Ultrasonic Thickness Measurement of Steel
Ultrasonic Thickness Measurement (UTM) is used to measure the thickness of steel , aluminium and other materials where only one side of the material is accessible to the surveyor. Typically the measurements are taking of the hull structure in order to determine when the material is excessively corroded. Therefore it has lost significant thickness when compared to the original thickness and steelworks would be required.
Who requires UTM Surveys?
UTM surveys are performed for buyers in the context of a pre-purchase survey of a steel or aluminium craft. Steelworks are expensive and it is important their costs are taken into account when considering a purchase.
Owners may also require UTM surveys for their own use, as a periodic monitoring of the hull condition or to determine steelworks before proceeding to a shipyard.
Insurers may require a UTM survey of a steel boat of a certain age before insuring or continuing to insure it.
How does it work?
The gauge has a probe which is put in contact with the material. The probe sends a pulse of high frequency ultrasonic sound through the material which echoes off the back wall of the material and the time taken for the echo to be received is recorded.
The velocity of sound passing through typical materials is known. For instance Mild Steel has a velocity of 5920m/s but the actual steel being measured may differ slightly from this velocity depending on it’s manufacture.
The gauge will make a calculation using the time taken for the echo to be received and also the velocity of the material in order to calculate the thickness measured.
What is Multiple Echo Ultrasonic Thickness Measurement?
There are two types of Ultrasonic Thickness Gauge. The first is a Single Echo Gauge which measures using one echo. This will measure the thickness of the material but it will also include the thickness of the coating in the measurement. The problem is that coatings have velocity generally 1/3 of steel therefore the thickness value will include the steel and 3 times the coating.
The second and subsequent echoes reflect backwards and forwards between the front and back walls of the material. A Multiple Echo Thickness Gauge will consider these echoes which ignore the coating.
Therefore with a Multiple Echo Thickness Gauge accurate through coating measurements can be made. So there is no need for the inconvenience of removing coatings for Thickness Survey.
In practice the Single Echo gauges can still be used over coating as long as the surveyor considers that the values measured will include about 3 times the coating thickness.
What is the Accuracy of these measurements?
Accuracy depends also on the operator but the gauges generally are quite accurate to 0.1 or 0.05mm. It is likely the operator will set the gauge to 0.0 resolution rather than 0.00 resolution however because for the purpose of determining steelwork renewals the higher resolution is not necessary.
What is the minimum thickness that can be measured?
This depends on the probe and gauge used. The higher Mhz probes like 5Mhz can measure thinner minimum thickness.
The operator may have difficulty measuring through thick coatings or on corroded material with a 5Mhz probe and may select a 3.5Mhz or 2.25Mhz probe which give stronger signals. The lower probe frequency (in Mhz) and the larger the size of the probe gives more penetrating power.
Considering Cygnus probes, the 5Mhz measures down to 1mm, the 3.5Mhz down to 2mm and the 2.25Mhz down to 3mm.
Therefore by lowering the probe frequency there results an increase in minimum thickness measurable. So there is a trade off between this and penetrating power.
In order to take measurements a gel or oil must be used in-between the probe and the surface of the material. This is called couplant. Typical couplants are water based and contain glycerine, others are oil based.
After the measurements are taken, spots of couplant will remain on the paint. With water based couplants, these will dry and can be washed off with water without any problem. Oil based couplants are more difficult to remove therefore are less preferable.
It is important to properly remove couplants before recoating otherwise paint defects will occur when recoating.
Water based couplants can also be hydroscopic therefore promoting corrosion if applied on bare steel and not removed. Some water based couplants include rust inhibitors.
What are the Benefits of Triple Echo used by Cygnus and Tritex Gauges?
Cygnus and Tritex Gauges use the Triple Echo method. They measure three echoes and then compare them. If the echoes do not match then the reading is not displayed. This is a more accurate way of working because the gauges are checking the readings taken.
How long will a UTM survey take?
It is not a small job to make proper measurements of a hull. Expect 1-2 days for the measurements of a say 50ft hull and then the surveyor will need more time to make the report and diagrams if required.
How will the Surveyor take the readings?
The surveyor will usually divide the hull into sections of 300mm or 500mm squares and then take minimum 5-9 readings within these squares. The general method differs between surveyors and this is a typical method where the welds between plates cannot be seen as they have been ground down.
Where thin areas are detected the surveyor must start again at these areas and take multiple measurements at further distances away from the original thin readings in order to determine the full extent of the thin areas.
What are the limitations?
The surveyor cannot see the backwall of the material while taking measurements so he is working partly “blind”. It is also likely that the plating which is heavily corroded on the interior surface is inaccessible in the interior of the boat due to furniture and other constructions being in the way. These are the areas that have remained with coatings not maintained over the years due to their inaccessibility and therefore have corroded over the years due to leaks, standing bilge water and condensation.
It is not possible to take measurements so close that they cover the entire hull perfectly. This would be economically impossible as it would take too much time. Therefore the surveyor has to take sampling readings and then further concentrate on areas where thin material is found from these sample readings. Therefore it is possible that the surveyor may miss some small thin areas.
What are the Factors which affect Measurements?
It is more difficult to take measurements on heavily corroded material. It may be impossible on occasion and a disc grinder may be needed to clean the material so measurements can be taken.
If the coatings are delaminating from the metal, ie they have become not properly adhered then thickness readings cannot be taken over the delaminated coatings and these must be removed at the measurement points using a chipping hammer.
Some coatings can be problematic to measure through and layers of different coatings can also be problematic. Some antifouls are easier to measure through when they are wetted with water.
Thick coatings can be a problem but the Cygnus and Tritex Gauges can measure through 6mm or even 20mm of coatings on Deep Coat mode.
These factors are not affecting the accuracy of the readings but may result in readings not being possible.
Considerations of Steelwork Renewals
What is Diminution?
Diminution is the term used to describe the loss of thickness the material has sustained from corrosion in comparison to the original thickness. The surveyor will consider the diminution and recommend repairs where plating has lost significant thickness. Typical diminutions where steelworks are required would be in the range of 20-30% depending on the original thickness of the material and the part of the vessel’s structure that the material serves.
What is an Insert?
An insert is where the hull plating has been cropped back to sound steel where it was corroded and a new plate has been inserted and welded in. This is considered to be an ideal repair.
What is a Doubler?
A doubler is a plate which is welded to the original plating of the hull where the plating has corroded to the point where steelworks are required. Doublers will greatly exceed the thin area and be welded to sound steel plate. Of course doublers remain only as good as the original plate they are welded to. Doublers may also be used when the boat is constructed in order to strengthen areas such as areas where keels are attached.
What other Methods will the Surveyor Use in a Hull Survey?
UTM is only one of the techniques used. The surveyor will also use more traditional techniques. Hammer sounding, where the surveyor uses a hammer along the hull and listens to the change in note may identify problem areas. Similarly brutal use of a large hammer and use of a centre punch or a chipping hammer where UTM readings were not possible to obtain, may identify plating with very severe diminution.
The surveyor must also assess factors like pitting corrosion. These are localised losses of material caused by galvanic corrosion resulting in pits. The depth and percentage coverage of the pits over a plate must be assessed in order to determine steelwork renewals. The pits may be to an depth that spot welding is required to fill them in or the coverage may be to an extent where plate renewal would be more cost effective than spot welding.
The surveyor must also assess the quality of welds and if there was any defect when they were made or if they have become fatigue cracked or are corroding.
Buckling of plates and localised structural damage to structural stiffening must be assessed also.
Galvanic corrosion and Cathodic protection (anodes) also should be assessed to confirm there is not excessive fast galvanic corrosion occurring and poor cathodic protection present, requiring corrective action.
So there is much more than just the readings that are important. The readings must be interpreted in order to determine steelwork renewals and the hull structure must also be properly surveyed.
In Small Craft it is ideal if a Small Craft Surveyor is performing or at least supervising the UTM measurements and performing also a proper Hull survey, rather than just a specialised UTM company which was mainly working on ships for classification societies.
Steel is a specialist area of boat surveying and should be done by surveyors with particular experience in Steel.
Yacht Surveyor – Athens, Greece