Section 147.843 - Thunder Horse Semi-Submersible safety zone(a)Description. Thunder Horse Semi-Submersible, Mississippi Canyon 778 (MC 778), located at position 28°11'26" N, 88°29'44" W. The area within 500 meters (1640.4 feet) from each point on the structure's outer edge is a safety zone. These coordinates are based upon [NAD 83].(b)Regulation. No vessel may enter or remain in this safety zone except the following: (2) A vessel under 100 feet in length overall not engaged in towing; or(3) A vessel authorized by the Commander, Eighth Coast Guard District.CGD08-05-019, 70 FR 43770 , July 29, 2005