Prediction of the Remaining Fatigue Life of Tubular Joints
Authors: Connolly M.P. 
Published: October 2008

A fatigue fracture mechanics model is described which can be used to predict the remaining fatigue life of cracked tubular joints. To perform this analysis knowledge of the maximum stress at the intersection (termed the hot-spot stress), together with the distribution of stress through the thickness and around the intersection is required. These stresses have previously been obtained, [1], [2], and [3] from detailed finite element analyses of tubular joints and the results are presented in the form of parametric equations describing the maximum stress, the through-thickness stresses and the distribution of the stress around the intersection. These equations are directly compatible with each other and provide a predictive capability for the stresses acting on the anticipated crack plane, that is at the welded intersection, of tubular joints. These equations are used here together with closed-form stress intensity factor solutions in order to estimate ?K'sfor cracks in tubular joints and are integrated with representative crack growth rate data in order to estimate the remaining fatigue life for any defect size. Example cases are considered which illustrate the importance of the through-thickness stresses in the remaining life assessment.

INTRODUCTION

The original concern for many tubular welded joints used in offshore oil and gas construction was the prediction of the total fatigue life and this issue has been addressed through the development of lower bound stress-life (S-N) curves [4]. These curves relate the number of cycles to failure, N, to a stress range, S, and are obtained from an empirical relationship established from large-scale fatigue tests conducted on tubular welded joints. The stress, S, in this case corresponds to the hot-spot stress which is the maximum geometric stress around the intersection. The general applicability of this approach has been demonstrated by the results of fatigue life tests on different tubular welded joint geometries, where the fatigue lives plotted as a function of the hot spot stress, S, fall on an approximate straight line. However, closer examination shows that systematic differences exist between the fatigue lives.

Calculated for joints with different geometries or modes of loading but with identical stresses calculated solely from the hot-spot stress approach, [5]. These differences are probably due to the through-thickness stress distributions, where tubular joints with different geometries and modes of loading have different distributions of stress through the thickness. This effect is not so important in other fatigue applications, but for tubular joints where a significant portion of the fatigue life is expended in the growth of :racks at the welded intersection these effects can be significant and must be accounted for when calculating the remaining fatigue life. The S-N curve approach requires no consideration of the through-thickness stresses since such effects are ameliorated by :hosing a S-N curve which is two standard deviations below the mean Life curve through the experimental data, Furthermore, the S-N curve can only be used to predict the total number of cycles to failure.