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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.

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

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  1. 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

  2. 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.

  3. Contributory factors. • Temperature. • TMCP high tensile steel. • Vibrations and flexing of structure. • Detachment of protective scales. • Microbial corrosion. • Sulphur from cargo and inert gas.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

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