فهرست مطالب

  • Volume:2 Issue:2, 2014
  • تاریخ انتشار: 1393/07/30
  • تعداد عناوین: 7
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  • Amin Mahmoudi, Habib Hakimzadeh, Mohammad Javad Ketabdari Pages 1-13
    In this paper a space-averaged Navier–Stokes approach was deployed to simulate the wave propagation over coastal structures. The developed model is based on the smoothed particle hydrodynamic (SPH) method which is a pure Lagrangian approach and can handle large deformations of the free surface with high accuracy. In this study, the large eddy simulation (LES) turbulent model was coupled with the weakly compressible version of the smoothed particle hydrodynamics (WCSPH) method to simulate the wave propagation over coastal structures. The WCSPH model was employed to simulate the periodic wave propagation over impermeable trapezoidal sea wall and submerged breakwater. The numerical model results were validated against the experimental and numerical data found in the literatures and some relatively good agreements were observed. Afterwards, solitary wave propagation over impermeable trapezoidal sea wall on a sloped bed was carried out and the results of numerical simulations were compared both qualitatively and quantitatively with experimental data of Hsiao and Lin (2010). The results of this study show that WCSPH method provides a useful tool to investigate the wave propagation over coastal structures.
    Keywords: Sea wall, Submerged breakwater, Wave propagation, WCSPH Method, LES Model
  • Babak Ebrahimian, Aida Nazari Pages 15-28
    Analyzing piles subjected to lateral loads significantly depends on soil resistance at any point along the pile as a function of pile deflection, known as p-y curve. On the other hand, the deformation characteristics of soil defined as “the soil strain at 50% of maximum deviatoric stress (ε50)” has considerable effect on the generated p-y curve. In this research, several models are proposed to predict ε50 specifically for designing very long pile foundations of offshore oil and gas platforms in South Pars field Persian Gulf Iran. Herein, ε50 is evaluated from extensive soil data of marine clays including in-situ and laboratory test results using evolutionary polynomial regression (EPR). It is demonstrated that the normalized cone tip resistance, which is an indication of soil undrained shear strength, leads to more realistic ε50 values compared with the laboratory-derived undrained shear strength parameter. Furthermore, the results of full scale lateral pile load tests in different sites are used in order to validate the performance of the proposed models in predicting lateral pile behavior. The results of a numerical study on lateral pile-soil system also show the efficiency of the proposed model in predicting lateral pile response
    Keywords: P, Y curve, Laterally loaded pile, Piezocone penetration test (PCPT), Marine clay, South Pars field
  • Ali Beigi, Pedram Edalat, Mohammad Reza Khedmati, Manouchehr Fadavi Pages 29-41
    In the current study, the effect of such parameters as the length, position and angle of crack on free vibrations of a cracked plate was examined by finite element method. The assumed crack was internal and through and its growth was ignored. In the proposed finite element model, crack was considered as a discontinuity on the plate surface and the results were compared with the results of other studies for validation which proved its acceptable precision. The studies show that the thickness and mechanical properties of the plate do not influence the ratio of natural frequency of a cracked plate to natural frequency of an intact plate and that the other parameters change the stiffness and natural frequencies of a plate through affecting the modal shapes and stress intensity factors.
    Keywords: Free vibration, Crack, Plate, Natural frequency, Finite Element Method (FEM)
  • Mohammad Kazem Sharifian, Kourosh Hejazi Pages 43-54
    Gravity currents are very common in nature and may appear in rivers, lakes, oceans, and the atmosphere. They are produced by the buoyant forces interacting between fluids of different densities and may introduce sediments and pollutants into water bodies. In this study, the hydrodynamics and propagation of gravity currents are investigated using WISE (Width Integrated Stratified Environments), a 2DV hydrodynamic numerical model. An Explicit Algebraic Reynolds Stress Model (EARSM) has been deployed and implemented in the hydrodynamic model and the simulated results have been compared against the laboratory measured values and the results obtained from the k- buoyant turbulence model originally implemented in WISE. The numerical simulations focus on three types of gravity currents generated for laboratory experiments, namely: Lock-exchange gravity current, buoyant wall-jet flow and intrusive gravity current. The simulated evolution profiles and propagation velocities are compared with measured values. The numerical model shows good quantitative agreements for predicting the temporal and spatial evolution of the gravity currents. The simulation results show better agreements in case of EARSM compared to buoyant k- turbulence closure. A sensitivity study also has been conducted to investigate the influence of the values of spatial and temporal increments on the accuracy of the prediction for the turbulence closures.
    Keywords: gravity currents, turbulence modelling, explicit algebraic Reynolds stress model
  • Alireza Sadat Hosseini, Mehdi Shafieefar Pages 55-65
    A reshaping berm breakwater is a type of rubble mound breakwater in which, its seaward slope is allowed to reshape under wave attacks. There are some key parameters in the reshaped seaward profiles, which can schematize the reshaped profile of a berm breakwater. A total of 412 test results was used directly to cover the impact of sea state conditions and structural parameters on these reshaping parameters. In this study, the key parameters are derived using the M5' model trees. According to these new reshaping parameters, a computer program is written in MATLAB to predict the reshaped profile of a berm breakwater. The performance of the new program is compared with BREAKWAT software in the range of variation of the experimental data.
    Keywords: Berm breakwater, Reshaping parameters, M5\ model tree, Computer program, Seaward profile
  • Reza Akbari Alashti, Seyed Ali Ahmadi Pages 67-74
    In this study, buckling analysis of panel types of marine and offshore structural components with initial imperfection under the combined action of lateral pressure and axial compression is carried out. The governing differential equations for thin and moderately thick shells are developed in terms of components of the displacement field. The governing ordinary differential equations are then discretized and reduced to a linear system of homogeneous equations employing the differential quadrature method. The results obtained by the present method are verified with results obtained by finite element method and those reported in the literature. Design rules of two classification societies, namely American Bureau of Shipping (ABS) and Det Norske Veritas (DNV) are briefly presented and results are compared with rules requirements of these societies. It is observed that DNV rules are more conservative than ABS rules for calculation of buckling loads of marine and offshore structures. Effects of several parameters including the curvature, initial imperfection, geometric ratios and loading conditions on the buckling behavior of a cylindrical shell and a curved panel are investigated.
    Keywords: differential quadrature method, buckling, offshore structure, curved panel, classification society
  • Taha Nasseri, Naser Shabakhty, Mohammad Hadi Afshar Pages 75-84
    In the structural optimization problem, the aim is to decrease the amount of structural costs and weight, but the safety of platform should not violate the individual limits enforced by offshore codes. The outer diameter and thickness of members are two important variables in the optimization process and their final dimensions should be obtained according to the optimization algorithms such as genetic algorithm. In this process, weight of the jacket is the objective function of optimization problem and constraints are design criteria such as axial and flexural stresses, buckling of members and displacement of offshore structure that should satisfy the limitations imposed by offshore design codes. The drag forces of wave, current and wind on a unit length of structural tubular members of the jacket which are located below and above water surface are directly related to their outer diameter. However, the inertia force of wave is related to the square of the outer diameter. Thus, by changing outer diameter of structural elements during the optimization process, sea environmental forces on these members and their resultant forces on the platform will change. The structural members of the jacket are classified in four main groups including legs, horizontal members, diagonal braces and vertical braces. Each of these groups has different contribution in the optimization process and their degrees of importance are investigated in this research. The results show that horizontal members of jacket have major contribution in the optimization process among other groups. Afterward legs and diagonal braces have the second and third ranks in the contribution percentage respectively. Finally, the lowest contribution in optimization process belongs to the vertical braces.
    Keywords: Optimization, Fixed Offshore Platform, Genetic Algorithm, Contribution Percentage