Shape and Topology Optimized TSHD Midsection

Abstract

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.