Failure of the towing arrangement caused the sinking of the towing vessel Mangilao towed by the towing vessel Chamorro off Guam, in August 2019, while in heavy weather conditions, according to a preliminary investigation report by the NTSB.
The incident
On August 05, 2019, about 0439 local time, the towing vessel Mangilao was under tow by the towing vessel Chamorro, about 800 miles northwest of Guam in the Pacific Ocean, en route to a drydock in Subic Bay, Philippines, in a developing storm.
The towline to the unmanned Mangilao parted, and the vessels separated.
The Chamorro retraced its route and found the Mangilao taking on water, and the Mangilao eventually sank.
The Chamorro continued to the Philippines with its crew of 10.
There were no pollution or injuries. The Mangilao was lost, with an estimated value of $437,227.
The National Transportation Safety Board determines that 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 captain was aware of the impending tropical storm, but at the time he made the decision to get under way, a typhoon had yet to be forecast. Meanwhile, the crew stated, and the vessel’s logbook confirmed, that they monitored the tow regularly during their watches. Although they were in heavy weather conditions throughout the transit, they said there were no issues with the Chamorro or Mangilao between the departure on July 29 and the morning of August 5, aside from restricted visibility due to the rain and sea conditions.
According to NTSB, it is likely that the chain from the bitt on the foredeck of the Mangilao was not long enough for the attached shackle to clear the fendering on the bow. In heavy seas, as the vessel pitched and the bow of the vessel lifted up over the waves, the shackle likely made repeated contact with the fendering. Around 0400 on August 5, the towline’s 50-ton shackle (closer to the Mangilao) opened, causing the towline to drop from the Mangilao.
Crew statements and the recovered shackle indicated that at some point, the shackle’s securing cotter pin broke or worked loose, thereby likely allowing the shackle bolt’s securing nut to work loose.
The bend in the recovered shackle indicated the load was uneven for a time, which could have occurred when the pin was partially retained in only one ear, bending the ear with the pin remaining inserted. Once the pin worked out, or snapped off, the Chamorro became immediately disconnected from the Mangilao.
Had the chain been longer and the shackle extended out beyond the bow fender, the chain, rather than the shackle, would have contacted the bow. This likely would have prevented the shackle pin securing mechanism (cotter pin) from failing, and the tow would have remained connected.
Because the towline remained intact for the first 4 days of the voyage, it is likely that seas were able to board and began to slowly flood the vessel through fittings on deck. It was known that the watertight integrity of the vessel was suspect, as evidenced by the company’s plan to replace the watertight doors and hatches. Boarding seas may also have dislocated one or more of the watertight fittings, leading to more rapid flooding. If the Mangilao was flooding, the added water weight would have put greater stress on the towing components. Once the towline arrangement for the Mangilao failed, leaving it dead in the water, it is likely that seas more easily boarded the vessel and continued to flood it, leading to its eventual sinking.
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