I asked this question about J range before. As I understand it the J will routinely be able to transit Lye Akr Lye, but there will be occasions, heavy load, winds etc when this is very tight. I have been told that external tanks were available at purchase for £1 mil approx per ac, but to retrofit would be more expensive. OBIGGS has its limitations. More modern system are continual gas generators, which solves the problem.