Hull integrity: thickness, NDT and hatch covers

A ship’s hull is steel that rusts from the day it is launched. Class surveys exist to find that loss of steel before it becomes a fracture, a flooded hold or wet cargo. Port State control officers in the Paris MoU region recorded 6,004 deficiencies under SOLAS chapter II-1 (structure, subdivision and machinery) in 2025, 11.6% of everything they found, and another 2,637 under the Load Line Convention, which covers hatch covers and other closing appliances. The detention rate rose to 4.18%. Behind those numbers are thin plates, cracked brackets, leaking hatch covers and survey records that do not add up.

This course follows the hull through its five-year class cycle. It shows when thickness has to be measured and who may measure it, how an ultrasonic gauge actually reads steel through paint, which non-destructive testing (NDT) methods find which defects, how hatch covers are proved weathertight with ultrasound, and when a diver or remotely operated vehicle (ROV) may inspect the bottom instead of a dry dock. It is built on the IACS Unified Requirements Z3, Z7, Z7.1, Z10.x and Z17, the 2011 ESP Code, the IMO Survey Guidelines under the Harmonized System of Survey and Certification (HSSC) and the IACS weld-testing requirements W33 to W35. Changes already adopted but not yet in force, such as UR Z17 Revisions 22 to 24 (2027 and 2028), are flagged as coming.

Every rule statement is cited to its paragraph so you can check it against your own class society’s rules, which always decide the details for your ship.

What you will learn

• Lay out the five-year class cycle and say when thickness measurement, close-up survey and bottom inspection fall due, and what an extension can and cannot do.
• Explain how a pulse-echo thickness gauge measures steel through coating, what goes wrong, and what “substantial corrosion” means in numbers.
• Check that a thickness measurement, ultrasonic tightness or diving firm is properly approved under IACS UR Z17, and what its report must show.
• Choose the right NDT method (visual, penetrant, magnetic particle, ultrasonic, radiography) for a weld or a crack, and know the personnel levels under ISO 9712.
• Plan and witness an ultrasonic hatch cover tightness test, read the results against the open hatch value, and spot what the test cannot see.
• Decide when an in-water survey can replace a dry-docking, and prepare one that the class surveyor will credit.