Danish firm Universal Foundation has begun to make waves in the wind-turbine industry with its novel foundation concept – a deep embedded gravity base, designed to lower the costs and improve the efficiencies of offshore installations. Now, thanks to years of development and live sea trials and with further demonstration projects planned, it appears that commercialisation of the technology is just on the horizon.
The Mono Bucket (MB) foundation is tailored for use in waters 25m-55m deep and combines a monopole and a suction bucket. The all-in-one steel structure consists of a ‘multishell’ foundation, with vertical stiffeners, a robust lid and a shaft for interfacing with a wind turbine, met mast or other topsides structure.
A prototype has been supporting a Vestas 3MW turbine in Denmark since 2002, and a second has supported a meteorological mast at the Horns Rev 2 windfarm off Denmark since 2009. Further trials in UK waters at Dogger Bank have helped to provide Universal Foundation with reams of data to refine its original design. The firm is confident that the design can work well in rough seas and be installed in a wide variety of site conditions, including sand, silt, clay and layered strata. This will be put to the test when the firm provides foundations for a demonstration offshore wind project planned for the Ohio waters of Lake Erie.
Technology transfer
Søren Andreas Nielsen, director of technology at Universal Foundation, explains that the concept is inspired by the oil and gas sector, which has deployed more than 2,000 suction cup substructures over the past 30 years. “In 2001 we had a team looking at wind turbines in northern Denmark and they were in contact with a Norwegian company working with suction anchors for the oil and gas industry,” he says. “That was when we started wondering if the same technology could be used for wind turbines.”
The team at Universal Foundation then began working with universities to develop guidelines to help design the MB foundation, looking at the structure interactions, how it would handle loads coming from the turbine and waves, and developing a methodology to determine the size of the foundation that is to be embedded in the seabed. For example, for a 6-8MW turbine, an MB would weigh 540 tonnes, with a 17m diameter and a skirt height (the scalloped rim of the foundation that embeds in the seabed) of 5-12m, depending on soil type.
Nielsen says that the foundation is lighter than a monopile by 20%, making it easier for handling. “Our plate thickness is up to 60mm, so it is not very heavy construction sites that are needed. It gives us more flexibility on the installation side where we have access to a broader range of vessels for future projects in deep waters. We are able to self-float the foundation horizontally using a small vessel and then up-end it in the sea. That is why we call them Universal Foundation as there is a lot of flexibility we can play on.”
Without the need for heavy machinery or divers, the MB installation method saves valuable time and money. The first-series MB is designed to be lowered by crane to the seabed and is suctioned into place using built-in water jets and a three-pump system before being fitted with a turbine. This process, explains Nielsen, lowers the pressure in the cavity between the foundation and the seabed and generates water flow, which lowers resistance around the edge of the foundation’s skirt to allow seabed penetration.
Speedy process: In 2013 two meteorological masts with Mono Bucket foundations were installed on the Dogger Bank in just seven hours
Trial results promising
As the jet and suction system is used as the driving force, the MB requires no pile driving or dredging and so eliminates any major noise and soil disturbance that is associated with offshore installation of traditional monopile structures.
Nielsen adds that a patented system controlling the vertical alignment ensures a foundation well within the prescribed installation tolerances, removing the need for a transition piece to adjust verticality.
To prove the technology, the MB has been put through various demonstration projects, including a scheme to install met masts in the North Sea, run by the Carbon Trust’s Offshore Wind Accelerator (OWA) programme. Marc Costa-Ros, offshore wind senior manager at the Carbon Trust, explains that the main aim was to de-risk the MB concept against rough environmental conditions.
Nielsen says that the control system also lived up to its name during installation at Dogger Bank, and it works “extremely efficiently when installed within 0.1° accuracy”. The 93m lattice tower was manoeuvred into place using a “human free” technique employing guide cones, enabling the crew to stay on deck and avoid lifting hazards. Costa-Ros says: “The installation of the two met masts at Dogger Bank took only seven hours from vessel deck to final position – significantly faster than monopile-type installations.”
Sensor readings taken from one of the Dogger Bank units have revealed that minimal movement of the structure has occurred, of 3-5mm, when hit by 20m waves. Costa-Ros feels that these early results are promising.
During last summer, the OWA was also involved in a project to test the feasibility of suction bucket installation in the North Sea. The work used two types of suction buckets, one of them being a scaled-down Mono Bucket-type system. An impressive 29 successful installations were achieved in 24 days at 12 different sites. Costa-Ros says: “This proved that suction buckets can be installed in a wide variety of soil conditions. According to Universal Foundation, suction buckets can be installed in 80% of the North Sea.”
Further to such trials, Universal Foundation has been able to refine its guidelines for penetration prediction previously based on data from the oil industry. The new calculating process, explains Nielsen, has enabled the firm to be able to penetrate the soil deeper than the old guidelines predicted. This has been achieved by reducing the diameter of the structure, and increasing skirt length, which can “significantly” reduce costs.
It was recently announced that MB foundations will be used by the Lake Erie Energy Development Corporation for its Icebreaker offshore wind project planned for the Ohio waters of Lake Erie. This decision followed a six-month review of the technology by UK-based Offshore Design Engineering (ODE).
The project will consist of six wind turbines in the 3-4MW class located seven miles north-west of Cleveland. It is the first freshwater project in North America and will come up against significant foundation icing conditions.
Simon Spolton, regional director for ODE’s London office, was involved in the firm’s evaluation of two innovative foundation designs: a monopile foundation with a supporting friction wheel structure around it at mud line, and the MB. While he explains that both were deemed to address the two main challenges of the site – soil conditions and the presence of ice – it was the significant difference in installation costs and the potential for reduced fabrication costs that made the MB the most attractive option. He says: “Reducing the capital and installation costs could really make an impact on the viability of projects.”
Spolton agrees that the MB may not have the history of mature installations but says the OWA programme has done much to de-risk the concept. “The natural progression now is to install full-scale MB foundations at a test site. The Icebreaker project is ideal,” he adds.
Nielsen says: “We should be fully commercial by late 2016 or early 2017.”