Page 146 - Pipelines and Risers

Vortex-induced Vibrations (VIV and Fatigue 119

b) For a) verified, a fatigue analysis will be performed.

c) The fatigue damage shall not exceed the allowable fatigue damage, q, that is normally 0.1.

M$rk et al. (1997, 1998) gave a series of papers on VIV and fatigue of free-spanning
pipelines.

9.2 Free-span VIV Analysis Procedure

9.2.1 Structural Analysis
The structural properties of the given span configuration are to be characterized in terms of
static and dynamic properties. The output are key parameters that can be applied in
subsequent analysis involving hydrodynamic loading etc. on span.
Key parameters and relationships to be deducted are mainly:
Relationship between loadingldeflection of span and associated stresses and sectional
forces/moments in pipe wall (static analysis)
0 Eigenfrequencies and mode shapes of span, relationship between vibration amplitudes and
stress cycles (dynamic analysis)
0 Soil damping in terms of soil static and dynamic interaction with pipe.

Structural models of various complexity, analytical as well as computer based models, may be
applied, ranging from simple models for simplified desk calculation to advanced finite
element models for computer analysis.

Static model
Basically the static model is applied to determine stresses due to static or quasi-static loads
such as deadweight of span, quasi-static wave and current loads, trawl boards, anchors.
Frequently an elastic approach is selected for the pipe itself, whereas elasto-plastic soil
behavior most often are adopted. This is particularly important in case of large spans
supported on soft seabed.
For analysis of impact loads it is usually relevant to consider elasto-plastic behavior of the
pipe as well as the soil.

Dynamic model
Basically the dynamic model is applied to determine stresses corresponding to flow induced
vibrations (in conjunction with Response Amplitude Data BaseModel) for subsequent
calculation of fatigue damage (in conjunction with the Fatigue Model) and for comparison
with criteria for maximum allowable stresses. In-line and cross-flow vibration may be treated
integrated or separately.