How does the ship survey cycle work?
Under most class society rules, a ship's class runs on a five-year cycle. The ship has an annual survey around each anniversary date, an intermediate survey in place of the second or third annual, and a special (renewal) survey every five years that renews class. Bottom surveys, usually in dry dock, take place twice in each cycle. Missing a survey window typically leads to suspension of class.
The five-year class cycle
A classification society assigns class when a ship is built, or when it transfers in, and issues a class certificate that is typically valid for five years. During those five years the society checks the ship at fixed points to confirm it still meets the rules. At the end, a thorough renewal survey starts a new five-year period.
The schedule hangs off the ship's anniversary date, which is normally the day and month the class certificate expires. Each survey has a window around a due date rather than a single fixed day, which gives owners room to fit surveys around trading.
The exact rules differ between societies, ship types and ages. The 12 IACS members follow common minimum requirements set by IACS, so the broad pattern below applies to most ocean-going ships. Tankers, bulk carriers and older ships are generally subject to extra requirements, often called enhanced surveys.
Annual surveys
An annual survey takes place each year, usually within a window of three months either side of the anniversary date. It is a general examination of the hull, machinery and equipment to confirm the ship is being maintained and that nothing has changed since the last visit. The surveyor typically looks at:
- Hull and deck condition, hatch covers, closing appliances and watertight integrity.
- Machinery spaces, steering gear and emergency equipment.
- Any outstanding conditions of class or recommendations from earlier surveys.
An annual survey is not a full inspection. It is the regular check that keeps class valid between the bigger surveys.
Intermediate surveys
The intermediate survey replaces either the second or the third annual survey, so it usually falls around the middle of the cycle. It covers everything in an annual survey plus more detailed checks. Depending on the ship's type and age, that commonly includes examination of selected cargo and ballast tanks, thickness measurements and checks on coatings.
For older ships the intermediate survey gets closer to the scope of a renewal survey. Under most class rules, the extent increases noticeably after a ship passes roughly ten or fifteen years of age.
Special (renewal) surveys
The special survey, also called the class renewal survey, is the main event of the cycle. It is due every five years and, when completed, renews class for another five years. It is the most extensive survey: internal examination of tanks and spaces, thickness gauging of structure, pressure testing where required, and detailed checks of machinery and systems.
Because of its size, a special survey can usually be started before the due date, from around the fourth annual survey, and carried out in stages. Many class rules allow it to be completed up to three months before the due date without changing the anniversary date. Some ships use a continuous survey arrangement instead, where hull and machinery items are surveyed on a rolling basis so that every item is covered within the five years.
Societies may allow a short extension of a special survey in exceptional circumstances, but this is limited and decided case by case. Owners should not assume one.
Dry-docking and bottom surveys
The outside of the hull below the waterline is checked in a bottom survey. Under most class rules there are two bottom surveys in each five-year cycle, one of them as part of the special survey, and the gap between them is usually limited to around 36 months.
Traditionally both are done in dry dock, where the ship is out of the water and the hull, rudder, propeller, sea chests and valves can be seen directly. Some ships, depending on age, type and class notation, may have one of the two replaced by an in-water survey carried out by divers with the society's approval. This saves a dry-docking but is usually not available to older ships or those with certain conditions.
Dry-dockings are expensive and take a ship off hire for days or weeks. That is why owners plan them years ahead, and why a missed dry-docking is often the reason a survey falls overdue.
What "date of next survey" means in IACS data
The IACS Vessels in Class dataset includes two survey fields for each ship: date_of_survey, the most recent survey date reported by the society, and date_of_next_survey, the next survey due date it reported. The dataset does not label which type of survey each date refers to, and societies may report them differently.
In the records we have checked, the next survey date often sits about five years after the previous renewal, which suggests it tracks the class renewal due date rather than the next annual. EVER GIVEN (IMO 9811000), delivered in 2018, shows a last survey on 12/07/2023 and a next survey date of 24/09/2028. You can see it at vesselclassfinder.com/?imo=9811000. Treat this as a pattern, not a definition: confirm the actual survey schedule with the society's survey status report when it matters.
What the fields do give you is a quick check. A next survey date in the past, on a ship still shown as in class, is worth a question. A last survey date several years old on an older ship is also worth a look.
What "Survey overdue" means
If a required survey is not completed within its window, class is typically suspended, often automatically under the society's rules. The society may then withdraw class if the survey is still not done after a further period. Once the overdue survey is completed and any findings are dealt with, a suspended class can usually be reinstated.
In the IACS data this shows up in the reason_for_status field. In the 30 September 2026 snapshot, Survey overdue is recorded as the reason for the latest status change on 1,589 ships, the second most common reason after by society for other reasons (3,645). By society, the largest counts are Bureau Veritas (295), ClassNK (279) and RINA (245).
Read the reason together with the status. A ship showing Suspended with Survey overdue is out of class now. A ship showing Withdrawn with the same reason has left that society's class. If the status is Reinstated, the problem has been resolved, though the history may still matter for vetting or underwriting. The status codes guide and suspended vs withdrawn cover each case, and charter vetting and marine insurance explain why an overdue survey matters in those settings.
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Get a free API keyFrequently asked questions
How often is a ship surveyed by class?
Under most class rules, every year. There is an annual survey around each anniversary date, an intermediate survey in place of the second or third annual, and a special survey every five years.
What is a special survey on a ship?
It is the class renewal survey, due every five years. It is the most thorough survey in the cycle and, when completed, renews the ship's class certificate for another five years.
How often does a ship need to go into dry dock?
Typically twice in each five-year class cycle for bottom surveys, with the interval usually limited to around 36 months. Some ships may replace one of these with an in-water survey.
What happens if a ship's survey is overdue?
Class is typically suspended until the survey is completed. If it stays overdue, the society may withdraw class. Survey overdue is the recorded reason for 1,589 ships in the IACS data snapshot of 30 September 2026.
What is the difference between an annual and an intermediate survey?
An annual survey is a general check of the ship's condition. The intermediate survey replaces the second or third annual survey and adds more detailed checks, such as tank inspections and thickness measurements, depending on the ship's type and age.