Dear friends
After complete the petroleum vessel hydrostatic test for (pipes), should I displace the nitrogen used for purging? If so, can I use the natural gas to displace the nitrogen and how?
Regarding tanks cleaning after cleaning and drying the tank, should I blanket by nitrogen first and then displace the nitrogen by natural gas or just using natural gas directly to purge the internal air?

I would appreciate any comments related to the petroleum vessels preparation after finish test

Thank a lot for your help