Offshore wind is an increasingly viable source of renewable energy and the UK is arguably the world leader in its development and exploitation. According to a report published by the Global Wind Energy Council in March 2021, the UK accounts for 29% of the world’s total offshore wind power capacity of 35.3 gigawatts.
Currently, the biggest obstacle to the adoption of offshore wind power is cost due to the logistical complexity of erecting huge wind turbines miles out to sea. But this cost is rapidly coming down as new and more efficient methods of construction are developed.
This article was first published in the July 2021 issue of The Construction Index magazine. Sign up online.
In very shallow seas it is usual to mount turbines directly on top of large monopiles driven into the seabed. But as turbines become larger and developers venture further out to sea, designers are turning to floating turbines anchored to the seabed. That in turn presents a challenge: how to construct anchors or foundations at greater depths.
Two Irish firms – Dublin-based Mincon and Subsea Micropiles, headquartered in Shannon – are collaborating to develop a solution. With €2.9m (£2.5m) funding from the Irish government and research support from University College Dublin and the University of Limerick, the two companies are developing a new robotic seabed drilling system that could slash costs and even reduce environmental impacts.
The concept of ‘micropiling’ is not as exotic as it might sound. It will be familiar to many in the form of earth drilling and rock anchoring to stabilise slopes and reinforce excavations.
According to Derek Robertson, chief executive of Subsea Micropiles, the concept was pioneered in the 1950s by Italian engineer Fernando Lizzi who invented the so-called ‘palo-radice’ or ‘tree-root pile’ for use in post-war reconstruction work.
“Lizzi’s challenge was to preserve historic structures in places where physical access was difficult,” explains Robertson. “He took his inspiration from nature and the way the root structure of trees mobilises a large soil mass to absorb dynamic loadings,” he adds.
A micropile is broadly defined as a slender bored pile or anchor ranging in diameter from as little as 30mm up to 300mm. Mincon Group, founded in Shannon in 1977 but now a global business, specialises the design, development, manufacture and service of rock drilling equipment for a variety of applications.
Subsea Micropiles, on the other hand, is a new business that aims to take technology developed by Mincon for terrestrial use and adapt it for the construction of “high performance” seabed foundations and anchors.
“We’re a start-up, as of 2017, and the purpose of our company is to commercialise micropile technology for offshore construction,” says Roberston.
“Micropiling is a proven foundation and anchor solution for onshore infrastructure; it’s attractive as a low-cost, low-noise, low-vibration solution to a lot of big challenges and we see a fantastic opportunity to take that offshore and realise a number of benefits that the offshore sector needs right now,.”
The new venture hopes to develop a foundation anchor design that could interface with a mooring line for a floating structure or the leg of a fixed structure. This will comprise an anchor ‘template’ – a steel structure through which multiple piles are installed by a robotic drill rig to create a foundation.
According to Robertson, micropiles – like conventional piling – were designed to withstand compressive loadings. “But over time the range of applications has been expanded to exploit the fact that, with the strength of the grounded pile structure, they can provide a lot of shear and lateral load resistance,” he says.
“And it goes further in that they are useful in tension loading. If you think of a really large radio tower; you know how those towers have guy lines coming down in a wide spread? Those would typically be micropiled anchors and they’re obviously going to be in tension.
“That’s where it gets more interesting for us because, as we go offshore we’re looking at much more dynamic load regimes…We would be looking at a wider range of both horizontal and vertical forces in our load profile,” explains Robertson.
Robertson says that, until recently, offshore wind development has been uniquely dependent on monopile structures in relatively shallow waters. “But a lot of seabed real estate that can support that kind of solution has now been taken up,” he says.
“We’re at an interesting juncture in the offshore wind industry now where turbines and the structures that support them are getting larger and there’s less and less of that shallow 30m-to 40m-depth real estate. And now the growth of the industry depends on being able to go further offshore into deeper water where there’s greater wind resource,” argues Robertson.
Venturing into much higher-energy environments and greater depths where monopile foundations become less feasible has seen a migration towards alternative structures, such as gravity-based structures and steel jackets, says Robertson. But the accepted wisdom, and the investment, is moving towards floating structures.
Offshore structures are expensive to build and to install. But Robertson says that the adoption of subsea micropiles will not only offer a technical solution but will have the potential to radically reduce costs.
First, he says, there is the obvious saving in materials employed – i.e. the quantity of steel. But the biggest saving is in what he terms “vessel spread”. Traditional piling operations conducted from the surface demand large, highly specialised vessels – and usually more than one vessel plus ancillary equipment such as large cranes.
“The daily rate on that vessel spread is an order of magnitude higher than the smaller non-specialised supply vessel we’d be aiming to use,” he says. “There are far more of those in circulation and they are more locally available.”
Such a vessel could deploy a submersible drill rig which would drill and grout hollow steel micropiles through the steel ‘template’ structure on the seabed.
So it would be cheaper, easier – and even environmentally more sustainable than traditional piling, says Robertson.
“We are seeing increasing concern about a wide range of impacts … the level of ambient noise in our oceans has fundamentally changed,” he argues. “In recent years there’s been an acute awareness of the effects on marine mammals and other species and now we’re seeing really stringent regulatory restrictions introduced to mitigate these impacts.”
Germany, having monitored offshore wind developments, has introduced very tight restrictions on how much noise can generated in its waters, explains Robertson. The industry, in response, is trying to develop new technology to mitigate that impact. “But it’s still a big expensive challenge,” he says.
He believes that the Mincon/Subsea Micropiles collaboration offers a unique solution: “I think we are arguably at the vanguard: there are no known micropiled anchor solutions that I am familiar with,” he says. “Of course micropiles can be installed from the surface in marine environments – you see that around ports and harbours – but not going into deep water to support the kind of things we want to do with offshore energy projects.”
Recent years have seen a rapid development in the area of subsea robotics and Robertson says it is this enabling technology that allows his business to adapt micropile technology for offshore construction.
Although Subsea Micropiles is a relatively new business, the initiative goes back to 2008 – before the company was founded. Robertson, a former US Navy officer, previously worked with US aerospace and defence technology group Lockheed Martin on subsea robotic systems.
As part of Lockheed Martin’s ‘new ventures’ group, Robertson helped develop technology for the company’s Ocean Thermal Energy Conversion programme. Then in 2018, having left Lockheed Martin and launched Subsea Micropiles, Roberston acquired exclusive worldwide rights to that technology. “One of the challenges that [Lockheed Martin] faced was the potential cost and impact of conventional anchor solutions. So what we were facing then is very much what the offshore wind sector is facing now: how to avoid that cost and address environmental concerns,” he explains.
“We therefore have experience with subsea robotic drilling systems – which to date, have been mostly limited to geotechnical surveying, like drilling core samples. But they are, reassuringly, exactly the kind of tool we need to do micropiling offshore,” says Robertson.
The project still has some way to go before the team has refined its technology and actually built the kit needed to install any subsea micropiles. But Robertson says he expects to be able to demonstrate a prototype system some time next year.
This article was first published in the July 2021 issue of The Construction Index magazine. Sign up online.
Ireland, unlike say the east coast of Scotland, is not known for its offshore expertise. But Subsea Micropiles is fortunate to have been able to team up with specialists with the ideal capabilities.
“We were delighted to find, here in Ireland, a drilling technology company like Mincon,” says Robertson. Historically Mincon Group has been focused on mining and geothermal well drilling, “but they have an incredible engineering resource and technology that is relevant to what we want to do offshore”.
Furthermore, in addition to mechanical engineering input from University College Dublin, the team includes the University of Limerick, which has a respected Mobile & Marine Robotics Research Centre.
“It’s a very happy coincidence,” admits Robertson. “That’s no small challenge in Ireland where we don’t have a big marine industry.
This article was first published in the July 2021 issue of The Construction Index magazine. Sign up online.
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