I not sure where but I did read on one website that so long as the crude price remains below $60/65 then there would be refunds or credits to onboard accounts. Maybe someone else will come up with a better response to your query.
If you have paid the fuel surcharge and the price of Brent crude oil remains below £40/bbl ($60) the before you sail then the fuel charge will be reimbursed through on-board credit. Go to the excellent Fred. Olsen website and select "our ships" and all will be explained in detail.