Page 160 - Pipelines and Risers

Vortex-induced Vibrations (Vlv) and Fatigue 133

The cross-flow and the in-line mode shapes need to be correlated in order to take account of
the cross-flow induced in-line fatigue. For the first three modes in the figures above, the in-
line and the cross-flow modes are clearly linked. There are two clear cross-flow modes at the
same location as the fourth in-line mode. Both the seventh and the eighth cross-flow show
large excitations over the same span. The seventh mode is conservatively chosen because the
natural frequency is lower. For the tenth in-line frequency the corresponding mode was found
to be the twelfth cross-flow mode (not shown).
9.6.2 Fatigue Assessment

For the long-term environment description typical North Sea omni-directional wave and
current distributions are applied.
joint-frequency spectrum (Hs and Tp) of 3 hourly sea states
3 parameter Weibull current distribution of the 10 minute average current measurement at
3 meters above the seabed

The water depth is approximately 120 metres so the longer period waves will have an effect
on the pipeline. The loads are initially considered acting at 90" to the pipe. This is a
conservative assumption, which reduces the run time during the first stage of the analysis. It is
used for screening purposes in order to determine which spans are critical and therefore
require a more detailed assessment.
Structural damping of 1% is taken for each of the pipelines. The fatigue resistance is
determined from the two-slope F1 S-N curve in seawater with cathodic protection.
The damage is found for an operational life of 50 years for all expected environmental
conditions. Ten percent of the total fatigue damage is allowed during the temporary phases,
i.e. empty and water filled conditions and a further ten percent for the installation. The results
are shown in Table 9.5 where the damage acceptance criterion for the operating condition is
0.48.

The results come from the Fortran based program VIVA (Grytten and Reid, 1999) for
calculating pipeline free span fatigue.
As expected due to the rough seabed the pipes experience a high level of fatigue damage and
intervention is required. There is unacceptable damage for the both the cross-flow and the in-
line directions. The lowest modes show high force model damage, in particular at KP 1841
and KP 3319. Most of the spans have cross-flow damage and therefore also experience cross-
flow induced in-line damage.