Each ground wire must be able to handle the full OCPD.
The reason is that in the event of a short in one of the pipes... ALL of the fault current floods the short, which is going to be huge. The grounding conductor in the undamaged pipe has to be able to handle the surge.
Example, a forklift or crane loses a heavy load that crushes surface mounted pipes in parallel... earthquakes... vehicle strikes... errant backhoes... subsidence crushes pipes... landslides...
If something can go wrong, it will. The instantaneous fault energy that is released from a paralleled XFMR faulting is truly astounding.