In order to accurately calculate the stress intensity factor at the deepest point during the crack propagation process of high-pressure vessels, a method was proposed for fitting the stress data with high-order polynomials and then calculating the stress intensity factor. Taking the crack at the opening of a high-pressure vessel as an example, based on stress data collected with varying data volumes, polynomial fitting of varying degrees was employed to calculate the stress intensity factor at the deepest point of each crack depth. The influence of polynomial degree and data collection volume on the calculation results was analyzed, and the calculation results of this method were compared and validated against the linear interpolation method in the literature. The research results indicate that as the increase of polynomial degree, the characterization accuracy of the fitted curve improves, and the calculation results gradually converge and stablize. The relative error between the calculation results using low-order (third-order) and high-order polynomial fitting shows an "inverted S" trend, with a minimum relative error of about −20%; As the amount of data increases, the calculation results gradually converge, and the relative error of the calculation results under lower and higher data volumes shows a trend of oscillation attenuation, with a maximum relative error of about 7.1%; The calculation results based on high-order polynomial fitting and piecewise linear interpolation are basically consistent, indicating that this method has certain reliability and is suitable for calculating the stress intensity factor at the deepest point during crack propagation.

The study of probability characteristics of ice loads can offer more precise input loads for the assessment of ship structural safety, taking into account the uncertainties involved in the interaction between polar ships and sea ice. By integrating the ice load estimation formula with a statistical characteristic analysis method, a Monte Carlo-based approach was proposed to investigate the probabilistic characteristics of ice loads on polar ships. According to the typical sea area and route, the scenarios of interaction between the midship and bow of the vessel with sea ice were considered separately. Based on the study of the probability distribution types and statistical parameters of the relevant variables, the ice loads at different positions was calculated by the method presented in this paper, and the corresponding probability density function was obtained. It can be found that the probability characteristics of ice force in the midship can be accurately modeled by a normal distribution, and the Weibull distribution can be used to describe the probability characteristics of both horizontal and vertical ice forces at the bow. Moreover, the horizontal force exerted on the ship due to ice crushing is significantly greater than that resulting from its bending failure.

In view of the fact that it is difficult to obtain analytical solutions for vibration problems of combined shells and it is hard to solve strongly coupled acoustic and vibration control equations, a Ritz-Legendre spectral element method was proposed to discuss the vibration characteristics of underwater conical-cylindrical-spherical shells. Based on Reissner shell theory, virtual spring technology and the displacement angle relationship of adjacent subshells, the theoretical structural model of the combined shells was established. The Legendre spectral element method was introduced to avoid the problem of discontinuity of normal derivative and discretize the Kirchhoff-Helmholtz boundary integral equation, then the theoretical model of underwater external sound field was constructed. Based on Fourier transform and coupled surface Euler equation, the coupled vibration control equation of underwater combined shells was obtained. Compared with FEM simulation results, the convergence, reliability and correctness of this method were verified. This method can provide theoretical reference for engineering application in the design stage.

Numerical simulations of a ship in cavitation flow before and after the installation of a stern flap were conducted for the DTMB5415 benchmark ship model, considering factors such as ship speed, propeller speed, and ship wake. The results show that owing to the dual effects of change of propeller inflow and decrease in propeller speed, the cavitation area and cavitation start-stop angle of the propeller increase after the installation of a stern flap. The blade frequency amplitude of the propeller excitation force/torque decreased by an average of 9% at Fr=0.28 with the stern flap installed. Owing to the negative impact of the propeller cavitation, the corresponding reduction was only 2.5% at Fr=0.413. The hull surface fluctuating pressure was mainly affected by a decrease in the propeller speed after the installation of a stern flap. The average reductions in the blade frequency amplitude at low and high speeds were 17% and 12%, respectively.

In order to study the thrust deduction of waterjet propelled high-speed amphibious platform, the self-propulsion flow field of the platform was solved, based on RANS equations and VOF model. The trim and heave motion of the platform were calculated by adopting overlapping grid method, and the effect of waterjet pump was simplified using body force method to realize the numerical simulation of self-propulsion of waterjet propelled high-speed amphibious platform. The inlet surface of the propeller was obtained by streamline tracing method, and the total thrust of the propeller was calculated by momentum flux method. The results show that the thrust deduction fraction of amphibious platform exhibits different characteristics at different speeds. At low speed, the thrust deduction fraction is positive. Negative thrust deduction occurs at medium and high speeds. In the whole speed range, the resistance increment is always positive and the jet thrust deduction fraction is always negative. The reason for the negative thrust deduction at medium and high speeds is that the resistance increment decreases gradually with the increase of speed and approaches zero.

Uniaxial compressive strengths tests were carried out in the field and in the low-temperature laboratory to investigate the mechanical properties of granular sea ice, with a strain rates ranging from 10⁻⁵ s⁻¹ to 10⁻² s⁻¹. The test temperatures were set at −3 ℃, −5 ℃, −7 ℃, −10 ℃, and −15 ℃, respectively. The loading direction was parallel to the ice surface. The test results show that the uniaxial compressive strength of sea ice increases with the strain rate in the ductile zone, decreases with the increase of the strain rate in the brittle zone, and reaches its peak in the ductile-brittle transition zone. Comparing the ice temperature-peak strength curve with historical data, it is found that the peak of compressive strength of granular sea ice in Bohai is relatively low, and increases with the decrease of ice temperature, but its upward trend gradually slows down, which reflects the influence of sea ice crystal structure on ice mechanical properties. The sea ice porosity was introduced to establish the statistical relationship between sea ice uniaxial compressive strength and strain rate, as well as porosity, across a wide strain-rate range. The feasibility of a unified mathematical description for mechanical properties of Bohai Sea ice and polar sea ice was discussed.

Vibration comfort is one of the key technical indicators to evaluate the passenger experience of large cruise ships. Due to the high superstructure and the large difference from ordinary ship types, there is no mature approximate calculation method for global vibration of cruise ships. According to the stiffness and mass distribution of a large cruise ship, this paper proposes a design method for the steel global vibration test model, and designs two schemes: the equiscale model and the non-equiscale abnormal model. Then the abnormal model is used to complete the modal test in the air and the pool, and the test results of the dry/wet modes under the loading and unloading condition are given. The results show that the ratio of the natural frequencies of the wet mode to the corresponding dry mode is basically unchanged under the loading or unloading condition. For the same order mode, the ratio of the natural frequencies under the loading condition to the corresponding unloading condition also remains basically unchanged. The steel-hull global vibration test model can better consider the influence of added water mass, which can provide a valuable reference for the design of the global vibration model and the natural frequency prediction of special ship types.

Marine operations, such as takeoff and landing of carrier-based aircraft, ship fuel supply, and ship lifting operations, have strict requirements for the movement of ships and need to be carried out within the quiescent period window to ensure operational safety. However, the incidence probability of a quiescent period window under high sea conditions is relatively low. Therefore, it is crucial to assess the probability of the target sea area’s quiescent period in advance for the planning and deployment of offshore operations. This article establishes a ship motion probability model based on the statistical characteristics of ship motion to study the short-term statistical characteristics of quiescent periods. Simulation data is used to verify the statistical characteristics of motion, the joint statistical features of motion amplitude-period, proving the accuracy of the model. Subsequently, the probability of continuous small movements below the operational threshold during the quiescent period have been analyzed. The results indicate that the established ship motion probability model and the ship continuous small amplitude motion probability model can accurately reflect the motion characteristics and probability of quiescent periods of ships under corresponding sea conditions. This study can provide the probability of a quiescent period meeting the operational requirements before the ship enters a certain sea area for operation, and also provide the probability of a quiescent period occurring in a certain period of time before the operation, providing auxiliary decision-making for the navigation operation plan of offshore ships.

Tip clearance flow is a complex phenomenon that occurs between the rotor blade tip and the inner surface of the duct of a pump-jet propulsor. The tip clearance size significantly influences both the tip clearance flow and the performance of the pump-jet propulsor. Previous studies on tip clearance flow primarily focused on cases with tip clearance sizes less than 4 mm on model scale. Tip clearance flow of pump-jet propulsors with tip clearance sizes of 1 mm and 16 mm were simulated based on large eddy simulation in this paper. The study focuses on the characteristics of tip clearance flow in the large tip clearance pump-jet propulsor and the effects on cavitation inception, hydrodynamic performance, and duct pressure fluctuation. The results indicate that, compared to smaller tip clearance, the starting position of tip-separation vortex of pump-jet propulsor with large tip clearance is closer to the leading edge of rotor, while the intersection position of tip-separation vortex and tip-leakage vortex is closer to the trailing edge of rotor. Furthermore, the propulsion efficiency of the pump-jet propulsor behind SUBOFF is reduced by approximately 10%. The vorticity and circulation of tip-leakage vortex are larger, and cavitation inception of tip-leakage vortex occurs earlier. The amplitude of fluctuating pressure on duct inner surface is significantly decreased by about 80%. Therefore, the design of the pump-jet propulsor should be made based on comprehensive balance of the above-mentioned performance characteristics to find the optimal tip clearance size.

To investigate the turning maneuverability of polar ships in floating ice area, in this study a combined CFD-DEM approach was adopted to numerically simulate the turning motion process of a medium-sized polar ship in floating ice area. In the simulation the ship’s turning motion at different rudder angles and ice concentrations have been considered, and the parameters of the ship’s turning motion have been predicted. The results show that the existence of floating ice will significantly hinder the ship’s turning motion, and the range of the tactical diameter in floating ice area is 1 to 1.5 times than that in open water conditions. The forces and moments acting on the hull exhibit strong randomness, and the instantaneous fluctuations of the ship’s speed and yaw rate are more pronounced. At the same rudder angle, the ice longitudinal force increases with the increase of ice concentration, while the variation of the fluid longitudinal force is not significant. The average total lateral force and total yawing moment are in the same direction as the turning maneuver, while the average ice yawing moment tends to be in the opposite direction.