Now the Norwegian Public Road Administration (Vegdirektoratet) has updated the number of dimensions in the general transitional rules for the requirement of winter tires for heavy vehicle trailers that will take effect January 1, 2015,
see link: http://www.vegvesen.no/en/Vehicles/Seasonal/Tyres+and+snow+chains/Tyres+on+heavy+vehicles/transitional-arrangements-for-winter-tyres
If you have a tire size where it is not possible to get your snow tires, but that is not on this list, it is necessary to apply for a special dispensation.
Information on how to apply for special dispensation, and application forms are available on Vegdirektoratets website.
The definition of winter tires for heavy vehicles and their trailers are also on Vegdirektoratets website, see link: http://www.vegvesen.no/en/Vehicles/Seasonal/Tyres+and+snow+chains/Tyres+on+heavy+vehicles
NB! The Public Roads Administration emphasises that even though you are exempted from the new requirements due to the transitional arrangements, all vehicles in use must have sufficient road grip for the weather conditions. Sufficient road grip must if necessary be secured by the means of winter tyres with or without studs, chains or similar traction devices. Vehicles that does not have sufficient road grip may be prohibited from use in accordance with the Act of 18 June 1965 relating to road traffic (The Road Traffic Act) section 36.