JOURNAL OF MARINE ENGINEERING
Year:2018 | Volume:13 | Issue:26 |Papers Count:12

the absorption performance of anechoic coatings depends on the material properties، layer thicknesses and cavity distribution density and cavity size. In this paper a design method based on numerical simulation was presented by combining FEM and acoustic duct method (ADM). Analyzing of anechoic coatings was performed under active sonar impinging plane wave by normal incident angle. In this approach considering a unit cell of anechoic coating and two ducts of the fluid on its both sides، the reflection characteristics، transmission and reflection coefficients of anechoic coating were calculated using acoustic duct theory. Validation was performed by comparing the results of simulation with the available experimental data. Also a design code in ANSYS software was developed. Finally using this code and the proposed model، the acoustical behavior of anechoic coatings was investigated. The results show that sound transmission coefficient for coating with cylindrical cavity is larger than that of spherical cavity. Furthermore unlike the transmission coefficient، for all frequencies upper than 1500 Hz، coating with cylindrical cavity has a greater echo reduction and therefore its anechoic performance is better than that of spherical cavity.

A marine gas turbine rotating disk is one of the most critical components which operates under nonstationary varying mechanical and thermal loads. In this research، the homotopy method is used for stress analysis of rotating disk subjected to thermal load. Apply the thermal loading on the disk with exponential thickness distribution by using the homotopy method and Considering the physical and mechanical properties variations with temperature، are the most important aspects of innovation in this paper. Disc thickness variations are considered exponential and the inner and outer rims of disk are considered constant thickness. Due to the use of homotopy analysis method، important and effective parameters in this method are selected and presented. Furthermore، results are compared and validated with finite element code and good agreement of results with this method is showed. Finally، stress distribution in disks with constant، exponential and hyperbolic thickness distribution are considered and compared. The results showed the efficiency and acceptable accuracy of the method for analysis of rotating disk subjected to thermal and mechanical loading.

Permanent magnet motors have some advantages including high power density، high efficiency and good thermal characteristic and are used in many industrial applications. The aim of this paper is to present the optimal design and fabrication of a counter rotate brushless permanent magnet motor for using in marine systems. Due to the motor space limitations and lack of heat transmission with the environment، the designed motor must have high power density meanwhile high efficiency. For this purpose، the motor dimensions are calculated with goal of increasing power density by using motor’ s electromagnetic equation and genetic algorithm. Furthermore، some considerations have been performed in order to increase the motor efficiency. In order to validate the calculated results، the motor has been simulated with finite element method. At the end، the motor has been fabricated according to the calculated parameters and the experiments results have been presented.

Rock breakwaters are used mostly in south costs of Iran compared to the other typesof breakwaters، while enough source of good rock deposits is quite rare. A numbers of Lomashel rock samples of Holor mine of Dargahan in Qeshm Island were taken and subjected to three different improvement materials of limewater، resin epoxy and biogrout and physical and mechanical properties of both natural and treated rock samples were measured in laboratory. The results show that limewater decreases the rock quality up to 8. 5% while biogrout، and resin epoxy increase rock quality up to 7% and 17% respectively compared to the natural rock samples. Resin epoxy known as a surface improvement because of its different physicochemical attributes with rock and its small penetration depth. Therefore this treatment should be used periodically، while biogrout treatment enjoys of more compatible mineral composition with Lomashel with a deeper dispersion rate into the rock.

One of the main challenges in low-cost inertial navigation systems (INSs) is inevitable errors of MEMS-grade inertial sensors which lead to time-growing navigation error. This paper aims to develop AHRS/GPS/DR integrated navigation algorithm for longrange autonomous underwater vehicle (AUV). Proper performance in deals with longterm GPS outage is the main advantage of the proposed algorithm. Online estimation of gyro biases plays noticeable role in the performance of the presented algorithm. Using Dead Reckoning (DR) method based on an axial speed sensor، an applied algorithm is developed for position estimation during the GPS outages. The algorithm is evaluated by field test executed on a long-range AUV with high underwater durability. Experimental results indicate that in spite of 500 seconds GPS outage، the estimation error in the heading angle does not exceed 5 degrees.

For structural reliability assessment or risk analysis of aging offshore steel structures، it is essential to have a probabilistic model، which contains specific statistical parameters، and predicts long term corrosion loss as a function of time. The aim of this study is to propose such model for offshore jacket platforms in the Persian Gulf. Field measurements for material loss due to uniform corrosion for submerged members in seawater have been collected and statistically analyzed to identify the probability density distribution function for dispersion and variation of corrosion loss value according to the passage of time. To predict the corrosion loss، two probabilistic models with different approaches and time dependent parameters are suggested، based on the statistical analysis results. For a comparative study، the application of aforementioned models on the reliability analysis of a simple plate structure is investigated. Prediction results، obtained from the proposed probabilistic models، are consistent well with filed measurements.

In this paper، an underwater targets classification system is presented using the noise released from them. This system is proposed for shallow water environments such as Persian Gulf. Acoustic noise is non-Gaussian and due to the near-surface and properties of the environment acoustic noise is nonlinear and nonstationary. This fact affects the quality of the classification system. To classify acoustic noise، a target identification system is developed using a feature extraction section based on Hilbert Huang's Transform (HHT) and Support Vector Machine (SVM). Then the performance of the proposed method is illustrated using simulation results. Simulations take on two groups of signals. The first group of noise was related to three different type of vessels، and the second group of noise was related to a number of heavy and a number of light vessels، which were used in the Persian Gulf. Classification results show that the proposed method has a better performance than the wavelet transform method.

Using finite element method، free vibration of very large circular floating structures is investigated based on the first order shear deformation theory. The very large floating structure is modeled as an elastic plate with free edges. The free vibration of a circular plate in contact with air (the dry plate) is investigated and the natural frequencies and mode shapes of the plate are determined using a code written in MATLAB as well as with ABAQUS software. Then a circular plate in contact with water (the wet plate) has been considered. Using ABAQUS software، The effect of the presence of water on vibrating plate is reflected by plate added mass. The good agreement between the present results with those of previous studies approves the accuracy of the applied finite element model for free vibration analysis of very large floating structures.

The aim of this study was to design and develop a three-dimensional numerical baroclinic model with the ability to accept complex bottom topography and variable wind in space and time to investigation wind-induced current at 10 layers in the south Caspian Sea. The finite difference method Was used for numerical solution of the primitive equation in spherical Sigma pressure coordinate system on staggered modified Arakawa C grid. The wind data available from ECMWF ERA 15 data sets and the bathymetric data obtained from the GEBCO data sets with spatial resolutions of 0. 125o were utilized in the model. The result shows there are found a cyclonic eddy in middle of the basin and an anticyclonic current in close of the western coast that continued over the year. In addition، the circulation in the extensive continental shelve near the eastern coast is especially sensitive to wind direction and speed Also the maximum of surface current always take place in this area.

Hydrodynamic design of the exterior hull shape is a main step of body shape design. The effective parameters in this process include the length of the nose and tail، the blunt sections of nose and tail as well as the profiles of the nose and the tail. In the present study to investigate the effect of each of these parameters on the drag coefficient of the body، Design of Experiments (DOE) method is used. For this purpose، by introducing the Hydrolab family profiles، the results of numerical simulation of flow around the Hydrolab500 body was applied to design of experiments. In the first phase، a sample experiments in water tunnel was conducted to validate the pressure profile around Hydrolab500 body. Comparing experimental and numerical results shows the validity of the employed numerical methods. The results of the current work shows that by presented method the drag coefficient of an AUV in the final design can be estimated with high accuracy.

Prediction of hydrodynamic loads during water impact has great significance in the structural design of vehicle. Impact load of fast crafts with water surface and the slam force of waves on the members of offshore structure is common examples of this phenomenon. There is not any available theoretical tool to exactly handle this complicated phenomenon and the experimental procedures in the laboratory are both time-consuming and expensive. Several studies has been done on impact to various structures from the water. Dependence of the solution of the problem to the change in the boundary shape of the water surface، makes the problem extremely complex and difficult to solve analytically. In this project، impact of a cylindrical projectile on the water surface at different entry velocities and entry angles is examined using Euler– Lagrange method in LS-DYNA software. Three-dimensional simulation and meshing of the body has been perform in LS-DYNA and then the water and the air model has been created. Simulation results with Specified boundary conditions indicate significant effect of velocity changes on the stress، plastic strain and reduce the velocity of the projectile on water impact.

The oil and gas pipelines are among the most important offshore structures. In this study، the turbulent flow past piggyback pipelines and vortex shedding are numerically simulated under steady currents. Numerical simulation has been conducted for different values of the flow velocities، diameter ratios، and gap ratios; in order to investigate how the hydrodynamic forces of the small and large cylinders are affected. Evaluation of the numerical model revealed that the drag coefficient of small and large cylinder is increased as the flow velocity increase. For the large values of gap ratios، increasing of the flow velocity has less effect on the magnitude of the RMS lift force of the large cylinder whereas the RMS lift force of the large and small cylinders considerably increased by decreasing the gap ratio.