نشریه مهندسی دریا
پیاپی 19 (بهار و تابستان 1393)

Exact dynamic analysis of piers due to lack of information about their constituting sub systems، is very complicated. Ordinary finite element methods cannot be used in modeling of complex dynamic systems such as these structures since many uncertainty factors exist which cannot be identified exactly. In addition to physical modeling of a pier structure، existence of uncertainty in dynamic behavior of the structure is investigated in this article. This investigation is carried out by means of random attachment of number of mass-spring oscillators to deck. Furthermore، stochastic finite element method is implemented for numerical study of its behavior. Random matrix distribution concept is used for modeling of dynamic characteristic of system. And finally، by means of comparison between frequency response function derived from experimental study and numerical modeling of random finite element، it is observed that Wishart matrix probability density function assesses uncertainty parameters in dynamic response of the structure acceptably.

Climate change caused the changes in earth surface temperature، precipitation، sea surface level، wind speeds، wave heights، coastlines، etc. Estimation of the effect of climate change is of great importance in long-term studies of such parameters. In this study، the effect of climate change on average wind speed near Chabahar، Gulf of Oman is assessed until the year 2100. For this purpose، wind speed was obtained from a global circulation model; CGCM3. 1 using the data for three scenarios، i. e. A2، B1 and A1B. The results indicated that there is no significant change in average wind speed due to the climate change near Chabahar. Furthermore، the seasonal distribution of the wind speed illustrated that wind speed will increase slightly in spring and summer and decrease slightly in autumn and winter.

When studying the structural response of rubble mound breakwaters to wave loading، the knowledge of water surface fluctuations، pore pressure variations and related wave attenuation inside the porous structure is important since the pore pressures affect most responses، such as wave run-up، wave overtopping، reflections، transmission and the hydraulic and geotechnical stability of the breakwater. For this purpose a numerical model was developed to investigate the water surface and pressure fluctuations outside and inside the statically conventional rubble mound breakwater by using FLOW-3D software. The water - structure interaction is obtained by model to simulate the corresponding turbulence field and volume of fluid (VOF) theory to simulate the free surface. The comparison between numerical and experimental results shows a good agreement in terms of both water surface and pore pressure Fluctuations in conventional rubble mound breakwaters.

In this paper a multi-body system of barge type wind turbine under stochastic wave and wind has been modeled within MSC ADAMS. For wind loading، the stochastic turbulent wind data have been extracted using TurbSim software. Also، the AeroDyn module has been used for calculating lift and drag forces on the blades of turbine.. The hydrodynamic loads have been calculated using HydroDyn module within time domain. It composed of hydrostatic restoring; nonlinear viscous drag from incident wave kinematics; the added mass and damping contributions from linear wave radiation، including free surface memory effects and the incident wave excitation. By linking these modules with ADAMS/Solver milieu، the time domain، aero-hydro-elastic simulation of Barge type wind turbines has been achieved. The derived results have been compared with FAST’s outputs. The comparison shows the prosperity and accuracy of implemented analysis. The generality of analysis ensures that the simulation tool is applicable for any other types of the wind turbine، floating support platform، and mooring system configurations.

In the current study، the step floating breakwaters were under consideration to improve the π type floating breakwater performance. By means of physical modeling 180 experiments were carried out in the wave flume for different geometric and hydrodynamic parameters including 2 and 3 step FBs. Result were then compared with standard π type FB with the same width. To be able to compare experimental results، kHi50 (which is an incident wave characteristics with transmission coefficient less than 0. 5) was introduced. Results obtained through this study show that using step floating breakwater can be enhanced the hydrodynamic performance as well as reducing the construction material. In addition by increasing the width of steps، FBs performance was improved. Comparison of 2 and 3 step floating breakwaters reveal that the performance of each type is dependent on the draft depth. However it can be concluded that with increasing the step width، the difference of their performance is decreased.

Many empirical methods for estimating LSTR have been introduced by scientists during the recent decades، but these methods have been calibrated and applied under limited conditions of bed profile and specific range of bed sediment size. The existing empirical relations are linear or exponential regressions based on the observation and measurements data and there’s a great potential to build more accurate models to predict sediment transport phenomena by means of Soft computation approach. Contemporarily soft computing (SF) models، Artificial Neural Networks (ANN)، Adaptive Neuro-Fuzzy Inference System (ANFIS) have been accepted as potentially valuable tool for modeling and forecasting complex nonlinear systems. In other words، SF is very helpful when the mathematical and physical scheme couldn’t propose accurate solution to the encountered problem. The main advantage of SF model is that the accurate detail of the problems is not required. A comprehensive comparison between both ANN and ANFIS models and the existing empirical formulae will be presented to demonstrate capacity of ANNs.

In this study، Homotopy analysis method is employed for nonlinear vibrational analysis of marine riser subjected to variable axial loads. Mid-plane stretching effect has been considered in the model. Galerkin''s decomposition technique is used to convert the Partial differential equation of the motion to nonlinear ordinary differential equation. Homotopy analysis method (HAM) is applied to find analytical expressions for nonlinear natural frequencies of the riser. Effects of design parameters such as riser’s length and Initial static displacement of upper support on riser frequency are investigated. The analytical expressions are valid for a wide range of vibration amplitudes. Comparing the semi-analytical solutions with numerical results، presented in the literature، indicates proper agreement.