Well centre assembly for an offshore drilling rig and associated method. There is described a well centre assembly 10 for an offshore drilling rig. The well centre assembly 10 comprises a diverter assembly 16 and a movement control system 18. The movement control system 18 is operable between the diverter assembly 16 and a supporting structure 12 of the rig. In use, the movement control system 18 controls or permits relative movement between the diverter assembly 16 and the supporting structure 12. A connector such as a riser 34 connected to the diverter assembly 16 during deployment, while deployed or during retrieval of the riser 34 may experience external forces such as from tidal, wind and/or wave movement at a water surface 50, which may cause the positioning and/or orientation of the riser 34 to vary. The movement control system 18 may control the movement of the diverter assembly 16 so as to compensate for movement of the riser 34.