DNV Bearing Damage Story

Stern Bearing DamageStern Bearing Damage

Mr. Geir Magne Bakken (M.Sc.)DNV Rotating Machinery Marindagen Gøteborg; 2007-03-22

DNV Rotating Machinery/GMB Slide 220 March 2007

Stern Tube Bearing Damage – The story!Stern Tube Bearing Damage – The story!

An oil tanker was rebuild

Earlier conversed from fixed pitch to controllable pitch

A tunnel thruster to be installed in the aft ship

After conversion The ship in service Stern bearing damage


The Ship’s PropulsionThe Ship’s PropulsionConventional direct propulsion; 19620 kW @ 91 rpmOriginally design with fixed pitch propellerShaft supported by two stern tube bearings & intermediate bearing


Troubleshooting CaseTroubleshooting Case

DNV was contacted by owner to assist the owner in the site investigation and to follow-up the repairing process

The vessel was build in 2001

The vessel had recently been rebuilt

Tunnel thruster installed in the aft ship

Bearing damage occurred after conversion

DNV was requested to identify the cause of the damage and recommend actions to be taken in order to rectify the problem


The Thruster Tunnel InstallationThe Thruster Tunnel Installation


Tunnel thruster installed in the aft hull structureExtensive welding work in the aft shipResidual stresses from the welding work deform the aft part of the hull

The conversion – Thruster installationThe conversion – Thruster installation


The Detection of the DamageThe Detection of the Damage

The Damage was detected due to leakage of oil from the aft sterntube of the vessel


On Site Investigation On Site Investigation

Flakes of white metal in way of the shaft sealing


Visual Inspection of the DamageVisual Inspection of the Damage


Spalling of the Bearing MetalSpalling of the Bearing Metal


Circumferential Crack in Bearing Metal Circumferential Crack in Bearing Metal


Visual Inspection of the DamageVisual Inspection of the Damage


Cause of the DamageCause of the DamageThe original alignment of the shaft

Stern bearing pressure; Actual 8.3 bar (criterion 8 bar)

Relative slope: Actual 0.33 mm/m (criterion 0.3 mm/m)


Measurement of the Present AlignmentMeasurement of the Present Alignment

Laser

FlywheelStern Tube Int.Bearing

The measurements were carried out and the alignment was recalculated based on these laser measurements


Cause of the DamageCause of the Damage

Calculated alignment

according to measurements

Initial calculated alignment

according to specification


Shaft Deflection in Stern TubeShaft Deflection in Stern Tube

Static; zero pitch

Running (in operation)

Edge contact

Running MCR


Max oil pressure in Stern BearingMax oil pressure in Stern BearingAlignment after conversion Stern bearing pressure:

Peak pressure

> 100 bar (at edge)(Projected pressure slightly above the 8.7

bar)

Stern bearing pressure:

Peak pressure

~ 40 bar (at edge)(Projected pressure slightly above the

8.3 bar)

Original alignment


Cause of the DamageCause of the Damage

Misalignment caused by:

The original installation “Designed at the limit”

Conversion of the vessel

- Tunnel thruster installation without pull out the propeller shaft

- The alignment not checked after extensive welding work


Suggested AlignmentSuggested AlignmentRestriction was not to change the main engine position

Stern bearing pressure:- Modified slope bore - Improved load distribution

Peak pressure < 30 bar


Benefits to the CustomerBenefits to the CustomerSuggested repairs that solved the problem

Forward sterntube bearing lifted upwards

Aft sterntube bearing lifted upwards

Aft sterntube double-slope bored

Suggested repairs that minimized the time and cost for carrying out the repairs

Improved understanding of shaft alignment

Revised procedure for conversion of vessel


Documents

DNV Bearing Damage Story