A Study of Fatigue Design Procedure for Tubular Connections of Offshore Structures

It is the most important thing to consider the fatigue strength of tubular connections at the design stage of offshore structures. Some design curves have been provided to estimate the fatigue strength of the tubular connections. One of the most popular curves is the X-curve in AWS Structural Welding Code. The ordinate of this curve is defined as the hotspot stress or strain. However, the definition of the term “hotspot” is ambiguous, and a procedure to estimate the hotspot stress or strain seems not to be established even now.In this paper, the meaning of “hotspot” was clearly defined as the location where the fatigue crack initiates, and the value of strain at the hotspot was estimated by a simple procedure. Namely, the factors of stress concentration at a tubular connection were devided into two components due to structural deformation and the existence of weld bead, and Stowell's equation was used to evaluate the plastic strain concentration factor. Subsequently, more than 100 pieces of fatigue test specimens were reanalyzed by this procedure and a fatigue design curve was obtained.The main conclusions are as follows : (1) A simple procedure to estimate the value of strain at a hotspot was developed.(2) The value of stress concentration factor due to structural deformation can be estimated by Kuang's equations with a fairly good degree of accuracy when it is compared to the results obtained by a finite element method.(3) The following fatigue design curve of tubular connections was proposed on the crack initiation life basis.ea= 0. 0180 Nc-0.270A similar expression for a failure life was as follows.ea=0.0235 Nf-0.252where ea : estimated strain amplitudeNc : crack initiation lifeN f : failure life