TSC Inspection System’s David Parramore discusses the benefits of ACFM®, and the company’s innovation – the ACFM® MagCrawler™
It often feels like there isn’t much you can’t attach to an ROV these days. The advent of more manoeuvrable and versatile vehicles, coupled with major advances in electronics and sensing technology, has transformed industry capabilities and expectations. That brings immense benefits to operators, but also new challenges in incorporating complex equipment into an enormous number of vehicle designs.
Now, alternating current field measurement (ACFM®) joins the ranks of inspection techniques, which can be deployed remotely. ACFM® is an electromagnetic inspection technique which uses an alternating current to detect defects.
It works by introducing AC into the surface of a structure, component or pipeline. Cracks in the material’s surface disturb the current’s electromagnetic field, and the returned signal is analysed in real time, alerting the user to the presence, size and depth of a defect or crack.
The immediacy and accuracy of the technique offers major advantages when compared to other non-destructive testing (NDT) methods. Independent testing has shown that ACFM matches the performance of rival techniques such as method magnetic particle inspection (MPI) for inspecting underwater structural welds, but offers significantly lower instances of missed and spurious signals when compared to MPI and conventional Eddy current testing.
It can also be performed through coatings and paints, meaning it can be carried out far more quickly, economically and safely than techniques like MPI.
ACFM was developed by TSC Inspection Systems, with support from BP, BG, Conoco and Royal Dutch Shell who were keen to improve the reliability of underwater inspection dramatically, reducing the reliance on the operator and provide auditable inspection records. It has since become a mainstay for use in diver and diverless inspection operations.
TSC Inspection Systems is a UK-based firm established in 1984 as a spin-off from University College London. Its founding team were recognised leaders in the fields of NDT, robotics and fracture mechanics, and the company now offers an array of inspection services and equipment.
Having begun as a software company for testing fatigue in components and tubulars, TSC developed ACFM as a method of checking and verifying the results and effects of test procedures in the early 1990s. Since then, NDT has become its core business, with ACFM at the forefront.
Most recently, this includes the ability to carry out ACFM inspection with a magnetic crawler deployed from an ROV. InnovOil spoke to TSC’s engineering & operations director, David Parramore, to find out more.
Enter the MagCrawler™
The ACFM® MagCrawler™ allows this inspection technique to be carried out by a tracked magnetic crawler vehicle. This is of particular use for situations requiring accurate inspection of subsea welds, but where access can be difficult or costly to reach with divers.
The crawler is magnetically attached to the inspection surface. Two rubber caterpillar tracks provide both traction and accurate steering. The vehicle can then be piloted directly down the structure, if near the splash zone, or can even be delivered and deployed by ROV, to the inspection site.
Parramore adds: “Although it’s possible to get good access to structures with ROVs, sometimes space constraints prevent you getting in close enough. Placing ACFM on a MagCrawler was a way we could overcome that, and particularly overcome the issue of ROV stability near the water surface.”
In order to ensure the operation could be performed accurately and remotely, TSC developed a new ACFM array probe. While divers might use a pencil-shaped probe to enable very accurate inspection, Parramore explains: “We recognised that as soon as you go to remotely operated inspection tasks, the precision capability of a diver is no longer there…so we overcame that by making arrays.” These are sensitive to changes over the surface area of the probe, allowing for wider and more accurate coverage for remote use.
The resulting array probe is accurate to within 10mm or so, and means that that in addition to crawler deployment, it can be used on ROV manipulator arms and in other configurations. ACFM can still detect and size defects as small as 15mm long by 1mm deep in welded connections, even when inspected through a protective coating.
Advanced control software means the crawler can also navigate the wide range of complex weld geometries found on both fixed and floating structures. Power is supplied to the array by the ROV or crawler, typically requiring 24 V at a couple of amps.
It can manoeuvre easily on flat surfaces or tubulars with a diameter greater than 30 inches (760mm). Motorised adjustments of probe position can be made in both parallel and transverse directions, allowing full coverage of the area to be inspected. The probe is held in contact with the inspection surface using passive compliance, ensuring that the probe is aligned correctly at all times. Additionally, 360° rotation means that the probe can fully inspect flange welds.
Crawler deployment also means that ACFM can be used to inspect ship hulls, “particularly large ship hulls which are semi-permanently in position, such as drilling ships and semi-subs,” Parramore says.
The ability to perform checks on welds with ROVs and crawler vehicles significantly reduces costs – an ROV-equipped vessel can be typically three or four times cheaper than a similarly capable diving support vessel (DSV).
2016 and beyond
December 2015 saw the ACFM® inspection technique accepted by classification society Lloyds Register. The endorsement covers manual and robotic deployment of ACFM® for topside, subsea and splash-zone inspection applications.
Under Lloyd’s Register certificate RSS/MNDE/0009, the TSC Amigo™ and U31™ Systems have been accepted for the in-service examination of structural welds, heat-affected zones and adjacent parent materials, in uncoated or coated steel structures, with the intention of detecting surface breaking discontinuities.
In 2016, the company will also be expanding the use of the ACFM® NodeScanner™, a dedicated device for complex node inspection on welded tubular joints.
“Now that we can perform ACFM without using a diver, it opens up the opportunity to further reduce inspection costs – and we believe we’re the leaders in the field,” Parramore says. While there are comparable ultrasonic techniques, they require much higher degrees of accuracy, making them extremely difficult to deploy via ROV.”
TSC’s year is already filling up with potential projects in both the Norwegian North Sea and Gulf of Mexico. If everything goes to plan, the ACFM MagCrawler may well be visiting a structure near you very soon.
TSC’s David Parramore will be speaking about the capabilities of the ACFM MagCrawler® and the NodeScanner™ during the Spotlight on Technology session, Thursday, February 4, at SubseaExpo 2016.