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The Marport SQX AUV and its Application in the Subsea Battlespace

06/14/2011

Conceived as a Commercial Survey Tool, Evolving to a Mine Countermeasures Platform

By Karl Kenny
CEO, Marport Deep Sea Technologies Inc.
St. John’s, Newfoundland, Canada

Marport Deep Sea Technologies Inc. is the largest sonar company in Canada with 120 employees and operations in St. John’s, Newfoundland and in Cornwall, Ontario Canada, as well as in the USA, France, Iceland and Spain. It develops and manufactures sonar products for underwater defence, ocean survey and science and deep sea fishing applications. Marport is the originator of the software defined sonar (SDS) processor, a software-centric sonar architecture implemented on reconfigurable embedded hardware.

In 2006 the company, in collaboration with the National Research Council of Canada (NRC) and Memorial University of Newfoundland, embarked on the development of an autonomous underwater vehicle (AUV) as a test platform for sonar systems under development and, later, as a product in its own right. The first version was developed as a subsea survey tool and the design is now evolving to a vehicle capable of advanced military mine countermeasures (MCM) work. Some elements of the design process are presented below along with a description of the proposed MCM variant.

The first version is a compact, medium weight AUV that uses a vertical twin-pod configuration, based on the Woods Hole SeaBED AUV concept, and is designed for inspection and mapping applications in waters down to 500 meters in depth. A large vertical separation between the centre of gravity (CG) and centre of buoyancy (CB) afforded by the twin pods provides a passively stable platform optimal for deployment of sonar or optical imaging payloads. An innovative 3D thrust vectoring propulsion system unique to the SQX-500 creates an extremely manoeuvrable platform, allowing for capabilities such as hovering, crabbing and a zero meter turning radius around the vertical axis. The thrust-vectoring system consists of identical forward and aft rudders each capable of +360° rotation. On each rudder is mounted a thruster and propeller, which in turn is mounted on an elevator assembly capable of +30° pitching rotation. In operation the rudders and thruster elevator assemblies can be configured dynamically by the vehicle control computer to provide hovering and a very high degree of maneuverability.

The design process involved extensive use of the expertise and facilities of the NRC Institute for Ocean Technology (IOT) in St. John’s, Newfoundland. These include a 200 meter long, 12 meter wide and 7 meter deep clear water towing tank with a carriage capable of controlled speeds between 0.001 meters / second and 10 meters / second and equipped with a planar motion mechanism (PMM); a cavitation tunnel with a 2.2 meter long, 0.5 meter square test section equipped with propeller thrust and torque dynamometers. Marport worked with IOT’s staff of AUV researchers, hydrodynamicists and computational fluid dynamics specialists to develop the overall vehicle and thrust-vectoring propulsion system.

To progress from the initial conceptual SQX-500 AUV design to a prototype design for construction, estimates of hydrodynamic drag and power requirements were necessary. Previously published scientific literature was reviewed to locate existing hydrodynamic drag data for similar designs of each component. This information was a critical input into the initial propulsion system design – propellers and thrusters -- and an essential component for an initial performance estimate of the AUV. In order to simplify the estimation process, the hydrodynamic loads acting on the system were evaluated as separate components; cylindrical hulls with ellipsoidal nose cones, vertical rudders, and horizontal elevators. The contributions from these individual components were then summed to estimate the total vehicle forward drag. Previously published scientific literature was reviewed to locate existing hydrodynamic drag data for similar designs of each component.

Cylindrical hull forward drag values were based on published measured data for single pod vehicles. Rudder and elevator forward drag values were based on standard test data available for NACA airfoils. Forward drag estimates for all components were then combined, and an initial estimate for total vehicle forward drag based on forward speed was produced. These results provided a useful staring point, but as the vehicle design was evolved into a functional prototype design, significant shortcomings were identified, particularly the fact that the data available were only for forward drag; little published data were available for cylindrical hulls in very high angles of attack, such as when the vehicle is crabbing sideways or holding station in a current. As a result, it was decided to perform a series of hydrodynamic drag tests with an accurate scale-model of the SQX-500. A 0.88 scale model of the SQX-500 was constructed and subjected to model testing in the IOT Clearwater Towing Tank.

The three primary manoeuvres of the SQX-500 AUV are forward transit, hovering while holding station, and transverse motion or crabbing. In order to provide sufficient hydrodynamic drag data for these manoeuvres, three sets of drag tests were performed with the scale model to measure resistance in forward transit, sway and heave. In each set of tests, the angle of attack of the scale model was varied in order to obtain data for the vehicle in all orientations relative to the direction of vehicle motion. Angle of attack was controlled using a yaw table mounted to a tow carriage. These tests have resulted in a large number of drag force data sets for all forward and transverse motions at a large number of angles of attack. This data provided input to the design of the propellers and the thruster, and a 6 DOF motion simulator.

After searching for a commercially available propeller, it was decided to perform a custom design using the scale model resistance data as input. Extensive use was made of the IOT cavitation tunnel (interestingly, a device built by the German Navy in the 1930’s for the design of U-Boat propellers and given to Canada as part of reparations after World War II.) The testing showed that the theoretical performance of a custom propeller design (estimated mathematically) was verified with real-world test data; real-world efficiency was up to 70%, agreeing well with theoretical efficiency. It was demonstrated that the custom design and production of a cast urethane propeller is a viable and cost-effective solution for the design of AUV propulsion systems. An important observation was that high propeller blade rigidity is desirable in this design. As blade deflection increases while under load, performance decreases as the propeller shape deviates from the design.

All remaining aspects of the design and prototype development were performed in the same way – analysis followed by mathematical modeling followed by full or scale model testing using the various state-of-the-art marine hydrodynamics facilities available in St. John’s. The data already collected will be used in the development of future versions of the SQX type and additional work will be performed as design input data requirements arise.

The SQX-500 was designed to be a commercial survey tool. During its development Marport has been germinating ideas for other applications, particularly in undersea defence. Working in collaboration with Defence Research and Development Canada (DRDC), a branch of the Canadian Department of National Defence, and with the NATO Underwater Research Centre (NURC), an MCM concept has evolved and is being developed based on the use of a larger version of the SQX type AUV in a multi-agent application. It also includes an underwater ad hoc communications networking system based on disruption tolerant design principles from development partner General Dynamics Canada, a high precision positioning system, a novel interferometric synthetic aperture sonar (InSAS) operating on an SDS processor, with a real time automatic target recognition (ATR) system and a surface Gateway for “acoustic – RF” communications and a navigation reference. The complete system is called Ocean Shield.

Because of the MCM requirements the proposed SQX-SAS AUV will have to be larger and heavier than the SQX-500 AUV design and will require enhanced yaw stability for InSAS and additional payload capacity, including additional energy capacity. The propulsion system itself will have to be augmented with larger propellers and possibly two (2) drive/gearbox units per rudder as opposed to one each in the current production version.

The naval mine is an efficient force multiplier and is one of the most cost-effective weapons in the naval arsenal. Mines are small, easy to conceal, inexpensive to acquire, need little maintenance and can be easily laid from any platform. Countering threats from sea mines and underwater improvised explosive devices (IEDs) involves the use of robust solutions to detect, classify, and then neutralize these devices.

It is generally recognized that, with the increasing threat from the proliferation of mines and ever decreasing resources for MCM, AUVs provide a promising option for meeting challenging MCM requirements currently and into the foreseeable future. Advantages of AUV-based MCM include potential for operating multiple systems simultaneously (increasing coverage efficiency), low risk to personnel because vehicles are unmanned and can be forward-deployed at safe stand-off ranges and higher sonar data quality. As well as its capability for mine search and disposal, the data received from an AUV reconnaissance mission may be used to assess the limits and characteristics of a mine field, with the intention of establishing diversion routes. The data may also be used for mapping lead-through routes to give safe passage to shipping and amphibious assault vessels.

The classic end-to-end response to a mine threat is often abbreviated as DCLIRN (detect, classify, localize, identify, re-acquire, neutralize). Ocean Shield includes the D, C, L and possibly I capabilities, but not N. An important requirement is the provision of real time communications to manned assets with the information that a mine like object (MLO) has been detected, where it is located and possible specific identifying features. In the case of a positive identification, the end game would be the deployment of another undersea asset to neutralize the object.

The inclusion of the InSAS is noteworthy. The use of a twin pod AUV as a platform for an InSAS has not been done before. In the planned configuration two receive transducers will be attached to each hull, one receiving echoes from the port side of the AUV, the other from the starboard side. On each side of the vehicle, therefore, echoes from the same transmission will be received by two transducers separated by a vertical distance called the interferometric baseline, the basic principle of interferometry. The significance lies in the fact that the SQX twin pod design offers the possibility of a separation distance of up to 1.0 m, which will be much larger, possibly by a factor of 3, than what is used in any other AUV mounted interferometric side-looking sonar system, and will result in a measurement quality higher than that offered by any competitor or even by systems known to be under development.

The ATR requirement is particularly salient in the case of an AUV equipped with a high-resolution InSAS. Its purpose is to detect and discriminate targets of interest from the many other naturally occurring or man-made yet benign objects. In a time-critical application such as minehunting there must be a quick reaction to the presence of objects of interest. Current military doctrine states a change in tactics is called for once the presence of a mine is detected. ATR generates a large amount of data but the Ocean Shield system will be designed to generate a short message that is small enough to be transmitted on an underwater link, and that indicates whether or not an MLO has been identified and its location. The message will be received by a surface Gateway that will retransmit it via RF to an above water platform or base.

Ocean Shield will be deployed in 2013 and promises to be a game changing MCM technology suite in the underwater battlespace.

Acknowledgements
This article is based on several papers published in various journals and proceedings by the Marport development team and their collaborators at the NRC Institute for Ocean Technology, Memorial University of Newfoundland, DRDC and NURC.

Karl Kenny was formerly a Maritime Surface Officer in the Canadian Navy. For the last 25 years he has been an ocean technology entrepreneur, is a co-founder of Marport and originator of the software defined sonar processor concept.

(As published in the May 2011 edition of Marine Technology Reporter)