11kV /13.8 kV System Grounding Methods for Offshore FPSO

Could you please somebody explain advantages disadvantages of following grounding methods
1. Individual grounding of each generator neutral
2. Common grounding through a grounding transformer on the busbar
i) through zig-zag grounding transformer
ii) through wye/delta or wye/open delta grounding transformer

I have seen both methods have been used in FPSOs (I have only seen wye/ open delta not wye delta).