نشریه مهندسی دریا
پیاپی 1 (تابستان 1383)

In order to predict the residual life of offshore platforms and establish efficient schedule for underwater inspection and repair, it is necessary to estimate the fatigue crack growth rate in tubular joints properly. Linear Elastic Fracture Mechanics and Stress Intensity Factor are applicable tools for evaluating growth rate of existing fatigue cracks in offshore tubular joints. In the past several approaches based on Paris crack growth law, have been proposed in this regard. Each of these approaches use different methods for estimating the Stress Intensity Modification Factor (Y). In this research the capability of Artificial Neural Networks for evaluating the fatigue crack growth rate in offshore tubular T-joints under axial loading is investigated, when the crack depth is more than 20% of chord wall thickness. For this condition the crack growth process is highly affected by joint geometry and loading mode. Two types of artificial neural network are developed for predicting the Y factor: Radial Basis Function (RBF) and Multi Layer Perceptron (MLP) networks. The required input data consist of the crack shape and the percentage of crack penetration through thickness. Experimental data from NDE center in University College London are used for training and testing the networks. The results of this research are compared with other existing theoretical and empirical solutions.

This paper holds a parametric study on seakeeping performance for one kind of submersible vehicles, a tourist submarine. This analyis has been performed from the aspect of motion sickness incidence by using a strip theory hydrodynamics solver. The parameters include sea state, sea direction, depth, speed and duration of operation. Results prove head seas to be more violative than following seas. Moreover, Effect of trip duration, operating depth and vessel forward speed on operability index has been investigated.

Propeller is one of the main propulsors for marine vehicles which produces thrust to overcome the resistance of the ship. Propeller works in the place behind the ship where the flow is non-uniform and with three dimensional velocities, so it causes the fluctuating thrust. The first part of this paper employs the boundary element method (BEM) to evaluate and analyze the hydrodynamic performance of propeller in the behind of the ship. After calculating the hydrodynamic characteristics of the propeller, the second part is the systematical method applied to determine and select the optimum propeller. Selection of the propeller depends on the design criteria which are minimum torque, high thrust, best efficiency and without (or with minimum) cavitation. In second part of the the paper, by those criteria and having the hydrodynamic characteristics of the propeller, three types of propeller have been calculated by the BEM and then one of the best the propellers have been matched for the PV11 ship.

Analytical analysis of a two-dimensional submerged body moving with a constant velocity in an inviscid incompressible fluid under gravity near a free surface is studied in this paper. This notion is one of the fundamental concepts in applied hydrodynamics. Experimental observations of water waves in large scale or laboratory observation in open channels is indicative of complex phenomenons taking place in the downstream of an object placed under a free surface. Also, in the downstream of floating or submerged bodies, surface wave generate. This is due to the fact that energy transferring takes place from the moving body to the fluid which is known as “wave drag”. Considering the mechanism of surface wave formation and its motion, describing the wave drag phenomenon in submerged and floated bodies has become one of more complex topics in hydrodynamic theory. In this present work, it has been attempted to take a step in more precise comprehension of hydrodynamic phenomenons adjacent to free surface. This has been down by the study of fundamental laws governing the fluid motions. Analytical results obtained provide insight in ability to interpret the formation of surface waves and wave drag phenomenon. A submerged cylinder and surrounding stream has been considered as a study model upon which the obtained results and the utilized procedure would be extendable to all other two dimensional problems. Moreover, based on this procedure, it is conceivable to extend the conventional methods of problem solving for infinite domain to problems bounded by a free surface.

Dhow (LanJ) is a wooden vessel which is traditionally operating in Persian Gulf and Oman Sea. In contrast of its frequent lost, ordinary people believe that it has very good behavior at waves. In this study, by means of seakeeping computer software, authors have calculated its dynamic behavior in waves. Comparison between dhow performances in waves against seakeeping criteria has been made. Then, its operable sea waves is concluded. Then the reason behind people believe has been discussed, thoroughly. The end result is that the typical wooden dhow may withstand in waves of significant wave height of not more than 1 meter.

Among compliant offshore platforms, tension leg platforms (TLP) are referred to, as hybrid structures. This kind of platforms resemble to ordinary semi submersibles when their serge, sway and yaw motions are considered, since they are compliant whereas with respect to pith, roll and heave motions they respond more like a fixed platform rather than a floating body. This paper presents a method to analyze and evaluation of hydrodynamic forces of a regular simple wave on a TLP and a general solution to expand the method to be applied for a randomly changed water surface with a known spectrum e.g. pierson-Moskowitz. This analysis was made, using a 3D model with 6 coupled degree of freedoms. Stiffness of mooring system and drag force are assumed to non-linear and the effect of diffraction and radiation are neglected. Hydrodynamic forces and response were modeled, using modified Morison equation. The dynamic behavior of mooring lines and hydrodynamic forces on tensioners were neglected. The resulting equation was solved in time domain by Newmark-Beta algorithm.

Nowadays, for managing coastal zones and littoral lands, we need accurate and updated information on hydrography of coastal areas. These data can be obtained by local measurements, airborne imagery and satellite data. Using satellite data is not time-consuming and expensive like than other methods, thus it is most economical method in the world. Unfortunately, this method is not developed in Iran, although it has many advantages. In this paper, the advantages of Landsat data have been taken to provide hydrographical maps after applying some specific image processing techniques. Landsat TM images were used.

In designing for ship production, the production method should be considered and it’s possible to reduce the manufacturing cost in designing procedure. A review of background of ship design that had minimized production costs, shows the principles and problems of these methods. Modern designing methods for manufacturing, considers hydrodynamic efficiency and less manufacturing cost. Quantifying the results of required designing changes in modern methods, is only possible with Bottom-Up method which is described in this paper. This method needs sufficient data and information of production process.

In line with the current research on the ultimate strength assessment of ship hull girders, careful understanding of the collapse behaviour of unstiffened plate elements within the ship hull girders is an important and necessary task. In this paper, the collapse behaviour of unstiffened plates under longitudinal and transverse thrust is studied by elastoplastic large deflection FEM and the characteristics of the collapse modes are highlighted.