Hull Thickness Measurement
Ultrasonic gauging in survey: substantial corrosion at 75 percent of the allowable margin, the reporting regime, and the audit of measurement firms.
Thickness measurement is the ultrasonic gauging of hull structure that runs alongside close-up survey under the 2011 ESP Code. Paragraph 1.5 requires it to be carried out simultaneously with close-up surveys in the areas where close-up survey is required, and paragraph 2.1.2 bars it from being taken before the fourth annual survey in a renewal survey cycle.
The finding that drives the survey is substantial corrosion, defined at ESP Code 1.2.9 as wastage in excess of 75 percent of allowable margins but within acceptable limits. For ships built under the IACS Common Structural Rules the same definition is expressed as a gauged thickness between the net thickness plus 0.5 mm and the net thickness. Finding it expands both the extent of close-up survey and its frequency.
The firms that take the readings are approved rather than merely engaged. Resolution MSC.553(108), in force 1 January 2026, amended the ESP Code annexes so that a thickness measurement firm shall be audited by the Administration, not only assessed on submitted documents. The full article will cover the measurement plan, the reporting format, IACS PR 19 and Recommendation No. 77, and how renewal thickness is derived.