Dispensation for a heavier aircraft
I suspect you are considering the AW139 when you mention 6.8 tonnes? I have known decks to be given an approval for the 139 to utilise a 5.4 Tonne deck previously used for the Bell 212.
To give approval for this type of deck to be used, the deck and structure has to have a recent structural inspection certificate and dispensation from the local regulator. The regulator will then base their decision on the performance of the aircraft type and the likelihood of a heavy landing occurrence and the deck having a sound structure. As previously mentioned, the S92 operates to the 9.3 tonne S61 decks that have similar considerations.
A number of AW139 aircraft are operating to the old B212 decks as they fit under the B212 D code.
Further info can be found in ICAO annex 14 but does not mention the above dispensations. Gulf Helicopters have dispensation for their 139 fleet to operate to some decks so they might be able to help you with the process.
There is no dispensation for smaller D coded decks, I think you know that.
Hope that helps.