These surveys are carried out using a non-destructive tester (NDT) employing ultrasound. This testing is done by measuring the metal thickness at multiple points on the hull looking primarily for general wastage. The tester used is a highly sophisticatd triple-echo machine that is able to shoot through the hull coatings without their removal. The tester emits three pulsed echos into the metal and measures the time for the echo to return (much like a depth sounder). The circuitry analyses the timing between each echo and subtracts the coating thickness providing only the metal thickness.
In addition to having the proper equipment, proper training is just as important. The results have to be interpreted and inconsistent or odd values have to be spotted and evaluated. The tester has to calibrated annually by the manufacturer to ensure accuracy.
The report will discuss general hull construction, general hull condition, evaluate galvanic corrosion issues found, evaluate any wastage for patterns and degree of concern. It will make repair recommendations, coating recommendations, and galvanic protection recommendations. The report will contain hull sketches with each “shot” located.
While the coatings typically do not have to be removed or damaged, poorly bonded coatings, surface corrosion, or marine growth can prevent reliable readings. It is always recommended that the vessel be hauled and pressure washed prior to preforming a UTT survey. The vessel should be on a flat, stable surface (preferably asphalt or concrete), and be well blocked. There must be adequate lighting and access around and under the vessel. Deficiencies in these areas require far more survey time and will result in increased costs. Wind and rain will also slow the survey and can result in increased costs.
Access to the vessel interior is often required to understand internal structures that affect readings. Also, interior corrosion, bilge liquids, etc. can be evaluated.
Typically, we grid off the hull with a laser level and chalk into 2′ to 3′ squares. This is done so each shot can be located on the sketches. Each shot is drawn onto the hull with chalk for later recording. The final sketches give a graphical representation of the overall hull condition.