Hi, I am wanting to know which collapse pressure equation will be appropriate for H-40, OD 30", thickness 0.375" D/t= 80" (is this correct?). In the future how do I select an appropriate equation according to D/t?
You need to get a copy of API TR 5C3 (ISO TR10400), if you don't already have one. Pay particular attention to Annex E.
You will have to rely on experience to let you know the appropriate equations. Most of the collapse equations are based on emperical data, not first principles, so you will need to find the ones that are better taylored for your specific applicaton.
30 inch diameter is more like conductor pipe than casing, so you will probably be "pushing the envelope" using equations developed for casing in this applicaiton.