It’s either 18 months or 12 months in an approved program, bboth have to be while qualified as RFPNW. So you would first need to get the 6 months of time for RFPNW and complete the assessments. Note that “while qualified as RFPNW” does not mean you need to have held RFPNW. You will have qualified for RFPNW on either the last day of the 6 months of service, or completing the last RFPNW assessment, whichever happens later. So if you finish your RFPNW assessments and sea time in the middle of your time on a vessel, you can still start the clock on AS-D.