ISSN  0890-5487 CN 32-1441/P

2009 Vol.(3)

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Reliability-Based Load and Resistance Factors Design for Offshore Jacket Platforms in the Bohai Bay: Calibration on Design Factors
JIN Wei-liang, HU Qi-zhong, SHEN Zhao-wei, SHI Zhong-min
2009, (3): 387-398.
For the fulfillment of the probability-based structural design for the offshore jacket platforms in the Bohai Bay, the design factors of loads, resistance and load combinations are much necessary to be calibrated according to the proposed target reliability index. Firstly, the limit states function for the offshore jacket platforms is introduced. Then, four approaches to calibrate the factors of load and resistance are presented and compared. Afterwards, the methods to calibrate the load combination factors are developed. Finally, the factors of load, resistance and load combination for the offshore jacket platforms in the Bohai Bay are calibrated and the corresponding design formulae are recommended. The results are proved to be rational in practice, and also illustrate that the proposed target reliability index for offshore jacket platforms in the Bohai Bay is also appropriate.
A Method for Calculation of Low-Frequency Slow Drift Motions Based on NURBS for Floating Bodies
LIU Wen-xi, REN Hui-long
2009, (3): 399-414.
Through a higher-order boundary element method based on NURBS (Non-uniform Rational B-splines), the calculation of second-order low-frequency forces and slow drift motions is conducted for floating bodies. In the floating body’s inner domain, an auxiliary equation is obtained by applying a Green function which satisfies the solid surface condition. Then, the auxiliary equation and the velocity potential equation are combined in the fluid domain to remove the solid angle coefficient and the singularity of the double layer potentials in the integral equation. Thus, a new velocity potential integral equation is obtained. The new equation is extended to the inner domain to relieve the irregular frequency effects; on the basis of the order analysis, the comparison is made about the contribution of all integral terms with the result in the second-order low-frequency problem; the higher-order boundary element method based on NURBS is applied to calculate the geometric position and velocity potentials; the slow drift motions are calculated by the spectrum analysis method. Removing the solid angle coefficient can apply NURBS technology to the hydrodynamic calculation of floating bodies with complex surfaces, and the extended boundary integral method can reduce the irregular frequency effects. Order analysis shows that free surface integral can be neglected, and the numerical results can also prove the correctness of order analysis. The results of second-order low-frequency forces and slow drift motions and the comparison with the results from references show that the research on the NURBS technology to the second-order low-frequency problem is of high efficiency and credible results.
Snap Tension in Mooring Lines of Deepwater Platform
ZHANG Su-xia, TANG You-gang, LIU Hai-xiao
2009, (3): 415-428.
Based on the theory of impact dynamics, the motion equations for a mooring line-floating body system before and after impact loading are established with consideration of the viscoelastic property of mooring lines. The factors that influence the taut-slack conditions of a mooring system are analyzed through classifying the taut-slack regions, which are defined by non-dimensional ratios of displacement, frequency, and damping of the system. The mooring system of Jip spar platform is analyzed, and the snap tension characteristics of mooring lines are given. The factors that influence the maximum tension in mooring lines, including the mass of the floating body, length of mooring lines, frequency and amplitude of external excitation, and pretension in mooring lines, are also analyzed through computing the dynamic response of system and parametric study. It is shown that the maximum tension increases with the increase of the mass of the floating body, external excitation and pretension. Also, it is found that the influence of the non-dimensional ratio of damping increases with the increase of the pretension in mooring lines.
Efficient Focusing Models for Generation of Freak Waves
ZHAO Xi-zeng, SUN Zhao-chen, LIANG Shu-xiu
2009, (3): 429-440.
Four focusing models for generation of freak waves are presented. An extreme wave focusing model is presented on the basis of the enhanced High-Order Spectral (HOS) method and the importance of the nonlinear wave-wave interactions is evaluated by comparison of the calculated results with experimental and theoretical data. Based on the modification of the Longuet-Higgins model, four wave models for generating freak waves (a. extreme wave model + random wave model; b. extreme wave model + regular wave model; c. phase interval modulation wave focusing model; d. number modulation wave focusing model with the same phase) are proposed. By use of different energy distribution techniques in the four models, freak wave events are obtained with different Hmax/Hs in finite space and time.
A Numerical Study on Water Waves Generated by a A Submerged Moving Body in a A Two-Llayer Fluid System
YANG Jia-zhen, NG Chiu-on, ZHANG Dao-hua
2009, (3): 441-458.
This is a numerical study on the time development of surface waves generated by a submerged body moving steadily in a two-layer fluid system, in which a layer of water is underlain by a layer of viscous mud. The fully nonlinear Navier–Stokes equations are solved on FLUENT with the Volume-of-Fluid (VOF) multiphase scheme in order to simulate the free surface waves as well as the water–mud interface waves as functions of time. The numerical model is validated by mimicking a reported experiment in a one-layer system before it is applied to a two-layer system. It is found that the presence of bottom mud in a water layer can lead to large viscous damping of the surface waves. To investigate the problem systematically, the effects of the Froude number and the mud layer thickness, density and viscosity relative to those of water are evaluated and discussed in detail.
An Efficient Model for Transient Surface Waves in Both Finite and Infinite Water Depth
NING De-zhi, TENG Bin, ZANG Jun, LIU Shu-xue
2009, (3): 459-472.
A numerical model is developed to simulate fully nonlinear extreme waves in finite and infinite water-depth wave tanks. A semi-mixed Eulerian-Lagrangian formulation is adopted and a higher-order boundary element method in conjunction with an image Green function is used for the fluid domain. The boundary values on the free surface are updated at each time step by a fourth-order Runga-Kutta time-marching scheme at each time step. Input wave characteristics are specified at the upstream boundary by an appropriate wave theory. At the downstream boundary, an artificial damping zone is used to prevent wave reflection back into the computational domain. Using the image Green function in the whole fluid domain, the integrations on the two lateral walls and bottom are excluded. The simulation results on extreme wave elevations in finite and infinite water-depths are compared with experimental results and second-order analytical solutions respectively. The wave kinematics is also discussed in the present study.
RANS Modeling of Solitary Wave Propagation over A Submerged Rectangular Breakwater
Tai-Wen HSU, Chin-Yen TSAI, Jian-Wu LAI
2009, (3): 473-488.
In this paper a series of numerical simulations are performed to investigate the vortex shedding mechanism for a solitary wave propagating over a submerged breakwater by use of Reynolds averaged Navier-Stokes (RANS) model combined with a k-ε model. Flows of different Reynolds numbers up to Re=1.4×105 corresponding to varying incident wave heights are considered in which the characteristic fluid velocity is represented by the maximum horizontal velocity above the submerged breakwater. For the verification of the accuracy of the numerical model, the incident waves and the velocity field in the vicinity of the breakwater are compared with experimental data. The result shows that the model is capable of describing vortex shedding for a solitary wave propagating over a rectangular submerged breakwater. Key features of vortex generation, evolution and dissipation are investigated. It is found that the vortex shedding and their evolution due to separated boundary layer over the breakwater are strongly related to the Reynolds number. A considerable number of vortices and complicated vortex pattern are observed as the Reynolds number increases.
Dynamic Modeling and Three-Dimensional Motion Analysis of Underwater Gliders
WANG Yan-hui, Wang Shu-xin
2009, (3): 489-504.
For Cconsideration ofing both the eccentric rotatable rigid body and the translational rigid body, the dynamic model of the underwater glider is derived. Dynamical behaviors are also studied based on the model and can be used as the guidance to underwater gliders design. Gibbs function of the underwater glider system is derived first, and then the nonlinear dynamic model is obtained by use of Apell Equations. The relationships between dynamical behaviors and design parameters are studied by solving the dynamic model. The spiral motion, swerving motion in three dimensions and the saw-tooth motion of the underwater glider in vertical plane are studied. Lake trials are carried out to validate the dynamic model.
Dynamic Modeling and Investigation of Maneuver Characteristics of A Deep-Sea Manned Submarine Vehicle
XIE Jun-yuan, XU Wen-bo, ZHANG Hua, XU Peng-fei, CUI Wei-cheng
2009, (3): 505-516.
A deep-sea Manned Submarine Vehicle (MSV) is usually required to move at a low forward speed and a low rotational speed when it executes investigation tasks. In this condition, the motion is in large drift angles, and the maneuverability hydrodynamic forces cannot be expressed properly in the conventional mathematical model of submersible motion. In this paper, firstly, a general equation of MSV with six-freedom motion is presented, and the numerical simulation of descent/ascent motion and helix motion is conducted to reveal the general maneuver characteristics of MSV. Secondly, according to the data of captive model tests of large drift angles of MSV, the regression analysis of position hydrodynamic forces and rotation hydrodynamic forces is carried out, and the results of regression analysis of maneuverability hydrodynamic characteristics are analyzed to reveal the special maneuver characteristics. Thirdly, a special new mathematical model of MSV with the whole range of drift angles motion is presented, which can be used to predict hydrodynamic performance of motion in the 0o~180o range of drift angles. The results are applied to the design of maneuverability hydrodynamic forces, development of control system and simulator of a practical MSV.
A Unified Computational Method for Simulating Dynamic Behavior of Planing Vessels
Mohammad Saeed SEIF, Ebrahim JAHANBAKHSH, Roozbeh PANAHI, Mohammad Hossein KARIMI
2009, (3): 517-528.
High speed planing hulls have complex hydrodynamic behaviors. The trim angle and drafts are very sensitive to speed and location of the center of gravity. Therefore, motion simulation for such vessels needs a strong coupling between rigid body motions and hydrodynamic analysis. In addition, free surface should be predicted with good accuracy for each time step. In this paper, velocity and pressure fields are coupled by use of the fractional step method. On the basis of integration of the two-phase viscous flow induced stresses over the hull, acting loads (forces and moments) are calculated. With the strategy of boundary-fitted body-attached mesh and calculation of 6-DoF motions in each time step, time history of ship motions including displacements, speeds and accelerations are evaluated. For the demonstration of the software capabilities, circular cylinder slamming is simulated as a simple type of water slamming. Then, a high-speed planing catamaran is investigated in the case of steady forward motion. All of the results are in good concordance with experimental data. The present method can be widely implemented in design as well as in performance prediction of high-speed vessels.
2-D Numerical Simulation of Natural Gas Hydrate Decomposition Through Depressurization by Fully Implicit Method
SONG Yong-chen, LIANG Hai-feng
2009, (3): 529-542.
Natural gas hydrate, as a potential energy resource, deposits in permafrost and marine sediment with large quantities. The current exploitation methods include depressurization, thermal stimulation, and inhibitor injection. However, many issues have to be resolved before the commercial production. In the present study, a 2-D axisymmetric simulator for gas production from hydrate reservoirs is developed. The simulator includes equations of conductive and convective heat transfer, kinetic of hydrate decomposition, and multiphase flow. These equations are discretized based on the finite difference method and are solved with the fully implicit simultaneous solution method. The process of laboratory-scale hydrate decomposition by depressurization is simulated. For different surrounding temperatures and outlet pressures, time evolutions of gas and water generations during hydrate dissociation are evaluated, and variations of temperature, pressure, and multiphase fluid flow conditions are analyzed. The results suggest that the rate of heat transfer plays an important role in the process. Furthermore, high surrounding temperature and low outlet valve pressure may increase the rate of hydrate dissociation with insignificant impact on final cumulative gas volume.
Numerical Simulation of Water Level Under Interaction Between Runoff and Estuarine Dynamics in Tidal Reach of the Yangtze River
LEI Zhi-yi, ZHANG Jin-shan, KONG Jun
2009, (3): 543-551.
The Yangtze Estuary, the largest estuary in China, is under an obvious interaction between runoff and astronomical tide. The research on the interaction is very important for the exploitation and utilization of water resources in this area. A horizontal 2D hydrodynamic numerical model is established and verified in the present study with the modeling range from Datong to the Yangtze Estuary. Based on the comparison of high water levels under the interaction between different runoff and estuarine dynamics, some conclusions are drawn. By revealing the general laws about the relationship between the astronomical tide and flood, the flood level is forecasted to lay a foundation for further research on storm surge in the Yangtze Estuary.
Wave Run-up on A Vertical Seawall Protected by An Offshore Submerged Barrier
ZHANG Jing-xin, LIU Hua
2009, (3): 553-564.
Submerged barriers are constructed in coastal zones for shoreline or harbor protection or to prevent the beach erosion. In the present study, the wave run-up on a vertical seawall protected by a submerged barrier is analyzed. The physical configurations include a rigid barrier and a long channel of finite depth. For linear water waves, by matching the velocity along the barrier and along the gap, the systems of linear equations about the velocity potentials are obtained. The wave run-up is further analyzed for various settings of barrier height and distance between the barrier and the wall, i.e. the chamber length. For nonlinear waves and random sea waves, a numerical model is extended to investigate the effect parameters of the barrier on the wave run-up against the seawall. Not only the numerical simulations, but also the analytical results illustrate that the wave run-up on the seawall depends very much on the distance between the barrier and the vertical seawall.
Experimental Investigation on Flow Characteristics at Leeside of Suspended Flexible Curtain for Sedimentation Enhancement
LI Yan-hong, YU Guo-liang
2009, (3): 565-576.
A new patent registered device – suspended flexible curtain (SFC) for sedimentation enhancement is proposed in this paper, which consists of two parts: a light-weight curtain with sediment-passing windows and a heavy pillar for the device stability. The mechanism of trapping nearbed sediment by the SFC is such that a primary and a secondary circulations with horizontal hubs would be formed at the leeside when it is placed on beds in running flow; the velocities within the sediment-passing windows are locally accelerated, the nearbed sediments would be brought by the flow through the windows, and then settled down within the primary circulation zone. Experimental tests on hydraulic characteristics are conducted in a laboratory flume with rigid bed. It is found that the dimensions of the curtain and the sediment-passing windows determine the characteristics of the primary and the secondary circulations. The intensity of the primary and the secondary circulations is dominated by the size of the sediment-passing window. Whether the secondary circulation would contact the bed or not depends on the height of the sediment-passing window. The length and the height of the primary circulation zone demonstrate quasi linear relationships with the effective height of the SFC. And the tests on sandy bed show that the sediment deposition happens just in the primary circulation zone.
Study on General Performance of Jack-up Under Elevated Condition
LI Hong-tao, YANG Qing-xia, LI Ye
2009, (3): 577-584.
Method of checking for jack-up elevated performance including leg structure strength, fixation system or jacking system bearing capacity, pre-load requirements, spud can bearing capacity and overturning stability is suggested in this paper. As an example, a jack-up with truss legs is analyzed by finite element analysis method. This paper may be helpful to the rig owners, operators and designers.
Nonlinear Dynamic Behavior of Moored Buoy Excited by Free Surface Waves
CHANG Zong-yu, XU Chang-mi, LI Hua-jun, TANG Yuan-guang, ZHANG Xi-chao
2009, (3): 585-592.
In this paper, studied are the dynamics of a moored buoy near the surface subjected to wave excitation. According to the physical structure, submersible buoy moored by tethered line is modeled firstly. Then from the differential equations, the natural frequencies are estimated by neglecting the coupling between tangential and normal direction. By use of numerical integration method, solutions are obtained. On this basis, strange attractors and bifurcation phenomena are obtained by applying Poincare map, phase plots and bifurcation diagram, showing the existence of the chaotic response in this system when wave steepness is high enough.

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