Averting Personnel Injuries

Written by Peter Buxbaum

SOTECH 2010 Volume: 8 Issue: 9 (December)

United States Navy special operations personnel perform many of their missions on smaller water craft. These vessels—and the personnel inside them—can take quite a pounding, especially in severe sea states.

Repeated exposure to wave shocks and to the forces of the ocean can impair performance, produce discomfort, and cause acute and chronic injuries. That is why the Naval Special Warfare Command tests smaller boats for their ability to absorb shocks and protect the crew.

In the past, naval personnel often preferred standing to sitting while operating their vessels, believing that they could use their legs from a standing position to absorb impacts. While there is some truth to that belief, special operations personnel are often exposed to forces that cannot be handled merely by standing or bending the legs. Much of the activity surrounding shock mitigation involves equipping combatant craft with seats that cushion the blows to which crews are subjected.

In one case, the seats in an entire fleet of small boats were replaced with seats equipped with shock mitigation technologies. These special seats contain their own suspension systems, in the form of shock absorbers that smooth the ride for naval special warfighters. The command keeps on eye on such technologies that can improve the safety of crew members.

“Combatant craft crewmen work in an environment dictated by the mission, not at the convenience of sea-state conditions,” said Bruce Holmes, a science and technology adviser at Naval Special Warfare Command. “Boat-related musculoskeletal injuries occur as a result of the environment in which the crew and passengers are required to operate. The purpose of the seat is to shield the operator from the high shock environment generated by the sea-state and craft speed.”

Techno-Sciences Inc. (TSI), in collaboration with Lord Corp., has developed what the companies term a reliable, revolutionary shock absorber system using magnetorheological (MR) fluid technology for seated occupants aboard high-speed watercraft, in a cost-effective package providing optimal shock and vibration protection.

Traditional seats using passive shock absorbers in these vessels provide limited benefit to the warfighter because they can only be optimized for one occupant weight (typically the 50th percentile male) and a single shock condition. Outside these ideal conditions, they provide subpar protection.

But in maritime operations, especially onboard special operations craft, ideal conditions do not exist. Conditions such as high sea states, boat speed, course relative to wind direction, and boat payload/weights are highly variable—leading to erratic shock and vibration conditions.

In addition, MR devices provide advantages over competing active damping technologies due to their simplicity, company officials explained. MR fluid shock absorber systems are enabled by versatile MR fluid technology, which allows the system to respond instantly and controllably to varying levels of vibration, shock or motion with simple, robust designs.

This novel seat suspension by TSI and Lord automatically varies the damping and energy attenuating capabilities, providing protection for all warfighters, regardless of their size and weight and the operational conditions. This system leverages decades of Lord production experience of various MR devices.

In particular, the Lord Motion Master Ride Management System has provided vibration and shock isolation in over 75,000 commercial truck seats since initial production in 1998. While this system has been developed and tested with the U.S. Navy’s RHIBs and Mark V SOC in mind, the system can easily be adapted into seats for all armed services and any high speed watercraft.

“The biggest problem with smaller craft is wave shock,” said Doug Taylor, CEO of Taylor Devices in North Tonawanda, N.Y. “It can cause damage to equipment and to people. A severe wave shock can knock someone out. The most important thing about any armed vessel is to optimize the blending of man and machine.”

Naval Special Warfare has pursued existing and future technologies in pursuit of mitigating musculoskeletal, boat-related injuries. After it came to light that operators on the Mark V Special Operations Craft were subjected to a litany of debilitating injuries, NSW performed a market survey in 2004 of available systems and directed minor design changes of existing shock mitigating seat technologies. As a result, the standard rigid STIDD V-4 seats in the Mark V were replaced with STIDD-Taylor V-5.3 seats equipped with passive shock absorbers. All 21 operator seats were replaced on all 20 Mark Vs.

“The STIDD-Taylor Shock Mitigating Seat has received wide acclaim for reducing injuries during severe sea-state conditions,” said Doug Taylor. “The seat was ergonomically designed by STIDD to restrain the operator so that he is not working his muscles to hold himself into the seat.”

The shock absorbing device used in this seat developed by Taylor Devices involves a spring and damper combination that has the ability to sense the incoming force so that it can change its output accordingly.

STIDD comes to the design and production of shock mitigating seats with a background in orthopedic and sports medicine research. “Our seats are compliant with medical guidelines as well as military specifications,” said Walter Gezari, CEO of STIDD Systems Inc., in Greenport, N.Y. “Before we get to the designing stage of a seat, we study where the force is going and how to dissipate it. We also figure out how to position and stabilize the user and place him in the proper orientation to accept the highest amount of load with the least amount of damage. The STIDD-Taylor seat solved the problem on the Mark V.”

But Holmes views passive shock absorbing as a less-than-optimal solution. “The problem with a passive system is that the craft must operate in both heavy and light sea-state and craft speed environments,” he explained. “What might work well to protect the operator during a high shock environment may not be ideal for a low shock environment and vice versa.”

Seaspension Technologies in Largo, Fla., produces shock mitigating seating equipped with a patented damper and coil system that automatically reacts to the force being exerted on it. “We basically had a global damper manufacturer tweak an existing shock absorber with our input,” said company President Peter Burer.

Seaspension, which started in the recreational boating market, first supplied shock mitigating seats to the Canadian Coast Guard around eight years ago. “We’ve had no failures with these shocks,” said Burer.

More recently, Seaspension delivered custom shock mitigating seating for use in 35 U.S. Coast Guard specialized emergency rescue airboats, known as Iceboats. The tripod-mounted seat is designed to protect the crew from jolts experienced when the boat transitions from land to ice and water, and during operation in rugged conditions. Seaspension also has supplied seating to Edgewater Power Boats of Edgewater, Fla., for vessels that were delivered to the Trinidad Coast Guard.

Active Shock LLC of Manchester, N.H., which concentrates on shock absorbers in the automotive space, developed a device for the U.S. Special Operations Command designed to be incorporated in a rigid inflatable boat (RIB). That project, developed under a SBIR grant in 2008, has not been acquired by the U.S. military.

“What makes our shock absorbers unique is that they are controlled by computer,” said Scott Martineau, the company’s director of business development. “We use sophisticated algorithms that take information from a suite of sensors contained in the shock absorbers themselves to determine the optimum damping rate. In high sea states, the sensor can detect when the boat goes airborne and prevents the seat from bottoming out and transmitting the shock to the crew.”

Seaspension, meanwhile, will soon be introducing a shock mitigating seat for use as either a bulkhead seat or as a replacement for the Marine Corps’s 11-meter RIB. “The damper system will allow anywhere from the fifth percentile female by weight to the 95th percentile male in the Marine Corps to be seated and protected from shocks without the use of tools or any kind of adjustment,” said Burer. That would cover women as light as 100 pounds to men as heavy as 250 pounds.

“The mechanism in the damper responds directly to forces,” said Burer. “The Navy is looking for a seat that can accommodate different sizes of personnel on a variety of missions subjected to different sea states without having to adjust the damping mechanism.”

Seaspension’s new device uses a dual-damper system in order to accomplish that task. “We found it better to incorporate two independent shock absorbers to regulate those weights,” said Burer. “The seat is designed to be able to slip right into existing craft in the inventory of U.S. special operations and foreign militaries as well.”

The principles central to shock mitigation are applicable not only to seating and the protection of crewmembers, but also to the protection of equipment mounted on vessels. “We are working on several new applications for our pedestals to be used as weapons and equipment platforms,” said Burer. “We recently bid our heavy duty pedestal as a mount for a .50 caliber gun on a 100-boat contract. We expect shock mitigation to be used much more in this application in the future and are actively testing and designing products to meet this demand.”

Taylor Devices makes shock mitigation products that have been used on Navy warships for shock isolation on missiles and missile systems, radars, electronics consoles, boat davits, and for recoil isolation on heavy machine gun mounts. These “soft mounts,” which are a combination coil spring and hydraulic shock absorber, reduce the firing recoil from the .50 caliber heavy machine gun from 4,400 pounds to 500 pounds, according to Taylor. This allows the .50 caliber guns to be used on smaller combatant craft.

The application of shock mitigation technologies can have the unintended side effect of allowing crews to push a vessel to the limits of its performance, a phenomenon which has both positive and negative implications. As crews become more comfortable, they push the boats harder, and as they push the boats harder, the more they are likely to be subject to additional wear and tear. That is especially the case when shock mitigation has been retrofitted onto a legacy platform.

“As operators feel secure in the seat and comfortable at higher and higher speeds they have been driving the boats faster,” said Gezari, referring to the Mark V. “This allows them to run their missions faster and more successfully. But the boats have also experienced structural issues as a result.”

For Holmes, special operations crews will best benefit when vessels are properly designed and evaluated for their shock absorbing properties from the beginning. The Special Operations Craft Riverine, he pointed out, does not have the same history of musculoskeletal, boat-related injuries as the Mark V or the Naval Special Warfare Rigid Inflatable Boat (NSW-RIB).

“Health monitoring of the shock environment in manned areas of combatant craft needs to be implemented on all combatant craft,” said Holmes. “Monitoring of the shock environment will provide data that will be used to assist future research and measure how effective proposed systems are against mitigating shock.”

Research is yielding a set of emerging standards that will likely find eventual widespread adoption and which will help in the measurement of the shock conveyed to the human body through vessels. The International Organization for Standardization, a Switzerland-based group, has released ISO 2631-5, a standard to measure vibration on the human body over time. Another emerging industry standard relating specifically to shock mitigating seats is known as SED-8 and is based on another ISO standard.

“SED-8 measures the amplitude and frequency of incoming impacts and then measures the outgoing response” of the seat, explained Burer. “Different seat manufactures employ different kinds of tests to measure the shock absorbing capacity of their systems. With this standard, we can measure the effectiveness of the damper system.”

At some point in the not too distant future, Burer predicted, a certification program for shock mitigating seats under SED-8 will be established.

Meanwhile, NSW will be assessing all future combatant craft for operator exposure to shock levels that could cause musculoskeletal, boat-related injuries, said Holmes. The current combatant craft, medium requirements document contains a maximum spine stress dose shock exposure level, as defined by SED-8 and ISO standards. ♦

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