Page 101 - Pipelines and Risers

14 Chapter 4

4.6.2 Fatigue Assessment based on S-N Curves
The S-N curves to be used for fatigue life calculation are defined by the following formula:
log N =log a- melog ACT
where N is the allowable stress cycle numbers; a and m are parameters defining the curves,
which are dependant on the material and structural detail. Aa is the stress range including the
effect of stress concentration.
For the pipe wall thickness in excess of 22 mm, the S-N CUNe is to take the following form:

where t is the nominal wall thickness of the pipe.
The fatigue damage may be based on the accumulation law by Palmgren-Miner:

where:
Dfat = accumulated fatigue damage
q = allowable damage ratio, to be taken as 0.1
Ni = number of cycles to failure at the i th stress range defined by S-N curve
ni = number of stress cycles with stress range in block i
A cut-off (threshold) stress range So may be specified below which no significant crack
growth or fatigue damage occurs. For adequately cathodic protected joints exposed to
seawater, So is the cut-off level at 2x108 cycles, see Equation (4.17).
-- I
so=($] (4.17)

Stress ranges S smaller than So may be ignored when calculating the accumulated fatigue
damage.
4.6.3 Fatigue Assessment based on As-N Curves
The number of strain cycles to failure may be assessed according to the American Welding
Society (AWS) Standards A&-N curves, where N is a function of the range of cyclic bending
strains AE. The A&-N curves are expressed as below:
AE = 0.055N4'4 for AE 2 0.002 (4.18)
and
AE = 0.016N4.25 for A& I0.002 (4.19)
The strain range A& is the total amplitude of strain variations; i.e. the maximum less the
minimum strains occurring in the pipe body near the weld during steady cyclic bending loads.
A study of low-cycle fatigue conducted as part of the DEEPPE JIP was summarized by
Igland et al. (2000).