Once the chair is fully disassembled and hydraulic pump is removed ( underneath 4 bolts), flip the base upside down and you will see the screws holding the ring on. They will need some penetrating oil applyed a few times and set for a few days to soak in. they are pretty corroded and gummed up from years of being so close to the floor and being exposed to floor washing and grime etc. so they will be rusted in and a pain to get out. With the base upside down you can apply the force required to remove them, try tapping the screwdriver like a chisel in the screw slot to help loosen the screws.