Page 277 - Pipelines and Risers

250 Chapter 14

3. probabilistic analysis in the conditional assessment.

For the simplified analysis, the observed corrosion is compared with the corrosion allowance.

Estimated corrosion: 0.33 = 7.8mm
Corrosion allowance: 1.6m

The observed corrosion is larger than the corrosion allowance.

For the deterministic analysis, the experienced degree. of corrosion (stationary corrosion rate)
is assumed to be valid over the remaining service life.

Corrosion after 15 years of service

corrosion rate: = 0.35t I T = 0.78mm I year
corrosion after 15 years: = $15 = 11.7mm
Specified yield strength for X60 steel: cry = 413MPa
Acceptable hoop stress: omleS 297MPa
=
Specified design pressure: AP = 13.4Mpa

Hoop stress for uncorroded pipe:
hP.D 13.4.914.0
OH=-= = 276MPa
2.t 2.22.2

Hoop stress for 11.7 mm corrosion:
hP*D 13.4.914.0
OH=-- = 583hPa
2. (t - 2) - 2 * (22.2 - 11.7)

Based on the observed corrosion, the estimated stress after 15 years service is larger than the
acceptable stress.