Assesment of ship structural reliability with using first order reliability methods

Development of technology has increased the demand for different type constructions. Structures should be completed different type of jobs successfully at an assigned time, because requests can be changed swiftly in short periods. In parallel with technological developments, structure can be exposed to different environmental conditions even within a short period. Different type of structures needs unusual designs to answer different demands. Deterministic analyzes or solutions cannot be adequate for these demands. On the other hand, the data or variables used to construct the designs contain different uncertainties. Probality of failure can be occurred under specified operating conditions of structures due to uncertainties. It is necessary to determine how safe the structures can be within the limits determined during the design phase or under different conditions that they may encounter. Reliability analyses are used for determining safety level of structures. In addition, reliability analyses are dependent on probability of failure for structures. Determining the probability of failure is important to calculate safety of structures. Structural reliability analyzes help to find worst cases for structures. In addition, these cases guide to identify safety-working constraints of structures Moreover, the structural behavior of ships has researched with reliability methods in this study. Assessing the structural reliability of ships should be a planned process. Reliability analyses of structures are remarkable process and all of the details scrutinize very attentively. One of the important step in the process is to determine the structural reliability methods to be used. In reviewing the structural safety of ships, two different methods can be used in general to be based on direct reliability and resistance factors Two different methods can be used interviewing the structural safety of ships. First, one is direct reliability method and the other one is resistance based method. These methods are examined at four different levels depending on the usage, random variables and the intended use of the structure. Reliability is explained by determining the probality of failure of the studied structure. Therefore, the method of reliability to be used has been defined, the conditions under which the investigated structure successfully performs its conditions should be determined. These conditions can be identified as design limits. These limits depend on the environmental factors, similar variables and operating conditions of the ship structures. These limit conditions, which can be defined as limit states, are the boundary between the expected behavior of the ships and failed states. Limit states conditions, can be evaluated according to four different approaches; Ultimate limit state, accident limit state, service limit state and fatigue limit state. The limit state function is modeled mathematically. This function is examined in which states the vessels can be structurally safe. When the limit state function takes values lower than 0, the region is called unsafe. Where the limit state function is greater than 0, the region is called safe. This function specifies the borderlines between the safe and unsafe regions. Ship indices and reliability levels of structure are specified with the help of defined limit state function by calculating the probability where the ships are in the unsafe regions. Two main reliability methods are detailed in this study. They are analytical and simulation methods. First and Second Order Reliability Methods, Hasofer Lind Reliability Method are analytical methods and Monte Carlo Method is example of simulation method. All of these methods are clarified with ship structural reliability process. The main objectives in these methods are to calculate the reliability index at least failure rate. Solution procedures of analytical methods are similar to each other but there are some important details in analyzing period. Derivation and consideration of data operations are different. First, limit state function is checked in analytical process. If it is nonlinear, limit state function should be linearized. Generally, Taylor series are used for this linearization analysis. Average values used in the linearization of First and Second Level Reliability Methods. On the other hand, series expand at design points in the Hasofer Lind method. The Monte Carlo approach is a simulation method that collecting or defining of the random variables is an important process for the success of this method. Structural reliability methods are examined in detail in the first three chapters. In the fourth section, different ship loads affects identified. These load effects examined as still water and wave-induced bending moments. The Poisson approach that used for the identification of still water effects has been examined in detail. Rayleigh distributions, Narrow and Wide band Gaussian methods are used for explained wave induced loads .On the other hand, most of the researchers select Ferry-Borges and Castanheta to explain still water and wave induced load combination effects. In chapter 5, the structural safety level of a sample dry cargo ship is examined. Bulk carrier was selected for example analysis. The reliability state of the dry cargo ship is calculated for hogging and sagging conditions Mid ship section details and ship main specifications are used to examine of example ship reliability situation. On the other hand, these details were used for the reliability calculations which 2Rrel program is used. Reliability analyzes were calculated using the 2Rrel program according to the First Order Reliability Methods. In the example of this study, the limit state function for the final strength state of a ship was determined and reliability analyzes were performed. Along with adding uncertainty effects to the limit state function, the solution equation used in the sample reliability analysis was obtained. In structural ship designs based on reliability principles, uncertainties are particularly important when design impacts of loads are determined. The effects of the resulting uncertainties, along with standard deviation values and coefficient of variation, have been added to the structural safety analyzes. Ultimate strength model uncertainty, still water bending moment uncertainty, wave bending moment uncertainty and the nonlinear calculation uncertainty parameters are included in the analysis of the limit state functions for the ultimate strength state as well as in the case study. The reliability level of the example ship evaluated with ultimate reliability approach. Ultimate bending moment, still water bending moment and wave induced bending moments are used to define limit state function for ultimate strength state. They are defined with uncertainty parameters and their statistical data. Evaluation methods for variables selected from reference researches. The results obtained from the sample examined and the overall situation of the thesis evaluated in Chapter 5. After that study, we can easily said that reliability approaches are necessary for structural analysis and designs Different design approaches can be gained by analyzing ship structures based on reliability principles. Benchmarking between reliability levels and structural designs is easier and more accurate with reliability analyzes. Reliability approaches are also being used in the rules of the classification societies, which are frequently referred to for the structural design of ships. We hope that this study will help engineers, students and researchers, which are interested in structural design, to evaluate structural designs from other approachments.