C4/2009M M/S SILJA EUROPA (FIN), breaking of the starboard rudder shaft in the Aland Archipelago on 22 November 2009

The M/S SILJA EUROPA was on her way from Stockholm to Turku on 22 November 2009 when irregularities in the manoeuvring of turns began to occur west of Sottunga at approx. 15:17. There were approx. 1660 persons onboard the vessel. At 15:40 it was decided that the vessel would be manoeuvred to a nearby extension of the shipping channel, and the Master was called to the navigating bridge. He started to manoeuvre the vessel by using thrusters and succeeded in stopping the vessel in the extension. The vessel was later taken to the Airisto open sea with tug assistance. There a diver concluded that the difficulties in manoeuvring were caused by the breaking of the vessel’s starboard rudder shaft.

On the basis of the analysis completed in the investigation, it can be concluded that the rudder shaft broke at approx. 15:10. The breaking of a rudder shaft does not cause any direct indication on the navigating bridge; the failure can be detected only indirectly by observing the vessel’s motion state, its changed manoeuvring behaviour and the manoeuvring variables. The bridge crew noticed manoeuvring irregularities in the first major turn after the failure from approx. 15:17 onwards. The voyage was continued and the bridge crew grew gradually aware of the gravity of the situation as they got more observations of the manoeuvring difficulties. The breaking of a rudder shaft is, however, highly unusual so it was not even considered to be the reason for the manoeuvring difficulties. The situation was so unclear that it took time for the bridge crew to gather observations before the decision to interrupt the voyage was made. Even if it took time to understand the severity of the situation and the voyage was continued with reduced manoeuvrability, the crew, who had in simulator training got readiness to steer a vessel with irregular behaviour, managed to keep the vessel in the fairway and finally to stop her in a safe manner.

In the docking after the accident in 2009 it was found out that the starboard rudder shaft had broken because of bending fatigue stresses. The extensive corrosion on the pintle bearings housing of both rudders had been repaired with cast epoxy filling in the docking in 2004. The corrosion had caused in the pintle bearings a significant empty space, a transverse free play, which might have made the rudder shafts bend more than intended. In the docking in 2009 it was discovered that the cast epoxy filling had disappeared entirely of the starboard side pintle bearing thus the large transverse free play was now only on this rudder. The changing stresses generated by the rudder forces on the shaft then considerably exceeded the design values. In scheduled traffic rudder forces recur similar each service period. The stresses caused by these forces depend, however, on the transverse free play of the pintle bearing. When the free play increases, the stress in the rudder shaft grows.

Three factors working simultaneously made the bending fracture of the rudder shaft possible. The two most important of these are anything but common. Firstly, in the production stage a sharp, 0.3-mm-deep notch was left on the starboard shaft and, secondly, there was sustained galvanic corrosion in the bearing housings caused by the structure of the pintle bearings. These two together would have been enough to cause the failure, but the breaking process was significantly accelerated by a third factor, i.e. the resonant vibration of the rudder.

The fatigue breakage developed in 17 years in three stages. In the first stage the housing of the pintle bearings of both rudder shafts corroded because of galvanic corrosion. This was made possible by the structure of the housing, in which a bronze bush and the cast steel of the housing were in contact without any insulation. After the housing had corroded around the bushes, they could rotate. The rotation wore the corrosion rust and this accelerated significantly the increase of free play. Because of the corrosion, the transverse free plays of the pintle bearings had for quite a while been manifold when compared with the rated value, which had caused the stresses in the rudder shafts to be higher than normally. The first stage of the process used a large part of the fatigue life of the starboard shaft, up to 85% on the basis of the calculations made in connection with the investigation.

The corrosion was not detected until in the docking in 2004. The pintle bearing housing was repaired by filling the corroded space with cast epoxy and the bearing bushes were replaced with new ones. Additionally, the hull areas (the surface of the cast iron) around the upper bearings of the rudder shafts were inspected from outside. The hairline cracks of the early fatigue breakage then possibly existing in the rudder shafts were impossible to be detected, because the inspection did not include the rudder shafts.

In the second stage of the process, the corrosion of the pintle bearing housings continued after the installation of cast epoxy, but the process was slower because of the insulating effect of the epoxy. In the docking in 2007 the epoxy still looked intact.

In the third stage the cast epoxy of the starboard pintle bearing broke entirely. The bush came loose and could now rotate in the housing due to the axle rotation. Therefore a large free play was generated in the pintle bearing for the second time, the stresses in the rudder shaft became high and the shaft broke. According to the calculations, the third stage only took a couple of days.

The Safety Investigation Authority of Finland recommends to Bureau Veritas and other classification societies that the measuring methods concerning clearances be specified and the method be entered into the inspection report of the classification society, and that the reparation history of a vessel be used as the basis for deciding whether a more detailed inspection of the steering gear is in order. In addition, it is suggested that classification societies consider adding an inspection of a removed rudder shaft at specified regular intervals in their rules. The use of cast epoxy when repairing a pintle bearing must be a temporary solution and the reason for the need for repair must established in order to plan and realise a correct repair method.

It is recommended to the shipping company that simulator training should be developed further in such a way that the preparedness of the trainees to work out and analyze in a systematic way a situation in which the maneuvering systems of the vessel do not function in a normal way will improve. The objective must be the development of gathering data to support the decision-making on whether a voyage should be interrupted or continued.

C4/2009M Report (pdf, 5.5 Mt)

 
Published 9.11.2012