With the long-term service of a semi-submersible platform at sea, the basic design parameters of the platform will change. If the platform is still analyzed based on the original design parameters, the accuracy of the results will be affected. In this paper, the basic parameters (displacement, center of gravity height and three-degree-of-freedom radius of rotation) that are easy to change during the service of a semi-submersible platform were taken as the updating parameters, the objective function was constructed by the variances, and wave frequency power spectral density of platform roll, pitch and heave, and a Bayesian parameter updating method of semi-submersible platform based on the monitoring data was established. With the monitoring data of a semi-submersible platform in China taken as an example, the parameter updating of the platform was carried out, and based on the updated parameters, the response of the platform under a multi-year return period environmental load was compared and evaluated.

Mono-shell submarine's acoustical coating is supposed to possess the multifunctional characteristics of mechanical noise, hydrodynamic noise and acoustic target intensity control. Two difficulties need to be addressed for this purpose: the first one is to improve the mechanical noise low-frequency control effect; the other one is to achieve multifunctional compatibility of acoustical coating. In this paper, from the perspective of reducing the sound radiation of mechanically excited shells, modal superposition method was adopted to establish the vibro-acoustic coupling and sound radiation model of multilayer graded acoustical coating and finite length ribbed cylindrical shell in ideal aqueous medium of infinite size. Based on the acoustic coating modal transfer function characteristics, the acoustic coating gradient distribution characteristics, thickness, the number of layers and other parameters were analyzed to reduce sound radiation of the ribbed cylindrical shell. The characteristic acoustic impedance asymptotic parameter distribution of the layered gradient acoustic cover was given and compared with the sound reduction characteristics of laying a uniform acoustic cover. The results indicate that the acoustic coating with increased acoustic impedance and slow wave velocity from the inner to the outer layer of the ribbed cylindrical shell has the property of reducing the vibration displacement of the outer surface more effectively than the uniform coating, which can significantly increase the sound reduction effect and extend the low frequency range of noise reduction. Optimisation of the acoustic coating should increase the acoustic impedance mismatch of the inner layer and the outer layer adaption effect, in order to help reduce the low frequency acoustic radiation of the ribbed cylindrical shell and to take the reduction of the acoustic target intensity into account.

Based on the concept of ‘casing treatment’ technology in aero-engine, a certain number of axial groove structures were set up in the inner wall of a pumpjet propulsor duct to weaken the tip vortex intensity, so as to reduce the pulsating pressure on the inner wall of the duct. By applying DES method and sliding mesh method, the flow around tip vortex field region of an elliptical hydrofoil was numerically simulated. The numerical calculation results are in good agreement with the experimental values, which verifies the reliability and applicability of the numerical calculation method.On this basis, the numerical simulations of the pumpjet propulsor with and without groove structures were carried out, and the effects of groove structure on the tip vortex core pressure, the pulsating pressure on the inner wall of the duct and hydrodynamic performance were compared and analyzed.The results show that the groove structures can significantly increase the tip vortex core pressure, reduce the amplitude of fluctuating pressure on the inner wall of the duct, and has little effect on the hydrodynamic performance.

Double-layer gearbox casings can not only provide elastic support for the gear transmission system, but have a good performance in the reduction of vibration noise. To efficiently and accurately predict the control effect of the vibration noise for double-layer gearbox casings, the flexible dynamic condensation model of a marine double-layer gearbox casing with vibration isolators was established based on the modal synthesis method. The condensation model was verified by calculating the natural characteristics of the complete finite element model of the double-layer gearbox casing. The vibration isolation performance of the double-layer gearbox casing was evaluated by analyzing the modal decoupling rate, modal contribution, vibration level difference, and inner casing inclination of the established condensation model. The results show that the modal decoupling rate of the double-layer gearbox casing in the translation direction is above 94.9%. The modal contribution is mainly dominated by the second to sixth mode shapes. The maximum vibration level difference can reach 34.2 dB in the excitation frequency range of 1-6000 Hz. The excitation frequency has a significant effect on the variations of the vibration level difference and inner casing inclination when the excitation frequency is less than 2000 Hz.

As a new type of structure, there is no mature design and strength analysis method for large-span reticulated shell structures of offshore tourism platforms. In this paper, the relative vertical displacement of the reticulated shell structure column was taken as the wave load control condition, and the wave load was calculated based on the design wave method and the three-dimensional hydroelastic theory.With the once-in-a-century wind speed taken as the wind load working condition, the turbulence model of k-ε was adopted to simulate the wind field for calculation of the wind load, the panel integration method was used to calculate the equivalent node load of the reticulated shell structure, and the calculation method of the external load for large-span shell structures was thus established. On this basis, taking the“Heart of the Sea”tourism platform as an example, the response analysis of the reticulated shell structure was carried out, and the responses of the reticulated shell structure under the action of wind load, wave load and combined loads of both were obtained. The effects of wind load and wave load on the response of the reticulated shell structure were compared and analyzed.

PTO (power take-off) system is an important part of wave energy converters (WECs). In this paper, a mechanical PTO system was proposed for a WEC with a float arm. Co-simulation study of the hydrodynamic performance of float-arm buoy and PTO system under the influence of generator load and mechanical transmission was carried out.The motion responses of the buoy under different stroke modes were analyzed, the parameters of the PTO system of the co-simulation model were also optimized.The results show that the power of the WEC with double strokes is significantly higher than that of the WEC with a single stroke, and that the transmission ratio corresponding to the optimal power of the WEC with double strokes is about 72.5. For the buoy working with a single stroke, the ascending stroke power is nearly equal to the descending stroke power under a low transmission ratio (less than 60). With the increase of transmission ratio, both the power of the ascending stroke and the power of descending stroke increase compared with that of dual-stroke mode, while the descending stroke power is larger than the ascending stroke power. The co-simulation method may provide a reference for the design and optimization of various point absorber WECs under different sea conditions.

Through long-term evolution and natural selection, fish have excellent swimming ability in water. With the help of the caudal fins, fish can perform fast and efficient straight-line swimming and fast start/maneuver. With the help of the pectoral fins, fish can move forward, backward and turn flexibly. Based on computational fluid dynamics (CFD) method, a mesh division strategy was proposed to solve the rigid motion of the caudal/pectoral fins and the flexible motion of the fish bodies. The hydrodynamic performance of the caudal/pectoral fins and the fish in uniform flow and the fish’s self-propelled swimming in still water were numerically simulated. The results show that the dynamic mesh method can simulate the rigid or flexible movement of fins and fish bodies effectively by using the hybrid mesh strategy of structure and non-structure based on CFD method. The effectiveness of the method in solving the hydrodynamic performance was verified by comparing with the experimental results. The numerical calculation method and validation examples have theoretical reference significance for the study of bio-hydrodynamics.

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.

Developing a reliable potential flow solver is necessary for the ship CAE software. The method on the three-dimensional time-domain Green function of ships with forward speed in infinite depth was studied. Firstly, a three-dimensional transient free-surface Green function was introduced, and its Rankine part was calculated by Hess & Smith’s method, and its free-surface memory part was calculated by the method of Beck team from the University of Michigan, US. Secondly, the memory part of the Green function and its derivatives were obtained in the mathematical expressions for making the program in five regions. Thirdly, the source method was used to calculate the source and the radiation potential, and then the radiation force was obtained by integrating the pressure around the floating body. Finally, the radiation force of the Wigley I ship was calculated, and compared with the published experimental and numerical results. The reliability of the method and the code in this paper is confirmed.

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.