نشریه مهندسی دریا
پیاپی 32 (پاییز و زمستان 1399)

Experimental measurements of dynamic instabilities in water environments are challenging due to their fluctuation and small amplitude. Most of studies use electrical circuits, this paper aims to use only mechanical methods to measure vortex induced vibrations. Measuring the fluctuations amplitude can help designing mechanisms for movement and for mechanical energy harvesting. This paper uses the novel particle image velocimetry method. The test comprises of a cylinder moving in water with different velocities. Uniform and tapered cylinders are used. Study of tapered cylinder is unprecedented. Output data is in the form of dimensionless amplitude-velocity graphs. From this test velocity for producing largest amplitude is obtained. It is also seen that in the efficient velocity, amplitude of uniform cylinder is larger. While tapered cylinder is suitable for using in currents with variable velocities.

One of the natural solutions to counteract the waves is using of coastal forest cover (Green Belt). Therefore, the focus of this study is generally to evaluate the impact of green belt on attenuation the effect of solitary waves at the presence it. Research experiments aimed at evaluating a mangrove tree effectiveness to reduce tsunami energy, by using simulated shrub structures, for two tree layouts with longitudinal intervals in transverse, different cover widths and different input wave heights. Wave-induced force to the coast and green belt was calculated by direct measurement and using an electronic force meter (load cell) mounted in front of the movable part of the flume and also the wave velocity were measured via an acoustic doppler velocity meter (ADV). The experiment results showed that the presence of green belt leads to 71.27% reducing the wave destructive force and 91.52% increasing the wave attenuation. Also, was presented the proposed equation for estimating of drag coefficient with the correlation coefficient of 0.84.

standard processing schemes of altimetry lead to the solution with relatively lower accuracy in shallow waters and inland water bodies. In order to get more accurate solution one needs to process the data with more specific processing algorithms. In this paper, we intend to review some retracking methods for shallow water level determination. Moreover, we introduce a new method for time series of water level using the well-known outlier detection of Baarda hypothesis testing. The achieved results are then compared with the dataset which is publicly available from Technical University of Denmark (TUD) as the standard data. The results show that the water levels estimated by the Baarda test are consistent with those of the robust mean water level. The accuracy of results obtained by the new method is at the same order of the robust mean water levels. the differences of baarda method with the standard method and tidegauge data in optimistic method are 2.7 and 28 cm.

The limitations of shallow foundations in terms of bearing capacity and settlement in soft clayey soils show the importance of the studying about so-called skirted foundations. The skirted foundations with penetrating into the soil and lateral confinement of soil can improve the bearing capacity and settlement behavior. These foundations are suitable for use in various applications such as seawater, beaches and dry lands. The objectives of this research are to evaluate the behavior of the skirted circular foundations on soft clay bed, and to estimate the improvement of bearing capacity and settlement reduction of skirted foundations using the numerical modeling. Various parameters including foundation diameter, skirt depth ratio, un-drained strength of clay, roughness of interface between foundation-soil and soil heterogeneity were investigated. The analysis results showed a significant improvement in skirted foundations performance in terms of bearing capacity enhancement and settlement reduction compared to the surface foundations. The improvement values increased with increasing the ratio of skirt depth, the roughness of the foundation surface and decreasing the shear resistance of the clay. The bearing capacity values of skirted foundations for the same depth and diameter were obtained close to the values of pier foundations.

In this study, spray behavior of the cavitation flow and the diesel fuel in a fixed volume combustion chamber for different nozzle hole geometries and needle lift profiles has been investigated using Fire software in order to improve the fuel spray characteristics and marine diesel engine performance. Thus, Firstly fuel flow inside the injector with cylindrical, convergent and divergent conical nozzle holes have been simulated with the base needle lift and then in the following, different needle lift profiles have been used in converged and diverged conical nozzles. Numerical results show that increasing the nozzle hole diameter and rounding edge and duration of needle opening lead to increase the injected and evaporated fuel mass and spray penetration length. With the increase in the amount of evaporated mass,SMD is decreasing. Thus spray characteristics can be controlled by varying the nozzle geometry and needle lift profile. Numerical results and experimental data was validated from previous researches.

Squat is a steady downward displacement consisting of a translation and rotation due to the flow of water past the moving hull. Nowadays, with the considerable increases in the size and speed of ships, the importance of accurate determination of the squat parameter in the design of waterways has increased. Squat has an important role both in economic costs of design and construction of new waterways and safe movement of ships in existing waterways. Various methods can be used for squat estimation including empirical relations, laboratory test models and numerical modeling. In this paper, based on PIANC recommendations, different empirical formulas are used to estimate the squat of the incoming ships to some of the main ports of Iran on the horizon of 1404. Finally, by concluding the results, suggestions are given for calculating the squat of different vessel types in the design of Iranian ports waterways.

This paper analyzes CALM terminal sensitivity under various operating parameters such as water depth, chain mass, current velocity, wave period, hawser length as well as terminal displacement in different directions, and the impact of tanker presence on the behavior of the riser (lazy S) during unloading/loading operations. The hydrodynamic response characteristics of the tanker and CALM buoy are calculated using ANSYS-AQWA software and the outputs are imported in OracleFlex software for simulation of the probable operating scenarios considering the terminal, tanker, mooring lines and Environmental conditions. The results indicate that the terminal dynamic response is most sensitive to the current velocity changes. It is also concluded that for the riser, when Near offset, an effective tension and for far offset, the bending moment includes critical states. The bending moment at the hang-off the riser and the effective tension at PLEM receive the most impact from the interactive mode of operation between the tanker and the terminal.

This paper evaluates the dynamic response and forces acting on the leg mating unit for the weather conditions in the Persian Gulf in the four seasons of the year. A six-degree-of-freedom model dedicated to the Floatover installation has been developed to provide a dynamic response and the forces acting on the leg mating unit, which acts as a shock absorber for the impacts caused by movements of the vessel. In addition to evaluating the dynamic response of the barge and deck in the frequency domain, the forces applied to the leg mating unit are obtained in the time domain. These forces have been obtained for 100% loading for different environmental conditions. The impact forces are assessed for a 2 minutes time history. In the hydrodynamic model, significant wave height, wave period, velocity, and wind direction were considered as input parameters. The effect of increasing degrees of freedom on the variations in Mooring forces was also considered. According to the results of the forces on the leg mating units, it is recommended that this installation be done in the summer due to the fewer changes in the forces, and to prevent overdesign conditions or possible failure, mooring forces be calculated in every six degrees.

The regional waterways, especially Persian Gulf and Oman Sea, are witnessing the passage of giant oil tankers daily. Due to oil extraction in the oil fields, wars in the region, unexpected incidents, and various explosions, southern areas of Iran are contaminated with oil derivatives. In this paper, the impact of crude oil and gas oil contamination on clayey fine soil and its impact on coastal structures is investigated. For this purpose, in addition to preliminary tests, hydrometer test, direct shear test, unconfined compressive strength and solidification were carried out on contaminated soils. It was found that the presence of more than 20% of petroleum derivatives on clayey soil leads to a sharp decrease in soil shear strength, which is considered as a serious risk for the coastal structures and oil field structures. Also, by using type II cement with 10% of dry soil weight, in addition to the relative increase in soil strength, it can also prevent the movement of contamination in water and soil substrates.

According to increasing use of composites in high speed craft and exposing continuously to cyclic load, in this research, implementation of fatigue life estimation algorithm of composite shell of high speed craft has been done. The selected fatigue model which is based on reduction of mechanical properties and damage variable is introduced. This model has been implemented in finite element code (abaqus) with UMAT subroutine and validated with available results. Then, by using slamming load calculated from DNVGL standard and craft properties, the shell thickness is determined and fatigue life of the shell is analyzed with the faigue model due to the cyclic sea pressure. The results show that, the used faigue model has adequate ability for impelementation in the FEM software to fatigue life estimation and if the shell is designed with slamming pressure and assumed safety factor, the fatigue life due to cyclic sea pressure will be very high.

Four-leg jacket is one of the most widely used offshore structures that is exposed to various damages during its service life. Unlike land structures, offshore structures are always exposed to waves, corrosion and damages, especially in the splash zone. At shallow to intermediate water depths, these structures are commonly used as the substructure of offshore wind turbines.  Passing their service life has made the structural health monitoring of offshore wind turbines necessary. In this research, a modified modal strain energy method is used for damage detection in deck and splash zone members which are more exposed to periodic loads of waves and high corrosive seawater than other members. The results showed the higher accuracy of the improved modal strain energy method compared to the original method (Stubbs index). Also, using improved method resulted in prediction of single and multiple damages, with low and high intensities with appropriate accuracy.

Longitudinal vibration of marine propulsion shafting system is a problem and the world have spent a lot of money to solve this problem. Oscillations occur at the propeller due to small variations in thrust when the propeller blades rotate through the non-uniform wake, resulting in longitudinal vibration in propulsion shafting system. Then, vibration will be transmitted to the hull through the thrust bearing. In this study propulsion shafting system modeled with decomposition of FRF’s method. Comparing between numerical solution of system in Simulink and decomposition of FRF’s method show that, the numerical solution has less 7 percent error. For reduce transmitted vibration to the hull we use Goodwin resonance changer (RC).An optimization method used to optimize RC’s stiffness, damping and mass coefficients. The result show that if we select appropriate coefficients the transmitted vibration can be reduced 99 percent.