Better dredger - Embedded computers power undersea ROV

March 8, 2007
The Spider — an underwater ROV (remotely operated vehicle) — uses a 3D model of the sea floor to manage its telescoping arm and thrusters while operating 1,000 ft below the surface.

Visible in this view of the Nexans Spider ROV is the telescoping arm (here equipped with a dredging tool) and its undercarriage. The chassis is based on a design from a machine for cutting down trees on steep slopes. The wheels normally carry tracks which have been removed for service.

When seawater is too murky to give a clear view from Spider's onboard video cameras, operators get a virtual image of what the ROV is doing from sensor feedback and a 3D model of the seabed as shown here. LabView software presents the model data to operators who control the ROV from a surface ship.

The French cabling company Nexans developed the Spider in 2004 to level a rough area of seabed for a gas pipeline being laid off the coast of Norway. To date, it is the only way of leveling seabed slopes of up to 35°. To get around on the seabed, the Spider combines powered tracks with articulated legs. Nexans borrowed the design from a forestry machine built by the Swiss firm Menzi Muck to cut logs on steep slopes.

The Spider measures 7 × 4 × 2.5 m and weighs 15,000 kg. The frame is titanium. The craft moves in the water using eight 17-in. thrusters each generating 550 kgf. To help move seabed material, it has two 400-kW jetting systems that put out 14 and 25 bar, respectively. The Spider uses a combination of water jetting and suction to move large volumes of soil. It also carries a 130-kW telescopic arm that extends 9 m. Various tools fits on its working end for tasks such as dredging and cutting pipe. Operators can change tools without bringing the Spider to the surface.

The Spider carries six video cameras, sonar, a pressure sensor, altimeters, gyros, eight angle sensors, and six proximity sensors. Sensor data goes to the surface over an umbilical cord using an Ethernet protocol. There, operators run Spider from a support vessel. Spider's video cameras let the operators follow its movements. But sometimes material stirred up from the seabed reduces visibility to such a degree that operators must use a virtual 3D computer image of Spider and its surroundings to follow what's going on. The underlying terrain model is prepared in advance from survey data. It is also updated in real time using a National Instruments Corp. LabView human-machine interface to show how the terrain is changing.

Operators control the Spider and its telescoping arm with a joystick. The LabView program reads joystick commands and transmits them down to the Spider. Three National Instruments CompactRIO computers in IP62 enclosures perform heave compensation, power and winch control, and communicate with the main LabView application.

National Instruments Corp.,

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