Page 423 - Pipelines and Risers

390 Chapter 20

The VIV analyses of the MCR could follow two different approaches: using SHEAR7 (h4IT,
1995 and 1996) and using VISFLOW (DNV 1998).

2) Combination of VN-Induced and Wave-Induced Fatigue Damage:
As the VIV- and wave-induced fatigue damage is established independently, results from both
calculations must be combined to get the total distribution of fatigue damage for the MCR’s.

The areas where significant wave-induced fatigue damage occurs are very distinct. The VIV-
induced fatigue damage occurs more evenly distributed (according to the larger variations in
mode shapes and their superposition).

The total fatigue damage is then obtained by a simple sum of the two contributions. The fact
that VIV- and wave-induced response will be more or less perpendicular to each other is
conservatively not accounted for (“hot-spots” are assumed to coincide).

20.4 Stresses and Service Life of Flexible Pipes

Calculation of ultimate capacity may be performed with good accuracy by tools estimating the
average layer stress. All the available flexible pipe analysis tools, including the manufacturers
design programs calculate the average stresses in each layer.

Service life prediction on the other hand requires detail knowledge of the mechanism leading
to failure. The manufacturers have established estimation methods based on theory and test
results. These analysis methods must be calibrated for each manufacturer, each wire geometry
and type of pipe (i.e. additional hoop spirals). The advantage with such empirical methods is
that residual stresses from manufacturing, actual tolerance on wire geometry, etc are present
in the tests and hence incorporated in the analysis. The problem is that design optimization is
hardly possible and independent verification is impossible.

Lprtveit and Bjerum (1995) has found that by combining detailed knowledge of flexible pipes
with state of the art non-linear FEM programs it is possible to develop an analysis tool that
can predict the stresses sufficiently accurately to provide input to service life prediction.
SeaFlex has recently developed a second-generation analysis tool, PREFLEX, for analysis of
flexible pipes. PREFLEX is based on the general non-linear FBM program MARC.
PREFLEX can model each wire with a mesh sufficiently detailed to calculate local hot spot
stresses.

Examples of attractive features of PREFLEX are:

- Virtually no modeling limitations. End fitting areas, damaged pipe etc., can be modeled.
- Service life predictions based on a minimum of test results. PREFLEX can accurately
calculate the stresses and small-scale tests of the wires may hence be used to define the