Offshore Wind Sustained Observation Programme (OW-SOP): scoping report

1 Introduction

The Scottish Government has set a range of targets to cut greenhouse gas emissions and to generate more energy from renewable resources. The Climate Change (Scotland) Act 2019 commits the Scottish Government to reach net zero emissions for all greenhouse gases by 2045. Offshore wind (OW) energy plays an important role within these targets, as it is widely regarded as one of the most credible sources for renewable energy production towards a resilient and decarbonised energy supply (e.g. Galparsoro et al., 2022). This energy sector has rapidly expanded within shelf seas over the past decade motivated by the high-quality and reliable wind energy resources (Esteban et al., 2011), the space availability and site accessibility for installation of large and efficient turbine systems (Sun et al., 2012), as well as the reduced visual impact compared to OW infrastructures in populated areas (Wen et al., 2018).

Government programmes have helped drive development of renewable OW energy from offshore wind farm (OWF) arrays, increasing from tens to hundreds of offshore wind turbines (OWT) supported by various fixed foundation designs. Now, new floating foundations are being designed to access deeper water sites and further expand offshore development opportunities.

However, interplay of social and economic drivers already places significant stress on shelf seas and further industrialisation of shelf seas will enhance these stresses, with the potential for significant long-term environmental impact (Kröger et al., 2018). The dynamic physical oceanographic processes in shelf seas directly control the magnitude and timing of primary production: the growth of microscopic marine plankton. However, from OWT scale to coastal scale, the impact of OW development on shelf seas has yet to be fully considered (Dorrell et al., 2022). Therefore, it is important to further our understanding of the potential impacts of OW energy developments on shelf sea dynamics. This is important in establishing potential risks to marine ecosystems due to wind energy production and infrastructure, and assessment of these risks needs to be relative to baseline(s) that include climate change. Critically, advancing our understanding of the interaction between offshore infrastructures and shelf sea environments will enable balance of key global societal goals, i.e., to ensure access to affordable, reliable and sustainable energy, and to conserve and sustainably use the oceans, seas and marine resources (United Nations, 2015). Therefore, understanding the potential physical and bio-geochemical risks to marine ecosystems from OW energy production is both timely and vital, and will support the adoption of management measures that minimise impacts and the environmental sustainability of this energy sector (Galparsoro et al., 2022).

Most research to date covers areas of the southern North Sea that are either fully mixed or only weakly/intermittently stratified during the summer season, leaving some unanswered questions, such as:

i. What changes may occur by installing OW infrastructure in stratified waters that could influence primary productivity, which could impact higher trophic levels through potential changes to biodiversity, carbon and nutrient fluxes, and the broader ecosystem functioning (e.g. Dorrell et al., 2022).

ii. What physical and biogeochemical changes could influence predator-prey interactions by changing locations where prey feed (e.g. Russell et al., 2014).

These physical effects are not considered a consenting risk at the scale of current developments, but possible resulting changes to the ecosystem have the potential to be of increasing interest as the industry expands. This is particularly relevant given the expected rapid expansion into deeper, more stratified waters made possible by the development of robust fixed foundations and the increasing utilisation of floating platform solutions which form the bulk of the ScotWind project proposals.

The challenge for this type of study lies in discriminating changes in the physical and biogeochemical parameters directly caused by the presence of OWF infrastructures from natural variability, with appropriate consideration of the influence of climate change. For this reason, the ScotMER Physical Processes Receptor Group has highlighted the need to identify the key parameters to be monitored as well as where and how to monitor them (Scottish Government, 2024).

Based on the above, a detailed review is necessary to assess:

i. How OWFs may change near-/far-field water column structure?

ii. How OWFs in seasonally stratified shelf seas may change primary production?

iii. What are the potential mechanisms (fixed vs. floating) that can have the greatest impact?

iv. Do we have sufficient baseline data to characterise and understand baseline conditions across the Scottish shelf areas and how these may alter due to climate change?

v. What relevant baseline monitoring is currently underway and what opportunities exist for a cost-effective improvement (e.g. using autonomous vehicles)?

vi. How can models be used to assess impacts and help inform monitoring programmes?

vii. What sustained observation/monitoring is required?

To answer the questions above, a review process and stakeholder engagement were carried out including government bodies, industry, academia, and research organisations involved with, but not exclusively, the Scottish marine environment. The results of this review process and of active stakeholders’ engagement are summarised in this report, in which recommendations for future monitoring and observational programmes are ultimately drawn in the form of a roadmap.

To provide a clear overview of the subject, this chapter (Chapter 1) provides an overview of potential benefits and issues associated with the fast-growing OW sector (Section 1.1)

Further insights on the history, current status and future trends of OW energy development in the wider North Sea as well as more details on the Scottish developments are provided in Appendix A.

1.1 Challenges facing the rapidly-expanding Offshore Wind sector

The OW energy sector has rapidly expanded over the past two decades, providing a renewable energy solution for coastal nations, including Scotland. Such a rapid scale utilisation of the offshore space does not come without environmental challenges and unknowns.

1.1.1 Large-scale utilisation of the offshore environment

To date, most OWFs have been installed in the near-shore shallow water regions, up to 60 m depth, of shelf-seas. Near-shore, shallow-water installations have been preferred due to the cost reduction from ease of access for installation, grid connection and operation and maintenance (Jacobsen et al., 2019). With global sector aspirations for an additional 208 GW of operational capacity in the next decade, and targets of 1.4 TW total by 2050 (Offshore Renewable Energy Action Coalition, 2020), near-shore and shallow-water sites are rapidly becoming limited.

Advances in engineering design have allowed larger turbines to be constructed, thus increasing energy output capacity. New foundation types have allowed OW expansion into deeper waters: mainly due to the development of floating foundations which can be used in water depths > 200 m (Figure 1.2) (Soares-Ramos et al., 2020).

The transition from near-shore and shallow water environments to deeper water further from shore could mark a fundamental change in the physical and biogeochemical structure of the water column. Shallow waters are typically well-mixed; however, deeper waters may be subject to seasonal stratification where density varies vertically with depth (Dorrell et al., 2022) (see Chapter 2). Stratified waters are a vital part of shelf seas, controlling primary production and biogeochemical cycling (Simpson & Sharples, 2012). As mentioned by Dorrell et al. (2022), ‘expansion into this new environment means that OWFs could increasingly come into conflict with its environmental functioning, controlled by natural mixing of water column stratification’ (Figure 1.2). Thus, there is a need for recommendations for the appropriate methodology necessary to assess the potential impact of OWFs.

1.1.2 OWFs and their interaction with water column structure

Stratification is a term used to describe when two distinct layers occupy the vertical water column in the sea: the near-surface one is less dense than the near-bed one (see also stratification/potential energy anomaly (PEA) assessment; Chapter 2 and Appendix B). This can be due to differences in temperature (warm layer overlying a cooler layer), salinity (fresh water overlying saltier water), or both. The balance between inputs of fresh water and/or heat from the sun and mixing from tidal currents, wind, and waves determines whether the water column is stratified. The interface between the two layers is very efficient at limiting the exchange of water and its properties such as nutrients (Figure 1.2) (Marine Scotland, 2020).

In areas with significant river inputs (Clyde, Forth, Inverness, Beauly, Cromarty and Dornoch Firth), stratification due to river run-off (input of fresh water) can occur year-round. There is currently no evidence that the strength of stratification (the density difference between the two layers) in these regions is systematically changing, although it can change due to particular rainfall events.

In large parts of Scottish shelf sea waters, stratification occurs on a seasonal basis: once the heat input from the sun is sufficiently strong, stratification develops in regions where the water is sufficiently deep or the tidal currents sufficiently weak (see Chapter 2). In these regions, the strength of mixing is insufficient to keep the water column vertically homogenous (Marine Scotland, 2020).

The rapid growth in OW infrastructures within stratified waters may alter the physical properties of the water and, with that, create a cascade of potential impacts. However, until now, research has primarily been focussed on assessing and proposing mitigation solutions on the direct impacts of OWF development on well-mixed shallow water marine ecosystems, such as benthic habitats (Dannheim et al., 2020), fisheries (Gray et al., 2005) and seabirds (Exo et al., 2003). Whilst this research is translatable with sector growth, the seasonally stratified regime offers a fundamentally new challenge: the introduction of infrastructure may lead to enhanced ‘anthropogenic’ mixing of stratified waters which may ultimately lead to profound impacts on shelf sea dynamics and thus marine ecosystem functioning. The dynamics of atmospheric wakes from OWTs are already of key interest, given their influence on available wind power from turbine to array scale (Howland et al., 2019). However, the dynamics of sub sea surface wakes from foundations within the immediate vicinity of OWFs are poorly understood, particularly in stratified waters, as are the modification of near-surface mixing due to changes of the near-surface atmospheric flow over the near-field extent of wind farm wakes (see Chapter 3). Despite this, the > 20 m minimum draft of current floating foundations is already large enough to penetrate the thermocline and directly mix seasonally stratified shelf seas (Figure 1.2) (Dorrell et al., 2022).

This document sets the scene for sector development into these new environments, reviews the potential physical and environmental impacts of large-scale industrialization of seasonally stratified shelf seas and identifies areas where research and best practice modelling strategies and observational programmes are required to quantify, manage, and mitigate environmental change. To support the above, the following chapters discuss:

• Chapter 2 – What is changing? Baseline parameters and existing data. This chapter focuses on the water column structure and provides a review of the prevailing physical and biogeochemical properties and how they change. It also looks at exiting data and potential data gaps.
• Chapter 3 – How to predict change using models? This chapter focuses on numerical modelling simulations used to predict water column natural variability, climate change effects, and also changes induced by anthropogenic activity (OWFs). The mechanisms through which an OWF can change the physical water column are outlined. This chapter also provides a review of existing modelling capabilities, limitations and observations required to enable improvements of the methods.
• Chapter 4 – Where and when to observe change - sustained observation programmes. This chapter outlines existing monitoring/observational efforts, as well as discussing best practice and standards needed to support our understanding of the interaction between the natural environment and anthropogenic stressors. Furthermore, it provides considerations for planning sustained observations and monitoring programmes.
• Chapter 5 – Roadmap towards sustained observational programmes supporting Offshore Wind. This is the key chapter of the report, summarising the outcomes from the review and from discussion held during the workshop. It proposes recommendations and a roadmap towards a sustained observational programme which could be implemented in order to support the sustainable development of the OW industry.