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Development of screw anchors for floating Marine Renewable Energy system arrays incorporating anchor sharing

Periodic Reporting for period 1 - SAFS (Development of screw anchors for floating Marine Renewable Energy system arrays incorporating anchor sharing)

Reporting period: 2017-08-01 to 2019-07-31

The EU has set up ambitious objectives to increase the renewable energy production in the Union to reduce its global environmental impact and ensures it energy independence. Offshore wind turbines have many advantages as wind is stronger offshore than onshore and there is more space available. The EU is at the leading edge of offshore wind development and many offshore wind farms already exist along the coast of several countries such as the UK, Denmark, Belgium or Portugal.

Conventional offshore wind turbines are usually founded on long steel piles (named monopiles), whose diameter reaches up to 6-8m, which penetrates the soil over several tens of meters. However, the deployment of new wind farms faces several challenges. Firstly, the space available in easily accessible shallow waters is already occupied. Secondly the installation of these monopiles generates a great disturbance for marine mammals and mitigation techniques are very expensive.

The development of floating wind turbines may be the solution to these challenges. Floating wind farms can be deployed further away from the coasts, harvest stronger and more consistent winds. In addition, the existing chain of supply of the Oil and gas industry can be easily adapted to these more sustainable applications, maintaining job and EU leadership. However, floating wind turbines require suitable mooring and anchoring systems to stay in place during operation and storms.

The objective of the SAFS project is to develop a new generation of foundations (i.e. screw anchors) for floating offshore applications (wind turbines but also wave energy converters or tidal turbines), by increasing the reliability of their design method. Screw anchors are literally screwed into the soil, which generates a low level of noise and vibration. They are easily removed and have a great capacity. The behaviour of screw anchors while subjected to complex loading generated by wind and waves will be studied through experimental small-scale modelling and numerical modelling. A comprehensive and reliable design procedure will be developed based on all these results and made available for the industry.
The work has been mainly focused on the understanding of the uplift behaviour of screw anchors and their installation. The SAFS research project dedicated to floating offshore renewable energy devices has reinforced an existing UK-funded research project on bottom-fixed offshore wind obtained by the University of Dundee, increasing the impact of both projects. Different geometries of anchors were considered as the potential applications are not limited to offshore wind, but also to wave energy converters and tidal turbines.

Predicting the installation requirements (force and torque) and ensuring the screw anchor will be installed at a desired depth are essential for project design. The installation has been simulated numerically, through the discrete element method in collaboration with another member of the team. A theoretical model has been developed, which allows a relatively accurate prediction of the installation requirements depending on the soil density and geometrical properties of the anchor.

The main part of the work has been focused on the understanding and modelling of the vertical anchor behaviour, namely what is the vertical displacement associated with a given vertical load (stiffness) and what is the maximum vertical load (capacity) that can be sustained by the anchor before it moves out of the soil. This work has been based on experimental (centrifuge testing) and numerical modelling. When the anchor is pulled out, a wedge of soil is formed and moved upwards, along what is called a failure mechanism. Determining the (inclination, volume involved) of this soil wedge is the key to accurately predict the capacity of the anchor.

A numerical procedure has been developed to incorporate the installation effects on the prediction of uplift behaviour of screw anchors. The method has been developed in a commercial finite element software which is available for practitioners and designers. It is applicable to a wide variety of geometries and soil properties.

Several laboratory experiments have been undertaken to characterise a sand material representative of offshore conditions under monotonic or cyclic loading. A long-term cyclic modelling model has been calibrated during a scientific stay to simulate the behaviour of the anchor while subjected to hundred thousand of cycles.

A simplified comprehensive procedure incorporating the installation requirements, uplift capacity and structural constraints has been developed to assess easily the resistance properties of an anchor, based on the equipment available for its installation and its geometry. This procedure includes the state-of-the-art knowledge related to the installation method or uplift capacity. Some charts have been derived to allow a rapid optimisation of the anchor geometry for a given purpose and front-end engineering design.

Preliminary numerical simulations combining lateral and vertical loading have been undertaken, which are representative of catenary mooring lines. Results show that in most cases, the vertical and lateral behaviours are uncoupled. However, only a single geometry has been investigated and results still have to be validated against centrifuge tests. These results can already be used to enhance the vertical method developed previously for purely vertical loading.
All of these results have been/will be published in international journal and presented at conferences.
The SAFS project combined with the UK funded project have proven that the screw anchor technology can be used for offshore applications. The investigations have shown that design methods applied for the relatively small-scale anchors used onshore are not suitable for upscaled anchors offshore. New numerical procedures and design methods have been introduced, paving the way to further developments.

The societal impact of this research is expected in the medium to long term, as the development of floating energy devices is still at an early age. The use of the screw anchor technology is likely to decrease the cost of the floating device anchoring and subsequently the overall cost of offshore renewable energy. Several companies have expressed their interest for the findings of these projects and are expecting to undertake field tests, based on the different methodologies developed at the University of Dundee.

Some national and international collaborations have been initiated with other researchers to disseminate the results and adopt a transversal approach to the problem. The influence of the anchor on the modelling of the mooring lines and floating wind turbines has never been tested before. The results obtained during the SAFS project have been made available to simulate a realistic non-linear behaviour of the anchor in fluid-structure interaction simulations.

A special attention has been devoted to the dissemination of the results to a broader audience. A public engagement kit has been designed and funded by the SAFS project. It helps explaining how anchors in general and screw anchors in particular are installed and how they resist vertical uplifting. It has been showcased at the ‘Science is Wonderful’ event in Brussels, targeting the general public and emphasizing the investment of EU funding into MSCA fellowships.
Graphical summary of the numerical prediction method
Comparison of driven piles and screw anchor capacity for a case study
Different types of mooring arrangements for floating wind turbines
Comparison of the numerical and centrifuge uplift load-displacement relationships
Maximum capacity and optimised geometry of a screw anchor as a function of the maximum torque