نشریه مهندسی دریا
پیاپی 30 (پاییز و زمستان 1398)

Single point mooring terminals are, as the name implies, facilities of small horizontal dimensions, to which large vessels are moored by means of a bow hawser or by any other means which allows the vessel to rotate 360 around the mooring point. Generally, the single point mooring terminal can have two functions. Primarily, it affords a safe mooring to the vessels. Secondly, it can form a link in the chain for the transport of oil. In this study, with the feasibility of 3D image processing, the SPM translational motion is evaluated. This translational motion includes Surge, Sway, and Heave. In order to ensure the results of the analysis of SPM translational motion, verification is carried out. In the following, by constructing SPM and situating it in reaction condition with regular wave, translational motion is obtained. Finally, the results indicate that image processing is acceptable in translational motion assessment of SPM.

The noise emitted by ships is one of the most important noises in ocean and the propeller noise is one of the major components of the ship noise. Measuring the propeller noise in laboratory despite the high accuracy and good reliability, costs a lot, has some complexities and is very time consuming. For this reason, the calculation of propeller noise using numerical methods has been considered in recent years. In this study, the noise of propeller in non-cavitating conditions is calculated by solving the Ffowcs Williams-Hawkings (FW-H) equations. A software for numerical calculation of propeller noise, based on FW-H equations, is developed and results are validated by experimental results. This study show that the results of the FW-H noise code have also good agreement with experimental data.

Extreme wave height prediction is very challenging due to its very high non-stationarity and non-linearity nature. The main aim of the present study is to propose a new hybrid method based on Local Outlier Factor and Artificial Neural Networks classifier, called LOF-ANN, to accurate prediction of extreme wave height occurrence using historical meteorological data. In this study to create models two major hurricanes Dean 2007 and Irene 2011at two locations (NDBC wave buoys stations: http://www.ndbc.noaa.gov) namely; 41004, 41041 in the Gulf of Mexico, is used. TO detect extreme waves, LOF method is used. The outputs of this method are considered as ANN targets. Extreme and normal waves are considered as Class 0 and class 1, respectively. The inputs of ANN models are historical metrological data, including: Wind direction (WDIR), Wind speed (WSPD), Sea level pressure (PRES), Air temperature (ATMP), and Sea surface temperature (WTMP). To create and evaluation of models, the input data sets are randomly divided into training (80%) and test set (20%). The performance of created models is evaluated using three popular criteria Root Mean Square Error (RMSE) and Receiver Operating Characteristic (ROC) and accuracy parameter. The experiment results show that the proposed method is able to predict the occurrence of extreme wave heights with height accuracy (up to 99%).

In this paper, the effect of composite patch on cylindrical pipe with circumferential through-wall crack has been investigated under internal pressure. For this purpose, using three-dimensional finite element and J-integral, stress intensity factors is determined before and after the repair. In order to ensure the accuracy of modeling, a pipe with circumferential through-wall crack under uniform tension load is simulated and the results are compared with theoretical data. Then, the cracking cylinder is repaired using four composite patches of Boron/Epoxy, Carbon/Epoxy, Kevlar/Epoxy and Glass/Epoxy in a local manner. The results of study are shown the significant effect of composite patches on reducing the stress intensity factor of cracked cylinder. It is observed that the use of composite patches with higher stiffness has a more decreasing effect on stress intensity factor. Also, the effect of composite patch, properties and thickness of adhesive on stress intensity factors along crack front is evaluated.

Assessing the stability of the breakwaters against incident waves is the most important issue in structure designing. Due to this fact, the effects of different parameters on the stability of multi-layer berm breakwater (MLBBs) have been presented in this study. A 2D experimental model of an MLBB with a JONSWAP spectrum of irregular waves is conducted. The effects of berm parameters such as berm elevation from SWL and water depth at the toe of the structure have been evaluated on MLBBs damage. The new parameter of stone class I height is assessed to find its effect on MLBBs stability. Outcomes disclose that an increase in stone class I height would increase the wave energy dissipation and consequently decrease the structure’s damaged eroded area. Moreover, with a 23% increase in water depth, the forces on the structure would increase and as a result, the structural damage is enhanced to 250%. The results indicate as, with a 40% increase in berm elevation from SWL, the eroded area is enlarged to 67%.

In this research, the main focus is on monitoring the dynamic instability created in a semi-planing catamarn and reducing the amplitude of these movements using a stern wedge. This is done using the model analysis in different Froude numbers in both the original and modified body conditions. Numerical analysis is also provided by solving the Reynolds-averaged Navier–Stokes equations and OpenFOAM open-source code. In addition to verifying the numerical solution method based on experimental data, it can be seen that the installation of a stern wedge of the ship has a very positive effect on reducing the range of dynamic motions in calm water. The effect of this motion decrease can also be seen in the resistance-speed chart. The most important achievement of this research is the creation of a numerical tank for simulating dynamic motions and accurate recording of details of fluid flow around the planing and semi- planing hulls. The result of the stern wedge installation and numerical and experimental analysis is a 12 percent reduction in ship resistance at design speed.

Hovercraft is a modern air-cushion vehicle (ACV) that has the ability to move on a variety of surfaces, such as water and land. The distinguishing feature of the hovercraft and other vessels is that the hovercrafts are deployed on a layer of air and then propelled, which is a major factor in the faster movement of them. In fact, this is a unique feature of the hovercraft, which makes it possible to use various military, recreational, transportation, and, research limitations/implications and so on. In this paper, the numerical study of the incompressible turbulent air flow in a steady state of the hovercraft suspension system is discussed. For this purpose, hovercraft is modeled in CATIA software. After transferring to the Ansys-Fluent software and applying the boundary conditions, and solving the equations governing the problem, the results are presented as pressure, velocity and curve of the lift force. In this research, we have tried to observe the effect of changing the profile of the flow transfer part and air channel on the distribution of pressure and velocity as well as the lift force applied to the substructure. Finally, the most suitable form for the distribution of the pressure and the amount of lift force from the presented profiles was the flow transfer part profile with a gradient of 8 degrees, which showed improvement in the result from the initial profile. It was also found that the deformations applied to the channel sectional profile did not improve the rate of initial lift force of the hovercraft.

The present study aims to introduce a new lattice wall breaker. The wall of this breaker is made up of small PVC cubic elements (PVC). Each of these cuboid shapes (blocks) has two holes, which, when placed on each other and through the metal cable through them, blocks are woven together. In the present study, three models of structures (with different porosity percent) are evaluated by examining three wave heights and at least 10 periods of rotation in a depth of water and a constant nourishment depth of the structure. The results showed that the decrease of wave height by 50%, in the porosity of the wall is 15%. The behavior of each model of the structure against the waves was also justified by accurate examination of the shock wave power.

Proper recognition of the coastal area helps to make better use of this area. Due to high risk conditions and wide variations in the coastal area, studies of this area usually cost a lot. In this research, the relationships among the breaking wave height, the width of the breaking zone, and number of breakings have been investigated as visual assessment and processing of video images taken by conventional and UAV cameras. Study of correlation between these parameters indicates the possibility of estimating the target parameter from the available parameters data using the relationships provided. The breaking zone width and the height of first breaking wave are directly correlated with coefficient of 0.92 and the correlation coefficient between the number of breaking and the breaking zone width is 0.86. The maximum, minimum, and averages of breaking zone width are 100, 0 and 27 meters, respectively.

Seasonal circulation of coastal currents of the Persian Gulf with a horizontal resolution of 2-minutes of latitude and longitude was simulated using COHERENS numerical model. The currents of the Iranian coasts flew northwestward from January to April, reaching the highest level from June to August, when surface inflow currents through the Strait of Hormuz gradually became stronger with establishment of the seasonal thermocline. The simulation results indicated thermocline expansion with the onset of the heating season. Under the mentioned conditions, stronger coastal currents were generated in summer. The simulation results showed the presence of coastal jet with a speed of approximately 30 cm/s from May to October. The coastal jet near the Iranian coasts is only confined to the surface, moves toward southeast; however, the coastal jet in the vicinity of Saudi Arabia is entirely present at all columns of water from surface to the seabed and moves toward southeast.

Acoustic Tomography (AT) technique is widely used in developed countries for water resources monitoring of the oceans, seas, and rivers. The 10-kHz Coastal Acoustic Tomography System (CATS) is previously applied to monitor coastal seas in the scale of kilometers. In this study, we evaluated the capability of the 10-kHz CAT system. The equation of sound propagation in water was developed to introduce a new equation to estimate the maximum operational range of 10-kHz CAT system. The results showed that with the assumption of clear seawater, the minimum and maximum operational ranges are 57 and 18000 m, respectively. Moreover, the velocity resolution is better than 0.1 cm/s in the range of greater than 7000 m. Choosing a suitable M Sequence due to the distances between acoustic stations is another point of operation to have the best performance. It is suggested that scientists apply this technique to monitor the Persian Gulf currents.

Design and analysis of the semi-submersible propeller due to meet with geometric and hydrodynamic parameters such as profile cross-section of blades, cavitation, ventilation, the effect of free surface and multiphase flow is very complicated. Designed and optimized for the semi-submersible propeller propulsion systems, requiring the solver is able to consider all the geometric parameters and flow in a short time with accurate results. In this paper, analyzes of the semi-submersible propeller using the boundary element method has been developed. Wake in the downstream of SPP is modeling and boundary condition using Taylor expansion and taking into account the long wave length is extracted. The results compared with experimental data shows that the boundary element method capable for analysis of SPP and using obtained numerical results by this method, the separation of initial, transient and fully ventilated areas during the rotation of the semi-submersible propeller is possible. In this paper, cross-flow effect on local results is discussed. By comparing the performance results of the propeller with considering the cross flow effect, the error reduction at low advanced coefficients is observed.