Complex sentence: The Fremantle Whaling Company in 1837 requested that tunnel be dug through Arthur Head to High street, as part of the agreement they constructed a break water to protect shipping up to 150 tons.
Split into simpler sentences: The Fremantle Whaling Company in 1837 requested that a tunnel be dug through Arthur Head to High street. As part of the agreement they constructed a breakwater to protect shipping up to 150 tons.