Spider 2 navigates by sound waves and has articulated walking gear from alpine forestry machine. (Images courtesy of Norsk Hydro)

Building a route for pipelines and service cables down a narrow undersea ravine and over a cliff into a plunging badlands of 60-meter peaks, valleys, boulders, ice water, high seas and strong currents is a challenge for equipment and technique.

Norsk Hydro invested three years and $100,000 surveying and planning the route and then called in undersea cable specialist Nexans Norway AS, Oslo, for new equipment to groom the way.

Nexans developed and now is operating two electrically powered machines called Spiders. They are underwater excavators that can take conventional attachments as well as water-jet and suction devices. With their powered tracks and articulated walking legs that move independently, they can work on slopes as steep as 35° in depths of up to 1,000 meters.

The Spiders were created by combining undersea pipe-burying technology and control systems Nexans has built for years with the boom, walking system and hydraulics of an advanced alpine forestry machine made by Menzi Muck AG, Widnau, Switzerland. Nexans purchased undersea rights to the technology from Menzi.

Two versions were built, Spider 1 weighing 10 tons and Spider 2 weighing 15 tons. The larger Spider is 3x6 meters, and weighs 15 tons. The larger Spider machine, Including the umbilical, umbilical winch and the launch and recovery system costs about $6 million. The smaller one costs about $4.5 million. A lot of the development costs have been towards developing the virtual control system.

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Nexans owns and operates the Spiders, but Hydro has paid for everything. Its total excavation contract with Nexans is $33 million.

When offshore weather permits, the Spiders work around the clock in 12-hour shifts. They are excavating about 10,000 cu m of material to prepare a 5-m-wide by 2-m-deep bed for two 30-in. gas lines. Two 6-in. antifreeze supply lines and several control cables will be buried with Nexan’s conventional equipment under 1 m of protective cover. Pipeline spans are limited to 50 m. Ship-based rock placers are dropping 2.4 million tons of stone to build supports in seabed depressions.

Tough Trenching

The Spiders get their power and control signals from an umbilical cable to a shipboard control room where operators direct them on a 3-D digital plan. The controllers use underwater video to navigate the machines when they can. But visibility disappears during digging, so most navigation is done using transducers that pick up acoustic signals from an array of two hundred transponders placed along the route.

Siting the transponders required careful planning. It had to take into account signal-warping effects of undersea formations and the shifting boundary layer between two water masses that have pronounced differences in temperature and salinity. The system also can be affected by a deep sea phenomenon that produces waves called solitons far below the surface, traveling in the boundary layer.