We have a 4000 t lpg storage tank, and have a requirement to changeout the primary isolation valve on the 8" relief valve riser. Is the equipment for this available on the market or do we have to develop our own equipment and methodology?

If there's room, you could look at stoppling the line.  TDW is one company that does this.  It involves installing a hot tap fitting between the tank and your block valve and then inserting a temporary plug in the line.  Then, the block valve can be removed and replaced and the plug is then removed.

However, you might not have physically enough room to do this.

There's also the technology of plugging the line by freezing the contents using liquid N2 on the outside.  While I know it's done for hydrocarbon services (and others), I suspect you can't do this for piping in the vertical as your piping likely is.

If this valve is leaking at the end flange or bonnet flange, you could consider "Furmaniting" the flange to stop the leak.  The valve could then be replaced on your next outage.

If it is a packing leak, the vessel should be taken out of service and have the valve replaced.

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