DNV GL has revised its class rules for single stern tube bearing installations and introduced two new class notations, to help shipping companies manage the risks associated with stern tube bearing failures.
Shaft alignment rules are designed to permit an acceptable distribution of loading on the shaft bearings and lubrication of the aft bearing, taking into consideration the bending caused by the operation of the propeller. However, according to DNV GL, when ships turn at higher speeds, there is an exaggerated propeller bending, potentially resulting in a reduced shaft-bearing contact area and an exponential increase in local pressure and thermal loading. Such a result can cause damage to the aft bearing.
Reportedly, most bearing damages occur in the aft-most part of the aft bearing. Typically, this occurs during a starboard turn on a right-handed propeller installation. DNV GL’s new rules will focus on this and the impact of these temporary hydrodynamic propeller forces and moments.
The revised class requirements for single stern tube bearing installations will require a multi-sloped bearing design. Also required will be an additional evaluation of the aft bearing lubrication condition that considers the increased propeller-induced hydrodynamic bending moment on the aft bearing in the downward direction. There will be further design and installation criteria to be met that will increase the operating margins and enhance bearing performance and fatigue lifetime in normal operating conditions.
Shaft align (1) is intended for propulsion systems installed on vessels with conventional hull forms and includes enhanced aft bearing performance during normal and turning operating conditions. Shaft align (2) is designed for propulsion systems requiring additional calculations to predict hydrodynamic propeller loads during turning conditions, such as vessels with non-conventional hull forms, including those with asymmetric sterns, twin skegs, and more.
Geir Dugstad, Director of Ship Classification at DNV GL – Maritime, says “We are always looking to push the development of our rules forward to help our customers operate and maintain more reliable and safe ships. 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.”
“We hope that by introducing the revised main class rules for single bearing installations and ‘Shaft align (1) or (2), we can substantially reduce stern tube bearing failures,” comments Oddvar Deinboll, Head of the Machinery section at DNV GL – Maritime. “We’ve received a lot of positive responses from the industry and are already working on some concrete projects.”
Shaft align (1) and shaft align (2) can be assigned to both newbuilds and vessels already in service in conjunction with propeller shaft withdrawal.
Design and in-service follow-up rules for the class notations are included in the updated DNV GL rules for the classification of ships, Part 6, Chapter 2, Section 10 and Part 7 Chapter 1, Section 6 (38), respectively.