Page 355 - Marine Structural Design

Chapter I7 Fatigue Capacity 33 1

In some of the design codes, there is a cut-off limit, and low fatigue damage is assumed when
the stress range is below the cut-off limit.
For the sake of consistency, discussions of the fatigue criteria in this chapter will be mainly
based on NORSOK (NTS, 1998). However, readers are recommended to refer the codes
relevant to their projects such as IIW (Hobbacher, A. (1 996), Eurocode 3 (1992), IACS (1 999),
ABS (1 992) and DNV (2000), among of others.

17.1.2 Effect of Plate Thickness
The thickness effect is due to the local geometry of the weld toe in relation to the thickness of
the adjoining plates and to the stress gradient over the thickness. It may be accounted for by:

logN=logK-mlog S - (17.3)
[ (tY]

where, tref = Reference thickness which in some design codes is 32 mm and 25 mm for
tubular joints and other types of welded connections respectively (NTS, 1998).
t = Thickness through which a crack will most likely grow.
k = Thickness exponent on fatigue strength in the range 0.00 to 0.25 depending
on the code employed, the S-N curves selected etc (NTS, 1998).

In other words, the thickness effect may be accounted for by multiplying a factor of (r / t Rf )" to
the stress range. In HSE (1995), the value of k and reference thickness I,~, is 0.25 and 22 mm,
respectively. In general, the thickness correction to the design equation for the S-N curve is
required when the plate thickness is thicker than the reference thickness. To some extent, the
thickness correction also accounts for the size of the weld and its attachments. However, it
does not account for the weld length or the length of component different from the tested
component.
17.1.3 Effect of Seawater and Corrosion Protection
In Figure 17.2, three types of S-N curve are compared for the tubular T S-N curves in air,
seawater with CP, and seawater under free corrosion. The relationship between in-air and in-
seawater with cathodic protection (CP) varies between codes. Using NORSOK (NTS, 1998),
the fatigue life at high stress ranges (when N is less than106 cycles) in seawater with CP is
considered to be 40% of that in-air. However, there is no difference between the S-N curves at
lower stress ranges (when N is in excess of lo7 cycles).
In general, the fatigue life in seawater under free corrosion is 33% of the life in air at high
stress ranges (when N is less than lo7 cycles). There is no change in slope for the free
corrosion S-N curve and hence the fatigue lives are around lo%, of the equivalent lives for in-
air S-N curve when N is more than lo7 cycles.
17.1.4 Effect of Mean Stress
Compressive mean stress has a beneficial effect on fatigue capacity. Normally it is not
required to account for the effect of mean stress. However, in some special cases, it is
necessary to take into account the mean stress effect to modify the selected S-N curves, e.g.
for the fatigue assessment of TLP tethers and mooring lines whose non-linear response is