Thursday, June 25, 2020

Stern tube leakage

Causes
  • Misalignment of bushes 
  • Poor material, design of liner and seals
  • Contaminated oil supply with foreign materials
  • Fishing net, rope and similar material entering into seal due to defective rope guard.
  • Electro static pitting on shaft due to defective shaft earthing device.
  • Prolonged low speed operation, in which hydrodynamic oil film could not attain.
Remedies
  • Keep good alignment of bushes. 
  • Use good material and an improved seal design.
  • Always maintain L.O level due to draught condition. 
  • Keep sufficient aft peak tank water level.
  • Use good design of rope guard. 
  • Keep earthing device in good order.
Action to be taken at sea to continue the voyage
  • When stern tube seal leaks at sea, minimized as possible. To continue the voyage, following actions are to be taken.
  • Correct the vessel trim, as possible as minimum astern, to certain allowable trim
  • Use more viscous oil (e.g. Cylinder oil)
  • Use lower header tank. If not possible, reduce the oil pressure-head by using a 'temporary header tank with flexible hose to obtain only slight pressure head above SW pressure.
  • If required, reduce rpm to avoid boundary lubrication.
Repair in port
  • Repairs or renewal of aft seal done either in dock or afloat.
  • If vessel is due for dry docking survey or a dry dock facility is available, arrange to enter dry dock and renew both seals.
  • If dry dock facility is not available, renewal of shaft seals done by Bonding method in port without removing propeller and tail shaft.
  • Necessary vessel trim by ahead until propeller shaft exposed to correct position above water level and to set up "scaffolding.
  • Renewal of sealing ring done by cutting and joining it on shaft with special tool and high temperature bonding method.
Insurance Claim
  • When leakage occurs, inform to head office or owner through captain.
  • When arrive in port, invite Class Survey and Underwriter Surveyor.
  • Survey damage and by their recommendation, repair to be carried out as per Class requirement.
  • Quotation of repairs and repair cost submitted to Underwriter Surveyor to negotiate for any reduction appear necessary.
  • Both surveyors will survey the repaired work when completed.
  • For insurance claim purpose, repair cost and bills endorsed by Underwriter Surveyor, but exclude transportation charges.
For insurance claim purpose the following items are necessary,
  • Damage report
  • log abstracts
  • Damage report form for insurance claim
  • Class Surveyor recommendation 
  • Repair bills endorsed by Underwriter Surveyor

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