Applications for Underwater Ultrasonic Testing

History

Ultrasonic testing (UT) has been deployed in underwater environments for industrial applications since the first offshore installations where being installed in the late 1940s and 1950s. Until 1953, there were no regulations or requirements for the inspection of structures, except where the owner/operator elected to write their own. It was not until 1970 that the Occupational Safety and Health Administration (OHSA) obtained statutory permission to conduct and/or require inspection of structures in US waters, but still little had changed.

Operators still bear the ultimate responsiblity and are the primary source of inspection requirements, although not all of them are uniform in their approach. In this blog post, we will discuss two things: applications of ultrasonic testing inspection methods and techniques used in underwater environments, and recommendations for a minimum training or certification level to perform NDT underwater.

Ultrasonic Testing Underwater

UT methods have been deployed underwater for many of the same reasons that we use the method “topside” (above water)—to detect, locate, identify, and size discontinuities, as well as obtain thickness measurements of materials, such as steel, concrete, and wood. The equipment used underwater is very similar to topside ultrasonic equipment, in some cases it is simply the same equipment modified for underwater use. Whether it is modified equipment, or built to suit, there are two primary styles; those that are self-contained, and those that are “wired” or cabled to relay information and data to the surface.

In the early days of using UT underwater, divers simply used a transducer connected to a long cable that reached a topside UT scope. The diver would place the transducer at the location of interest and a reading would be taken from a topside technician who controlled the UT instrument. Nowadays, equipment has been designed and built specifically for use underwater. Whether self-contained or relayed topside, equipment is more robust and able to reach deeper depths. Underwater video monitors help the diver to see and interpret a live feed of data, as well as support the inspector in the “zero-vis” (no visibility) conditions they often work in. These systems have also been designed and adapted for installation on remotely operated vehicles (ROVs) which deploy the system to the area of interest. In some cases, we even deploy both a diver and an ROV to complete certain inspection types.

Underwater UT Methods & Techniques

Ultrasound has many uses underwater and has been constantly developing into new applications that can take advantage of its capabilities. Technologies are rapidly advancing, bringing new materials to the market that have not been inspected before, as well as new equipment capabilities to inspect these materials. Because this catalog is constantly evolving, we may never touch on all possible solutions, but here we will touch on all the viable options available now.

Ultrasonic Thickness Testing (UTT)

Industrial UTT was first used underwater to obtain simple thickness measurements in steel structures. Now it’s used to inspect thicknesses on a variety of materials that can propagate a mechanical waveform, but is mostly used for steel, concrete, and wood. Remaining wall thickness is important to know for structural integrity, as well as for pressure burst calculations. UTT is traditionally a manual technique and, just like topside applications, is operator driven. This is accomplished by the diver manually manipulating the probe over the surface of the area of interest to attain thickness readings, or by the ROV placing the transducer against the surface.

UTT can also be incorporated into a semiautomated process known as automated ultrasonic testing (AUT). This is where the probe is secured into a manipulator arm in a robotic system that can then scan the surface of a part within the scan parameters set. This is used for faster and more reliable acquisition of data and can incorporate a C-scan view (top-down) used for detecting, sizing, and visualizing corrosion or midwall discontinuities. These AUT systems have been designed for both diver and ROV deployment, and have been used to inspect structures, as well as underwater pipelines, vessels (such as ship or barge hulls), and walls and floors of tanks, etc.

Ultrasonic Angle Beam (UTSW, PAUT)

Angle beam inspections using transverse (shear waves) waves are used underwater for the same reasons we utilize the technique topside: full volumetric inspection of welds, crack detection, and for sizing discontinuities. Both single element shear wave techniques, as well as multielement phased array systems have been adapted for use underwater.

Time of Flight Diffraction (TOFD)

TOFD can be a very powerful tool when used properly, even on underwater assets. TOFD is very sensitive to sizing midwall discontinuities, as well as measuring accurate depths. We have seen TOFD deployed for attaining remaining wall thicknesses in welds due to preferential girth weld corrosion/erosion. TOFD generates much more reliable thickness readings in welds than angle beam techniques, which need a perpendicular reflector to work around weld reinforcements.

Guided Wave (Bulk Wave or Long-Range UT)

Guided wave UT systems have also been adapted for underwater use, where the transducers are set into a ring that matches the outside diameter (OD) of a pipe, and a torsional wave is sent out through the pipe wall to detect issues such as corrosion or cracking.

Acoustic Emission (AE)

AE has grown in popularity over the years as its usage case has seen positive results in not only detecting discontinuities as they propagate, but because AE also has the ability to remotely locate where issues are happening. AE was first used on offshore structures as early as 1975, only a decade after the technique was first developed. AE utilizes multiple piezoelectric transducers, coupled to the structure at multiple locations. All the received signals are sent back to a computer system that can amplify, condition, and process the signals for interpretation by a technician or scientist. AE senses and records elastic waves that are generated during the rapid release of energy from a localized source within a material. For example, AE is sensitive enough to not only “hear” cracks when they propagate, but can also detect active corrosion as it is happening. Positioning of and locating these discontinuities occurs from triangulation or time of arrival of the elastic waves to each of the transducers.

AE monitoring can be either continuous or intermittent. Transducers can be installed in a semipermanent manner and left to continuously monitor a system, or be retrieved and moved to another location. The upside to AE is that once the system is installed, it can monitor the asset for years, then direct the scheduled Level III close visual inspection to areas of interest.

3D Sonar Imaging

Do not forget about sonar! Yes, we all know that sonar utilizes ultrasound and ultrasonic principles, but it can and has been adapted for use as an industrial underwater inspection tool. Echoscopes and multibeam sonar imaging systems are gaining popularity in the industry to “see” underwater in areas where little or no visibility is available. These systems are now even able to capture “live” data and display it in real time. For example, say a diver or ROV needs to perform an inspection at a particular joint or node, but in little to no visibility—3D sonar imaging can see and direct the diver or ROV to the correct area of interest, verifying that the data being collected is accurate. 3D sonar can also create 3D models of data collected, and give accurate distance measurements between features, point-plottable, and be directly tied into an RTK-GPS system for real-world location. This is very useful when determining scour profile around structures on the seafloor (to maintain structural integrity), determining whether structures or pipelines are static or have moved, and locating and identifying features.

Acoustic imaging can be used for topographical geographic imaging systems, which is large-scale mapping, and also for highly sensitive localized inspections, such as mapping corrosion on the surface of a pipe. Acoustic imaging today can measure the profile of the numbers, letters, and Lincoln’s face on a penny! With such a broad range of applications, it is exciting to see what these technologies will be able to accomplish in just a few more years.

Personnel & Training

The question of whether or not the inspector/diver or ROV operator should be NDT qualified and certified to conduct inspections and interpretation of data is significant. Currently, there are no ASNT or BINDT/ PCN guidelines or recommendations for underwater NDT inspection personnel training or certification. Some companies do have in-house training programs.

The need for qualification becomes more apparent when realizing that some methods of inspection or techniques require significant training and experience topside to become proficient, let alone performed in environments such as a confined space (underwater) in limited or restricted access and/or visibility. Simply observing a diver or ROV operator by a qualified technician through a video system still presents problems.

As self-contained and portable instruments for use underwater become more readily available, one must question the wisdom of sending an unqualified NDT technician to perform these examinations without supervision. One diver’s assessment of “general corrosion” can be quite different from another. Such observations may need to have a more precise meaning and boundaries in order to attain a more uniform description of conditions. Though video systems can be used to observe and record the unqualified diver to allow surface monitoring by a qualified observer, the judgment concerning what is significant to video is still in the hands of the diver.

The only organization currently offering training and certification is the Certification Scheme for Weldment Inspection Personnel (CSWIP). These schemes include underwater training for Diver Inspectors (3.1u, 3.2u), ROV Inspector (3.3u), and Underwater Inspection Controller (3.4u). Although this is a great start, these training courses are limited to practical use of traditional NDT methods such as visual/tactile, ultrasonic thickness, magnetic particle, video/still photography, and cathodic potential readings.

The change is starting, and now a lot of companies are starting to require personnel to attain these certifications to conduct inspections on their assets. This trend will continue to grow, as will the need for more underwater inspection personnel with advanced inspection training and professional certifications that are internationally accepted and recognized.

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Joshua de Monbrun, IEng, is with SubSea NDT, LLC, Anchorage, AK (josh@subseandt.com) and MISTRAS Group, Inc., Diving COE (josh.demonbrun@mistrasgroup.com).

Photos courtesy of Joshua de Monbrun.