Offshore wind farms are now critical to meeting the UK’s electrical energy requirements for the rest of the 21^st Century and beyond. New turbine farms are being installed at a rapid rate.

Meanwhile, the first farms, installed two decades ago, are already reaching the end of their lives and are being decommissioned, creating new and demanding demolition challenges.

Ultra-High Water Pressure (UHP) jetting specialist RGL, a member of the Water Jetting Association,  is developing technology that enables power companies and demolition contractors to overcome these challenges safely and sustainably.

Once the above-sea-level turbines have been lifted away, what is left is a steel or concrete monopile that has been driven into the sea bed many metres below sea level.

Jeremy Twigg, Commercial Director of RGL, based in Romsey, Hampshire, said: “Removing these monopoles is the big challenge, not least because of the over-riding need to do it safely.

“In most cases, they have to be cut below the sea bed to ensure they don’t pose a risk to or can snag on fishing nets. In many cases, they must also be removed without the use of divers.

“We’re developing bespoke ultra-high pressure water jet cutting tools that allow us to cut these monopiles remotely no matter how big they are or what the depth of water.”

An example has been the project to remove two monopiles from the Blyth Offshore Wind Farm, sited a mile off the coast of Northumberland.

It became the first wind farm to be built in UK waters, developed by a consortium comprising of E.ON UK Renewables, Shell Renewables, Nuon UK and AMEC Wind.

After 18 years of service, two of the wind turbines had reached the end of their lives and it was decided they were to be decommissioned.

E.ON appointed marine contractor Fugro Geoservices Ltd to carry out the demolition work. In turn, Fugro Geoservices appointed RGL to undertake the subsea cutting.

RGL manufactured a bespoke abrasive water jetting tool to cut through the 50mm thick steel walls of the 3.4m diameter monopiles.

The challenge was complicated by the presence of a 200mm cementitious grout annulus. This also had to be cut through at the same time.

Working from an offshore jack-up maintenance barge alongside Fugro Geoservices colleagues, the RGL team’s first task was to use a different water jetting tooling to cut through redundant grout tubes and electric power cables inside the monopile so they did not interfere with the cut line location.

Water jetting was also used to create holes used as lifting points.

The pile cutting tool could then be lowered inside the monopile to the specified one metre below the sea bed and fixed into position with hydraulic arms.

The rig has two jetting heads to accelerate the cutting process. The combination of potable water jets, operating at 2,800bar (40,600psi) with a flow rate of 24 litres per minute, and an abrasive additive of garnet chips, easily cut through the grout and the steel pile in less than 12 hours.

“Once the water jetting has started, we want to complete it in one sweep,” explained Jeremy Twigg. “This ensures we have a clean and complete cut. Once the cut was complete, each monopile could be lifted from the sea bed, with our cutting tool still in it.”

In another project at the Horns Rev 2 wind farm off the coast of Denmark, RGL had to first dredge a monopile to remove silt and sludge. Its teams used UHP water jets and compressed air to push the material out of the pile, a process that is faster and more thorough than the  conventional use of a grab bucket.

High pressure water jetting was then used to clean the inside of the pile so the cutting rig could be safely lowered into position at a depth of 24.2m from the top of the 44-tonne pile. Eight hours later, the cut was complete and the monopile could be lifted to the surface.

Jeremy Twigg said: “Hydrodemolition is proving to be the most sustainable, safest and most certain method for removing wind turbine piles. We’ve removed piles where previous attempts using diamond cutting had failed.

“Over the coming decades, there is no doubt hydrodemolition will be crucial to maintenance and development of off-shore wind farms.”