Mike Daniel / Trenching Business Line Manager / Fugro Subsea Services Limited
It is important to bury wind farm cables, oil and gas umbilicals, and pipelines to protect them from damage, particularly in the crowded, relatively shallow waters off European coasts. 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 present a serious hazard for trawlers which can lose gear or even be pulled under. In the oil and gas sectors the burial of pipelines offers protection from thermal problems as well as upheaval buckling.
A few solutions
Seabed geology varies widely, so two types of tractor-based systems are used for trenching. Conventional systems are deployed from separate vessels, using water jetting for loose granular soils and chain cutting for hard clay and boulders.
Fugro collaborated with Soil Machine Dynamics Limited (SMD) in the design of an innovative, all-purpose trenching system. The first of two Q1400 systems was delivered in June 2012. Conventional trenchers use either water jetting or chain cutting. The Q1400 however, provides interchangeable water jet and chain-cutting skids which can be exchanged, on board the vessel while at sea.
The Q1400 trenching system, which can operate in water depths from 10 to 3,000m, performs jet trenching in soils of up to 100 kPa shear strength, i.e. sands and softer clays. Medium and harder clays up to 500 kPa call for a mechanical chain cutter. Jetting speeds are usually from 300 to 500 m/h but with chain cutting that falls to 100 to 200 m/h.
When jet trenching, the Q1400 can use as much as 1,459 hp. Of this, 1,000 hp is delivered through variable speed drive electric motors to direct-drive water pumps. The jetting tool has twin-legged parallel jet swords and can trench up to 3-m deep in soil conditions from 5 to 100 kPa using 2 or 3m jetting swords. The system is capable of accommodating pipelines, cables, and umbilicals up to 900-mm diameter. 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.
With pre-laid rigid pipe, the trenching-jet legs fluidize the soil on either side of and underneath the pipe causing it to sink into the seabed. 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 diameter.
“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 type. When chain cutting a trench to bury a cable or umbilical, the trench naturally backfills. The trench, which is narrow relative to its depth, will normally and partially collapse.”
The vessel’s deck-transfer system has been 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. The Q1400 launch and recovery system (LARS) uses an A-frame equipped with cross beam winches and cursor. The LARS is certified by Lloyds to sea state 6, letting operations continue even in a heavy swell up to 3m significant wave height.
“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.”
The Fugro Saltire has been adapted to operate as a dedicated trenching support vessel. The trencher normally operates with a Fugro work class FCV 3000 ROV for pre and post-trenching surveys, as well as route clearance when debris is found along the cable paths. Fugro’s second trencher, based in Scotland, can be deployed for projects as required on alternative vessels.
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.
In 2014 Fugro’s trencher successfully undertook the trenching and burial of inter-array cables for CT Offshore A/S at Gwynt y Môr wind farm off the North Wales coast. Cables were trenched in the hardest soil area of the wind farm in a minimum water depth of 11m, where ploughing was deemed too difficult. Ploughing was through a mixture of soils and hard clays interspersed with boulders and cobbles, gravels, and sand.
“The Q1400 has the manoeuvrability and compact size to trench right up to a cable protection system which minimises the need for rock dumping or matressing, potentially saving on cost,” explains Watt.
“We met many challenges, especially with the demanding soil conditions,” said Jimmy Laursen, Project Manager at CT Offshore A/S. “With Fugro as our trenching partner, we were able to overcome the challenges and accomplish the installation and burial of the inter-array cables in the safest and most efficient way.”
The 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.
More than trenching
In addition to trenching and burial, Fugro recently introduced a cable-laying service. In September 2015 the company was awarded a contract for the installation and burial of array cables, and commenced engineering and planning for Rampion Offshore Wind Ltd. Installation will be carried out in two phases in 2016 and 2017.
Fugro will lay and bury the cables with its construction and installation vessels, Fugro Symphony and Fugro Saltire, using its trenching system. The array cables will be pulled in and laid between the wind turbines and the offshore substation, from where the power will be then transmitted onshore.
The Rampion Offshore Wind Farm, 13 km off the Sussex coast in the English Channel, will consist of 116 turbines, each with a generating capacity of 3.45 MW. Construction is expected to complete in 2018.
The Q1400 trenching system represents an important step forward, enabling jet and mechanical chain-cutter trenching from a single vessel. This provides operators of offshore wind farms along with oil and gas facilities with the flexibility to trench a wide range of seabed terrain, from silty and sandy soils to hard clays, in less time and lower cost than previously possible.
Watch an animated video of the Q1400 trencher in action, here: