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.

The development of the potential flow solver for the CAE software of a ship requests a reliable method to solve the potential flow. The numerical method of the diffraction force with a three-dimensional time-domain panel method was studied. With a three-dimensional time-domain Green function introduced, its Rankine part was calculated by Hess & Smith’s method while its free-surface memory part was calculated by the method of Beck team from the University of Michigan, followed by the derivation of the diffraction impulse function in the mathematical expressions for making the program. Then the source method was used to calculate the source and the diffraction potential, and the diffraction potential force was obtained by integrating the pressure around the floating body. Finally, the diffraction force and Froude-Krylov force were calculated with the Wigley I ship. The verification was carried out by comparing the results with the published experimental and numerical results. The method and the code in this paper are reliable for developing the potential flow solver of the CAE software and predicting the nonlinear stability failure models in waves.

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.

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.

The calculation method for the slamming load of manned submersibles floating on the water surface at zero speed was studied in this paper, the calculation processes for predicting slamming pressure were provided based on the frequency domain and time domain response, the calculation formula for the extreme value of slamming pressure was derived, and the frequency domain and time domain slamming pressure prediction on a certain manned submersible were performed. According to the calculation results, the effects of ship motion and wave surface inclination on the water entry angle were not considered in the frequency domain method, so it is only suitable for ships whose motion responses have little effect on the water entry angle. For a manned submersible, the motion response is significant, which will reduce the entry angle and lead to obvious increase of the slamming pressure coefficient. At this point, the slamming load should be calculated based on the time domain method to avoid the underestimation of the slamming load. At zero speed, the slamming pressure of the submersible on the wave facing side is significantly greater than that on the other side. So the slamming pressure on the stern is greater in following waves, while the slamming pressure on the bow is greater in head waves.

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.

In order to study the vortex-induced motion characteristics with coupled multi-degrees of freedom of Spar platforms under a uniform flow, a numerical model about the vortex-induced motion of a Spar platform considering fluid-structure interactions was established based on STAR-CCM+, and the coupling effects of five-degrees of freedom (surge, sway, heave, pitch and roll) were investigated. The results show that the vortex-induced resonance can be observed obviously at three degrees of freedom (sway, heave and roll), and the velocity interval of vortex-induced resonance is the same at the two degrees of freedom (heave and roll). Meanwhile, the coupling effects between sway and roll are obvious. In the case of sway resonance, the relationship of the dominant frequency between sway and roll is 1:1, and in the case of roll resonance, the secondary peak frequency of sway is the same as the dominant frequency of roll. Moreover, there are complex nonlinear coupling relationships among the three degrees of freedom (surge, heave and pitch), not only the dominant frequency of surge and heave always exists in the pitch spectrum, but also the coupling effects between pitch and surge, pitch and heave are various in the different ranges of reduced velocity.

Offshore wind energy resources is richer than land wind energy, and water depth of the continental shelf in China's waters increases slowly as the distance offshore increases. Based on the characteristics, how to optimize the design of mooring systems to adapt to the water depth conditions in China is one of the major problems encountered in the development of floating wind turbines. In this paper, a 5MW-OC4 semi-submersible floating wind turbine was used as the research object, the floating wind turbine was moored by suspended chain lines, and the frequency domain and time domain calculations of the floating wind turbine were performed under 40 m water depth in a sea area of Bohai Sea using SESAM software. Mooring accessories were used separately and in combination for parameter sensitivity analysis, and then the mooring system was optimized by combining buoys and clump weight blocks. The results of the study show that under shallow water conditions, the combination of mooring fittings has the same effect on the overall response of the floating wind turbine as changing the same mooring parameters when used alone, but the effect of changing the mooring parameters on the optimisation of the performance of the mooring system is more obvious when used in combination, the optimisation difference in the mean value of the counterweight block position parameters when used in combination can reach 15.7%, and the optimisation difference in the longitudinal oscillation, longitudinal rocking and the tension response are all within 10% of each other. The optimisation difference of longitudinal oscillation, longitudinal rocking and tension response is basically within 10%. Therefore, choosing a reasonable combination of accessories can significantly change the overall characteristics of the floating system and affect the safety and cost of the system.

High-precision monitoring of the thrust of the propulsion shafting is of great significance for the ship rapidity prediction, hull-engine-propeller matching, and health management of the shaft. However, for the super-long propulsion shafting, because of its large span, small slenderness ratio (ratio of the radius to the length of the shaft) and large thrust, the shaft will produce severe bending-longitudinal coupling nonlinear deformation, which will have an important influence on the thrust measurement based on deformation information such as strain or displacement. Therefore, the bending-longitudinal coupling nonlinear effect of the propulsion shafting on the thrust measurement accuracy was studied. Considering the Von Karman nonlinear displacement-strain relationship, a nonlinear mechanical model of the propulsion shaft was established by using the Hamilton variational principle combined with the finite element method. The bending-longitudinal coupling effect on the displacement, strain and thrust measurement errors was studied. The results show that at low rotation speeds, the bending-longitudinal coupling effect is weak and has little effect on the thrust measurement. However, at high rotational speeds, ignoring the bending-longitudinal coupling effect will cause large measurement errors (the error can reach 13.95% at 240 r/min). Besides, the closer the measuring point is to the propeller, the stronger the bending-longitudinal coupling effect will be, and the larger the thrust measurement errors will be. Therefore, arranging the measuring point near the thrust bearing at the front end of the shaft can effectively reduce the measurement errors caused by the bending-longitudinal coupling effect. The research results have guiding significance for the thrust measurement of super-long shafts with large spans and small slenderness ratios.

In general, in the calculation of the hull girder ultimate strength and the residual strength, the bilge plate should be considered as a hard corner element in Common Structural Rules for Bulk Carriers and Oil Tankers, but the specific conditions are not clear. In this paper, a bilge curved plate with typical geometric shape inside and outside the midship region was studied by means of the buckling capacity evaluation method and the nonlinear buckling finite element analysis method. The brief conditions in which the bilge plate can be regarded as a hard corner element in the calculation of hull girder ultimate strength and residual strength required by common structural rule were obtained.