Class' role in the thickness measurement process
Thickness Measurement (TM) is a manditory part of the survey process of a ship. Measurements of the vessel's structure are required to be taken at varying intervals. At a minimum, this would be every 5 years at Special Survey (Class Renewal). Many types of ships require measurements at Intermediate Survey as well. Older vessels' TM requirements at Intermediate Survey are to be taken at the same extent as the previous Special Survey.
On vessels which are older or have notations such as POOR coatings in ballast tanks or Substantial Corrosion at the previous survey, it may very well mean that measurements are required at Annual Survey as well!
This is all to keep vessels safe on our seas and to try and avoid terrible catastrophies which have destroyed our coastlines. This is one of the responsibilities of the Class Surveyor.
It is therefore crucial that the Class Surveyor is involved in the Thickness Measurement process from the absolute start.
Before planning the TM survey, your local Class Surveyor should be notified. A meeting will be scheduled, usually on board, with the Surveyor, the TM company and the Owner's representative.
At this meeting the extent of measurement will be decided by the Surveyor. It is his / her word that is final on the subject. This is usually according to a template showing minimum TM requirements. In this template there is a point called Critical Areas as deemed by the Surveyor. The surveyor will decide this after a thorough inspection of the ballast tanks and other areas has been made.
During the TM process, the Surveyor has the right to accompany the TM operator while measurements are being taken or until such time as he is satisfied that the TM survey is being carried out correctly.
AlfaTest has a good reputation with the Surveyors that have worked with us.
When all measurements are completed, the autherised TM company submits a TM report, written in table form which shows the structure which was measured, the original thickness of the steel when constructed, the measured thickness and the difference between - both in millimeters and in procent. Supporting sketches should accompany this report in order to make it easier to read.
All measurements taken of the transverse sections of the vessel are added and compared with the original thickness. In this way a single figure can be calculated representing the Longitudinal Strength of the vessel.
Regulations between the various Classifictaion Societies are similar regarding TM but there are some differences. These regulations are subject to change from time to time so it's important to check.
Please see the links to the various Classification Society's regulations on Thickness Measurement.