NTSB: Loss of Tow-Line Shackle Pin Led to Sinking of U.S.-Flagged Tug in Pacific
The NTSB released a Marine Accident Brief on Thursday detailing its investigation of the sinking of the towing vessel Mangilao on August 5, 2019, about 800 miles northwest of Guam.
No pollution or injuries were reported as a result of the sinking. The sunken vessel, however, has not been recovered.
The US-flagged Mangilao, a 114-foot-long tugboat, was being towed from Guam to a drydock in Subic Bay, Philippines, by the US-flagged Chamorro, a 97-foot-long tugboat, when the accident occurred.
Both vessels were owned and operated by Cabras Marine Corporation. No one was aboard the Mangilao during the tow.
According to the NTSB, the Mangilao was being towed behind Chamorro on a 2-inch wire rope with approximately 1,000 feet extended; a 14-inch-diameter-by-60-foot PolyDac plaited 8-strand hawser; a 1.25-inch chain terminal; and two anchor-type shackles connecting each part—one 35-ton shackle closest to the Chamorro and one 50-ton shackle to the 1.25-inch chain closest to the Mangilao.
From the NSTB’s Marine Accident Brief:
During the investigation, the captain of the Chamorro told investigators he believed the chain, the one from the bitt on the foredeck of the Mangilao, was not long enough to clear the bow fendering and the shackle pin likely worked loose from repeated contact with the fender.
According to the NTSB, the recovered shackle was bent, indicating that the load was uneven for a time, and, as the pin was working itself loose from the shackle, it may have fallen or snapped off, resulting in the Chamorro immediately disconnecting from the Mangilao.
A U.S. Coast Guard marine inspector completed a dead ship movement inspection before the Chamorro’s departure, and according to the marine inspector, a survey was conducted of the primary and emergency towing arrangements and verification that the exterior structure of the vessel was watertight, the NTSB said.
In its analysis the NTSB indicated that had the chain from the Mangilao’s bow been longer and the shackle extended out beyond the bow fender, the chain, rather than the shackle, would have contacted to bow. This likely would have prevented the shackle pin securing mechanism (cotter pin) from failing, and the tow would have remained connected, the NTSB said.
The NTSB determined the probable cause of the sinking of the Mangilao was the failure of the Chamorro’s towing arrangement due to the loss of a towline shackle pin, which left the Mangilao adrift and resulted in the ingress of water from boarding seas in a developing typhoon.
The NTSB’s Marine Accident Brief can be found here: MAB-20-33
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