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Station-Keeping Autonomous Surface Vehicle

By Jonathan Burton • Vince Dobbin

The C-Stat 2 is lifted onto the trials ship offshore Japan.

The C-Stat developed by Autonomous Surface Vehicles Ltd. (ASV) of Portchester, England, is a station-keeping vessel for applications where anchoring maybe difficult or the cost of continuously operating a manned vessel is prohibitive. For some applications, solutions using wave- or wind-powered station-keeping platforms struggle to support the necessary payloads (mass, volume, electrical power) or positioning accuracy in strong currents.

The aim of the C-Stat program is to produce a rugged, cost-efficient and practical vessel capable of operating for weeks and months at low speeds, while reliably holding position in high currents and powering payload equipment. The concept of a station-keeping buoy was envisaged to have a wide range of uses in both the security and commercial markets, such as coastal reconnaissance, perimeter security, communications nodes, metocean studies, subsea positioning, marine-mammal monitoring, surveying and seismic-support applications. The C-Stat is a long-endurance, station-keeping buoy capable of surviving and holding position in harsh conditions while providing significant power to onboard payloads.

Algorithm Testing
The algorithms required to control the C-Stat were developed in-house at ASV in Portchester. Simulations were carried out using Simulink as part of the Matlab software suite, which predicted power requirements and energy usage in differing wind, wave and current conditions for a system with two differential thrusters. The main aim of the simulation was to produce a set of energy-efficient algorithms that could be tested on a physical prototype.

Tests were carried out in Portchester throughout 2012 using a 2.55-meter prototype based on a commercially available boat, with test motors mounted on each side of the vessel. The vehicle was equipped with a GPS and compass, and the entire system was controlled using an ARM CPU. The orientation and position data were recorded on board, and commands were sent to the buoy through a radio connection. The test system was powered using a 12-volt lead-acid battery bank with a capacity of 170 amp-hours. The tests verified the algorithms and showed a good correlation between the test data and computer predictions.

Naval Architecture
The development of the first production C-Stat 2 started in May 2012. The design requirements included self-righting, with a low above-water profile and compact design for transportation (including via air freight). The vessel was to be capable of station keeping in strong tides and currents, with a peak speed up to 4 knots using protected propellers and electric propulsion, allowing for adaptability of power sources (diesel, fuel cell, solar). The hull was to be rugged with high roll and pitch damping, and an allowance for a folding keel to stabilize the hull further when cameras or sensitive sensors are to be mounted.

The C-Stat's purpose as a long-endurance, station-keeping buoy requires it to survive harsh sea states and environmental conditions, which is why self-righting is necessary. The height of the deck and the fender design are the main contributors to righting. However, it was also deemed important to keep the above-water profile to a minimum to reduce air drag. Thus, an optimum design was reached with a 0.3-meter deck height above the load water line.

A propulsion system was developed that would offer a good compromise between protected propellers, anti-ingestion of material and efficiency. The use of two motors for differential steering reduces the need for a rudder and its potential failure modes (rudders are susceptible to fouling and failure). Two propellers are mounted behind stainless-steel guards. Two brushless permanent magnet asynchronous motors with electrical efficiencies around 85 percent were chosen for their low maintenance and long lifetime.

For a vessel to station keep it must be able to withstand the effect of the current, waves and wind. For this particular vessel, it was decided to allow for 3 knots as a continuous speed, leaving a 1-knot reserve. Accurate power (resistance) estimates for the craft were an important part of endurance calculations. The novel design generated for this role is quite different to conventional ship forms and therefore did not benefit from historical resistance data or traditional empirical series.

To predict the required installed power, a mixture of methods were used, including estimations of drag using coefficients from Fluid-Dynamic Drag by Dr. Sighard F. Hoerner, simplified potential flow theory using the software Michlet, computational fluid dynamics calculations using Open Foam software and measurements from two physical prototypes of different sizes. To continue this article please click here.

Jonathan Burton is a naval architect at ASV Ltd. He graduated from the University of Southampton in 2011 with a master's in engineering in ship science with small craft and yacht design. Since joining ASV Ltd., he has been involved in a variety of projects for the military and commercial markets, including the C-Stat 2 program.

Vince Dobbin is an ex-submarine officer who has served on diesel, fast-attack and strategic submarines, and has held various shore appointments. He left the U.K. Royal Navy in 2005 and has held a number of positions within the defense industry in technology companies. He joined ASV Ltd. in November 2011.

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