نشریه مهندسی دریا
پیاپی 7 (بهار 1387)

Fatigue analysis and estimation of safe life of structures that are subjected to cyclic loadings, such as ships and offshore structures is one of the most important steps of structural design. Fatigue failure in the form of crack will start from details and propagate in structure. In steel structures these cracks will started from welds. Most of the methods for fatigue life assessment in welded connections are based on stress values at critical points, called "hot spot". Determination of hot spot stress is done by finite element analysis with using some empirical relations, which is expected to be the most practical method. These empirical relations are mostly based on linearization of stress through the thickness or extrapolating the stress to the hot spot locations. In this paper, different methods are reviewed for the evaluation of hot spot stress at a typical ship structural detail by finite element analysis and the results are compared with an experimental data. Accuracy and sensitivity of hot spot stress to finite element meshing is investigated, and the best method for calculation of hot-spot stress based on required accuracy, time and money is proposed.

One of the important items in offshore structure design is fatigue analysis. Because of existence of periodic loads derived by wave, sea current, wind, etc. this analysis has become one of the major analysis in basic design. Estimation of structure’s life that is generated from fatigue analysis is more important in operation of the offshore unit. In this paper according to the environmental conditions of Caspian Sea, a jack up was modeled by use of ANSYS. According to available statistics of this specified region the wave by 12 m significant height and sea current by the speed of 1 (meter/second) is exerted on the structure. API rules are considered in modeling of soil and spudcan interaction and the stiffness of the elastic springs also selected by this standard. After that, nonlinear dynamic analysis has been carried out for all directions of wave motion so that the dominant wave direction and critical joint has been detected. This critical joint has been modeled by considering the effective length of members and appropriate support condition. The design loads (axial loads, out-of- plane bending, in-plane bending) have been exerted, and Hot Spot Stress “HSS” which had calculated by API formula has provided for stress combination in order to indicate the critical node. After that fatigue analysis has been done for this node by considering S-N curves in API recommendation. Finally the roughly life of the jack up rig has been determined. In addition to that analysis the effect of fatigue damage in each wave direction has been investigated and compared by using wave-energy distribution method. In this paper we try to generate a simple method for detecting the critical joint in order to calculate the structure’s life approximately.

In this paper, the procedure of developing an unconstrained Genetic Algorithm code to solve mooring pattern optimization problems for floating structures is described. How to spread mooring lines around the offshore structures and tension level of these lines are the factors that have direct effect on response or offset of floating units which experience environmental conditions such as waves, wind and current. A GA may be used to search optimum mooring pattern and the best tension of lines to minimize floating structures offset. In this work, dynamic analysis is used to estimate floating units responses in a mooring optimization problem. Waves and wind are defined by appropriate spectra and analysis is performed in frequency domain. The goal for optimization is to minimize the objective function which is the sum of squares of the floating unit’s displacements for each set of environmental conditions. The considered variables of optimization are: position of anchors, line lengths and vessel heading relative to environmental conditions. Also size of mooring lines, and their material and strength are considered to be constant. It is assumed that all lines are from the same size and material and no buoy or counter weight is attached to the lines. To demonstrate the capability of the developed code, its application in a case study is presented and some conclusions are derived.

In this article, the accuracy of empirical and numerical methods in prediction of wind wave characteristics in AmirAbad and Bushehr Ports have been studied, evaluated and compared. First, the wave characteristics have been calculated using Babolsar and Bushehr synoptic stations and employing empirical methods including SMB, SPM and CEM. Moreover the errors of empirical methods have been determined by comparing the predicted values of wave heights and periods with wave characteristics recorded by wave buoys of AmirAbad, Neka and Bushehr. Then the results of empirical methods have been compared with the wave characteristics obtained from Iranian Seas Wave Modeling Project. This modeling has been implemented using the modified ECMWF wind field and the SW module of MIKE21 software. A more complete assessment of the results has been carried out by extreme value analysis of the predicted and recorded wave height and calculating the significant wave height with 2~10 years return periods, totally and in dominant directions. Results show that the SPM method is the most suitable method among the empirical models for determining the wave characteristics of AmirAbad Port and the SMB and SPM methods are more suitable for predicting the wave heights and periods in Bushehr Port, respectively. The accuracies in predicting wave parameters were found to be compatible for numerical and empirical methods in Bushehr Port and agreed with recorded characteristics. However, in comparison to numerical method, the empirical method yields to poor prediction in Amirabad Port. According to the investigations the errors of empirical methods are mainly due to unsuitable wind data and it is predicted that in the presence of reliable wind data, the results of empirical and numerical methods would be comparable.

In this paper, the analysis of two dimensional viscous incompressible flow around bodies with multiple industrial applications, is presented. Both of boundary layer flow and potential flow have been analysed repeatedly and in this way, the major parameters of flow are calculated. Laminar and turbulent boundary layer flow are solved with the aid of an virtual flow normal to the surface of the body as boundary conditions. The flow around the body has been made by combining the uniform potential flow and distribution of potential source and sink and vortex on the surface of body and constantly powered vortex on the separation streamline. The rotational flow in wake region is also made. With a development of computational program, the numerical method is applied in which, the power and situation of distributed singularities are obtained. Considering wake length and minimum pressure coefficient from empirical data, the shape of streamlines and consequently boundary layer around the body have been obtained. Also, distribution of pressure and velocity and curve of streamline beginning from separation point, have been calculated and compared with empirical values.

The morphological states of Guyana’s coastal system, at various spatial and temporal scales, are found to be influenced by the formation and migration of mudbanks. Stationary and propagating mudbanks along the coast are investigated with the use of multiple data sources, including aerial photographs, satellite imagery, GPS measurements, and a time series of coastal profile data (1941-1987, 1992-2005). Multi-temporal data from different sources are assimilated, integrated, analyzed and visualized with a geographical information system. Six mudbanks are recognized from 1941-2005. Each mudbank had a different length and moved at a different rate in a westerly direction. Mudbanks are associated with recurring episodes of erosion and accretion. Statistical analysis of variance demonstrated significant spatial and temporal variations in the patterns and amounts of erosion and accretion along the coast.

This article describes the 11-year wave simulation (1992-2002) in the Persian Gulf and the Gulf of Oman using the input data derived from European Center for Medium-Range Weather Forecasts (ECMWF). The ECMWF 10 meter wind field and spectral wave boundary condition at 18 ْN degree are input into one of the latest versions of numerical wave models (3rd generation) after a few local modifications. Tropical cyclones during the last 30 years in the northern Indian Ocean which affect the Gulf of Oman are regenerated and wave simulation for individual cyclones is carried out. Open boundary of continuous hindcast is also modified in cyclone periods. In-situ and satellite wind and wave data sets are used to evaluate the accuracy of input wind and simulated wave fields. Extreme Value Analysis (EVA) is the next taken stage in which the wave characteristics were calculated for different return periods. Similar analysis is performed on the directional data to find out significance of storms in each direction. Finally, a user-friendly engineering and management tool is developed and verified.