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Hull thickness and NDTLesson 1 of 5

The class survey cycle and when thickness is measured

You will be able to map any ship's hull surveys on a five-year timeline and say when thickness gauging, close-up survey and bottom inspection are due.

14 minutes · Open to everyone · Rules checked 20 October 2026

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Every classed ship lives on a five-year clock. The Classification Certificate runs for five years, the hull is examined every year, more deeply in the middle of the cycle, and is taken apart, cleaned and measured at the end. Miss a date and the consequences are not paperwork: under the IMO survey guidelines, a statutory certificate ceases to be valid if a required survey or bottom inspection is not completed in time.

The clock also decides cost. Thickness measurement on a 15-year-old bulk carrier means staging, cleaning, gas-freeing and days of gauging with a class surveyor on board. Superintendents who know which survey brings which scope can plan the dock, the gauging firm and the steel budget years ahead. Below is that clock, the way IACS Unified Requirement (UR) Z7 and the 2011 Enhanced Survey Programme (ESP) Code set it.

Try it

The five-year hull survey cycle of a cargo ship

In plain words

Think of the hull survey cycle as three kinds of visit.

The annual survey is a walk-round. The surveyor looks at what can be seen from the deck and from inside a few spaces: hatch covers, coamings, air pipe heads, ventilators, the deck plating. If an earlier survey found a “Suspect Area”, a place where corrosion is heavy or likely to be fast, that spot is checked and measured again every year.

The intermediate survey comes in the middle, at the second or third annual survey. It goes into the ballast tanks. On young ships it is a sample; on old ships it is close to a full special survey.

The special survey (some societies call it the class renewal survey) comes every five years. Everything is opened, cleaned and examined. The ship goes into dry dock, and a specialist firm measures the steel thickness with ultrasonic gauges at hundreds or thousands of points. The older the ship, the more points.

Two ideas drive the amount of measuring. A close-up survey means the surveyor is close enough to touch the structure. Wherever a close-up survey is required, thickness is measured at the same time. Substantial corrosion means the steel has lost more than 75% of the margin the rules allow, but is still within acceptable limits: a warning, not yet a repair. Once a surveyor finds it, the area becomes a Suspect Area and the gauging spreads out to find its edges.

Underneath the class cycle sits the statutory one. The Cargo Ship Safety Construction Certificate, issued for the flag under SOLAS, needs two inspections of the outside of the ship’s bottom in every five years, never more than 36 months apart. Class and flag surveys are usually done together by the same surveyor, so in practice one plan covers both.

In detail

Dates and windows

Survey Window Source rule
Annual 3 months before or after each anniversary date UR Z7 3.1
Intermediate At or between the 2nd and 3rd annual survey UR Z7 4.1.1
Special Every 5 years; may start at the 4th annual survey UR Z7 2.1.1, 2.1.4
Bottom inspection 2 per 5 years, max 36 months apart UR Z3.1.2; HSSC 4.6.1

A special survey started before the 4th annual survey must be finished within 15 months if the work is to count. If it is finished within 3 months before the expiry date, the new class period runs from the old expiry date; finish it earlier and the new period starts from the completion date. Work credited to the intermediate survey cannot be credited again to the special survey.

The 36-month bottom rule has no extension under the HSSC guidelines. The IMO text warns that if the first bottom inspection falls between months 24 and 27, the 36-month limit may stop the certificate being extended at all. Under class rules (UR Z3), a 3-month extension of the bottom examination is possible only in “exceptional circumstances”, which UR Z3 defines narrowly: no dry dock or repair facility available, essential materials or spare parts unavailable, or delay caused by avoiding severe weather.

What gets measured at the special survey

UR Z7 Table 1 sets the minimum thickness measurement for all self-propelled ships. The scope grows with age:

Special survey Ship age Minimum gauging (UR Z7 Table 1, summarised)
No. 1 up to 5 years Suspect Areas
No. 2 5 to 10 Suspect Areas; one transverse section of deck plating amidships
No. 3 10 to 15 Suspect Areas; two transverse sections amidships; all cargo hatch covers and coamings; internals of fore and aft peak tanks
No. 4 and later over 15 Suspect Areas; three transverse sections; hatch covers and coamings; peak tanks; all exposed main deck plating; lowest strakes of transverse bulkheads in cargo spaces; all wind-and-water strakes; all keel plates; sea chests

A transverse section is a full girth belt of longitudinal members: deck, sides, bottom, inner bottom and longitudinal bulkheads. Ships under 100 m may take fewer sections at special surveys No. 3 and later. Where the hard coating is in GOOD condition, the surveyor may specially consider (reduce) the gauging of internals.

Bulk carriers (UR Z10.2), oil and chemical tankers (Z10.1, Z10.3, Z10.4), double-skin bulk carriers (Z10.5) and general dry cargo ships of 500 GT and above (Z7.1) have their own, heavier tables. For a single-skin bulk carrier at special survey No. 3 or later, for example, every deck plate outside the line of the hatch openings within the cargo length is gauged, plus wind-and-water strakes and the members under close-up survey.

When gauging spreads: substantial corrosion

When a reading shows substantial corrosion, the gauging must be extended to map the area before the survey is credited. UR Z7 Table 2 gives the guidance pattern: for plating, the suspect area and adjacent plates on a 5-point pattern over 1 m²; for stiffeners, three readings in line across the web and across the flange. The area is then recorded as a Suspect Area and gauged at every following annual survey.

Coating condition steers the survey

UR Z7 grades coating as GOOD (only minor spot rusting), FAIR or POOR (general breakdown over 20% or more of the area, or hard scale on 10% or more). A ballast tank other than a double-bottom tank whose hard coating is POOR and not renewed, or which has soft or semi-hard coating or none, must be examined at every annual survey; double-bottom tanks in that condition may be examined annually. For bulk carriers under the ESP Code, the bar has been higher since 1 July 2024: amendments in resolution MSC.525(106) require annual examination where the coating is “less than GOOD”, which means FAIR as well as POOR. Lloyd’s Register applies the change at special or intermediate surveys commenced on or after that date.

The ESP layer

The ESP Code is mandatory under SOLAS regulation XI-1/2 for bulk carriers and oil tankers of 500 GT and above. On top of the class scope it adds a survey programme agreed in advance, two surveyors for the third and later renewal surveys (and the intermediate surveys after the third) of bulk carriers and tankers of 20,000 dwt and above, a ban on taking the renewal thickness measurement before the 4th annual survey, and the Survey Report File (survey reports, the Executive Hull Summary and the thickness measurement reports) kept on board for the ship’s life.

Coming on 1 January 2028: resolution MSC.584(111) amends the ESP Code with a framework for using remote inspection techniques (drones, ROVs, climbers) for close-up surveys and the related thickness measurements on ESP surveys commenced from that date, supported by the guidelines in MSC.1/Circ.1700.13

Who measures

The thickness measurement must be done by a firm approved by the class society under UR Z17 (lesson 2). The only exceptions are non-ESP ships of less than 500 GT and fishing vessels. The surveyor must be on board while the gauging is taken, to the extent needed to control the process, and a survey planning meeting with the master, the owner’s representative and the gauging firm must be held and recorded before an intermediate or special survey.

What the rules say

  • UR Z7 paragraph 2.1.1 requires special surveys at 5-year intervals to renew the Classification Certificate; 2.1.2 allows an extension of up to 3 months “in exceptional circumstances” only, and 2.1.4 lets the survey start at the 4th annual survey.1
  • UR Z7 paragraph 3.1 sets annual surveys within 3 months before or after each anniversary date, and 3.2.4 requires Suspect Areas from earlier surveys to be examined and gauged, extending the gauging where substantial corrosion is found.2
  • UR Z7 paragraph 4.1.1 places the intermediate survey at or between the 2nd and 3rd annual survey; 4.2.1 requires a general internal examination of representative ballast tanks at 5 to 10 years and of all ballast tanks over 10 years.3
  • UR Z7 paragraph 1.2.8 defines substantial corrosion as wastage “in excess of 75% of allowable margins, but within acceptable limits”, and 1.4.1 requires thickness measurement simultaneously with any close-up survey.4
  • UR Z7 paragraph 2.2.11 and Table 1 set the minimum special survey gauging by age; Table 2 gives the extended pattern for substantial corrosion; 2.2.3 requires the special survey to include the underwater parts per UR Z3.4
  • UR Z3 paragraph Z3.1.2 requires at least two examinations of the outside of the bottom in each five-year special survey period, one at the special survey, never more than 36 months apart; Z3.1.3 requires dry dock for ESP ships of 15 years and over.5
  • The HSSC Survey Guidelines (resolution A.1207(34)) paragraph 4.6.1 repeat the two-in-five-years, 36-month limit for the statutory bottom inspection, and paragraph 5.3 says no extension of the 36 months should be permitted.6
  • The 2011 ESP Code, annex A, part A, paragraph 2.1.2 says the renewal thickness measurement “should not be held before the fourth annual survey”, and paragraph 1.4 requires two surveyors for the third and later renewal surveys, and the intermediate surveys after them, of bulk carriers of 20,000 dwt and above.7
  • UR Z10.2 paragraphs 4.2.3 and 4.2.4 require the intermediate survey of bulk carriers over 10 years old to match the previous special survey in extent, and over 15 years to include a dry dock survey.8
  • IACS Procedural Requirement PR 19 paragraphs 1 and 2.1 require a surveyor to witness thickness measurements and a recorded survey planning meeting with the master, owner and gauging firm before intermediate and special surveys.9
  • Resolution MSC.525(106), in force 1 July 2024, requires ESP bulk carrier ballast tanks (other than double-bottom tanks) with coating “less than GOOD” to be examined at annual surveys.10
  • Resolution MSC.584(111), coming into force on 1 January 2028, adds rules to the ESP Code for remote inspection techniques used for close-up surveys and thickness measurements.13

Flag and class notes

  • Lloyd’s Register Class News 15/2024 applies the MSC.525(106) and UR Z10.x changes to special or intermediate surveys commenced on or after 1 July 2024, judged by the first day of attendance. It also applies the MSC.525(106) requirement to examine the coating condition of double-side-skin void spaces on bulk carriers of 150 m and over once they are 20 years old, with annual re-examination where the coating is POOR or there is none.11
  • The Hong Kong Marine Department promulgated MSC.525(106) to owners and class societies of Hong Kong ships in Merchant Shipping Information Note 10/2023.10
  • Your class society’s rules decide the acceptance criteria (allowable diminution) for each member; UR Z7 paragraph 1.5 leaves them to the individual society’s rules and specific URs such as UR S21 for hatch covers.4
  • Extensions of the special survey or bottom inspection are decided by class (for the class certificate) and by the flag (for the statutory certificate). Check both before you plan a late docking.

Key numbers

What Value
Class period 5 years1
Annual survey window ± 3 months of anniversary date2
Special survey extension, exceptional only 3 months maximum1
Special survey started before 4th annual: must finish within 15 months1
Bottom inspections per 5-year period at least 25
Maximum interval between bottom inspections 36 months6
Substantial corrosion threshold more than 75% of allowable margin used4
POOR coating general breakdown over 20% or more of area, or hard scale at 10% or more4
Extended gauging pattern for plating 5 points over 1 m²4
Two surveyors on ESP renewal surveys from the 3rd renewal (and intermediate surveys after it), ships of 20,000 dwt and above7

What inspectors find

  • Structure and closing appliances are among the largest deficiency groups: in 2025 Paris MoU inspectors recorded 6,004 SOLAS II-1 deficiencies (11.6% of all) and 2,637 load line deficiencies (5.1%).12
  • Suspect Areas recorded at the last special survey but not re-gauged at the following annual surveys. UR Z7 3.2.4 requires them to be examined and measured every year.2
  • Missing or incomplete Survey Report File on ESP ships: no Executive Hull Summary, or thickness reports kept only ashore.8
  • Ballast tanks with FAIR coating not added to the annual survey list on bulk carriers after July 2024.11
  • Bottom inspection planned at month 26 that blocks a later certificate extension because of the 36-month limit.6

Questions people ask

Can the intermediate bottom inspection always be done afloat?

Not always. For bulk carriers and oil tankers of 15 years and over, the bottom must be inspected in dry dock. Other ships of 15 years and over need special consideration before an in-water survey is accepted, and the in-water survey must be done by a firm approved under UR Z17. Lesson 5 covers this in detail.

Does a good coating reduce the gauging?

Yes, within limits. Where the hard protective coating is in GOOD condition, the surveyor may specially consider the extent of close-up survey and thickness measurement of internals. The minimum gauging of Suspect Areas and transverse sections still applies.

Can we start thickness measurement early to save time in dock?

For ESP ships, not before the 4th annual survey. For any ship, the surveyor decides the final extent and locations after the overall survey, so readings taken before the surveyor has seen the spaces may have to be repeated or extended.

Who keeps the thickness reports?

On ESP ships the reports form part of the Survey Report File kept on board for the life of the ship, with copies in the owner’s and the class society’s offices.

Check yourself

6 questions. Pass with 60% to mark the lesson as done.

  1. 1 A ship’s special survey is due on 1 June. Which plan meets UR Z7?
  2. 2 A reading shows a deck plate has lost 80% of its allowable diminution margin. What happens?
  3. 3 The last bottom inspection was in dry dock 34 months ago. The special survey is due in 5 months. What is the problem?
  4. 4 On a 12-year-old ESP bulk carrier, how deep is the intermediate survey?
  5. 5 Who must attend while an approved firm gauges the hull for the special survey?
  6. 6 Since 1 July 2024, which ESP bulk carrier ballast tanks are examined at every annual survey?

Sources

  1. 1 IACS Unified Requirement Z7, Hull Classification Surveys, Rev.29 Corr.1 (May 2024), section 2.1 (Special Survey schedule), paragraphs 2.1.1 to 2.1.6. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2024/05/21130428/UR-Z7Rev.29Corr.1.pdf
  2. 2 IACS UR Z7 Rev.29 Corr.1 (May 2024), section 3 (Annual Surveys), paragraphs 3.1, 3.2.3, 3.2.4 and 3.2.5. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2024/05/21130428/UR-Z7Rev.29Corr.1.pdf
  3. 3 IACS UR Z7 Rev.29 Corr.1 (May 2024), section 4 (Intermediate Survey), paragraphs 4.1.1 to 4.2.6. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2024/05/21130428/UR-Z7Rev.29Corr.1.pdf
  4. 4 IACS UR Z7 Rev.29 Corr.1 (May 2024), paragraphs 1.2.3, 1.2.7, 1.2.8, 1.2.10, 1.4.1, 1.5, 2.2.3, 2.2.7, 2.2.11, 5.3.1, Table 1 (minimum thickness measurements at special survey) and Table 2 (additional measurements in way of substantial corrosion). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2024/05/21130428/UR-Z7Rev.29Corr.1.pdf
  5. 5 IACS Unified Requirement Z3, Periodical survey of the outside of the ship’s bottom and related items, Rev.8 (April 2019, applied from 1 July 2020), paragraphs Z3.1.2, Z3.1.3 and footnote 2. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/18150117/ur-z3rev8.pdf
  6. 6 IMO Resolution A.1207(34), Survey Guidelines under the Harmonized System of Survey and Certification (HSSC), 2025 (adopted 3 December 2025, revoking A.1186(33)), Annex, paragraphs 4.6.1, 4.6.2.2, 5.3, 5.6 and 5.7. https://wwwcdn.imo.org/localresources/en/OurWork/IIIS/Documents/A%2034-Res.1207%20-%20SURVEY%20GUIDELINES%20UNDER%20THE%20HARMONIZED%20SYSTEMOF%20SURVEY%20AND%20CERTIFICATION%20(HSSC),%202025%20(Secretariat).pdf
  7. 7 IMO Resolution A.1049(27), International Code on the Enhanced Programme of Inspections during Surveys of Bulk Carriers and Oil Tankers, 2011 (2011 ESP Code), as amended, annex A, part A, paragraphs 1.1.1, 1.4, 2.1.1, 2.1.2 and 2.2.1 to 2.2.3. https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/AssemblyDocuments/A.1049(27).pdf
  8. 8 IACS Unified Requirement Z10.2, Hull Surveys of Bulk Carriers, Rev.37 (February 2023, applied to surveys commenced on or after 1 July 2024), paragraphs 4.2.2 to 4.2.4, 6.2 (Survey Report File), 6.3 (supporting documents) and 7.1. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/08/10152246/ur-z102rev37.pdf
  9. 9 IACS Procedural Requirement PR 19, Procedural Requirement for Thickness Measurements, Rev.1 (July 2017), paragraphs 1, 2, 2.1 and 2.2. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/02/10150422/pr19rev1.pdf
  10. 10 IMO Resolution MSC.525(106), Amendments to the 2011 ESP Code (adopted 10 November 2022, in force 1 July 2024), operative paragraphs 2 and 3 and Annex; and Hong Kong Marine Department, Merchant Shipping Information Note No. 10/2023 (8 February 2023). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/MSCResolutions/MSC.525(106).pdf and https://www.mardep.gov.hk/en/msnote/pdf/msin2310.pdf
  11. 11 Lloyd’s Register, Class News 15/2024 (26 June 2024), New survey requirements for bulk carriers and oil tankers. https://www.lr.org/en/knowledge/class-news/15-24/
  12. 12 Paris MoU on Port State Control, Annual Report 2025 (published 1 July 2026), Major categories of deficiencies 2023-2025. https://parismou.org/year-report/year-report-2025/
  13. 13 Lloyd’s Register, Class News 17/2026 (12 October 2026), New IMO ESP Code framework for Remote Inspection Techniques (resolution MSC.584(111), in force 1 January 2028; MSC.1/Circ.1700). https://www.lr.org/en/knowledge/class-news/17-26/

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