Cases   Techno Fysica BV is a Dutch-based engineering company, highly specialized in solving all kinds of problems related to the dynamic loading of machinery and installations. For this, several measuring and analysis techniques are used. In a series of articles, we will give examples of what can go wrong in the engineering and operating of vessels, and what errors lie at the root of the damages observed.

Case 15:  Failure analysis of an aluminum gangway which collapsed during                 transfer of passengers

This case is an example of a multidisciplinary approach used in the failure analysis of an aluminum gangway which failed during the unloading of passengers from a ship in the Rotterdam port.

A short description of the problem is as follows: The gangway was composed of four parts. The parts each had a length of approximately 4.6 meter, resulting in a total length of some 18.5 meter. The parts of which the gangway was composed were standard items. However, up to now, the maximum number used in combination was 3. This was the first time four elements were used to form a single gangway. The gangway was meant for loading supplies to the vessel and for discharging items such as empty crates garbage and so on. However, some of the passengers had discovered that it brought them closer to the buses waiting ashore, as a means of transport into the city, so, soon, the passengers all used this gangway instead of the official one (which was for more rugged and suited for the application). During the event, there was some additional information with regards to the conditions during which the failure occurred. For one, most witnesses confirmed that there were some 16 to 20 persons on the gangway at the moment of collapse. Furthermore, most mentioned that the gangway "danced" considerably. Something that was confirmed by all passengers on the gangway. Furthermore, it was noted that, just before the collapse, one of the passengers went back to the ship, probably she had forgotten something on the ship. This forced the other passengers to walk on one side of the gangway. Shortly after this, the gangway collapsed, fortunately without any significant injuries to the people on it.

During the investigation of the cause of the failure, preliminary metallurgical examination of the fracture parts did not reveal any pre-existing defects, and all observations indicated towards an overload.

However, calculations, made by the manufacturer, indicated that the gangway was operating within ample safety margins, and should not have collapsed. Material testing confirmed the requested material properties. Since the gangway was rented by the port authorities, liability for the damage was an issue, and the insurance companies involved requested a more thorough investigation into the cause of failure.

For this end, we performed a failure analysis of the failed components, and performed numerical analysis in order to find the most likely cause of the failure. Since the failure was attributable to a mechanical overload, the cause of this overload was to be established. After some deliberation with representatives of the insurance companies (3 parties were involved, namely the ship, the port, and the rental company), it was decided that the best way to perform a numerical check was by using the finite element method. This, especially since the report was to be read by insurance people also. The graphics produced by a finite element post-processing programs makes a calculation very "readable" for non-technicians.

From the numerical static analysis, it could be learned that, with four elements, and 20 average persons, the stresses were considerate, but well within what can be considered as "safe". There was some increase when the fact that all passengers were leaning towards one side because of another passengers traveling in opposite direction, but still there should not have been a problem.

Stress distribution in area of bolt holes.

1st mode of vibration:       torsion  at 2.02 Hz             2nd mode of vibration: vertical bending