Offshore structures such as wind turbines, wave machines and tidal stream turbines are mounted in a previously located foundation including a socket. The foundation includes an upper body with a conical inner surface which acts as a guide for inserting a leading end of the structure into the socket. The conical surface also provides a reaction surface for setting the vertical alignment of the structure when inserted in the socket. Clamps are provided supported by an A frame for controlled lowering of the structure into the socket.