ISSN  0890-5487 CN 32-1441/P

2002 Vol.(3)

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Finite Element Analysis of Deformed Legs of Offshore Platform Structures
2002, (3): -.
[Abstract](340)
Abstract:
The element stiffness matrix of the equivalent beam or pipe element of the deformed leg of the platform is derived by the finite element method. The stresses and displacements of some damaged components are calculated, and the numeri-cal solutions agree well with those obtained by the fine mesh finite element method. Finally, as an application of this method, the stresses of some platform structures are calculated and analyzed.
Analysis of Wave Loads on A Semi-Submersible Platform
2002, (3): -.
[Abstract](338)
Abstract:
Simulation of the Whole Process of Ship-Bridge Collision
2002, (3): -.
[Abstract](324)
Abstract:
The simulation of the whole ship-bridge collision process can be effectively carried out by nonlinear dynamic finite element method. Based on the simple description of the theory, a scenario of a 40000 DWT oil tanker colliding with a bridge across the Yangtze River is designed for simulation. The technology of structure modeling and the determination of related parameters are introduced. The deformation of the bulb bow, the history of collision force change, the exchange of collision energy and the stress distribution of the bridge pier are described in detail, which are of great value to bridge de-sign and bridge pier damage estimation. Some mechanical characters in the process of ship-bridge collision are described. More accurate results can be produced by finite element method than that by empirical formulas and simplified analytical methods.
Analytical Crack Model for 2-D Reinforced Concrete Structures
2002, (3): -.
[Abstract](326)
Abstract:
A two-dimensional smeared crack model for reinforced concrete members is presented. Special emphasis is placed on the bond between concrete and reinforcement as the main factor influencing tension stiffening in cracked reinforced concrete. With the derived tangential stress-strain equations for concrete in the direction perpendicular to the cracks, the constitutive relationship for cracked reinforced concrete is established. Experimental specimens have been analyzed with the analytical model, and the analytical and experimental results are found to be in good agreement.
Vibration Control of Multi-Tuned Mass Dampers for An Offshore Oil Platform
2002, (3): -.
[Abstract](337)
Abstract:
The purpose of this study is to investigate the effectiveness of multi-tuned mass dampers (MTMD) on mitigating vi-bration of an offshore oil platform subjected to ocean wave loading. An optimal design method is used to determine the optimal damper parameters under ocean wave loading. The force on the structure is determined by use of the linearized Morison equation. Investigation on the deck motion with and vvithout MTMD on the structure is made under design condi-tions. The results show that MTMD with the optimized parameters suppress the response of each structural mode. The sensitivity of optimum values of MTMD to characteristic wave parameters is also analyzed. it is indicated that a single TMD on the deck of a platform can have the best performance, and the small the damping value of TMD, the betler the vibration control.
An Approach to Stability Analysis of Embedded Large-Diameter Cylinder Quay
2002, (3): -.
[Abstract](319)
Abstract:
The large-diameter cylinder structure, which is made of large successive bottomless cylinders placed on foundation bed or partly driven into soil, is a recently developed retaining structure in China. It can be used in port, coastal and off-shore works. The method for stability analysis of the large-diameter cylinder structure, especially for stability analysis of the embedded large-diameter cylinder structure, is an important issue. In this paper, an idea is presented that is, em-bedded large-diameter cylinder quays can be divided into two types, i.e. the gravity wall type and the cylinder pile wall type. A method for stability analysis of the large-diameter cylinder quay of the cylinder pile wall type is developed and a method for stability analysis of the large-diameter cylinder quay of the gravity wall type is also proposed. The effect of sig-nificant parameters on the stability of the large-diameter cylinder quay of the cylinder pile wall type is investigated through numerical calculation.
Energy Anaiysis for TMD-Structure Systems Subjected to Impact Loading
2002, (3): -.
[Abstract](348)
Abstract:
This paper investigates the characteristics of reduction of the lateral vibration by use of a Tuned Mass Damper (TMD) for offshore jacket platforms under impact loading. Unlike traditional analysis, the present analysis focuses on the energy concept of TMD/structure systems. In this study, a time domain is taken. The platform is modeled as a simplified single-degree-of-freedom (SDOF) system by extraction of the first vibration mode of the structure and the excited force is assumed to be impact loading. The energy dissipation and energy transmission of the structure-TMD system are studied. Finally, an optimized TMD design for the modeled platform is demonstrated based on a new type of cost function - maxi-mum dissipated energy by TMD. Results indicate that TMD control is effective in reducing the Standard deviation of the deck motion but less effective in reducing the maximum response under impact loading.
Comparison of Linear Level I Green-Naghdi Theory with Linear Wave Theory for Prediction of Hydroelastic Responses of VLFS
2002, (3): -.
[Abstract](322)
Abstract:
Very Large Floating Structures (VLFS) have drawn considerable attention recently due to their potential significance in the exploitation of ocean resources and in the utilization of ocean space. Efficient and accurate estimation of their hydroelastic responses to waves is very important for the design. Recently, an efficient numerical algorithm was developed by Ertekin and Kim (1999). However, in their analysis, the linear Level I Green-Naghdi (GN) theory is employed to describe fluid dynamics instead of the conventional linear wave (LW) theory of finite water depth. They claimed that this linear level I GN theory provided betler predictions of the hydroelastic responses of VLFS than the linear wave theory. In this paper, a detailed derivation is given in the conventional linear wave theory framework with the same quantity as used in the linear level I GN theory framework. This allows a critical comparison between the linear wave theory and the linear level I GN theory. it is found that the linear level I GN theory can be regarded as an approximation to the linear wave theory of finite water depth. The consequences of the differences between these two theories in the predicted hydroelastic responses are studied quantitatively. And it is found that the linear level I GN theory is not superior to the linear wave theory. Finally, various factors affecting the hydroelastic response of VLFS are studied vvith the implemented algorithm.
Reflection of Oblique Incident Waves by Breakwaters with Partially-Perforated Wall
2002, (3): -.
[Abstract](333)
Abstract:
The reflection of oblique incident waves from breakwaters with a partially-perforated front wall is investigated. The fluid domain is divided into two sub-domains and the eigenfunction expansion method is applied to expand velocity poten-tials in each domain. In the eigen-expansion of the velocity potential, evanescent waves are included. Numerical results of the present model are compared with experimental data. The effect of porosity, the relative chamber width, the relative water depth in the wave absorbing chamber and the water depth in front of the structure are discussed.
Bragg Reflection of Waves by Different Shapes of Artificial Bars
2002, (3): -.
[Abstract](323)
Abstract:
Experiments are performed in a wave flume to demonstrate the Bragg reflection of linear gravity waves by artificial bars. Three different artificial bars with rectangular, triangular and rectified cosinoidal shapes are placed discretely on the seabed for measurement of the Bragg reflection. A series of experimental conditions including the number of bars, the pe-riodic bar spacing, the water depth and various wave conditions are tested. Key parameters influencing the Bragg resonances are investigated. The experimental data are compared with the values from both theoretical and numerical models. Some key parameters have proved to be effective in describing the primary resonances. Predictive equations of the charac-teristics for the Bragg reflection are proposed in this paper.
A Computational Model for Velocity Separation in Shallow Sea
2002, (3): -.
[Abstract](325)
Abstract:
Based on the hydrodynamical feature and the theoretical velocity profiles of tidal flow and vvind-induced flow in shal-low sea, a computational model is established for the first time, which can separate observed velocity into tidal velocity and wind-induced velocity by use of the least square method. With the model, not only the surface velocities of tidal flow and vvind-induced flow are obtained, but also the bed roughness height is determined and the wind velocity above the wa-ter surface is estimated. For verification of the model, the observed velocity in the Yellow River Estuary and the laborato-ry test is separated, then it is applied to the Yangtze River Estuary. All the results are satisfactory. The research results show that the model is simple in method, feasible in process and reasonable in result. The model is a valid approach to analysis and computation of field dala, and can be applied to separate the observed velocity in shallow sea; at the same time, reasonable boundary conditions of the surface and bottom can be obtained for two-and three-dimensional numerical simulation.
Application of DDA Approach to Simulation of Ice Breaking Process and Evaluation of Ice Force Acting on A Structure
2002, (3): -.
[Abstract](341)
Abstract:
The sea ice is idealized as an elastic-brittle material. When an ice sheet moves toward a structure, the dynamic in-teraction between ice and the structure is analyzed by the DDA (Discontinuous Deformation Analysis) approach, where the ice sheet and the structure are considered as assemblages of blocky masses. This has the advantages that the whole process of collision between the ice and structure can be shown visually vvith a series of pictures. Meanwhile, the dynamic response of the structure at each time step after the bumping of the ice against the structure is calculated. And with the aid of inverse analysis developed by the authors, the time history of the resultant ice force exerting on the structure is evaluated. A numerical example shows that the proposed approach is suitable to the simulation of the ice-breaking process and reasonable result of ice force acting on the structure can be obtained.
Analytically Derived Wind Wave Growth Relations
2002, (3): -.
[Abstract](340)
Abstract:
By the use of the 3/2 power law presented by Toba combined with the significant wave energy balance equation for wind wave, wind wave growth at a limited fetch is analytically investigated. The new wind wave growth relations (WWGRs) are analytically derived with shehering coefficient and wind drag coefficient as parameters. The geometrical average of observational values of shehering coefficient and the arithmetic average of observational values of wind drag coefficient are applied to detennine the new WWGRs. Comparisons with existing empirical WWGRs are made.

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