Chang, Enrong; (1997) Parametric study for non-destructive fatigue strength evaluation of offshore tubular welded joints. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with the development of tools and methodologies for non-destructive fatigue strength evaluation of offshore tubular welded joints. The sizing capability of new Non-Destructive Technique(NDT), alternating current field measurement(ACFM), was investigated. The University College London(UCL) underwater Probability Of Detection(POD) trial results were re-analysed to make them suitable for reliability fracture mechanics procedures. Comprehensive thin shell finite element analyses were conducted for 660 tubular Y, T, X and DT-joints, with geometries typical of those used in offshore structures, subject to different modes of loading. The results have been used to produce a new set of stress parametric equations. These were assessed by comparing the predicted values with available test data. The equations can be used to predict hot spot Stress Concentration Factor(SCF) and Degree Of Bending(DOB) at all critical positions for X and DT-joints and also the full SCF distribution along both chord and brace toes for Y and T, X and DT-joints. Combination of these parametric equations with original UCL HCD equations allows one to recreate the full 2D stress distribution for tubular Y and T, X and DT-joints. Moreover, stress distribution concentration factor(SDCF) has been proposed and parametric equations were derived to predict average SCF and SDCF for Y and T, X and DT-joints. Combining available weight functions and the UCL database of T-butt through-wall stress analysis results, a new set of weld toe Stress Intensity Factor(SIF) parametric equations were derived for both the deepest and surface points of semi-elliptical surface cracks in T-butt welded joints. Using thin pipe weight function as reference data, a novel weight function and the corresponding SIF solutions were derived for the deepest point of a semi-elliptical cracks at the saddle of tubular welded T-joints. A comparison of the predictions with the UCL experimental results, especially early fatigue crack growth data, showed that this new model can work well with the constant force shedding and the non-linear moment shedding derived from previous line spring FE analysis.

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