نشریه مهندسی دریا
پیاپی 20 (پاییز و زمستان 1393)

Given the increasing importance of vessels carrying liquefied natural gas (LNG)، the high cost of incidents and also projects related to the design and construction of LNG transport vessels requires further investigation of fluid sloshing inside tanks of these vessels. This paper، reports the towing tank، equipped with a wave-maker، test results on a one-to-176 model of a product carrier with a rectangular tank exposed to head seas. This type of sloshing test in which the tank is installed within the ship model is one of a kind. Wave amplitude has a major effect on the amplitude of the vertical displacement (heaving) of the vessel. On the other hand، the test results show that the sloshing water inside the tank increases the range of heave and pitch motions of the vessel. However، in most frequency ranges، with increased wave amplitude، sloshing effect is such as a damper for the heave motion of the vessel.

Finite elements method can considerably increases the computational works of free vibration analysis of offshore wind turbine with fixed monopile platform depending on the modeling type of foundation،. In this paper، transfer matrix method is used to reduce computational works and increase the speed of analysis instead of the finite elements method. For this purpose، the wind turbine foundation is modeled using CS، DS and AF models are used and required relations are extracted to determine the natural frequencies and mode shapes for each of the models. Then، factors affecting the frequency such as water depth and added mass coefficient are examined in several case studies. Finally، the transfer matrix method obtained results are compared with the results of the finite elements method، multi body dynamics and experimental data which show good agreement in spite of low computational cost.

Wave endurance time (WET) is a time domain method for evaluating the response of offshore structures under different storm conditions. In this method، intensifying artificial records named wave functions (WFs) are generated in place of various random records of extreme sea states. In this regard، the computational costs are reduced، and reliability of structures is evaluated by the endurance time. In this study، a description of WET concept is presented، and also generation of initial type of WFs is thoroughly addressed for the Persian Gulf region. To consider the accuracy of this method، the results of WET method are compared with typical 3h extreme wave simulations for maximum base shear and deck displacement. The results indicate that there is acceptable agreement between two approaches.

Construction of ports and coastal structures actually affects the coastal flow pattern and results in the changing of the sedimentation pattern. The research carried out in this paper، aims to simulate sedimentation process following the construction of the Bandar-e Anzali New Breakwaters at the site، using Iranian developed PMO hydrodynamics and well known MIKE21 modeling package developed by DHI. These two models utilize different mesh technique to solve the governing flow hydrodynamics and transport equations. So to compare the results obtained from these two models، sensitivity analysis and calibration procedures using field data were undertaken firstly. Then wave propagation pattern of waves with heights of 0. 5، 1 and 1. 5 m in the spectral wave module (SW)، long shore currents in the hydrodynamic module (HD) and sediment transport simulation in the sediment transport module (SD) have been considered using both models. Various statistical parameters were obtained and the results showed good agreement between the two mentioned models with at least 90% correlation coefficient in all cases. Furthermore، the study showed acceptable results can be obtained by center point mesh-type technique using PMO Dynamics model with much easier calculation procedures that reduces the computation time significantly even by a factor of 4.

In this study، Plasma Electrolytic Oxidation (PEO) technique was used for generating ceramic layer on the surface of aluminum alloy AA2024-T6; and the effect of coating time on the electrochemical corrosion behavior، micro-hardness، morphology of coating، and the phase structure was studied. The alloy was coated in 1، 3، 5 and 7 minutes at a constant electrical potential 550 V in a sodium silicate solution 30 g/l. Coating surface morphology was studied by Scanning Electron Microscopy (SEM); and the electrochemical corrosion behavior was investigated by potentiodynamic and Electrochemical Impedance Spectroscopy (EIS) tests in 3. 5 %wt. NaCl solution. Phases that generated during coating process were studied by X-Ray Diffraction (XRD) and hardness of coating was investigated by micro-hardness test. The results revealed that، as the coating time increases، the hardness value increases too. But، since the sample coated for 5 minutes exhibit the best morphology، it has shown the maximum corrosion resistance.

Oscillating water column (OWC) is one of the wave energy converting devices available. In this article، a numerical model has been introduced to simulate wave interaction with OWC in one dimensional plane. Mild slope equations have been used in one dimension. The pressure and water elevation inside OWC have been calculated using an iterative method. Also an absorbing boundary is used at the beginning of the canal. Conventional test cases were used in order to verify the model such as sinusoidal waves. After the model verification، the interaction of wave and OWC has been simulated. The effect of different wave conditions and also the immersion depth of the OWC’s front wall have been investigated. The numerical results were consistent with available experimental results.

Nowadays، the power industry with fixed offshore wind turbines through the renewable energies is one of the progressing industries. The installation of scour protection structures is done against the scouring process. The collars are one of the protective structures. In this study، distribution of the bed shear stress around the pier is investigated via the FLUENT software. The results have been observed based on the relative heights of the collar to the substrate. On the other hand، in a hydraulics laboratory، some experiments have been done on a physical model with and without the protective structures. The charts of the scour depths، the equilibrium scour depths and the patterns of the scour hole around the pier have been obtained for different test scenarios. Finally، comparing of results of the numerical analysis and experimental results showed that there was direct relationship between bed shear stress and the equilibrium scour depths as well as the collars with less height from substrate had proper effect on protection of pier.

In the current study a numerical simulation approach has been chosen to simulate the force-deflection response of tubular member under lateral impact in the absence of internal pressure or axial force and the results have then been compared with analytical relations. Two series small scale experiments were carried out by the authors on X70 steel tubes and D/t of 22 and the numerical model has then been verified against experimental results. The results show that the analytical relations estimate greater lateral resistance than experimental results for the spherical indentor and in the unpressurized tubular members. The numerical model has then been used to study the effect of D/t and d/D and yield stress on lateral resistance of steel tubes. It has been noticed that، the lateral resistance was governed by: 1) D/t ratio 2) Yield stress 3) d/D ratio. The comparison between analytical and numerical results show that analytical relations are more accurate results for large D/t and d/D ratios.

Brace and chord members of a tubular joint always have a degree of geometrical imperfection due to the manufacturing، transportation and installation processes and also the action of various loads during the lifetime of the structure. However، during the analysis and design of tubular joints، geometrical imperfection is not usually taken into account. Considering the geometrical imperfection in the analysis of a tubular joint، can lead to a more realistic prediction of the fatigue life. In this paper، the effect of geometrical imperfection of the brace member and dimensionless geometrical parameters on the stress distribution in tubular T-joints under axial loading is studied. The stress concentration factors (SCF) is used to investigate the stress distribution. The research is focused on the SCF values at the saddle، crown 0˚ and crown 180˚ positions. After the numerical modeling of the considered tubular joints using FEM-based software package ANSYS، verification of the FE models by experimental data، and the extraction of SCFs، a set of parametric equations is presented to calculate the SCFs at the saddle، crown 0˚ and crown 180˚ positions. The results show that the in-plane geometrical imperfection of the brace member can change the SCFs at the crown 0˚ and crown 180˚ positions، but does not have a considerable effect on SCF values at the saddle position.

Leca lightweight particles are used for structural and semi structural concrete in different countries. In Iran it is usually used as Mass lightweight concrete، blokes and partitions. In this research an experimental work carried out using Leca concrete to assess its physical and mechanical properties and possibility of constructing high strength lightweight concrete to use as structural element in marine environment. Using leca، microsilica gel (silica soot plus super plasticizer) and cement type II، structural high cement content semi lightweight concrete was constructed. Then different characteristics such as specific weight، percentage of abrasion strength، compressive strength، specific gravity of fresh and hard concrete and intensity of bare steel bar corrosion by comparing bar corrosion potential in lightweight and ordinary concrete was obtained. The results show that the leca concrete is in the range of lightweight concrete and is suitable as semi structural and structural element. Furthermore it is found that this kind of lightweight concrete in corrosive environment behaves similar to or better than ordinary concrete. Therefore it can be used in onshore and offshore structures.

Acoustic wave propagates properly in water، whilst most forms of electromagnetic waves، particularly in salty and dense sea water، are attenuated after a few tens or hundreds of meters so that it will be impossible to trace and detect them. This paper presents a special method for recognizing probable underwater currents، and reconstruction of their intensities، widths، and directions by using the acoustic waves. The method relies on the differences in travel times of sound in the locations where the water density varies from one place to another. The design is based on the «rotating sound source-receiver» system. This technique is somehow similar to the customary methods of image reconstruction in diagnostic tomography. A computer program was written in Visual C in the Windows media. The efficiency and accuracy of the program was consequently evaluated by simulating an ocean tomographic region and its postulated currents.