Audemars Piguet - A Gentleman's Stainless Steel Royal Oak Chronograph
possibly the model 15510ST.OO.1320ST.06 assuming it measures approximately 41mm stainless steel case with fixed octagonal stainless steel bezel, scratch resistant and anti-reflective sapphire crystal, blue dial with mega-tapisserie pattern, silver hour markers, window back, and automatic movement, case No:K 43 663, on a stainless steel bracelet with AP double folding clasp.

These are faked in very large numbers but a reputable secondhand watch dealer would immediately verify whether fake or genuine. If fake it's insurance value is $450 approx.