By Ryan Marson, Code 1000i, Technology Insertion Office PSNS & IMF

Puget Sound Naval Shipyard & Intermediate Maintenance Facility supported Naval Surface Warfare Center Carderock with the temporary installation of two new structural health monitoring systems March 28 - April 1, aboard ex-Buffalo, a decommissioned Los Angeles-class submarine here.

Structural Health Monitoring systems are an offshoot of traditional non-destructive inspection, where sensors are permanently integrated on an asset, and data can be collected on an automated basis. Carderock is funding Metis Design Corporation through the Small Business Innovative Research program to develop an SHM system to monitor specific hull weld locations for fatigue cracks that are normally inspected by qualified personnel during availabilities.

Code 1000i, Technology Insertion Group; and Code 350, Inactive Fleet, Reactor Compartment Disposal and Recycling; helped facilitate the installation of the SHM systems at PSNS & IMF. These systems may eventually be permanently installed aboard active U.S. Navy ships and submarines, if testing proves them to be effective.

Permanently installed SHM systems that monitor welds in real-time would provide data trends over time to allow remote maintainers to judge the condition of these components. This could eliminate the need to continually inspect these areas during shipyard availabilities.

“The main benefit of SHM is asset availability,” Kessler said. “Vessels might not need to go to port for inspection as often. SHM could shorten inspection durations by guiding on-site expert inspectors to areas of concern. Also, by enabling condition-based maintenance, sustainment costs can be reduced and logistics improved for when repairs or replacements are necessary, based on sensor outputs.” Kessler said all of these benefits have already been realized using a similar approach—health and usage monitoring systems— for rotorcraft.

Kessler said SHM is best implemented at “hot spot” locations, including:

• Locations where damage has been previously identified on similar assets in the fleet. Since vessels are not designed to crack, it's hard to know where to look until cracks start showing up in service. At that point, typically every similar vessel in the fleet will need to undergo a periodic phased inspection to make sure that same crack is not present.

• Areas that are challenging to access by inspectors. These can be enclosed areas, hidden behind insulation or high up where additional heavy equipment is needed to facilitate inspection. SHM sensors can be installed to provide data without the need for disassembly.

• Failure-critical locations, in order to provide real-time warnings of even very small cracks starting to form.

Seth Kessler, principal investigator with Metis Design, said two different SHM systems were installed aboard ex-Buffalo. “The first system is called MD7, and it is an ultrasonic guided wave detection system,”Kessler explained. “Essentially squareinch beamforming arrays are bonded to the structure like strain gauges, and they scan 2-D hull structures identically to active sonar systems on submarines. The microelectronics are locally embedded to create a digital daisy-chain that can cover up to 100 feet on a single cable bus.”

“The second system is called WISP, which stands for Witness Integrity Sensor Platform,” he said. “WISP is a completely passive system that permanently encodes changes such as fatigue cracks as physical damage to the sensor. The WISP “reader” is powered over USB from a tablet or phone, and temporarily magnetically couples with the WISP sensor to charge the hardware and collect data in a matter of seconds.”

Ben Grisso, Structural Health Monitoring R&D lead, Code 654, Service Ships Structures Branch, at Naval Warfare Center Carderock, said while SHM systems are also being evaluated for use on surface ships, there is simply no substitute for the scale of materials and structural geometry testing an inactive submarine provides.

“Demonstrating the damage detection capability of the SHM sensing systems on the real, inactive structure provides a tremendous amount of technical risk reduction and increased readiness in preparation for seeking approval for installation on an active boat,” Grisso said. “PSNS & IMF Codes 1000i and 350 enabled initial critical technology feasibility studies without the need for expensive test article fabrication, and we look forward to working with PSNS & IMF in the future for other technology demonstrations.”

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