A Fracture Mechanics Approach to Nonoverlapping, Tubular K-Joint Fatigue-Life Prediction
Authors: Pan R.B. , Plummer F.B. 
Published: December 2007

A fracture mechanics approach has been developed for predicting the fatigue lives of nonoverlapping, tubular K-joints. The approach is derived from experimental test results and can be generalized for other types of tubular joints or complex connections for which crack-growth fatigue data are available.

Introduction
A technique using fracture mechanics for predicting the fatigue life of axially loaded, nonoverlapping, tubular K-joints has been developed. This method relies on an experimentally determined stress-intensity factor, KI, as shown in Fig. 1. In this application, KI was derived from fatigue data and the corresponding crack growth records of six tubular-joint tests. The expression is general and has been verified by comparing predicted fatigue lives with the results of eight sets of published, nonoverlapping K-joint data that include more than 50 individual tests. This method for predicting fatigue life could be extended to other types of joints, if suitable experimental data are available for derivation of stress-intensity factors.
This work has provided an improvement in offshore tubular-joint fatigue technology through the use of fracture mechanics techniques. Data on small-specimen fatigue-crack growth now can be used to estimate the fatigue-crack propagation life of full-scale, tubular K-joints. Effects such as random load histories, corrosive environment, and other important factors known to affect fatigue life can be incorporated in a more direct manner. Fracture mechanics principles also can benefit conventional fatigue analysis by providing a means of generating appropriate S-N curves for specific applications. This type of life prediction appears to be less prone to the large scatter that typically characterizes tubular-joint S-N results.
For the application of this method to the fatigue problem for typical offshore platforms, stress-intensity factors problem for typical offshore platforms, stress-intensity factors are needed for overlapping and nonoverlapping tubular joints subjected to bending and axial cyclic loads. Progress is expected in this area as more tubular-joint data Progress is expected in this area as more tubular-joint data become available.
Semisubmersible drilling vessels, deep-water platforms, and platforms in hostile environments experience platforms, and platforms in hostile environments experience more damaging stress cycles during their lifetime than do structures in the Gulf of Mexico. Thus, metal fatigue and the resulting fatigue cracks in highly stressed members and joints may be controlling design factors. Consequently, industry, certification authorities, and government agencies have directed increased attention to consideration of fatigue in offshore facilities. Thus, there has been consider-able effort toward developing the fatigue analysis techniques required for offshore design.
In recent years, new technology based on fracture mechanics principles has received increasing attention in the fatigue analysis of tubular joints. But, because of complicated joint geometries, the analytical development of the stress-intensity factor, KI, has been protracted. This paper presents a fracture mechanics approach to predicting the fatigue lives of nonoverlapping, tubular predicting the fatigue lives of nonoverlapping, tubular K-joints that circumvents these difficulties. It is derived from experimental test results rather than purely analytical methods.
JPT
P. 461