it isn't really an sub tender. it is dutch called torpedowerkschip, I don't know how you say it in english, but literally translated it is 'torpedo work ship'. it is mostly for practising purpose: it is used as an target for torpedo's, loading torpedo's on subs and lauching them against the subs. I guess that the dutch submarine's are that efficient because of this ship
the ship isn't even armed (unless in wartime, then it is armed with 2 20 mm's, but I have no idea were these are placed, as this ship was never in a warzone as far as I know.
EDIT: found the right name: it is called an Torpedo Trials Ship