For high speed ships, impact produced by cavitation is the source of abnormal noise and even cavitation erosion. However, cavitation is actually a complex process containing multi-scale structures coupled with many influencing factors such as cavitation shedding, cavity breaking, local merging and collapse, it is still a tough work to make clear the mechanism for the impact in full-scale cavitating flow and to control its effects. In this paper, cavitation around a twisted hydrofoil was selected to clarify the relation between the cavitating impact and instantaneous cavitation behavior, using a synchronized system composed of a high speed camera and a hydrophone to make the measurements synchronously in time domain, and a soft paint layer to show the cavitation impacted region. The results indicate that for a typical sheet-to-cloud cavitating flow, the high magnitude impulse composed of high frequency component is related to the local collapse and rebound of cloud cavitation shedding from sheet cavity. The study provides the physical understanding and control of cavitation impact in the industrial field.

Vortex-induced vibration (VIV) is one of the main factors leading to fatigue damage of marine risers, and the endeavor to suppress vibration is one of the important contents in the design of cylindrical ocean structures. In this paper, based on a two-way fluid-structure interaction simulation method, the VIV response characteristics related to airfoil pipe, textured pipe and smooth pipe were investigated, taking into account the function of VIV passive suppression attributed to special-shaped surfaces such as spoilers and surface textures. The simulation results show that both the spoiler and the textured shapes can effectively reduce the vibration response, shortening the vibration lock-in range. By affecting the vortex shedding modes and reducing the fluid force coefficients, the special-shaped surfaces covered onto the flexible pipes play a significant role in suppressing VIV, especially under conditions with higher reduced velocities. This research work provides a reference for the design of marine risers covered by special-shaped surfaces, such as airfoil pipes and textured pipes, aiming at passive vibration suppression.

At present, iFEM (inverse finite element method) is one of the most promising methods to construct a structural strain field. The purpose of this method is to obtain a structural strain field that meets the precision requirement with the least number of real measuring points in the process of discrete strain acquisition. However, when it is difficult to collect strain data in some local areas, discrete strain data can be collected by combining virtual and actual measuring points. In this paper, the stiffened plate of a typical ship structure was taken as an example, according to the measured data, combined with the simulation model and the measurement point regression method of Xgboost, the strain field reconstruction accuracies of the three methods of measurement, simulation and virtual and real combination were calculated based on the iFEM technology, and the causes of error were analyzed. The prediction results showed that the average error of 47 physical measurement points was the lowest, which was 1.92%.When 15 points and 21 points were input through the virtual-real combination path, the errors between the results and the verification points were all less than 3%, which verified that the strain field reconstruction method of virtual-real combination with a quick supplement of the missing data has strong operability and high accuracy.

The Arctic region is rich in oil and gas resources. The exploitation and utilization of oil and gas resources in this area have great prospects, but are also very challenging. Arctic areas have ice-free and ice-covered seasons. As current design specifications for offshore platforms do not fully consider the impact of sea ice, the effect of wave loads and ice loads should be considered together to enable the platform to operate all year round. In this paper, the dynamic response of a semi-submersible offshore platform under different conditions was studied, such as wave, level ice, and ice floes. Potential flow theory and Morrison equation were used to calculate wave loads while the discrete element method was used to solve the ice loads. The slender body finite element method was used for analysis of the slender system. The motion of the platform and the force of the slender system were solved by nonlinear time-domain calculation. The results show that compared with other environmental loads, the tension of the mooring line is the highest under level ice. In a broken ice field with smaller ice thickness, ice concentration, and ice floe size, the platform motion conforms to current safety specifications. The research of this paper will provide a reference for the safe development of oil and gas resources in the Arctic region.

Ship roll damping estimation based on roll decay curves is very common in engineering. There are many methods to estimate damping from roll decay curves, which lead to difference in damping estimation results. In order to achieve the purpose of obtaining high-precision damping prediction methods, the principle of estimation of roll damping based on roll decay curves in recent years was analyzed in detail first in this paper and then its scope of application was expanded. The methods were divided into two categories according to the principle, namely, one based on piecewise linear assumption and the other based on parameter identification. The data of standard model (DTC) were used to verify and compare the characteristics of different methods, and the error sources of damping coefficient estimation were analyzed. The results show the accuracy of estimation is strongly related to the used method. The Froude energy method with clear physical meaning, few error sources, and insensitivity to measurement noise and error is recommended as the preliminary estimation method. If a damping model with sufficient accuracy can be found, estimation based on the Prony-SS method, which has an inherent noise suppression mechanism and a unique solution, can be further done to improve the accuracy.

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.

Deep draft cylindrical floating nuclear platforms are a new type of multi-functional and efficient platforms, which can effectively solve the problem of energy supply in the development of offshore islands and reefs in the South China Sea. At a certain inflow velocity, the wake area will induce vortex-induced motions (VIMs) of a platform, which will seriously accelerate the fatigue damage of the mooring and riser system, and adversely affect the operation of nuclear reactors inside the platform. Based on the improved delayed detached eddy simulation (IDDES), the sway, surge and yaw motion responses of a platform at different reduced speeds were numerically simulated, and the key characteristics of vortex-induced motions were analyzed from the perspectives of motion trajectory, motion frequency and three-dimensional flow field. The results show that when the reduction speed of U_r ranges between 5.45 and 9.08, the amplitude of sway and yaw increases gradually, the motion trajectory is similar to a“banana”, the frequency of sway and yaw is basically the same, and there is no obvious change of“locking”interval in sway. However, when 7.26 < U_r < 9.08, the amplitude of yaw increases approximately linearly with several peaks appearing near the dominant frequency, and the motion trajectory gradually widens in the downstream direction. In the three-dimensional flow field, it is found that the three-dimensional vortex structures is quite complex, and that the skirt structure at the bottom of the platform interferes with the surface flow separation, which results in a certain vortex reduction.

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.

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.

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.