Corrosion Mapping · Vlissingen, Netherlands

Encoded ultrasonic corrosion mapping of ship plating and piping in Vlissingen-Oost

A spot thickness gauge cannot say how far wastage spreads, and for some plates in Vlissingen that is the question that decides the yard job. Is the hopper floor of a dredger worn evenly or through in patches? How far does the wet-area wastage run under a ferry's car deck? Atlantis NDT sends a team from India with an encoded ultrasonic scanner to map those areas, producing a colour thickness map that sizes the renewal. The work is done in the dock at Vlissingen-Oost, at the yard quay or alongside at the Sloe berths.

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Dredger hoppers mapped, not sampled

Abrasion by spoil wears hopper plating unevenly. A scan across the floor shows the thin band where flow concentrates and the area it covers, so the insert plate is cut to measured extent and not to a guess drawn from scattered readings.

Wet-area wastage on car decks

Water finds its way along car-deck drain lines and under tank-top plating. A map of the plating along those paths shows how far the loss has spread and which plates are near the limit, for the yard's insert plan.

Scrubber pipework at dock

Wash-water and overboard lines on scrubber-fitted ships waste from the inside. When the ship is in dock and pipework is accessible, mapping a spool shows the length and depth of loss, so only the worn section is replaced.

Where can corrosion mapping be done for ships at Vlissingen?

The main location is the dock at Damen Shiprepair Vlissingen. With the ship on blocks, bottom plating, sea chests, hopper floors and bottom doors can be scanned from outside, and tank structure from inside once the tank is gas-free. The yard supplies staging and lighting, and the scanner needs a clean, reasonably flat surface with a couplant film, so the yard's preparation sequence matters.

Alongside at the yard quay or a Sloe quay, the team scans decks, hatch coamings, tank tops accessible from the hold and piping on deck or in the engine room. The Westerschelde anchorages are rarely suitable, because the scanner needs a steady setup and enough time on each area. If a ship is waiting for the tide, we recommend gauging with a spot gauge for the estimate and mapping later at a berth.

In dock: bottom plating, hopper floor, bottom doors and sea chest areas.
In gas-free tanks: tank tops, bulkheads and ballast tank bottoms.
On deck at the quay: car-deck plating, hatch coamings and working deck.
In the engine room and on deck: scrubber wash-water and overboard piping.

Which ships in Zeeland benefit most from mapping, and why?

Dredgers lead the list, because spoil abrasion produces localised, fast wear that a normal TM grid will miss. Ferries are next: car-deck plating and the steel beneath, along drains and wet zones, shows pattern wastage that mapping resolves. Offshore support vessels have working decks and ballast tank bottoms where corrosion pits and patches. Bulk carriers and general cargo ships come for tank tops and hold bottoms where cargo lay wet.

Ships fitted with scrubbers belong here for pipework, particularly on older ships where the scrubber was retrofitted to hull and piping already some years old. Mapping is a decision tool, so we recommend it where the answer will change what the yard does: where a hopper floor might be renewed in part or whole, or where a deck might be patched or replaced.

Offshore support vessels deserve a note of their own. Their ballast tanks are cycled between full and empty during a campaign, which wets and dries the plating repeatedly, and that pattern favours pitting on the tank bottom and along the lower stiffener flanges. A scan of one or two representative tank bottoms at docking tells the owner whether the tanks across the fleet's sister ships need attention, which is a decision that one scan can inform.

How is mapping scoped on a ship, and what does it need?

We choose the areas with the superintendent from the earlier thickness readings, choosing those close to the allowable limit or where the trade predicts rapid loss. Each area gets a datum point and a grid, marked on the plate so the same area can be scanned again at the next survey. The scan resolution is set from the expected size of the wastage: finer for pitting, coarser for general loss.

The surface needs preparation. Loose scale and coating are removed so the probe sits flat, and a couplant film is applied. We scan in a raster pattern with an encoded scanner, which records position with each reading, then check the lowest readings and the deepest pits by hand. The colour scale is fixed and stated on the map, so a reader comparing two maps sees the same colours for the same thickness.

We also state the limits of the method in the report. A scan reads the thickness under the probe path, so areas the scanner could not reach, such as the edges of a plate or the toe of a stiffener, are gauged by hand and marked as such. Where coating or heavy scale prevented good contact, the map shows the gap as no data, not as a thin reading, so no one reads a blank as sound steel.

How is a corrosion map turned into a steel renewal decision?

The map shows the area below each threshold, the minimum and mean thickness and the location of deep patches. From it, the yard can draw an insert boundary that includes the plate below the renewal limit and leaves sound plate alone. For a hopper floor, this might be a band across the flow path. For a car deck, it might be a strip along a drain line.

Mapping also shows the shape of the loss, which tells the owner why it happened. A band along a flow path points to abrasion, a patch under a drain points to water, and scattered pits point to coating breakdown. The owner can then decide whether to renew, recoat or change operating practice. If the vessel was mapped at a previous survey, a comparison shows the rate of loss, which sizes how long the remaining steel will last.

For scrubber pipework the map is read differently. The lowest wall is usually at bends, tees and downstream of valves, where flow turns, and the map locates them along the spool. That tells the yard to replace the fittings and the first section of straight pipe beyond them, while the straight runs between often remain serviceable for several more years.

How is a mapping job planned with the yard in Vlissingen?

Mapping takes longer per area than spot gauging, so planning matters. We ask the superintendent for the areas of interest, the earlier TM report, the yard's preparation plan and the dock days available. Scanning follows cleaning and precedes blasting for external surfaces, so we agree the dates with the foreman. For tank areas, we agree the gas-free plan with the chief officer.

A team from India needs visas before travel, and the yard needs names for the induction. The agent arranges gate passes. Scanner equipment travels with the team, and we check it on arrival against a reference block. Tell us the plate areas and their approximate size, and we estimate the scanning days. If time is short, we prioritise the areas where the answer is most likely to change the yard's plan.

Areas of interest with drawings and approximate sizes.
Earlier TM reports for the same areas.
Yard's cleaning and preparation plan.
Dock days available and the blasting start date.

Where corrosion mapping sits in the survey cycle

Every renewal survey asks for more thickness measurement than the last, and for oil tankers, chemical tankers and bulk carriers the ESP Code adds close-up survey on top. The table is the usual shape of the work; the class rules and the surveyor set the exact extent, which is why the measurement plan for a job at Vlissingen is agreed in writing before mobilisation.

Survey	When it falls	What the thickness measurement usually covers
Annual survey	Every year, within three months of the anniversary date	General examination. Gauging where the surveyor finds suspect areas or where substantial corrosion was recorded at an earlier survey.
Intermediate survey	In place of the second or third annual survey	Suspect areas, and on older ships and ESP ships selected ballast and cargo spaces with close-up survey; the extent grows with age.
Renewal survey No. 1	Around 5 years	The first full baseline: suspect areas and selected plating and stiffeners, usually with modest steel renewal.
Renewal survey No. 2	Around 10 years	Wider gauging of deck, bottom and side plating, transverse sections across the cargo area on ESP ships, and more close-up survey.
Renewal survey No. 3	Around 15 years	Extensive gauging of cargo and ballast tank structure and more transverse sections. The survey most likely to turn up steel renewal.
Renewal survey No. 4 and later	Around 20 and 25 years	Gauging extended to most structural members in cargo and ballast spaces. Charterers often add a Condition Assessment Programme.
Docking survey	Twice in each five-year period	Bottom and side shell, sea chests, rudder and stern area while the hull is dry; suspect bottom plating gauged.

How the job runs at Vlissingen

The sequence for a thickness measurement job at Vlissingen is short and written down, so the yard, the surveyor and the superintendent know who arranges what.

Send the vessel name or IMO number, ship type, class society, the survey due (annual, intermediate, renewal survey number, CAP or owner's condition survey), the port and dates, and the scope you expect.
We confirm the scope against the class rules for that survey, prepare the thickness measurement plan with frame and section references, and say whether the measurement will be credited to a class survey.
Mobilisation is planned around visa lead times and the vessel's schedule; the ship's agent arranges port access, and the yard or ship arranges staging, lighting, ventilation and gas-free certificates.
Gauging with calibrated ultrasonic thickness gauges: through sound coatings only where the gauge's through-coating mode has been verified on a coated reference, otherwise on cleaned bare steel, with every reading tied to its frame, strake and location on the sketch.
Readings are compared with original thicknesses as work proceeds; substantial and excessive corrosion and any cracks are reported to the attending surveyor and the superintendent the same day.
The final report is issued on the class thickness-measurement formats with sketches, a summary of locations exceeding the allowable margins, piping tables where piping was in scope, and the gauge calibration and technician certificates.

Who does the work, and when class approval applies

We send the team from India to the ship for each job rather than keeping staff at the port, and plan the mobilisation around visas and the vessel's itinerary. The work is planned and supervised by the founder, our Responsible NDT Level III (ASNT NDT Level III in UT, RT, MT, PT and VT, with more than eleven years of international field experience). Technicians hold certification under the company's written practice to ASNT SNT-TC-1A; Atlantis is ISO 9001 and ISO 14001 certified.

Thickness measurement that a classification society credits to a class survey has to be carried out by a firm approved by that society under IACS UR Z17. Atlantis is in the process of obtaining UR Z17 approval from DNV, Lloyd's Register and ABS. Until a certificate is issued, the readings we take are for the owner's own decisions, not for credit to a class survey, so tell us the vessel's class and the survey due when you enquire and we set the scope for the right purpose. Owner's and charterer's condition gauging, pre-docking steel estimates, pre-purchase and on-hire or off-hire gauging, and scrubber and piping condition checks between surveys do not need class approval.

Governing codes and standards

ISO 16809 – Ultrasonic thickness measurement
ISO 16810 – Ultrasonic testing: general principles
IACS UR Z7 – Hull classification surveys
IACS UR Z10 series – Hull surveys of tankers and bulk carriers
IACS Common Structural Rules for oil tankers and bulk carriers
ASNT SNT-TC-1A – Personnel qualification and certification in NDT

What you receive

C-scan thickness maps with a fixed, stated colour scale and datum references
Minimum and mean thickness and the area below each threshold, per mapped area
Hand-checked minimum readings and deepest pit depths
Insert boundary drawings for the yard, with approximate sizes
Comparison with earlier maps where a previous scan exists
Raw data files, calibration records and the scanning procedure

Industries served

Trailing suction hopper dredgers
Ro-pax and ro-ro ferries
Offshore wind support and installation vessels
Bulk carriers and general cargo ships
Chemical and product tankers
Ships fitted with exhaust gas cleaning systems
Ships under pre-purchase inspection

Frequently asked questions

Can a dredger hopper floor be mapped in the dock at Vlissingen?

Yes. With the ship on blocks and the hopper cleaned, the floor can be scanned from inside. The scanner needs a clean, flat surface with couplant, so the yard's cleaning matters. The map shows where abrasion has thinned the plate and how large the area is, which sizes the insert.

Is mapping better than spot gauging for a ferry car deck?

For the wet areas along drains, yes, because it shows how far wastage spreads from a spot reading. Spot gauging remains the right tool for the general grid. We recommend mapping where the answer will change what the yard does, such as renewing a strip or a whole plate.

Can you map a ship at anchor in the Westerschelde?

Rarely. The scanner needs a steady setup and enough time on each area, which a launch transfer and sea state seldom allow. For a ship waiting for the tide, a spot gauge gives the estimate, and mapping is done later at a berth or in the dock.

How is a scrubber pipe spool mapped?

With the pipe accessible and clean, the scanner is wrapped around the spool or run along it, and the map shows the length and depth of internal loss. Only the worn section needs replacement, which is why mapping at dock saves renewing sound pipe.

Can maps from different surveys be compared?

Yes, if the datum marks and the colour scale are the same. We mark each area with a permanent datum and state the scale, so a later scan overlays the earlier one. The comparison shows the rate of loss, which tells the owner how long the remaining steel will last.

Can corrosion mapping at Vlissingen be credited to a class survey?

Planning a survey or a drydock?

Send the vessel name or IMO number, the class, the survey due and the port. We reply with the scope, the mobilisation plan and a quote for that vessel.

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Request a ship gauging quote

Send the vessel name or IMO number, the class society, the survey due and the port. We reply with the scope, the mobilisation plan and a quote for that vessel.

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