DNV GL upgrades rules for single stern tube bearing installations
DNV GL has advanced its class rules for single stern tube bearing installations having rolled out two new class notations, “Shaft align (1)” and “Shaft align (2)” enabling customers to better manage the risk of stern tube bearing failure. The new class notations can be assigned to both new builds and vessels in service in conjunction with propeller shaft withdrawal.
Classification rules range from covering shaft alignment to achieving an acceptable distribution of loading on the shaft bearings and lubrication of the aft bearing, taking into consideration the bending moment induced by the propeller during operation. However, during turning manoeuvers at higher ship speeds, exaggerated propeller bending moments can occur, potentially resulting in a reduced shaft-bearing contact area and an exponential increase in local pressure and thermal loading. This could cause damage to the aft bearing. Most of the reported bearing damages have been observed in the aft-most part of the aft bearing damages have been witnessed in the aft-most part of the aft bearing, during a starboard turn on a right-handed propeller installation.
In the “Shaft align” class notations and the revised main class requirements for single stern tube bearing installations, a multi-sloped bearing design is mandatory. This is further supplemented by an additional evaluation of the aft bearing lubrication condition considering an increased propeller-induced hydrodynamic bending moment on the aft bearing in the downward direction.
Geir Dugstad, Director of Ship Classification at DNV GL – Maritime said “With this revision to the DNV GL class rules and the two additional class notations, we will enable owners to enhance bearing performance, and benefit from a longer lifetime in their stern tube installations.”