Fatigue life prediction of offshore tubular T-joints using fracture mechanics approach

Bhuyan, Gouri Shankar (1986) Fatigue life prediction of offshore tubular T-joints using fracture mechanics approach. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The thesis presents the results of an analytical and experimental investigation of the fatigue behaviour of tubular T-joints, using a linear fracture mechanics approach. The analytical study includes the development of a finite element computer program to analyse tubular joints with/without weld toe cracks. The experimental investigation consists of quantifying the effects of sea water, temperature, frequency, load ratio and wave form on fatigue crack-growth-rates in CSA G40.21 M 350 WT steel, which has been'proposed for the Canadian Offshore. -- The actual curved surface and the brace/chord intersection are approximated by the assemblage of flat quadrilateral and triangular plate elements for two dimensional analysis. The analysis uses a plane stress element to account for membrane stiffness and a plate bending element to account for flexural stiffness. For three dimensional analyses, chord/brace/weld regions are modelled using incompatible brick and prism elements. In order to reduce the computer storage requirements as well as the solution costs, the region of high stress gradient is analysed using the rezone technique. Hot spot stress and stress concentration factors obtained from these analyses are compared with the experimental values available in the literature. -- To study the effect of a shallow weld toe crack on through-thickness and surface stress redistribution, the region at the vicinity of the crack is modelled using special elements which produce singular stress fields at the crack front. The crack-tip stress intensity factors are determined by comparing the crack-tip stresses, obtained from the analyses, with the available theoretical solutions. -- Fatigue crack-growth-rate data and material coefficients, C and m, for various temperature, load ratio and frequency ranges are obtained from compact tension specimen tests in air as well as in sea water. Knowing the crack-growth behaviour in the base metal, the through-thickness-cracking lives of tubular T-joints are predicted analytically using the modified Paris' equation and the estimated lives are compared with reported experimental results. The accuracy of the computer program and the validity of the proposed method for predicting fatigue life of tubular joints are checked by testing a prototype tubular T-joint.

Item Type: Thesis (Doctoral (PhD))
URI: http://research.library.mun.ca/id/eprint/8637
Item ID: 8637
Additional Information: Bibliography: leaves [209]-219.
Department(s): Engineering and Applied Science, Faculty of
Date: 1986
Date Type: Submission
Library of Congress Subject Heading: Tubular steel structures; Offshore structures--Dynamics; Welded joints--Fatigue

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