Qatar Navigation warns of challenging 2015

* Q1 net profit 365 mln riyals vs 349 mln riyals year ago * Investment business earnings decline 31 pct * Chairman warns of "challenging" year (Adds detail, context) DUBAI, April 27 (Reuters) - Qatar Navigation (Milaha) warned of a challenging 2015 as conditions in its offshore business remained weak and its investment business was exposed to instability in local equity markets. "While we hope to maintain the momentum, we know that general market conditions for our offshore segment are weakening