首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 437 毫秒
1.
A fully coupled 6-degree-of-freedom nonlinear dynamic model is presented to analyze the dynamic response of a semi-submersible platform which is equipped with the dynamic positioning （DP） system. In the control force design, a dynamic model of reference linear drift frequency in the horizontal plane is introduced. The dynamic surface control （DSC） is used to design a control strategy for the DP. Compared with the traditional back-stepping methods, the dynamic surface control combined with radial basis function （RBF） neural networks （NNs） can avoid differentiating intermediate variables repeatedly in every design step due to the introduction of a first order filter. Low frequency motions obtained from total motions by a low pass filter are chosen to be the inputs for the RBF NNs which are used to approximate the low frequency wave force. Considering the propellers’ wear and tear, the effect of filtering frequencies for the control force is discussed. Based on power consumptions and positioning requirements, the NN cen-ters are determined. Moreover, the RBF NNs used to approximate the total wave force are built to monitor the disturbances. With the DP assistance, the results of fully coupled dynamic response simulations are given to illustrate the effectiveness of the proposed con-trol strategy.  相似文献   

2.
Dynamic response analysis of a floating mooring system   总被引：1，自引：0，他引：1  
An innovative floating mooring system with two or more independent floating mooring platforms in the middle and one rigid platform on each side is proposed for improving efficiency and safety in shallow water. For this new system, most of collision energy is absorbed through the displacement of floating platforms. In order to illustrate the validity of the system, a series of model tests were conducted at a scale of 1:40. The coupled motion characteristics of the floating mooring platforms were discussed under regular and irregular waves, and the influences of wave direction and other characteristics on dynamic response of the system were analyzed. The results show that the mooring system is safest at 0° of wave incident angle, whereas the most dangerous mooring state occurs at 90° of wave incident angle. Motion responses increase with the increase of wave height, but are not linearly related to changes in wave height.  相似文献   

3.
An investigation on the dynamic response of a top tensioned riser (TTR) under combined excitation of internal solitary wave, surface wave and vessel motion is presented in this paper. The riser is idealized as a tensioned slender beam with dynamic boundary conditions. The KdV-mKdV equation is chosen to simulate the internal solitary wave, and the vessel motion is analysed by using the method proposed by Sexton. Using finite element method, the governing equation is solved in time domain with Newmark-β method. The computation programs for solving the differential equations in time domain are compiled and numerical results are obtained, including dimensionless displacement and stress. The action of internal solitary wave on the riser is like a slow powerful impact, and is much larger than those of surface wave and vessel motion. When the riser is under combined excitation, it vibrates at frequencies of both surface wave and vessel motion, and the vibration is dominated by internal solitary wave. As the internal solitary wave crest passes by the centre of the riser, the maximum displacement and stress along the riser occur. Compared to the lower part, the displacement and stress of the riser in the upper part are much larger.  相似文献   

4.
The dynamic response of the steel lazy wave riser(SLWR) subjected to the internal solitary wave is a key to assessing its application feasibility. The innovation of this paper is to study the dynamic response properties of the SLWR with large deformation characteristics under internal wave excitation. A numerical scheme of the SLWR is constructed using the slender-rod theory, and the internal solitary wave(ISW) with a two-layer seawater model is simulated by the extended Korteweg–deVries equatio...  相似文献   

5.
This paper presents the results from a numerical study on the nonlinear dynamic behaviors including bifurcation and chaos of a truss spar platform. In view of the mutual influences between the heave and the pitch modes, the coupled heave and pitch motion equations of the spar platform hull were established in the regular waves. In order to analyze the nonlinear motions of the platform, three-dimensional maximum Lyapunov exponent graphs and the bifurcation graphs were constructed, the Poincaré maps and the power spectrums of the platform response were calculated. It was found that the platform motions are sensitive to wave frequency. With changing wave frequency, the platform undergoes complicated nonlinear motions, including 1/2 sub-harmonic motion, quasi-periodic motion and chaotic motion. When the wave frequency approaches the natural frequency of the heave mode of the platform, the platform moves with quasi-periodic motion and chaotic motional ternately. For a certain range of wave frequencies, the platform moves with totally chaotic motion. The range of wave frequencies which leads to chaotic motion of the platform increases with increasing wave height. The three-dimensional maximum Lyapunov exponent graphs and the bifurcation graphs reveal the nonlinear motions of the spar platform under different wave conditions.  相似文献   

6.
CHEN  Zhen-lin XU  Qiang    HU Xiao 《山地科学学报》2013,10(6):996-1007
Currently, scant attention has been paid to the theoretical analysis on dynamic response mechanism of the ＂Dualistic＂ structure roek slope. The analysis presented here provides insight into the dynamic response of the ＂Dualistie＂ structure rock slope. By investigating the principle of energy distribution, it is shown that the effect of a joint plays a significant role in slope stability analysis. A dynamic reflection and transmission model （RTM） for the ＂Dualistic＂ structure rock slope and explicit dynamic equations are established to analyze the dynamic response of a slope, based on the theory of elastic mechanics and the principle of seismic wave propagation. The theoretical simulation solutions show that the dynamic response of the ＂Dualistic＂ structure rock slope （soft-hard） model is greater than that of the ＂Dualistic＂ strueture rock slope （hard-soft） model, especially in the slope crest. The magnifying effect of rigid foundation on the dynamic response is more obvious than that of soft foundation. With the amplitude increasing, the cracks could be found in the right slope （soft-hard） crest. The crest failure is firstly observed in the right slope （soft-hard） during the experimental process. The reliability of theoretical model is also investigated by experiment analysis. The conclusions derived in this paper could also be used in future evaluations of Multi-layer rock slopes.  相似文献   

7.
An offshore wind-wave hybrid platform could consistently and cost-effectively supply renewable power.A multi-objective optimization process is proposed for a hybrid platform with hydrodynamic coupling interaction.The effects of various critical structural parameters,spacing values,and wave directions are studied for higher energy capture and offshore platform stability.Approximation models of various key parameters are established to optimize the hybrid system,with the objects of the power captu...  相似文献   

8.
The removal of bed material from active river channels usually affects the bed profile of the streambed, causing progressive degradation upstream and downstream of the extraction site. These effects can extend for kilometers affecting hydraulic structures located in the vicinity of the river reach. In this paper, the geomorphic effects of gravel mining are reviewed and summarized. Some cases in Venezuelan streams are presented to illustrate the problem. To describe the processes of erosion and sedimentation in a gravel extraction pit, a recent developed mathematical model for the simulation of flow and sediment transport in gravel-cobble bed streams is applied to a hypothetical case of gravel mining in a river channel. A simple rectangular dredge pit is imposed as initial condition in the channel bed, and changes in bed elevations and grain size distribution of bed material are calculated by using the numerical model. The process of deposition within the pit, and the downstream and upstream migration of the erosion wave are well simulated by the model and closely resemble the phenomena observed in laboratory experiments. The response of the friction coefficient to the changes in flow and bed elevations shows the importance in modeling adequately flow resistance and sediment transport in gravel-cobble bed streams.  相似文献   

9.
Simplified wave models- such as kinematic,diffusion and quasi-steady- are widely employed as a convenient replacement of the full dynamic one in the analysis of unsteady open-channel flows,and especially for flood routing.While their use may guarantee a significant reduction of the computational effort,it is mandatory to define the conditions in which they may be confidently applied.The present paper investigates the applicability conditions of the kinematic,diffusion and quasisteady dynamic shallow wave models for mud flows of power-law fluids.The power-law model describes in an adequate and convenient way fluids that at low shear rates fluids do not posses yield stress,such as clay or kaolin suspensions,which are frequently encountered in Chinese rivers.In the framework of a linear analysis,the propagation characteristics of a periodic perturbation of an initial steady uniform flow predicted by the simplified models are compared with those of the full dynamic one.Based on this comparison,applicability criteria for the different wave approximations for mud flood of power-law fluids are derived.The presented results provide guidelines for selecting the appropriate approximation for a given flow problem,and therefore they may represent a useful tool for engineering predictions.  相似文献   

10.
Return periods calculated for different environmental conditions are key parameters for ocean platform design.Many codes for offshore structure design give no consideration about the correlativity among multi-loads and over-estimate design values.This frequently leads to not only higher investment but also distortion of structural reliability analysis.The definition of design return period in existing codes and industry criteria in China are summarized.Then joint return periods of different ocean environmental parameters are determined from the view of service term and danger risk.Based on a bivariate equivalent maximum entropy distribution,joint design parameters are estimated for the concomitant wave height and wind speed at a site in the Bohai Sea.The calculated results show that even if the return period of each environmental factor,such as wave height or wind speed,is small,their combinations can lead to larger joint return periods.Proper design criteria for joint return period associated with concomitant environmental conditions will reduce structural size and lead to lower investment of ocean platforms for the exploitation of marginal oil field.  相似文献   

11.
In this paper, the dynamic response of a marine riser under excitation of internal waves is studied. With the linear approximation, the governing equation of internal waves is given. Based on the rigid-lid boundary condition assumption, the equation is solved by Thompson-Haskell method. Thus the velocity field of internal waves is obtained by the continuity equation. Combined with the modified Morison formula, using finite element method, the motion equation of riser is solved in time domain with Newmark-β method. The computation programs are compiled to solve the differential equations in time domain. Then we get the numerical results, including riser displacement and transfiguration. It is observed that the internal wave will result in circular shear flow, and the first two modes have a dominant effect on dynamic response of the marine riser. In the high mode, the response diminishes rapidly. In different modes of internal waves, the deformation of riser has different shapes, and the location of maximum displacement shifts. Studies on wave parameters indicate that the wave amplitude plays a considerable role in response displacement of riser, while the wave frequency contributes little. Nevertheless, the internal waves of high wave frequency will lead to a high-frequency oscillation of riser; it possibly gives rise to fatigue crack extension and partial fatigue failure.  相似文献   

12.
With the increasing application of floating platforms in deep waters and harsh environments,a proper assessment of the reliability of floating structures is important to ensure that these structures can operate safely during their design lives.This study outlines a practical methodology for reliability analysis of a semi-submersible platform based estimating the probability distribution of the extreme response in rough sea conditions(survival conditions).The Constrained NewWave(CNW)theory combined with Monte Carlo simulations was first applied to simulate the random wave surface elevation process in the time domain.A Gumbel distribution was the best fitting to describe the dynamically sensitive extreme response statistics under extreme waves(drift and mooring tension).The derived probability distribution of the extreme response was subsequently used in estimation of the associated limit state func-tion,and a reliability analysis of the floating structure was conducted using the Monte Carlo method.A semi-submersible platform in a water depth of 1500 m subjected to extreme wave loads was used to demonstrate the efficiency of the proposed methodology.The probability of failure of the semi-submersible when considering mooring lines tension is greater than considering drift.  相似文献   

13.
Based on the principle of turned mass damper(TMD) systems,the conceptual design of semi-submersible platform with a moveable heave-plate(MHS) has been put forward.The heave motion response amplitude operator(RAO) and viscous damping of the MHS platform are calculated by iteration,and the coupling stiffness between the MHS hull and the heave-plate is optimized to decrease the maximum heave motion response of the MHS hull under 10-year survival conditions in the South China Sea.The nu-merical results indicate that the heave motion RAO of the MHS hull can be decreased in the range of predominant wave frequencies,which may provide some reference to the heave motion control of offshore platforms.  相似文献   

14.
The newly developed Coupled Ocean-Atmosphere-Wave-Sediment Transport(COAWST) Modeling System is applied to investigate typhoon-ocean interactions in this study. The COAWST modeling system represents the state-of-the-art numerical simulation technique comprising several coupled models to study coastal and environmental processes. The modeling system is applied to simulate Typhoon Muifa(2011), which strengthened from a tropical storm to a super typhoon in the Northwestern Pacific, to explore the heat fluxes exchanged among the processes simulated using the atmosphere model WRF, ocean model ROMS and wave model SWAN. These three models adopted the same horizontal grid. Three numerical experiments with different coupling configurations are performed in order to investigate the impact of typhoon-ocean interaction on the intensity and ocean response to typhoon. The simulated typhoon tracks and intensities agree with observations. Comparisons of the simulated variables with available atmospheric and oceanic observations show the good performance of using the coupled modeling system for simulating the ocean and atmosphere processes during a typhoon event. The fully coupled simulation that includes a ocean model identifies a decreased SST as a result of the typhoon-forced entrainment. Typhoon intensity and wind speed are reduced due to the decrease of the sea surface temperature when using a coupled ocean model. The experiments with ocean coupled to atmosphere also results in decreased sea surface heat flux and air temperature. The heat flux decreases by about 29% compared to the WRF only case. The reduction of the energy induced by SST decreases, resulting in weakening of the typhoon. Coupling of the waves to the atmosphere and ocean model induces a slight increase of SST in the typhoon center area with the ocean-atmosphere interaction increased as a result of wave feedback to atmosphere.  相似文献   

15.
A reasonable initial state of ice concentration is essential for accurate short-term forecasts of sea ice using ice-ocean coupled models. In this study, sea ice concentration data are assimilated into an operational ice forecast system based on a com- bined optimal interpolation and nudging scheme. The scheme produces a modeled sea ice concentration at every time step, based on the difference between observational and forecast data and on the ratio of observational error to modeled error. The impact and the effectiveness of data assimilation are investigated. Significant improvements to predictions of sea ice extent were obtained through the assimilation of ice concentration, and minor improvements through the adjustment of the upper ocean properties. The assimilation of ice thickness data did not significantly improve predictions. Forecast experiments show that the forecast accuracy is higher in summer, and that the errors on five-day forecasts occur mainly around the marginal ice zone.  相似文献   

16.
Sloshing phenomenon in a moving container is a complicated free surface flow problem. It has a wide range of engineering applications, especially in tanker ships and Liquefied Natural Gas (LNG) carriers. When the tank in these vehicles is partially filled, it is essential to be able to evaluate the fluid dynamic loads on tank perimeter. Different geometric shapes such as rectangular, cylindrical, elliptical, spherical and circular conical have been suggested for ship storage tanks by previous researchers. In this paper a numerical model is developed based on incompressible and inviscid fluid motion for the liquid sloshing phenomenon. The coupled BEM-FEM is used to solve the governing equations and nonlinear free surface boundary conditions. The results are validated for rectangular container using data obtained for a horizontal periodic sway motion. Using the results of this model a new arrangement of trapezoidal shapes with quadratic sidewalls is suggested for tanker ship storage panels. The suggested geometric shape not only has a maximum surrounded tank volume to the constant available volume, but also reduces the sloshing effects more efficiently than the existing geometric shapes.  相似文献   

17.
Unbalanced parental contribution and small effective population size(N e)are common issues during the artifi cial breeding of marine bivalves.The impact of hatchery-spawning practices on parental contribution,effective population size,the N e/N ratio,and genetic diversity are largely unknown.To address this,we conducted a parentage analysis on a complete 3×3 diallel cross of clam M eretrix meretrix using eight microsatellite markers.The genetic diversity of the parents was higher than that of their respective offspring in most crosses(8/9).Sires or dams from the same family contributed unequally to the pool of offspring from a particular cross,and the same parent clam exhibited large variation in parental contribution among different crosses.The variance in male contribution was higher than that of the female contribution in most crosses,suggesting that male contribution was more skewed than for females.The N e/N ratio for nine crosses ranged from 0.58 to 0.86.There was no linear relationship between the sex ratio and the N e/N ratio(P0.05).Moreover,a sex ratio closer to one-to-one does not necessarily mean a larger effective population size.A solution to small effective population size in commercial breeding programs is increasing broodstock numbers and attempting to maintain a balanced sex ratio.  相似文献   

18.
To investigate the mechanism of secondary circulations in rip current systems, and to explore the relationship between wave conditions and secondary circulation intensity, a series of numerical experiments is performed using coupled nearshore wave model and circulation model. In these experiments, the rip currents and secondary circulations generated above barred beaches with rip channels are simulated. A comparison experiment is conducted to investigate the formation and hydrodynamics of the secondary circulations. Model results indicate that the secondary circulations consist of alongshore flows driven by wave set-up near the shoreline, part of the feeder currents driven by the wave set-up over the bars, and onshore flows at the end of the rip channel driven by wave breaking and convection. The existence of the secondary circulation barely affects the rip current, but narrows and intensifies the feeder currents. Three groups of experiments of varying incident wave conditions are performed to investigate the relationship between wave conditions and secondary circulation intensity. The velocity of the alongshore flow of the secondary circulation is sensitive to the variation of the incident wave height and water depth. It is also found that the alongshore flow intensity is in direct proportion to the alongshore variation of the wave height gradient between the bars and the shoreline.  相似文献   

19.
With the increase of petroleum and gas production in deep ocean, marine risers of circular cylinder shape are widely used in the offshore oil and gas platform. In order to research the hydrodynamic performance of marine risers, the dynamic mesh technique and User-Defined Function （UDF） are used to simulate the circular cylinder motion. The motion of a transversely oscillat-ing circular cylinder in combination of uniform flow and oscillating flow is simulated. The uniform flow and oscillating flow both are in x direction. SIMPLE algorithm is used to solve the Navier-Stokes equations. The User-Defined Function is used to control the cylinder transverse vibration and the inlet flow. The lift and drag coefficient changing with time and the map of vorticity isolines at different phase angle are obtained. Force time histories are shown for uniform flow at Reynolds number （Re） of 200 and for the com-bination of uniform and oscillating flows. With the increase of amplitude of oscillating flow in combined flow, the change of lift am-plitude is not sensitive to the the change of cylinder oscillating frequency. Lift amplitude increases with the increase of oscillating flow amplitude in the combined flow, but there is no definite periodicity of the lift coefficient. The drag and inertia force coefficients change when the maximum velocity of the oscillating flow increases in the combined flow. The vortex shedding near the circular cylinder shows different characteristics.  相似文献   

20.
《山地科学学报》2020,17(9):2203-2213
Mercury intrusion porosimetry(MIP) is a simple and fast way to obtain the pore distribution of soil and can be used to estimate the soil-water characteristic curve(SWCC). In previous studies, soil was assumed to be a perfect wettability material, and the contact angle(CA) of the soil-water interface was taken as zero in the SWCC prediction method.However, the CA has proved to be much greater than zero even for hydrophilic soils according to some soil wettability experiments, and it has a significant effect on predicting the SWCC. In this research, a method for predicting the SWCC by MIP, which takes the CA as a fitting coefficient, is proposed. The pore size distribution curves are measured by MIP, and the SWCCs of two loess soils are measured by pressure plate and filter paper tests. When the CA is taken as70° and 50° for the wetting and drying process,respectively, the SWCCs predicted by the pore size distribution curves agree well with the measured SWCCs. The predicted suction range of the proposed method is 0-105 k Pa. The consistency of the results suggests that utilizing the MIP test to predict the SWCC with a proper CA is effective for loess.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号