Tanker safety Lifetime reduction structural compromise where are

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

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. • Severe corrosion of ullage space. • Accelerated

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. • • • Temperature. TMCP high tensile steel. Vibrations and flexing of

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. • Major factor is temperature, due to insulation effects from

Contributory factors - temperature. • Major factor is temperature, due to insulation effects from double hull. • Effects include: • Corrosion rate can double with every 10 o. C 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. • Used because of superior uniformity of

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. • Scales are loose & friable due

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. • Design tanks to avoid excessive vibration and flexural “hot spots”.

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. • Install flush mounted sacrificial anodes in critical areas. • Avoid

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.