API 650 was updated on November 2009. This addendum consists of the pages that have changed since the June 2007 printing of the 11th edition and Addendum 1 dated November 2008.

Except some minor changes, I noticed that Figure 4-1 was plotted for SI and Imperial Units as well. Also, this figures are given as tabular value on Table 4-3a and 4-3b. This changes are very useful.

On the other hands, the most critical change is Impulsive Natural Period equation (Eq. no : E.4.5.1-1a) in Seismic Provisions, Appendix E . This equation was changed all edition and addendum. In previous one:

T1 = 1/(2000)^1/2*(Ci*H^(ρ/E)1/2)/(tu/D)^1/2 for SI Unit

In addendum 2:

T1 = (1/(2000)^1/2)*(Ci*H/(tu/D)^1/2)*(ρ/E)^1/2 for SI Unit

This change seems to be affects many calculations. If you want to download free Addendum 2 only, please check this link. And here is the free Addendum 1 download link.