@notnevernotevernotnohow
OP, these threads always attract the extreme type of pet people, so you can't win. In reality there are very few people who could or would cope with an elderly cat covering their house in urine. But rehoming is not really an option, no-one will take her on. Probably time to pts, at 15 it's likely that something is going on that can't be fixed anyway.
This 100% ^
It's a huge mistake putting something like this on here, as you will get the furious angry 'how very dare you' and 'you are a monster!' kind of responses. When in actual fact, as the poster I have quoted says, many people will find it incredibly hard to live with a pet who pisses over everything, even though they claim they would be just fine and dandy with it, or 'it wouldn't happen, because THEY would get the problem fixed!' 
What makes me laugh are the posts saying 'how would you feel if your children put YOU out to pasture or gave up on YOU when you become an inconvenience, and put YOU in a home?'
PMLS that's what many people do with their elderly parents when they become very infirm, and they CAN'T cope with them, and find them difficult to handle; they put them in a home!!! So people need to climb down off their moral high horses, and quit acting like a cat pissing all over their house, wouldn't bother them, and like they would never put their elderly parents into a home if they became very infirm and frail.
As has been said, the OP needs to consult the vet. They will be able to advise her best, and may even be able to help the peeing issue. If nothing can be done, the vet can advise the OP what her options are. I am sure the OP isn't going to just dump the cat in the middle of the fast lane of the M6, so don't get your knickers in a bunch!