نشریه مهندسی دریا
پیاپی 13 (بهار و تابستان 1390)

In general, Lay and Tow methods are the two most common methods of offshore pipeline installation. In the shallow waters, considering the restrictions of Lay barge, Tow method is usually more appropriate. In this method, pipestring is fabricated on-shore and then it is launched into water. Then, according to environmental conditions and restrictions, the pipeline is towed to the installation site. In the present study, the evaluation of stress and deformation along pipeline during transportation and installation in shallow water has been presented. All analyses have been carried out using Orcaflex software. A parametric study has been carried out on the effect of environmental loading (wave and currents) and other parameter such as tugboat pull capacity and reserve buoyancy rate. Also lateral and vertical displacements and stress on pipeline for different conditions including static, transportation and laydown conditions have been investigated. For modeling properties of the pipeline, an existing pipeline in the Persian Gulf is considered. Due to the pipeline floating condition, the effect of the tug motions due to wave is ignored. The results are shown that lateral deflection of the pipeline due to wave spectrum is less than monochromatic waves with a maximum height of the wave spectrum. Also, by increasing tug speed, lateral curvature is closer to the end of the pipeline and increases the bending stress due to lateral curvature.

In this research study, effects of the draught and sheet height in pontoon floating sheet-breakwaters on the transmission and reflection coefficients against irregular sea waves was investigated. The project has been carried out using an experimental method within a wave flume of Soil Conservation and Watershed Research Institute. The incident waves were irregular and the wave spectrum was JONSWAP. The range of wave heights varied between 2 and 12cm and their mean periods were set to be between 0.63 and 1.26 seconds. To investigate the effect of draught in pontoon breakwaters, three experimental model with similar breadth and different draughts of 8, 10 and 12 cm were tested. Also, in order to investigate effect of sheet height in pontoon sheet breakwaters, sheet heights with 8, 16 and 24 cm were examined and compared without sheet case. The water depth was set to be 60 cm. The number of waves for each test was 250.

Optimisation of stacking sequence for composite plates under slamming impact loads using genetic algorithm method is studied in this paper. For this purpose, slamming load is assumed to have a uniform distribution with a triangular-pulse type of intensity function. In order to perform optimisation based on the genetic algorithm method, a special code is written in MATLAB software environment. The prepared code is so coupled with the commercial software ANSYS that in each step of optimisation, it could analyse the composite plate under study and calculate the central deflection. After validation, different cases of stacking sequence optimisation are investigated for a variety of composite plates. The investigations include symmetric as well as anti-symmetric conditions of stacking sequence. Results obtained from these analyses reveal the fact that the adopted approach based on genetic algorithm is highly capable for performing such optimisations.

Springback in metal forming process is tendency to partially return to its original shape because of the elastic recovery of the material upon release of the forming force. In order to increase accuracy of dimension in products, it is desirable to eliminate or reduce this phenomenon. It is the main objective of present work to study the amount of springback generated during deep drawing forming of titanium sheets and influence of plate thickness and heat on springback. For this purpose, LS/DYNA computer code is used to simulate forming process. For tools (mandrel, clamped, matrix) preheating of 200 degree centigrade and for plating 200, 300 and 400 degree centigrade are used for first, second and third stage of forming respectively. Results are compared with available results in literature for V-shape forming. This comparison shows that springback changes directly with material and geometry of tools and with increasing temperature, formability of Titanium alloys has increased and springback are decreased.

Biomimetic underwater vehicle design has attracted the attention of researchers for various reasons such as ocean investigation, marine environmental protection, exploring fish behaviors and detecting the leakage of oil pipe lines. Fish and other aquatic animals have good maneuverability and trajectory following capability. They also efficiently stabilize themselves in currents and surges leave less noticeable wake than conventional underwater vehicles equipped with thrusters. This paper presents the hydrodynamic simulation of a biomimetic robot fish that is fabricated at the Advanced Dynamic and Control Systems Laboratory (ADCSL), University of Tehran. In order to simulate a fish-like swimming robot, a comprehensive hydrodynamic analysis was performed. Extensive study of the biology of fish particularly their motion was performed. Carangiform swimming mode, which is the swimming mode of fish that use their tail and peduncle for propulsion was chosen. A hydrodynamic simulation is performed using computational fluid dynamics. In simulation Reynolds averaged Navier-Stokes equations (RANS) and Large Eddy Simulation (LES) method were employed to solve turbulence conditions. In this analysis the fluid was supposed to be single phased and the flow to be distributed and incompressible. The presence of a complex shaped moving boundary makes a difficult proposition for the computational fluid dynamics. Dynamic mesh method is employed to simulate moving boundaries. A hydrodynamic model of a robot fish permits us to determine the properties of the robot fish and facilitates the development of control algorithms. In order to verify these numerical models, an experimental test bed is fabricated at ADCSL. Experimental results show a smooth, repeatable and controllable motion of the robot fish. Data gathered through this method demonstrated reasonable agreement with simulation results. Consequently, the numerical model maybe utilized for further analysis and optimization process to reduce the experimental trail and error process cost.

The goal of this study was to simulate the hydrodynamics induced by con-current wind and tide loading in the southern part of the Persian Gulf. The model was calibrated with a 28 day lunar cycle, in which both neap and spring tide were modeled. The model boundaries were forced using tidal water levels obtained from the United Hydrographic Tide Tables. Wind data extracted from PERGOS database was used as input to the model. Current data recorded at Ghasha buoy, as well as current and water level data from PERGOS database were used for calibration of the model. The error in water level prediction is typically less than 25% of tidal range. Currents were also predicted with accuracy of 0.1 m/s (95% of time). Model results are insensitive to variations of calibration parameters over the recommended range, which demonstrates that the model is robust and can be used with confidence.

In this study two breakwater models with different conditions of bed, waves and water level are employed to analyze the toe stability and scour depth. The tests are conducted with regular waves. It is found that the scour depth in front of the rubble-mound breakwater depends on wave height, structure slope, water level and wave period, but the results reveal the effect of wave period toe is more than others. So that long period waves with more energy can be a threat for rubble-mound breakwater. Also the results of the toe stability and scour depth are given in the form of diagrams.

Due to the much higher maintenance and replacement expenses of the offshore wind turbine structures, more attention should be paid for a reliable lifetime analysis of them. Meanwhile, the cyclic nature of wave and wind loads together with the corrosive effects from the sea water are major factors for the creation and growth of flaws and cracks in offshore structures. These cracks can be the cause of instantaneous failure in marine structures. The “Tripod” model is one of the favorite support structures which are used for offshore wind turbines particularly in deeper waters. In this paper, a crack is considered in a support structure that can be typically used in the offshore wind turbine constructions. The finite element code ABAQUS is employed to determine the related stress intensity factors for different types of loads. It is shown that the variations of stress intensity factors with crack length differ significantly for various loading types. Also the mode II stress intensity factor is not negligible compared to the mode I stress intensity factor. Therefore, for analyzing the fatigue life or the fracture load of similar cracked offshore structures, appropriate mixed mode crack growth criteria should be employed together with the curves derived in this research for stress intensity factors.