Container ships are becoming larger and larger in recent years, requiring more evident assurance of the structural safety. To achieve this, it is essential to grasp actual stress history experienced by the ship structures to facilitate efficient design and maintenance, and to use them for optimal operation of the ship. To perform accurate estimation of these stress histories, it is important to precisely estimate the sea state which the ship is actually encountering. In this study, the authors studied a new method to estimate directional wave spectra using measured ship responses and discussed the following three cases. The first one is the combination of two components, vertical bending stress and horizontal bending stress. The second one is the combination of three components, vertical bending stress, horizontal bending stress and double bottom bending stress. The last one is the combination of three components of ship motion (pitch, roll and heave). The estimated sea states are compared with the ocean wave hindcast database and radar data, and then, accuracy and selection of appropriate combination of the responses are discussed.

There is plenty of energy contained in the ocean waves, and great efforts have been made over recent decades to make full use of these sustainable energy resources. As a novel device to utilize ocean wave energy, the authors propose in this paper a whipping energy converter, which utilizes a linear generator extracting power from ship’s whipping and springing (hull girder 2-node vibration) responses. The device is designed resonant to the hull girder 2-node vibration frequency. Numerical simulation of the responses of the whipping energy converter was carried out under irregular head sea condition, using time domain ship motion analysis by Rankine source method and subsequent three-dimensional whole ship FE analysis, and the main parameters of the whipping energy converter was studied. A 14,000 TEU large container ship was utilized in this study, because large container ship is prone to constant whipping and springing responses in real seas. As a result, it was found that the whipping energy converter can effectively generate power from the whipping and springing responses, and at the same time, the whipping energy converter functions as a dynamic damper. With the piston weight of 5–10 tons, the whipping vibration is visibly reduced, while if the piston weight is about 100 tons, the spectral peak is reduced to less than one-third. This implies that the whipping energy converter contributes to the prevention of fatigue damage due to whipping and springing, which is an imminent concern in the design of large container ships. It was also found that larger power generation can be expected as the piston weight is heavier, and the bow is the most effective location for this device.

Topology optimization (TO) is currently a focal point for researchers in the field of structural optimization, with most studies concentrating on single-loading conditions. However, real engineering structures often have to work under various loading conditions. Approaches addressing multiple-loading conditions often necessitate subjective input in order to determine the importance of each loading condition, aiming for a compromise between them. This paper proposes a so-called bisection constraint method (BCM), offering a unique, user-preference-independent solution for TO problems amidst multiple-loading conditions. It is well-known that minimizing the system’s compliance is commonly used in TO as the objective. Generally, compliance is not as sufficient as stress to be used as a response to evaluate the performance of structures. However, formulations focusing on minimizing stress levels usually pose significant difficulties and instabilities. On the other hand, the compliance approach is generally simpler and more capable of providing relatively sturdy designs. Hence, the formulation of min–max compliance is used as the target problem formulation of the proposed method. This method attempts to minimize compliance under only one loading condition while compliances under the remaining loading conditions are constrained. During the optimization process, the optimization problem is automatically reformulated with a new objective function and a new set of constraint functions. The role of compliance under different loading conditions, i.e., whether it is to be treated as an objective or constraint function, might be changed throughout the optimization process until convergence. Several examples based on the solid isotropic material with penalization (SIMP) approach were conducted to illustrate the validity of the proposed method. Furthermore, the general effectiveness of the compliance approach in terms of stress levels is also discussed. The calculation results demonstrated that while the compliance approach is effective in several cases, it proves ineffective in certain scenarios.