In order to improve the propulsion performance of contra-rotating propellers and reduce the cavitation effect, an end plate was applied to the contra-rotating propeller. The cavitation performance and propulsion performance of the end-plate contra-rotating propeller were analyzed. The RANS method with Schnerr-Sauer cavitation model was used for analyzing. Then the propulsion performance of contra-rotating propellers composed of conventional skewed propellers and end-plate propellers were checked for comparison. It is found that the addition of an end-plate makes the contra-rotating propeller possess better anti-cavitation performance. The sheet cavitation range is reduced by about 59% in the bollard state (J=0), and the cavitation is delayed when J=0.1. In addition, under the condition of low advance speed, the end-plate contra-rotating propeller shows a higher propulsion efficiency by 0.9%~3.1% than the conventional one. The open water performance of the end-plate contra-rotating propeller was tested at different rotation speeds, and the data were in good agreement with the simulation results considering the cavitation model. In this study, an end plate is innovatively applied to the contra-rotating propeller, which is suitable for the propulsion and operation requirements of low-speed submersibles.

An eccentric semi-submersible foundation was proposed considering the characteristic of wind energy direction concentration in actual environment. AQWA-Fast software was used to establish the floating wind coupling analysis model, and the kinematic response characteristics of symmetric and eccentric wind turbine were compared and analyzed under different incident angles of wind waves. The results show that the variation of wave incidence angle hardly affects the average motion response value and average power generation of the floating wind turbine. When the incident angles of wind and wave are the same, the eccentric platform has better motion performance in terms of sway, roll, and yaw degrees of freedom, and to some extent the mooring tension is reduced. When the wave incidence angle constantly changes, the eccentric design will improve the motion performance in terms of sway, roll, pitch, and yaw degrees of freedom. In addition, the eccentric floating wind turbine has better comprehensive power generation performance.

A marine riser usually works in the form of multiple riser clusters. The hydrodynamic interference characteristics between adjacent risers are obviously seen, and the interference can accelerate the fatigue damage. Therefore, it is meaningful to investigate the vortex-shedding patterns and the hydrodynamic interference characteristics of twin cylinders in order to ensure safe operation of risers. This study was aimd to parametrically investigate flows around twin moving cylinders at a staggered arrangement by a ghost cell method, in which, the incompressible Navier-Stokes equations were solved on a Cartesian staggered grid by using an in-house developed time semi-implicit finite difference method. A ghost cell method was used to enforce the no-slip boundary conditions. A radial basis iso-surface function was used to track the moving boundary implicitly and identify the properties of background grid. Based on the present numerical model, flows around twin forced moving cylinders at a staggered arrangement were simulated. Vortex patterns and force coefficients were analyzed under different gaps and oscillation frequencies. Typical interference phenomena such as synchronized, deflected and merged vortex patterns were observed. The results can provide theoretical guidance for the arrangement optimization of the riser clusters.

Thick-walled pipelines are widely used as transmission pipes for (ultra) deepwater petroleum and natural gas, and buckle arrestors for shallow water pipelines. However, the current international authoritative regulations may underestimate their ultimate bearing capacity significantly so that their economy and safety are hot topics in industrial circles. After deriving the calculation formula of vector form intrinsic finite element (VFIFE) method solid element, an analysis model of thick-walled pipelines considering the nonlinearity of geometry, material and boundary was established to solve the key mechanical problem of local collapse of thick-walled pipelines. And its accuracy was verified by comparison with 8 sets of thick-walled pipe scale tests, the DNV code, and ABAQUS simulations. Sensitivity analysis of diameter-to-thickness ratio, initial ovality and material yield strength were carried out to quantify the calculation errors of the DNV code method. Then, a more accurate formula for calculating the local collapse pressure of thick-walled pipes was obtained by fitting the VFIFE results. The results show that the simulation results of the VFIFE constant strain tetrahedral element are in line with the actual situation and can provide a new analysis strategy for the collapse behavior analysis of thick-walled pipelines. However, attention should be paid to determining the maximum load rate under the requirement of the quasi-static loading. Under high external pressure, the pipeline will collapse locally and propagate buckle dynamically and the deformation of the pipe section changes from an ellipse to a "dumbbell" shape with certain folds on the inner wall. During local collapse, the change trend of the stress distribution conforms to the general features of solid structure buckling instability. The calculation error of the DNV code of thick-walled pipelines’ local collapse pressure increases with the decrease of the diameter-to-thickness ratio, the decrease of the initial ovality, and the increase of the material yield strength respectively. The corrected formula for local collapse pressure calculation of thick-walled pipelines has a fitting error of -2.49%~1.72% for homologous data and a calculation error of -6.11%~1.70% for heterologous data. It can accurately calculate the local collapse pressures of deepwater pipelines with diameter-to-thickness ratio of 8~18, initial ovality of 0.5%~3.0%, and material yield strength of 300~500 MPa. The results can be used to guide the design and verification of submarine thick-walled pipelines.

Aiming at the initial stress field generated by welding of ring-ribbed cylindrical shell structures with initial geometric defects after shape correction, this paper presents the study on the influence of initial stress field on the strength and stability of ring-ribbed cylindrical shell structures under the premise of considering geometric nonlinearity caused by large deformation. The finite element model of ring-ribbed cylindrical shell with initial geometric defects was constructed by using ANSYS software and the initial stress field was calculated. The strength and stability of ring-ribbed cylindrical shell with initial geometric defects and stress field were solved by arc length method. The comparison between the experimental results and the calculated results verifies the validity of the model. The analysis shows that the ultimate bearing capacity of the ring-ribbed cylindrical shell under hydrostatic external pressure is slightly reduced, and the regularity of the instability waveform is reduced, but the failure position remains unchanged.

The additional strengthening effect caused by the non-proportionality of loading-path under cyclic loading is an important factor that shortens fatigue life of material. To solve this issue, the plane of maximum shear strain amplitude was treated as the critical plane, and the non-proportionality of material and loading-path were both considered. A new non-proportionality factor was introduced to quantify the impact of non-proportionality loading on fatigue life of material based on the equivalent strain model and critical interface theory. Secondly, the damage mechanism and fatigue failure mode of the specimen were also considered, the maximum normal stress on the critical plane was adopted to characterize its contribution to fatigue failure. On this basis, a multiaxial fatigue life prediction model was proposed by combining the non-proportional factors. Finally, the proposed model was verified by using the fatigue test data of four materials under multiaxial loading, and the prediction results were compared with five proposed models. The results show that the proposed model can effectively improve the fatigue life prediction accuracy under non-proportional loads compared with the existing models.

In view of the numerous and complex structures of ship machinery, equipment and the coupling of vibration transmission paths, a method of underwater radiated noise prediction based on BP (Back Propagation) neural network was proposed in this paper. A BP neural network based on gradient descent algorithm and Bayesian regularization algorithm was constructed respectively. Vibration data was taken as input, hull radiation noise was taken as output, and root mean square error (e_RMSE) and mean absolute error (e_MAE) were taken as evaluation indexes of model prediction accuracy. The results show that the generalization and robustness of Bayesian regularization BP neural network is better than that of gradient descent algorithm BP neural network. The error of Bayesian regularization is less than 3 dB, and the proposed method has good applicability in the field of ship radiation noise prediction.

This paper aims to propose an improved theory of homogenization for acoustic coatings, the acoustic coating with periodic cavities can be considered equivalent to a uniform layer, thus improving the computational efficiency of sound absorption. Based on the traditional homogenization theory, parameters of equivalent density, modulus, and thickness for the acoustic coating by are derived incorporating the potential flow theory and the Minnaert resonance scattering theory. The sound absorption coefficient of the coating is then obtained through analytical calculation. This paper specifically addresses the effectiveness, efficiency, and adaptability of the proposed algorithm, thus offering technical support for accurately predicting the sound absorption coefficient of acoustic coatings within the middle and low-frequency range.

With the development of high flow turbopumps, the size and speed of impellers have increased rapidly, resulting in the coupling vibration phenomenon of the turbopump. In order to keep the turbopump running smoothly and reduce its vibration and noise, the coupling dynamics characteristics of the turbopump rotor system were studied in this paper. Firstly, based on the simplified finite element model of the turbopump, it was found that when the rotor frequencies of different orders approach each other in the Campbell chart, two coupling characteristics phenomena of curve turning and merging would occur. Further analysis of the rotor modes conducted for the coupling characteristic phenomenon showed the different orders of the coupling characteristic modes would affect each other. Secondly, a two-degree-of-freedom system was used to simulate the coupling characteristics of the rotor, and then the effects of coupling degree, damping and gyroscopic torque on the characteristics were analyzed. Finally, investigation on the coupling dynamic characteristics of the two-degree-of-freedom system revealed that the dynamic response would change abruptly when the curve turning and merging occurred, leading to the increase of the coupling vibration amplitude and even the instability of the system. The research results could provide theoretical guidance for the safe and stable operation of rotor systems.

In this paper, numerical simulation and comparative verification tests of two ship models in heading regular waves, which was recommended by ITTC, were conducted. Based on the Reynolds time averaged method for solving the N-S equation, a calculation method of wave interference and 6DOF motions for two ships under horizontal mooring constraints was established. With the model and mooring parameters in the test taken as inputs, the numerical simulation of wave interference between the two ships under typical wave conditions was carried out. The time curves of 6DOF motions, wave surface rise between the two ships and mooring tension were obtained and compared with the results of model tests. The comparison shows that the motions of surge, heave, roll and pitch as well as the wave surface rise of the comparative conditions (wave period: 1.5 s) are in good agreement with the test results. There are relatively large errors in motions of sway and yaw compared with the test results, because these motions result from the coupling of wave system interference between two ships and mooring constraints, and the superposition of multi-factor simulation errors reduces calculation accuracy.