Page 408 - Pipelines and Risers

Use of High Strength Steel 375

In the context of future developments beyond X80, it is worth noting two points:

1) Marine sacrificial CP systems are available with potential control (as opposed to full open
circuit potential capability of normal systems) to allow the use of steels with higher
hardness values. Open circuit is the condition of maximum negative potential (or
polarization) of protected steel from a conventionally mounted sacrificial anode when no
current flows as can (almost) occur in practice at low current demands. This condition is
the worst for hydrogen evolution and consequent hydrogen cracking. Steels
conventionally need to be compatible with this potential which is more negative than that
required for corrosion protection. Smart CP systems now exist which have local, potential
sensing devices to control the applied potential only to the value required for corrosion
protection, thus risks of hydrogen cracking are minimized. These systems have been used
on high strength steels of jack-up rigs which previously have been known to crack due to
hydrogen uptake.

2) Developments of linepipe for sour service will impose lower hardness limits, typically 250
- 275HV10.

Corrosion fatigue in the presence of CP is a secondary consideration in that pipelines would
not normally be designed against a specified fatigue life. However fatigue concerns may arise
in the event of spanning of subsea pipelines and so it is prudent to confirm that candidate
materials do not have degraded fatigue properties relative to established grades. The concern
arises from the unwanted uptake of hydrogen under the influence of CP. Hydrogen uptake
adversely influences toughness and fatigue crack growth rates. Healy and Billingham (1993)
indicates that fatigue properties of high strength grades under CP are comparable to
conventional steels but information should be obtained that is specific to candidate linepipe
steels.

Pipelines on land similarly require compatibility with CP and the above hardness criteria are
also conventionally applied. Occurrences of external stress corrosion cracking (SCC) do not
correlate with steel grade. Hydrogen embrittlement is associated with hydrogen uptake,
normally in seawater. External SCC is fundamentally different and is a known risk for land
pipelines and can be potentially a problem for all lines.

19.5 Fatigue and Fracture of High Strength Steel

It is recommended to obtain fatigue data for the proposed materials and apply the data to
mechanical design. Fatigue life is used as the basis for many of the limits placed on offshore
pipeline strength design. These limits have often been established based on empirical data
from tests on low strength steels, with a safety margin applied. In general, the ability of steels
to resist fatigue failure increases with increasing yield strength. Fatigue analysis data from
linepipe manufacturers can be used to challenge the requirements of pipeline codes in the
areas of thermal buckling analysis, freespan and pipeline stability analysis.