Determining susceptibility to scour and using physical modelling to develop solutions is essential to protect turbines and other structures from excessive vibration, that could lead to damage and potential loss of revenue
As offshore windfarms are built farther from shore, in deeper water, so the wind and wave load on turbines has grown. Monopiles, which remain the foundation of choice for many if not all offshore windfarms, need to be installed and remain in place with very tight tolerances if the turbines they support are not to be adversely affected by these loads.
However, turbines, foundations, wind and wave load and the nature of the seabed in which foundations are installed differ widely, making it essential that scour protection solutions are developed and tested on a case-by-case basis.
Without adequate protection in place, scour around foundations can adversely affect the overall ‘stiffness’ of a foundation and turbine, potentially leading to excessive vibration and failure, so developers are increasingly turning to research facilities in Europe that can model exactly the conditions that will prevail offshore.
They need to do so because although all turbine structures vibrate to some degree during operation, with the frequency of resonant vibration determined by the foundation, tower, nacelle and blade dimensions and interactions of the foundation with the soil, the challenges associated with far-offshore windfarms are growing all the time.
The design process for a foundation-tower-turbine system addresses this behaviour to determine the depth to which a foundation must be installed, but scour can cause the exposed length of the monopile at the seabed to change and can alter the frequency at which the system resonates. If, as a result, the turbine structure begins to vibrate excessively, it may need to be shut down, loss of revenue will occur, and potentially significant remediation costs could be incurred. If a turbine affected in this way happens to be 60-70 km or more offshore, repairs to components and installation of scour protection can be very expensive.
The University of Oxford's Professor Byron Byrne, from the department of engineering science and HR Wallingford's chief technical director, sediment dynamics, Professor Richard Whitehouse, told the 9th International Conference on Physical Modelling in Geotechnics that research exploring the interaction between scour, foundation stiffness, and structural dynamic behaviour is “critical.”
Together they have undertaken experimental work on monopile foundation and tower-nacelle structures in HR Wallingford’s Fast Flow Facility. The flume tanks allow for realistic scour protection solutions to be developed, providing a means to explore the effectiveness of different remediation strategies for the sometimes challenging conditions found in the North Sea and elsewhere.
Leading windfarm developers such as Ørsted have used the facility to improve their knowledge of scour and scour processes that occur around foundations and to increase confidence in the performance of the scour protection.
A research project funded by Eon also provided insight into the effects of scour around the foundations of offshore wind turbines and provided insight on mitigation strategies. HR Wallingford’s work with Oxford University’s department of engineering science and Eon examined different forms of scour protection to assure the stability of monopiles and identified scour remediation strategies that would maintain foundation performance. The project, Foundation response to scour protection, saw a number of physical experiments undertaken at HR Wallingford.
“We scaled monopile and turbine structures and exposed them to scour and erosion processes,” Professor Byrne explained. “We measured the structural dynamic responses, quantifying the amplification of the vibration as it changed with erosion at the base of the monopile and for different scour protection systems.”
Eon Climate and Renewables offshore foundation engineer Phillippa Cassie said, “By conducting this research, we are aiming to quantify the contribution of scour and scour protection measures in structural terms to inform proactive management of the operational fleet and designs on future projects.”
However, it is not just monopile foundations for turbines that need to be protected from scour, as research carried out by the University of Rostock and HR Wallingford demonstrated. Together the institutions have assessed the development of scour around jacket-type foundations as part of a research project funded by the German Government.
As they noted, the integrity of a substation jacket and its cables is vital to the operation of an offshore windfarm. Failure or damage to the substation could mean lost transmission for the entire windfarm, with repair costs and loss of revenue running into tens of millions of euros, but physical model tests allow for the effects of scour to be accurately assessed under realistic conditions in the laboratory, which can be used to inform and optimise jacket design.
Dr Peter Menzel from the sediment transport research group in the department of ocean engineering at the University of Rostock, said “For scour tests we provided two models of steel jacket foundations at a scale of 1:60. The data gathered in HR Wallingford’s facilities deepened our understanding of the effects of scour on substation jackets in the German sector of the North Sea, where substantial offshore wind development is both ongoing and being planned.”
HR Wallingford is acknowledged as a leader in the field of scour protection and physical modelling, but work is also ongoing at other research institutions, including the Environmental Hydraulics Institute of Cantabria (IHCantabria) where, in collaboration with Iberdrola Renovables and ScottishPower, work is under way to evaluate potential scour protection solutions for jacket-type foundations on the East Anglia One offshore windfarm in the UK.
In these physical tests, the characteristics of the location at which the jackets will be installed is being simulated at a scale of 1:30. The experiments are being carried out in the Cantabria Coastal and Ocean Basin (CCOB) at IHCantabria, a facility that has a multi-directional wave generator, portable wind generator and a current generator that allow for any floating or fixed offshore foundation to be tested.
To maximise the efficiency of the physical experiments, two jacket foundations with different scour protection systems were tested simultaneously in the CCOB. The protection for the first jacket foundation was a ‘frond mat’ system developed by UK-based Seabed Scour Control Systems; the second was a system consisting of conventional rock protection. The aim of the test was to compare the performance of the scour protection systems and collect data for the definition of monitoring and maintenance requirements.
During the physical experiments, the interaction among the jacket foundation, the scour protection and the seabed were evaluated under several combinations of wave and current conditions. Because of the presence of sand waves in the area in which the windfarm will be built, physical experiments were also carried out reproducing their presence.
New approaches to scour protection are among the new technology that will be tested on the Borssele V offshore wind innovation site in the Netherlands. The Two Towers consortium, consisting of Van Oord, Investri Offshore and Green Giraffe, recently won the tender for Borssele V windfarm. MHI Vestas Offshore Wind is the preferred supplier for the wind turbines.
Borssele V, designated as an innovation site, is situated in site III of the Borssele windfarm zone. The offshore windfarm is located more than 20 km off the coast of Zeeland, the Netherlands and will consist of two turbines, each of 9.5 MW. The project will see a new type of scour protection used on the seabed, including a new concept that will see part of the scour protection system for the project forming an artificial oyster reef, which will promote species diversity.