MFC actuators, widely used in intelligent sensing/actuation, energy harvesting, underwater bionic robots and other fields, have increasingly attracted attention due to their good flexibility, large actuation force and excellent waterproof performance. In this paper, the vibration characteristics and dynamic response of underwater flexible structure driven by MFC actuators were studied. The driving force of the MFC actuators were calculated, and the additional inertia force and additional damping force of the fluid were derived according to Morrison's semi-empirical formula. Based on the Euler-Bernoulli beam theory, the assumed mode method and the second kind of Lagrange equation, a coupled nonlinear dynamic model of MFC-actuated underwater flexible structures was established. The harmonic balance method was used to convert the nonlinear damping into linear. The numerical simulation and experimental results show that the local stiffness of the flexible beam structure with the MFC actuator is increased, and the measured modal shape is basically consistent with the simulated one. Affected by the hydrodynamic force of the surrounding fluid, the first two orders resonance frequency and dynamic response of the MFC-actuated cantilever beam underwater decrease significantly. The amplitude-frequency response curve predicted by the model is in good agreement with the measured curve, which confirms the validity of the coupling dynamic model. This study provides a reference for underwater bionic propulsion devices based on smart materials.

The complexity of the stern appendage of a four-screw ship and the differences in the spatial layout of the internal and external propellers will lead to the unbalanced load distribution of the internal and external propellers. Based on the CFD method, numerical prediction simulation for the viscous wake field of a ship with four propellers was carried out. According to wake field distribution and turbulence characteristics, the difference in wake fields between the inner and outer propellers was compared and analyzed, and also the results was contrasted with the test values. The study results show that the inner propeller in the turbulence zone is affected by the hull boundary layer, which makes the axial velocity at the inner propeller disk less than that at the outer propeller disk, resulting in uneven load distribution between the inner and outer propellers. The flow at the outer propeller is a mixed one of the hull boundary layer and the turbulent wake, which makes the wake at the outer propeller disk more uneven. The research can provide basis and support for four-screw ship design and stern layout optimization.

The directional dimensional analysis method is commonly used in the design of similar scale-down models of hull structures. However, the traditional dimensional analysis method based on elastic theory cannot reflect the nonlinear response process of a structure, which limits its application in the scale down model test of hull structures. In this paper, based on the finite similarity method, the scale-down factors of the geometric dimension, material density and time of a structure were obtained by matching the transport equations in the physical space and the trial space. The nonlinear similarity relationship between the scaled down model of the stiffened plate structure and the prototype was derived, and the influence of the material parameters on the nonlinear similarity process was analyzed. By calculating the ultimate strength of the stiffened plates subjected to plain compression, the effectiveness of the scaling criterion based on the finite similarity method was verified. The result shows that the present method can well reflect the nonlinear characteristics of materials, and achieve a good prediction on the ultimate strength of the original model through the results of the scale down model.

The installation of spool pieces of submarine pipelines is important in the construction of newly-laid submarine pipelines. Due to the manufacturing errors in the process of producing marine pipelines, as well as the offset between the spool piece and offshore riser or the submarine horizontal pipes, the problem concerning the limit size offset of spool piece naturally emerges during installations. In this paper, the finite element analysis software ABAQUS was employed to establish the simulation model of a spool piece with flange joint based on the practical structural configuration and material parameters. As for the three typical situations in the practical process of the spool piece installation, the stress and strain distribution of the structure were obtained. Furthermore, the limit size offsets of the spool piece were calculated under the condition of satisfying the practical strength requirement. The mechanics and deflections of the spool piece during installation were discussed and analyzed in detail. The relevant conclusions are expected to provide the corresponding theoretical basis for the practical construction of the offshore platforms. And the research could have some engineering significance for the development and construction of offshore oil and gas fields.

Effective use of numerous wave energy resources in the ocean remains a problem to be solved. Due to the wave conditions for low periodic and small wave height in the South China Sea, a model combining a split heave point absorber with a tension leg platform (TLP) was proposed. Through a physical model test, the hydrodynamic characteristics and wave energy capture efficiency of the split float were measured and analyzed, then compared with the whole float. And the optimal wave energy capture of the split float was discussed. The results show that the split float presents different hydrodynamic characteristics, and the energy capture efficiency of the device for small periodic waves is greatly improved in the low sea state, but the pitch motion of the platform is significantly increased in the specific wave direction, which requires further optimization of the tension leg system.

The bow configuration of the tumblehome hull has a certain influence on the motion and the characteristics of green water loads in waves. In this paper, based on the inclination angle of stem, three kinds of bow configurations with inclination angles of 30°, 45° and 60° were selected. The motion response of the tumblehome hull in regular waves and the load characteristics of green water were studied by using the CNT-CGFDM method. The simulation of object boundary and motion was realized by immersed boundary method, and the free surface was captured by THINC/SW method. The ship models with 45° and 60° inclination angles of stem were selected for model test. The numerical simulation results are in good agreement with the experimental results. The results show that the bow configuration has little effect on the motion response of the tumblehome hull in regular waves, and show that some local differences exist only in some sea conditions. It has a certain influence on the slamming load of green water on the tumblehome hulls. Compared with the configuration scheme with the inclination angles of 30° and 60°, the load performance of green water on tumblehome hull with the inclination angle of 45° is more excellent.

Mn25Al7 steel is a new type of lightweight and high-strength marine steel, and the fatigue properties of Mn25Al7 steel have not been studied in relevant experiments. In this paper, the fatigue tests of typical nodes such as base metal, butt welded joint and T-welded joint of Mn25Al7 steel were carried out, and the fatigue grade curve of the typical nodes was obtained based on the nominal stress method and the hot stress method, respectively, and compared with the fatigue grade curve of the existing standard. The fracture morphology of the three samples was observed, and the crack propagation law and fracture mechanism were analyzed. The test results show that the fatigue life of Mn25Al7 steel base metal is higher than that of ordinary steel designed by the specification. The specification underestimates the fatigue life of butt weld joints of base metal and smoothed toes, but can accurately evaluate the fatigue life of T-weld joints. By comparing and analyzing the difference in damage rate of the three specimens, the fracture morphology was further systematically analyzed. It is found that the initial crack source and welding residue will reduce the fatigue strength. This study can provide a theoretical basis and experimental support for the prediction of fatigue life of marine high-strength steel.

This paper proposes a hydrodynamic mathematical model of a tethered underwater robot system by introducing boundary conditions and coupling relations into the existed governing equations for umbilical cable systems. A feed-forward and feedback control method was used for adjusting the length of the umbilical cable while the incremental PID algorithm was applied on regulating rotating speeds of propellers for establishing the integral hydrodynamic and control model of a tethered underwater robot. The experimental validation and hydrodynamic responses under the two control manipulations were simulated numerically. The simulation results showed that the proposed model was valid and reliable. In the depth control, the maximum errors of pitch, roll and submerged depth of the underwater robot between simulation and experiment are 2°, 1° and -50 mm respectively. The errors of trajectory tracking simulations in X direction and Z direction are 10% and 15% respectively. The motion in Z direction of the underwater robot is determined mainly by the feed-forward and feedback control strategy for the cable, and the motion in X direction of the underwater robot is primarily controlled by the PID algorithm for regulating the rotational speeds of the duct propellers. The hydrodynamic loadings on the robot are influenced by the flow fields around the robot, and the changes of the flow fields are determined by the changes of the robot velocity and the propellers rotating speed.

Ice ridges are one of the typical features of polar ice underlying surface. Understanding the interaction between ice ridge and fluid flow is important for the navigation of submersibles. Five groups of typically spaced ice ridges were established based on the polar field data to investigate the influence of ice ridges on the fluid flow beneath the ice surface and its influence range. By solving the steady-state Reynolds stress equation model (RSM) through Fluent software, the effect of ice ridge spacing on fluid flow beneath the ice was studied. The relationship between spacing and wake vortex oscillation was explored. The radiation depth of ice ridge interference with the ice flow field was also studied. The numerical result shows that the continuous ice ridges have a tensile effect on the tail vortex. Depending on the level of interference, radiation depth can be divided into three ranges: strong radiation area, stable radiation area and no influence area.

For a high-speed planing craft, remarkable variation of the sailing state may cause abnormal distribution of air-water on the bottom for numerical calculation. In order to match the mesh layout and free-surface, a numerical wave tank based on Reynolds-averaged Navier-Stokes (RANS) method was established with dynamic mesh and manual six degrees of freedom (6-DOF) motion model. The high-resolution interface capturing with volume-of-fluid model (HRIC-VOF) scheme was applied to calculate the bottom’s water-air distribution on the ship model. The influences of angle factor, sharpening factor, Courant number’s upper bound, Courant number’s lower bound and time step on the calculation results of water-air distribution and total resistance were explored. The comparison of calculation and experimental results indicates that the current method is feasible for high-speed crafts’resistance forecast and for capture of free-surface. The relative error is less than 4.5% for ship model’s velocity at 2-13 m/s when FV=0.96-5.78.