Page 457 - Pipelines and Risers

424 Chuprer 22

This section addresses this issue and asks if the acceptance criteria we are using is still sound
and are the analytical approaches valid as we approach water depths of 10,000 ft for steel
pipes employed as risers. The approaches adopted for flexibles and, in part, new riser
materials such as composites are not addressed here. These materials (unlike steel) are usually
expected to undergo physical failure testing to demonstrate their suitability for application,
and merit a separate review.
There are many failure modes for a metallic riser to fail, however two modes of failure are
expected to be dictating as greater water depths are experienced are:
Riser local buckling capacity due to combined axial load, external pressure and bending;
0 Riser fatigue.
For each area a comparison of the recently issued riser codes and analyticaVFEA results is
performed, leading to observations on the existing approaches and proposed approaches by
the authors.
22.8.2 Riser Fatigue

Vortex Induced Vibrations
VIV is perhaps more sensitive to the current profile than to any other parameter. For short
riser spans the current magnitude determines whether or not VIV will occur, and determines
whether the response is in-line or transverse to the flow direction (or both). The cross-flow
response is more significant than the inline response. For deepwater risers a low current will,
for a catenary with low horizontal components of tension, produce some VlV due to the low
natural frequency of the riser. The variation of the current along the riser span (i.e. with depth)
then determines which modes will be present in the response. Here it should be noted that:
Current profiles that are conservative for platform offsets are not necessarily conservative
for deepwater riser VIV prediction (this is because VIV of deepwater risers is much more
dependent upon the shape of the current profile with depth);
The current profile should be varied during the analysis to determine the sensitivity of the
results to current profile shape;
Currents change with time, so some kind of probabilistic description of the current
magnitudes and/or profile shapes is necessary for a sufficiently accurate VIV analysis;
It is possible that even if numerous modes are potentially excited by a current profile, a
single mode (or a small number of modes) can dominate the response due to “lock-in” in
which the vortex shedding tends to adjust to the vibration frequency within certain limits
(dependent upon mass ratio and Reynolds number etc.;
Even in a highly sheared current it is possible for a single mode (or a small number of
modes) to dominate the response.
Time domain analysis can identify the governing modes because interaction between
vibrations and axial loadings is modeled.

Analyzing VIV
The most recognized used program to predict VIV is the MJT program SHEAR7 (Vandiver
and Li, 1998) which is a non-linear, fluid-stmcture interaction, frequency domain model. The
interaction model allows for the local lift coefficient and local hydrodynamic damping
coefficient to depend on the response amplitude. SHEAR7 is based on mode-superposition