Page 40 - Pipelines and Risers

14 Chapter I

To provide additional resistance to forces by use of anchors (rock-berms) or additional
weights on the pipeline.

In the latter case the spacing of the anchors must be designed to eliminate the potential for
sections of line between the Axed points to undergo large movements or suffer high stress
levels. The safety of the line on the seabed is again the most important criterion in the stability
design.

A finite element model for on-bottom stability analysis is discussed in Chapter 8.

1.4.5 Expansion Analysis
The expansion analysis determines the maximum pipeline expansion at the two temrination
points and the maximum associated axial load in the pipeline. Both results have significant
implications in the design as:

Axial load will determine if the line may buckle during operation, and hence additional
analysishestraint will be required;
End expansions dictate the expansion that the tie-in spools (or other) would have to
accommodate.

The degree of the expansion by the pipeline is a function of the operational parameters and
the restraint on the pipeline. The line will expand up to the “anchor point”, and past this point
the line does not expand (hence fully restrained). The distance between the pipeline end and
this length is determined based on the operational parameters and the pipeline restraints. The
less the restraint the greater the anchor length becomes and hence the greater tie-in expansion
becomes (see Figure 1.5 for terminology).

1.4.6 Buckling Analysis
Buckling of a line occurs when the effective force within the line becomes so great that the
line has to deflect, and so reduce these axial loads (i.e. takes a lower energy state).

As more pipelines operate at higher temperatures (over 100°C) the likelihood of buckling
becomes more pertinent.

The buckliog analysis will be performed to identify whether buckling is likely to occur (see
Figure 1.6). If it is, then further analysis iS performed to either prevent buckling or
accommodate it.

A method of preventing buckling is to rock dump the pipeline. This induces even higher loads
in the line but prevents it buckling. However, if the rock dump should not provide enough
restraint then localized buckling may occur (i.e. upheaval buckling) which can cause failure of
the line.