Aiming at the fatigue crack growth behavior of marine HTS-A steel at low temperature, the CT sample of HTS-A steel with a thickness of 25 mm was adopted in this paper, and the fatigue crack growth tests at room temperature and different low temperature environments were carried out. On the basis of experimental research, a low-temperature fatigue crack growth prediction method with improved McEvily formula including temperature term was proposed. The results show that the fatigue crack growth rate of HTS-A steel decreases gradually with the temperature decrease. At -60 ℃, the low temperature fatigue crack growth rate of HTS-A steel does not show low temperature brittle fracture, and the test results provide data reference for the low temperature fatigue design of marine HTS-A steel. At the same time, the low temperature test data of HTS-A steel in this paper and the low temperature test data of titanium alloy in the literature were used to verify the rationality and correctness of the low temperature fatigue crack growth rate prediction method. The method can be used to predict the metal fatigue crack growth rate in different low temperature environments.

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.

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.

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.

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.

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.

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.

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.

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.

The multi-spherical shells composite structure is a common structurural form of manned submersibles. A vibro-acoustic coupling model of multi-spherical shells system considering acoustic field coupling between shells was established by using the translationnal addition theorem for spherical wave function. The acoustic field coupling characteristics and their influencing factors of a series of spatially-distributed spherical shells were studied. The results show that the acoustic field interaction can change the coupling between the modes of the spherical shells. The contribution of a single mode to the vibration of the spherical shell is no longer limited to a single peak, and thus affects the spectral characteristics of the acoustic radiation. Both shells of the double spherical shells system generate acoustic radiation. There is a strong field coupling effect between two spherical shells under axial excitation. When the excitation force deviates from the axial direction, the field coupling effect gradually weakens. The double spherical shells system composed of spherical shells with different scales expands the frequency range of effective coupling, and the modal characteristics of both shells appear in the acoustic field. When the spherical shells are arranged in series, the acoustic field coupling has a transmission effect. While the spherical shells are distributed at different angles, the spatial distribution of acoustic field becomes more complex.