Ballast tanks are the most aggressive environment on board: seawater alternating with damp air. They are also the place from which the submerged plating is measured without going into dry dock.
- ✓ Access to the submerged plating from inside, in the dry
- ✓ Scan mode against localised corrosion and pitting
- ✓ High gain on heavily corroded plate
- ✓ A-Scan to validate readings on an irregular backwall
- ✓ A handy, long-running instrument for confined spaces
- ✓ Data logger so nothing has to be written down by hand
A ballast tank alternates filling with seawater and emptying into damp, salt-laden air: it is the combination that accelerates corrosion more than any permanent immersion. Cargo tanks have a different history according to what they carry, and structural bulkheads suffer from both faces. In all three cases the damage is typically localised: patches, pitting, attack along the weld seams and behind the stiffeners.
This changes the way measurements are taken. A regular grid with one reading per node can pass right by the thinnest point without seeing it: scan mode is needed, which travels across the area and holds the lowest value met. Where the backwall is very irregular the echo becomes weak and noisy, and high gain and an A-Scan are needed to tell whether the reading is real.
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There is then an important operational advantage: the submerged part of the hull borders on these tanks. Measuring the plating from inside gives the same data that dry docking would give, but in the dry and without stopping the vessel for hull work.
What changes from one area to another
| Area | Prevailing mechanism | Point to watch |
|---|---|---|
| Ballast tanks | Alternating seawater and damp air | Widespread pitting: scan mode indispensable |
| Cargo tanks | Depends on the product carried | Internal coatings: check the measurement mode |
| Structural bulkheads | Corrosion on both faces | The figure belongs to the structure, not to the face measured |
Working in a confined space
Inside a tank the conditions penalise anything bulky, fragile or demanding of the operator’s attention.
- The instrument has to be used one-handed, in awkward positions
- Battery life has to cover the entry with no battery changes
- The data logger avoids having to note readings down on wet paper
- The display has to stay readable under artificial light
- IP65 ruggedness is not a detail: damp and knocks are constant
Time inside is limited by the safety procedures: an instrument that wastes time is paid for in extra entries.
Recommended instruments
| Instrument | Why in this application |
|---|---|
| Dakota CMX2-DL | 110 dB gain range and A-Scan: heavily corroded plate |
| Dakota CMX10-DL | Colour A-Scan readable even in poor light |
| Dakota ZX6-DL | IP65, rugged and handy: the instrument for going into a tank |
| Dakota MX1-DL | Large grid files when there are many tanks to survey |
Frequently asked questions
Because they alternate seawater with damp, salt-laden air. It is the wet-dry cycle that speeds the process up, more than permanent immersion would: at every emptying the surface stays damp and oxygenated, which is the ideal condition for attack.
Yes, and it is one of the main reasons for going in: the submerged part of the hull borders on the ballast tanks and the double bottoms. Measuring from inside gives the same data that dry docking would give, but in the dry, with standard instruments and without stopping the vessel for hull work.
Because the corrosion here is localised: pitting, patches, attack along the seams and behind the stiffeners. A grid with one reading per node can pass right by the thinnest point without picking it up. Scan mode travels across the area and holds the minimum, which is the value decisions are made on.
Back to Marine and Offshore. For a survey in tanks and holds, write to us giving the number of spaces and the condition of the internal coating.
Further information and contacts
Systems division and sensor integration partner : www.roder.it
More information about RODER SRL : about us
Contact for general information : info@roder.it
Contact for information about these products : info@roder.it
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