A clever all-purpose trenching machine intended for burying cables on the sea floor provides interchangeable water jet and chain-cutting skids which can be swapped on board its installation vessel while at sea. The Q1400 trenching system, a collaboration of Fugro Subsea Services and Soil Machine Dynamics, operates in water depths from 10 to 3,000m and uses water jet trenching in soils of up to 100 kPa shear strength, typically sands and softer clays. Medium and harder clays up to 500 kPa call for the mechanical chain cutter. Jetting speeds are usually from 300 to 500 m/h but with chain cutting that drops to 100 to 200 m/h.
“It is important to bury wind farm cables as well as oil and gas umbilicals, and pipelines to protect them from damage, particularly in the crowded, relatively shallow waters off European coasts,” says Mike Daniel, Trenching Business Line Manager at Fugro. A 2009 report by the International Cable Protection Committee suggested that two-thirds of all breaks to telecommunication cables are caused by ship anchors and commercial fishing trawlers. Unburied cables and pipelines also present a serious hazard for trawlers which can lose gear or even be pulled under.
When jet trenching, the Q1400 can use as much as 1,459 hp. Of this, 1,000 hp is delivered through variable-speed electric motors to direct-drive water pumps. “The jetting tool has twin-legged parallel jet swords and can trench up to 3m deep in soil conditions from 5 to 100 kPa using 2 or 3m jetting swords. The system can work with cables, pipelines, and umbilicals up to 900-mm diameter,” adds Daniel. The jetting system also provides backwashing and eduction (suction) of seabed material at the same time as the jet trenching is being carried out, which uses an additional 300 hp. The trenching machine also provides sidewall backfilling.
For trenching pre-laid cables and flexible pipes, the Q1400 uses a 150 hp, 2 x 400-mm chain cutter and two loading arms which can take flexibles, cables, and umbilicals up to 250-mm dia. With pre-laid rigid pipe, the trenching-jet legs fluidise the soil on either side of and underneath the pipe causing it to sink into the seabed.
“In jetting mode, separate water-pump systems can either backfill or keep the trench open depending on client requirements,” explains Mike Watt, Trenching Project Manager at Fugro Subsea Services. “The method of backfilling depends on the soil. A trench naturally backfills when it is chain cut to bury a cable or umbilical. The trench, which is narrow relative to its depth, normally collapses.”
The vessel’s deck-transfer system was also developed by Fugro and SMD to let the trenching team change between cutting and jetting modes. The cutting or jetting skids are switched by a fixed pallet attached to preinstalled skidding beams, which allows making changes while at sea, without a crane, and in less than 18 hours.
“Once deployed, the trencher runs along the seabed on its tracks for jetting and cutting operations,” says Mike Watt. “Thrusters can be used for adjusting its position, and lifting the trencher off the seabed to aid movement in soft soil. Thrusters can also be used to hop along the seabed.”
Although the trencher has cameras, they are of limited use while trenching because the process throws up a great deal of sediment reducing visibility to nearly zero. The solution is to equip the Q1400, the ROV, and all other tools with multiple sensors and sonar systems to allow most operations in zero visibility.
The first system cut its teeth successfully in September 2012 at an offshore wind farm on the UK’s east coast. This involved post-lay trenching of 16 x 120-mm diameter array cables over a distance of around 16 km to a trench depth of 1.2m. The work involved mechanical cutting through 300 kPa soil consisting of cobbles, flints, and chalk with boulder clay, and at speeds from 100 to 150 m/hr. Despite the difficult terrain, overall performance exceeded expectations with array cables being completed from deck-to-deck in less than eight hours.
Filed Under: News, Offshore wind