In order to solve the problem of path redundancy and long algorithm execution time, this paper proposed a path planning method that combines the Northern Goshawk Optimization (NGO) algorithm with the improved rapidly-exploring random tree (RRT*). First, a fitness function with obstacle avoidance and goal orientation was designed to optimize the initial NGO population. Additionally, the adaptive sampling step size of the RRT* algorithm was designed according to the fitness function to improve the search efficiency in a large-scale map. Then, the optimal neighbor node sampling mechanism was designed to simulate behavior of the northern goshawk transmitting information to its nearest companions, while also the RRT* node sampling was constrained by considering the USV’s (unmanned surface vehicle) heading angle. Finally, in order to improve path smoothness, the Metropolis criterion was introduced and the smoothness and minimum rudder angle design fitness function were combined to select a more suitable parent node for dynamic rerouting. The experimental results show that compared with RRT*, Informed-RRT* and RRT*-smart algorithms, the improved algorithm reduces the path length by 19.36%, 3.36% and 5.98%, and decreases the search time by 49.33%, 57.01% and 59.16%, respectively. At the same time, the curvature of the path also decreases significantly.

Aiming at the problem that the ship's course is easily disturbed by the external environment during navigation, an improved non-singular terminal sliding mode control strategy based on Nonlinear Extended State Observer (NESO) is proposed. Firstly, a nonlinear mathematical model of ship's heading system is established, and a nonlinear expanded state observer based on the hyperbolic tangent function is designed to estimate the external disturbances encountered during the ship's navigation and perform feedforward compensation. Then, based on the improved non-singular terminal sliding mode surface and the double power reaching law, an improved non-singular terminal sliding mode control law is designed, which reduces the chattering of the sliding mode control law and improves the convergence speed. The nonlinear extended state observer and the improved non-singular terminal sliding mode control law are combined to form a composite control law. Finally, the stability of the control law is analyzed based on Lyapunov stability theory. Simulation results show the proposed control strategy can effectively improve the tracking performance and robustness of the ship heading control system.

The safety of a mooring system is one of the key performance that require verification for floating offshore platforms. Based on the requirements of lightweight design, precise control of active breaking and avoidance of measurement interference, this paper developed an active broken simulation device that can be used in marine engineering model tests. The semi-submersible floating wind turbine platform was used to carry out an active broken model test of a mooring chain in an extreme environment. The results show that the device can be triggered based on tension threshold judgment and timing, and has stability, accuracy and real-time performance. It can effectively capture the whole process of dynamic response of floating structure and mooring system after mooring failure. After breaking occured near the maximum mooring tension, the tension of the adjacent mooring chain surges, the safety factor decreases sharply, the platform undergoes a long-distance offset, and experiences severe oscillation, and the safety and positioning ability of the mooring system are seriously reduced.

In order to explore the plastic accumulation characteristics of box girders under cyclic hogging and sagging bending moment loads and to study the attenuation law of ultimate strength of box girders based on incremental plastic failure, this paper creatively designed a test device for continuous cyclic bending moment loading, and carried out a series of four-point bending ultimate strength tests on box girders under different loading conditions based on replaceable box girder specimen. Combined with numerical simulation, this study analyzed the variation characteristics of cumulative plasticity critical point, cumulative plastic deformation and neutral axis height of box girders under cyclic loading, and then the influence of constant and variable amplitudes of rotation angle loads on the ultimate strength attenuation of box girders was evaluated. After these investigations, the fluctuating range for the ultimate bearing capacity of ships encountering random extreme waves bending loads was determined to ensure the safety and reliability of ships sailing in severe sea conditions.

Ship structures can fail due to extreme or cyclic loading, corrosion and erosion, so it is necessary to perform structural health monitoring (SHM) on ships to ensure the safety, reliability and integrity of ship structures to avoid major accidents. Displacement reconstruction is one of the main objectives of structural health monitoring. The inverse finite element method (iFEM) is a structural health monitoring method for real-time reconstruction of full-field displacement in plate and shell structures. In the case of shells with large curvature, existing iFEM algorithm will suffer from the warping problem, which affects the fitting accuracy. In this paper, an improved iFEM algorithm is proposed, which does not change the displacement-strain relationship of the element itself. The new algorithm improves the calculation accuracy of the shell element by estblishing tangent plane coordinate system at the element’s Gaussian integration points and improving the element stiffness integration, without increasing the mesh and node quantities. The result of numerical examples shows that the improved inverse shell element can effectively reduce the displacement error after inverse finite element reconstruction.

Large-scale offshore floating platforms belons to special offshore structures. Their bearing pin shaft connection structures have good flexibility and applicability. In order to ensure the safety of the large-scale offshore floating platform connection system during service, it is necessary to analyze its fatigue life. Firstly, the large-scale offshore floating platform connection system with multiple sets of bearing pin shaft was studied. A nonlinear contact analysis method of various contact forms was adopted to investigate the overall strength of the structure under the design working condition load. Then, the fatigue life of the platform connection system was calculated and analyzed before and after the local structural improvement. The results show that, for large-scale offshore floating platform with multiple sets of bearing pinsnajt, the lowest fatigue life appears at the flat steel of the connection between the outermost bearing and the plate-frame structure of the loading end. In addition, the fatigue life could be effectively improved by increasing the overall plate thickness of the outermost bearing base plate-frame at the loading end. The thickness of the plate-frame and flat steel near the opening should also be increased. In this paper, a set of fatigue strength analysis methods for large offshore floating platform connection system with multiple sets of bearing pin shaft is finally formed.

The polar environment is cold, complex, and characterized by a lack of data. Aluminum alloy multi-environment boats navigating in polar regions will encounter four types of interface conditions, including waves, water ice, snow, and ice-water interfaces. Therefore, it is necessary to conduct research on the fatigue damage of aluminum alloy multi-environment boats in low-temperature environments. This paper is based on a time-domain analysis method analyzing the stress response of the boats under four types of interface loads. The rainflow counting method was used to statistically analyze the time history of hot spot stresses, and the final damage and fatigue life were calculated based on the Miner linear cumulative damage theory. On this basis, the damage sensitivity to ship speed, interface height, and ice density was analyzed. The results indicate that wave impacts and water ice loads contribute significantly to hull damage, with ice density being the most significant factor affection damage. Fatigue tests were conducted on low-temperature aluminum alloy materials, welded joints, and actual ship nodes. The S-N curve of low-temperature aluminum alloy was fitted, and the reliability of the S-N curve was verified through fatigue tests of actual ship nodes. The research findings can be utilized for the fatigue assessment of 5059 aluminum alloy boats in low-temperature environments.

Due to the significant differences in material properties between the sandwich panel skins and core, sandwich panels exhibit different vibration characteristics in different frequency bands. This leads to different sound insulation mechanisms and calculation methods in different frequency bands. In this paper, theoretical methods applicable to low-frequency, high-frequency, and full-frequency ranges respectively were adopted to investigate the sound insulation performance and mechanisms of sandwich panels in different frequency bands. The results indicate that the sound insulation of sandwich panels in the low-frequency band is mainly determined by bending vibration mode, while the sound insulation in the high-frequency band is more affected by normal longitudinal waves. In the transition frequency band between the low-frequency and high-frequency bands, the sound insulation is affected by both bending waves and longitudinal waves. The phenomenon of the sound insulation trough caused by bending modes of sandwich skins when the core material behaves as an elastic foundation was analyzed in this paper. The influence of material damping on sound insulation in different frequency bands was studied, and the mechanism of damping weakening the sound insulation trough was analyzed. Finally, the effect of core material parameters and panel areal density on their sound insulation performance were presented.

Accurately identifying the far-field radiation hotspots of structures is of great significance for vibration and sound radiation control. In this paper, an open-source boundary element program is used to obtain the radiation impedance matrix of the cylindrical shell, and the normal vibration velocity of the shell is extracted. Based on the singular value decomposition, the non-negative intensity and radiated sound power of the underwater cylindrical shell are obtained, and the far-field acoustic radiation model is acquired, moreover, the method is verified by finite element software. On this basis, the influence of large stiffener at the stucture, where an excitation force is applied, on non-negative intensity is discussed. Furthermore, the mathematical model of active control is established with the aim of minimizing the acoustic radiation power. The influence of control force on structural sound radiation mode is analyzed, and its influence mechanism is revealed using the non-negative intensity distribution. The results show that the radiation mode of the structural surface must be changed to reduce the radiation acoustic power. And the essence of secondary force controlling structural vibration radiation is to change the structural vibration from strong radiation mode to weak mode, thus reducing the radiation efficiency of the structure.

There still remain many challenging topics for CFD to numerically simulate flow around a bluff body at subcritical Reynolds numbers, such as the high-fidelity resolving and capturing for instability structures in the shear layer, as well as the periodic shrinkage and enlargement of the recirculation region. This paper presents the development of a RANS-based wall-modeled large eddy simulation method (RANS-WMLES) to provide a high-fidelity CFD tool for numerically simulating such complex flow phenomena around a bluff body. Such a new method is different from traditional hybrid RANS/LES models. In particular, for the new method the transition from RANS to LES can be achieved through a filtering parameter which is only related to local grid parameters. Moreover, the transition can be pre-controlled through two customizable parameters and for not only the transition boundary positions between RANS and LES, but also the ability of resolving turbulent kinetic energy. A series of numerical simulations for flow past a sphere at subcritical Reynolds number Re=3700 show that the new method is capable of resolving and capturing with high-fidelity temporally/spatially developed coherent structures for such complex three-dimensional flows around a sphere.