Page 524 - Pipelines and Risers

Design Examples 491

flowline. It was further concluded that if a more sophisticated approach was used, this could
reduce the high cost of large-scale span corrections. The adopted approach was to allow both
in-line and cross-flow VIV to occur provided it is demonstrated that the allowable fatigue
damage is not exceeded in the span during the design life. The methodology used was in
accordance with the recommendations given by the Multispan project. For VIV, the natural
frequency of a span is both a function of the span lengwshape, effective force and restraint at
the shoulder. The conventional approach does not account for the effective force or the actual
shoulder constraints. This project assessed the natural frequencies of the spans by developing
a 2D FBM modal multispan analysis model, which takes the in-situ condition of the pipe into
consideration.

This approach typically reduced the number of spans requiring corrections to prevent VIV
from 20 spans to 2-3 spans for a lOkm production flowline.

Buckling Control
It was apparent that the behavior of a high pressure - high temperature flowline resting on a
very uneven seabed is extremely complex yet the cost of installing and maintaining full cover
protection would be exorbitant. In order to gain further insight into how expansion, seabed
friction and free spans influence each other as the flowlines heat up, non-linear Elasto-Plastic
2D and 3D FE models were developed in which the available seabed survey data is imported
directly into the model. As a result of these studies and the improved understanding of the
buckling behavior, a cost-effective seabed intervention strategy was adopted. Global buckling
is controlled by allowing the flowline expansion to be absorbed into spans and to further
control the expansion behavior by using strategically placed discrete rock berms.

Note that results from these studies were published at the ISOPE’97 conference- See Tames
et al. (1997) and Nystram et al. (1997).

The adopted intervention strategy is estimated to give a saving of 300 million NOK ($40
million US) compared to a conventional design in which the flowlines are trenched and
buried.

Wall thickness design
With the right control of the material characteristics for linepipe manufacture it was possible
to use a hoop stress usage factor of 0.80.

Reliability design
Corrosion allowance is optimized using reliability methods. A thicker pipe wall may not
reduce operational and maintenance costs, even at the expense of increased initial
construction costs. It is therefore necessary to optimize the life cycle costs in the wall-
thickness design, by investing in measures that have greater effects on corrosion resistance.

When carbon steel pipe is used corrosion allowance is to be added to the minimum wall-
thickness. Nedland et al. (1997) conducted a detailed study on the corrosion engineering and