Study On Fracture Characteristics And Load Bearing Capacity Of Cracked Cylinder In Offshore Platform

With the engineering background of offshore platforms, research is carried out on the fracture characteristics and load bearing capacity of the cylindrical shells with crack damage, making a foundation for the accurate evaluation of overall load bearing capacity on the offshore platform structure system. This research is of great importance for taking protection and maintenance measures scientifically, controlling the crack effectively, exerting the whole structure potential maximally, and extending the service life of the platform adequately. The theoretical analyses, finite element numerical simulations and experiment research are proposed in this paper around the very problem.Firstly, based on the semi-membrane theory and Dugdale model, the elastic-plastic solution of cylindrical shells on which the crack lies on the fixed boundary is obtained under the tension and bending load. Taking the CTOD and CTOA as crack initiation and extension parameters respectively, the elastic-plastic solution covering the whole process, the static crack, stable extension and plastic collapse, is derived as the crack grows, which makes a contribution to the present theoretical solution of these problems.Then the linear elastic solution of cylindrical shells with circumferential surface crack located in the fixed-end is obtained under the load of axial tension or bending, and calculation formula of stress intensity factor on crack front is given at the same time. The result shows that stress intensity factor on the maximum depth of the crack is insensitive to the shape of the crack for the relatively long crack on cylindrical shells.Afterwards, a series of calculations are carried out for the cylindrical shells with circumferential surface crack under different constraints by means of the elastic-plastic finite element method, revealing the variation of stress distribution and plastic length in the crack section. The elastic-plastic linear spring model applicable for the stripy slender surface crack is established. Combined with the Sanders' theoretical solution of through-wall crack, elastic-plastic solution with small scale yielding is gained. The calculation formula of crack opening displacement on the crack front is given in the end.In the following research, for the cylindrical shell with circumferential through-wall crack damage, with series of calculations based on the existing theoretical analytical solutions of elastic-plastic crack and crack extension, various parameters, which including constraint condition of crack section, initial length of crack, diameter thickness ratio of cylindrical shell, critical CTOD and CTOA of material, yield stress and Young's modulus etc., are analyzed about their effect on the ultimate strength, and the sensitivity of these parameters is proposed, too. Dimensionless parameters, such as the ultimate strength loss coefficientκ, strength redundancy coefficientλand crack remaining extension lengthξare proposed and the quantitative evaluation is given, considering variation of structural bearing capacity with the crack extension. The result shows that the ultimate strength obtained under the small-scale geometric ratio tends to be dangerous and it could not be applied directly in the structure with large-scale geometric ratio, which means that the increasing of load bearing capacity caused by the geometric size could not be ignored. The initiation length of crack and yield stress of material should be taken into consideration when assessing the effect of crack damage on the ultimate strength of structure.Then, under the load of bending, the load bearing capacity experiments are conducted with constraints of free support, different elastic supports, and rigid fixing, providing essential experiment data for further theoretical research.Finally, three new crack elements, comprising an elastic-plastic through-wall crack element, a linear elastic surface crack element and a small-scale yielding elastic-plastic surface crack element, are developed based on theoretical analytical solutions above and could be used in the computation of structures with crack damage.