The defects of a submarine pipeline caused by laying, non-uniform seabed support and impact of falling objects are often distributed in many places. In order to simplify the analysis, the buckling model of a pipeline with multiple elliptical defects in the axial direction is generally simplified as the pipeline with uniform elliptical defects or the pipeline with a single local elliptical defect in the axial direction. However, there are few studies on the differences of these three models and their application in engineering. Based on the numerical simulation, this paper presents a study on the buckling and instability behavior of the pipeline with three kinds of elliptical defects. The RIKS method was used to solve the buckling pressure of the pipeline, and compared with the calculation results of DNV-OS-F101, focusing on the analysis of the excitation position and influence parameters of the buckling and instability of the pipeline with multiple elliptical defects in the axial direction. The results show that the buckling pressure of the pipeline model with different elliptical defects is obviously different, and the order is the buckling pressure of the pipeline with uniform elliptical defects < the theoretical solution of DNV-OS-F101 < the buckling pressure of the pipeline with multiple elliptical defects < the buckling pressure of the pipeline with single local elliptical defects. For the buckling analysis of the pipeline with multiple elliptical defects in the axial direction, if the pipeline model is simplified with axial uniform elliptical defects, it is more conservative. However, if the pipeline model is simplified with axial single local elliptical defects, it is more dangerous. Besides, the buckling pressure of the pipeline with multiple elliptical defects is affected by the size of elliptical defects, the distance between elliptical defects and the length of elliptical defects.

The cooling system can be carried out by the flow pressure difference through the outside protrusion of an underwater vehicle. This system can prevent the low line frequency pipe noise from the pump’s excitation. However, the protrusion is external, generating the hydrodynamic noise and destroying the acoustic stealth of underwater vehicles. Two protrusion models with a rectangular inlet and a circular inlet were created. The flow field and sound field were numerically calculated. The generation property of the hydrodynamic noise from the two protrusions on the condition of different flow velocities was analyzed. It is indicated that the total radiated sound power of the protrusion with the rectangular inlet is slightly lower than that with the circular inlet. To further reduce the hydrodynamic noise, the protrusion with the rectangular inlet was carried out by the flow control through the serrations. The noise reduction effect of the protrusion with the rectangular inlet by the leading-edge serrations is better than that by the tail-edge serrations. The optimized parameters of the serrations were summarized. To validate the numerical calculation results, four protrusion models were designed and fabricated, and the experimental tests were done in the gravity low-noise water tunnel. The results show that the protrusion with the rectangular inlet by the serrations optimization has a better noise reduction effect in the frequency range from 10 Hz to 2000 Hz. The results in this paper can provide some references for the low-noise design of protrusions of underwater vehicles.

With the continuous development and consumption of traditional land resources, the development and utilization of new water energy has become a new trend, and a large number of various floating structures have appeared. As the key to ensure the safe and stable operation of floating structures, mooring systems have always been the focus of the industry. In this paper, a large number of literature review and research were carried out on the existing floating structure mooring systems, the types of floating structures were summarized, the mooring system structures were analyzed from the aspects of the classification of the mooring systems, the way of chain distributions, the bottom anchorage foundation types and the new mooring systems, and the characteristics and advantages and disadvantages of various mooring cable materials were discussed. The static characteristics and dynamic response of the mooring systems were analyzed according to a large number of existing literatures, and the applicability evaluation and recommendation of various mooring methods were given through a comprehensive analysis of the water depth, seabed topography, geology, platform function, wind and wave conditions, economy and other aspects of the mooring engineering, and the shortcomings of the existing research were pointed out, and the current research direction still needs to be further developed.

Ventilation law of supercaviting vehicles plays an important part in cavity shape control. This paper presents the development of an experimental method in high speed water tunnel to study the supercavity stability which describes the relationship between cavity shape and ventilation rate. An empirical formula was established based on test results of cavity shape and ventilation flow rate under different cavitator sizes and angles of attack of the model. By making analysis and comparison of the experiment results with and without tail wings, the influence of tail wing on ventilation law was preliminarily obtained. The results show that ventilation rate needs to be kept in certain range in order to maintain a steady and smooth ventilated supercavity.

Aiming at the actual demand of improving the numerical simulation of unsteady submarine motion in maneuvering condition, the overset mesh was used to directly model six degree of freedom coupling motion of a submarine and the independent motion of each control surface, and the sliding mesh was used to directly simulate the propeller rotation at the stern of a submarine, so as to carry out the research on the numerical simulation of a generic submarine in self-propulsion and free running maneuvering conditions. Through the procedure of numerical simulation in typical working conditions, such as submerged self-propulsion, turning maneuver, zigzag maneuver, crashback, etc., the issues of submarine/propeller/rudder hydrodynamic coupling modeling and PD (proportional-derivative) numerical control realization of autopilot in maneuvering condition were emphatically solved, and the technology of free running submarine model numerical simulation based on overset mesh was established. At the same time, the surrounding flow phenomenon and the time-history change process of kinematics and dynamics parameters of the submarine in maneuvering condition were analyzed. By comparing the numerical simulation results of typical dynamics parameters with the model test results, the practicability of the approach for engineering prediction was verified. The research can provide a guidance for the prediction and evaluation of submarine maneuverability and seaworthiness. It is beneficial for the improvement of free running submarine model tests.

Path planning is one of the key technologies for autonomous navigation of unmanned vehicles. A good path planning method is of great significance to the intelligent development of unmanned vehicles. In the existing path planning research, the maneuvering performance of unmanned vehicles is not considered. In order to make the planned path have shorter voyage time, shorter path length and better path tracking ability, it is necessary to combine the maneuvering performance of unmanned vehicles with the path planning algorithm. In order to accurately predict the ship maneuverability, the channel-type unmanned catamaran was taken as the research object, and simulation tests of three planar motion mechanisms were carried out by CFD technology. Simulation results were fitted with different hydrodynamic models, and corresponding hydrodynamic derivatives were calculated. The MMG model was used to establish a mathematical model of ship maneuvering motion to simulate the turning motion and Z-shape motion of the unmanned catamaran. The influence of different hydrodynamic models on the simulation results was analyzed, and the maneuvering pre-diction of the unmanned catamaran was realized.

With a scale model of tuned liquid multi-column damper (TLMCD) and floating substructure established, experiments were carried out in a flume to study the control effect of TLMCD on the pitch motion response of the floating foundation under regular wave excitation. The numerical model was established and verified by OpenFOAM. The coupling mechanism of TLMCD and floating foundation was analyzed from the aspects of flow field, hydrodynamic loads, floating body motion and damping force. The results show that TLMCD has the best pitch suppression effect under resonant excitation, and that the liquid with a mass ratio of 2% reduces the maximum pitch response of the floating body under resonant excitation by 10.84% to 18.53%, and achieves at least 7.32% damping effect in the range of 0.9<T/T₀<1.1. By numerical method, it was observed that under the condition of resonance, the hydrodynamic force generated by tank sloshing took up 89.52% of the time to do positive work, and that the sloshing of liquid in the liquid columns periodically provided reverse damping moment for the floating body.

In order to evaluate the 3D effects of wave glider spring hydrofoil mechanism in waves on its dynamic performance, a numerical computational model of the wave glider spring hydrofoil mechanism was developed. Based on the overset mesh technology, the dynamic performance between 2D and 3D hydrofoils was analyzed and studied by using CFD FLUENT software. The results show that due to the limited span of the 3D hydrofoil, the tip vortex phenomenon is generated at the wing tip, resulting in reduced hydrofoil dynamic performance, and that the forward propulsion efficiency of the 3D hydrofoil is reduced by 22.1% compared with that of the 2D hydrofoil. Then, the bionic principle was used to design the wave glider bionic hydrofoil. It is found that the bionic hydrofoil reduces the loss of hydrofoil power performance by the tip vortex, while the forward thrust of the bionic hydrofoil is increased by 17.6% and the efficiency by 10.4% compared with the 3D hydrofoil of the same spreading chord ratio. Finally, the experimental comparison shows that the CFD simulation data and the experimental data have the same trend, and the reliability of the CFD simulation model is verified.

The two compulsory conditions for boundary layer separation are fluid viscosity and positive pressure gradient. By designing the shape of a vehicle so that its surface has a negative pressure gradient area as large as possible, the flow transition and separation are delayed so as to achieve the purpose of drag reduction. Based on the theoretical flow non-separation shape design method of slender bodies, the shape of a vehicle with a critical speed of 100 m/s was designed, and numerical simulation was used to analyze its flow characteristics at different speeds and angles of attack. It is found that the simulation results at zero angle of attack are consistent with those of the theoretical calculation, which proves that the surface of the vehicle can be in a state of non-separation of laminar flow through the shape design. A small attack angle will not destroy the fluid adhesion state on the surface of the vehicle, but whirlpools will appear in the flow when the attack angle is greater than 2 degrees.

In order to improve the efficiency of multi-parameter, high dimensional and high nonlinear optimization of ship structure reliability optimization design and make up the lack of uncertainty factors affecting structural safety in traditional deterministic optimization design, a river-sea-going ship was taken as the research object. BP (Back Propagation) neural network agent model technique and SMOTE (Synthetic Minority Oversampling Technique) algorithm were used to increase the number of sample points near the failure surface, in order to obtain a high-precision limit state agent model of ship structure with fewer sample points. Combined with Monte Carlo simulation method, the reliability calculation program of hull structure was developed. Structural reliability optimization analysis was performed adopting the simulated annealing optimization algorithm in order to reduce the structural weight. A set of complete and effective reliability optimization design system based on agent model technology was established to improve the efficiency of reliability optimization design, which has guiding significance to the reliability optimization design of river-sea-going ship structures.