Hi

Blast drag coefficient can as high as 10. This is possible when low reynold number acheived. But blast loads generally significant, where reynolds number of fluid is quite high. Again depend on reynolds number/Mach no. to could go upto 3. Not to add more ambiquity, i have been working in offshore for last 3 years and practice followed by companies is drag coefficient factor of 0.67 and DLF of 1.5 to 1.67. The product of drag coefficient and DLF is 0.67 * 1.5 = 1 or 1.1 are given as input in CAESAR.

You may read DNV-RP-D101, there is a good guidance. It is available on internet for free. Just use google to find location of DNV report.

best regards

Rao