A Guide to Thickness Testing

Test metal thickness onboard your vessel without having to remove marine paint layers. The Portagauge® 4 uses triple echo technology which allows operators to inspect the underlying metal thickness, independent of any surface coatings such as paint. 

In the maritime industry, mild steel remains the number one metal for constructional purposes by virtue of its relatively low cost, mechanical strength and ease of fabrication. However, it corrodes easily in seawater and unless adequately protected, rapidly loses strength which may result in structural failure. Corrosion of ship structures and machinery has multiple causes including with seawater or with salt-laden air, microbial action, corrosive cargoes and even the sulphur within the fuel used in main and auxiliary engines. 

Using high performance corrosion sprays, coatings and paints are readily available and used widely to combat corrosion issues. However, where protective coatings fail, any corrosion will very likely not be visible.  Even new paint can hide severe internal corrosion which eventually creates failure.

Corrosion can lead to costly repairs, prolonged maintenance, material losses, poor performance and, if left untreated, failure. The total annual direct cost of corrosion to the U.S. shipping industry is estimated at $2.7 billion, with maintenance and repairs costing $810 million and corrosion-related downtime at $785 million*. UTM (ultrasonic thickness measurement) is the simple and effective solution for testing for corrosion. Coltraco Ultrasonics are proud industry experts in ultrasonic technology. Coltraco manufacture two industry leading ultrasonic thickness gauges, ideal for the regular testing of marine boats.

The Portagauge® 3 is a low-cost single-echo portable thickness gauge. The quick, simple and hardy Portagauge® 3 allows accuracy of ±0.1mm even on corroded, challenging and some plastic surfaces. With a 50 hour battery life, measuring thickness ranges between 1.5mm to 99.9mm and a variety of verified testing materials such as steel, quartz and glass PVC, the Portagauge® 3 provides the great amount versatility needed to suit a diversity of safety needs. The handheld highly dependable unit is ideal for scrubber systems.

As well as this Coltraco Ultrasonics also have the Portagauge® 4 which is an advanced thickness gauge. Offering unparalleled accuracy, the Portagauge® 4 uses triple echo technology which allows operators to inspect the underlying metal thickness, independent of any surface coatings such as paint. However, unlike single echo technology, the multiple echo unit measures the time delay between 3 consecutive back wall echoes, and it is the distance between the 2nd and 3rd echoes that gives a true and accurate reading of the material thickness only, ignoring any paint or coating.

For applications where true metal readings and a higher degree of accuracy is required, the Portagauge® 4 is the ideal solution. With a variety of sensor options available and underwater variants, this range of units can easily be adapted to a variety of different roles and requirements.

The Portagauge® 4 boasts a range of benefits for testing metal thickness:

  1.  Quick testing allows the user to test a large range of metals within a short period of time. Intelligent Probe Recognition (IPR automatically adjusts settings at the same time as transmitting recognition data.
  2. Ideal for a wide range of industrial applications: fire cylinders, ship hulls, pipework, storage tanks. Ability to work on heavily corroded metal.
  3. Triple Echo technology gives true metal thickness ie excluding coating and paint thickness.
  4. Highly accurate units – measurements in steps of 0.1mm from 1.5mm – 99.9mm.
  5. Portable unit, lightweight, compact and with long-term reliability.

Using an ultrasonic thickness gauge, such as the Portagauge® 4 allows for regulatory compliance in corrosion testing:

NFPA 25 – Standard for the Inspection, Testing and Maintenance of Water-Based Fire Protection Systems

9.2.7 Tests During Interior Inspection. Where a drained interior inspection of a steel tank is required by 9.2.6.4, the following tests shall be conducted:

(3) Non-destructive ultrasonic readings shall be taken to evaluate the wall thickness where there is evidence of pitting or corrosion

(5) Tank bottoms shall be tested for metal loss and/or rust on the underside by use of ultrasonic testing where there is evidence of pitting or corrosion NFPA 13 – Standard for the Installation of Sprinkler System

References made to NFPA 25 for regular inspection, testing and maintenance.

Class Guideline DNVGL-CG-0285 (Dec 2018) – Section 3 – Basic Requirements – General – 5) Only multiple echo instruments may be used for UTM onboard all ships

CASE STUDY: Built in 1933, the HQS Wellington is the last surviving member of the Grimsby class of sloops, which served valiantly during the Second World War. The Wellington Trust ensure the preservation of this historic ship.  The engineering team from the Trust invited members of the Coltraco team to test metal thickness and corrosion onboard.  Testing of the Portagauge® 3 and its sister product the Portagauge® 4 triple echo multigauge led to sound results helping to prove the ship’s worthiness for years to come.

Using ultrasonic thickness gauges accurately measure and check material thickness and corrosion, to keep maintenance costs low, whilst providing adequate protection and quality.

* https://www.rustbullet.com/cost-of-corrosion/advantage-ships-html/