Complex sentence: Upon delivery of her human cargo, "Lord Sandwich" sailed to Newport, which had be occupied by British forces in December 1776, where she was retained at anchor and intermittently used as a prison ship under the British flag.
Split into simpler sentences: Upon delivery of her human cargo, "Lord Sandwich" sailed to Newport, which had been occupied by British forces in December 1776. There she was retained at anchor and intermittently used as a prison ship under the British flag.