Abstract
To meet national energy security needs, to reduce national greenhouse gas emissions, and to decarbonise the United Kingdom (UK) economy to meet a Net Zero target by 2050, the UK government has (amongst other plans and policies for power), committed to developing up to 50 gigawatts (GW) of offshore wind by 2030. The required expansion of the UK offshore wind (OSW) energy sector from today’s 13 GW (approx.) to the 2030 target, will require associated subsea and onshore cabling. This means a potential for a significant scale-up of the amount and the overall spatial footprint of external cable protection (typically hard substrata), to be placed on the seabed within UK waters, where required and permitted. The likely upscaling of the spatial footprint of subsea cabling and cable protection for OSW, requires scientific assessment of the potential impacts (whether positive or negative) to benthic biodiversity and marine ecosystem structure and functioning (including linkages with marine ecosystem goods and services). The potential impacts associated with cable protection will occur during both the operational phase of an Offshore Wind Farm (OWF), and during decommissioning of the infrastructure.
With the expansion of OSW (in the UK and globally), there is a growing awareness and interest to investigate the potential environmental benefits of using nature-inclusive design (NID) or nature-based design principles for OSW infrastructure. NID can be defined as “options that can be integrated in or added to the design of an offshore wind infrastructure to create suitable habitat for native species (or communities) whose natural habitat has been degraded or reduced” (Hermans et al., 2020). The interest in NID for OSW infrastructure is, in general, linked with the aims of seeking to enhance ecological functioning, marine biodiversity and marine ecosystems, with the eventual policy goal of achieving a net positive impact.
This report forms part of the Phase 1 of the delivery for the Nature Inclusive Cable Enhancement and Protection (NICE) project. The NICE project is a project led by Cefas and in partnership with Accelerating Reef Creation (ARC) Marine, that is funded under the Crown Estate’s Offshore Wind Evidence and Change (OWEC) Programme. In summary, the NICE protection project aims to provide evidence on the potential ecological impacts (focused on benthic epifauna), of NID technologies for cable protection, compared to existing ‘standard’ cable protection technologies. This is to be achieved through phases of work consisting of:
- Phase 1: Literature review (this report).
- Phase 2: Deployment of a standard and a NID type of subsea cable protection at a UK study site.
- Phase 2: ROV survey to determine presence of epifauna and development stage.
- Phase 3: Data analysis and reporting.
In fulfilment of Phase 1 of the NICE project delivery, this report contains a composite of information that will support further phases of work during the NICE project. The Phase 1 review findings and summary will help to shape the study design and analyses in Phases 2 to 3, and input to the Phase 3 report.
This Phase 1 report contains a summary of methods for seabed preparation (sand wave/ mega-ripple presweeping, boulder clearance and pre-lay grapnel run) and subsea cable burial methods (ploughing, jetting, vertical injectors and mass flow excavators). This is coupled with information about the different types of standard external cable protection measures used in the offshore wind sector (as well as those used in other marine sectors) – including rock dump, concrete mattresses, sand/grout bags, tubular protection. There is also information on indicative quantities and other types of protection measures used in the UK presently.
A high-level review of direct benthic ecological impacts associated with the installation, operation and decommissioning of subsea cables and external cable protection was undertaken, based on peer-reviewed studies and reviews that include post-consent monitoring data in the UK. Where relevant information about external cable protection / subsea protection systems exists from other sectors e.g., oil and gas sector, wave and tidal sectors, this has also been incorporated. Twelve relevant studies of benthic community presence and development on external cable protection types, and/or subsea protection systems for pipelines (in the case of the oil and gas industry) were identified during this review. These reflect the different sectors of offshore wind (fixed and floating wind), tidal energy devices, wave energy devices, oil and gas, plus a study that compared benthic communities on oil and gas infrastructure with those observed on fixed offshore wind farms. The studies include research conducted in the UK, France, The Netherlands, Belgium and Australia.
From the studies reviewed, the key ecological metrics (i.e., species density, biomass, relative species abundance, and/or species richness and community composition), have typically been used to evaluate changes (over time and/or space) of epibenthic communities, from the presence of subsea cable protection, scour protection, turbine foundations, pipelines, oil and gas platforms. Depending on the datasets, the studies were able to characterise the benthic community assemblages on the various structures and indicated ecological patterns from initial settlement through to succession of communities over time and space. A variety of data collection methods have been utilised in the respective studies: towed video camera system, ROV, scuba diver surveys and scrape sampling, with varying monitoring sampling designs.
This report also identifies relevant information from decommissioning guidelines in the UK on options for decommissioning subsea cables and cable protection, including highlighting whether future use of NID cable protection solutions will affect decommissioning options. A high-level summary of potential decommissioning impacts to benthic communities has also been presented.
The principles of NID in the OSW sector are introduced and examples of NID cable protection measures available are presented, together with an overview of where NID measures are being trialled. An overview is given of ecological and technical considerations, opportunities and risks associated with NID forms of cable protection, based on studies that have drawn on expert knowledge, including some comparative experiences in artificial reef research outside of the OSW sector.
The evidence gathered from the review process has identified pertinent knowledge gaps. These include improving understanding of ecological impacts of different types of cable protection (including NID types) in connection with marine benthic ecological settlement and community succession over time. Also, increasing knowledge of impacts to higher trophic levels arising from benthic artificial reefs forming on the NID and non-NID protection systems, both at individual and regional scales.
From the review conducted, there appears to be a relatively limited number of studies that assessed the ecological effects of standard external cable protection measures on subtidal benthic communities over long timescales (i.e., >5 years), and more specifically, in connection with detecting functional changes, and implications for marine biodiversity and wider marine ecosystems.
There appear to be relatively few documented studies of NID features at OWFs and associated monitoring datasets. This information would help address questions and understanding about whether NID features for subsea external cable protection can potentially enhance marine biodiversity and contribute to a net positive impact. Evidence is being acquired via OSW research programmes, such as the Dutch Governmental Offshore Wind Ecological Programme (Wind op Zee Ecologisch Programma, WOZEP), The Monitoring and Research, Nature Enhancement and Species Protection (MONS) Programme, or via obligations for building a windfarm via site decisions (in the Netherlands).
Information including survey methods, indicators and metrics of benthic community change, and recommendations identified during this review, will help to shape the study design and analyses to be conducted in the Phase 2 / 3 of the NICE project. The information and discussion points about benthic ecological impacts of cabling / cable protection, NID options of cable protection, information on cable/ cable protection decommissioning and evidence gaps identified during the review, will feed into the content of the Phase 3 report for the NICE project.