Numerical wave simulation is a significant research topic. In this paper, the open source computational fluid dynamics (CFD) platform, OpenFOAM, is utilized to simulate Stokes fifth-order waves. Since geometrical volume-of-fluid (VOF) could better capture free surface due to its geometrical reconstruction step, the free surface simulations are accomplished by applying OpenFOAM built-in geometrical VOF method-isoAdvector, and the relaxation zone scheme is introduced through secondary development for wave absorption. The mesh density and Courant number convergence analyses with geometrical VOF are conducted. The simulation shows that satisfactory results could be obtained with a large Courant number. The algebraic and geometrical VOF simulated data with respect to wave elevation and phase at varied wave steepnesses and frequencies are recorded and compared with the theoretical value of Stokes fifth-order waves, which demonstrates that geometrical VOF is better than algebraic VOF in the prediction of wave elevation. Finally, the lengths and weights of the wave absorption zone are discussed, and the results imply that the best practice for the wave absorption is assigning the wave absorption zone length at least two times of the wave length along with applying exponential weight distribution.
In the initial design stage of a semi-submersible platform, the main particulars of the platform are the key factor affecting the hydrodynamic performance and construction cost. Therefore, multi-objective optimization of the main particulars of the semi-submersible platform is of great engineering significance. First, the design variables of each platform and sample database are determined by design of experiments. Then, the hydrodynamic performances of the semi-submersible platform are analyzed by using the panel method and Morison’s equation. The distribution of probes for estimating the wave elevations on the calm water surface is arranged, and the airgap can be computed. Based on the database obtained by numerical simulation, the surrogate models based on radial basis function (RBF) are established. Next, the formal parameters in RBF are obtained by using the leave-one-out cross validation method. The surrogate model can greatly improve the optimization efficiency. Finally, by using the multi-objective particle swarm optimization (MOPSO) method, taking safety and economy of offshore platforms as two optimization objectives, and taking platform stability, airgap and horizontal motion performance as constraints, the optimization program for the semi-submersible platform can be obtained. Through the detailed analyses of the optimization program for the semi-submersible platform, the most efficient design strategy for the three-column semi-submersible platform is proposed.
To realize the practical scale application of the spar-type floating offshore wind turbine (FOWT) in the medium depth sea areas, a novel 6 MW spar-type floating offshore wind turbine is analyzed by model test and numerical simulation under extreme conditions. The response of main freedom degrees, the mooring tense and the stress at the danger point are explored by a 1∶65.3 scale model at the State Key Laboratory of Ocean Engineering in Shanghai JiaoTong University. Coupled motion response of the spar-type floating wind turbine is calculated by using numerical simulation software in time domain. The results of the numerical simulation and model test are compared and analyzed in time and frequency domain. The maximum deviation between numerical simulation and model test is less than 12%, which shows that the numerical simulation results are in good agreement with the model test results. The dynamic response energy of the FOWT is mainly concentrated at low frequency and wave frequency. Moreover, the whole FOWT system has an excellent survivability under extreme conditions. Finally, the ultimate load of the wind turbine is predicted, which provides the necessary theoretical basis and calculation parameters for the structural strength calculation.
To improve the safety and operating efficiency of underwater slender bodies, the hanging underwater slender body is divided into several micro segments for analysis. The equilibrium equations of each segment are listed according to the mechanical equilibrium and deformation coordination conditions. Then, the equilibrium equations are solved with MATLAB programming. When the upper end moves horizontally and periodically, the upper end motion amplitude, upper end motion period, and lower end weight are changed separately to obtain the amplitude envelope of hanging underwater slender bodies. Based on the results of numerical calculations, the response envelope characteristics of hanging underwater slender bodies are analyzed. The maximum amplitude point on the amplitude envelope of the slender body is generally at the upper end. As the parameters change, it can be converted to the lower end. Changing the amplitude and period of upper end motion has a greater impact on the minimum amplitude point. The lower end weight has a small influence on the minimum amplitude point. By adjusting the ranges of these parameters, the positions of maximum amplitude point and minimum amplitude point on the amplitude envelope of slender body can be controlled.
In connection with the elastic-plastic failure problem of offshore jacket platform subjected to a powerful earthquake, an improved modal pushover method based on performance design is proposed. The elastic-plastic seismic performance and failure modes of the jacket platform are obtained to solve the problem of elastic-plastic performance evaluation of the offshore jacket platform in strong earthquakes. The elastic-plastic seismic responses of the platform in 8-degree seismic fortification and rare intensity are calculated by using different methods, and the differences between these responses are compared. Besides, the influences of combined modes, mode shape vectors, and uncertainties of seismic are discussed. The results show that the high-order vibration modes and mode shape vectors have a great influence on the elastic-plastic seismic performance of the platform. The first 9 or higher order modes and mode shape vectors should be adopted. The seismic-resistant weak links are located at the top of the platform in 8-degree seismic fortification and rare intensity, which should be paid more attention to. The seismic responses of the platform show significant differences and discreteness in different seismic activities, which have the same peak seismic acceleration. The improved modal pushover method is suggested to be used to evaluate the elastic-plastic seismic performance of jacket platform.
In order to study the liquefaction of the seabed around the pipeline under wave loading, a two-dimensional numerical model was established based on Biot’s partly dynamic equation (u-p model). The dynamic response of the seabed around the pipeline under wave loading was investigated in detail, and the wave loading was applied on the seabed surface through pore pressure boundary. Based on the validation of the numerical model, the response and liquefaction of the seabed around the pipeline under wave loading were studied. The pore pressure, vertical effect stress, and liquefaction range of the seabed at different pipeline buried depths were investigated and the effects of wave height, soil permeability, and saturation were discussed. The results show that the pipeline buried depth has significant effects on the response and liquefaction of seabed under wave loading. The pipeline induces the vertical effective stress concentration of seabed around the pipeline. The effects of wave height, soil permeability, and saturation on the seabed response under wave loading are significant. The results provide a theoretical basis for the safety and stability of submarine pipeline in marine environment.
In order to study the characteristics of vortex-induced motion and galloping of the classic Spar platform, a model test with a mooring system is conducted in basin. The sway motion characteristics of the Spar platform at different current velocities are analyzed. By comparing the results in currents, waves, and wave-current coupled conditions, the coupling effects of current and wave on vortex-induced motion and galloping of the Spar platform are also studied. The results show that galloping is induced by the currents with high reduced velocities. Compared with the vortex-induced motion, the galloping phenomenon has a longer period, larger amplitude, and randomness. The coupling of current and wave would not change the mode of flow-induced motion, but it significantly affects the motion amplitude.
In view of the low-frequency sound energy propagation in range-dependent waveguides in shallow water, in cylindrical coordinates, the sound energy flux is used as the study object based on the finite element method (FEM). The influence laws and the corresponding mechanisms of three types of complex seabed topographies, wedye-shaped seabed, seamount, and trench on sound energy propagation characteristics in sound field are discussed combined with specific simulation examples. The simulation results reveal that the FEM can accurately calculate sound field distribution in any seabed topography. For the up-sloping wedge-shaped seabed, the greater the inclination angle is, the stronger the sound energy leaks to the bottom, the more rapid the sound energy attenuates in water. But for the down-sloping wedge-shaped seabed the opposite is true. A small seamount enhances the sound energy above it, but hinders the reception of the sound energy behind it. The influence laws of small trenches are related to the grazing angle of sound energy. Only when the horizontal angle of the trench is smaller than the grazing angle of each normal mode in waveguide, would the sound energy be influenced in propagation.
