Page 448 - Pipelines and Risers

Fatigue of Risers 415

22.2.2 2"d Order Floater Motion Induced Fatigue
Mean floater drift motions can have a significant influence on riser TOP fatigue damage and
must be accounted for in linearisation analyses. In addition, the slowly varying component of
drift motions provides a further contribution to total riser fatigue damage.

The approach to analysis of low frequency drift motions may follow that used for first order
fatigue analysis in that scatter diagram windowing and response linearisation is used. For each
window, linearisation analyses are conducted in pairs, using the mean drift offset and mean
plus root mean square (RMS) low frequency drift motion, each applied statically to the riser,
with no wave or current loading. The difference in stress between the two static analyses, at
each point along the riser, is assumed to represent the RMS stress amplitude due to drift
motions. Assuming the low frequency stresses are Rayleigh distributed, the fatigue damage
from each seastate, and hence each window, may be calculated. For each scatter diagram
window, a representative EMS drift offset and drift motion mean crossing period must be
selected.
DISPLACEMENT
DUE TO WAVES
1 st ORDER &
2"d ORDER
FLOW-
VELOCITY
1 st ORDER
FF'S OR TLP
MWL

b

t

Analysis of slow drift fatigue damage is based on static analysis of floater motions with no
current or wave applied. The scatter diagram is first split into 6 linearisation windows, the
seastates in each having similar drift characteristics. For each window, linearisation analyses
are conducted in pairs, using a representative mean drift offset and mean plus RMS low
frequency drift motion, each applied statically to the riser. The difference in stress between
the two static analyses, at each pint along the riser, is assumed to represent the RMS stress
amplitude due to drift motions. The fatigue damage from each linearisation seastate is