Historical approach to fatigue design
The fatigue design of forgings in steel catenary riser (SCR) systems is commonly based on the BS 7608 Class B S-N curve, which was derived from longitudinal butt weld data. However, there is very little direct evidence to support this approach. As the surface finish of machined components is better than that of welded components featuring weld ripples, it follows that their fatigue performance should be better than Class B.
In addition, the welded specimens used to generate the Class B data will have contained very high tensile residual stresses, resulting in fatigue performance that reflects the most severe applied tensile mean stress conditions. On the other hand, there is substantial data suggesting that the Class B S-N curve may be too high.
Generating data for forged parts
TWI carried out a testing programme to produce a relevant fatigue database for establishing a suitable fatigue design procedure for plain steel components, based on Class B if appropriate. The database focused on large forged steel pipeline connectors, taking into account the effects of stress ratio and surface finish.
Programme of work
Fatigue tests were performed on strip specimens extracted from two forged J-lay connectors with different surface finishes (Figure 1). TWI used finite element modelling of the stress distribution, with the aim of designing strip test specimens that retained the features expected to influence fatigue strength. This modelling indicated that complete removal of the collar, leaving that region flush with the neighbouring outside surface, considerably reduced the bending stress induced by the thickness change, without significantly changing the stress concentration factors (SCFs) on the outside surface. The final design of the test specimen is shown in Figure 2. It retained the original surface finish and geometry, and hence SCFs at the critical locations, while reducing secondary bending stress.
The specimens were tested under constant amplitude tensile axial loading in a 1000kN servo-hydraulic fatigue testing machine at a frequency of 2–5Hz in laboratory air at ambient temperature. Two stress ratios were used, R=0.1 and 0.4. The fatigue lives of all strip specimens, including those that ran-out, are plotted in Figure 3 in terms of local stress range, which is the product of nominal stress range and SCF. The data showed that the stress ratio significantly affected fatigue endurance.
The fatigue test results were assessed as the basis for the fatigue design of full-scale forged components, with particular reference to SCRs. In this context, it is often necessary to consider higher tensile mean stress conditions than those investigated here. To illustrate a possible approach to fatigue design under higher stress ratios, the present results were analysed by modifying the well-known Goodman correction to calculate the fatigue limit at a higher mean stress.
Figure 3 illustrates possible design curves for various R values, including R=0.1 and 0.4, in comparison with the present experimental data. Each set of data lies well above the corresponding design curve. The Class B curve coincides with the calculated curve for R=0.76. This seems entirely reasonable since the Class B curve was derived from fatigue test data obtained from welded specimens containing very high tensile residual stresses. These would have had the effect of producing a very high effective stress ratio for any applied cyclic stress. Based on the test data and the Goodman mean stress correction, TWI was able to develop an alternative fatigue design approach that included the effect of the applied stress ratio.