Tanker safety: Lifetime reduction & structural compromise - where are we now? Drs Les Callow & Jane Lomas. Amtec Consultants Ltd. Tel: Fax:

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 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 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 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 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”. 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 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.