Wall Thickness Calculation for 30 Year Service Life

From AWS Forum:

In this situation you can try this approach; since the design life of the pipe is controlled by corrosion exposure versus other time dependent damage mechanisms, and some level of inspection had been performed after "x" years of service, one could conclude that the design life of the pipe is in excess of "y" years.
Your rationale for determining what "y" years should be may be as simple as; periodic inspection was used to extrapolate loss of wall thickness over "x" years. Assuming a negligible corrosion rate based on the past inspection over "x" years, the remaining service life will be at least 25 years (use a graph to show the calculated corrosion rate based on past inspection). Once the nominal wall thickness reaches a value below design (using a minimum wall thickness calculation referencing ASME B31.3 or other suitable piping code) minimum wall thickness based on the extrapolated corrosion rate form above, this would be the point in time where repair or replacement is necessary.
The reason 25 years is typically used as a benchmark is because most economic models for equipment use a 25 year "design life" for economic modeling. In reality, the "real design life" could far exceed 25 years, it could be 50 or 75 years.

API RP 581 Part1 Clause 4.

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