DNV pipeline plastic moment approximations

Compare two formula

Numerically (dt ≈ 15–45): the constant 1/3 is tiny compared to dt^2, so the two formulas are practically identical.

• Formula 1 is the exact CHS plastic moment written via dt and t.

• Formula 2 is a slightly simplified DNV approximation—replace 4/3 with 1 to get a neat (dt−1)^2 term.

• For realistic pipeline D/t, that simplification changes the result by well under 1%, which is why they feel “almost identical” in practice.

See some comparisons below for D/t of 10, 15 and 20.