Page 422 - Pipelines and Risers

Design of Deepwater Risers 389

point (TDP) region of the riser, transverse (out-of-plane) motions will occur as a consequence
of oscillatory forces caused by transverse wave acting on the free hanging part of the riser.

A proper description of the pipe-soil interaction is therefore important for the accuracy in
calculation of riser fatigue damage. Depending upon the stiffness and friction of the seafloor,
out-of-plane bending stresses will be more or less concentrated in the TDP region when the
riser is subjected to oscillatory motion.

In riser response analysis tools, the pipe-soil interaction is commonly modeled by use of
friction coefficients (sliding resistance) and linear springs (elastic soil stiffness). However,
these parameters must be selected carefully in order to properly represent the complex pipe-
soil interaction.

During small and moderate wave loading (the seastates contributing most to the fatigue
damage) the riser TDP response in the lateral direction is very small (in the order of 0.2 pipe
diameters). This will cause the riser to dig into the top sand soil layer and create its own
trench. This effect will gradually decrease as the riser gets closer to the underlying stiff clay
soil, where very limited penetration is expected. The width of this trench will typically be 2-3
pipe diameters, which leaves space within the trench for the pipe to move without hitting the
trench edges. During a storm build-up, the trench will gradually disappear as a result of larger
riser motions in addition to natural back fill. For the ULS condition, the pipe-soil interaction
is found to be of minor importance even if higher lateral soil resistance is mobilized.

20.3.6 TDP Response Prediction
It is necessary to further compare FLEXCOM and RIFLEX (SINTEF, 1998), for models close
to and within the buckling regime. Also, the effects on analysis results of structural damping,
hydrodynamic drag coefficients, element refinement, pipe imperfections and seabed stiffness,
should be investigated.

Use of the general finite element analysis program ABAQUS, is an alternative to FLEXCOM
and RIFLEX.

20.3.7 Pipe Buckling Collapse under Extreme Conditions
Within the industry, there are considerable differences between recommended methods for
sizing riser pipe for resistance to collapse and propagation buckling in deepwater particularly
for low D/t ratios. Existing formulations are based on empirical data, which attempt to
account for variations in material properties and pipe imperfections. Application of these
codes to deepwater applications provides scatter of results. Additionally, the effects of tension
and bending (dynamic and static) are uncertain, depending on the nature of the loading
condition.
20.3.8 Vortex Induced Vibration Analysis

1) Analysis Procedure and Modeling Assumptions: