Abrupt annular shear flows induced by internal solitary waves will cause submersibles to produce a large depth drop and pitch motion, threatening the safety and operation performance of submersibles. Based on the velocity inlet wave generation method and overset grid technique of CFD, the motion response prediction method of submersibles in internal solitary wave was established. With the prediction results compared with the experimental results, it is proved that the method can accurately simulate the large amplitude motion of submersibles in internal solitary waves. Through the simulation analysis of motion process of unpowered floating submersibles at the stratified interface under the action of internal solitary waves, the mechanism of depth drop and pitch motion of submersibles in internal solitary waves was revealed, and the effects of stratified thickness ratio, wave amplitude and depth of navigation on the longitudinal motion of unpowered floating submersibles were investigated. The results show that the amplitude of depth drop and pitch of the submersibles in internal solitary waves were determined by the wave amplitude and the maximum slope of wave surface; The factors such as amplitude, stratified thickness ratio and depth of navigation have great influence on the longitudinal motion property of submersibles.

The floating cylindrical structures, such as the floating frame of the fish farming and the floating bodies of wave energy convertor and so on, are generally semi-submerged. However, the investigation on the hydrodynamics on such floating cylinders are few. This status makes the design and analysis of the relevant new structures face the essential problems of the force input uncertainty. The hydrodynamic forces on a semi-submerged cylinder were experimentally investigated under the combined influence of steady and oscillatory flow. To simulate the equivalent combined flow, the cylinder towed by the carriage was forced to oscillate with different amplitudes and periods. A significant mean downward lift force was observed for the semi-submerged cylinders, which was obviously larger than that of the mean drag force. Moreover, the relationship between the hydrodynamic coefficients and multi-sensitive parameters was revealed. It is confirmed that the KC number has a weak influence on the coefficients. The Froude number is seen to be the most important parameter affecting the hydrodynamic coefficients. The empirical models for predicting hydrodynamic coefficients in combined flow are initially developed. This work can provide useful references for the design of the related marine structures and serve as the valuable guideline for future research.

Ships are susceptible to wind and waves causing the declines of installation accuracy and maintenance safety of offshore wind turbines. The most seriously affected case is the non-stationary ship roll motion under long-peaked random wave spectrum. To ensure the stability of offshore operations under complex sea conditions, it is necessary to improve the generalization of the prediction model. In this paper, a preferential feature federation method was proposed. Firstly, the non-stationary ship roll motion was decomposed into multi-component stationary sequences by using the variable modal decomposition method. Then, the long and short-term memory neural network with attention mechanism was used to build a local multi-dimensional multi-step prediction model with error correction. Finally, in order to improve the prediction effect of new type ship motions in complex sea conditions, a federation method was used to combine some ship motion data holders for best model parameters, which were selected with the maximum mean discrepancy method with high similarity for preferential feature federated training. The experimental results show that the federated model has higher prediction accuracy and better generalization ability, which can help the stability control of wave compensation during offshore wind turbines installation.

With the continuous development of large-scale offshore floating structures, the rain load of large-scale offshore floating structures in extreme environment has become one of the focus issues of designers. Based on the discrete particle model, the motion prediction of wind driven rain field was made and the distribution law of spatial characteristic parameters of rain field was quantified. The calculation formula of structural rain load was deduced, and the rain load prediction of offshore platform under different wind speed and rainfall intensity combination conditions was completed. The research results show that the rain load of offshore platforms under the action of DNV wind profile and Davenport wind spectrum follows Gamma distribution. Compared to the average wind load, when the rainfall intensity R is 800 mm/h, the significant value of rain load accounts for 5.07%, and the maximum value accounts for 8.87%; when the rainfall intensity R is 20 mm/h, the significant value of rain load accounts for 0.36%, and the maximum value accounts for 0.6%. The research results of this paper can provide guidance for the design of offshore structures and their mooring systems.

For floating structures deployed in waters near the coast or island lagoons, the shallow water depth makes the impact of seabed topography changes on the mooring system non-negligible, and the seabed can no longer be simplified as flat when exploring the characteristics of the mooring system. To study the static characteristics of the anchor chain under the condition of uneven seabed terrain, an anchor chain model was established based on the lumped mass method, and effects of the seabed inclination angle and the arrangement of the anchor chain on the tension and the tension angle at the top of the anchor chain, and the length of the catenary were discussed through numerical simulation, in an attempt to guide the design and safety performance evaluation of floating mooring system in shallow waters.

To solve the path tracking problem of ROV deep-sea mining equipment, where a Mining Robot (MRT) is towed by a Remote Operated Vehicle (ROV), the motion model of a simplified ROV deep-sea mining equipment was established first. Then, a path tracking algorithm based on Linear Model Predictive Control (LMPC) and Nonlinear Model Predictive Control (NMPC) was proposed. Different from the traditional Model Predictive Control (MPC), the proposed Double Model Predictive Control (DMPC) algorithm consists of two parts: (1) the LMPC controller of MRT for calculation of the speed control law of MRT, which is used to quickly converge the tracking error of MRT; (2) the NMPC controller of the ROV for calculation of the control input of the ROV, which is used to follow the speed control law of the MRT. In the design of DMPC, constraints of state quantity and control quantity were considered effectively. In order to ensure the smoothness of ROV control input, the incremental control quantity constraint of ROV was introduced. Finally, simulation experiments were designed to verify the path tracking performance of MRT. Numerical simulation results demonstrate the effectiveness of the proposed algorithm.

In this paper, the open-source computational fluid dynamics software OpenFOAM was used to simulate the effect of plunging breaker waves and non-breaking waves on the typical column of an offshore platform in the extreme ocean environment. The purpose was to compare and analyze the variation of wave loads and structural stress in the column under the above two wave types. The motion of the fluids was simulated based on the Reynolds-averaged Navier-Stokes equations combined with k-ω SST turbulence model. The numerical results show that the breaking wave load on the column is much higher than the non-breaking wave load under the same design wave parameter input. The maximum stress values for each structure of the column under non-breaking wave loads are less than the yield limit of material. However, under the plunging breaker load, the maximum stress of some structures exceeds the yield limit of materials and cannot meet the requirements of structural safety. Therefore, the impact of breaking wave load on structural strength should be considered in the safety design of marine structures to ensure that the structure has sufficient safety margins.

Aiming at the difficulty in modelling the diving process of spherical shells, a data-driven algorithm for diving process modelling and anomaly detection of deep-sea pressurized spherical shells was proposed in this paper. Firstly, the spherical shell structures and historical diving data of manned capsules were analyzed. Then, the diving process modelling algorithm was established based on the long short-term memory network (LSTM), taking the diving depth as the input and the key hot spot strain as the output. The deduction results were analyzed and compared with the DNN model and BP model, the derivation error was reduced by 35.89% and 63.80%, respectively. Finally, based on the LSTM model, a data anomaly detection algorithm was proposed. The proposed algorithm can diagnose and correct abnormal data when a sensor fails.

Aluminum alloys have been widely used in the ship industry for their low density, high specific strength and specific stiffness. However, compared with steel, the strength of aluminum alloy is low. To improve its strength, JDA1b aluminum alloy, which is often used in die casting and outfitting parts of ships, was taken as the research object. Pre-strain tensile, pre-bend tests and dynamic tensile tests were carried out to investigate the effects of pre-strain and pre-bend on the mechanical performance of JDA1b alloy, and the effects of strain rate on the flow behaviors of the alloy were also investigated. The results show that increasing the strain rate can increase the flow stress and tensile strength of the material; the yield stress and tensile strength of this material increase with the increase of pre-strain, and the elongation decreases, and that the increase of pre-bend is favorable to improve the bending yield load, bending stiffness and damage displacement of the material. A new dynamic constitutive model considering ultimate stress and critical strain was proposed, which allows the flow stress to be predicted more precisely at various strain rates over a wide range of 1~800/s, and the fitting goodness-of-fit value reached 0.999. This study provides methods to improve the strength of aluminum alloys, and the proposed constitutive model is conducive to the improvement of the accuracy of the simulation of mechanical performance of parts.

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.