Apparatus for providing an offshore drilling operation in a sea, the apparatus comprising a drilling derrick support structure (2) having a base (4) to rest on the sea-floor (5) and a deck (7) to support a drilling derrick (1), the structure being in the form of a concrete block (2) having a hollow cavity to enable the block (2) to be floated to the site where drilling is to take place and being adapted to receive ballast into the cavity in such a way and in a sufficient quantity as to cause the block (2) to sink base-first and rest upon the sea-bed with the deck (7) clear of the surface of the sea, a drilling derrick (1) for mounting on the deck (7) of the block (2) and a tender assist barge (3) for transport to the site in down to 2 m of water to provide services, supplies and equipment to the drilling derrick (1).