Thickness monitoring underneath coating systems using permanently installed ultrasonic sensors

Thickness monitoring underneath coating systems using permanently installed ultrasonic sensors

Dr. Tim Coope, Composites Manager (

Coating systems are critical to maintaining the integrity of metallic oil & gas infrastructure assets but can also pose challenges for future thickness monitoring after the coating system has been applied. To inspect the underlying metal thickness of a coated pipe or tank the coating is typically removed and reapplied, at increased labour and material cost. In this blog we discuss the use of permanently installed ultrasonic sensors to access otherwise inaccessible locations underneath three different coating systems.

The challenge with thickness monitoring underneath protective coating systems

Traditionally, multi-coat thin film reactive paints are employed to inhibit external corrosion, and some coated assets are then over clad with insulation for higher temperature applications. Due to the cost of coating inspection, reapplication and insulation removal/replacement the oil & gas industry is now transitioning to more cost effective, innovative coating solutions. These include products from Seal For Life, such as Stopaq WrappingBand and Mascoat. For applications where wall thickness is approaching or has reached a critical level, composite repair systems such as those supplied by CSNRI, are typically installed to reinstate structural integrity.

Due to the coating thickness and material types there is high signal attenuation when attempting manual ultrasonic thickness (UT) measurements through coatings. As these coating systems are permanently installed, when they are removed to carryout UT (if it is possible to do) they are non-reusable. For annual inspections it is not cost effective or practical to apply this inspection methodology. Manual UT measurements can also be subject to human error.

Permanently installed ultrasonic thickness monitoring sensors from Inductosense have recently been embedded and validated underneath Stopaq, Mascoat, and CSNRI composite repair coating systems. This technology can eliminate the necessity to remove coatings for acquiring a UT measurement. The sensors installed under coatings can be wirelessly powered up and activated through the coating, to provide highly accurate, repeatable and trendable thickness data from the same location. This capability can then be used for predictive maintenance.

The Inductosense WAND solution is an accessible approach to monitoring under coatings

The Inductosense WAND sensors are permanently installed, ultrasonic sensors. They offer accurate repeatable, ultrasonic thickness measurements from the same location each time. Uniquely, the sensors are wireless, do not have any batteries and have a very low thickness profile (<1mm). This enables them to be fully embeddable under coatings and following installation does not impact on the coating integrity. They are activated wirelessly, through the installed coating, using a handheld or remote data collector – the thickness measurement can be acquired in a few seconds. Each sensor has an RFID associated with it and the measurements can be tracked over time to provide accurate rates of internal wall loss.

A key advantage of the WAND system is the simplicity of installation and data collection.  The sensors are low cost and require no maintenance. WAND is more than an order of magnitude more cost effective per thickness monitoring location than alternative permanently installed sensors on the market. Minimal training is required to deploy and take readings from the sensors, meaning it is possible for the coating applicator to also install sensors. This ensures that a reactive sensor installation approach can be utilised, to apply sensors just prior to coating application.

Expanding the application use case for innovative coating solutions

Where a pipe, vessel or tank, has had a permanently installed coating applied, either for corrosion protection or to provide thermal insulation or structural reinforcement, it is often not possible to carry out an inspection of the wall thickness. Inductosense have validated the WAND wireless TMS sensors with a number of innovative coating products and an overview is detailed below.


STOPAQ’s primary layers within a system consist of a fully amorphous, non-cross-linkable, non-polar polymer composition which is totally impermeable once applied. It is impenetrable to water, oxygen and bacteria, the elements that commonly cause corrosion. Unlike conventional coating types, the polyisobutene-based coating features a liquid-like behaviour to flow and ensure a full wetting of the entire surface of a substrate. This behaviour does not change with time, meaning that STOPAQ retains its corrosion preventative properties (non-ageing).  STOPAQ provides superior adhesive to any surface (steel pipe or existing coating) through a permanent molecular bonding with the substrate. With a glass transition temperature of -67°C (-88°F), STOPAQ will flow and adhere even in the coldest working environments, and it will self-heal in case of minor damages.


Validation testing

The validation testing was carried out to determine the WAND TMS sensor readability after application of the Stopaq WrappingBand product, and the measured thickness value taken using the WAND handheld data collector during continued thermal and humidity exposure at 80°C (176°F) (the sensor maximum operating temperature when installed with room temperature cure adhesive).

WAND TMS sensors were installed on 4 inch diameter pipe sections and Stopaq WrappingBand applied to a cleaned bare metal surface (Figure 1). Thickness measurements were collected before and after applying Stopaq, noting that the same standoff distance was used for both measurements. It was shown that the presence of Stopaq over the sensor does not affect the signal amplitude or the measured thickness. This is shown in Figure 2. The test pipe sections were then exposed to continuous heat at 80°C (176°F), with and with humidity exposure. An ECHO100 extender coil was added on top of the STOPAQ coating to enable thickness measurement from outside of the laboratory oven, while at temperature. The thickness and signal amplitude remain constant and stable (no internal corrosion present) over the current testing period. In addition, when exposed to continuous high humidity and temperature conditions the Stopaq WrappingBand provides excellent protection against moisture ingress.

In summary, the installation of WAND TMS sensors underneath STOPAQ WrappingBand has shown to be effective in providing in-situ internal corrosion inspection and monitoring, without having to remove the coating. The STOPAQ WrappingBand is also effective in providing protection to demanding high humidity conditions, and at temperature, where sensors are typically deployed.



Mascoat manufactures innovative spray-applied coatings for thermal and sound protection that eliminates the need for installing separate insulation and cladding. This unique property saves total installation time when compared to conventional insulation, anti-condensation, and damping methods while providing long-term asset protection. Because of this it is easy to install with no cutting, tapping, or special hangers. The coatings adhere to virtually any substrate and eliminate the potential for surface corrosion or corrosion under insulation (CUI), and can be applied to working substrates. As such, the long application life (25+ years) reduces year-over-year maintenance and the associated costs.

Validation testing

The validation testing was carried out to investigate a method for ultrasonic thickness monitoring without the need for removing Mascoat’s Industrial DTI insulating coating. The WAND TMS sensors were installed on two step-machined carbon steel panels, and each panel was then gradually coated with Mascoat thermal coating and subjected to a series of heat tests (Figure 3). The purpose of this investigation was to validate the performance of the coated TMS sensors across a range of temperatures from 27°C to 149°C (80°F up to 300°F), by confirming that thickness readings could be successfully acquired using the WAND handheld data collector across this temperature range. Thickness measurements were also captured following a series of heat aging and heating cycling tests.

In summary, the sensors’ signal amplitudes were unaffected by the application of Mascoat coating (Industrial-DTI insulating coating), and readings could be successfully acquired from the TMS sensors using the WAND handheld data collector (Figure 4). Once the sensors were heated up to 149°C (300°F), readings were successfully acquired across this temperature range through the Mascoat coating (Figure 5). Despite some expected signal amplitude drop above the operating temperature limit of the sensors of 130°C (266°F), the sensors still provided accurate thickness measurements. For prolonged exposures of up to 121°C (250°F) there was no significant reduction in signal amplitude, where the signal strength was replenishable during ramp down. For prolonged exposures at 149°C (300°F), although readings could be successfully acquired, there was some observable amplitude drop due to the fact this exceeded the operating temperature limit of the sensors.

CSNRI Composites

CSNRI composite products consist of highly engineered repair systems made of proprietary cloth and resins for process piping and pipeline applications. ThermoWrap™, A+ Wrap™, Atlas™ and Clock Spring® composite wrap products have been evaluated with the Inductosense WAND technology.

ThermoWrap™ fiberglass composite is a field-saturated repair system made of fiberglass cloth applied with a two-part epoxy and a high-modulus filler material. It is used globally in plants, refineries, tank farms, terminals and on offshore assets as a pressure-containing repair to seal leaks and as a reinforcing repair to restore the strength of a pipe in the axial and circumferential directions. It installs easily in challenging environments for repairs involving complicated geometry, including tees, flanges, and varying diameter pipe.

A+ Wrap™ composite is an easy-to-install, factory saturated, bi-directional fiberglass composite solutions for on-site permanent repair of corroded and dented pipelines. By eliminating field saturation, resin and fabric measuring, and compressing installation time, A+ Wrap™ ensures the highest repair quality while reducing time to backfill.

Atlas™ carbon composite is the most tested carbon fiber composite solution for the permanent repair and rehabilitation of pipelines and piping structures. Atlas™ materials are highly engineered to permanently restore the original strength of pipelines suffering from both common and complex pipeline anomalies while maintaining strict compliance with corresponding regulations, codes and standards.

Validation testing

The validation testing was carried out to examine how effectively the WAND TMS sensors can function beneath the CSNRI composite repair systems. More specifically this was to determine the maximum thickness of composite material that can be placed over the installed sensor (on 10 mm thick mild steel blocks) and for the Inductosense WAND device to be able to capture a reading from the sensor.

In summary, the Inductosense system was shown to be capable of reading through a standard ThermoWrap™ MT, A+ Wrap™, and ClockSpring® repair systems, up to a repair thickness of 15mm (0.6 in). For the carbon fibre reinforced Atlas™ system, reading through a repair thickness of 6mm (0.25 in) was achieved. An example is shown in Figure 6, for a reading taken with a fixed stand-off distance through air and an A+ Wrap™ repair of 13mm (0.52 in) thickness.

In addition, tests were carried out on 6 inch diameter pipe to examine how the physical presence of the WAND TMS sensors affects the leak sealing and mechanical characteristics of the typical composite repair systems (Figure 7). The first test, energy release rate testing (As per ASME PCC-2 Article IV), involved installing a sensor over a through-hole of known size to act as a nucleation site for crack growth within the repair system. The acceptance criteria for this test was for the repair failure to occur due to edge leakage. The second test, the spool-type test (ASME PCC-2 Article IV), consisted of a pipe with known wall loss that was reinforced using the composite repair system to return its structural strength to its original value without the wall loss.

From the spool pipe testing, the inclusion of Inductosense sensors under the composite repair did not cause change/loose performance during the test pipe pressurisation. Throughout the pressurisation cycle, pipe wall thickness readings could be acquired and there were no signs of de-lamination of the sensor from the test pipe beneath the repair. Similarly, energy release pipe testing from all the repair systems with the Inductosense sensors installed passed the testing in terms of a theoretical target values and ultimately failed via edge leakage.

Benefits for installing sensors underneath coatings

The Seal For Life and CSNRI Composites products provide a lot of key benefits to the Oil & Gas sector to mitigate and manage corrosion. Where Stopaq is applied to prevent external corrosion, it then introduces a challenge for inspecting the structure for internal corrosion. Typically, at inspection locations the Stopaq would have to be removed, the thickness measurement taken using manual UT, and then a repair patch and additional overwrap applied to maintain the corrosion barrier protection. There is obviously a significant cost and time associated with this process for each single measurement point, and this would ultimately multiply with every subsequent thickness measurement. Installing WAND TMS sensors on pipework, tanks and vessel inspection locations, prior to Stopaq application, eliminates any coating removal and replacement steps. This methodology and approach to using WAND sensors under coatings would similarly apply for Mascoat coatings.

Where CSNRI composite repairs are installed to provide structural reinforcement to significantly corroded and thinned pipes it becomes challenging or not possible to inspect the remaining wall thickness. By installing sensors the remaining wall thickness can be known and this can inform the remaining lifetime of the composite repair. This is especially prevalent where the composite repair design has been specified based on a certain remaining pipe thickness. Without installed sensors an in-service composite wrapped pipe may have to be prematurely cut out based on historic maintenance records, at significant cost, rather than being able to extend the pipe service life through monitoring.

The unit price of the WAND TMS sensors when purchasing at high volume is <£100 per sensor and requires minimal installation time (a few minutes). Therefore, it could be expected that the sensor cost could be recouped after the first thickness reading using the WAND data collector. More importantly by having a sensor installed under a coating there is no limitation on the number of measurements or frequency of measurements that can be collected from a single sensor. Accurate, precise, and reliable data collected using WAND can further support better and more cost-effective asset integrity management.  In addition, locations that require scaffolding access, installed WAND sensors could be retrofitted with the WAND RDC to enable remote data collection. Thickness measurements can then be collected to a pre-determined time schedule and the saved data can then be downloaded via Bluetooth from a ground-based location. The use of the WAND technology with permanently installed coatings can further support the key benefits already shown by using Seal For Life and CSNRI composites products and further increase their uptake in the Oil & Gas sector by enabling time and cost-effective thickness monitoring.


Post a comment