Bi-metallic mechanical clad pipes are one of the main measures for anti-corrosion control of oil and gas field gathering and transmission pipelines. The failure of their liner layers restricts the clad pipes' engineering application. In order to explore the buckling failure mechanism of liner layers of bi-metallic mechanical clad pipes, the mechanical model of a clad pipe under bending load was established. The influence of forming pressure, working pressure and composite pipe structural parameters on the failure mode of the liner layer lining was studied. The results show that the buckling resistance of the liner layer is improved by increasing the residual contact pressure. The buckling time of the liner layer is delayed and the fold amplitude is reduced by increasing the working pressure. The reduction of the initial layers gap of the clad pipe before forming is beneficial to improve the buckling resistance of the liner layer. With the increase of the wall thickness of the outer base pipe, the wall thickness of the lining pipe and inner diameter of the clad pipe, the buckling resistance of the lining pipe increases.

As a typical representative of the propulsion mode of BCF (body-caudal fin), tuna swims fast, which has become one of the important biomimetic research objects.In this paper, based on spring-based smoothing model and local remeshing model, a numerical calculation method for solving RANS equations and analyzing vortex structure extraction established for the flow field of caudal fin were verified by grid independence verification, and the results were compared well with those published in the literature. The investigation on the variation of the hydrodynamic force and vortex structure with the St number shows that the vortex ring structure in the wake presents a staggered distribution in a“zigzag”shape, and that, with the increase of St number, the vortex ring structure in the wake gradually evolves from a single column to a double column, and that the thrust coefficient increases.

Level ice is one of the main ice types encountered in the operation of offshore structures while vertical structures are a typical configuration of offshore structures, so it is of great significance to carry out the study of the ice loading on vertical structures under the action of level ice. Firstly, the interaction process between level ice and vertical structures was analyzed generally. Then, the model test of ice loading on vertical cylindrical structures was performed, and the failure model of level ice and the characteristics of the ice loading were investigated. Finally, based on the cohesive element model and the degraded constitutive model of sea ice, a numerical model was developed to simulate the crack propagation and crushing process of level ice, and test data were used to verify the correctness of the numerical simulation model. The work can lay an important foundation for the numerical and experimental research of ice loading and provide technical reference for the design and construction of offshore structures.

In this paper, a new frequency-domain analysis method based on the modified Tovo-Benasciutti (T-B) method was proposed to calculate the fatigue damage under wide-band Gaussian random processes. According to the parametric power spectrum with different spectral shapes, a new nonlinear function model for the key parameter b_TB of T-B method was developed through the time-domain fatigue damage analysis. Compared with the original T-B method, the modified T-B method was proposed by introducing the slope parameter m of S-N curves into the new function model of parameter b_MTB. Through the numerical tests with parametric power spectrum and real power spectrum, the results of time-domain rain-flow counting (RFC) method was used as reference, and the accuracy and robustness of the modified T-B method were verified against several existing frequency-domain methods.

Pump jet thrusters have gradually become the first choice of modern submarines, and vector thrusters have also been widely used in the aerospace field. In order to solve the problems of low control efficiency under low speed and improve the turning performance of submarines based on traditional rudder control, the application of vector thrusters in submarines has gradually become a hot spot at home and abroad. In this study, considering the nonlinear influence of various parameters of submersibles under large rudder angle (nozzle deflection angle), a nonlinear model of the horizontal maneuvering motion of a submersibles was established. By analyzing the turning performance of the submersible under three different control methods of ship rudder propeller, ship vector propeller and ship rudder vector propeller, the simulation was carried out under three different conditions: low speed, controlling the rotating speed of the pump jet propeller and controlling the axial speed of the submersible. The simulation results show that the vector pump jet propeller can effectively improve the turning performance of the submersible.

For the influence of cavitation effect on the forward and reverse performance of two-way propellers, the ice-class propeller model test in a cavitation tunnel was adopted to discuss the hydrodynamic effects of the cavitation number and the advance coefficient on the propeller forward and reverse performance in the uniform flow environment, as well as the effects of the cavitation number, advance coefficient and ice-propeller spacing in ice blockage environment. The results show that in the uniform flow environment with constant flow speed and variable rotating speed, severe cavitation phenomenon reduces more thrust and torque than the increase of thrust and torque due to the increase of rotating speed. In the ice blockage environment with constant rotating speed and variable flow speed, the thrust and torque of the propeller are affected by the ice blockage and cavitation. When the cavitation is severe, the thrust and torque no longer increase with the decrease of the blockage distance. The reverse performance of the two-way propeller is worse than the forward performance. The larger the advance coefficient is, the greater the performance difference is. When the advance coefficient is 0.7 in the uniform flow environment, the difference of the thrust coefficient is about 80%. With the increase of ice-propeller spacing, the hydrodynamic difference increases insignificanty. Cavitation is continuously generated on the blades, and quickly collapses when it is separated from the blades. With the decrease of the ice-propeller spacing, the cavitation phenomenon on the surface of the blade near the ice is more serious, the larger the area of cavitation is, the more irregular the shape of the cavitation will be.

Aiming at the inherent bottleneck of low efficiency and narrow frequency band of energy capture for traditional linear hinged module floating wave energy converters (WEC), a simple negative stiffness mechanism for hinged two-module floating WEC was proposed, which could be used as a passive method to improve the energy capture efficiency. Firstly, a simple and compact negative stiffness device was proposed, which was realized by placing simple stretch elastic elements between articulated floating bodies. Secondly a dynamic model of two-module nonlinear WEC in the time domain was established based on linear wave theory and Cummins equation. At the same time, the convolution integral term induced by wave radiation force was replaced by the state space model to improve the calculation speed. Finally, the numerical simulation of the two-module nonlinear WEC was carried out, and its energy capture characteristics under regular waves were analyzed. The numerical results show that the equivalent natural frequency of the system can be effectively reduced by introducing the nonlinear negative stiffness mechanism. When the negative stiffness mechanism was adjusted to appropriate parameters, the elastic force of the system can form an elliptical potential well in the phase plane of pitch motion, and its long axis is close to the mode direction of pitch motion of the floating module. Thus the pitch motion of the two modules tends to anti-phase and the nonlinear negative stiffness mechanism plays the role of phase control. Due to the above mechanism, the nonlinear negative stiffness mechanism with appropriate parameters can effectively improve the energy capture efficiency and broaden the energy absorption band.

This paper aims to study the influence of immersion and liquid filling on the acoustic radiation characteristics of cylindrical shells, and to provide theoretical basis and research methods for the evaluation and measurement of acoustic performance of liquid-filled shells near the interface. The frequency immersion depth spectrum of the radiated sound pressure of a semi-filled infinite cylindrical shell and the frequency liquid-filled height spectrum of a semi-immersed infinite cylindrical shell were calculated by using the finite element numerical simulation method. The results show that there are two obvious resonance phenomena, which are respectively excited by the bending waves and the low-order fluid additional waves. According to the dispersion curve of phase velocity, the prediction formula of resonance frequency was deduced, which can accurately predict the resonance phenomenon in the radiated sound field and provide theoretical support for the control and evaluation of the characteristics of the vibration acoustic radiation line spectrum of the internal tank structure in the mooring state.

As a kind of body-force model capable of replacing real propellers, the Blade Element Momentum Theory (BEMT) has a great application potential in simulating propeller performance and hull-propeller interaction. In order to deal with the problem that the traditional BEMT based on the ideal fluid hypothesis cannot accurately represent the real propeller model in a wide range of working conditions when coupling with the viscosity solver, an improved induct factor calculation method based on the real propeller was proposed in this paper. Open-water simulation of KP505 propeller body-force model at J=0.4-0.8 was carried out, and the simulation results of the body-force model, such as open-water performance, propeller load distribution and induced velocity field, were compared with those of the real propeller model. The results show that the maximum error of open water performance is 1.05% at each advance ratio, and the induced velocity field of the improved body-force model can reflect the wake momentum transport of the real propeller.

The joined shell with complex boundary condition is widely employed in the marine propulsion. And the traveling wave mode of the joined shell with rotational motion usually plays an important role in the marine propulsion. For prompting the application of functionally graded materials (FGMs) in ships and ocean engineering, the boundary conditions of a shell structure were simulated by the spring, the dynamical model of the rotating FGMs joined conical-cylindrical shell was derived, and the traveling wave mode of the rotating FGMs joined conical-cylindrical shell was analyzed. Firstly, considering the influence of the Coriolis force and centrifugal force produced by rotation, energy equations of the joined shell with the boundary spring and connecting spring were derived based on the Love’s thin shell theory. Then, the displacement function could be assumed based the Chebyshev polynomial, and the modal frequency equation was derived. Finally, the modal frequency of the traveling wave was solved by the Rayleigh-Ritz method. Based on the convergence analysis, the stiffness values of corresponding springs and the truncated terms of the Chebyshev polynomial were given. The effects of the circumferential wave number, volume fraction exponent, cone angle, rotational speed and the general boundary condition on the traveling wave mode were discussed. Results indicate that the bifurcation behavior with respect to the forward wave and backward wave are notable with the increase of rotating speed; the stiffness of axial spring has a greater effect compared with other springs; compared with the traditional energy method, the efficiency can be reduced for the repeated calculation and the elastic boundary condition has a large influence on the traveling wave mode, meaning the necessity of employing the spring to simulate the boundary condition.