The Magazine for Underwater Professionals
Mike Daniel, trenching business line manager at Aberdeen, UK-based Fugro Subsea Services, introduces the all-purpose Q1400 trenching system
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 suggests two-thirds of all telecommunication cable breaks are caused by ships’ 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, burial of pipelines offers thermal as well as upheaval buckling protection.
Seabed geology varies widely and 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 Ltd (SMD), UK, over the design of an innovative, all-purpose trenching system, and the first of two Q1400 systems was delivered in June 2012. Conventional trenchers use either water jetting or chain cutting. Uniquely, the Q1400 provides interchangeable water jet and chain cutting skids which can be exchanged, on board the vessel while at sea.
The Q1400 trenching system can perform jet trenching in soils of up to 100kPa shear strength (i.e., sands and softer clays). For medium and harder clays up to 500kPa, the mechanical chain cutter is used. Jetting speeds are usually between 300 and 500 metres per hour, but with chain cutting that falls to 100 to 200 metres per hour. The Q1400 can operate in water depths from ten metres up to 3000 metres.
In jet trenching mode, the Q1400 has a total available power of 1459hp; 1000hp of this 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 three metres deep in soil conditions from 5kPa to 100kPa using two-metre or three-metre jetting swords, with the system capable of accommodating pipelines, cables and umbilicals up to 900 millimetres in 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 a further 300hp. Sidewall backfilling is also possible using the Q1400 trenching machine.
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. For trenching pre-laid cables and flexible pipes, the Q1400 uses a 150hp, two-metre x 400-millimetre chain cutter and two loading arms which can take flexibles, cables and umbilicals up to 250 millimetres in diameter.
“In jetting mode separate water pump systems can either backfill or keep the trench open depending on the client’s needs,” explains Mike Watt, trenching project manager at Fugro Subsea Services. “The method of backfilling depends on the soil type. With chain cutting, the trench backfills naturally because, as the umbilical or cable is trenched with the chain cutter, the trench, which is very narrow relative to its depth, will normally partially collapse.”
The vessel deck transfer system has been developed by Fugro and SMD to enable the trenching team to change between cutting and jetting modes. The cutting or jetting skids are switched via a fixed pallet attached to preinstalled skidding beams, which allows changes to be made while at sea, without a crane, and in less than 18 hours. The Q1400 launch and recovery system uses an A-frame equipped with cross beam winches and cursor. The LARS is certified by Lloyds to sea state 6, allowing operations to continue even in a heavy swell up to three metres significant wave height.
“Once deployed we use the trencher’s tracks to run it along the seabed for both 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 very soft soil. They can also be used to hop along the seabed.”
The Fugro Saltire has been adapted to take the Q1400 and operates as a dedicated trenching support vessel. The trencher is normally operated in conjunction with a Fugro work-class FCV 3000 ROV for pre- and post-trenching surveys, as well as route clearance if debris is found along the cable paths. Fugro’s second trencher, based in Montrose in Scotland, can be deployed for projects as required on alternative vessels.
Although the trencher has cameras, they are of limited use while trenching as the process throws up a great deal of sediment, reducing visibility to virtually zero, so the Q1400, the ROV and all other tools are equipped with multiple sensors and sonar systems to allow most operations in zero visibility.
In 2014, Fugro’s innovative trencher successfully undertook the trenching and burial of inter-array cables for CT Offshore A/S, Denmark, 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 11 metres – where ploughing was deemed too difficult – with a mixture of soils and hard clays interspersed with boulders and cobbles, gravels and sand.
“The Q1400 has the manoeuvrability and compact size to be able to trench right up to the cable protection system, which minimises the need for rock dumping or matressing, potentially saving on cost,” explains Mike Watt.
“We experienced outstanding cooperation with the Fugro team throughout the Gwynt y Môr project,” says Jimmy Laursen, project manager at CT Offshore A/S. “We met many challenges, especially with the demanding soil conditions. With Fugro as our trenching partner, we were able to overcome these challenges and accomplished the installation and burial of the inter-array cables in the safest and most efficient way.”
The system had 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-millimetre x 120-millimetre diameter array cables over a distance of around 16 kilometres to a trench depth of 1.2 metres. The work involved mechanical cutting through 300kPa soil consisting of cobbles, flints and chalk with boulder clay, at speeds between 100 and 150 metres per hour. Despite the difficult terrain, overall performance exceeded expectations with array cables being completed from deck to deck in less than eight hours.
In addition to trenching and burial, Fugro recently introduced a cable-lay 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, UK. 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 Q1400 trenching system to bury the cables. The array cables will be pulled in and laid between the wind turbines and the offshore substation, where the power is then transmitted onshore.
The Rampion offshore wind farm is located in the English Channel, 13 kilometres off the Sussex coast, and will consist of 116 turbines, each with a generating capacity of 3.45 megawatts. Construction is expected to be completed in 2018.
Fugro has also carried out oil and gas trenching projects. The first in 2013 was at the Corrib gas field off Western Ireland and involved jet trenching an umbilical from the shore to an offshore installation, a distance of 16 kilometres. The umbilical was successfully trenched in a single pass to the required specification, at speeds ranging from 400 to 800 metres per hour.
The next project involved chain cutting for a replacement ten-kilometre umbilical between a North Sea oil platform and the subsea production manifold. The work involved cutting at 100 metres water depth in mixed soil conditions with sand and hard clays.
Chain cutting was also needed on a major field north-east of Aberdeen. Here, a 4.5-kilometre umbilical was successfully and speedily trenched over five days in September 2013. Probably the most challenging oil project was chain cutting on the Bittern field in 2013, 200 kilometres east of Aberdeen. Here, two umbilicals of 2.5 kilometres and 20.8 kilometres respectively had to be trenched into hard clays at a water depth of 125 metres. Despite the challenges of the long distance and the very hard seabed, trenching was completed in less than six days at speeds between 120 and 200 metres per hour.
The Q1400 trenching system represents an important step forward in enabling both jet and mechanical chain cutter trenching to be carried out from a single vessel. This provides oil, gas and offshore wind farm operators with the flexibility to trench a wide range of seabed terrain, from silty/sandy soils to hard clays with consequent savings of time and money.