igst or cgst N sgst

we have service provider state of maharashtra ,
our business is offshore n onshore oil platform make.
1.we have prepared one bill to our customer . customer also state of maharashtra no. we done fabrication work in maharashtra but execute the same at onshore under other territory code 97.

my query is in bill i will charge IGST or CGST n SGST??

2.above subject connection i had prepared three bill month of Nov Dec and jan charged CGST n SGST, if I had done could i passed the credit note

3. If I will prepare new bill against credit note n charged IGST, then can I take the set off against IGST of CGST n SGST