S-N Fatigue Assessment

The S-N high cycle fatigue assessment is conducted to determine the pipeline fatigue life under the high cyclic temperature and pressure fluctuations. A fatigue assessment of response that is associated with number of cycles more than 10000 is denoted high cycle fatigue, where the assessment is performed based on calculation of elastic stresses.

The S-N fatigue assessment is based on Palmgren-Miner rule, which does not take into account of mean stress, frequency and loading sequence. The fatigue assessment is carried out for the following locations:
•    Girth weld cap;
•    Girth weld root;
•    Longitudinal seam weld (except seamless pipe).
Accounting for:
•    axial stress;
•    hoop stress;
•    Bending stress.
•    Stress concentration factor (SCF) at the girth weld including the effect of misalignment;
•    Type of S-N curve i.e. D, E, F, F1 or F3;
•    Condition of weld i.e. seawater cathodic protection (CP), in air or free corrosion.

The longitudinal stress range at each single location along the pipeline route can be obtained from Finite element analysis (FEA) model or calculated in accordance with DNV-OS-F101 Section 4.1.1 conservatively assuming the pipeline is fully restrained.

References:
1.    DNVGL-RP-0005:2014-06 (supercedes DNV-RP-C203);
2.    DNV-OS-F101