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Tanker safety: Lifetime reduction & structural compromise - where are we now?. Drs Les Callow & Jane Lomas. Amtec Consultants Ltd. Tel: +44 1928 734996 Fax: +44 1928 734998 Email: lcallow@amteccorrosion.co.uk www.amteccorrosion.co.uk. Cargo Oil Tanks - the problem.

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tanker safety lifetime reduction structural compromise where are we now

Tanker safety: Lifetime reduction & structural compromise - where are we now?

Drs Les Callow & Jane Lomas.

Amtec Consultants Ltd.

Tel: +44 1928 734996

Fax: +44 1928 734998

Email: lcallow@amteccorrosion.co.uk

www.amteccorrosion.co.uk

cargo oil tanks the problem
Cargo Oil Tanks - the problem.
  • Severe corrosion of ullage space.
  • Accelerated pitting of tank tops.
  • Failure of uncoated areas to form a protective scale.
  • Faster wastage of uncoated tank structures in the mid-tank area.
  • Thinner scantlings, corrosion from both sides & reduced rolling allowances, lead to shorter service life.
contributory factors
Contributory factors.
  • Temperature.
  • TMCP high tensile steel.
  • Vibrations and flexing of structure.
  • Detachment of protective scales.
  • Microbial corrosion.
  • Sulphur from cargo and inert gas.
contributory factors temperature
Contributory factors - temperature.
  • Major factor is temperature, due to insulation effects from double hull.
  • Effects include:
    • Corrosion rate can double with every 10oC rise.
    • Corrosion deposits become porous at higher corrosion rates.
    • Microbial corrosion increases to a maximum at typical operating temperatures.
    • Deposition of sulphur from inert gas occurs at high rates on porous surfaces.
    • Sulphur in corrosion products makes later stages of pitting worse.
contributory factors tmcp high tensile steel
Contributory factors - TMCP high tensile steel.
  • Used because of superior uniformity of properties & faster welding speeds during ship building.
  • Very lamellar structure.
  • Side to side variation in surface condition during manufacture.
  • Lower sides can retain contaminants from production process.
  • Contaminants lead to pit initiation sites.
  • Pitting van be more severe with TMCP steel.
  • Behaves differently in laboratory tests and in service.
contributory factors detachment of protective scales
Contributory factors - detachment of protective scales.
  • Scales are loose & friable due to high corrosion rates.
  • Scales contain weak sulphur layers like rings in a tree.
  • Scale removal occurs due to:
    • Mechanical shock.
    • Vibration from winches.
    • Structural flexing.
    • Impacts from COW systems.
remedial actions 1
Remedial actions 1.
  • Design tanks to avoid excessive vibration and flexural “hot spots”.
  • TMCP steel requires extra attention to surface condition during shot blasting at new building.
  • Use of high zinc shop primers during construction - especially on areas that will not be coated.
  • Ideally coat the whole tank with high quality coatings.
  • Careful cleaning of tanks prior to the first cargo.
remedial actions 2
Remedial actions 2.
  • Install flush mounted sacrificial anodes in critical areas.
  • Avoid sea water washing whenever possible.
  • Modify COW systems to lower the rate of scale removal in uncoated or partially coated tanks.
  • Design Inert Gas systems to remove more sulphurous gases.
  • Increase corrosion allowances where possible, especially in uncoated, critical areas.