Fatigue Analysis Of FPSO Hull Structure In Service Based On Fracture Mechanics

The fatigue damage has become an important reason for failure of offshore structures,FPSO as an important ship type of offshore oil development and production,since it is not like other ships in port to avoid bad storm,not regular docking repair,so the fatigue problem is particularly distinctive.With the rapid development of shipbuilding industry and related subjects,the theory and method of fatigue life assessment of offshore engineering structures have also become more and more perfect.The current method for calculating the fatigue life of structure are the simplified method,Spectral method based on S-N curve and Miner linear cumulative and fracture mechanics;Since domestic research of fracture mechanics started late and the subject characteristics,It is not widely used in ship design and verification at the present stage,Spectrum fatigue analysis is the main method in the calculation of ship fatigue life.In the actual project,due to service for a period of time,The structure details may have been formed for fatigue crack under the alternating load,For initial details with existing defects,Using the method of fracture mechanics checking is a appropriate manner.This paper is done to checking the fatigue life of typical details in FPSO tank based on the fracture mechanics theory and combined with the steps in British Standards Committee BS7910.expect to get a suitable checking method for engineering application,The main contents of calculation include the following:(1)According to the non-destructive testing data,the initial crack shape and size of the hull member are determined,and the initial defects are considered in the structural model;(2)Typical fatigue details are selected at different locations of the ship,the bending stress and the membrane stress components are calculated under different loading conditions;(3)Calculate the Kr and Lr,according to the 2B-FAD diagram to determine whether the crack is in a acceptance range;(4)To determine the stress intensity factor threshold of ?K0 and the actual stress intensity factor range ?K;(5)Calculate the cumulative number of cycles of each typical node.In this paper,the finite element method is used to solve the problem,first,it can solve the complex loading situation,compared with the analytical solution,can be more convenient to obtain the stress intensity factor K value and more suitable for engineering applications,on the other hand there have been lots of mature commercial finite element software such as ANSYS,MSC-Nastran,SESAM and so on,can make the calculation of fracture mechanics analysis of structure of personnel is more convenient.