Why would Norwegian exchange 737 landing gears?

[Rengineer]

Forgive me if this is the wrong forum to post this question, or if the answer is blindingly obvious, but this article caught my attention:
Norwegian becomes Boeing's largest landing gear exchange customer (http://www.flightglobal.com/news/articles/norwegian-becomes-boeings-largest-landing-gear-exchange-customer-374438/)

Apparently, Norwegian Airlines are exchanging lots of landing gears on their 737s. What's the deal there?

[Exup]

Landing Gears are time lifed items so need swapping out at some stage during scheduled maintenance. Nothing unusual.

[Rengineer]

OK thanks Exup. That makes it clear of course.

[Mikehotel152]

Maybe they're banging them in on short strips in the arctic circle! :p