| It is of crucial importance to estimate the ultimate strength of structures during the design and service of offshore platform structures. Offshore platform structures are prone to suffering various types of accidents and damages, which may cause ultimate strength decrease and structures destroy. So it is vital to estimate the ultimate strength of the damaged structures.The crack is one of main damage types of offshore jacket platform. According to the current research of residual ultimate strength of cracked structures, this thesis fo-cuses on the structural residual ultimate strength assessment of cracked components of offshore platform. The main works in the study are as follows:(1) Because of the complexity of stress distribution in the cracked structures and the diversity of impact parameters on residual ultimate strength, the assessment methods for residual ultimate strength are complex. Based on existing research, the assessment methods on residual ultimate strength of cracked structures under one direction of ten-sive stresses have been analyzed comparatively. The analysis results indicate the finite element method is more reasonable than other methods to access to residual ultimate strength of the cracked components.(2) For the diversity of impact parameters on residual ultimate strength of cracked structures, the parametric analysis on the residual ultimate strength of cracked pipes have been execurated by the finite element method. The analyzed parameters including material properties as yield stress and Poisson's ratio, crack characteristics as crack length, crack angle and crack position represented by two eccentric ratios, the pipe's geometrical dimensions as pipe thickness and aspect ratio, the boundary loading condi-tions of uniform stress and constant displacement. The reasonable empirical formula for residual ultimate strength calculation of cracked pipe is obtained.(3) In order to consider the impact crack on structural reliability in service, based on the empirical formulas for residual ultimate strength of crack pipe, the attenuation model of resistance on crack-pipe is obtained. Combined with time-dependent reliability theory, the reliability of cracked structures are analysed, and the impact properties of crack on the reliability are obtained.This paper indicates that the residual ultimate strength of the cracked structures can been calculated by using the nonlinear finite element method, and the crack weakens the strength of the structures seriously and the safety level of structure. So the effect of the cracks must be given sufficient attention when the strength of the offshore platforms is evaluated. |
