Special Survey Thickness Measurement: Renewal and Intermediate Survey Gauging

The hull survey cycle: annual, intermediate, docking and renewal surveys

Class keeps a ship's hull on a five-year cycle. Annual surveys fall within a window around each anniversary date; an intermediate survey takes the place of the second or third annual survey; the bottom is examined at docking surveys twice in each five-year period, one of them with the renewal survey; and the renewal survey closes the cycle. It may be commenced at the fourth annual survey and progressed through the following year to completion by the fifth anniversary. Under the general hull survey requirements, work started earlier is credited only if the whole survey is completed within 15 months.

The table summarises what each survey typically asks of thickness measurement. The unified requirements set minimums; the surveyor extends them wherever findings warrant, the survey programme adds suspect and critical areas on ESP ships, and the detail differs between general cargo ships and ESP tankers and bulk carriers. Under the ESP requirements, the same survey and thickness measurement of a space cannot be credited to both an intermediate and a renewal survey.

Why the 15-year and 20-year surveys are the expensive ones for steel renewal

Three things coincide at the third renewal survey. Coatings come first: the IMO performance standard for ballast tank coatings, made mandatory by SOLAS regulation II-1/3-2, is built around a target useful life of 15 years, and ships built before it applied may lose their original coating sooner. Once coating has gone, corrosion accelerates in the warm, wet, cycling environment of a ballast tank. Margins come second, because fifteen years of general wastage has consumed much of the allowable diminution on members built with modest corrosion additions. Extent comes third: the gauging widens sharply, so diminution that existed at ten years but was never measured is now recorded and has to be dealt with.

The fourth renewal survey compounds all three. Bottom plating and wind-and-water strakes are gauged far more completely, more transverse sections are taken, and the substantial corrosion found at fifteen years has had five more years to progress. The steel renewed at the 20-year survey is frequently the largest quantity of the ship's life, and it concentrates in predictable places: on tankers, under-deck structure in cargo tanks and the webs, brackets and longitudinals of ballast tanks; on bulk carriers, hold frames and brackets, topside and hopper tank structure, hatch coamings and corrugated bulkheads with their stools; on container and general cargo ships, double-bottom and side ballast tanks and hatch covers.

The yard's steel invoice is only part of the burden. Steel found late extends the docking, and the extra days off hire can matter more to the owner than the steel itself. That is the argument for knowing the scope before the ship arrives.

Pre-docking gauging and the steel renewal estimate

Pre-docking gauging is the owner's own thickness measurement, taken months before the docking at a berth, at anchorage or during a lay-by. It concentrates on the structure most likely to need renewal: ballast tanks, accessible cargo holds and, on tankers, cargo tanks that have been cleaned and gas-freed; decks, hatch covers and coamings; side shell above the waterline; and targeted transverse sections. It is not credited to the renewal survey and needs no class approval, but it follows the same method and reporting standard, so the yard and the surveyor can compare it directly with what they find later.

The output is a steel renewal estimate: the plates and members at or beyond allowable diminution, those in the substantial corrosion band whose trend suggests they will cross the limit before the survey, and marked-up drawings showing where they are. Each item carries its dimensions and original thickness, so the superintendent or the yard can calculate steel weight. The estimate becomes the steel section of the repair specification, which means the yards' quotations describe the same work and can be compared, and steel of the right grades and thicknesses, with the certificates the society will ask for, can be ordered before the ship arrives.

An estimate remains an estimate. External bottom plating, upper structure that needs staging, and tanks that cannot be cleaned in service are often out of reach before docking. The report lists what was not gauged and suggests how to allow for it, and the class gauging in the yard may still find more. Planning against a known core of renewal, with an allowance for the unseen remainder, is still far better than planning against nothing.

Intermediate survey thickness measurement

The intermediate survey sits between renewal surveys and is easy to underestimate. Under the general hull survey requirements, ships between five and ten years have representative ballast tanks examined and ships over ten years have all ballast spaces examined, with cargo spaces added on some older ships; gauging follows the surveyor's findings and any substantial corrosion carried forward. On ESP ships the scope widens much further from about ten years, as the enhanced survey programme page explains. On older double-hull tankers the rules allow the intermediate survey to be started at the second annual survey and completed by the third, which lets an owner spread the gauging over two port calls instead of one.

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