NOVELTY - The system has power conversion module (13) coupled with a frequency conversion module (7) by a power conversion module mounting (6) and a frequency conversion module mounting. The power conversion module is common to turbine assemblies (2). One of the module mountings is remote from the turbine assembly. The module mountings are installed on a seabed. The modules comprise controllable buoyancy, and are retrieved from and returned to the mountings.    USE - Modular water current turbine system for use with a submersible modular turbine array (claimed) for a heavy offshore oil and gas lift ship in tidal flows.    ADVANTAGE - The turbine assembly can be repaired and maintained on an electrical module in an easy manner with low manufacturing cost as the electrical module is small in size and light in weight for containing components with a shorter mean time to failure, where the electrical components are an integral part of the powertrain. The system ensures modularising sub-systems of the energy conversion chain at low cost, provides frequent access to the items of the system with low mean time before failure (MTBF) so as to maintain availability of the system at a high level while minimizing the exposure to operation cost escalation due to costly operations with large high day rate offshore oil and gas ships. The system reduces downtime and lifting costs by the turbine system of a modular design so as to allow easy retrieval and replacement for reliable components. The assembly ensures forcing frequent lifting arid lowering of the turbine and a powertrain and the associated electrical component to and from the subsea mounting structures in strong tidal currents, so that smaller lighter modules can be lifted frequently.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a submersible modular turbine array.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a modular water current turbine system.    Turbine assemblies (2)    Power conversion module mounting (6)    Frequency conversion module (7)    Vessel (10)    Power conversion module (13)