Page 266 - Pipelines and Risers

Remaining Strength of Corroded Pipes 239

Effective Area, AREA
Two levels of AREA assessment are recommended in Section 4.3.

Closely Spaced Corrosion Pits
A distance of t (wall-thickness) is used as a criterion of pit separation for a colony of
longitudinal oriented pits separated by a longitudinal distance or parallel longitudinal pits
separated by a circumferential distance.

Interaction of Longitudinal Grooves
For defects inclined to pipe axis, if the distance x, between two longitudinal grooves of
lengths L1 and Lt, is greater than either L~ or L2. then the length of corrosion L is the
maximum of L1 and L2; if the distance x, between two longitudinal grooves of lengths L1 and
Lz, is less than L1 and Lz, the length of corrosion L is the sum of x, L~ and L~,
L = L1 + L2 + x. For two longitudinal grooves separated by a circumferential distance x, the
wall thickness t is used as groove separation criterion.

Spiral Correction Factor
The spiral correction factor Q is determined as:
1-Q W
Q=---~-+Q~ (14.26)
32 t

in which W is defect width, and coefficient Q1 is a function of the spiral angle cp (cp = 90' for
longitudinal corrosion, cp (=O( for circumferential corrosion)

Flow stress
Consideration should be given to factors affecting flow stress, e.g., fabrication process (e.g.
hot rolled versus cold expanded), material aging, possible size effect, installation process and
possible crack in corrosion defect bottom. Use of the actual value of the flow stress is allowed
provided the value has been obtained from a reliable approach (e.g., material testing of the
pipe in situ. etc.).

If the ultimate tensile stress is known, the flow stress can be estimated as the ultimate tensile
stress. For API 5L materials, SMTS (Specified Minimum Tensile Stress) is recommended as
flow stress.

Folias Factor M
The Folias factor is estimated based on the following equations:

2.51(L/2)'- 0.054(L/ 2)4 for
M= Dt (Dt? Dt (14.27)
I 0.032- +3.3 for - 50
L2
L2
>
M
Dt