10-centimeter undersea precision

Halvor Snellingen, Nexans -- Control Engineering, 12/1/2006

In 1997, the Ormen Lange gas field was discovered off the western coast of Norway. It was the second-largest natural gas discovery on the Norwegian shelf and has the potential to produce around 20 billion cu m of gas each year. Ormen Lange, which means “the long serpent,” is approximately 40 km long and eight km wide and lies about 3,000 m below sea level.

The gas field will be operational in 2007 and equipped with seabed installations at depths from 800 to 1,100 m. The gas will be transferred through a pipeline from production platforms to a processing plant at Nyhamna, Norway, then exported via a 1,200-km undersea pipeline to Easington, on the east coast of the UK, and to other locations in Europe via a distribution center on the island of Sleipner in the North Sea.

Extreme conditions at the site include below-freezing temperatures, stormy seas, and strong underwater currents. Because of these conditions, the Ormen Lange gas field will not use conventional offshore platforms. Instead, wellheads on the ocean floor will be connected directly by pipes to the onshore processing facility at Nyhamna.

The pipelines must be routed through the rugged topography of the ocean floor in such a way that unsupported sections of pipe will not be vulnerable to damage. To solve this problem, Nexans developed Spider, a remote-controlled underwater excavator designed to level the seabed for pipe-laying. Spider is based on a Swiss forestry machine outfitted to work on steep slopes and rocky terrain below the water.

Nexans engineers used the National Instruments Labview platform to develop Spider's human machine interface (HMI) and the embedded control system running on National Instruments CompactRIO distributed industrial control and acquisition hardware that controls the hydraulic systems.

Spider can be controlled with 10- to 20-cm precision, even at a depth of 1,000 m. This is accomplished using 3-D software, sensors on all movable parts of the machine, and a network of acoustic transmitters on the ocean floor. A 3-D model of the seabed is updated in real time using Labview HMI. A separate remote-controlled underwater vessel with an echo sounder carries out a detailed inspection daily.

The software for Spider is operated from a control room onboard a ship, where an operator has a complete overview through Labview screens. Live video is displayed from several cameras mounted on Spider. The excavation process is displayed in a 3-D ActiveX control on the Labview front panel, which shows a model of the seabed and, through a number of sensors on the Spider, a real-time image of the machine's position.

The Spider and its grabber are controlled using an off-the-shelf joystick. Labview reads the commands from the joystick through the joystick VIs and sends control signals over a fiber link to Spider—even at depths of up to 1,000 m.

The three CompactRIO systems are protected from extended temperatures, salty sea air, and high humidity by IP62 enclosures. They perform heave compensation, winch, and power control, and communicate with the main Labview application. The algorithms run in real time on CompactRIO. The system is easy to maintain, in part, because of the consistency in programming both the controls and the HMI with Labview.