What an ESP Survey Demands of the Thickness Measurement Technician
SOLAS chapter XI-1, regulation 2 makes the 2011 ESP Code (IMO resolution A.1049(27), as amended) mandatory for bulk carriers and oil tankers of 500 gross tonnage and above. Its thickness measurement is reserved for a company certified by an organisation the flag Administration recognises, using operators certified to a recognised NDT standard who can select representative positions on ship structure.
The ESP Code splits into Annex A for bulk carriers and Annex B for oil tankers, each divided again by hull form — single-side skin and double-side skin bulk carriers, double-hull and other oil tankers. A technician works to whichever branch the vessel sits in, because the required extent of gauging differs between them. Three survey types drive the workload. Annual surveys call for thickness measurement when the surveyor considers it necessary or where extensive corrosion exists. Intermediate surveys start at suspect areas for ships five to ten years old and expand to full renewal-survey extent once the ship passes ten. Renewal surveys escalate in four age bands, moving from suspect areas alone to every deck plate, every bottom plate and three transverse sections once the ship exceeds fifteen years. Substantial corrosion — wastage above 75% of allowable margins — forces extended measurement before the survey is credited.
Source: IMO resolution A.1049(27) (2011 ESP Code), as amended by resolutions MSC.461(101), MSC.483(103) and MSC.525(106); SOLAS chapter XI-1, regulation 2; IACS UR Z17 Annex 1, section 1.
| Age band | Deck plating | Transverse sections | Bottom plating | Wind and water strakes | Always included |
|---|---|---|---|---|---|
| Age 5 years or less | Not required | None | Not required | Not required | Suspect areas |
| Over 5 up to 10 years | Two transverse sections of deck plating outside the line of cargo hatch openings | Two, outside the line of cargo hatch openings | Not required | In way of those two transverse sections, plus selected strakes outside the cargo length area | Suspect areas; members subject to close-up survey |
| Over 10 up to 15 years | Each deck plate outside the line of cargo hatch openings | Two, one of which is in the amidship area, outside the line of cargo hatch openings | Not required | All wind and water strakes within the cargo length area, plus selected strakes outside it | Suspect areas; members subject to close-up survey |
| Over 15 years | Each deck plate outside the line of cargo hatch openings | Three, one of which is in the amidship area, outside the line of cargo hatch openings | Each bottom plate | All wind and water strakes, full length | Suspect areas; members subject to close-up survey |
Where ESP sits in the regulatory stack
SOLAS chapter XI-1, regulation 2 makes the International Code on the Enhanced Programme of Inspections during Surveys of Bulk Carriers and Oil Tankers mandatory. The Code itself was adopted as IMO resolution A.1049(27) on 30 November 2011 and has been amended three times since — resolution MSC.461(101), resolution MSC.483(103) and resolution MSC.525(106), the last of which entered into force on 1 July 2024. A technician who quotes the 2011 text alone quotes a superseded document.
The Code applies to self-propelled bulk carriers of 500 gross tonnage and above and to self-propelled oil tankers of the same threshold. It covers surveys of hull structure and piping systems in way of cargo holds, cofferdams, pipe tunnels, void spaces and fuel oil tanks within the cargo length area, and all ballast tanks. Annex A carries the bulk carrier regime, split into Part A for single-side skin and Part B for double-side skin construction; Annex B carries oil tankers, split into double-hull and other than double-hull.
What the Code publishes is a minimum. Clause 1.1.3 states plainly that the extent of examination, thickness measurement and tank testing set out in it is the floor, and that the survey is extended when substantial corrosion or structural defects are found. Every figure in an ESP scope of work is a starting position the attending surveyor enlarges on the day.
The certificate that must exist before you board
Clause 7.2 reserves ESP thickness measurement for a qualified company certified by an organisation recognised by the Administration. Annex 5 sets out how a company gets there. It submits an outline of its organisation and management structure, its experience gauging hull structures, its technicians' careers, the ultrasonic equipment it uses with maintenance and calibration procedures, a written guide for thickness measurement operators, training programmes, and its measurement record format.
The recognised organisation then audits the company, and certification is conditional on an on-board demonstration of thickness measurement together with satisfactory reporting. Passing the paperwork is not enough — someone watches a technician gauge a real structure and reads the report that comes out of it. Firms preparing for that audit work through it on our IACS service supplier approval engagement.
IACS UR Z17 adds the personnel layer. The responsible supervisor holds Level II under a recognised national or international industrial NDT standard, with ISO 9712 named as the example. Operators hold Level I under such a standard and must have adequate knowledge of ship structures sufficient to select a representative position for each measurement. Z17 also requires a minimum of one year of tutored on-the-job training for operators and two years of operating experience for the supervisor, a route mapped on our UTM operator qualification page.
The survey planning meeting is the technician's briefing
Clause 5.6.2 requires a survey planning meeting before any part of a renewal or intermediate survey commences. The thickness measurement company operator sits in it alongside the attending surveyor, the owner's representative on board and the master or a qualified representative the master appoints. Clause 7.1.2 repeats the requirement from the thickness measurement side, which tells you how seriously the Code treats it.
The indicative agenda in 5.6.3 reads like a technician's job card. It covers the vessel's schedule, provisions and arrangements for gauging — access, cleaning and descaling, illumination, ventilation and personal safety — the extent of measurement, the acceptance criteria drawn from the list of minimum thicknesses, and the extent of close-up survey and gauging in light of coating condition and known suspect areas.
Three agenda items are pure craft: execution of the measurements, taking representative readings both generally and where uneven corrosion or pitting is found, and mapping areas of substantial corrosion. The ninth item fixes communication — how the operator reports findings to the surveyor and the owner's representative. Annex 14 requires the survey report to name where and when the meeting happened and who attended.
Extent of measurement by survey type
An annual survey carries no fixed gauging quota. Thickness measurement is carried out when the surveyor considers it necessary, or where extensive corrosion exists. The consequence sits in the next sentence of the same clause: if those readings show substantial corrosion, the extent increases per annex 10, and that extended work is completed before the annual survey is credited. A technician called for a handful of readings finishes a much larger job.
Intermediate surveys split on age. For a single-side skin bulk carrier five to ten years old, gauging is sufficient to determine general and local corrosion levels at the areas subject to close-up survey, with the stated minimum being suspect areas identified at previous surveys. Where the surveyor is satisfied by close-up examination that there is no structural diminution and hard protective coatings are in GOOD condition, that extent receives special consideration.
Past ten years the picture changes hard. The intermediate survey for a bulk carrier of 10 to 15 years, and again for one exceeding 15 years, runs to the same extent as the previous renewal survey. Internal examination of fuel tanks and pressure testing of all tanks drop out unless the surveyor calls for them. Beyond 15 years a survey in dry-dock becomes part of the intermediate survey, bringing the lower portions of cargo holds and ballast tanks into scope.
How renewal extent escalates with age
Annex 2 of Annex A Part A sets four age bands for renewal survey gauging on single-side skin bulk carriers. Up to five years, the requirement is suspect areas. Between five and ten, two transverse sections of deck plating outside the line of cargo hatch openings join the list, together with members subject to close-up survey and the wind and water strakes in way of those sections.
Between ten and fifteen years, deck coverage goes from two sections to each deck plate outside the line of cargo hatch openings, transverse sections stay at two but one sits in the amidship area, and all wind and water strakes within the cargo length area come in. Above fifteen, transverse sections rise to three, each bottom plate is gauged, and wind and water strakes are measured over the full length of the ship.
Transverse sections are chosen where the largest reductions are suspected to occur, or where deck plating measurements have already revealed them. A transverse section means every longitudinal member at that cut — plating, longitudinals and girders at deck, side and bottom, inner bottom and hopper side plating, longitudinal bulkheads, and bottom plating in top wing tanks. Reading that definition as the deck and the bottom is how a technician under-scopes a job.
Substantial corrosion and the coating trigger that changed in 2024
Substantial corrosion is defined at 1.2.9 as an extent of corrosion whose pattern indicates wastage in excess of 75% of allowable margins while remaining within acceptable limits. For ships built under the IACS Common Structural Rules the definition is numerical: a gauged thickness between tnet plus 0.5 mm and tnet. Two ships in the same berth therefore hit the trigger on different arithmetic.
Annex 10 sets what the extended measurement looks like. Suspect shell plate plus the four adjacent plates, with a five-point pattern for each panel between longitudinals. Three measurements in line across the web and three on the flange, over a minimum of three longitudinals in way of suspect areas. Transverse bands within 25 mm of the welded connection to the inner bottom on lower stools. Five-point patterns over one square metre of bulkhead plating.
Resolution MSC.525(106) tightened the coating trigger with effect from 1 July 2024. Where the text once required annual examination of a ballast tank whose hard protective coating was in POOR condition, it now reads less than GOOD condition. FAIR coating — local breakdown at stiffener edges and weld connections, or light rusting over 20% or more of the area — now pulls that tank into the annual cycle, with thickness measurement carried out as the surveyor deems necessary.
What the surveyor controls once gauging starts
Annex 14 hands direction of the gauging operation to the attending surveyor. Clause 3.1 gives the surveyor final say on extent and location after the overall survey of representative spaces. Clause 3.3 requires the surveyor to select locations so that readings represent, on average, the condition of the structure for that area. A technician who picks the shiniest plate in the tank works against the requirement.
Where owners want gauging to start before the overall survey, clause 3.2 lets it proceed on the understanding that planned extent and locations are subject to confirmation, and that findings generate additional work. Clause 3.4 requires measurements aimed at evaluating corrosion affecting hull girder strength to be taken in a systematic manner, so that all longitudinal structural members are gauged as required. Partial systematic coverage of a hull girder is no coverage at all.
The reporting duty runs live, not at the end. Annex 14 clause 2.2 requires prompt notification to the surveyor of excessive or extensive corrosion, pitting or grooving of any significance, structural defects such as buckling, fractures and deformed structure, detached or holed structure, and corrosion of welds. A technician who logs a fracture and mentions it at handover has breached the procedure.
The reporting format and the signatures that close it
Clause 7.3.1 fixes the minimum content of a thickness measurement report: the location of measurements, the thickness measured, the corresponding original thickness, the date the measurements were carried out, the type of measuring equipment, the names of personnel and their qualifications, and the operator's signature. Annex 8 supplies the reporting forms — TM1-BC through TM7-BC for bulk carriers — and requires the maximum allowable diminution to be stated.
The general particulars page carries the audit trail: ship's name, IMO number, class identity number, the company performing the measurement, who certified that company, the certificate number and its validity dates, the place and first and last dates of measurement, details of the measurement equipment, and the qualification of the operator. UR Z17 adds that the report includes a copy of the certificate of approval.
Clause 7.3.2 closes the loop — the surveyor reviews the final report and countersigns the cover page. UR Z17 requires the attending surveyor's signature as documented verification of each separate job. That signature converts a set of readings into evidence a classification society will act on, which is why sloppy member identification gets a report bounced rather than queried. The gauging craft behind the numbers is covered on our hull thickness gauging technician page.
Building the qualification as a US technician
American gauging technicians reach the standard by a different route than European ones. UR Z17 names ISO 9712 as its example of a recognised standard, and ISO 9712 certification is issued by an independent body. ASNT SNT-TC-1A, the scheme most US employers run, is employer-based — the employer certifies its own people against a written practice. The Z17 wording admits a recognised national standard, which puts the weight on the quality of that written practice.
That places weight on documentation most NDT shops treat as filing. A written practice that names ultrasonic thickness measurement as a method in its own right, sets the training hours and experience behind each level, and defines the general, specific and practical examinations, is the document an auditor reads first. We build those documents on the written practice development engagement and stress-test them against class expectations through an NDT programme audit.
Sea time counts too. Z17 clause 1.3 requires the operator to have adequate knowledge of ship structures sufficient to select a representative position for each measurement. No ultrasonic certification teaches that; it comes from learning where a hopper knuckle sits and why the aft end of a ballast tank wastes first. The structural grounding behind it is laid out in our maritime structures orientation.
Does an ESP thickness measurement technician need to be certified before the survey starts?
The company must hold a certificate of approval issued by an organisation recognised by the Administration before it gauges anything. Certification follows a document review, an audit, and an on-board demonstration of measurement plus satisfactory reporting. The operator carrying out readings must hold certification to a recognised national or international industrial NDT standard, and the responsible supervisor must hold Level II under such a standard.
Who attends the ESP survey planning meeting?
The meeting is held before any part of a renewal or intermediate survey begins. Attendance covers the attending surveyor or surveyors, the owner's representative on board, the thickness measurement company operator, and the master or an appropriately qualified representative appointed by the master or company. The survey report records where and when the meeting took place and who attended.
What triggers extended thickness measurement during an ESP survey?
Substantial corrosion triggers it — corrosion whose pattern indicates wastage above 75% of allowable margins while staying within acceptable limits. On ships built to the IACS Common Structural Rules the trigger is a gauged thickness between tnet plus 0.5 mm and tnet. Once found, the surveyor directs additional readings to delineate the wasted area, and that extended gauging completes before the survey is credited.
Must a class surveyor watch every ESP thickness reading?
Thickness measurement not performed by the recognised organisation itself must be witnessed by its surveyor, who stays on board to the extent necessary to control the process. That attendance is recorded, and the same rule covers readings taken during voyages. The surveyor selects final extent and locations after the overall survey, directs the gauging so readings represent average condition, and countersigns the report cover page.
How often must a thickness measurement company renew its class approval?
Every three years. IACS UR Z17 sets a five-year renewal cycle for service suppliers in general, then carves out firms engaged in thickness measurements for renewal or endorsement at intervals not exceeding three years, by verification that original conditions are maintained. The 2011 ESP Code annex 5 states the same three-year cycle. Approval is withdrawn where measurements were improperly carried out or results improperly reported.
Which ships fall outside the ESP Code but still need class-approved gauging?
The ESP Code reaches self-propelled bulk carriers and oil tankers of 500 gross tonnage and above. Everything else — container ships, general cargo, ro-ro, chemical carriers — gauges to IACS UR Z7 instead. UR Z17 still applies: its thickness measurement scope excludes only non-ESP ships under 500 gross tonnage and all fishing vessels, so a class-approved firm and certified operators remain mandatory.