请教一个石油套管的问题？

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Item No Describation of goods Quan. Unit 01 13 3/8''OD seamless steel,Grade L-80,68 IB/Ft,BTC Threaded and coupled. Length Range III 48000 meter 02 13 3/8''Casing Nipple,Grade L-80,68 IB/Ft,BTC pin X pin,3 M long 28 No 03 13 3/8''Casing Pup joint,Grade L-80,68 IB/Ft,BTC theaded and coupled, 6M long 8 No 04 13 3/8''Casing Pup joint,Grade L-80,68 IB/Ft,BTC theaded and coupled, 3M long 8 No 05 Casing coupling for 13 3/8''Casing 4 C' y& N- }. E( y5 O' o5 v 10 No 06 API casing drift for 13 3/8''Casing ,compelete with draw eyes at both ends 4 No . V' S+ I  p" s9 Q请问一般石油套管都是怎么理论计算重量？ 还是采用 =(外径-壁厚）*壁厚*0.02466 7 j4 {: h3 J, P  v) j+ H: [3 ?$ z/ }另外我看到 L80钢级 有三种类型 1,9Cr,13Cr, 一般什么情况下才采用9Cr,13Cr?