Global Marine has been involved in the installation of cables for the offshore renewables sector since the conception of the industry. In fact, in 2002, the company installed 85 kilometres of cable at Horns Rev, Denmark, Europe’s first-ever commercial wind farm. Here, power cables featuring around 160 connections were buried up to two metres below the seabed. Similarly, two years later, the company installed the cables for the UK’s first commercial wind farm at Kentish Flats, an array of 30 wind turbines close to Herne Bay.

This pioneering activity in combination with efficient, competent, high quality operations, has led to a plethora of landmark renewables projects over the years, including several in UK, Danish, Dutch, Belgian and German waters. Coming right up to date, Global Marine has recently secured a contract for an inter-array project, working for Prysmian, Italy, at the Wikinger offshore wind farm, Germany, which is due for comletion in 2016-17.

Transferrable expertise is one of the key reasons behind the company’s success in the renewables market, with a 165-year legacy in subsea engineering. For instance, customers benefit from access to one of the industry’s most capable fleets, in terms of assets and seafarer skill. This incorporates vessels such as the CS Sovereign, which is equipped with a pair of powered 2300-tonne ‘basket’ turntables that are designed to operate at a linear speed of 900 metres per hour, thus optimising the cost-efficiency of cable laying.

Of course, laying the cable is one thing, protecting it is quite another. For this reason, one of the core activities of all cable laying projects for the renewables sector is the development and implementation of engineered trenching and burial solutions.

PROTECTION
Trenching, which can take place pre- or post-cable laying, is necessary to ensure protection from the dangers posed by the seabed terrain, as well as trawlers, anchors, icebergs or any large objects, such as shipping containers, that occasionally fall from passing vessels.

Trenches are created by ploughing, jetting or cutting the seabed material. Ploughing is a proven, general purpose method that is suitable for various seabed soil types, including granular sand, clays and fractured rock. It can produce deep trenches with high quality, straight walls in a single pass, while impressive speeds can be achieved in softer seabed types, thus reducing time and costs.

Global Marine’s advanced ploughing technologies operate to depths of around 2000 metres. Each of the company’s ‘Hi Ploughs’, as they are called, offers 500 kilowatts of forward and under-heel jetting power to reduce active tow tensions. In addition, they can be fitted with a two-metre plough share or a 3.25-metre injector share.

Of course, there may be some applications where ploughing is not suitable. For example, the large size of Hi Ploughs means that launches are more complex, while some deepwater limitations also exist. In such situations, jetting can prove to be a viable and cost effective alternative.

Jetting is usually performed using remotely operated vehicles, self-propelled, mobile devices that are easily deployed, deep water capable and well-suited to granular type seabed materials. Here, Global Marine has dedicated cable trenchers that include the company’s Atlas 1 and Atlas 2 ROVs, and its ST200 free-swimming ROVs – the benchmark for modern cable working ROVs – which are depth rated to 2500 metres with a one-metre burial tool.

DEMAND
For the offshore renewables market, there is increasing demand for multi-purpose ROV jetting trenchers. This is because they are designed for both pre- and post-lay trenching, as well as simultaneous cable lay and burial. Global Marine’s multi-purpose ROV jetting trenchers include the XT601, along with the recently acquired SMD, UK, Q1000.

Offering 1000hp of total installed and variable jetting power, the Q1000 ROV can be easily mobilised on to vessels, and is suitable for trenching cables up to 20 inches (508 millimetres) in diameter to a burial depth of three metres at speeds of up to 400 metres per hour. Jet flow can be optimised to meet the numerous variables that could be encountered during trenching operations, while a range of eductors (jetting swords) for trench clearance are easily interchangeable with backfill tools.

The main jetting tool is a twin-leg implement of adjustable width. Jet leg lengths of one, two and three metres are available and can be deployed as stand-alone devices or in conjunction with eductor or rear backwash capability in both cohesive and non-cohesive seabed materials.

The rear-mounted eductors are designed for use in combination with two-metre jet legs to lift and discharge spoil beyond the trencher footprint in cohesive seabed soils, thus leaving an open trench. When deployed in non-cohesive soils, the discharged spoil from the eductors serves to extend the envelope of the fluidised material and aid the lowering of the product into the trench as it is cut.

Rated to an operational depth of 1000 metres, the Q1000 can be configured for use with either tracks or skids to suit the requirement, and can be deployed for trenching in the majority of seabed conditions, from fine sand to firm clay. State-of-the-art cable detection and tracking is designed to result in time-saving, precision burial for the customer. Furthermore, the Q1000’s high specification enables it to operate in extreme environments and challenging weather conditions. With this in mind, it offers heavy latch beam launch and recovery systems designed for sea state 6.

Of course, there may be conditions where jetting is simply not suitable. For instance, where there are exceptionally strong currents or where clays above 100 kilopascals or cemented materials are encountered on the seabed.

CHOICE
For hard seabed materials, self-propelled mechanical cutters are the preferred choice. These devices typically deploy chain, wheel or scoop type slot cutters to mechanically remove spoil, and can be used in both shallow and deep water for post-lay or simultaneous operations. Mechanical cutters can produce slot or V-type trenches, depending on whether cable protection or stability is the main priority.

It is worth noting that the trenching of more demanding seabed terrain brings certain additional factors into play. For instance, a greater deck space area will be required to accommodate a mechanical trencher. Furthermore, a large turning circle will be needed if a mechanical trencher is to be deployed successfully.

 
In general, asset selection comes down to the specific properties of seabed materials, such as the shear strength of clays and relative density of sands. Here, Global Marine’s geophysical and geotechnical analyses have helped to select the optimum asset – plough, jetter or mechanical cutter – for a wide range of offshore renewables projects over the past 15 years.

Looking to the future, the company recognises the growing demand for construction and operations and maintenance (O&M) services in the offshore renewable market. The acquisition of Essex, UK-based services provider CWind in February 2016 demonstrates Global Marine’s commitment to this growing sector. CWind offers particular specialisation in renewables, having supported the construction and operation of more than 26 wind farms in UK and European waters, and gives Global Marine an expert partner in this field.

Ultimately, the range of services provided by the team and the vessels within CWind complements Global Marine’s range of installation and maintenance services, and creates an organisation that is well positioned to address the developing needs of customers in the offshore renewables market.