SriLankan Airlines is joining the Oneworld Alliance on May 1, 2014.
Oneworld Alliance members always have a promo offering double award miles on new member’s flights for few weeks after the joining.
The period during which you should be able to earn double award miles on SriLankan Airlines flights is from May 15 to June 30, 2014.
You should check your airline’s website to register for this offer. Many airlines likely don’t have it up yet, but JAL’s version can be accessed here.
These double miles periods seems to vary. The period during which you can earn double award miles on partner Oneworld Alliance loyalty plans is from May 15 to June 30, 2014.