Marine Structures最新文献

In marine structures adhesive joint structures are often exposed to cyclic conditioning where the ambient humidity changes cyclically during their service. Though some comprehensive studies on the aging of adhesives exist, these researches mainly focus on monotonic aging conditions where the adhesive joints are exposed to a wet condition continuously for a long time. However, the few investigations performed on the cyclic moisture absorption of adhesive materials show that the parameters obtained in monotonic aging conditions are not suitable for estimation of the aging behaviour of adhesive exposed to alternating humidity. An important question that can arise is whether or not this frequency affects the mechanical behaviour of adhesive joints under cyclic aging condition. In this investigation bulk dogbone and square samples were manufactured, subjected to cyclic aging conditions with four different aging frequencies and tested. The results show that the moisture diffusion constant of adhesives exposed to higher aging frequencies increase more than those exposed to lower aging frequency conditions. In addition, the moisture content only affects the degradation of strength and stiffness of the tested adhesives in different aging frequencies.

Foundation damping holds large potential for design optimisations of monopile-supported offshore wind turbine structures. However, the contribution of foundation damping is not well understood, in part due to lack of suitable method for incorporating foundation damping in the time-domain analysis of wind turbine structures. This paper presents a practical approach for this purpose in which a dashpot is attached in parallel to each p-y spring along the monopile. In this way, the distributed soil-pile interaction stiffness and damping are accurately modelled. To demonstrate the validity of the proposed approach, a case study exploring the influence of foundation damping on the dynamic response of an IEA 15 MW reference turbine founded on monopile is carried out. The results demonstrate that the foundation damping has relatively limited impact on the overall dynamic response and fatigue loads in power production states. However, inclusion of foundation damping is shown to significantly reduce the structural response after emergency shutdown and under parked conditions. The findings of the case study show promising potential for wind turbine structure design optimization through consideration of foundation damping.

Models used in hydro-elastic experiments should not only reflect the geometries of the hull form, but also the elastic characteristics of the prototype. Two types of models have been generally developed for this purpose: segmented and fully elastic models. When scaling sectional properties, most studies have only considered similarity in the vertical bending mode. Container ships may also experience high torsional moments in addition to vertical bending moments, because of their large openings on the deck. To investigate the various hydro-elastic response of container ships, we develop and present a fully elastic ship-shape model in this study. Applying a novel structural design with internal structures resembling a container ship allows the vertical bending, horizontal bending and torsional vibration modes to be similar to those of a prototype container ship. Strain gauges instrumented throughout the model measure the hydro-elastic structural response including vertical bending moment, vertical shear force, and torsional moment. We studied the elastic parameters of the model comprehensively through a series of experiments and further tested the model in different wave conditions and forward speeds through wave experiments. We found the measured global structural responses agree with those of numerical simulations. The methodologies developed in this study provide new insights into how to design a fully elastic model which can be used to investigate the hydro-elastic responses of container ships.

An improved scaled method for box girders subjected to a hogging moment is developed based on maintaining the similarity of section properties and slenderness. The section area, moment of inertia, section modulus, plate slenderness, and column slenderness are considered as factors influencing the determination of the ultimate hogging moment. A smaller scale ratio is required to reduce the load-carrying capacity of scaled models, which poses challenges in ensuring that the designed scaled model aligns with the requirements of the construction process. Based on the discussion of several design strategies, a reasonable design of scaled models can be achieved through the following steps. First, the similarity of the plate slenderness is maintained by adjusting the number of stiffeners. Secondly, the similarity in column slenderness is maintained by adjusting the plate length. Third, the similarity of the section properties is improved by maintaining the similarity of the area of the local part. Simultaneously, the scaling strategy of the stiffeners is changed to ensure the similarity of the collapse modes. Fourth, modification methods for the similarity of the moment of inertia and section modulus are proposed to further improve the similarity of the ultimate bending moment. Furthermore, the criteria for choosing a modification of the section modulus and moment of inertia were proposed.

At present, there is no clear data to analyze the probability and impact of falling objects on the subsea Christmas tree. However, during the long-term operation in the complex deep-sea environment, objects may fall and hit the tree due to the anchored platform, fishing boat operation or other reasons, the impact of large heavy objects onto the tree can potentially give rise to the development of internal forces higher than those for which the tree was designed to. Therefore, aiming at the risk problem of falling objects, the paper presents the development process and results of an improved impact probability numerical method. And the safety model of falling object impact on the tree is established. Preliminary results indicate that the method offers valuable insight including and beyond a probability estimation of third-party damage, and the safety results were obtained. At the same time, the influencing factors of the tree impacted by falling objects are analyzed, and the serious influencing factors are obtained, which provides theoretical basis for the design and optimization of the protection frame of subsea square equipment.

Smooth bore unbonded flexible pipes are widely used to transport oil and gas in offshore fields. Due to its main application in shallow water, the radial bearing capacity is usually designed to be low for economic reasons. If the compressive load applied by tensioner equipment is too high during installation, the flexible pipe will fail due to excessive deformation. In this paper, the nonlinear behavior of smooth bore flexible pipes passing through tensioner equipment was investigated numerically and experimentally. A three-dimensional numerical model considering the nonlinear behavior of the materials, layer-to-layer contacts, and friction for flexible pipe radial compression analysis was established. A radial compression test was conducted to verify the accuracy of the numerical model; Then the numerical model was used to analyze the radial compressive behavior of each layer, and its load-bearing capacities and failure mechanisms were investigated. Finally, a parametric study was performed on the opening angle and number of tensioner tracks. The results of this study can provide a useful reference for the installation of smooth bore flexible pipes.

This study proposes a hydroelastic optimization method for multiseparated modules of high-specific-strength floating structures with low-weight cores and high-strength surfaces in two-dimension. A sixth-order dynamical model of this floating composite structure is deduced from the fluid-structure interaction condition of the wave surface. Drawing upon potential flow theory and separation of variables, we discretize the water domain into plate-covered areas and open water areas, in which the analytical solution for the interaction between waves and the floating composite structure with any number of separated modules is derived. The convergence and accuracy of the analytical solution are verified by calculating the transmission and reflection coefficients of the waves and various hydrodynamic parameters, including deflection, bending moment, and shear force. Furthermore, the effects of the number of modules, module spacing, and core thickness on the hydrodynamic parameters of the floating structure are investigated. The module number and structural-component-material parameters are coupled in influencing the hydroelastic behavior and mechanical responses of this floating cluster. This proposed method is an alternative optimization technique for considering the sea state, spatial modules, structural features and material properties in practical engineering applications.

Thickness loss due to corrosion wear generally occurs on ship structural members in corrosive environments. For corrosion wear, a probabilistic model can be successfully identified from the thickness measurements data to evaluate and estimate wear conditions. On the other hand, thickness loss due to mechanical wear may occur in some ship structural members in addition to thickness loss due to corrosion wear. In this study, a new probabilistic model for the case where mechanical wear is superimposed to corrosion wear is proposed. The condition observed in the actual structure is a mixture of corrosion wear condition and condition of corrosion wear with mechanical wear. When identifying a probabilistic model based on the plate thickness measurements data, it is impossible to distinguish whether the measurements data represent a corrosion condition due to corrosion wear or corrosion wear with mechanical wear. Therefore, a method is examined to obtain a model that would be the maximum likelihood by classifying the data using latent variables. By classifying the data and identifying both of a probability model of corrosion wear and a probability model of corrosion wear with mechanical wear, respectively, the posterior distribution of the latent variables is updated. These processes are repeated until the likelihood of the mixed probability models is maximized. For verification, a set of corrosion data consisting of mixed wear conditions were simulated and analyzed. It shows that both probability models could be properly identified with the proposed method, and in particular, the amount of diminution corresponding to high cumulative probability could also be properly estimated. Furthermore, this method was applied to thickness measurements data of ship structural members in the lower cargo holds of bulk carriers, where mechanical wear is also a concern, and it was confirmed that there was no difference in the corrosion wear condition of the members in the lower part of cargo holds of bulk carriers, however, significant differences were observed in the degree and extent of mechanical wear, indicating that the influence of mechanical damage is very significant in the wear condition of these members.

Top-tensioned risers are commonly used to transport fluid from seabed wells to floating platforms at the sea surface. Under wave action, the floating platform may heave. The tension at the top of the riser may change, potentially causing parametric excitations on the riser. The riser may transport oil and natural gas at the same time. The internal total fluid density fluctuations may induce parametric excitations on the riser. In this study, a top-tensioned riser under parametric excitations from periodically fluctuating top tension and varying internal fluid density is simulated and analyzed extensively. The variations in the top tension are modeled using a sinusoidal function over time. Meanwhile, the varying internal fluid density is described with a function of time and space. The governing equations of the riser are derived based on Hamilton's principle. The present model is solved numerically and validated. The analysis shows that if the top tension and internal fluid density fluctuate outside parametric resonances, the riser may become unstable. In different parametric resonances, different riser modes are excited, leading to irregular vibrations with multiple frequencies.

The expansion of aquaculture fish farms into deep sea has become a trend today. Several new types of offshore fish farming structures have been designed and tested in real sea, a typical of which is the semi-submersible structure. To ensure structural safety in harsh sea environment, detailed investigations on the hydrodynamic characteristics of offshore aquaculture structures are necessary and meaningful. The study focuses on the basic components of a semi-submersible net cage, namely the combined cylinder-net structure, and conducts CFD-based numerical research on its hydrodynamic coupling characteristics under uniform flow. The net structure is numerically simulated based on the porous media model. The drag and lift force and corresponding flow field of only the cylinder, only the nets, and the combined cylinder-net structure are measured under different inflow angles and solidity ratios of nets. The results indicate that the presence of the cylinder increases the drag of nets, and its impact on lift force of nets is closely related to the inflow angle. On the other hand, the presence of the net structure also amplifies the resistance of the cylinder. Meanwhile, the disturbance law of the flow field caused by the cylinder and the net structure is analyzed. By combining numerical and existing experimental results, the hydrodynamic coupling mechanism between the cylinder and the net structure is preliminarily revealed. This study indicates that when conducting hydrodynamic analysis of offshore structures such as the new semi-submersible net cage, the hydrodynamic interaction between the columns and the net structure should be considered.