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

Project Information

SEAFLOWER

Grant agreement ID: 891826

DOI

10.3030/891826

Project closed

EC signature date 19 March 2020

Start date 15 March 2021

End date 14 March 2024

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 257 209,92

EU contribution

€ 257 209,92

Coordinated by

ALMA MATER STUDIORUM - UNIVERSITA DI BOLOGNA
Italy

Periodic Reporting for period 2 - SEAFLOWER (Strategies for the Exploitation of Anchors for FLoating Offshore Wind Energy Reaping)

Reporting period: 2023-03-15 to 2024-03-14

Summary of the context and overall objectives of the project

The European Union's future goals on clean energy include ambitious targets for increasing the share of renewable energy sources in the energy mix, fostering innovation in clean technologies. These goals are outlined in initiatives such as the European Green Deal and the European Climate Law, which aim to accelerate the transition to a carbon-neutral economy by 2050. In today's energy landscape, the EU stands as a global leader in offshore energy development and aims at continuously shaping the future of renewable energy worldwide.
In recent decades, Europe has seen the development of offshore wind farms supported by monopiles, steel piles suitable only for shallow waters. However, the advancement of floating technology offers a solution for harnessing wind resources in deeper waters, expanding the potential of offshore wind energy beyond traditional bottom-fixed solutions. Floating wind farms hold particular promise for regions like the Mediterranean with narrow continental shelves, where wind resources are abundant but inaccessible to conventional installations.
SEAFLOWER contributes to the discussion on Europe's future energy supply by focusing on the geotechnical aspects of floating offshore wind turbines. Anchoring systems, which secure floating structures to the seabed, constitute a significant portion of the capital expenditure for large-scale wind farms, up to 30%. Optimizing anchoring solutions could lead to substantial cost savings. While various anchoring methods have been tested successfully to moor pre-commercial wind turbines, not all are suitable for real wind farms spanning wide areas with multiple turbines.
The objective of SEAFLOWER is to analyse the behaviour of different anchor foundations through a numerical investigation. The procedure make use of metamodelling techniques to emulate the response of finite element models able to represent the anchors’ performances. A metamodel is a surrogate numerical model that is able to encode the response of a complex and computational expensive FE model. Once built, the metamodel can be easily exploited for the design. The procedure developed by SEAFLOWER particularly suits cost analyses and pilot design activities. This would contribute towards overcoming the actual technical barriers to the deployment of offshore wind in deep waters.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The research primarily focused on applying metamodelling techniques to two different soil-anchor systems. Initially, the study investigated a pile anchor driven into a homogeneous sand deposit and subjected to monotonic uplift load. Subsequently, it examined a plate anchor installed in a spatially variable clay domain.
Driven pile anchoring solutions are pertinent to Tension Leg Platform (TLP) floating wind turbines, where the platform is tethered to the seabed by vertical tendons. The study analysed the pile’s ultimate capacity under pure pull-out load using a finite element solution that incorporated the installation effects simplistically. As for the metamodelling study, the Polynomial Chaos Expansion (PCE) method was employed to summarise the load-displacement response of the axially-loaded pile. The developed PCE showed able to replicate with good approximation the observed behaviour of some experimental evidences from literature.
For plate anchors, known for their efficiency and ease of installation, deterministic solutions assume homogeneous soil properties, overlooking variations across the deposit. This is significant for floating wind turbine projects covering large spatial extents. The study employed the Polynomial Chaos Expansion (PCE) to assess a plate anchor's holding capacity in randomly variable soil under general loading conditions.
The first investigation revealed uncertainties in quantifying the installation's impact on pile performance, prompting small-scale experiments at the Centre for Offshore Foundation Systems (COFS). Tests examined installation methods' influence on uplift capacity, especially under cyclic loading. Some tests also explored noise reduction during pile driving by applying suction to decrease effective stress at the skirt tip level. Preliminary results suggest this approach merits further investigation.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

SEAFLOWER has proven that metamodelling techniques can be used as time-cost effective solutions in the offshore context. The method was applied to two different case studies that analysed both different soil and anchor types. In particular, 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) is required to get a comprehensive description of the anchor’s performance, with a high computational cost. 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 generalised with the introduction of further input variables in the problem and/or for comparison purposes.
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.
The interest on using these artificial intelligence tools in the offshore geotechnical industry was highlighted by the recent OSIG international conference organised by The Society for Underwater Technology at the Imperial College in London, where a keynote lecture was specifically dedicated to the topic.
After SEAFLOWER, some national and international collaborations were established with other researchers, with the scope of applying the developed methods on other case studies. The first results of the project were presented to a broader audience at the Researcher’s night held in Bologna (Italy) in 2022, while the final outcomes will be proposed at the upcoming event of September 2024.

Impact of suction(flow) assisted pile driving on number of blows.

Plate anchor FE geometry and plot of a single realisation of the spatially variable domain.

Driven pile FE geometry (a) and PCE metamodel predictions Vs FE observations (b and c).

PCE metamodel predictions Vs FE observations of plate anchor's performance.