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.

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 performance of Autonomous Underwater Vehicles (AUVs) is significantly influenced by their shape design. This study presents a novel AUV shape optimization method that integrates data-driven approaches and machine learning technique to focus on the impact of the head and tail profiles, and the configuration of the fins and rudders on AUV performance. A parameterized hydrodynamic analysis workflow was developed to automatically generate the hydrodynamic parameters required during the optimization process. Based on this workflow, a data-driven framework was constructed for multi-objective optimization of AUV shapes, with the aims of minimizing drag and maximizing maneuverability. Surrogate models for the two optimization objectives above were built using Multilayer Perceptron (MLP) neural networks and ensemble learning methods respectively, and their performances were compared with traditional surrogate models. The optimization problem was solved using the Non-dominated Sorting Genetic Algorithm II. Comparative analysis of the initial and optimized AUV shapes demonstrates significant improvements in hydrodynamic performance, confirming the feasibility and effectiveness of the proposed method.

The identified nonparametric model was used to tune the control parameters of the ship motion. Local Gaussian Process Regression (LGPR) method was utilized to model and predict the ship motion based on the collected ship motion data. The tuning of control parameters was converted to an optimization problem. The fitness function was designed to improve the control performances, and Genetic Algorithm (GA) was applied to tune the control parameters. The simulation results demonstrate that the proposed method has satisfactory control accuracy and dynamic performances in the path following and obstacle avoidance control.

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 investigate the hydrodynamic characteristics of submarines moving in ice regions, a mathematical model and corresponding numerical method, considering the coupling interaction of submarine, water and ice, were established based on RANS equations for level ice and brash ice conditions. The ice sheet was regarded as a no-slip wall boundary with a tangential velocity equal to the incoming flow speed, while the discrete element method incorporated with a linear elastic model was employed to simulate the motion and collision of brash ice. The SUBOFF submarine model was selected as the research object. The influence of sea ice on the resistance was numerically investigated first after mesh convergence analysis. Then further calculations were conducted on resistance components for different submerged depths and speeds under various ice conditions. And wave patterns of the SUBOFF were analyzed for free surface and brash ice condition. Numerical results show that the sea ice predominantly affects the pressure resistance, especially the wave-making component, while there are little differences in frictional resistances for different submerged depths and ice conditions. As the submarine moves under level ice, a significant decrease in pressure resistance and total resistance is observed owing to the lack of wave disturbance. In contrast, when navigating in water covered by brash ice, waves are generated. However, the random and uncertain collisions of floating ice lead to strong fluctuations in the pressure resistance and total resistance curves. And the corresponding average resistance values are slightly smaller than those in open sea due to the wave attenuation induced by brash ice. Furthermore, the submerged depth and forward speed are found to have a significant impact on the hydrodynamic characteristics of the submarine sailing in ice regions. With the increase of submerged depth, both pressure and total resistances will gradually decrease and tend to be consistent. Generally, the increase of forward speed will lead to larger resistance. However, when the submarine navigates beneath the open sea or brash ice, the pressure resistance increases at first and then decreases slightly, which makes the total resistance to grow at a slower rate.

In order to investigate the effect of fillets at the leading and trailing edges of flow holes on the noise characteristics of submerged body, a submerged body with a single flow hole was taken as the research object. Based on the large eddy simulation turbulence model and FW-H (Ffowcs Williams-Hawkings) acoustic model, numerical simulations were carried out using STAR CCM+ software. After verifying that the simulation accuracy meets the requirements, the influence of different fillet positions and radii of the orifice on flow noise was then deeply explored at a flow rate of 10 m/s. The results show that the treatment of fillets at the leading and trailing edges of the flow hole can reduce the flow noise of the flow hole, especially the second-order line-spectrum noise. Regarding fillet position, the noise reduction amplitude by the fillets at the trailing edges is greater than that at the leading edge, and the noise reduction amplitude at the outer rounded corners is greater than that at the inner edge. In the studied operating conditions, when the outer edges of the leading and trailing edges of the orifice are simultaneously rounded with a radius of 10 mm, the minimum second-order line-spectrum noise is 94.03 dB, which is 9.48 dB lower than that in the reference operating condition, and the corresponding frequency is reduced by 4.72 Hz. The far-field radiation noise of the orifice is mainly affected by the pressure pulsation on the back wall and bottom of the orifice, and the influence of the orifice shear layer oscillation is relatively small. The research conclusions provide guidance for the optimization of submerged body flow hole structure and the low-noise design.

In order to study the effect of different stress ratios on the fatigue crack growth of HTS-A steel in low temperature environment, low-temperature fatigue crack growth tests of HTS-A steel CT specimens at stress ratios of 0.1 and 0.3 were carried out. The test results show that with decreasing temperature, the crack growth rate decreases and the fatigue life increases. At the same time, with the increase of the stress ratio, the fatigue crack growth rate also increases accordingly. However, with the decrease of temperature, the effect of stress ratio on fatigue crack growth rate becomes smaller and smaller. On the basis of experiments, an improved McEvily model considering the effects of temperature and stress ratio was proposed in this paper. The model can predict the fatigue crack growth rate of HTS-A steel under different low temperatures and different stress ratios. The predicting results were compared with the experimental data. This prediction model lays a foundation for the fatigue life assessment of marine equipment in low temperature environment.

In the process of oil and gas development, semi-submersible platforms operate relocation between wells by retracting and releasing mooring chains. In this paper, a fast well relocating optimization method of semi-submersible platform based on greedy algorithm was proposed in view of the traditional method that cannot obtain relocation strategy quickly. Three sets of relocation operations were selected in this study, an optimization analysis of relocation strategy was carried out for windlasses normal operation and windlasses failure operation. The results show that under normal operation, the optimized strategy has a significant improvement in terms of optimization effect and calculation cost compared with the original strategy and the best strategy. Under windlasses failure operation, it shows that the feasibility of relocation is closely related to the distance and direction of the relocation operation. A comparison of relocation strategy was made between the normal operation and the windlasses failure operation, it was found that the retracted length of the mooring chains and the number of operating steps increased under windlasses failure operation, and the stability keeps changing with different working conditions.