I mistakenly wrote ...Be8, i meant ...Bd8. Anyway, i had already looked at Ba5 and i'm sure white's bishop would be lost without a4.
17. Rad1 b5 18. Ba5 Bd8
19. Bc3 and if black plays Bf6 we're back at the beginning of move 18. If black plays Bc7, it would be different but a4 would still need to be played to save the bishop.
19. Rxd8 Rxd8. If white doesn't recapture, then black will be up the exchange, and white temporarily lost control of the d-file and the two bishops are preoccupied. 20. Bxd8 Qxd8 and a4 will need to be played to save the bishop.
19. Bxd8 Rxd8 is similar to 19. Rxd8. The black queen will end up at b6 and a4 is needed to save the bishop.
19. Qb4 Bxa5 20. Qxa5 Qb6 If 21. Qxb6 axb6 and the bishop remains trapped without a4. 21. Qa3 b4 and the white queen can no longer defend the bishop.
@JG27Pyth - I don't use an engine so there are possible holes in my line. I looked at Ba5 and thought it could be refuted by ...Be8, and the black queen will eventually get to b6.