In order to understand the motion characteristics of amphibious aircraft planing in head waves, in this paper, the Chinese numerical tank-Cartesian grid finite difference method was used to study the planing motion response characteristics of an amphibious aircraft in waves under different wave conditions. The effects of wavelength and wave height on heave and pitch response of the aircraft and the vertical overload of mid-ship and stem were summarized. Based on the step-by-step method, the Navier-Stokes equation was discretized in space. The immersed boundary method was used to capture the boundary of complex objects and accurately simulate its large motion. At the same time, the THINC/SW algorithm was used to effectively simulate the violent motion of free surface. The results show that the wavelength and wave height have a great influence on the planing motion response of the amphibious aircraft. With the increase of planing speed, the pitch resonance wavelength increases, and the heave resonance wavelength decreases first and then increases, while the linear correlation between the heave and pitch response of the aircraft, the vertical overload of the mid-ship and stem and the wave height will also be weakened.

Thick-walled pipelines are widely used as transmission pipes for (ultra) deepwater petroleum and natural gas, and buckle arrestors for shallow water pipelines. However, the current international authoritative regulations may underestimate their ultimate bearing capacity significantly so that their economy and safety are hot topics in industrial circles. After deriving the calculation formula of vector form intrinsic finite element (VFIFE) method solid element, an analysis model of thick-walled pipelines considering the nonlinearity of geometry, material and boundary was established to solve the key mechanical problem of local collapse of thick-walled pipelines. And its accuracy was verified by comparison with 8 sets of thick-walled pipe scale tests, the DNV code, and ABAQUS simulations. Sensitivity analysis of diameter-to-thickness ratio, initial ovality and material yield strength were carried out to quantify the calculation errors of the DNV code method. Then, a more accurate formula for calculating the local collapse pressure of thick-walled pipes was obtained by fitting the VFIFE results. The results show that the simulation results of the VFIFE constant strain tetrahedral element are in line with the actual situation and can provide a new analysis strategy for the collapse behavior analysis of thick-walled pipelines. However, attention should be paid to determining the maximum load rate under the requirement of the quasi-static loading. Under high external pressure, the pipeline will collapse locally and propagate buckle dynamically and the deformation of the pipe section changes from an ellipse to a "dumbbell" shape with certain folds on the inner wall. During local collapse, the change trend of the stress distribution conforms to the general features of solid structure buckling instability. The calculation error of the DNV code of thick-walled pipelines’ local collapse pressure increases with the decrease of the diameter-to-thickness ratio, the decrease of the initial ovality, and the increase of the material yield strength respectively. The corrected formula for local collapse pressure calculation of thick-walled pipelines has a fitting error of -2.49%~1.72% for homologous data and a calculation error of -6.11%~1.70% for heterologous data. It can accurately calculate the local collapse pressures of deepwater pipelines with diameter-to-thickness ratio of 8~18, initial ovality of 0.5%~3.0%, and material yield strength of 300~500 MPa. The results can be used to guide the design and verification of submarine thick-walled pipelines.

A marine riser usually works in the form of multiple riser clusters. The hydrodynamic interference characteristics between adjacent risers are obviously seen, and the interference can accelerate the fatigue damage. Therefore, it is meaningful to investigate the vortex-shedding patterns and the hydrodynamic interference characteristics of twin cylinders in order to ensure safe operation of risers. This study was aimd to parametrically investigate flows around twin moving cylinders at a staggered arrangement by a ghost cell method, in which, the incompressible Navier-Stokes equations were solved on a Cartesian staggered grid by using an in-house developed time semi-implicit finite difference method. A ghost cell method was used to enforce the no-slip boundary conditions. A radial basis iso-surface function was used to track the moving boundary implicitly and identify the properties of background grid. Based on the present numerical model, flows around twin forced moving cylinders at a staggered arrangement were simulated. Vortex patterns and force coefficients were analyzed under different gaps and oscillation frequencies. Typical interference phenomena such as synchronized, deflected and merged vortex patterns were observed. The results can provide theoretical guidance for the arrangement optimization of the riser clusters.

Based on the SPH numerical simulation method, this paper presents an analysis of the wave variation of regular and irregular waves propagating along an inshore island reef with a floating structure, and the dynamic response of a tethered floating structure under the island reef topography, respectively. The results show that the floating structure attenuates irregular waves better than regular waves when a tethered floating structure is installed in front of the shore reef, and changes in wave height have little effect on the attenuation of floating structure wave height. However, changes in wave period have a greater impact on the attenuation effect of the floating structure, and the floating structure is less effective in attenuating the wave height of long-period waves. The maximum vertical displacement, maximum longitudinal rocking angle and maximum transverse oscillation values of the floating structure all show an increasing trend with wave height increasing under different regular wave height conditions, with the maximum longitudinal rocking angle being the most sensitive to changes in wave height and the maximum vertical displacement being the least sensitive.

An eccentric semi-submersible foundation was proposed considering the characteristic of wind energy direction concentration in actual environment. AQWA-Fast software was used to establish the floating wind coupling analysis model, and the kinematic response characteristics of symmetric and eccentric wind turbine were compared and analyzed under different incident angles of wind waves. The results show that the variation of wave incidence angle hardly affects the average motion response value and average power generation of the floating wind turbine. When the incident angles of wind and wave are the same, the eccentric platform has better motion performance in terms of sway, roll, and yaw degrees of freedom, and to some extent the mooring tension is reduced. When the wave incidence angle constantly changes, the eccentric design will improve the motion performance in terms of sway, roll, pitch, and yaw degrees of freedom. In addition, the eccentric floating wind turbine has better comprehensive power generation performance.

The additional strengthening effect caused by the non-proportionality of loading-path under cyclic loading is an important factor that shortens fatigue life of material. To solve this issue, the plane of maximum shear strain amplitude was treated as the critical plane, and the non-proportionality of material and loading-path were both considered. A new non-proportionality factor was introduced to quantify the impact of non-proportionality loading on fatigue life of material based on the equivalent strain model and critical interface theory. Secondly, the damage mechanism and fatigue failure mode of the specimen were also considered, the maximum normal stress on the critical plane was adopted to characterize its contribution to fatigue failure. On this basis, a multiaxial fatigue life prediction model was proposed by combining the non-proportional factors. Finally, the proposed model was verified by using the fatigue test data of four materials under multiaxial loading, and the prediction results were compared with five proposed models. The results show that the proposed model can effectively improve the fatigue life prediction accuracy under non-proportional loads compared with the existing models.

In order to accurately evaluate the influence of irregular current load on the safety of semi-submersible platform towing operation, based on the coupled time-domain analysis theory and potential flow theory, this paper presents an analysis of the influence of different current velocities and flow angles on the dynamic response and towing tension of semi-submersible platform towing under the same wave environment and towing speed through the semi-submersible platform-towing-tug coupling dynamic model. The results show that under certain wind and wave conditions, the current velocity and flow direction angle have little influence on the heave and pitch of the platform, but have great influence on the roll and towing tension. With the increase of flow velocity and flow direction angle, the platform heave gradually increases, with the platform roll and pitch angles under different flow velocities and flow angles fluctuating within ± 3.5°. The maximum towing tension is obtained when the flow angle is 90°. Therefore, the angle of 90° between the heading and the flow direction should be avoided during the towing process, and it is not suitable for towing when the flow rate reaches 1 m/s.

In ships’voyaging conditions, the global hull girder is subjected to the combination action of cargo loads on ship decks, still water moment and wave moment on hull bottom, and correspondingly partial ship stiffened panels suffer complex loads including longtudinal, transverse forces and pressures. Lateral loads could to some degree influence the ultimate compressive strength in the longitudinal and transverse directions. Thus, it is needed to establish the interaction formulae of ultimate strength for stiffened panels under combined loads. The stiffened panel structures in hull bottom of bulk carrier was selected as the research object. Based on the two-bays-and-two-spans geometrical extent model of stiffened panel with periodic boundary conditions, a nonlinear numerical method was employed to investigate the interaction formulae of ultimate strength for hull girder structures under longitudinal, transverse and lateral loads. It is found that lateral loads always reduce the ultimate strength in longitudinal and transverse directions, meanwhile the larger the lateral loads are, the more the ultimate strength decreases. It is suggested to increase stiffener size and plate thickness for improving the longitudinal and transverse ultimate strength under lateral pressures, respectively. The interaction formulae were developed to assess the ultimate strength of stiffened panels under combined loads and the interaction relationship of ultimate strength under multiple loads. It can be used to perform rapid calculation on the structural strength design under complex combined loads.

A numerical study of vortex-induced vibration (VIV) related to a flexible pipe system subjected to external current and internal flow was performed mainly to investigate the complex vibration response of the flexible pipe due to the coupled effect of external current and varying-density internal flow. The numerical model was validated through mesh dependency and fluid-structure interaction (FSI) analysis. A coupled correlation analysis method, combined with a 3D position-frequency-energy (PFE) spectral analysis technique, was proposed to reveal the spatial multi-mode competition along the flexible pipe span. It is shown that the increase in the velocity and density of internal flow amplifies the spanwise in-line mean deflection, but has limited effect on the dominant vibration mode. The vibration modes at the amplitude peak and trough are significantly different. High order vibration modes, characterized by classical“8”-shaped vibration trajectories, are dominant around the amplitude peak, but low order vibration modes become predominant, and phenomena of spatial multi-mode competition with chaotic vibration trajectories are favorable at the amplitude trough.

The line spectra noise radiated by cavity flows greatly deteriorates the acoustic stealth of naval ships, whose formation mechanisms are related to the flow and acoustic modal effects, as well as the acoustic-vibration and flow-sound coupling effects. In this paper, the transient flow, equivalent sound source and acoustic fields of the simple square and typical cavity flows were numerically simulated, based on the CFD/CHA hybrid approaches. The applicability of the numerical methods was verified by comparison with experimental data of water tunnel test. The characteristics of the flow and acoustic modes of the cavity flows were concluded, particularly the effects of the acoustic-vibration coupling of elastic cavity walls and the complexity of cavity inner shapes upon the acoustic mode frequencies were quantitatively calculated. The important regularity, which the line spectrum induced by the effect of the first acoustic mode is the "decisive line spectrum" in the far-field radiated noise spectrum, is summed up. The tendency of the acoustic mode frequencies to shifting sharply towards lower frequencies under the actual cavity conditions was analyzed, indicating the necessity of avoiding the flow-sound coupling. The necessary condition of the cross-sectional area ratio for the related acoustic experiments in water tunnels was quantitatively established through an analytical solution. The research has an important guiding value for the designs of experimental modals.