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Hull thickness and NDTLesson 5 of 5

In-water survey in lieu of dry-docking (IWS and UWILD)

You will be able to decide whether an in-water survey can replace a bottom inspection in dry dock, and prepare one that the class surveyor will credit.

15 minutes · For signed-up learners, free · Rules checked 20 October 2026

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A dry-docking takes a ship out of service for days or weeks, plus the passage to the yard. For the intermediate bottom inspection, the one that falls in the middle of the five-year cycle, many owners ask whether a team of divers or a remotely operated vehicle (ROV) can look at the hull while the ship stays afloat. Often the answer is yes. The international rules have allowed the “alternate” bottom inspection to be done afloat for decades, provided the ship, the water, the equipment and the people are right.

Getting it wrong costs more than a dock. An in-water survey done in murky water by an unapproved team is not credited, the 36-month limit keeps running, and the statutory certificate can lapse. This lesson shows when an in-water survey (IWS, the IACS term) is allowed, how it is run, and how the extended dry-docking schemes work. In US practice the same idea is called underwater inspection in lieu of dry-docking (UWILD). Work through the case first.

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The rest of this lesson, the interactive An in-water survey for a mid-life container ship and the quiz are open to signed-up learners. It's free, takes a minute, and opens every lesson in all 14 courses.

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In this lesson

  • Try it: An in-water survey for a mid-life container ship
  • In plain words
  • In detail
  • What the rules say
  • Flag and class notes
  • Key numbers
  • What inspectors find
  • Questions people ask
  • Quiz: 6 questions

Sources

  1. 1 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, Z3.2 and Z3.3.1 to Z3.3.6. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2022/05/18150117/ur-z3rev8.pdf
  2. 2 IMO Resolution A.1207(34), Survey Guidelines under the Harmonized System of Survey and Certification (HSSC), 2025 (adopted 3 December 2025), Annex, paragraphs 4.6.1, 4.6.2.2 and 5.10 (with footnote reference to MSC.1/Circ.1348). 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
  3. 3 IACS Unified Requirement Z17, Procedural Requirements for Service Suppliers, Rev.21 (January 2025, implemented from 1 July 2026), paragraphs 4.1.2, 4.2 and Annex 1, section 3 (firms carrying out in-water surveys by diver or ROV). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/14135618/UR-Z17-Rev.21-Jan-2025-CLN.pdf
  4. 4 IACS UR Z17 Rev.22 (in force 1 January 2027), Annex 1, paragraphs 3.2.1 and 3.2.2 and section 16 (survey using Remote Inspection Techniques). https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/05/05122440/UR-Z17-Rev.22-23-24-CLN.pdf
  5. 5 IACS Recommendation No. 133, Guidelines for Schemes of Extended Interval between Surveys in Dry-Dock: Extended Dry-docking (EDD) Scheme, Rev.1 (May 2026), sections 1 to 9. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2026/05/27081846/Rec-133-Rev.1-May-2026-CLN.pdf
  6. 6 IACS Unified Requirement Z10.2, Hull Surveys of Bulk Carriers, Rev.37 (February 2023), paragraphs 4.2.3.3 and 4.2.4.3. https://iacs.s3.af-south-1.amazonaws.com/wp-content/uploads/2023/08/10152246/ur-z102rev37.pdf
  7. 7 US Code of Federal Regulations, 46 CFR 31.10-21, Drydock examination, internal structural examination, cargo tank internal examination, and underwater survey intervals, paragraphs (a), (c), (d) and (e). https://www.ecfr.gov/current/title-46/chapter-I/subchapter-D/part-31/subpart-31.10/section-31.10-21

Free awareness training. Al Safwan Marine is not an approved training provider, so no certificate is given, and these courses are not a qualification. They don't replace STCW training, manufacturer training or the certification technicians need for this work. The manufacturer's manual and your flag's rules always come first.

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