As offshore wind turbines become larger, they require supporting structures such as those more usually associated with oil and gas facilities.
It is no surprise then that MENCK is increasingly being asked to apply its oil and gas expertise to projects such as Germany’s first offshore wind farm.
alpha ventus, Germany’s first offshore wind farm, is groundbreaking in many ways. The project will be a fully functioning set-up supplying power to 50,000 households. However, the developers and the German government are also seizing the opportunity to run a largescale field trial that will examine rival 5-MW turbine and foundation types in operation.
Deutsche Offshore-Testfeld und Infrastruktur GmbH (DOTI), a consortium of German utility giants EWE, E.ON and Vattenfall, is developing the wind farm, which is 45 km off the island of Borkum in the German North Sea in 35-m-deep water. The engineering, procurement, construction and installation contract for the 12 generating units has been split between two contractors, AREVA and NorWind. AREVA/PROKON Nord has installed six Multibrid M5000 turbines on tripod foundations, and NorWind has mounted six REpower M5 turbines on jacket foundations. MENCK installed the foundation piles for both companies in June–August 2009.
The offshore wind industry is still finding its feet. However, MENCK has deep-rooted experience of installing similar structures for the oil and gas sector. For this reason, both AREVA and NorWind have worked closely with MENCK from an early stage.
The combined weight of an alpha ventus tripod foundation, tower and nacelle is 1000 t, which makes these structures comparable with unmanned oil and gas facilities. In addition, offshore wind farms are being planned with more and larger turbines and in deeper waters. Whereas one or two foundations may be installed to support an oil and gas topside, the new breed of offshore wind farms will require hundreds of foundations and piles, which makes installation a long and continuous process. The scale of these projects increases the pressure to lower costs, enhance efficiency and extend installation windows by mitigating underwater construction noise.
“Planning is critical,” says Jeremy Tygielski, director of marketing, MENCK. “The supply chain must be fast and efficient, and installation vessels cannot waste time waiting for equipment. During our work on the alpha ventus project, we pre-installed the piles for the six jackets, which reduced the total installation time. We also tested our ‘little bubble-curtain’ noise mitigation concept while installing one of the tripod piles. Techniques like these may be critical for the success of future offshore wind-power schemes.”
The pre-installation of piles through a template has several cost and time advantages. It removes the need for substructure pile sleeves, which make the jackets lighter, and vertical installation of the piles helps to standardise the substructure design for mass production. It also means that one vessel can focus on piling while a second installs the substructures.
Martin Ros, MENCK senior sales manager, says, “We have pre-installed many piles for the oil and gas industry. This method was decided on for the alpha ventus jackets during consultation with NorWind. Our initial plan was to use a follower to install each pile in a single operation through the 7-m template sleeve so that 1.2 m of it protruded above the seabed. However, we discovered that the site experiences strong currents that prevent the use of followers.
“To solve this problem, we redesigned the MHU 500T hammer so that it has a shorter pile sleeve. This creates a smaller overlap with the pile head. Optimising the space tolerance between the pile and the pile sleeve ensured that the hammer retained its vertical position.
“We also modified the installation process. Each 35-m-long pile was driven as far as possible through the template, then the template was removed and the piles were driven to their final depth. The jacket was lowered onto the pile heads and grouted in place. Although the twostage piling sequence takes longer, it considerably reduces the cost of jacket fabrication by removing the need for pile sleeves.”
MENCK also installed the piles for the tripod foundations using its MHU 500T hammer. These piles were driven through the tripod sleeves. Ros adds, “One of the many lessons the we learned from this pioneering project is that the additional use of a vibratory hammer for the initial pile stepping of pre-installed piles for tripods is advantageous. It ensures pile verticality, removes the need for levelling equipment and reduces installation time. We now recommend this process for similar work.”
Bubble-curtain noise mitigation
Concern for marine wildlife has led to restrictions on the length of installation windows. However, the scale of offshore wind-power projects means that longer installation periods will be required to deliver projects on time.
MENCK is at the forefront of research into pile-driving noise mitigation. Ros was a co-author of the 2007 COWRIE (Collaborative Offshore Windfarm Research Into the Environment) report that assessed engineering solutions for mitigating underwater construction noise. The engineering solutions included in the study are patented MENCK designs.
Sound attenuation by creating a bubble curtain is a key mitigation measure. Ros says, “We used the ‘big bubble curtain’ concept during the August 2008 installation of the FINO3 offshore wind research platform, which is mounted on a monopole 80 km west of the island of Sylt in the German North Sea. The entire construction site was surrounded by a 60-m-diameter seabed air hose that released a curtain of bubbles. Underwater construction noise during the installation was measured at a standard distance of 750 m from the pile. The big bubble curtain was effective, as it reduced the noise by 10 dB, but its installation costs make it uneconomic.
“During the alpha ventus construction, we teamed up with the University of Hanover to test our ‘little bubble-curtain’ concept during the installation of one of the tripod foundations. Instead of surrounding the whole construction site, this system creates a wall of bubbles around a single pile.
“For reasons beyond our control, the full system of eight vertically stacked bubble rings was not tested. However, three of the eight rings were tested. The results were encouraging, with a 12-dB reduction in the down-current direction. We are looking forward to measuring the full potential of the system in the near future. We also have patented mitigation concepts such as inflatable sleeves and telescopic foamfilled tubes that we are keen to test.”
As MENCK finished its alpha ventus work, it was preparing for another groundbreaking project, a BARD Tripile installation. This will complete MENCK’s wind-farm piling portfolio, as the company will have driven piles for every offshore foundation type that requires piling. With over 40 years of offshore pile-driving experience, a large offshore windfarm portfolio and many more jobs being planned, MENCK is in a good position to support this growing industry.