Page 262 - Pipelines and Risers

Remaining Strength of Corroded Pipes 235

Effect of Corrosion Width
Figure 14.4 shows the effect of defect width on burst pressure with a longitudinal defect (Mok
et al. (1991)), for the case of X52, OD=508mm, t=6.35mm, dltS.4. It can be concluded in
Mok’s studies that the width effect is negligible on the burst pressure of pipe with long
longitudinal defects.

Irregular Shaped Corrosion
The major weakness of the existing B31G criterion is its over-conservative estimation of the
corroded area for long and irregular shaped corrosion (Bai et al. (1994), Kiefner and Vieth
(1990), Hopkins and Jones (1992)). Therefore, the key to the irregularly shaped corrosion is
the accurate estimation of the corroded area.

Two shapes were considered in the development of the original B31G criterion. One was the
rectangle area method. The other was the parabola area method. Tests of Hopkins and Jones
(1992) indicated that irregularly shaped corrosion could be conservative assessed using the
B31G criteria when the accurate cross-sectional area of the corrosion defect was used.

We recommended two levels of AREA assessment. In the level 1, the AREA is estimated as:
2
L I (Dt)< 30 AREA =-L* d
3 (14.15)
L /(Dt)> 30 AREA = 0.85L. d

In the level 2, the exact area (AREA) of the corrosion profile is estimated by Simpson
integration method.

14.2.5 Material Parameters
The major material parameters in the B31G criterion are flow stress, Specified Minimum
Yield Stress (SMYS), Folias Factor M.

Flow Stress and SMYS
In the B31G-1993 manual, the flow stress was defined as 1.1 SMYS which is an appropriate
value for the new pipelines. However, the flow stress is influenced by a number of factors,
fabrication process (e.g. hot rolled versus cold expanded) and material aging. Furthermore,
the flow stress used in burst strength criteria is influenced by possible cracks in the pit bottom
due to corrosion fatigue. Therefore, specific attention should be made for accurate estimate of
flow stress for aging pipelines. Many researchers (Hopkins and Jones (1992), Klever (1992),
Stewart et al. (1994)) indicted that the flow stress for base material could be estimated as
ultimate tensile stress. An approximation of the ultimate tensile stress is the Specified
Minimum Tensile Stress, a statistic minimum of the ultimate tensile stress:
oflow = SMTS (14.16)

The value of SMTS are available in some design specification (API 5L).