Clearing the PGB of a large cement patio

Does anyone have experience clearing large amounts of hard cement from the internal framework of a PGB?  We have a wellhead to recover and I'm concerned about dropped objects when we get the PGB back to surface so I'd like it to be as clear from cement as possible before recovering it.

The cement is 20 years old, the well has been plugged for almost as long.  I've considered both an ROV mounted HP water jet and similarly mounted percussion hammer but while each looks to be effective over time neither looks to be particularly quick. I'm also considering just having at it with a rock-bit on drillpipe.  Any pointers out there as to what may be the best way to get this done?

Thanks

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