Thickness gauging for class renewal: plan, measure, report

#Why gauging decides the docking

Most of the steel a ship renews at a special survey is decided by numbers: the thickness readings taken in the holds, the tanks and on the shell. When the readings arrive late or incomplete, the steel list grows during the docking — and so do the days in dock. A campaign planned before the ship arrives turns those numbers into a repair list that can be priced and scheduled.

#Planning on the drawings

The plan starts from the survey requirements for the ship's type and age and from her structural drawings: midship section, shell expansion, deck and bulkhead plans. On them we mark:

• the transverse sections to gauge and the members in each — plating, longitudinals, frames, brackets;
• the strakes and plates to read along the length;
• the areas for close-up survey, where readings go together with the visual inspection;
• the spots where earlier reports showed substantial corrosion.

Every point gets an identity — member, frame, strake — so a reading can always be traced back to the same place.

#Taking the readings

Readings are taken with a calibrated ultrasonic gauge and a dual-element probe, checked on a reference block before and during the work. Each spot is cleaned to sound metal, couplant is applied and the reading is logged against its point. Where a value is low, a few readings around it show whether it is general wastage or a pit. Coatings in good condition can be measured through when the instrument works echo-to-echo; otherwise the spot is cleaned.

The surveyor attends the measurements or verifies them, as the class requires — and the gauging firm must be approved by the classification society for this work.

#Reporting against the rules

A reading on its own says little; what matters is how much of the original thickness is gone. The report lists, for every point, the original thickness, the reading and the diminution in per cent, and compares it with the allowable diminution for that member. Members beyond the limit are highlighted, and the areas and weights to renew are totalled — the basis of the steel list.

Illustrative values.

#What makes a campaign go well

• Drawings and the previous thickness reports sent before the ship arrives.
• Access planned with the ship: staging, lifts or rafting, gas-free spaces.
• One team that gauges, then renews and tests, so findings become repairs without hand-over delays.