نشریه مهندسی دریا
پیاپی 14 (پاییز و زمستان 1390)

In order to model real behavior of jackup platforms subjected to environmental conditions, a nonlinear dynamic analysis considering soil-structure interaction subject to wave loading is investigated. For this purpose, spudcans behavior, soil nolinear behavior near the structure, contact phenomena at the interface of the soil and foundation such as gapping as well as geometrical nonlinear behavior of the structure are considered. The soil which is used in this study is assumed to be granular with varying friction angles. Beam on Nonlinear Winkler Foundation method (BNWF) is a suitable model for soil media modeling. For wave loading two methods were used, monochrome wave and irregular waves representing a sea state. Results of this study show that inelastic behavior of the soil causes an increase in displacement at hull elevation and permanent unequal settlement in soil below the foundations which is increased by decreasing friction angle of soil. In fact spudcans and the underlying soil cause a relative fixity at support of the platform which changes dynamic response of structure compared to the case where the structure is assumed to have a fixed support.

In the present research, two sets of data were analyzed for the purpose of obtaining the oscillatory derivatives of the sway force and yaw moment with respect to the sway velocity and sway acceleration. The results of the analysis show that the values of the derivatives which were calculated from the dynamic test data are somewhat different from the values of the same derivatives which were calculated from the static yaw test data. Moreover, the test data from the dynamic manoeuvres show that the values of these derivatives depend both on the frequency of the forced oscillations and hull length.

In recent years, unmanned vehicles have intensively been developed to reduce human dangers for marine applications. Predicting the hydrodynamic coefficients of an autonomous underwater vehicle is important during the vehicle’s design phase. In other words to design an AUV, one must clarify its maneuverability and controllability based on a mathematical model. The mathematical model contains various hydrodynamic forces and moments expressed collectively in terms of hydrodynamic coefficients. Therefore, the correct values of the coefficients must be known to precisely design the controller of AUVs. Despite of remarkable progress in numerical analysis in recent years, But the experimental methods test modes is still more reliable. No marine vehicle will be manufactured unless its model been tested clearly first. SUT-2 is an AUV, being developed by Marine Engineering Research Center Sharif University of Technology in Iran. Model test done into marine engineering laboratory towing tank. In this paper hydrodynamic coefficients calculated with model test of autonomous underwater vehicle and hydrodynamic forces analyzed. This coefficients are used for dynamic modeling and autonomous controller design.

So far many case-study researches have been undertaken to propose suitable models for hull optimization of planning high-speed crafts. In this paper a systematic and generic method is illustrated for the first time to introduce the optimized hull form of craft considering the desired speed and weight. Using semi-empirical Savitsky's method, optimal geometrical parameters of planning high-speed craft are defined to minimize the hydrodynamic resistance. Of course some variants i.e. beam, deadrise and the distance of center of gravity to transom and two objective functions are introduced for hull optimization via Pareto-based genetic algorithm. The first objective function is the ratio of pressure and frictional resistance summation to displacement and the second one is the ratio of water spray's wetted area to displacement volume powered two-thirds. Optimization involves constraints such as: governing limitations on Savitsky method, hydrostatic stability, proposing instability and corkscrew dynamic stability. Finally a bunch of variants optimal quantities are presented to be applied in conceptual designing of high-speed monohull crafts.

Diffusion coefficient is one of the parameters which can describe the rate of chloride ion diffusion in concrete. This can be obtained by the chloride ion profile. By determining this factor,one can predict when the chloride ion concentration on the reinforcement surface reaches a critical value,and the cover tickness which is needed for preventing corrosion. Time and distance dependency of chloride ion diffusion coefficient has been proved by researchers,but the exact equation of dependency is ambiguous. In this paper,Researchers try to collect the various opinions about the quality of chloride ion diffusion time-dependency and then compare the experimental approaches with a new analytical approach to calculate diffusion decay index.

Bed-load transport is effective on river bed stability and Morphology. Therefore, modeling this phenomenon is very important in the field of hydraulics engineering. Bed-load transport occurs by saltation at low to medium shear stress and by sheet-flow at high shear stress. In the present paper, the hydrodynamics of transport in thesaltationand sheet-flow regimes under unidirectional flow is studied using a Lagrangian model based on I-SPH method. The fluid and sediment phases are modeled separately as Newtonian and non-Newtonian fluids respectively. Fluid-sediment interaction is modeled using drag force to define interaction mechanism. To calibrate and validate the numerical model, the result is compared with experiments. There is a good agreement between them that illustrates the applicability of the present model in the simulation of both saltation and sheet-flow regimes hydrodynamics.

Tidal behavior of esturies is more complicated than open seas. It arises from the effects of geometrical parameters. Also other parameter such as wave reflection from the end of estuaries, sieche, bed friction and coriolis forces cause the water level changes in the esturies to be several times as in the open sea. This paper evalutes the effects of gemetric parameters (such as length, wide and depth), on the tidal behaviour of macro-tidal estuaris. It was done specifically for Mahshahr estuary, located in north west of Persian Golf. This research was done by using numerical simulation and field measuements. As a result, it can be said that: wide of estuary has no effect on phase and high tide elevation in the estuary. Also reduction of water depth causes the phase increament and reduction of tide amplitude, while it has no effect on high tide elevation in the estuary. It also shows that, flood wave transformation from sea into the estuary is associated with phase lag. The amount of this lag is about 10 and 20 minutes per kilometer of the channel length in the deep part and shallow part of the estury, respectively. The results also show that Mahshar estuary to behave as a rectangular–shallow-long estuary.

Calculation of the reflected waves against obstacles is a prominent step in designing coastal structures. In addition, a complex process occurs during wave collision with beaches or breakwaters. These processes cause wave characteristics to be (energy dissipation, reflected wave, etc.). The new wave characteristics are dependent on reflected wave height straightly which are not measureable. In other words, finding an appropriate method for identifying reflected waves is an essential part of each marine project. In recent years analytical and experimental methods have been established for the decomposition of incident and reflected waves. An analytical solution for estimation of the reflection coefficient is time domain analysis. In this method, waves which radiate in opposite directions could be separated by zero crossing method. Alternative analytical way is using fast Fourier method which is called spectrum analysis. In this paper, above methods are compared by using experimental data. Two-dimensional experimental studies are carried out in the Hydraulic laboratory of Amirkabir University of Technology (Tehran, Polytechnic) to collect appropriate time series. Results reveal that although spectrum analysis, compared with time domain analysis, allocates higher wave height for both incident as well as reflected waves, reflection coefficients are virtually same in both methods. Furthermore, evaluated incident wave height in time domain analysis is close to wave height in wave maker theory.

Prediction of wave height is of great importance in marine and coastal engineering. In this study, the performances of artificial neural networks (feed forward with back propagation algorithm) for online significant wave heights prediction, in Persian Gulf, were investigated. The data set used in this study comprises wave and wind data gathered from shallow water location in Persian Gulf. Current wind speed (u) and those belonging up to eight previous hours are given as input variables, while the significant wave height with leading time of 1-24 hour are the output parameters. Results show that the artificial neural networks can perform very well in predicting significant wave height, when shorter intervals of predictions (6 hour) were involved. Small interval predictions were made more accurately than the large interval ones. Results of artificial neural networks were compared with those of regression trees. Results indicate that error statistics of neural networks and regression trees were nearly similar