The German BSU has shared lessons learned following two ships collided on a canal after loss of steering.
What happened?
Two ships were due to pass each other on a narrow channel. Shortly before the encounter, a malfunction occurred in the steering system of one of the ships. This malfunction caused the rudder to veer sharply to port. Switching to emergency steering was unable to correct this rudder position.
As a result, the ship swung in this direction and collided with the bow of the oncoming vessel shortly afterwards. The other ship was unable to take evasive action. Crew members on both ships suffered minor injuries. The ship that was hit suffered severe water ingress in the fore section. The ship that caused the collision also suffered damage to its fore section.
Why did it happen?
The technical cause of the rudder system malfunction could not be determined with certainty. The entire system had a long history of repairs.
Hydraulic oil was used in the steering system via a 4/3-way solenoid valve to control/adjust the axial piston pump. It was kept in an oil circuit that also included the hydraulic oil tank. During the investigation, it was found that the hydraulic oil was not filtered when it was taken from the tank. No filter was provided for in the design.
Foreign objects were found in the hydraulic oil system. The impairment of the 4/3-way solenoid valve and/or the control piston of the axial piston pump as well as the function of the safety valves (pressure relief valves) by these foreign objects is considered a possible cause of the accident.
What can we learn?
- For safety reasons, the entire hydraulic oil flow from the tank in a rudder system should be filtered.
- Regular laboratory tests of the hydraulic oil are helpful in detecting wear processes. However, they do not provide any indication of possible foreign objects in the filters or in the tank.
- Tanks and filters for hydraulic oil in steering systems should be checked regularly for foreign objects. If foreign objects are found, the system should be cleaned to prevent malfunctions of individual components.