Sea Technology

APR 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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(Top) C-Stat 2 is capable of 3.5 to 4 knots speed, dependent on payload, as seen here at ASV���s base in Portchester. (Bottom) C-Stat 2 deployed offshore Japan in the Pacifc Ocean. nection. The test system was powered using a 12-volt leadacid battery bank with a capacity of 170 amp-hours. The tests verifed the algorithms and showed a good correlation between the test data and computer predictions. Naval Architecture The development of the frst production C-Stat 2 started in May 2012. The design requirements included self-righting, with a low above-water profle 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 profle 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 effciency. 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 effciencies 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 beneft 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 coeffcients from Fluid-Dynamic Drag by Dr. Sighard F. Hoerner, simplifed potential fow theory using the software Michlet, computational fuid dynamics calculations using Open Foam software and measurements from two physical prototypes of different sizes. The choice of power installation was based on future adaptability. The current vessel has a 2.5-kilowatt diesel generator and a fuel tank combined with lead-acid batteries. This hybrid system allows for the generator to be replaced with any sort of fuel cell or even just batteries for missions in environmentally sensitive areas or where there is a requirement for a low acoustic signature. The vessel hull is constructed from rugged, lightweight marine aluminium. An integral D-section fender was custom manufactured, incorporating a bow-impact section. This all-round fendering makes the C-Stat practical to launch, recover and approach from other craft. The low maximum vessel speed, light weight, rounded hull shape and fendering design reduce the risk of damage from potential collisions to as low as reasonably practicable. A pivoting keel with adjustable mass is installed on the C-Stat 2. It is not necessary to install the keel on all designs. Its selection depends on the sensors deployed, above-water payload mass and customer stability requirements. Trials and Testing In late 2012, ASV delivered the frst C-Stat to Tohoku University in Japan for use in a long-baseline acoustic system to monitor the movement of tectonic plates. The C-Stat was deployed off a charted 40-meter oceangoing tug about 150 miles offshore the eastern coast of Japan in conditions with a maximum 1-knot tide, winds up to 20 knots and waves up to 2 meters. In these conditions, the vessel was capable of moving against wind and tide at 2 knots over the ground while following a programmed line. Initial indications have shown a good correlation between predicted and actual power requirements. The resistance www.sea-technology.com April 2013 / st 11

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