ISSN  0890-5487 CN 32-1441/P

2008 Vol.(3)

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Moving-Particle Semi-Implicit Method for Vortex Patterns and Roll Damping of 2D Ship Sections
2008, (3): -.
[Abstract](733)
Abstract:
A meshless method, Moving-Particle Semi-Implicit Method (MPS) is presented in this paper to simulate the rolling of different 2D ship sections. Sections S.S.0.5, S.S.5.0 and S.S.7.0 of series 60 with CB=0.6 are chosen for the simulation. It shows that the result of MPS is very close to results of experiments or mesh-numerical simulations. In the simulation of MPS, vortices are found periodically in bilges of ship sections. In section S.S.5.0 and section S.S.7.0, which are close to the middle ship, two little vortices are found at different bilges of the section, in section S.S.0.5, which is close to the bow, only one big vortex is found at the bottom of the section, these vortices patterns are consistent with the theory of Ikeda. The distribution of shear stress and pressure on the rolling hull of ship section is calculated. When vortices are in bilges of the section, the sign change of pressure can be found, but in section S.S.0.5, there is no sign change of pressure because only one vortex in the bottom of the section. With shear stress distribution, it can be found the shear stress in bilges is bigger than that at other part of the ship section. As the free surface is considered, the shear stress of both sides near the free surface is close to zero and even sign changed.
A Non-Linear Rheological Model of Soft Clay and Its Application in Settlement of Sunk Ship
2008, (3): -.
[Abstract](806)
Abstract:
A one-dimensional consolidation-creep model test on the creep deformation of soft muddy clay in the littoral area of Tianjin is performed. A non-linear rheologic model is established and the model coefficients are determined, in consideration of the characteristics of soft muddy clay. Furthermore, a settlement equation is deduced from the rheologic model and verified by the field settlement measurements of Beitang Reservoir dam in Tianjin littoral area. Finally, the settlement equation is applied in calculating the settlement of "FAIRWAY" suction dredger, which sunk in the external channel of Tianjin Port, induced by the soft clay consolidation of seabed. These results provide useful information for the decision of salvage plan.
Mitigation of Ice-Induced Vibrations for Offshore Platforms Using Tuned Mass Damper
2008, (3): -.
[Abstract](768)
Abstract:
The problems of ice-induced vibration have been noticed and concerned since the 1960s, but it has not been well resolved. One reason is that the dynamic interaction between ice and structure is so complicated that practical ice force model has not been developed. The recent full-scale tests conducted on jacket platforms in the Bohai Sea presented that ice could cause intense vibrations which endanger the facilities on the deck and make discomfort for the crew. In this paper, the strategy of mitigation of ice-induced offshore structure vibration is discussed. Based on field observations and understanding of the interaction between ice and structure, the absorption mitigation method to suppress ice-induced vibration is presented. The numerical simulations were conducted for a simplified model of platform attached with a Tuned Mass Damper (TMD) under ice force function and ice force time history. The simulation results show that TMD can favorably reduce ice-induced vibrations, therefore, it can be considered to be an alternative approach to utilize. Finally, the application possibilities of utilizing TMDs on other miniature offshore structures in ice-covered areas of marginal oil fields are discussed.
2008, (3): -.
[Abstract](176)
Abstract:
Simulation of Wetting and Drying Processes in A Depth Integrated Shallow Water Flow Model by Slot Method
2008, (3): -.
[Abstract](757)
Abstract:
A particular porosity method named "slot method" is implemented in a depth-integrated shallow water flow model (DIVAST) to simulate wetting and drying processes. Discussed is the relationship between the shape factors of the "slot" and the preset depth used in "wetting-drying" algorithm. Two typical tests are conducted to examine the performance of the method with the effect of the shape factors of the "slot" being checked in detail in the first test. Numerical results demonstrate that: 1) no additional effort to improve the finite difference scheme is needed to implement "slot method" in DIVAST, and 2) "slot method" will simulate wetting and drying processes correctly if the shape factors of the "slot" being selected properly.
Design Methods for Geotextile Tubes Application in Dike Construction
2008, (3): -.
[Abstract](1048)
Abstract:
The technique of geotextile tubes used to construct dikes for land reclamation has been widely used. The tubes are usually filled with slurry of soil, such as sand, silt or clay. The tensile stress developed in the geotextile during filling the tube is the dominant factor for construction of a safe dike. The existing design methods are good for designing sausage shaped tubes and can not be directly applied for designing flat tubes, which are commonly used in dike construction. This paper presents a procedure that can determine the relationship among the tube size, the pumping pressure, the unit weight of the slurry, and the tensile stress developed in the geotextile during the tube filing. When the tubes are piling up to form the dike, the tubes in the bottom will sustain the load from the weight of the upper ones. A procedure is also developed to calculate the changes of the mechanical and geometrical behavior of the tubes under the load with a similar method. All these approaches have been programmed, which can help dike designers to select the suitable geotextile and determine the profile of the dike.
Failure Loci of Suction Caisson Foundations Under Combined Loading Conditions
2008, (3): -.
[Abstract](1053)
Abstract:
Suction caissons are widely used to support offshore fixed platforms in coastal areas. The loadings transferred to suction caissons include the eccentric lateral force induced by waves and self weight of the platform structure. However, under this kind of combined loading conditions, the failure mechanism of caissons with shallow embedment depths is quite different from conventional deep foundations or onshore shallow footings. The behaviour of caissons subjected to combined loadings may be described with the "failure locus" in force resultant spaces. Here the failure loci of smooth caissons are studied by use of finite element approach, with the embedment ratio of caissons varying in the range of 0.25~1.0 and eccentricity ratio of horizontal loadings in 0~10. The platform settlement and tilt limits are involved into determination of failure loci, thus the platforms can avoid significant displacements for the combined loadings located inside the failure locus. Three families of loading paths are used to map out the locus. It is found that the shape of failure loci depends on 3 non-dimensional parameters, and the failure locus of a given caisson changes gradually from the elliptical curve to hooked curve with increasing shear strength of soil. The lateral capacity of short caissons may be enhanced by vertical forces, compared with the maximum lateral capacity of long caissons occurring at the vertical force being zero. The critical embedment ratios partitioning elliptical and hooked loci are proposed.
2008, (3): -.
[Abstract](292)
Abstract:
A Method for Analyzing System Reliability of Existing Jacket Platforms
2008, (3): -.
[Abstract](958)
Abstract:
Owing to the ageing of the existing structures worldwide and the lack of codes for the continued safety management of structures during their lifetime, it is very necessary to develop a tool to evaluate their system reliability over a time interval. In this paper, a method is proposed to analyze system reliability of existing jacket platforms. The influences of dint, crack and corrosion are considered. The mechanics characteristics of the existing jacket platforms to extreme loads are analyzed by use of the nonlinear mechanical analysis. The nonlinear interaction of pile-soil-structure is taken into consideration in the analysis. By use of FEM method and Monte Carlo simulation, the system reliability of the existing jacket platform can be obtained. The method has been illustrated through application to BZ28-1 three jacket platforms which have operated for sixteen years. Advantages of the proposed method for analyzing the system reliability of the existing jacket platform is also highlighted.
An Approximate Method for Calculation of Fluid Force and Response of A Circular Cylinder at Lock-in
2008, (3): -.
[Abstract](771)
Abstract:
In this paper, equations calculating lift force of a rigid circular cylinder at lock-in in uniform flow are deduced in detail. Besides, equations calculating the lift force on a long flexible circular cylinder at lock-in are deduced based on mode analysis of a multi-degree freedom system. The simplified forms of these equations are also given. Furthermore, an approximate method to predict the forces and response of rigid circular cylinders and long flexible circular cylinders at lock-in is introduced in the case of low mass-damping ratio. A method to eliminate one deficiency of these equations is introduced. Comparison with experimental results shows the effectiveness of this approximate method.
Information for Cintributors
2008, (3): -.
[Abstract](224)
Abstract:
Probabilistic Assessment of Wave Overtopping of Nampo Dike in the West Coast of Korea
2008, (3): -.
[Abstract](946)
Abstract:
The Nampo dike, which is located at the west coast of Korea, was destroyed by wave overtopping during the storms on 30 August and 17 September in 1989. In this paper, is performed the probabilistic assessment of wave overtopping of Nampo dike by use of Owen model, Van der Meer & Janssen model and Hedges & Reis model for wave overtopping of seawall. Based on the available tidal and wave data for storm surges in 1989, the risk assessment of wave overtopping of the Nampo dike has been carried out by both Level II and Level III reliability methods. The calculated results show the general agreement of failure probability between the two methods. By utilizing the rehabilitated cross section of Nampo dike, the failure probability of wave overtopping for the Nampo dike after rehabilitation will be rapidly reduced to that of initial design at crest level of 9.0 m with the improved slope from 1:2 to 1:4 at seaside. Since the sea level may only rise 1.0 m in the next few decades, the failure probability of Nampo dike will be still in the safe range.
Simulation of Random Waves and Associated Laminar Bottom Shear Stresses
2008, (3): -.
[Abstract](769)
Abstract:
This work presents a new approach for simulating the random waves in viscous fluids and the associated bottom shear stresses. By generating the incident random waves in a numerical wave flume and solving the unsteady two-dimensional Navier-Stokes equations and the fully nonlinear free surface boundary conditions for the fluid flows in the flume, the viscous flows and laminar bottom shear stresses induced by random waves are determined. The deterministic spectral amplitude method implemented by use of the fast Fourier transform algorithm was adopted to generate the incident random waves. The accuracy of the numerical scheme is confirmed by comparing the predicted wave spectrum with the target spectrum and by comparing the numerical transfer function between the shear stress and the surface elevation with the theoretical transfer function. The maximum bottom shear stress caused by random waves, computed by this wave model, is compared with that obtained by Myrhaug's model (1995). The transfer function method is also employed to determine the maximum shear stress, and is proved accurate.
Accuracy Evaluation of A Diagnostic Test by Detecting Outliers and Influential Observations
2008, (3): -.
[Abstract](726)
Abstract:
Logit regression analysis is widely applied in scientific studies and laboratory experiments, where skewed observations on a data set are often encountered. A number of problems with this method, for example, outliers and influential observations, can cause overdispersion when a model is fitted. In this study a systematic statistical approach, including the plotting of several indices is used to diagnose the lack-of-fit of a logistic regression model. The outliers and influential observations on data from laboratory experiments are then detected. Specifically we take account of the interaction of an internal solitary wave (ISW) with an obstacle, i.e., an underwater ridge, and also analyze the effects of the ridge height, the lower layer water depth, and the potential energy on the amplitude-based transmission rate of the ISW. As concluded, the goodness-of-fit of the revised logit regression model is better than that of the model without this approach.
Study on Interaction Relationship for Submarine Pipeline with Axial Corrosion Defects
2008, (3): -.
[Abstract](946)
Abstract:
Corrosion is one of the main reasons to cause the operation accident of submarine oil and gas transmission pipelines. As the major corrosion pattern in submarine pipelines, the effects of corrosion clusters consisting of the adjacent corrosion defects on failure pressure are investigated through non-linear large-deformation finite element method. Typically, the failure behavior and limit strength of submarine pipeline with axial groove-groove corrosion defect pair exposed to internal pressure are analyzed. The effects of corrosion depth and axial spacing between a pair of corrosion defects on failure pressure are concluded. An interaction relationship for corrosion defects in pipelines, as well as prediction formulations for assessing the remaining strength of corroded pipelines are proposed. The expressions based on the proposed interaction relationship give more accurate results than the methods used in the existing design guidelines.
An Overview of Deepwater Pipeline Laying Technology
2008, (3): -.
[Abstract](691)
Abstract:
The technology and methods involved in pipeline laying in shallow water have evolved to the level of routine and commonplace. However, regarding the unexpected deepwater complexity, the traditional pipeline laying techniques have to confront many new challenges arisen from the increase of the water depth, diameter of the pipe and the welding difficulty, all of which should be modified and/or innovated based on the existed mature experiences. The purpose of this investigation is to outline the existing and new engineering laying techniques and the associated facilities, which can provide some significant information to the related research. In the context, the latest deepwater pipeline laying technology and pipe laying barges of the renowned companies from Switzerland, Norway, Italy etc., are introduced and the corresponding comparison and discussion are presented as well.
Hyperbolic Mild Slope Equations with Inclusion of Amplitude Dispersion Effect: Regular Waves
2008, (3): -.
[Abstract](996)
Abstract:
A new form of hyperbolic mild slope equations is derived with the inclusion of the amplitude dispersion of nonlinear waves. The effects of including the amplitude dispersion effect on the wave propagation are discussed. Wave breaking mechanism is incorporated into the present model to apply the new equations to surf zone. The equations are solved numerically for regular wave propagation over a shoal and in surf zone, and a comparison is made against measurements. It is found that the inclusion of the amplitude dispersion can also improve model's performance on prediction of wave heights around breaking point for the wave motions in surf zone.
Numerical Simulation of Large Diameter Cylindrical Structure Slamming
2008, (3): -.
[Abstract](850)
Abstract:
The water entry of large diameter cylindrical structure is studied by applying numerical simulation method. The processes of different diameter cylindrical structures impacting water with various constant velocities are calculated numerically. Thereafter, analyzed are the distribution of slamming pressure on structure during slamming course and the influence of slamming velocity and cylindrical diameter on slamming process. Furthermore, presented herein is an equation being used to forecast the peak slamming force on a large diameter cylindrical structure.

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