The National Transportation Safety Board (NTSB) has published an investigation report into an incident where a cargo vessel ran aground following an anchor chain failure.
The incident
On 4 October 2023, about 0328 local time, the cargo vessel Bonnie G ran aground in the Caribbean Sea, less than 1 mile south of Cyril E. King Airport on St. Thomas, US Virgin Islands, while maneuvering following an anchor chain failure in heavy weather. The twelve persons on board abandoned the vessel and were rescued by a US Coast Guard small boat. The Bonnie G remained hard aground for several weeks until salvors could successfully refloat it and tow it to a dock. There were no injuries, and no pollution was reported. Damage to the vessel was estimated at $1.5 million.
Analysis
While the cargo vessel Bonnie G was anchored, waiting out poor weather about 1 mile south of Cyril E. King Airport on the island of St. Thomas, its anchor chain parted. As the captain attempted to navigate the vessel away from shore, the vessel grounded. As the weather deteriorated further, the captain and crew abandoned the vessel and were later rescued by the Coast Guard.
Before departing St. Croix, the captain of the Bonnie G reviewed the weather forecast for the vessel’s intended route to St. Thomas. The forecast indicated that a small craft advisory (triggered due to the potential for seas greater than 7 feet) was in effect. The winds, which were the greatest concern to the captain and vessel owner, were forecasted to be between 10–15 knots and gusting to 25 knots after midnight (by which time they expected to be tied up to the dock). However, when the vessel arrived in Crown Bay, the winds were sustained at 27 knots—which was higher than what had been forecasted and unusual for St. Thomas. In addition, a loaded barge had broken free of its moorings and was partially blocking Bonnie G’s mooring location.
Given the barge blocking the Bonnie G’s docking location and the worse than expected weather conditions, the captain made a reasonable decision to anchor the vessel until the weather improved. Once the decision was made to anchor the Bonnie G and the captain navigated the vessel into position, the port anchor was deployed with 1.5 shots (135 feet) of chain. He likely based his decision to pay out 135 feet of chain on his erroneous assessment that the water depth in the area was only “around 7 to 8 meters” (23-26 feet). In actuality, the navigation chart for the location indicated the water depth was 68 feet.
Considering the water depth and weather conditions at the time of anchoring, the Bonnie G captain did not deploy enough anchor chain for a secure anchorage (inadequate scope). The weight of the anchor chain resting on the bottom provides additional holding force and absorbs the tension in the chain resulting from the vessel’s natural Movement in seas. If not enough anchor chain for the depth of water (inadequate scope) is used, there is less holding power, and the anchor may be subject to shock tension. In these conditions when there is inadequate scope, the anchor is more likely to drag, and in extreme cases, the chain may be damaged or break.
About 3 hours after anchoring, the Bonnie G began to drift north-northeast, in the direction of land. When the AB attempted to heave the anchor, he discovered that the anchor chain had parted at the first shackle, and that the anchor was no longer attached to the vessel. The inadequate scope of anchor chain was likely a factor in the anchor chain parting. When anchoring the vessel, the captain relied on satellite imagery screen shots sent to him by the vessel owner to find the anchorage rather than using onboard charts and navigational equipment. Additionally, after anchoring the vessel, the captain did not look at the chart and identify the charted rock just 0.3 miles northeast of the vessel’s position (the direction the wind would set the vessel when drifting).
The captain of the Bonnie G was attempting to keep his vessel and crew safe after encountering heavy weather at night, experiencing a broken anchor chain, and then observing his vessel drifting toward shore. However, because the captain did not use the available onboard navigational chart to identify a rock nearby, when maneuvering after the anchor chain failed, he inadvertently steered directly onto the rock and grounded the vessel.
As a result of the vessel grounding, the hull was breached just forward of the engine room. Floodwater filled the midships passageway and then poured into the engine room through the open watertight door.
Watertight doors are required to remain closed while not being traversed and while a vessel is underway so as to maintain watertight bulkhead subdivision and prevent progressive flooding (from a flooded space to another space). The forward watertight door in the engine room of the Bonnie G was open during the casualty events, and it was not secured before the crew abandoned the vessel. Had the watertight door been shut as required, it is likely that progressive flooding of the engine room and aft spaces would not have occurred, limiting vessel damage.
Lessons learned
When anchoring, mariners should review navigational charts and other sources of local information, such as the US Coast Pilot, to become familiar with nearby hazards. It is important to have this information available in heavy weather conditions, when there is heightened risk of drifting and crews may need to get a vessel underway on short notice.
