Marine Structures最新文献

Vortex-induced Vibration (VIV) responses of the three-dimensional cylinders inspired by Terebridae are numerically investigated at the Reynolds number ranges of 0.8 × 10⁴ ≤ Re ≤ 8.0 × 10⁴. By Terebridae, we mean that inspired by its slender structural shape, and whether it can be used to improve the VIV suppression performance of the traditional structures. Three different cylinders are considered, including the smooth cylinder (T₀), the Terebridae-inspired cylinder with four-start helical ribs (T₁), and the Terebridae-inspired cylinder with six-start helical ribs (T₂). The VIV responses of T₁ and T₂ is effectively suppressed, and T₁ shows better suppression performance. The results show that the maximum cross-flow amplitude ratio of T₁ is reduced by 70.7 % compared with that of T₀, and the maximum in-line amplitude ratio of T₁ is reduced by 85.7 % compared with that of T₀. Compared with the maximum mean-drag coefficient of T₀, it is reduced by 44.0 % for T₁. In the high Reynolds number ranges, the galloping phenomenon does not occur for T₁ and T₂, and the relationship between the mean-drag coefficients of T₀, T₁ and T₂ is $\overline{C_d}$ (T₂) > $\overline{C_d}$ (T₁) > $\overline{C_d}$ (T₀). It is found that the changes of Q-criterion vortex structure and wake flow play an important role in the VIV response. The Q-criterion vortex of T₀ is a slightly curved linear shaped, and the Q-criterion vortices of T₁ and T₂ are break up. The three-dimensional geometry of the Terebridae-inspired cylinder undermines the correlation of vortex-shedding along the span. The wake flow of T₀ can develop normally. The wake flow of T₁ quickly rolls up after passing through the top separation point, and the reattachment of the wake vortex destroys the development of the original wake flow. The continuous development of the boundary layer is destroyed by multiple Terebridae-inspired ribs of T₂, resulting in the broken wake. Through the comparison of vortex force, it can be seen that the vortex force of T1 is significantly weakened compared to that of T₀, and this also indicates that the three-dimensional geometry of T₁ can effectively reduce the intensity of the wake flow.

In this paper, the wave propagation mechanics theoretical model of hull grillage metastructures with local resonators is established, and the intelligent design method of hull grillage metastructures is proposed based on deep learning. The flexural vibration isolation characteristics of periodic hull grillage structures and the proposed hull grillage metastructures are theoretically studied using the spectral element method. Numerical calculation and experimental tests of the flexural wave vibration transmission of hull grillage metastructures are conducted to validate the proposed theoretical model. In addition, the data set is constructed by using the wave mechanics theoretical model of hull grillage metastructures, and the forward prediction neural network model of vibration transmission characteristics as well as the inverse design neural network model of hull grillage metastructures are established. Results show that compared with periodic hull grillage structures, the proposed hull grillage metastructures possess significant low-frequency flexural wave vibration isolation characteristics which almost completely covers the low-frequency range from 27 Hz to 169 Hz. Both forward prediction and inverse design network models have good performance, which verifies the correctness of the intelligent design method based on deep learning, and provides a new direction for the structural design on demand of low-frequency vibration reduction of ship and offshore structures.

This research investigates the collapse mechanisms and crossover pressure capacity of carbon fiber-reinforced polymer (CFRP) buckle arrestors in subsea pipelines. A nonlinear three-dimensional finite element (FE) model was formulated using ANSYS framework, incorporating a nonlinear geometric algorithm to simulate the collapse and crossover failures. The study underscored a good agreement between the FE model predictions, experimental observations done by the author, and previous research outcomes. Two principal crossover modes; flattening and U-shape modes were identified, though initiated similarly, differ in progression, attributed to the arrestor's attributes. The arrestor thickness ratio (h/t) emerged as a significant determinant of arresting efficiency, particularly impactful when h/t ≥ 1.5. An inverse relationship between σ_yp/σ_uc and P_X/P_P was identified. Furthermore, specific geometric parameters, like h/t and L/D, delineate the predominant crossover mode. Finally, new empirical expressions including the geometric and material parameters are proposed to predict the crossover pressure (P_X) for each crossover mode, separately. Also, the findings suggest that CFRP arrestors, especially those with configurations of L = 1.5D and h = 3t, can offer optimum resilience to external pressure.

The utilization of automatic mooring systems is under extensive interest with the growing technological demands for autonomous ships and smart ports. A vacuum suction pad with a rubber seal, which endures external loads to the moored ship, such as mooring forces, is a critical element in an automatic mooring system. To develop a high-performance automatic mooring system, a vacuum suction pad and a rubber seal need to be thoroughly designed, manufactured, and evaluated. This work demonstrates a protocol for evaluating the performance of a vacuum suction pad through both simulation and experimental testing. Uniaxial tensile testing was conducted to understand the mechanical behavior of a rubber seal. Stabilized stress-strain curves were utilized to find an optimal strain energy density function model and to extract material parameters for 3D finite element method (FEM) simulations. The FEM simulations were conducted to calculate strain distributions, contact status and the maximum load capacity, and along with the FEM simulation results, experimental evaluations of the vacuum suction pad were designed and conducted against static and cyclic loads. Based on the simulation and experimental results, we can conclude that the vacuum suction pad can maintain the stable suction at least up to 25 kN suitable for the use in automatic mooring systems.

The common ship structural optimization design is a deterministic method without considering uncertain factors, whereas the Reliability-Based Design Optimization (RBDO) can compensate for this deficiency. The RBDO of ship structure is a multi-parameter, high-dimensional, and high-nonlinear optimization solver. There exists a difficulty in guaranteeing accuracy and efficiency due to massive computation. In this study, the high-precision agent model for the limit state of ship hold structure is established based on agent model technology, including BP neural network, Radial Basis Function neural network, and Support Vector Machine combined with SMOTE oversampling algorithm. Furthermore, the reliability computation program is developed using Monte Carlo Simulation Method. A river-sea-going ship is considered the research object. The definition of rules, structural direct calculation result, and reliability requirement within all life cycles are considered boundary conditions. The RBDO system is constructed by the simulated annealing algorithm to investigate the lightweight structure. The established system can improve the efficiency and accuracy of the RBDO, which is significant for the ship's structural optimization design.

As the demand for harvesting offshore energy increases worldwide, the need for slender structures, such as marine risers and power cables, will increase. The dominating loads for deep water applications will primarily be caused by current-structure interactions, where vortex-induced vibrations (VIV) are known to be a challenging response type to handle correctly during design. Semi-empirical time-domain models are promising for VIV prediction, but uncertainties related to simplifications in the hydrodynamic load model and empirical parameters lead to over-conservative designs, where the parameters are estimated from model tests within a relatively low Reynolds number range. The aim of this paper is to present an efficient tool to estimate empirical hydrodynamic parameters directly from test data. To illustrate the method, the hydrodynamic parameters were estimated from pure cross-flow VIV model tests in a steady current with Reynolds numbers in the range of 40,000-120,000. The parameters were updated sequentially by using a Bayesian optimization framework, with the aim of minimizing an objective function that incorporates the response errors between time-domain VIV simulations and model test data. It is shown that three empirical parameters can be obtained simultaneously, resulting in small response errors but the optimal parameters were overly sensitive to variations in the flow velocity. The optimal parameters were used to reconstruct the drag amplification related to cross-flow VIV and in forced motion simulations to illustrate how the parameters in the load model would map into the traditional formulation in terms of the added mass and excitation coefficient.

Numerical investigations of Vortex-induced Vibration (VIV) suppression of a flexible cylinder with spanwise grooves in the uniform flow have been conducted by viv3D-FOAM-SJTU solver, which imports the thick strip model into OpenFOAM. The Reynolds-averaged Navier-Stokes (RANS) method is adopted to calculate hydrodynamic forces at each fluid strip. The vibrations of cylinder are solved through the Euler-Bernoulli bending beam hypothesis and the Finite Element Method (FEM) in two directions. During simulations, four spanwise grooves are arranged equally around the cylinder with an interval of 90° at two configurations. The spanwise groove keeps a constant width of w = 0.2D and a variable groove depth among 0.08D, 0.12D and 0.16D. Specific comparisons on Root Mean Square (RMS) vibration displacements, vibration modal responses, vibration frequency responses and vortex structures are conducted to analyze suppression effects of the spanwise grooves. Numerical studies indicate that the appropriate choose of groove geometry and arrangement can effectively reduce both crossflow and inline VIV responses.

The sandwich pipe, with its high hydraulic pressure resistance and thermal insulation properties, presents itself as a potential choice for deepwater oil and gas exploration. This study developed a 3D finite element numerical model using ABAQUS software to simulate the collapse propagation of sandwich pipes under the high external hydraulic pressure. A parametric analysis was undertaken using a numerical model, which had been calibrated by experimental tests. The study aimed to assess the contributions of material properties, such as yield strength and Young's modulus, as well as geometric dimension, to the collapse propagation behavior of the sandwich pipe. Moreover, a simplified empirical formula was formulated to research the propagation pressure of polymer sandwich pipes. The research also investigated the contribution of the interaction property between the core layer and inner/outer steel layer on the propagation pressure. An interfacial factor was incorporated into the present simplified empirical formula. The empirical formula was validated using experimental results and data from various sources, including real interlayer interactions observed during tests. The results indicate that this formula offers an accurate prediction of the propagation pressure for polymer sandwich pipes.

The single blade installation method for large-scale offshore wind turbine has the advantage of reduced deck and crane requirements on the installation vessel. Accurately simulating the coupled dynamic responses of the system throughout the entire hoisting process is crucial to avoid spatial interference with surrounding structures. Additionally, the aerodynamic loads, particularly the turbulent wind with spatiotemporal non-uniformity, further aggravate the uncertainty of the dynamic responses. Therefore, a numerical model capable of comprehensive analysis of mechanical-hydrodynamic-aerodynamic-control is urgently needed. However, it is challenging for the commercial software to achieve the coupling analysis. In this paper, the hoisting process is investigated by jack-up installation vessel, so the influence of wave loads on the structure is not considered. The aerodynamic loads are firstly calculated by the blade element theory, and then the numerical model is established by the Lagrangian method, finally the analysis program is developed in MATLAB. The accuracy of the program is verified by code-to-code, and the interaction between the ship-mounted crane and aerodynamic loads during the hoisting process are analyzed. The results show that the aerodynamic loads should be considered during the hoisting of the blade, and the established program could determine the trajectory of the blade inputting the sea state, which can avoid the possible collisions with the surrounding structures.

Characterizing multivariate ocean variables is quite critical for reliability design and risk assessment of marine structures. A robust, precise, and practical multivariate statistic model is necessary for comprehending ocean characteristics. As the time-varying characteristics exist in the ocean data, it is unreasonable to employ a simple constant statistical model to characterize all the multivariate data at one time. Therefore, in this paper, a time-varying copula approach is developed for modeling time-varying multivariate ocean data. Considering climate variations, a time-varying formula for return period and environment contour is also derived. The developed approach is demonstrated based on a site-specific ocean dataset collected from a buoy on the US coast. The climate effects associated with the multivariate ocean variables are characterized. The developed time-varying copula approach is also compared to the conventional copula and the conditional model in estimating the return period. The results showed that the time-varying model is helpful to explore the most critical environmental conditions for marine structures.