Technology
Universal Foundation’s Mono Bucket is the latest generation foundation structure. The Mono Bucket is specifically designed for offshore wind turbine application, combining the key benefits of a gravity base foundation, a monopile and a suction bucket, and building on a legacy of more than 2,000 suction technology-based foundations from the oil and gas industry.
The Mono Bucket has proven its ability, having been successfully installed in a wide variety of site conditions, including sand, silt, clay and layered strata.
The Mono Bucket foundation is installed using a patented installation control system as the driving force. Lowering the pressure in the cavity between the foundation and the seabed generates water flow, which lowers resistance around the edge of the foundation’s skirt. This reduces resistance and allows seabed penetration.
The patented installation control system controls vertical alignment ensuring the foundation is installed well within prescribed tolerances, removing the need for a transition piece to adjust verticality.
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The all-in-one steel structure consists of a multi-shell foundation with vertical stiffeners, a robust lid and a shaft for interfacing with a wind turbine, met. mast or other topside structure.
It does not require divers, in most cases does not require seabed preparation and there is also little or no need for scour protection. Installation is completely noise free and offers no risk to marine mammals, offshore installation crews or marine operators.
Looking ahead to decommissioning, the foundation can be removed for reuse or recycling by reversing the suction process. This involves applying water pressure into the foundation skirt so that it can be smoothly lifted from the seabed.
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Key Features
The Universal Foundation Mono Bucket is characterised by a number of key features that contribute to reducing the cost of energy.
Design verified up to 55m deep water sites with 8MW turbine
This result was verified by independent third parties under the Large Rotor study led by the Offshore Wind Accelerator (OWA) programme.
Suitable for a wide range of soil profiles
Installation has been completed across a range of challenging soil conditions including soft clay, moraine clay and boulder bank clay – with sand spikes and layers, clay crust, sand and silt. In 2014 Universal Foundation joined forces with Statoil, Dong Energy, E.ON and Statkraft in a Trial Installation campaign. The result was 29 successful installations in 24 days at Dudgeon, Dogger Bank and Hornsea. This campaign verified and furthered findings from previous installations which remain in situ in the North Sea.
Modular design, utilising existing supply chain
Manufacturing the Mono Bucket uses the established monopile and component supply chains. This enables access to the very competitive supply of tubular components and enables assembly in high volume at local staging sites using local competencies.
25% less steel than comparable monopile
Less steel translates into lower cost. The reduced weight is a result of eliminating the transition piece and the ability to use steel plate down to just 25mm thick.
No grouted connections – integrated transition piece
The topside is fixed directly to the foundation with no requirement for a separate transition piece. This simple design approach translates into fewer offshore operations during installation, with fewer critical connections prone to failure.
No seabed preparations
Installation makes use of an integrated seabed leveling system which, by circulating water inside the Mono Bucket, creates a water-vortex to level out any slope in the soil top layer thus enabling full contact with the seabed.
Integral scour prevention
A scour prevention system is inherent in the design, which monitored results have proven very effective for foundations installed at Horns Rev 2 and Dogger Bank.
Single-lift or self-floating installation
Flexibility in the installation concept allows a wide choice of installation vessels. The single-lift was demonstrated using the Fred. Olsen Windcarrier’s Brave Tern vessel at Dogger Bank and a floating installation was deployed for the DONG Energy Horns Rev 2 project.
Quick and cost-effective installation
Installation of the Mono Bucket depends on one key installation tool, a reusable click on suction pump unit. Installation cycles of only 7 hours were achieved at Dogger Bank. The reduced equipment spread and short installation cycles make installation highly cost-effective.
Noise-free installation
Noise-free installation results in minimal impact on surrounding marine life. It does not require expensive noise mitigating equipment and reducing exposure to project delay, offering further significant cost benefits.
Self-levelling
Thanks to the patented installation control system, virtual alignment is fully controlled during installation ensuring the foundation is well within specified tolerances. All installations have been achieved within 0.1 degree of perfect vertical. This negates the need for a separate transition piece with problematic grouting – reducing steel, offshore operations and exposure to delays, again resulting in significant cost reductions.
Completely removable
By reversing the installation process the foundation can be 100% removed, reducing conventional decommissioning costs in the magnitude of 75% and restoring the seabed to its original state.
Endorsements
“Since the selection of Universal Foundation as one of four finalists’ innovative foundation concepts under the Foundation Innovation Competition organised in 2011 by the Carbon Trust partnering with nine offshore wind developers under the Offshore Wind Accelerator Program (“OWA”, more information here), the Mono Bucket has been subject to continuous testing and evaluation. The main winning criteria for the selection of Universal Foundation was the ability to reduce cost of offshore wind energy significantly, and hence support the industry requirement of reducing the Levelised Cost of Energy (LCoE).The direct result of Universal Foundation’s continued commitment to the OWA industry programme has been a number of industry studies, where the design methods and technology of Universal Foundation has been subject to independent third party engineering consultancies and utility evaluations, and the deployment of two major offshore campaigns; two met. masts deployed at the UK Round 3 site of Dogger Bank in 2013 and latest the Trial Installation Campaign with 29 installation at the three major UK offshore sites: Dudgeon, Dogger Bank and Hornsea. The projects have been designed with the direct intention to de-risk the Mono Bucket technology and advance the technology’s readiness level towards full scale commercial demonstration. By combining the Mono Bucket with a multi-MW wind turbine in an offshore working environment, the cost reduction potential is substantial and we foresee it to be one of the key paths towards achieving the industry cost out on LCoE.”
Marc Costa Ros
The Carbon Trust
