RSTRENG exists because the shape assumptions in B31G are crude. The original method replaces the corroded region with a parabola of the maximum depth over the full length; the modified method uses 0.85 of that rectangle. Real corrosion is neither. It is usually a shallow general patch with one or two deeper pits, and idealising the whole length at maximum depth throws away most of the remaining metal on paper.

The effective area method uses the metal that is actually missing. The inspector records a river-bottom profile — the deepest remaining wall along each axial line through the corroded region — and the area under that profile is integrated. The failure pressure follows the same form as Modified B31G, with the area ratio A/A₀ replacing the idealised 0.85·d/t, where A₀ = L·t is the original metal in the same axial length:

S_f = (SMYS + 68.95) · [1 − A/A₀] / [1 − (A/A₀)/M], with the same Folias factor as the modified method. The full RSTRENG procedure then repeats this for every possible subsection of the profile — every start and end pair of measurement points — and reports the lowest predicted failure pressure of all of them, because a short deep subsection can govern over the full-length average. This calculation evaluates one subsection at a time; where a profile has distinct deep pits, run it for the full length and again for each deep region and take the lowest result.

The gain over Modified B31G is real but the method is only as good as the profile. It requires a genuine axial grid of depth readings — typically a pit gauge on a 25 mm or finer grid, or an accurate laser or ultrasonic C-scan map — not a single maximum-depth measurement. If all you have is a maximum depth and a length, you do not have an effective area and you should use Modified B31G. The same exclusions apply as for B31G: blunt metal loss only, internal pressure only, no cracks, gouges, dents or weld-zone defects.

Worked example

Pipe outside diameter	508 mm
Nominal wall thickness	9.53 mm
Axial length of the evaluated subsection	150 mm
How the metal loss is entered	area
Effective metal-loss area	300 mm2
Average depth over the length	2 mm
SMYS	359 MPa
Design factor / class	class1
Safety factor	1.39
Original area A₀ = L·t	1429.5 mm2
Effective area used	300 mm2
Implied average depth	2 mm
Area ratio A/A₀	0.2099
z = L²/(D·t)	4.648
Folias factor M	1.96
Safety factor applied	1.39
Predicted failure pressure	14.21 MPa
Safe operating pressure	10.22 MPa

Same NPS 20 X52 pipe and 150 mm corroded length as the B31G example, but the river-bottom profile integrates to 300 mm2 of lost metal. A0 = 150 x 9.53 = 1429.5 mm2, so A/A0 = 0.209864 and the implied average depth is 300/150 = 2.00 mm against a 3.0 mm maximum. z = 4.64757 and M = 1.960472 as before. S_f = 427.95 x (1-0.209864)/(1-0.209864/1.960472) = 427.95 x 0.790136/0.892953 = 378.675 MPa. P_f = 378.675 x 19.06/508 = 14.208 -> 14.21 MPa, and P_s = 14.208/1.39 = 10.22 MPa. That is about 4 % more than Modified B31G on the same defect, because the real profile holds less lost metal than 0.85 x 3.0 x 150 = 382.5 mm2.