Applied Ocean Research最新文献

The wave-driven vehicle is a surface vehicle powered by capturing wave energy, which is required to face harsh sea conditions when performing tasks in parts of the ocean. However, wave-driven vehicles are usually small in size, and their seakeeping and speed are generally poor in high sea conditions. Wave driven vehicles are usually equipped with rigid connected hydrofoils to capture wave energy, which can provide power for wave driven vehicles and enhance seakeeping. Aiming at the long-term survival and operation requirements of wave-driven vehicle under high sea conditions, this paper studies the effect of high sea conditions launching wing on the self- propelled performance of wave-driven vehicle. After installing rigid connected hydrofoils on wave-driven vehicles, the structural parameters of the hydrofoils are changed, and the kinematic and dynamic responses of wave-driven vehicles at 0–90 ° wave encounter Angle are numerically simulated based on CFD method. The effects of underwater wing depth, hydrofoil spacing and hydrofoil span length on the self- propelled performance of wave-driven vehicles are analyzed. Based on this, the structural parameters of rigidly connected hydrofoils are optimized, which improves the seakeeping and rapidity of wave-driven vehicles in high sea conditions.

This paper presents an effective approach for quantitatively assessing the structural performance of the corroded reinforced concrete (RC) columns with a circular cross-section in aggressive marine environments. Firstly, the material damage and structural strength deterioration models are constructed by analyzing the degradation mechanisms of the materials due to rebar corrosion. The general method for calculating the load-bearing capacity of the circular RC columns at various corrosion levels with different loading conditions is proposed, based on the assumptions of the equivalent steel ring and the rectangular stress block of the concrete. In order to improve computational efficiency, a simplified method is then developed by introducing a linear relation to replace the complex expression of the load-bearing capacity coefficients of rebar in the general method. Finally, a numerical example is employed to investigate the effect of design parameters on the load-bearing capacity and performance deterioration rate of the corroded RC columns, and the effectiveness of the proposed simplified method is examined. The obtained results show that the corrosion level, loading condition and design parameters have a significant impact on the residual load-bearing capacity of the circular RC columns, and the simplified method with the introduced linear expression can be used for the preliminary assessment of the residual resistance of the corroded circular RC columns.

Natural gas hydrates stored in the subsurface seabed of continental margins are one of the most important carbon reservoirs on Earth. Research on natural gas hydrates is of great significance to global warming and ecological protection. Methane plumes caused by crustal dynamics are usually considered as a sign of existence of natural gas hydrates. Detection of methane plumes thus becomes the first step of cold seep research. This paper conducts comprehensive research on detection of methane plumes based on deep learning methods and optical images. First, we proposed a method of creating high quality and balanced datasets for methane plumes detection tasks using open-source videos. We then proposed a FMAW-YOLOv5s method for methane plumes detection. The FMAW-YOLOv5s method improves the traditional YOLOv5s in design of backbone network, neck network and loss function. The FMAW-YOLOv5s method can realize accurate and fast detection of methane plumes with a precision of 96.9% and FPS of 141.7. The lightweight feature of FMAW-YOLOv5s also enables the deployment in edge computing devices such as AUVs and ROVs. This research can not only promote the study of cold seep activities, but also provide meaningful insights for detection of other underwater events such as gas pipelines leakage.

Near-trapping is an essential resonant phenomenon associated with multiple-column structures in water waves, which exhibits high wave profiles in the area enclosed by multiple columns. For engineering safety, a straightforward scenario is proposed in this study to suppress the near-trapping phenomenon by allowing the multiple columns to move longitudinally with respect to the symmetric axes. To evaluate the effectiveness of the scenario, a stable and efficient second-order numerical model in the time domain is developed and adopted, which is also robust for the simulation of multiple structures with complex geometry and undergoing individual motions. Since both the first-order and second-order boundary value problems are solved, the second-order nonlinear properties are highlighted and the second harmonic induced near-trapping is the main focus of this study. For the cases in this study, the numerical results obtained by the validated numerical model confirm that this scenario can reduce the maximum second harmonic of the wave elevation by 63% and the maximum second-order wave elevation by 59% at the second near-trapping frequency. The first-order wave elevation is also reduced, and it is even smaller than the incident wave in a large portion of the enclosed region. As a mass–spring system is considered in the simulation of body responses, by testing different body masses and stiffnesses, it is revealed that the wave profile is insensitive to those parameters and the reduction in the wave profile occurs for all those parameters tested. It is interesting to find out that the near-trapping frequency can shift in the suppression scenario, and a remarkable reduction (32%) in the second-order wave elevation is still observed at the shifted near-trapping frequency.

This study examines the effectiveness of buffer blocks in dissipating the flow due to extreme events along the coasts using the numerical modelling approach. Initially, the study explores the effect of the number of rows of buffer blocks on the reduction of momentum flux. After establishing that the three rows configuration are the most efficient, subsequent analysis were carried out which include the understanding in the variations of block heights, including adjustments in height ratios between rows and by arranging blocks in increasing or decreasing height within the rows. Through a systematic examination of these configurations, the study aims to determine the most effective setup for maximizing energy dissipation of flow characteristics during extreme events such as tsunamis and storm surges.

The wave glider is an unmanned surface vehicle propelled by wave energy, consisting of three main components: a surface float, a submarine glider, and a tether. The submarine glider serves as the primary propulsion mechanism, converting the wave-induced motions of the float into forward thrust, which is crucial for the wave glider’s energy absorption efficiency. However, predicting the motion performance of the submarine glider presents a significant challenge due to its complex and unique structure. In this study, we establish a kinematic and dynamic model of the submarine glider’s hydrofoils, considering the elastic effects such as spring stiffness, spring preload, and spring attachment positions. To support this model, wind tunnel tests were conducted to determine the lift and drag coefficients of the submarine glider under various motion states. Utilizing the elastic hydrofoil model and the experimentally obtained lift and drag coefficients, we developed a comprehensive kinematic and dynamic simulation model of the submarine glider under heave excitation forces. To validate the accuracy of this model, performance tests for the submarine glider were designed under different vertical excitation forces , with results compared to simulation outcomes. The findings indicate that the deviation between simulated and experimental outcomes is less than 5%, demonstrating the model’s precision. This accurate simulation capability allows for detailed analysis of the effects of various design parameters on the glider’s performance and lays a solid foundation for high-accuracy motion simulation of the entire wave glider.

Large-scale direct shear tests were conducted to investigate the shear behavior of the interface between coral sand and geogrid. Polypropylene biaxial geogrid was embedded in the coral sands with two grain size distributions, which were in-situ coral sand (ISG) and uniformly graded coral sand (UG) from the Paracel Islands in the South China Sea. The results revealed the strain-softening behavior of both coral sands. The peak shear strength of the ISG coral sand was higher than that of the UG coral sand since the relative density of the ISG coral sand was higher. A bilinear relationship of peak shear stress versus normal stress was observed, with a dividing point of 100 kPa normal stress. This is because the shear displacement of the coral sand transferred from shear dilatancy to shear contraction when the normal stress reached 100 kPa, which enhanced the cohesion. The irregular shape of coral sand particles and the strong interaction of the geogrid contribute to a higher interface shear coefficient of coral sand, compared with silicious sand. The relative particle breakage was found to increase as the growth of the normal stress, and breakage was more significant in the ISG coral sand. The particle breakage rate of the reinforced and unreinforced coral sand was very close to each other, indicating that the application of geogrid in coral sand has little effect on the particle breakage rate.

Catastrophic structural failure caused by fatigue damage under cyclic loads can be avoided by identifying critical locations of the damage initiation and the fatigue life during the design stage. Peridynamic (PD) theory defines structure as a collection of material points with non-local bond interactions where the structural discontinuity due to fatigue represented by instantaneous bond breakage is estimated through cumulative decrement of the bond’s life at each load cycle. In this work, we model a reference jacket presented under the Offshore Code Comparison Collaboration Continuation project (OC4) through peridynamic beam formulation. Initially, the static deformations of the beam and deformations of the OC4 jacket under static, harmonic, and irregular point loads are validated with ABAQUS results in all six degrees of freedom. Thereby, the PD fatigue parameters of the jacket’s steel material are calibrated from the experimental data of the corresponding material with a trial simulation for fatigue damage analysis. Later, different load cases from regular waves interacting with the jacket are generated in the PD framework by adopting linear wave theory. Based on the PD cyclic energy release rate model, a comparative study of all load cases to identify critical damage locations with the failure load cycles for damage initiation and fracture is performed for the considered OC4 jacket.

Steel cylindrical shells have significant applications in the field of ocean engineering. However, such shells possess lower actual load-bearing capacity due to their high sensitivity to geometric imperfection. To improve the load-bearing capacity of normal steel cylindrical shells, steel-composite cylindrical shells were proposed in this work, and their buckling behaviours under axial compression were investigated in depth. The theoretical formula of the linear elastic buckling for the steel-composite cylindrical shells was derived. The linear bucking numerical analyses were conducted to verify the correctness of theoretical solution. The imperfection sensitivity of the steel-composite cylindrical shells were also examined by nonlinear buckling numerical analyses. Results show that the maximum average deviation between the theoretical and linear numerical values did not exceed 20 % for all considered models, and most of the average deviations were lower than 10 %. This exhibited a good agreement between the theoretical prediction and numerical simulation. Compared to the normal steel cylindrical shell, the steel-composite cylindrical shell possessed lower imperfection sensitivity and higher load carrying capacity. These findings can provide theoretical guidance for designing and evaluating steel-composite cylindrical shells under axial compression.

The damage state of the crane-wharf structure is difficult to describe quantitatively in earthquake, which brings great challenges to its emergency rescue and post-earthquake repair. At present, the criterion for evaluating the crane-wharf structure damage reflect a certain section or point, such as bending moment, axial force, displacement, stress, strain curvature, etc., which cannot reflect the overall performance and can be easily obtained. In addition, in the current popular performance-based seismic design concepts, the definition of damage degree is an indispensable part, which shows that the extraction and identification of damage characteristics is imminent. Therefore, on the basis of results of centrifuge experiment that have been carried out under different damage conditions, the basic damage law of the crane-wharf structure in liquefiable stratum is analysed first in the study, and it is clear that the existing means cannot distinguish the damage degree in detail. Furthermore, the proven numerical modelling techniques and the Hilbert-Huang transform theory are applied to extract and identify damage characteristics of the crane-wharf structure, Finally, the corresponding damage index is constructed and damage criterion are developed, which provided a reference for damage evaluation of similar structures in liquefiable stratum.