Sharif Digital Repository

Nowadays, according to existence of oil resources in offshore areas and the necessity of extracting, constructing of offshore platforms is developed. The design of these structures has high effect on the cost of construction and maintenance and the performance under the loads such as wave, earthquake, impact, etc. Therefore, if the analysis of platform structure is more accurate the evaluation of the behavior and consequently the better design in different conditions is better. In this thesis, a modification of Endurance Wave Analysis (EWA) in evaluation of offshore structures is introduced in this study. EWA is a new method to predict the response of offshore structures in different sea... 

Tension leg platforms (TLP) are vertically moored with reserved buoyancy used for drilling and production in deep waters. Pre-tensioned tendons controls the heave, pitch and roll motions due to marine currents and waves efficiently. Seastar platforms are new generation of tension leg platforms that are similar to spar platforms that has the advantages of TLP besides spar simultaneously. This platform is usually considered as an alternative for large and cost consuming platforms. Seastar has a cylindrical surface piercing hull that creates the required buoyancy along with three submerged additional bodies. Pre-tensioned cables are attached to platform through these three submerged bodies. The... 

Planing crafts in waves are characterized by large and violent vertical motion and acceleration. The hull structure is subjected to high impact loads and the crew experiences high transient vertical acceleration and in most cases the crew needs to lower speed in order to avoid damage to the hull.In recent years, interest on increasingly faster marine vehicles has remarkably grown. Typical planing monohulls are: patrol vessels, pilot boats, rescue vessels, coast guard vessels and small navy vessels. So the behavior of planing monohulls in waves has been a widely researched topic since more and more planing monohulls appeared.The accurate prediction of large amplitude ship motions in severe... 

A Tension Leg Platform (TLP) can be difined as a vertically moored, buoyant and compliant platform, where the excess buoyancy of the platform maintains tension in the mooring system (tethers). This platform type is usually used for drilling in deep water and includes the following sections, foundation, tendons, risers, hull and deck which is the most important section in terms of hydrodynamic forces and weight. Enough initial tension can be used to inhibit vertical movement of the structure. one of The main properties of this oil platform is extraction at large oil fields. Body of TLP is consist of vertical column, horizontal circular or rectangular cross section Pantoons and circular... 

Oil is being discussed as the main source of producing energy in the world. Thus its exploration and exploitation from the sea or land is very important. Most of these resources exist in the depths of the sea. Exploitation of these resources demands to reach knowledge of design and technology of manufacturing platforms and devices for exploitation in the deep waters. There are massive oil reserves in the context of deep waters. Using fixed platforms in depth is very uneconomic; also we know the natural period of these structures is close to the wave period in deep waters. As a result researches and mining industry in the deep waters have gone to use comparative platforms such as: TBT, TLP,... 

In this thesis, multi-degree-of-freedom and finite element methods are utilized to simulate the dynamic behavior of fixed offshore structures under the wave and the earthquake excitations. In the field of seismic loading, seismic response of the platform is determined with the response spectrum analysis (RSA), time history analysis (THA) and endurance time (ET) methods. The accuracy and the computational cost of the ET method are compared with the RSA and THA methods. The results indicate that despite the RSA method, the ET method can estimate the nonlinear behavior of these types of structures. Moreover, in comparison with the THA method, the computational burden decreases to about 5% while... 

Reduction ships resistance is the major issues that always been about moving the ship into a water. In the course of research, many scientists have been interest to resistance reduction. It is important protect the body of ship against damage caused by sticking marine algae and crustaceans as protection against corrosive agents. Marine organisms such as algae, crustaceans and bacteria stick on submerged surfaces and increase the ship resistance. In this thesis resistance reduction using coating is evaluated. Foul release paint make smooth surface and low surface energy therefore ship with foul release coating experience lower drag contrast ship with another coating in the unfouled condition.... 

In positioning and navigation, Sonar with utilization of sound waves in water environments is similar to the role of Radar in not watered (irrigated) environments. In the Sonar field, modeling and simulation of propagation of sound waves in the undersea environment is a fundamental and very important topic. Nowadays simulation is significant tool that allow to the researchers to predict the quality of Sonar function respect to the experimental conditions. For simulation and modeling of sound wave propagation are used two different methods; Wave theory and Ray theory. In the wave theory, because of the large volume of data, the interpretation of the results is hard. Also in this method cannot... 

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.SUT-2 is an AUV, being developed by Marine Engineering Laboratory of Sharif University of... 

In this thesis, the potential flow including unsteady partial cavitation around axisymmetric bodies are studied using a boundary element method (BEM) based on potential theory. For this purpose the wetted surface of the body and cavity surface are divided (approximated) to panels. Then, by applying the Green’s third identity and expressing this theory in potential flow around every surface, cavitation can be modeled by distributing the source and the dipole rings on cavity/body surface. In order to approaching this goal we introduced dipole rings distribution on the cavity and body surfaces and source rings distribution on the cavity surface. In this work the cavitation number is assumed to... 

Nowadays, reaching higher speed in marine crafts and underwater vehicles mostly requires a modern propulsor with high efficiency and enhanced cavitation performance. Because of these requirements, numerous researches for developing the existing propulsion systems and invention of new systems have been conducted in recent years. One of the most sophisticated and developed propulsion systems that have high hydrodynamic efficiency and high cavitation performance and low acoustic signature is pumpjet. This propulsor contains a rotor, stator and a duct that surrounds two other parts. Stator reduces the swirl of flow coming from rotor and thereby reduces the loss of energy. High cavitation... 

The purpose of this study is extracting a dynamic model of longitudinal motions of high speed catamaran in regular and irregular waves, and then motion control of vessels by using of foil control elements and interceptor. For simulating longitudinal motions in regular waves, at first the longitudinal hydrodynamic coefficients of vessels should be extracted, then by substituting extracted coefficients in cumminz equation and using system identification methods, some equations are presented to estimate longitudinal two degree of freedom motions of high speed catamaran in waves. In this thesis the extraction of hydrodynamic coefficients is based on both numerical and experimental methods. ANSYS... 

Partially submerged propellers are new type of propellers which knowledges ragard them is limited. These propellers found a growing market ahead of them and expertsexpect to use these propellers for big vessels at high speeds. This type of propulsors do not lose their efficiences at high speeds and are assumped to be economical. Meanwhile due to continuosus hit to water surface, huge stresses are extered on tha blade witch needs to be fundamentally studied. Several researchers have studied the partially submerged propellers which unfortunately did not publish their results. Undergo less or more thrust and torques which in both cases the efficiency also increases. The use of numerical methods... 

High speed crafts due to their design, geometry, one of the parameters to improve the performance of hydrodynamic elements such as trim tabs, bow thruster, steps and Spray rails.Step is an element which change height of bottom and due to reduce wetted area and extra lift and finally cause to improve drag of vessel.In this thesis we have evaluated numerically and compared with experimental tests on planning crafts in calm water and waves, and the effects on the motions of floating and drag floating are compared.Numerical analysis is conducted to help Ansys.CFX software. First, to provide 2D modeling and analyzing real-bodies floating in calm water and waves with simple, step-paid. The... 

In this thesis, cavitating flow over 2D/3D geometries of cavitators and projectile bodies in stationary and moving conditions are simulated using the "Volume of Fluid" (VOF) technique. The main application of VOF model is for simulating free surface flows. In this work, another capability is added to the VOF algorithm to solve for gas phase and by applying analytical-numerical mass transfer models, cavitation phenomenon is simulated. The developed algorithm precisely simulates the geometrical parameters of supercavities such as dimension and shape in a wide range of cavitation numbers. In addition, the flow parameters of supercavity such as drag coefficient, pressure coefficient inside the... 

Performing research and development in the field of modern vessel types which serve several functions is considerably necessary because of their importance and rapid development. HARTH vessel is considered one of the important ships which has been recently attracted much attention from the world. In this respect, a full investigation of its performance could make a big contribution to its optimum design and appropriate exploitation.“HARTH” ship is the abbreviation for “High Aspect Ratio Twin Hull” is included in the catamaran vessel category with a high aspect ratio with respect to their underwater cross-section’s ratio of length to width.A host of crucial issues should be considered for... 

In this thesis ship propeller performance in maneuvering conditions have been investigated which means an oblique flow condition for working propeller. Then results of these investigations have been put to use to simulate maneuvering using MMG equations. These calculations have been done to investigate nominal wake variations in different drift angles and depth conditions. In this project displacement vessel KVLCC2 and propeller POTSDAM VP1304 have been studied due to the abundance of experimental data. Calculations in this study have been carried in three steps: 1, calculation of nominal wake of the vessel in different drift angles and depths via CFD. 2, Using the results of previous... 

Improving the performance of modular, low-cost autonomous underwater vehicles (AUVs) in such applications as long-range oceanographic survey, autonomous docking, and shallow-water mine countermeasures requires improving the vehicles’ maneuvering precision and battery life. These goals can be achieved through the improvement of the vehicle control system. A vehicle dynamics model based on a combination of theory and empirical data would provide an efficient platform for vehicle control system development, and an alternative to the typical trial-and-error method of vehicle control system field tuning. As there exists no standard procedure for vehicle modeling in industry, the simulation of... 

Efficient path-following control is essential for the exploitation of underwater resources, providing the ability to track predefined positions. Control, guidance, and navigation (GNC) serve as a common subject for marine vehicles. In this study, the aim is the development of an adaptive path-following control system for an underwater glider (UG), considering uncertainty parameters through GNC-based techniques. Uncertainty parameters within navigation, guidance, and control simulations can lead to instability in the control system. Uncertainty parameters in this method include some factors in the GNC loop such as hydrodynamic coefficients, sensor errors, biases, noise, and actuator errors....