The deep-sea pressure structure gradually presents the characteristics of a medium-thick shell, and their structural stress characteristics are significantly different from those of a traditional thin shell. Taking a deep-sea unmanned system head cover reinforced with rectangular ribs as the object, the strength of medium-thick shells was studied based on the three dimensional stress analysis method of ring-stiffened cylindrical shells. Firstly, the correctness of the calculation of the three dimensional stress method on the cylindrical shell was tested by examples in other literatures, then the typical stress calculation of the main structure of head cover and the stress characteristics of the medium-thick shell were studied, and finally the stress level and distribution of the shell and rib in the typical area of the head cover were calculated based on the assumption of the equivalent rib spacing. Compared with the finite element simulation calculation and model test results, the three dimensional stress analysis method has good applicability and engineering accuracy for the stress calculation of medium-thick shell pressure structures, which can provide a reference for the strength analysis of deep-sea pressure structures and the theoretical research of medium-thick shells.

In order to improve the anti-explosion protection performance of ships and explore the damage mechanism of marine sandwich structure under impact load, the close-range air explosion test of U-shaped folded sandwich plate was designed and carried out. The deformation and failure mode of the sandwich plate under shock wave load were analyzed, and the finite element simulation analysis was carried out. On this basis, the damage characteristics of sandwich plates under the combined action of shock wave and fragments were studied by prefabricating fragments at the bottom of explosives. The influence of panel thickness and detonation distance on the anti-explosion performance of sandwich plates was discussed from three aspects: panel break size, vertical displacement and energy absorption. The results show that the failure modes of the upper panel of the sandwich plate under the combined action of shock wave and fragment are large deflection, boundary tearing, plug break and scattered perforation. The bottom panel is dominated by large deflection and pits, and the shape of the upper panel break changes from' butterfly' to' circular' with the increase of the thickness of the upper panel. At this time, the upper panel and the core layer are the main energy absorbing components. The deformation of the central region of the sandwich plate is more sensitive to the decrease of the detonation distance, while the deformation of the edge region has no significant change. The total energy absorption of the sandwich plate, the energy absorption and proportion of the lower panel and the core layer increase with the decrease of the thickness of the upper panel. The change of the thickness of the lower panel and the core layer has little effect on the overall energy absorption of the sandwich plate and the proportion of each part.

Welding residual stress has a significant impact on the fatigue life of a welding structure. Meanwhile, welding residual stress is not invariable, but will be redistributed with crack propagation. Therefore, the coupling study of welding residual stress redistribution and crack propagation is very important to predict the fatigue life of a welded structure accurately. Based on thermal elastic-plastic finite element method and extended finite element method (XFEM), a fatigue life analysis method considering the coupling of residual stress redistribution and crack propagation was proposed in this paper. Taking the tensile fatigue sample of TC4 titanium alloy as an example, the redistribution of the welding residual stress along with crack propagation was studied with the extended finite element method. The welding residual stress distribution in front of the crack tip during crack propagation and the fatigue crack propagation a-N curve under the redistribution of weld residual stress were calculated by cyclic iteration. The calculation results show that the welding residual stress at the crack tip increases firstly and then decreases with the crack propagation. Compared with the results based on constant value of the residual stress, the extended finite element fatigue life analysis method considering the redistribution of the residual stress is more accurate to predict the fatigue lives of welding structures.

In order to explore the lubrication characteristics of the main bearing under heavy load and its interaction mechanism with wear, based on the micro-contact and fatigue damage mechanism of micro-convex body, the wear model of bearing material was constructed, and combined with the mixed thermal elastohydrodynamic lubrication theory, the coupling analysis model of mixed thermal elastohydrodynamic lubrication and wear of the main bearing was established, based on which, the wear distribution and evolution law of the main bearing under specific load at the time of ignition were investigated, and the effects of wear on lubrication characteristic parameters such as oil film pressure, oil film thickness and rough contact pressure were discussed. The effects of load, radius clearance, micro-convex friction coefficient and rotational speed on the wear characteristics of the main bearing were obtained. The results show that with the increase of wear time, the wear area increases from the center of the bearing zone along the circumferential direction of the bearing, the contact pressure decreases and the minimum oil film thickness increases, and that the increase of radius clearance and micro-convex friction coefficient and the decrease of rotational speed will aggravate the wear of bearing bush.

The exciting forces generated by the propeller during ship operation will cause the vibration of propulsion shafting, and the shafting vibration will also have feedback to the propeller, causing complex spatial motion. There is a two-way fluid-structure coupling problem in the propeller-shafting system. To investigate this complex dynamic problem, the numerical model for simulating the coupling of shafting multi-degree-of-freedom vibration and propeller viscous flow field was established in this paper, based on the finite element (FEM) and computational fluid dynamics (CFD) methods. The iterative solution of the two-way coupling was realized by carrying out secondary development in the flow field solver, and several numerical examples were simulated to study the variation characteristics of fluid exciting and vibration response. The results show that, the simulation method proposed in this paper has practical values, and the convergence and accuracy of the two-way fluid-structure coupling simulation can meet the engineering requirements. The computing speed for solving the coupling system of this method is fast, and no additional computing resources are required. The two-way coupling effect and unbalanced mass have significant influence on the amplitudes at rotating frequency while the non-uniform inflows have significant influence on the amplitudes at blade passing frequency.

A shallow water interface interference separation and sound field reconstruction method based on the equivalent source method was proposed. The combination of the ray acoustic virtual source method with the equivalent source method can effectively separate the impact of interface reflected sound in shallow water conditions, and accurately reconstruct the radiated sound field of structural sound sources. The virtual source method model was briefly described in this paper, the basic principle and algorithm implementation process of the interference stripping equivalent source method were given, the impact of different frequencies and holographic surface configurations on the algorithm was illustrated through simulation, and the advantages and applicability of this method compared to conventional equivalent source methods were summarized.

The accurate measurement of wave evolution is fundamental for the hydrodynamic studies of marine structures. An image-based method was proposed to measure waves in the laboratory based on binocular stereo vision. The light foam particles were used as trackers to mark wave surface for obtaining the wave images with sufficient textures. The images were rectified based on epipolar constraint, and threshold segmentation was used to avoid the interference of uneven light and bottom reflection. A stereo matching algorithm with GPU acceleration was used to achieve the real-time matching of wave images, obtaining the point cloud and mesh of the wave surface. Through the reconstruction of the image sequences, the wave time series, wave heights and periods were extracted. They were then compared with the measurements by wave probes, showing great agreements. The results indicate that the proposed method can accurately measure the 3D wave field in the laboratory, capturing the instantaneous evolution of wave and providing real-time monitoring.

Sail-assisted navigation is one of the effective ways to reduce carbon dioxide emissions in ship industry. In this study, a sail array consisting of six hard sails with a cross section of NACA0018 airfoil was designed. The lift and resistance of each sail were analyzed by numerical simulation under three conditions: upwind, crosswind and downwind. By comparing with the single sail case, the law of inter-sail interference on sail propulsion performance was explored. The confidence level of several independent geometric parameters on the propulsion performance of the sail array was evaluated by combining the experimental design method with ANOVA. The results show that the significance level of the independent parameters is highly dependent on the apparent wind direction. By changing sail layout and using non-uniform angle of attack, the propulsion performance can be better than that of single attack angle. This study is helpful to find the main parameters affecting the thrust performance of a multi-sail propulsion system and support for its global manipulation.

In order to study the propeller cavitation and induced pressure fluctuation in non-uniform wake, a multi-field synchronous measurement system was used to carry out tests in a cavitation tunnel for a highly skewed propeller (HSP) and explore the influence of advance coefficient and cavitation number on the propeller cavitation performance and fluctuating pressure characteristics. The results show that the propeller cavitation appears in the test mainly in three types: back cavitation, face cavitation and tip vortex cavitation. Low-frequency pressure fluctuation is highly correlated to the cavity volume oscillation. Under non-cavitation condition, the first blade passing frequency is the main component of pressure fluctuation while the amplitude of higher blade passing frequency component can be ignored by comparing the contributions of each blade passing frequency component of the whole pressure fluctuation. Under cavitation condition, the unsteady cavitation has a great impact on the pressure fluctuation. The higher blade passing frequency components can be obviously observed, and the amplitude of main frequency pressure fluctuation increases significantly.The variation of pressure will become more observable with the increase of intensity of cavitation evolution process. The higher order blade frequency component of pulsating pressure increases obviously when tip vortex cavitation is present.

Propeller open water tests, hull resistance tests and self-propulsion tests were carried out on two ship models propelled by submerged waterjet and conventional propeller respectively. Detailed analysis was done on the variations of propulsive performances of the submerged waterjet, propeller as well as corresponding hull-propulsor systems. Results show that: (1) the propulsive efficiency of submerged waterjet in open water is equivalent to that of the propeller in model scale, with the maximum efficiency around 0.72, while the high-efficiency operation range of submerged waterjet is wider than that of propeller in open water; (2) compared with propeller ship, the simpler stern-appendage arrangement of submerged waterjet ship can effectively reduce additional hull resistance caused by stern appendages; (3) at Fr = 0.20 and Fr = 0.26, the overall propulsive efficiency of submerged waterjet ship model in self-propulsion is 64.7% and 66.1%, which is 1.2% and 5.3% higher than that of propeller ship respectively. The corresponding absorbed power of the submerged waterjet is 9.41% and 15.4% lower than that of the propeller at two speeds respectively. Generally, the submerged waterjet propelled ship shows a better performance of propulsion and energy saving. This study provides a meaningful reference for the integrated design and optimization of submerged waterjet and its corresponding hull geometry.