Cases

Techno Fysica BV is a Dutch-based engineering company, highly specialised 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.

A Trailing Suction Hopper Dredger (TSHD) suffered from repeated fracture of the shaft of the jet pump which fractures were, as determined by  fractographical examination, clearly caused by bending fatigue. Design changes to the shaft, relating to material strength, radii and thread geometry to fit the impeller, did not restore the desired reliability. the metallographic examination did not reveal any initial material or manufacturing defect, so the cause of the failure must be related to stress in excess of the fatigue limit of the material. These stresses had to be related to specific operating conditions, as the stress according the engineering calculations were low and should not result in fatigue.

From an extensive vibration investigation it was clear that the installation was operated very close to a resonance situation where the natural frequency of the pump on it’s foundation was excited by the impeller vane passing frequency ( 3^rd order) and that the natural frequency for bending of the impeller shaft was also very close to this frequency. This resulted in resonance due to excitation of the pump shafts natural frequency (whirling).

The crew had repeatedly mentioned that it was frightening to stand close to the pump installation due to the high vibration levels.

Vibration response with 3-bladed impeller