It's ~ 12t derrick for loading goods & other stuff in ports. Also it might be used to launch the MOB (which is behind the port funnel as you can see on other shots) & store it back. Since Neftegaz-51 the design was modified & the derrick was replaced by a 12t crane.

Agreed Denis,But wait! Is that a HLD I see on the AFTER Deck?
What on earth will that do for the stability whilst on water, Heaven knows!
(Can't wait to be "informed")

An utter beautifulness of a classic offshore supplier, not like monsters that are recently being built by STX.

1a *** ;-)