All terrain trenching

Subsea trenching has become more versatile with Fugro and SMD’s Q1400. Mike Daniel explains.

Launching the Fugro Q1400 trenching system. Photos from Fugro Subsea.

It is important to bury wind farm cables, oil 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.

Seabed geology varies widely and two types of tractor based system 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 (SMD) over the design of an innovative, all-purpose trenching system. The Q1400 provided interchangeable water jet and chain cutting skids, which can be exchanged onboard 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 300-500m/hr, but with chain cutting that falls to 100-200m/hr. The Q1400 can operate in 10-3000m water depth.

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 3m deep in soil conditions from 5-100kPa using 2m or 3m jetting swords, with the Q1400 system capable of accommodating pipelines, cables and umbilicals up to 900mm in diameter.

With pre-laid rigid pipe the trenching jet legs fluidize the soil on either side of the pipe causing it to sink into the seabed. For trenching pre-laid flexible pipes the Q1400 uses a 150hp, 2m x 400mm chain cutter and two loading arms, which can take flexibles, cables and umbilicals up to 250mm diameter.

Backfilling can take place at the same time as jetting. Separate water pump systems can either backfill or keep the trench open depending on the client’s needs. The method of backfilling depends on the soil type. With chain cutting it 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 back.

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 under 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, allowing operations to continue even in a heavy swell up to 3m significant wave height.

Once deployed, the trencher’s tracks are used to run it along the seabed for both jetting and cutting operations. Thrusters can be used for adjusting its position, lifting the trencher off the seabed to aid movement in very soft soil; they can also be used to hop along the seabed.

The Q1400 swung out on its launcher alongside a Fugro FCV 3000 ROV.

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.

In 2014, Fugro’s trencher successfully undertook the trenching and burial of inter-array cables for CT Offshore 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 —with a mixture of soils and hard clays interspersed with boulders and cobbles, gravels and sand.

The Q1400 has the maneuverability and compact size to be able to trench right up to the cable protection system, which minimizes the need for rock dumping or matressing, potentially saving on cost.

The system had cut its teeth successfully in September 2012 at a wind farm offshore Skegness on the UK’s east coast. This involved post-lay trenching of 16 x 120mm-diameter array cables over a distance of around 16km to a trench depth of 1.2m. The work involved mechanical cutting through 300kPa soil consisting of cobbles, flints and chalk with boulder clay, at speeds between 100 and 150m/hr. Despite the difficult terrain overall performance exceeded expectations with array cables being completed from deck to deck in less than eight hours.

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 16km. The umbilical was successfully trenched in a single pass to the required specification, at speeds ranging from 400 to 800m/hr.

The next project, involved chain cutting for a replacement 10km umbilical between a North Sea oil platform and the subsea production manifold. The work involved cutting at 100m water depth in mixed soil conditions with sand and hard clays.

Chain cutting was also needed on a major new field 70km North-East of Aberdeen. Here a 4.5km umbilical was successfully trenched over five days in September 2013. Probably the most challenging oil project was chain cutting on the Bittern field in 2013, 200km east of Aberdeen. Here two umbilicals of 2km and 20.8km respectively, had to be trenched into hard clays at a water depth of 95m. Despite the challenges of the long distance and a very hard seabed, trenching was completed only six days at speeds between 120-200m/hr.

Mike Daniel is Business Line Manager for trenching and cable installation with Fugro Subsea Services. He has been involved in the subsea industry for over 33 years, starting in 1981 on early North Sea offshore oil and gas pipeline, umbilical installation and burial projects. He was at the forefront of UK offshore wind farm cable installation where he took his experience from oil and gas to the fledgling wind farm business.