Subsea manifold system

A modular, subsea hydrocarbon production hub 36 comprises a primary manifold 22 and a plurality of co-located extension structures 26, 30, 34. The primary manifold 22 has first connections 23 for connection a top-side facility 21, second connections 27, 35 for connection to at least one extension structure 26, 34, and third connections 25 for connection to at least one well 24. The extension structures 26, 30, 34may comprise a secondary manifold 26, 30, having first connections 27, 31 for connection to the primary manifold 22 or another secondary manifold 26, 30, second connections 31 for connection to at least one further extension structure 26, 30, 36, and third connections 28, 31 for connection to further wells 29, 32. The extension structures 26, 30, 34 may also comprise one or more functional units, such as a pumping unit 34 or a separator unit 51.