International classification society Bureau Veritas has introduced a new set of requirements for shaft alignment on high powered ships. The ESA notation will help ensure the integrity of the structure and shaft bearings on very large vessels such as ultra-large container ships and the next generation of very large LNG carriers.
Jean-Francois Segretain, deputy technical director, Bureau Veritas, says, “As ships grow in size and we seek more fuel economies we are now seeing vessels with shafts handling over 20MW of power driving large diameter propellers of over 30 tonnes weight. Critical attention has to be paid at the design stage to analysing the elastic shaft alignment and ensuring that the bearings and structure match the shaft response, especially when the vessel is turning. If this is not done, huge forces will either lead to bearing failure or excessive vibration. We have been called in to help solve incidences of both on new ships. This new ESA notation sets out standards of how the shaft must be analysed and the criteria the whole shaft, bearings, and aft structure must meet to avoid such problems.”
Bureau Veritas has unrivalled experience with shaft alignment studies dating back to the first ULCCs built in the 1970s. It is continually updating its expertise in this area and has developed special software for shaft analysis.
The Elastic Shaft Alignment (ESA) notation will apply to vessels with shafts in excess of 750 mm diameter, or smaller shafts handling over 20 MW of power, propellers over 30 tonnes or using synthetic bearings. The notation requires a full analysis of the criteria covering hull flexibility with respect to loading conditions, bearing material stiffness, shaft speed and oil film behaviour.
Says Segretain, “We will make this notation obligatory for ships meeting these conditions in our class, and will also make it available as a voluntary notation for smaller vessels and vessels not in BV class. Getting the shaft/structure right is critical for modern designs and we have the deepest expertise in this area available anywhere. This notation helps to codify that and guide yards with new designs.”
[mappress]