Page 91 - Pipelines and Risers
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64 Chapter 4
The strength criteria will be applicable for the following design situations:
Pipeline in-place behavior
0 Trawl pullover response
Free-spanning pipelines
Pipeline dynamic free-span
The pipeline route is divided into two zones:
Zone 1 is the zone where no frequent human activity is anticipated along the pipeline
route. For operating phases Zone 1 is classified as “Normal Safety Class”
Zone 2 is the parts of the pipelinehser in the near platform (manned) zone or in areas with
frequent human activity. The extent of zone 2 is 500 m from the maximum facility
excursion or determined based on risk analyses. For operating phases Zone 2 is classified
as “High Safety Class”
For temporary (construction) phases both zones are classified as “Low Safety Class”, when
the pipeline does not contain any hydrocarbons.
4.2 Out of Roundness Serviceability Limit
The pipeline out of roundness is related to the maximum and minimum pipe diameters (DmX
and Ddn) measured from different positions around the sectional circumference and is defined
according to the following equation:
Dmx -D&
fo = (4.1)
The out of roundness during the fabrication process is not to be more than 1.5%.
The out of roundness of the pipe may increase where the pipe is subject to reverse bending
and the effect of this on subsequent straining is to be considered. For a typical pipeline the
following scenarios will influence the out of roundness:
- The out of roundness may increase during the installation process where the pipe is
subject to reverse inelastic bending;
- Cyclic bending may occur as a consequence of shutdowns during operation if global
buckling is allowed to relieve temperature and pressure induced compressive forces.
Out of roundness due to point loads is to be checked. Critical point loads may arise at free-
span shoulders, artificial supports and support settlement.
The accumulative out of roundness through life cycle is not to exceed 4%. This out of
roundness requirement may be relaxed if:
