The National Transportation Safety Board (NTSB) has published an investigation report regarding an incident where debris in the steering system of the bulk carrier American Mariner caused it to collide with navigational aid in the St. Marys River, Michigan, on 28 March 2024.
The incident
On March 28, 2024, about 0018 local time, while transiting upbound in the St. Marys River, about 25 miles south of Sault Ste. Marie, Michigan, the bulk carrier American Mariner experienced a steering failure and struck the Munuscong Channel Junction Light, a 31-foot-diameter cylindrical concrete structure with a navigation aid on top (see figure 1 and figure 2).1 The vessel began taking on water; pumps stabilized the flooding. None of the 18 crewmembers on board were injured, and no pollution was reported. Damage to the vessel was estimated at $800,750, and damage to the Munuscong Channel Junction Light was estimated at $1.25 million for repairs.
Analysis
On March 28, the bulk carrier American Mariner struck the Munuscong Channel Junction Light in the St. Marys River near Sault Ste. Marie, Michigan, after the steering gear locked the vessel’s rudder at 20° left. The vessel was on its first voyage of the season following its winter layup period, during which hydraulic technicians performed annual preventative maintenance on the ship’s steering gear hydraulic system and electronic technicians installed a new steering electronic control system stand on the bridge. Coast Guard inspections and class society surveys, which included testing and inspection of the new steering control system, were completed. Before departing on March 26, the crew completed predeparture functional steering tests.
During the voyage leading up to the contact, alarms for the vessel’s steering system sounded intermittently on the bridge while the crew was operating in hand steering and running both steering pumps. Crewmembers investigated the alarms but could not determine the cause, and the rudder appeared to properly respond to input commands. Additionally, the steering hydraulic system was found to be hunting between left and right positions while the two pumps were running.
Once electronic technicians recalibrated the steering control system after the casualty, the steering alarms and hunting conditions no longer occurred—indicating likely issues with the newly installed control system’s calibration of parameters. Immediately after the casualty, crewmembers inspected the steering system and determined that the no. 1 control motor (which controlled the stroke of the main hydraulic pump) within the vessel’s steering system had failed. The motor was dissembled, and a piece of black, pliable debris was found to be lodged in between the rotating gears of the control motor, preventing it from rotating properly.
Once seized, the control motor’s corresponding linkages to the main steering pump remained stationary, preventing the main steering pump from changing output pressure and locking the rudder in its last ordered position at 20° left. Once a replacement control motor was installed, the steering system responded properly to steering commands from the bridge.
During a postcasualty inspection of the hydraulic sump, technicians noted a “fair amount of contamination of unknown materials, both ferrous and non” within the hydraulic oil. The suction filters were rated at 125 microns, meaning that particles larger than 0.125 millimeters (0.005 inches) would be captured in the mesh of the suction filter elements. The debris found inside the no. 1 control motor ranged from about 2–4 millimeters (0.08–0.16 inches) in length. Therefore, since the debris was did not originate from within the hydraulic sump of the steering system.
A FTIR analysis found that the two black pieces of debris from the steering system’s no. 1 control motor and an exemplar O-ring (replaced on the directional valve during the winter layup period the month before the casualty) were PVDF; however, results from another type of analysis, EDS, differed. Additionally, the colors of the materials differed: the debris found in the no. 1 control motor was black and the exemplar O-ring was brown. Because the debris was not an exact match to the exemplar O-rings, it is possible that a similar type of PVDF O-ring used previously within the hydraulic system or a material from another component in the system downstream of the suction filters could have been dislodged during operation.
Therefore, the origin of the debris found in the no. 1 control motor could not be positively identified, which indicates that the debris likely came from another hydraulic system component that was circulating through the unfiltered portion of the hydraulic control system. It cannot be determined whether conditions that triggered the alarms from the newly installed control system and/or the hunting condition contributed to debris that caused the failure of the no. 1 control motor.
The additional heat generated by, and material wear on, the steering system as it constantly hunted for position while running on two pumps in an attempt to adjust to ordered steering commands could have been potential factors. Within 20 seconds of noticing the steering gear locked at 20° left, deck crewmembers brought the engine from full ahead to full astern and dropped anchors. The bow thrusters and stern thrusters were not online during the river transit, nor were they required to be.
However, even if running, with the vessel’s speed about 13 mph, the thrusters would not have effectively changed the vessel’s heading. Although the deck crewmembers quickly used all available means to avoid striking the Munuscong Channel Junction Light, they could not stop the vessel or make any corrective course changes.
Conclusions
Probable Cause
The National Transportation Safety Board determines that the probable cause of the contact of the bulk carrier American Mariner with the Munuscong Channel Junction Light was O-ring-type material debris in the steering gear system’s hydraulic oil becoming lodged within a control motor, which caused it to seize, resulting in the rudder locking at its last ordered position.
