The generation of internal wave wakes by submerged objects in density-stratified environments is closely linked to the navigation speed. This study develops a technique of step-layer injection in a wide-scale density-stratified simulation tank and proposes a high-precision multi-array conductivity detection method. Experimental investigations on the excitation of internal wave wakes by an underwater sphere driven by cyclic towing were conducted. Using probability density statistics, root mean square analysis of wave amplitudes, and other analytical methods, this research delved into key issues such as spatiotemporal probability distribution density of Froude number correlated with internal wave, transition zone delineation, and vertical displacement field characteristics, etc. The findings demonstrate that the experimental system and techniques employed can accurately capture the fluctuation information within the stratified flow field and precisely determine the relationship between the characteristics of internal wave wakes and the Froude number. The probability distribution density of the internal-wave-correlated Froude number reveals the transition process of internal wave wakes and clearly identifies the transition zone as approximately 1.6 ⩽ Fr < 3.3. It is discerned that, after the transition, the dominant internal wave correlated Froude numbers within the wake wave system approximately fall within the ranges of [0.3, 0.4] and [2, 2.8] for the inner and outer layers, respectively. Additionally, Lee wave correlated velocity can still be detected in the outer layer region.

Vortex-induced vibration (VIV) of a marine riser is a great threat to its service safety. From the perspective of energy, the hydrodynamic force on the riser undergoing vortex-induced vibration was divided into three components, i.e. vortex-induced force acting as energy input, drag force acting as energy dissipation and added mass force acting a neutral role in energy. The energy competition between the first two components determines the final energy effect of the fluid on the structure. Furthermore, the identification method of hydrodynamic coefficients based on the flexible riser model experiment was derived for the new hydrodynamic force model in detail. Through the towing experiment of the flexible riser, the vortex-induced vibration response, and coefficients distribution characteristics under different flow velocities were identified. The results show that the vortex induced vibration response of the flexible riser under uniform flow has multi-mode participation characteristic, which leads to the "jump" phenomenon of hydrodynamic coefficients. The vortex-induced force coefficients and drag coefficients behave significant correlation with the amplitude of VIV. Based on the measured values between hydrodynamic coefficients and response amplitude, an empirical model for hydrodynamic coefficients under energy competition force model was preliminarily established. The research in this paper provides a valuable reference for the development of fast empirical prediction methods of marine risers in the future.

Aiming at the practical application of I-core metal sandwich structures in the main hull section, the four-point bending ultimate bearing capacity test of a I-core metal sandwich composite cabin model was carried out. The failure mode and ultimate bending moment of the I-core metal sandwich composite cabin model were obtained by test, and the test results were in good agreement with the nonlinear finite element calculation results. Test results show that the failure of I-core metal sandwich structure is dominated by the overall buckling while the local buckling is secondary. Meanwhile, I-core metal sandwich structure has a high load bearing capacity and can replace the deck and side stiffened plates in the hull structure. In addition, the metal sandwich composite cabin model is successfully manufactured, which verifies the feasibility of the application of metal sandwich structure in hull structure.

The extreme polar marine environment is harsh and the ice conditions are complex, requiring sufficient structural strength for navigation safety of ships. Existing design specifications and monitoring guidelines for polar ships senerally only consider the ultimate strength within the elastic phase of material under single loading conditions, resulting in overly conservative strength design. The ultimate bearing capacity of polar ship structures were analyzed based on the Combined Theory of Strength and Stability (CTSS) and plastic failure models, and a calculation formula for the ultimate strength of typical structures was derived in ship-ice collision areas under plastic deformation conditions. By using the finite element analysis method, a numerical analysis model for the ultimate strength of structures was established. By comparing the theoretical and numerical results, it was found that the theoretical calculation formula is highly accurate and can be used as a quick verification method for the ultimate bearing capacity of polar ship structures. Besides, with reference to the design requirements of specifications, the article provides recommendations for the arrangement of rib space in the ice belt areas of various ice-class ships based on the theory of ultimate strength, providing reference for the design of the ultimate strength of polar ship structures.

The traditional low-order finite element model is usually used to obtain the acoustic scattering field of a submarine structure, and then to evaluate the acoustic stealth performance. However, the traditional finite element method is affected by the numerical pollution effect, and requires very dense mesh to obtain reliable numerical solutions for problems with relatively medium and high frequencies, leading to prohibitive cost in mesh division. In this paper, the overlapping finite element method (OFEM) and Dirichlet-to-Neumann (DtN) mapping technique are combined to construct a coupled numerical model for the acoustic scattering of underwater elastic targets. When constructing local approximations in the OFEM, the virtual nodes are used to generate partition of unity functions, while no degrees of freedom are assigned to these virtual nodes. The novel OFEM can be directly applied to low-order finite element models and achieve higher-order approximations of the unknown variables. Numerical examples show that the OFEM can reduce the numerical error significantly and has broad application prospects in the prediction of underwater acoustic scattering by elastic targets.

Aiming at the problem of predicting the sound radiation characteristics of cylindrical shells with internal substructures, this paper carried out theoretical and experimental research on the sound radiation characteristics of cylindrical shells with internal substructures. In theory, a hybrid calculation method based on the combination of condensed transfer function method, direct stiffness method and precise transfer matrix method was proposed, which can calculate the sound radiation characteristics of cylindrical shells with internal substructures. In the experiment, the linear excitation method was used to obtain the vibration response at each measuring point and the sound pressure at the underwater reference point, which were compared with the analytical calculation results. The analytical calculation results are in good agreement with the experimental test results.

High-speed vessels face significant challenges in optimizing bow structures under slamming loads due to uncertainties in load magnitude and spatial distribution. This paper proposes a multi-stage topology optimization method integrating load uncertainty analysis and manufacturing constraints to balance lightweight design and engineering feasibility. Firstly, the uncertain loads are converted into multi-scenario worst-case loading problems. Through an iterative "critical load scenario-topology optimization" process, the critical load positions are dynamically updated. Then, a topology optimization strategy based on the Solid Isotropic Material with Penalization (SIMP) method is employed, incorporating geometric/manufacturing constraints to progressively derive an optimal stiffener layout that meets strength and stiffness requirements. Each iteration retains prior design outcomes and updates worst-case load scenarios to achieve progressive adaptation to uncertain loads. Finally, multiple iterations and geometric reconstruction convert high-density element clusters into manufacturable stiffener configurations. Finite element verification demonstrates that the optimized bow structure exhibits significantly reduced maximum displacement, more uniform multi-scenario responses, and compliance with lightweight and safety requirements. This method effectively addresses the computational burden of double-layer nested optimization, offering a novel approach for structural optimization of high-speed vessel bows under stochastic slamming loads.

The motion of ships and marine structures is a nonlinear motion with time series characteristics. The Long Short-Term Memory (LSTM) artificial neural network has the characteristics of memorizing time interval information and processing nonlinear data, which is very suitable for processing such nonlinear motion with time series characteristics. Therefore, LSTM has significant advantages in predicting the very short-term motion response of ships. In this paper, an improved LSTM method for the prediction of very short-term motion response of ships is proposed. This method converts the prediction of ship motion into the prediction of peak and valley values by means of extracting envelopes, which can reduce the data demand of the traditional LSTM model and simplify the complexity of the prediction curve, thereby significantly improving the forecast duration. In this paper, the improved LSTM was used to predict the regular wave curve, irregular wave curve and real ship motion curve. The results show that the improved LSTM prediction method can enlarge the maximum forecast duration of the traditional LSTM model from 6~8 s to about 20 s, and has ideal prediction results for special signals such as abrupt signals, which has high practical value.

An analytical model of pipe torsional stiffness in clockwise and counterclockwise directions is derived based on the helical winding structural characteristics of steel wires in the unbonded flexible pipe armour layers considering radial contraction and expansion phenomena. Taking a typical unbonded flexible pipe as an example, the bi-directional torsional stiffness analysis is conducted. The results show that the analytical model has a close match with the results of existing numerical model. It can be found that the error is 3.5% in clockwise torsion, and the error in counterclockwise torsion is 4.6%. This paper can provide a useful reference for the design and analysis of the torsional performance of flexible pipes.

Particle damping vibration absorber is a dynamic vibration absorber that uses a mass block containing several particles as the mass element. It has the effect of broadening the effective frequency band of dynamic vibration absorbers and suppressing the secondary line spectrum of dynamic vibration absorbers. However, dynamic vibration absorber control techniques are sensitive to parameter selection, and the vibration equivalent mass of particle damping vibration absorbers changes with variations in excitation amplitude, affecting their vibration absorption performance and practical engineering applications. The method of building a 3D network into the mass block was proposed to improve the above defects. The influence of adding obstacle network into the mass block of particle damping vibration absorber was studied by experiment and simulation in this paper. The results show that when the vibration intensity of the particle damping vibration absorber is high, a part of the particles in the mass block enter a suspended flow state, leading to these particles not participating in the vibration process. As a result, equivalent mass of particle damping vibration absorber is changed, and the vibration absorption frequency has shifted. And the 3D network can keep the vibrational equivalent mass stable by breaking up the suspended flow state of the particles. And the vibration reduction effect is effectively improved on the specific vibration amplitude.