Page 259 - Pipelines and Risers

232 Chapter I4

MAOP
dallow = [I-=] (14.9)

The Safe Maximum Pressure Level P'
The safe maximum pressure level P' for the corroded area is:

(14.10)

(14.1 1)

where:
(14.12)

14.2.3 Evaluation of Existing Criteria
The existing criterion ASME B31G (1993) for corroded pipelines was established based on
the knowledge developed over 20 years ago. This criterion is re-examined to develop an
improved criterion based on current knowledge. This evaluation is conducted based on the
corrosion mechanisms, parameters in the existing criterion and the applications which are not
included in the existing criterion.
/
Groove
m

Pits containing cracks in the bottom

Figure 14.1 Type of Corrosion Defects.

14.2.4 Corrosion Mechanism
Figure 14.1 shows the types of corrosion defects. For marine pipelines, internal corrosion is a
major problem (Mandke (1990), Jones et al (1992)). Many forms of internal corrosion occur,
e.g., (a) girth weld corrosion, (b) massive general corrosion around the whole circumference,
and (c) long plateau corrosion at about 6 o'clock position. External corrosion, on the other
hand, is normally thought of as being local, covering an irregular area of the pipe. However,
when the protective coating is failed, the external corrosion may tend to be pattern of long
groove.