Maritime

Shaft alignment and propeller shaft aft bearing performance – recent trends call for action

Recent experience reflects concerns on propeller shaft aft bearing performance on some oil lubricated installations, e.g. ships with single stern tube bearing, during turning conditions involving hard-over steering angles in the upper speed range (MCR). This also coincides with evolving trends comprising of larger and heavier propellers operating at a lower RPM and different types of stern tube lubricants. This technical news aims to elaborate the basic logic, criteria and recommendations associated with propeller shaft aft bearing performance.

Downward bending moment
Figure 1 – Downward bending moment induced during a starboard turn: right handed propeller

Relevant for shipyards, suppliers, owners/managers, flag states and vetting agencies.

The basic concept and industry challenges 

DNV GL rules for shaft alignment (Jan. 2018, Pt.4 Ch.2 Sec.4) are formulated to achieve an acceptable distribution of load on the shaft bearings, and include a hydrodynamic lubrication criteria of the aft bearing. Due consideration is also made to accommodate the bending moment induced by the propeller during continuous operation. 

During extreme transient turning conditions in the upper speed range, exaggerated propeller bending moments are induced, leading to reduction in shaft-bearing contact area and an exponential increase in local pressures and thermal loading (see Figure 1). The expected nature of aft bearing lubrication under these conditions, i.e. a combination of a mixed and boundary type, poses a challenge to retain a hydrodynamic oil film of acceptable thickness. Most of the reported bearing damages (resulting from abrupt overheating) have been observed in the aft most part of the aft bearing, typically during a starboard turn on a right-handed propeller installation.

DNV GL solution

As a part of the continuous rule development process in accordance with the industry demand to cater to evolving design trends and experience, DNV GL has now 

  • revised the main class shaft alignment rules for single stern tube bearing installations, and
  • introduced new optional class notations, Shaft align(1) and Shaft align(2), for oil-lubricated propeller shaft installations.

Shaft align(1) is a basic cost-effective option intended for propulsion systems installed on vessels with conventional hull forms and which incorporates enhanced aft bearing performance during normal and turning operating conditions. 

  • Multi-sloped aft bearing is mandatory
  • Increased propeller bending moment ranging from -30 to +30% MCR torque included in the aft bearing loading criteria 
  • State-of-the-art measurement techniques for installation sighting (laser or equivalent) 
  • Means of warning against incomplete propeller immersion

Shaft align(2) is intended for propulsion systems requiring additional calculations to predict hydrodynamic propeller loads during turning conditions. Typical installations are vessels with non-conventional hull forms such as asymmetric stern and twin skeg.

  • Design-specific hydrodynamic propeller load spectrum and transient forces using CFD
  • FE analysis for transient aft bearing contact pressure and area
  • Hull deflections where applicable
  • Shaft align (1) requirements apply as the basic criteria 

Shaft-aft bearing contact area mapping A
Shaft-aft bearing contact area mapping B
Figure 2 – Shaft-aft bearing contact area mapping: single sloped (upper picture) versus double sloped aft bearing, the colored plots show the shaft-bearing gap

How does the solution make a difference?

The DNV GL shaft alignment class notations and revised main class rules for single stern tube bearing installations introduce additional focus on the impact of transient hydrodynamic propeller downward-acting bending moments, which are induced in turning conditions at MCR speed, on the aft most propeller shaft bearing. 

This is supported by a mandatory requirement for a multi-sloped aft bearing, coupled with an additional aft bearing lubrication evaluation criteria, with an increased bending moment acting downwards on the bearing (30% MCR torque). Compared to a single-sloped bearing, a multi-sloped design better assists in optimizing the shaft-to-bearing contact area and surface pressure in all operating conditions, considering the hydrodynamically-induced propeller bending moments (see Figure 2). 

For Shaft align(2), input from CFD-aided prediction of hydrodynamic propeller moments and forces acting on the aft bearing are used in conjunction with the FE analysis to evaluate the bearing surface pressure and contact area under turning conditions. DNV GL class guidelines linked to the rules provide guidance on the criteria to be followed in this regard. 

Recommendations

DNV GL recommends that operators consider enhanced propeller shaft bearing performance solutions.

The class notations Shaft Align(1) and (2) may be assigned at the newbuild phase or during service in conjunction with a propeller shaft withdrawal. This is particularly recommended for vessels undergoing retrofits or re-metalling of propeller shaft bearings during dry dock.   

References

Contact

For customers: 
DATE – Direct Access to Technical Experts via My Services on Veracity

Otherwise: 
Use our office locator to find the nearest DNV GL maritime office

09 March 2020

IMO Sub-Committee on Ship Systems and Equipment

The 7th session of IMO’s Sub-Committee on Ship Systems and Equipment (SSE) was held in London from 2 to 6 March 2020. SSE 7 agreed on draft new requirements for on-board lifting appliances, and on draft new interim guidelines for safe operation of shore power. SSE 7 further progressed work on ventilation of survival craft and on fire safety of ro-ro passenger ships, and initiated work to improve the safety of commercial diving operations.

  • Maritime
24 February 2020

IMO Sub-Committee on Pollution Prevention and Response

The 7th session of the IMO’s Sub-Committee on Pollution Prevention and Response (PPR) was held in London from 17 to 21 February 2020. PPR 7 finalized guidelines for Exhaust Gas Cleaning Systems (EGSC – scrubbers), developed a basis for work to determine the impact of EGCS discharge water on the environment, drafted regulations for prohibiting the use and carriage of HFO as fuel in the Arctic, and revised the guidance on ballast water system commissioning.

  • Maritime
10 February 2020

IMO Sub-Committee on Ship Design and Construction

The 7th session of IMO’s Sub-Committee on Ship Design and Construction (SDC) was held in London from 3 to 7 February 2020. SDC 7 finalized the interim guidelines on the second-generation intact stability criteria, clarified the requirements to watertight integrity on passenger ships and cargo ships, developed mandatory provisions for OSVs and other cargo ships carrying more than 12 industrial personnel, and finalized safety guidelines for fishing vessels and pleasure yachts operating in polar waters.

  • Maritime
27 January 2020

IMO Sub-Committee on navigation, communications and search and rescue

This statutory news provides an update from the 7th session of the IMO’s Sub-Committee on Navigation, Communications and Search and Rescue (NCSR), held in London from 15 to 24 January 2020. NCSR 7 agreed to recognize the Indian Regional Navigation Satellite System (IRNSS) for the provision of positioning, navigation and timing services, and finalized draft SOLAS amendments for the modernization of the Global Maritime Distress and Safety System (GMDSS).

  • Maritime
View all