optimizations for the seven web frame spacings were performed, resulting in a new orientation of beams. The topology optimization results showed that constructing beams not in an orthogonal way and along the ship hull but rather under various angles could reduce the total mass of the web frame.

To see how the shape optimization result influenced the topology optimization, three studies were carried out where all the surrounding plates had the same thickness, except for one that would have a significant smaller thickness. This showed how the web frame supports the hull plating, but also how the web frame is dependent on the stiffness of certain panels to be able to transfer shear into them.

Finally a comparison was made between the reference ship and the optimized structure. The result was a decrease of 23\% in weight for the midsection. Due to practical production considerations this weight is likely to be higher in practice, however it is a promising start toward a more efficient ship design. The innovative combination of shape optimization combined with the MBESO procedure could help in early design stages where the main components of the construction are defined.