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The ecological performance of geosynthetics – opportunities and challenges in offshore applications

Introduction

The construction industry is of great global importance in achieving sustainable development goals. It accounts for around 50 per cent of the resources used globally, 30-40 per cent of energy demand, 20-40 per cent of greenhouse gas emissions and around 17 per cent of fresh water consumption. On the other hand, all players in the construction industry value chain contribute to generating up to 10 per cent of gross domestic product and 7 per cent of all employees can be attributed to this sector.

It is obvious that our current methods of production and consumption do not meet the requirements for sustainable development as defined, among other things, in the 17 Sustainable Development Goals of the United Nations (see Figure 1).

Fig. 1: The 17 Sustainable Development Goals of the United Nations

This is particularly true when we consider the continuing upward trend in population growth, the need for economic development in developing countries – which are still poor or poorer today – and the needs of future generations.

Even though this insight is only slowly gaining ground and the legal, economic and socio-cultural conditions are challenging, numerous decisions have been made at all political levels to counteract a "business as usual" approach. In recent decades, there has also been a shift from so-called remedial environmental protection, such as the use of filters for flue gas desulphurisation, to sustainable environmental protection, which addresses the root causes and, among other things, focuses more on prevention strategies. The revision of the Construction Products Regulation is a good example of this development in this context, as the current version also requires energy consumption, environmental impacts and the use of natural resources to be considered throughout the entire life cycle.

In recent years, there has also been increased work on instruments that are intended to enable the various stakeholders to make better-informed, more sustainable decisions and to enable supervisory bodies to perform their control tasks. The further development of the so-called life cycle analysis according to DIN/ISO 14040/14044 should also be seen in this context. This is used in some cases as a supplementary control instrument in the economy, but is also increasingly being used by the public sector, for example in public procurement procedures and procurement. In order to make life cycle assessments comparable, modules covering the entire life cycle were defined in the EN 15804 and EN 15978 "Sustainability of Construction Works" standards. In line with this development, NAUE, among others, has decided to use "life cycle assessment" as part of its product development and to evaluate the performance of geosynthetic materials on the basis of specific fields of application.

performance of geosynthetic materials. In doing so, it was able to draw on experience gained in 2013 as part of a detailed study on the life cycle assessment of geosynthetic materials in four fields of application commissioned by the European Association of Geosynthetic Product Manufacturers (EAGM).

As wind energy is considered a key pillar in the transition of the energy sector from fossil fuels to renewable energies, and as offshore energy generation is gaining in importance alongside land-based energy generation with wind turbines, a study was commissioned to assess the environmental impact of two scour protection variants from NAUE. The approach and the results achieved are briefly outlined below. A more detailed scientific publication in English is also available (Hoyme et al. 2019).

The life cycle assessment of scour protection methods in the field of offshore wind turbines

Overview of the study

Offshore wind farms have grown significantly over the past decade, from around 1 GW of installed capacity in 2006 to 11 GW in 2015 (EWEA 2015). With the ratification of the Paris Climate Agreement, the role of offshore wind farms in the European electricity grid will continue to grow. Among the various challenges facing offshore wind farms, ensuring scour protection poses a technical risk. Due to currents on the seabed, the sand surrounding the wind turbine tower (monopile) must be secured against washout (scour protection). Without scour protection, the natural frequency could change over time with the altered embedment length of the monopile, resulting in the wind turbine having to be shut down because the frequency is outside the operating limit. To counter this phenomenon, various protection methods have been developed, including a process using geosynthetic material.

This article examines scour protection options for an offshore wind farm. It evaluates a solution using geosynthetic materials compared to the conventional solution using rocks.

Functional unit

The function of scour protection for an offshore wind farm is to prevent scouring around the tower of a wind turbine. Therefore, the functional unit was defined as a solution to protect the tower with a diameter of 6 m from scouring.

System boundaries

The life cycle assessment carried out in this study follows a cradle-to-grave approach, which corresponds to modules A to C in

EN 15804. The extraction of raw materials, their processing into building materials, and the

installation, operation and end-of-life phases. It was assumed that no further expenses would have to be taken into account for the operation and end-of-life phases, except for land use. The service life of the wind turbine with scour protection was set at 25 years. Transport processes and infrastructure are taken into account accordingly. Manufacturing and installation processes represent site-specific conditions, while average European conditions were used for all other processes.

Cut-off criterion

The study covers all relevant inputs. Equipment for the manufacture of geosynthetics is excluded due to its minor significance.

Impact categories considered

The environmental performance of this study is assessed using the following impact indicators:

• Cumulative energy demand (primary energy consumption, broken down into non-renewable and renewable

components),

• Abiotic degradation potential,
• Greenhouse gas emissions (GWP100),
• USEtox,
• Photochemical ozone creation,• Acidification and
• eutrophication.

Limitations of the study

The life cycle assessments of scour protection covered in the study represent four existing cases of offshore wind farms: one with a sand-filled geosynthetic container and three rock-based solutions. This limits the generalisability of the study results, as construction methods may vary depending on the region. Nevertheless, the cases can be considered the best representative variants, according to the statements of the experienced industry partners consulted for this study (NAUE and Sellhorn Ingenieurgesellschaft).

Due to the limitations of life cycle assessment based on the available impact indicators, the results of the life cycle assessment do not provide a complete answer to the question of whether geosynthetic-based constructions are generally the more environmentally friendly option. Site-specific information, which can be obtained from environmental impact assessments or other types of environmental assessments, for example, is generally not included in life cycle assessment studies. Additional information and the application of different methods may be necessary in order to fully conclude the environmental compatibility of certain types of construction solutions for scour protection. One such study to supplement the life cycle assessment results may be the environmental impact assessment by (E.ON Climate and Renewables 2012).

Results

Figure 2 shows the environmental impact results for the entire life cycle of the scour protection variants. The results are normalised to 100% based on the environmental impact of the scour protection solution using rocks from the Amrumbank West. The results show clear environmental competitive advantages of GSC scour protection for all impact categories examined. The difference between the three cases examined for the stone wash protection solution ranges from -35% to +32%, depending on the environmental impact category.

Fig. 2: Environmental impacts of the four scour protection variants examined (normalised to 100% of the solution with rocks from the Amrumbank West)

Sensitivity analysis

Two aspects were examined in more detail in a sensitivity analysis. The first aspect concerns the end-of-life (EoL) treatment of the GSC scour protection solution. The second concerns the "allocation rule" of life cycle assessment using the recycled content approach available in Ecoinvent v3 (Weidema et al. 2013). A detailed explanation is not possible here due to space constraints, but reference is made to the publication by Hoyme, Su, Kono & Wallbaum 2019.

The results in Figure 3 illustrate the marginal influence of the end-of-life (EoL) consideration of the GSC solution in terms of competitive advantage over conventional solutions. The differences between the GSC reference assumptions and the three sensitivity analyses ranged from -2.6% to 26% of the GSC reference cases (-0.5% to 3.8% using the Amrumbank West rock solution as a reference). In addition, the influence of the allocation method was more pronounced in conventional scour protection than in the GSC solution. The results of the sensitivity analysis confirmed the clear advantage of the GSC solution within the sensitivity range considered.

Fig. 3: Results of the sensitivity analysis taking into account different end-of-life assumptions and allocation methods

Discussion and classification of the results

The investigation of the environmental impacts of the two scour protection variants, the use of GSC containers and conventional rock, in offshore wind farms led to a clear advantage of the GSC solution for the impact categories considered. The ecological advantages of the GSC solution were evident in a comparison of three different offshore wind farm locations, each of which used different quantities of rock. A sensitivity analysis was carried out to include end-of-life waste treatment for the GSC solution and the application of various LCA allocation methods. This analysis also did not lead to any different conclusions.

A detailed investigation of the environmental impacts of GSC scour protection revealed that the extraction and filling of the containers with sand are responsible for a large part of the environmental impacts. One way to further improve the environmental performance of GSC containers is to reduce their weight while maintaining the same technical performance.

Despite the necessary simplifications and assumptions, the results of this analysis can be considered significant and reliable. It should be noted, however, that life cycle assessment is not a method for mapping all environmental impacts. Other risk analyses, e.g. biodiversity and environmental impact assessments, are therefore indispensable.

References

E.ON Climate and Renewables (2012): Rampion Offshore Wind Farm Environmental Statement.

Frischknecht, R., S. Büsser-Knöpfel, R. Itten, M. Stucki and H. Wallbaum, "Comparative Life Cycle Assessment of Geosynthetics versus Conventional Filter Layer" (Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering), 2013.

Frischknecht, R., S. Büsser-Knöpfel, R. Itten, M. Stucki and H. Wallbaum, "Comparative Life Cycle Assessment of Geosynthetics versus Concrete Retaining Wall" (Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering), 2013.

Hoyme, H., Su, J. H., Kono, J. & Wallbaum, H. (2019): Nonwoven geotextile scour protection at offshore wind parks, application and life cycle assessment, Proceedings and Monographs in Engineering, Water and Earth Sciences, pp. 315–321.

Weidema BP, Bauer C, Hischier R, et al (2013): The ecoinvent database: Overview and methodology, Data quality guideline for the ecoinvent database version 3.