Research Objectives and Scope of Thesis

A considerable amount of research effort has been spent to develop the methodology for the assessment of fatigue behaviour of welded tubular and plate joints. Non destructive evaluation of offshore tubular welded joints involves crack detection, crack sizing, stress analysis and fracture mechanics based assessment of crack growth. So far a lot of significant developments have been achieved in each of these areas. However, each of these areas still poses challenging problems for the offshore industry as reviewed in previous sections. In order to use the existing methodology, it is crucial to have the accurate information such as POD in NDT reliability, SCF, DOB and stress distribution in stress analysis, and SIF in fracture mechanics analysis. However, the task to have these basic modelling tools is far from being finished. In this study, it is intended to concentrate on the computational aspect of fatigue strength assessment. It will try to provide more information on underwater NDT reliability and stress distribution for tubular joints in a format suitable for fracture mechanics analysis. With more information available, it will be possible provide the more accurate modelling that is needed to improve the accuracy of prediction of fatigue crack growth. So the final objective of this study is to develop the models to predict the stress intensity factor in welded T-butts and tubular joints. As a summary, this study aims to provide the POD information of underwater NDT in convenient format for reliability based scheduling and the more accurate models for stress analysis and fracture mechanics analysis by carrying out a parametric study.

This chapter overviews the several important aspects of non-destructive fatigue strength assessment of offshore tubular welded joints. The rest of the thesis can be split into the following three parts.

1) Underwater Non Destructive Inspection Reliabilitv

Chapter 2 will report the work on underwater NDT reliability. Fatigue cracks on tubular welded joints were measured using ACPD and ACFM techniques in order to clarify some characterisation data in UCL crack library. The UCL underwater Non destructive inspection reliability trial results (POD data) were re-analysed to make them suitable for reliability fracture mechanics procedures for the first time and these were incorporated into the Reliability based inspection scheduling(RISC) system.

2) Deriving the Stress Parametric Equations for Simple Tubular Joints

Comprehensive thin shell finite element stress analyses have been carried out for 330 tubular X, DT, and Y, T-joints. Based on the results of these systematic analyses, a series of stress parametric equations have been derived by regression analysis and will be presented from Chapter 3 to Chapter 6. Chapter 3 is on the derivation of a set of comprehensive SCF parametric equations for tubular welded X, DT-joints. A set of parametric equations to predict the degree of bending and stress distribution around the intersection of tubular X and DT-joints are reported in Chapter 4. In Chapter 5, efforts have been made to derive a new set of parametric equations to predict the stress distribution along the intersection of tubular Y and T-j oints in order to enhance the prediction capability of UCL HCD stress parametric equations. Chapter 6 deals with developing a set of characteristic parametric equations for tubular Y, T, X and DT-joints to represent the stress distribution around the intersection of simple tubular joints.

3) Developing a set of SIF Parametric Equations for T-butt and Tubular Welded Joints A series of SIF parametric equations was derived for both the deepest and surface(comer) points of semi-elliptical surface cracks in T-butts and the deepest points of semi-elliptical surface cracks in tubular welded joints, by using the weight functions and the database of T- butt through wall stress analysis results. This aspect is addressed in Chapter 7 to Chapter 9. Chapter 7 is on derivation of the deepest point SIF parametric equations for T-butt using the Niu-Glinka weight function. Chapter 8 reports on the derivation of the surface point SIF parametric equations for T-butt using the Wang-Lambert weight functions. In Chapter 9, taking the Wang-Lambert weight function for longitudinal cracks on thin pipe as reference

data, a new weight function for tubular welded joints is proposed and the corresponding deepest point SIF parametric equations are derived. Combined with the non-linear load shedding model, this solution can be used to predict the fatigue crack growth in tubular joints and this is confirmed by the experimental data.

Finally, Chapter 10 presents the conclusions of the study and proposes the areas which need further research and investigation.

F ig u r e 1.1 T y p ic a l J a c k e t S tr u c tu r e fPatigue H andbook 1985) T J o in t Y Join t K J o in t T K J o in t

J l _

r

YVP W P

ÿ n y / / / / / - / ? / / V/

/

Distance between and restraints or points o( contraf/e;cure

Geometric ratios: a = — 6 = — y = x = — C = —

D D I T I D

Figure 1.3 Geometric Notation for Simple Tubular Joint(UEG 1985)

b) Axial loading _{c) IPB loading}

d) OPB loading

Figure 1.4 Modes of Loading on Tubular Joint