Because of the advantages of good physical conservation and grid suitability along with low demand on computational consumption, VOF is now the main numerical method for the treatment of free-surface in ship hydrodynamics. But for the original VOF method, the problem of interface diffusion is rather serious and results in excessive thickness and insufficient resolution for interface simulation. It will also affect the computational precision of other correlative variables in the flow filed. This problem is especially serious in the simulation of unsteady free-surface flow. In this paper, anti-diffusion VOF method was developed by introducing an artificial convective term into the control equation to restrain interface diffusion and reduce interface thickness. Implicit scheme was adopted for discretization of the artificial convective term to ensure computational stability. The method was tested by typical cases of Zalesak and shear field under different meshes. The results show that the interface thickness is reduced remarkably and the mass error is also reduced obviously. The simulations of 3-dimensional test cases for dam break and flooding in damaged cabin show that the anti-diffusion VOF method can capture the free-surface better and improve computational precision in the CFD simulation of practical unsteady free-surface flow.

With the continuous development and consumption of traditional land resources, the development and utilization of new water energy has become a new trend, and a large number of various floating structures have appeared. As the key to ensure the safe and stable operation of floating structures, mooring systems have always been the focus of the industry. In this paper, a large number of literature review and research were carried out on the existing floating structure mooring systems, the types of floating structures were summarized, the mooring system structures were analyzed from the aspects of the classification of the mooring systems, the way of chain distributions, the bottom anchorage foundation types and the new mooring systems, and the characteristics and advantages and disadvantages of various mooring cable materials were discussed. The static characteristics and dynamic response of the mooring systems were analyzed according to a large number of existing literatures, and the applicability evaluation and recommendation of various mooring methods were given through a comprehensive analysis of the water depth, seabed topography, geology, platform function, wind and wave conditions, economy and other aspects of the mooring engineering, and the shortcomings of the existing research were pointed out, and the current research direction still needs to be further developed.

The defects of a submarine pipeline caused by laying, non-uniform seabed support and impact of falling objects are often distributed in many places. In order to simplify the analysis, the buckling model of a pipeline with multiple elliptical defects in the axial direction is generally simplified as the pipeline with uniform elliptical defects or the pipeline with a single local elliptical defect in the axial direction. However, there are few studies on the differences of these three models and their application in engineering. Based on the numerical simulation, this paper presents a study on the buckling and instability behavior of the pipeline with three kinds of elliptical defects. The RIKS method was used to solve the buckling pressure of the pipeline, and compared with the calculation results of DNV-OS-F101, focusing on the analysis of the excitation position and influence parameters of the buckling and instability of the pipeline with multiple elliptical defects in the axial direction. The results show that the buckling pressure of the pipeline model with different elliptical defects is obviously different, and the order is the buckling pressure of the pipeline with uniform elliptical defects < the theoretical solution of DNV-OS-F101 < the buckling pressure of the pipeline with multiple elliptical defects < the buckling pressure of the pipeline with single local elliptical defects. For the buckling analysis of the pipeline with multiple elliptical defects in the axial direction, if the pipeline model is simplified with axial uniform elliptical defects, it is more conservative. However, if the pipeline model is simplified with axial single local elliptical defects, it is more dangerous. Besides, the buckling pressure of the pipeline with multiple elliptical defects is affected by the size of elliptical defects, the distance between elliptical defects and the length of elliptical defects.

With an 80 000 DWT bulk carrier taken as the optimization object, two new hull lines design technologies based on the hull fusion method and regional feature analysis were applied by combining ship CFD resistance and self-propulsion assessment for the research on hull lines design and performance analysis, aiming at the hydrodynamic performance under design draft. With the whole design process starting with the reference ship, the hull line automatic deformation designs of the bow and stern part were made respectively in order to reduce the resistance and improve the wave making and stern wake distribution. By analyzing the CFD numerical simulation evaluation results under the same model scale, a series of design hulls with good performance were successfully obtained. Finally, the simulation results were verified by model test under the design draft in still water. Compared with the design prototype, the received power trial prediction of the final design scheme at the speed of 14.0 kn was reduced by about 5.6%.

Ventilation law of supercaviting vehicles plays an important part in cavity shape control. This paper presents the development of an experimental method in high speed water tunnel to study the supercavity stability which describes the relationship between cavity shape and ventilation rate. An empirical formula was established based on test results of cavity shape and ventilation flow rate under different cavitator sizes and angles of attack of the model. By making analysis and comparison of the experiment results with and without tail wings, the influence of tail wing on ventilation law was preliminarily obtained. The results show that ventilation rate needs to be kept in certain range in order to maintain a steady and smooth ventilated supercavity.

The existing fatigue S-N curves are no longer applicable to the low-temperature environment in the Arctic regions. In order to evaluate the low-temperature fatigue life of Arctic offshore engineering equipment, it is necessary to establish the low-temperature fatigue S-N curves of metal structures, especially welded structures. In this paper, based on the equivalent structural stress method, the master S-N curve of the girth weld of Q690 high strength steel pipes was calculated, and the method was verified by resonance fatigue test. On this basis, combined with a large number of test results of high-strength steel welded structures, the temperature sensitive factor c was introduced into the derivation of the low-temperature fatigue master S-N curve. The master S-N curve based on low-temperature metal welded structures was established for the first time, and the fatigue S-N test data in the literature and the correction method in ASME were compared and verified. The results show that the equivalent structural stress method can accurately calculate the weld fatigue S-N curve, and the derived low-temperature main S-N curve of metal welded structures is in good agreement with the test curve, which can meet the engineering requirements in low-temperature areas, and provide theoretical guidance for the wide application of high-strength steel welded structures in low-temperature environments in the Arctic regions. This method can save a lot of costs for the low temperature fatigue research of metal welded structures and reduce unnecessary errors caused by non-standard test operations. This research is of great significance to the design, safe operation and fatigue assessment of Arctic offshore engineering equipment.

The two compulsory conditions for boundary layer separation are fluid viscosity and positive pressure gradient. By designing the shape of a vehicle so that its surface has a negative pressure gradient area as large as possible, the flow transition and separation are delayed so as to achieve the purpose of drag reduction. Based on the theoretical flow non-separation shape design method of slender bodies, the shape of a vehicle with a critical speed of 100 m/s was designed, and numerical simulation was used to analyze its flow characteristics at different speeds and angles of attack. It is found that the simulation results at zero angle of attack are consistent with those of the theoretical calculation, which proves that the surface of the vehicle can be in a state of non-separation of laminar flow through the shape design. A small attack angle will not destroy the fluid adhesion state on the surface of the vehicle, but whirlpools will appear in the flow when the attack angle is greater than 2 degrees.

In order to clarify the influencing factors of the amplitude of local ice pressure and pressure-area curve of the upright wide structures, the calculation method of the local ice pressure in ISO 19906 was firstly analyzed in this paper, and its limitation was found. Secondly, the discrete element method was used to simulate the interaction between different sea ice conditions and multi-scale structures. It is found that the amplitude of local ice pressure increases with the increase of ice thickness and decreases with the increase of structure width. The amplitude and standard deviation of local ice pressure decrease with the increase of width-thickness ratio, showing obvious scale effect. Finally, the influence of ice thickness and width on pressure-area curve parameters was studied by square-averaging method, and one similar working condition was selected to compare with that of ISO specifications. The rationality of the discrete element simulation method in dealing with the local ice pressure-area curve was verified, which provides a certain basis and reference for the ice resistance performance analysis and structural design of the platform.

With a scale model of tuned liquid multi-column damper (TLMCD) and floating substructure established, experiments were carried out in a flume to study the control effect of TLMCD on the pitch motion response of the floating foundation under regular wave excitation. The numerical model was established and verified by OpenFOAM. The coupling mechanism of TLMCD and floating foundation was analyzed from the aspects of flow field, hydrodynamic loads, floating body motion and damping force. The results show that TLMCD has the best pitch suppression effect under resonant excitation, and that the liquid with a mass ratio of 2% reduces the maximum pitch response of the floating body under resonant excitation by 10.84% to 18.53%, and achieves at least 7.32% damping effect in the range of 0.9<T/T₀<1.1. By numerical method, it was observed that under the condition of resonance, the hydrodynamic force generated by tank sloshing took up 89.52% of the time to do positive work, and that the sloshing of liquid in the liquid columns periodically provided reverse damping moment for the floating body.

In order to improve the efficiency of multi-parameter, high dimensional and high nonlinear optimization of ship structure reliability optimization design and make up the lack of uncertainty factors affecting structural safety in traditional deterministic optimization design, a river-sea-going ship was taken as the research object. BP (Back Propagation) neural network agent model technique and SMOTE (Synthetic Minority Oversampling Technique) algorithm were used to increase the number of sample points near the failure surface, in order to obtain a high-precision limit state agent model of ship structure with fewer sample points. Combined with Monte Carlo simulation method, the reliability calculation program of hull structure was developed. Structural reliability optimization analysis was performed adopting the simulated annealing optimization algorithm in order to reduce the structural weight. A set of complete and effective reliability optimization design system based on agent model technology was established to improve the efficiency of reliability optimization design, which has guiding significance to the reliability optimization design of river-sea-going ship structures.