A design method for wake-adapted contra-rotating propellers (CRPs) with optimal circulation distribution was presented based on the vortex lattice model in lifting surface theory. The implicit relations of propeller thrust and torque with the radial circulation distributions of CRPs were modelled via neural network, where the input data for training of the neural network were yielded from an in-house vortex-lattice code. Subject to the requirements for total thrust and torque-balance, a genetic algorithm was employed to optimize the radial circulation distributions of the forward and aft propellers to maximize the total efficiency. Taking the optimal radial circulation distributions and a prescribed chordwise distribution of circulation as the objective, the camber surface geometries and pitch distributions of the forward and aft propellers were designed. Numerical example was presented with the CRPs for a high-speed underwater vehicle, and the design results were then validated in self-propulsion simulation by solving the unsteady RANS equations. While the efficiency and torque-balance of the designed CRPs are slightly improved against the prototype, the minimum pressure values on blade surfaces are significantly increased, which is favorable for retarding the inception of cavitation.

The hub effects are usually ignored in the propeller lifting-surface design based on potential flow theory, the circulation of root is constrained to zero, and the hub is neglected when dealing with boundary value problems. Herein, the theoretical design method of propeller considering hub effects was established by coupling image method of two-dimensional flow problem with lifting-surface design method. Two propellers were designed for a ship by the methods with and without consideration of hub effects respectively, the geometrical shapes, hydrodynamic coefficients and pressure distributions of two propellers were compared. It is shown that hub effects in lifting-surface design has no obviously influences on hydrodynamic coefficients, but the pressure distributions of the propeller designed by the method considering hub effects is in better agreement with design goals. At the same time, considering hub effects could help to reduce loading of leading edge in external radius, that would be beneticial to postponing cavitation inception.

The method of combining overlapping grid and sliding grid was applied to study the hydrodynamic performance and motion control of an underwater robot under incoming flow. The PID (Proportional, Integral, Differential) control method was used to realize the vertical heave motion under the presence of incoming flow through the joint operation of the umbilical cable and the ducted propeller, and the depth of the underwater robot was maintained under the condition of incoming flow change. The trim angle of the underwater robot could be adjusted by the propeller though PID method, maintaining the attitude close to balance at a fixed depth. Also the underwater robot can keep its trim angle constant throughout the entire movement process under the complicated heave working conditions.

This paper focuses on proposing a finite-time command filtered backstepping robust adaptive dynamic positioning control method to deal with the thruster dynamics, parameters uncertainty, input saturation and unknown external disturbance problems. The proposed method not only has the advantages of the command filtered backstepping, but also can guarantee the control system convergence in finite time. Firstly, adaptive neural network was used to estimate uncertain functions in the system. Secondly, the thruster input saturation issue was addressed with a finite-time auxiliary dynamic system. Finally, based on the uncertain estimation and the FTADS, a finite-time command filtered backstepping (FTCFB) control law was introduced, and the system tracking errors and parameter estimating errors were proved to be convergent in finite time by using Lyapunov stability theory. Additionally, the effectiveness of the proposed positioning control method was verified by numerical simulations.

Large vessel-shaped fish cages are promising large aquaculture structures developed in recent years, with maximum structure length of nearly 400 meters. The effects of the hydrodynamics on the nets and the frames will be significant for the cage deformation response in waves, which will increase the complexity of the cage design. In this paper, a coupled dynamic model of a large vessel-shaped fish cage is used to calculate the motion and structural response in the time domain. Firstly, the floating body of the cage is discretized into multi-module units, connected by equivalent elastic beams. The nonlinear effects of the net and steel frames are considered. A floating cage model considering the deformation of the floating body is then established in time domain for dynamic analysis. The radiation force of the floating body is solved by applying the added mass and damping directly or the state space method, and the hydrodynamic loads on the net and steel frames considering the disturbing effect of floating body are calculated by Morison formula. The results show that the hydrodynamics of the net and frames have an obvious influence on the response of the fish cage and the cross-sectional elasticity produces a certain degree influence on the net twine tension, which provides reference for structural analysis and cross-sectional design of the large vessel-shaped fish cages.

Reliable experimental data are crucial for understanding the performance of Floating Wind Turbine (FWT) systems in complex wind-wave-current marine environments. This paper presents the results from 1∶70 scale model tests conducted in a wave basin to investigate the motion response characteristics of a new 12 MW semi-submersible FWT. The experimental design incorporated improvements, including a large-scale wind generation system with a rectifier network, aiming to provide a stable wind field for the experiment. The experimental results indicate that wind loads primarily exert static effects, as reflected by changes in response mean values. On the other hand, increased wave parameters predominantly contribute to dynamic effects, which are demonstrated through changes in response standard deviations. Aerodynamic damping effect is primarily manifested in the coupling responses of pitch and surge, as well as at the natural frequency of pitch motion. The action of current significantly reduces response at the natural frequency of pitch, although it concurrently amplifies platform’s surge and yaw responses. This study contributes valuable insights into the dynamic behavior of large-scale semi-submersible FWTs under combined wind, wave and current conditions.

Oscillating-water-column (OWC) wave energy converters (WECs) have attracted worldwide attention because of their simple structures and easy maintenance. However, they still face the issues of low energy conversion efficiency and narrow effective frequency bandwidth. A numerical simulation of the hydrodynamic performance of an offshore stationary dual-OWC array was conducted. Based on the potential flow theory, a nonlinear aerodynamic model considering air-liquid coupling was constructed by introducing aerodynamic and artificial damping on the first and second-order free surface boundaries of the OWC chamber. A set of physical experiments were carried out to validate the numerical model, and a good agreement between the experimental and numerical results was achieved. The results show that the chamber resonant frequency shifts due to the multiple diffraction waves between component devices. Appropriate array arrangement helps to improve the relative capture width and effective frequency bandwidth of the OWC wave energy devices, with a maximum increase of 61.7% and 24.5%, respectively.

In order to analyze the impact attack angle on the oblique water-entry performance of projectiles, the oblique water-entry tests of a projectile under multiple conditions were carried out based on the high-speed photography technology. The effects of the attack angle on the cavity evolution and motion characteristics of the projectile were studied. Based on finite volume method and overlapping grid technology, a numerical method for oblique water-entry of projectiles was established. The water-entry stability and force characteristics of the projectile were compared and analyzed with the attack angle set as 0°, ±0.5°, ±1.5°, ±2.5°, and ±3.5° respectively. The reliability of numerical simulation was validated by comparing the simulated results with the experimental results. It is shown that when the attack angle is positive, the projectile will deflect counterclockwise and deviate upward, while it will deflect clockwise and deviate downward when the attack angle is negative. When the attack angle is zero, the projectile motion is stable, and the simulation results under the same condition are consistent with the experimental ones. With the increase of the absolute value of the attack angle, the absolute value of the deviation and deflection of the projectile presents an increasing trend simultaneously, and the time of the tail-slapping of the projectile is gradually advanced. When the absolute value of the attack angle is the same, the first tail-slapping of the projectile occurs earlier when the attack angle is less than 0°, and the water-entry stability of the projectile is worse. Influenced by the attack angle, both the drag and lift of the projectile will shock violently at the early stage of water-entry. With the increase of the attack angle, the lift of the projectile presents a decreasing trend, while the drag is not significantly influenced by the attack angle, except that the drag is relatively large at attack angle of −3.5 °.

In this paper, a NURBS-based geometric parametric level set topology optimization method is proposed to address the challenges faced by the traditional topology optimization method in seamlessly integrating CAD and CAE and dealing with the fragmentation between geometric modeling, structural analysis, and optimization design for complex ship structures. Firstly, the ship structure is immersed in a three-variable NURBS 3D solid structure. Then, a ray-tracing-algorithm is employed to quickly determine the relevant geometric information of the design domain, boundary, and load application area, such as units and control points, in order to establish the NURBS-based geometric parametric level set topology optimization method. By this method, the limitations of traditional NURBS-based topology optimization, which is restricted by regular NURBS topology, are overcome. The method can handle any complex CAD model. It is demonstrated through numerical examples that the computational efficiency of the algorithm can be improved by more than 30% compared to the traditional geometric SIMP method.

The soft yoke mooring system is one of the main mooring methods of FPSO. The motion state of the mooring system under the combined action of wind and wave and other environmental loads shows the dynamic characteristics of high aggregation of multiple degrees of freedom and modal mixing. In this paper, in view of the traditional clustering algorithm that cannot obtain the characteristics of small clusters characterizing extreme operating conditions, a BCALoD clustering algorithm based monitoring data extraction method for soft yoke mooring systems was proposed for compression and dynamical analysis of the massive data. The clustering results show that the cluster centers of multiple data clusters correspond to the mooring attitude under normal operating conditions, but the cluster centers of relatively few data clusters contain not only the mooring attitude under extreme operating conditions but also the mooring attitude under normal operating conditions. The horizontal and vertical mooring forces were calculated for each clustering condition by extracting the time course data of each clustering center based on the original data and combining with the multibody dynamic modelling of soft yoke mooring system. The results of the force analysis based on the clustering show that the mooring forces of the clustered normal and extreme conditions are as expected, and that the extreme force states under the normal behavior of the mooring attitude are found in particular, which characterize the inadequacy of the design consideration of the soft yoke mooring system. It is shown that the clustering method in this paper has the ability to effectively extract the dangerous conditions of the soft yoke mooring system, and that it is important to pay attention not only to the forces in extreme attitudes, but also to the extreme forces generated by the coupling of attitude motion.

With the rapid development of the transportation industry, ship-bridge collision accidents occur from time to time and bring about loss of life and property. Ship-bridge anticollision facilities can reduce the damage of ships while protecting the bridge structure. In view of the shortcomings of traditional steel box, such as high stiffness and unchangeable protection position, a new self-floating ship-bridge anti-collision device is designed based on gradient foam aluminum composite sandwich structure. A ship-anticollision device-bridge pier collision model considering pile-water-soil coupling effect was established by finite element software LS-DYNA. The damage characteristics of ship-anticollision device-pier under typical collision loads were studied, and the crashworthiness of anticollision device under cases with different ship speed and ship collision angle were evaluated. The results show that the anticollision device has excellent buffering and energy absorption characteristics, which can effectively reduce the peak value of collision force, prolong the collision time, reduce the damage of pier and effectively reduce the damage of bow structure.

As an important hub connecting offshore oil and gas fields and offshore platforms, unbonded flexible pipes play a vital role in marine production. As one of the main bearing parts of flexible pipes, the failure of the tensile armor layer will threaten the integrity of a pipeline system. Therefore, it is of great theoretical significance and engineering practical value to clarify the influence mechanism of various defects on the mechanical properties of the tensile armor layer and to study the mechanical behavior of the tensile armor layer. The axial compression performance of the tensile armor layer of an unbonded flexible pipe with defects was studied by numerical simulation method. A five-layer unbonded flexible pipe axial compression finite element model was established. The effects of non-metallic layer defect size, interlayer friction coefficient and steel strip fracture on the axial compression stiffness and critical buckling load of the flexible pipe were studied. The research results show that the decrease in the friction coefficient caused by the increase in the annular water content will significantly reduce the critical buckling load, and that the defects in non-metallic layer and the fracture of steel strip will significantly reduce the axial compression stiffness and critical buckling load. Therefore, in engineering, more attention should be paid to the annular water content and the structural integrity of each layer. The results can provide a reference for flexible pipe design and integrity management.

The control of low-frequency broadband excitation and acoustic radiation of ship thrusters is of great significance for the acoustic stealth performance of ship. The control of low-frequency broadband excitation and induced sound radiation in a combination thruster with a front stator was investigated in this paper. Starting from the theoretical prediction model of low-frequency broadband excitation for rotors, the influence of parameters such as flow field, geometry, and operating conditions on the broadband force of rotors was systematically calculated and analyzed. Three directions to control the broadband excitation of rotors were proposed: (1) improving the distribution of turbulent flow fields to make them as uniform as possible; (2) improving the geometric design of the rotor to reduce the pulsating load component;(3) improving the matching design of various components of the thruster and reducing the rotational speed at the same speed of ship. Based on several research achievements, control research was conducted on the low-frequency broadband force and sound radiation of the rotor of a combined thruster. It is concluded that the optimization of the hydrodynamic shape of the stern appendage and the optimization of rotor geometric parameters combined with the efficiency enhancement design of the duct and stator can control the low-frequency broadband force and sound radiation of the rotor to a certain degree, and that the optimization scheme reduces the peak and integral values of the low-frequency broadband noise of the rotor by about 3 dB. The research can provide certain reference for the control of low-frequency broadband force and induced sound radiation of ship thrusters.

The outboard discharge process of an underwater vehicle navigating in water results in an interaction between cross-flow and jet. The fluid dynamic characteristics generated by jet in cross-flow are important topics in the field of fluids. Based on Reynolds average and large eddy simulation method, a numerical calculation model of jet in cross-flow was established in this paper. Then the flow field characteristics of jet in cross-flow were explored in detail. The accuracy of the current method was verified by comparing the results of averaged velocity and fluctuating pressures with those in related references. The velocity and vorticity characteristics of the near-wall surface upstream and downstream of the orifice were investigated based on the results of numerical simulation. Additionally, the characteristics of the sound field in the vicinity of the orifice were analyzed. The results indicate that the impact of jet in cross-flow on the upstream near wall was confined within the range of 6 times the aperture from the orifice. Flow separation occurs in the downstreams of the orifice within a range of 2-14 times the bore size. Under the interaction between the crossflow and the jet flow, the characteristic vortex structure of counter-rotating vortex pairs (CVP) are formed in the downstreams of the orifice. The CVP is formed near the lower edge of the orifice and persists downstream. The vortex core gradually moves away from the wall along the flow distance, and the influence area gradually expands. The primary source of acoustic energy resulting from the interaction between the cross-flow and the jet is situated in close proximity to the wall. The acoustic energy level near the orifice is higher and exhibits a detached characteristic from the wall. Furthermore, the sound pressure level in the downstreams of the orifice is considerably higher than that upstream. The radiation of the sound field is dependent on the direction of flow and exhibits obvious directional characteristics.