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Strategies for the Exploitation of Anchors for FLoating Offshore Wind Energy Reaping

Periodic Reporting for period 1 - SEAFLOWER (Strategies for the Exploitation of Anchors for FLoating Offshore Wind Energy Reaping)

Reporting period: 2021-03-15 to 2023-03-14

SEAFLOWER is being conducted to contribute overcoming some of the barriers that have so far prevented the exploitation of wind resources with floating support structures, providing a key support to further add to the European supply of clean and renewable energy in the upcoming years.
The topic is being addressed from a geotechnical standpoint, focusing on the identification of anchor foundation solutions. An offshore wind farm project lasts, on average, ten years from site selection to the commissioning phase. The process develops along the following stages: (a) permitting and site selection; (b) preliminary site characterisation; (c) design concept and final investment decision (d) detailed site investigation and design; (e) certification; (f) manufacturing; (g) transport and installation.

SEAFLOWER is developed to support the current and upcoming needs of floating offshore wind developments, and research outputs are thereby mostly related to the initial wind farm project stages, connected to the areas of preliminary design.
SEAFLOWER results are therefore meant for offshore wind farm designers which are involved in the technical feasibility studies which underpin the successive detailed plan of site-investigations necessary to the successive phases of detailed design (phases (a) to (c)).
These project results targets researchers in the field of offshore geomechanics and offshore renewables, as well as practitioners (offshore geotechnical engineering consultancy, offshore contractors, energy industries, etc.), who are encouraged to embrace non-deterministic design approaches to duly account for uncertainties that underpin offshore geotechnical design in view of cost-performance optimisation and sustainability. The communication of the project aims and results targets a wider audience, which includes undergraduate and PhD students and future civil engineers, suggesting practical ways to decisively contribute to the current energy debate, developing new solutions with a clear understanding of the central role of rigorous geotechnical modelling in the development of such solutions.

The objective is to define a numerical procedure to emulate FE models representative of anchoring systems for floating wind developments through the use of metamodels. A metamodel is a surrogate model that is able to emulate the results of a set of FE analyses in a computationally efficient mathematical algorithm. Once built, the metamodel can be easily coupled with traditional probabilistic approaches and allows to account for the uncertainties intrinsic to any offshore foundation problems and any technology transfers.
SEAFLOWER started on March 2021 with a four months Secondment phase carried out at INRAE, followed by the outgoing phase at COFS in Western Australia. Due to the Australian international travel restrictions, the outgoing phase started remotely from UNIBO for the first nine months.
The study of the metamodelling techniques was focused since the project’s start carried out at INRAE. At this stage, the objective was to test the approach for a simplified case study as proof of concept of the project. The case of a driven pile anchor installed in homogeneous sand deposit and subjected to monotonic uplift load was investigated, which is relevant to the application of Tension Leg Platform (TLP) for FOWT. The numerical work was then developed during the remote outgoing phase.
The results highlighted the feasibility of the procedure and dissemination activities were undertaken. The outcomes were presented at two international conferences: the 5th International Symposium on Cone Penetration Testing (CPT’22) and the Indian Symposium for Offshore Geotechnics (ISOG 2022). The study being further improved found then publication in a scientific journal. Communication to industry and policy makers was addressed by participation at the Floating Offshore Wind Turbines (FOWT) event held in Saint-Malo (France) from 16 to 18 November 2021. SEAFLOWER also participated to the European Researchers Night organised in Bologna on September 2021.

The outgoing phase at COFS started on April 2022 where a more sophisticated case study was addressed, which implemented the spatial variability of the soil properties in the numerical model. The case of a plate anchor installed in a spatially variable fine-grained domain and subjected to combined loading was considered. Soil domains are typically modelled as homogeneous, rather than layered materials, but the reality is that soil properties are varying over the deposit. This is of further interest for floating wind turbines developments that have large spatial extent with respect to oil and gas structures. Even a complete soil investigation campaign cannot provide a detailed report of the deposit over such an area of intervention and the designer has to face with uncertainties, which can be quantified with the use of metamodels. A numerical strategy was developed to model the new case study and to conduct a FE parametric test. Then, the simulations results were used to build the metamodels.
This part of work will be presented at the 9th International Offshore Site Investigation and Geotechnics Conference (OSIG 2023), that will take place at the Imperial College in London from 12 to 14 September 2023. During the outgoing period, the results of SEAFLOWER were disseminated within the academic community with three seminar organised at different institutes (The University of Western Australia; The University of Melbourne; The Univeristy of Newcastle).
The first case study has shown the reliability of the approach that was tested for a simplified case study. The use of surrogate modelling is new in the offshore geotechnical research field and the results have received the attention of the community with participation to international conferences and seminar invitations, rather than the first journal publication.

Addressing the application of the metamodelling technique for analysing the challenging case of spatially variable domains represented a further development of the study. The soil randomness can be analysed with common FE models but a high number of realisations (FE simulations) would be required, with a high computational cost. The results can then be analysed in a probabilistic framework with a study that would be limited to the selected case study. The metamodel can overcome these limits. It is built on a limited number of FE simulations and -once developed- it can be used to provide thousands realisations in a negligible time.

Potentially the procedure can be extended to emulate the response of other FE models that implement other modelling features or investigate different model conditions.
As floating wind developments are in the pre-commercial phase, simplified tool –like the metamodel- would aid the design stage, particularly during the preliminary design state of advancements where the designer has to face with limited details for using more sophisticated numerical models.