Ultrasound to inspect external chain plates and keel bolts

[thinwater]

Note: I did ultrasound inspections of refinery equipment professionally for decades. Much or this I am pretty certain of, since I have done very similar work.

Has anyone used ultrasound to inspect accessible structures like external chain plates or keel bolts?

I've seen surveyors walking around with UT gauges (thickness only), but I hesitate to tell them just how limited they are. They can estimate the average thickness of steel, but since they only display a single number, they tell you nothing about the condition of the backside (beginning corrosion), cracks (partial echos) or laminations/delaminations. They need more advanced equipment.

For example, a cracked, or even parochially cracked bolt, is simple to spot by placing the transducer on the head. A thickness gauge will show some false thickness (since the bolt is too long for it), but an A-scan unit will show an echo at the crack, up to a foot away. I've tested this on damaged bolts.

In the case of metal corrosion, such as a chain plate, often by the time the damage is obvious as thickness loss (averaged over the head of the transducer), the plate is horrible. However, an A-scan unit will show surface roughness by echo scatter when only very slightly corrosion is present. This is really aided on chain plates by the fact that they are polished; the new echos are very clean.

I've also found it useful for confirming the positions, size, and thickness of imbedded backing plates, inspecting mast corrosion at the foot and near spreaders.

The thing of it is, the average surveyor is NOT a trained or certified NDT technician. In petroleum tank inspection, for example, inspectors must be not only certified in the standard, but also in the methods used.

So all that said... any experiences?

[a64pilot]

Not boats, but have used an ultrasonic thickness gauge for years on aircraft tubing, if tubing is .032 and thickness gauge shows .025, then tube is toast from internal corrosion. I assume the same for a boat hull, thickness variation is due to corrosion so you should be able to tell corrosion amount by measuring thickness?
Aircraft inspectors have to be either level 1 or 2 I believe level 3 guys usually write inspection protocols etc, they are the schiznit
I was level 2 but non current now.

On edit, old method for aircraft tubing and still used by many was an ice pick, or awl. If you can knock a hole in the tube, it's bad

[thinwater]

^^ for general corrosion this is true, but for pitting, looking at the echo width is a better indicator of incipient corrosion. The reason is that thickness is the average over the size of the transducer head, typically about 1/2-inch. Pits can be much smaller.

[Captsteve53]

I am in full agreement that the average Marine surveyor has not the experience or qualifications to under take full Ultra sound work other than the perhaps the normal thickness testing, but , if that degree of examination is required or asked for I'm sure every Surveyor I know would happily point the client in the right direction to gain such expertise, that said professional bodies such as the IIMS-Lloyd's with whom I am a full member have VERY high standards with training required on all Survey equipment, The IIMS run a points system for training/knowledge that must be met yearly other wise you loose your standing(so keeping up to date is mandatory), I'm not sure re American societies and their requirements,

Here's a few issues/points in reality regarding Ultra sounds on hulls etc versus static inspections in controlled situations:

-Ultrasonic signals don't like traveling through air. As a result, foam core acts as a significant barrier to signal penetration. If you are concerned with detecting water intrusion into core- use a moisture meter. If you would like a global image of water intrusion into a cored hull, then thermal imaging is your best bet.

-Low tech laminates like thick, hand laid hulls fiberglass tend to have many tiny air bubbles that cumulatively tend to disperse a UT signal. In some cases, UT is like using a scalpel when a blunt instrument would suit you fine. There are times when you just need to break out the hammer and grinder, mate.

-UT sends out a signal that is a beam. The thicker the laminate, the lower the transducer frequency and the wider the beam. This can limit the ability send signals into tight spots such as inside corners.

-The A-scan UT that field units offer cannot produce a pretty, global, color picture of the entire part. The only way to get that is with a monstrous jig and a robotic arm such as you would see in a production setting. The accuracy and information is the same- it's just that the individual wave forms cannot be collated and plotted into a single graphical image.

-UT records separate waveform images as you scan the part. You need to be realistic in deciding what size defects are significant as well as what kind of inspection grid you need to set up on the boat. You can't expect to ferret out every 1/4" bubble in the hull of an 80' boat. Well, you could...but you wouldn't want to pay me the time it would take to do it!

There is no single magic inspection tool that is ideal for all situations. Ask anyone in the composites industry and they'll tell you that marine structures can be horrendously difficult to inspect when compared to more conventional production laminates. UT is particularly adept at assessing the following situations:

-Solid vacuum bagged carbon spars. The same clean, dense laminates that make it virtually impossible to detect subtle damage with a hammer are easily scanned with UT. Ultrasound will not only tell precise overall thickness, but also the depth of the defect/anomaly. Poor consolidation (loss of bag pressure), dry fibers, and voids are all good examples of detectable flaws.

-Bond line issues. Ultrasound can pick up the transition between skin coat and structural laminate. A clean line will transmit the signal, where a delamination or "never bond" will show a clear interruption on the waveform.

-Secondary bond damage. As noted above, UT can pick up delamination fairly quickly. There's no need to tent off and demolish the interior of the boat in order to determine the extent of damage.

-Quality Assurance. A new part can be scanned to check for defects and laminate consistency. Critical areas of the part can be scanned and the readings can be archived so that, in the event of future trauma, the part can be scanned again to see if there are any anomalies compared to the "as built" baseline readings.

I offer the forgoing only as advice with no preconception, with no legal implications and without prejudice.

Regards Steve Warren (MIIMS-Lloyd's Accredited)

[bletso]

Looking for a crack in a chain pale can not be done with a simple thickness gauge. It will take a proper UT unit and angled transducer using a shear wave. With that approach it is very doable. Advantage is it can be don in situ. Set up properly it could find corrosion on the underside with an experience tech.

[UNCIVILIZED]

Captsteve53 thanks for the great & very informative post. Might you have any tips on where to look for people qualified to do such testing on boats, as well as what questions to ask them so as to winnow the wheat from the chaff (vet them) so to speak?


Also, I know that there are various NDT forums out there. What are your guys favorites, & why?

[thinwater]

CaptSteve53: I was never certified in composites, but I have fooled with them, and everything you say rings true. I've examined parts with known defects. As you say, inspecting the whole vessel with field equipment is impractical. I was focused on metal parts, since that is my experience.

Bletso. Yes, shearwave for cracks perpendicular to the surface. Sometimes it is also possible to access the edges of the plate.

Other NDT methods? I'm not a huge fan of penetrant or Magnaflux around boats. X-ray has its uses, particularly for new welds.

In the US the only certification group I am familiar with (referenced in most regulations) is ASNT. However it is a general certification, not marine-specific... although that should not matter. https://www.asnt.org/MajorSiteSections/NDT-Resource-Center/Codes_and_Standards/NDT_Certification_System.aspx.

A level III certification is generally required, unless there is a level III person guiding.

[Captsteve53]

Uncivilized ,

As a member of the IIMS (CHECK OUR WEB SITE:https://www.iims.org.uk/)

you can find surveyors world wide (and check their strengths and area's they specialize in on line- then perhaps a direct contact to establish a specialist in your area)

As mentioned re training being a member we are fortunate that many companies do online web video sessions (some open to public)all at a cost i should add! with recent relevant training from TRITEX NDT as: Ultrasonic Thickness Gauges to Check Metal Thickness | Tritex NDT

Others as:https://www.gemeasurement.com/inspec...-flaw-detector

Personally i dont really get to much time to visit any of the NDT forums, as you state there's quite a few out there, being affiliated with Lloyd's i get updates posted regularly on all aspects from Ultra sound use/issues to the latest hot item of using drones for marine inspections from masts to tanks,

As for experience /questions I would ask for a sample of previous UT work reports/and equipment they use,

Training again:/This weekend/next week the IIMS is running a three day workshop in Singapore (Basically a must attend in order to stay affiliated!)

Cheers Steve