1. Cut a slot in the riser pipe with circular saw
2. Slowly insert steel plate to gradually block most of the bore, slowing crude flow rate, gradually building backpressure.
3. With flow rate throttled down, do a successful top kill
Minor modification of a grapple like one below could attach an apparatus that could insert a steel plate into the riser pipe. The grapple is needed to anchor the operation.
Thanks to Dymax Inc for posting the pic.
Additional grapples can and should be fixed to riser pipe and chained to weights on ocean floor to provide anchoring support for when pressure builds in the riser pipe so the riser and BOP don’t get pulled off the well casing pipes.
With a steady and slow decrease of diameter, the pressure will build…
With a decrease in flow rate, all the drilling mud will not get “washed” out.