With the demand for wind energy continuously increasing, the offshore wind market is quickly developing. For the companies installing the windfarms, the demand for bigger turbines and foundations, lower costs and noise mitigation has prompted new solutions and smarter technology.
Released in September 2019, a report from the Global Wind Energy Council concludes that on a global scale, an additional 330GW of wind energy capacity will be installed from 2019 to 2023. Predicted to make up approximately 18 per cent of total wind energy capacity by 2023, up from nine per cent in 2018, the growing global role of offshore wind is a major contributor to the forecasted growth.
The rising focus on the offshore wind market has produced noticeable developments to increase the yield and size of individual wind turbines. This, on the other hand, has also entailed new challenges in the installation of turbines.
Michael Schaap, research & development manager at IHC IQIP, says: "The future challenge will come in two ways. I think you will mainly see monopile foundations growing, and due to the growing size of the turbines, and the growing size of piles, there will be a need for different equipment; everything will have to be designed for movement as it will be installed from floating vessels.
"The other challenge is underwater noise; with piling it's a very big issue. Currently, there are only hard legislative noise limits in Germany, the Netherlands and Belgium, but, over the years, we expect to see uniform standards all over, and a lot of measures are needed to meet these limits."
Tom Bauer, foundations manager at InterMoor, has on the other hand, also seen a growing demand for floating wind projects. "Floating wind projects are becoming more and more common as offshore wind projects reach deeper waters and fixed foundations no longer are commercially viable," he says.
Noticeably, with the production and installation of foundations representing up to 20 per cent of the capital expenditure of offshore windfarms, overall price competitiveness can be considerably improved with designs with low installation and production costs.
Steady at sea
While the increase in turbine size has happened gradually over the last decade, the last couple of years have seen growth intensify with turbine sizes increasing from 6 to 10MW. On the construction side, this has signalled the need for not just modifications but essential changes in installation methods and equipment, too.
"With monopile foundations, one of the challenges with larger turbines is the need for installation vessels with larger deck space and greater crane capacity. This will make it difficult to use the purpose-built jack-up vessels currently used for most monopile installations. Instead, installations will have to be done from floating vessels with no stable work platform and additional measures required to position the equipment," explains Schaap.
Noticeably, turbines are still growing; in 2021, GE Renewable Energy is planning to launch its new 12MW offshore wind turbine (Haliade-X 12MW).
One of the innovations introduced to the market to deal with the anticipated shift to floating installation vessels is the Motion Compensated Pile Gripper concept developed by TWD and Barge Master. During hammering of the pile, the Motion Compensated Pile Gripper measures the pile's inclination and hydraulic cylinders actively compensate the disturbing vessel motions in heave, surge and sway direction.
Another challenge caused by the requirement for larger monopiles is the limitation on the use of impact hammers, which cannot drive piles of more than 12m diameter. A solution to this is the BLUE Piling Technology currently tested by IHC IQIP. Developed in partnership with Jasper Winkes, the developer of the BLUE Hammer, the technology can be used to drive piles using hydropower rather than the traditional steel ram. This eliminates the limitation on the size of monopiles and reduces fatigue in the pile by up to 60 per cent. Furthermore, the BLUE Piling Technology significantly reduces noise at the source making noise-mitigating measures such as bubble curtains and noise mitigation systems superfluous. The BLUE Piling Technology is under development and expected to be launched in 2022/23.
The BLUE Piling Technology is not the first noise mitigating technology explored by IHC IQIP. The most recent product introduced is Pulse, a dampening system with two hydraulic plungers positioned between hammer and sleeve. The advantages of Pulse include a reduction in the sound equivalent level of 3-6dB and a 9-12dB reduction on sound peak pressure level. Combined with IHC IQIP's noise mitigation system, the Integrated Monopile Installer (IMI), this eliminates the need for a costly bubble curtain.
The high cost of bubble curtains (around €2-3 million (US$2-3 million) when produced around a wind farm of approximately 80 turbines) is the main driver for the search for piling methods that eliminate noise at the source.
One innovation offering this possibility is Cape Holland's Vibro Lifting Tool -Upend (VLT-U), a vibro hammer integrating a certified lifting and upending function. Lifting and upending the pile to the installation position, the VLT-U can then silently drive it down, measuring and adjusting verticality while driving, without the need of an ILT or trunnions, a gripper system or seabed frame. With the vibro technology tested in German waters, the results of industry tests and certification for the vibro installation method are expected soon.
Silent alternatives for different soil conditions are also being explored in Germany. One of them is the Mono Bucket foundation, a foundation developed by Universal Foundation and set to be tested at Northland Power's Deutche Bucht Offshore Windfarm. The foundation consists of a 'suction bucket', a single steel cylinder on which a shaft is mounted. The suction bucket structure becomes firmly embedded in the seafloor by its own weight and vacuum pressure, eliminating the need for pile driving and consequently for noise mitigation measures. Deutche Bucht Offshore Windfarm will include two of the mono bucket foundations which will be equipped with V164 8.4MW wind turbines by MHI Vestas. It will be the first windfarm worldwide to test this new type of foundation structure under commercial operating conditions.
XL monopile alternatives
Some years ago, many in the offshore wind market were predicting a shift from monopiles to alternative foundations such as jacket piles, gravity foundations, anchor piles, suction piles and tripods. And while monopiles are still, by far, the most widely used foundation, alternatives offering less noise, deeper depths (monopiles are currently considered suitable at depths of up to 60m) and easier installation in difficult ground conditions are continuously explored.
On deeper depths, floating wind projects - the foundations for which can include driven piles (small pin piles driven by subsea hydraulic hammers), suction piles, vertically loaded anchors and Intermoor's suction embedded plate anchors - often offer the most economic solution.
In difficult soil conditions, like those seen in many UK projects, jackets are often used, but alternative solutions are also being tested. In France, a new type of piling for a variety of types of subsoil has been tested by Bauer Spezialtiefbau GmbH. The company was contracted to design, install and load test a total of 14 onshore test piles to obtain friction values for a planned offshore windfarm in the Bay of Saint-Brieuc. "This will be the first commercial application for this type of piles worldwide. Bauer has developed a special drilling system for such projects in order to install temporary cased drilled and grouted piles offshore in a variety of different types of subsoil," explains Paul Scheller, head of Underwater Foundations at Bauer Spezialtiefbau.
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