Page 452 - Pipelines and Risers

Fatigue of Risers 419

the industry today, the likely error in the response amplitude prediction may be as high as a
factor of two. Much of the reason for this lack of accuracy is to be found in our poor ability to
model the hydrodynamics and in the lack of calibration data at high Reynolds numbers. The
hydrodynamics issues were mentioned in the previous section and the calibration issue is
addressed in a later section on field data. For the remainder of this section the focus is on the
limitations of current structural dynamic, modeling methods.

As mentioned before, SHEAR7 is based on the mode superposition method, which has
practical limitations when the number of excited modes becomes large. Many deepwater
production risers will require modeling of dynamic properties that may be best described as
typical of structures that behave as if infinite in length. For example, vortex shedding in high
velocity surface currents may produce travelling waves at the top of the riser that are damped
out before reaching the bottom end. Mode superposition models are poorly suited for such
scenarios.

SHEAR7 is not a EM program, but the input to SHEAR7 can be calculated in a FEM
program. The input needed are natural frequencies, mode shapes and modal curvatures from
the riser.

The results from SHEAR7 includes for every node, the RMS values of the displacement and
stress, fatigue damage, local drag coefficient, tension and current velocity.

SHEAR7 predicts the cross flow VIV response.

22.4 Flexible Riser Analysis Program

Riflex (SINTEF, 1998) is a program for analyses of flexible risers and other slender
structures, such as mooring lines, pipelines and MCR’s.

Riflex is based on finite element modeling, the most important features are listed below:

- Beam or bar element based on small strain theory.
- Description of non-linear material properties.
- Unlimited rotation and translation in 3D space.
- Stiffness contribution from material properties as well as geometric stiffness.
- Allowing varying cross-sectional properties.

Riflex analyses
In Riflex there are four main type of analyses:
- Static analysis
- Static parameter variation analysis
- Dynamic time domain analyses including eigenvalue analysis