Failure of a Concentric Pipe for a Controllable Pitch Propeller System

摘要

Propellers for ships can be distinguished into two main types: fixed pitch propeller and controllable pitch propeller (CPP). The CPP design is an alternative for improved efficiency, especially in the need of a wide range of thrust and load levels, while maintaining an optimum shaft speed, as in the case of diesel or gas turbines. A dynamic positioning ship presented a problem on its CPP system after 8 months of operation. By means of fractography, microstructural, microhardness, and chemical analyses, it was possible to determine that fatigue was the main responsible for the failure. Fatigue cracks were initiated at welded connections between the wire spacers and concentric pipes, positioned inside the hollow shaft of the propeller. The stress concentration caused by a sharp defects like lack of penetration and lack of fusion and the geometric restriction of this type of weld contributed to a shorter fatigue crack initiation phase and premature failure.