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 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.

When submarine cables are suspended in the air and dragged by ship anchors, they will be deformed or even fractured, which seriously threatens the power transmission and information communication. Therefore, it is of great application value to investigate the backfill protection scheme for the suspended section of submarine cables. Based on ABAQUS, we established a three-dimensional finite element model simulating the process of a ship anchor dragging a three-core composite submarine cable through soil. Using nonlinear dynamics, we obtained the stress changes in the cables armorlayer copper conductor, and fiber optic armoring when it was dragged through different soil materials by a ship anchor. We conducted comparative analysis of the protection effect of four kinds of soil properties on the marine cable. Then by applying different speed loads, we investigated the relationship between the longitudinal compression rate of the cable and the burial depth under different ratios, to propose a layered backfill protection scheme. The results show that the difference of the protection effect on the cable of different seabed soil material properties is significant, clay can effectively reduce the tensile stress on the cable, and gravel can effectively reduce the stress change amplitude and strain time of the plastic yielding stage of the cable, and it is proposed to backfill the submarine cable with layers of clay and gravel to protect the submarine cables; when the ratio of clay to gravel is 1∶2, the backfill protection can withstand the towing speed of 90 cm/s. The backfill protection can also be used to protect the submarine cable against the tugging of a ship with a speed of 90 cm/s. The backfill protection is also used for the protection of the submarine cable. When the ratio of clay to gravel is 1∶2, the backfill protection depth of 0.7 m can withstand the damage caused by the anchor drag with a speed of 90 cm/s, which provides theoretical references for the backfill protection engineering and operation and maintenance of submarine cable.

The stored air mass (i.e. muzzle gas cloud) ahead of a projectile nose in a vertical launch tube has significant influences on the flow field and loads during underwater launching. In this paper the process was simplified to an impulsively started projectile in a stationary vertical tube, which then passied through air mass with constant velocity. The flow was investigated using numerical simulation. The primary conclusions are as follows: The air mass is compressed, pushed out, entrained, and finally forms an annular oscillating bubble, which is accompanied by intense unsteady vortex around the muzzle platform and the projectile. The oscillating pressure induced by the bubble propagates through the flow field and attenuates with time and distance. The oscillating pressure can be considered as being linearly superimposed on original flow pressure, resulting in a periodic adverse pressure gradient along the projectile surface, which affects flow separation and cavitation. The oscillating drag coefficient for different volumes of the air mass can be normalized by dimensionless time defined using the velocity and equivalent spherical diameter of the air mass.

Ship structures operate continuously in the marine environment, where they are prone to fatigue crack growth (FCG) under complex alternating loading, therefore it is of great significance to accurately predict the FCG and ensure the safety of structures. In this paper, the load spectrum constructed by the spectral method was combined with an improved unique curve crack growth model, and a method was proposed to more accurately predict the FCG in the near-threshold regime for ship structures under spectral loading. A balcony opening corner in a cruise ship was taken as an example; the method for determining the shape exponents in the improved model was given, the FCG of this structure under spectral loading was predicted, and the effects of the initial crack length and crack growth model on the FCG were discussed. The results show that the prediction method can more accurately predict the FCG in the near-threshold regime, and the prediction result is more conservative than that predicted by the unique curve model recommended in the regulations of CCS. The method presented in this paper can also provide a reference for the fatigue life assessment of other marine structures.

Based on the slicing theory, a time domain model of flow-induced vibration of 3D rotating slender structure is established by combining computational Fluid dynamics (CFD) and finite element method, and the flow vibration characteristics under the action of water flow and rotation are studied. Under the action of water flow, the trajectory of non-rotating elongated structure is mainly "8" shaped. Under the combined effects of flow and rotation, the motion direction of rotating slender body is opposite to its rotation direction, resulting in backward whirling. When the flow velocity is 0.46 m/s, vibration is jointly influenced by the flow and rotation. As the rotational frequency increases, the trajectory of the rotating elongated body transitioned gradually from a "8" shape to a circle. When the flow velocity is 1.02 m/s, the frequency is close to the theoretical intrinsic frequency, and the main cause of vibration is Vortex-Induced Vibration (VIV). The vortex motion is completely suppressed. There is a frequency-locking interval near the intrinsic frequency of the cylinder. The relative amplitude of the transverse vibration of the rotating cylinder increases with the flow velocity in the locking interval, while the frequency ratio remains unchanged.

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.

The precise determination of low-frequency wave loads on ship hulls is an indispensable cornerstone and core challenge in hull structural design. Low-cost ship model testing is often employed in engineering to forecast wave loads. However, test data frequently suffer from deficiencies or abnormalities due to various reasons. Consequently, predicting wave loads from data with defects or anomalies, remains a major engineering challenge. This paper presented an efficient method for accurately determining the wave design loads on ship hulls, specifically tailored to handle deficient or abnormal test data. By integrating 5,400 sets of wave load data calculated using two-dimensional strip theory, a machine learning transfer network was constructed. To address deficient data, we innovatively introduced a fine-tuning network layer, and designed a novel loss function that ignores zero terms, thereby enhancing the network's adaptability. This method achieved rapid wave load forecasting by transferring simulation results to ship model tests, with an accuracy better than 90%. This technique enhances design efficiency, reduces labor costs, and maximizes data utilization, providing a reliable and efficient solution for wave load prediction in hull structural design.

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.

In recent years, shipowners have become increasingly concerned about the actual performance in the real marine environment. At present, ship design optimization is mainly conducted based on performance in still water which has a certain difference from real-sea performance. The factors influencing the powering performance of ships in wind and waves were investigated taking series oil tankers, bulk carriers, and container ships as the research subjects. The theory of two-dimensional strip was used to calculate the added resistance under typical sea states BF6 and BF8. The propeller was redesigned using graph method to research the influence of light running margin in the wind and waves. The results show that the principal dimensions have a significant impact on the added resistance of oil tankers and bulk carriers, and have a relatively small impact on container ships. Therefore, it is necessary to consider the ship type separately when optimizing the powering performance in wind and waves. In addition, with the increase of the light running margin, the reserved space for power becomes increasingly sufficient.