Electromagnetic Fields (EMFs) from subsea power cables in the natural marine environment

Abstract

To advance the understanding of anthropogenic electromagnetic fields (EMFs) from subsea power cables, an interdisciplinary expert workshop was facilitated by the Centre for Environment, Fisheries and Aquaculture Science (Cefas) and the Scottish Government at the Royal institution, London on 17 and 18 January 2023. This workshop project forms part of the Offshore Wind Evidence and Change programme, led by The Crown Estate in partnership with the Department for Energy Security and Net Zero and Department for Environment, Food & Rural Affairs. The Offshore Wind Evidence and Change programme is an ambitious strategic research and data-led programme. Its aim is to facilitate the sustainable and coordinated expansion of offshore wind to help meet the UK’s commitments to low carbon energy transition whilst supporting clean, healthy, productive and biologically diverse seas.

Fifteen interdisciplinary participants from England, Scotland, Sweden, Belgium and the USA attended the workshop. The participants were technical experts, in electrical engineering, biology/ecology, oceanography, physics and geophysics, represented by government, industry, academia and research institutes.

After an introduction to set the scene of EMFs in the environment and their relevance to marine species, the workshop was divided in four sessions. The first session focused on the cable characteristics and their influence on the EMFs. The second session tackled the interactions between the natural environment and cable EMFs, in order to estimate the total EMF environment, which presents the EMFs in a more realistic and integrated way. In session three, outputs from session one and two were considered in discussions of how to best incorporate data into models. The last session focused on adding the biological context and determined the key outputs and recommendations from the workshop.

The discussion highlighted several key points. Direct current (DC) and Alternating current (AC) cables are different, therefore there is a clear need to separate the consideration of EMF’s based on the type of current applied. The main cable components involved in EMFs for DC are the core (the conductor) and the sheath. In addition, for AC the spacing between the conductors and the time varying magnetic fields will induce time varying electric fields.

Simple estimation or modelling of the EMF of a conductor at a specified point along a cable can be achieved through a quasi-stationary solution and both the magnetic field and the electric field can be calculated by applying Maxwell’s equations. A simple 2D model (i.e. at a point along the cable) will provide much of the basic knowledge on the EMFs, whilst a 3D solution will provide greater insight and realism (along the cable and into the surrounding environment). Based on classical formulae accurate 2D estimates can be made for long straight cable runs assuming the quasi-stationarity is valid. However, when including the realities of bends and angles in the cables the complexities may be better evaluated with a non-stationary solution. In this case (and from a 3D perspective) more complex geometrical vector evaluations are required.

When considering cables within the environment there are factors that will affect the EMFs. The seabed (if it has no magnetic properties) will not change the magnetic field but may affect the electric field. Water movement through the magnetic field, will induce electric fields. At the interface between the water and the seabed there is a boundary layer, which is likely to be important in the magnitude of the electric fields produced. Hydrodynamic boundary interface conditions can be taken into account using 3D models and are most appropriate for HVAC cables. This may be particularly relevant when considering the 3D nature of animal movement through EMFs. The cable sheath and bonding arrangements will also affect sheath currents and therefore the EMF from cables.

Understanding the interaction between natural EMFs (e.g. the earth’s geomagnetic field) and power cable EMFs requires both measurement and modelling of the components making up the total field. Cable orientation, bundling, or any helical twist will influence EMFs measured at any point. An EMF sensor should measure along three axes. It is acknowledged that measuring the electric field in situ is difficult and may require a bespoke solution. The variability in the magnetic field was highlighted as a critical aspect of measuring the total EMF. When considering power cables associated with offshore wind devices, the power generation varies considerably by day and season therefore temporal fluctuations should be considered in both EMF measurements and modelling.

For permitting purposes there is a need for simplified models to define a minimum set of parameters. It was therefore suggested to differentiate between modelling that is useful to have for the purposes of permitting in addition to modelling for research purposes in to determine any effects on marine animals.

A series of key outputs and recommendations were developed, leading to the following actions to enhance the understanding of EMFs emitted by subsea power cables. For ease, they are separated into recommendations that can easily be applied now and those that require research and development, although some may be easily actionable. Collectively, these recommendations will facilitate continued progress towards a clearer understanding of the cable EMFs and potential impacts on species.

Recommended approach that can be applied now

  1. Two main approaches to EMFs modelling can be taken and broadly categorised as models applicable for permitting and those applicable for research:
    • For both permitting and research models, the essential knowledge, applicable to both DC and AC cables is:
      (a) the basic cable EMFs (i.e. energy emission only)
      (b) the cable EMFs in the marine environment
    • For the purposes of research, additional definition and resolution of the cable EMFs, and the interactions with the marine environment, can be gained by approaches (a) and (b) with:
      (c) research additions to better define the magnetic field component
      (d) research additions to define the motionally induced electric field
      (e) research additions to improve the AC model
  2. Permitting models of EMFs are regarded as simple models due to availability of parameters but models can be improved in accuracy once cables are operational.
  3. Optimum application of modelling to scenarios should be data driven.
  4. When reporting an EMF model or measurement the following should be defined:
    • if it is an AC or DC cable and specifically what is being modelled/measured (magnetic field, induced electric field)
    • if the geomagnetic field is combined in the model/measurement or only the cable EMFs are reported (applicable to DC models)
    • the grounding and bonding arrangement of the cable
    • total length of cable and position along cable of modelled/measured field
  5. Measurements of cable EMFs should report the same factors (see point 4 above) in addition to being accompanied with evidence of calibration and the method, including limitations in detection for the magnetic field and/or electric field as well as how the geomagnetic field was handled in the data processing.

Research and Development

  • Data access should be explored with developers/cable owners, taking account of confidentialities, with the goal of accessing data on power variability and burial depths after cables become operational.
  • Data assessment for optimum scenario model building is recommended through commissioned work to explore data in the context of realism and determine the best data to be made routinely available from cable operators.
  • Spatial configurations of cables should be verified with industry, particularly for floating offshore wind in order to improve/develop EMFs models; how cables will be positioned in three-dimensional space, the degree of cable movement and geometry of the cable relative to itself (i.e. how bent/straight).
  • Exploration of changes in electrical currents along an AC cable and modelling the associated EMFs to understand the realities of power cable EMFs in the marine environment (applicable to both modelling & measurement).
  • Incorporation of the water boundary layer through developing a combined model (EMFs, boundary layer, geomagnetic field) and scaling to determine the influence on the EMFs, with a laboratory validation.
  • Consideration of motionally induced fields in EMFs modelling, separate to the induced electrical fields associated with the AC cable.
  • Determine how to model the total AC field and consider its relevance from the species perspective as well as the ability for regulators and researchers to interpret the model.
  • Develop an agreed strategic approach with developers and cable/operators to measure EMFs to enable validation of models.
  • Determination of other power cable factors that can influence EMFs, such as temperature of cable materials, power surge protections and potential cable faults as well as potential scenarios to be considered with respect to defining effects on marine species.
  • The biological context is important when defining modelling scenarios of EMFs in the environment and this should include defining species detection ranges for intensities and frequencies of electric and magnetic fields. However, it is noted that the knowledge base on species sensitivities requires advancement hence this is a long-term goal to be met through studies of several model species and careful definition of appropriate metrics.
  • The knowledge from current literature on the effects of EMFs on species was discussed and it was recommended that where studies are not directly applicable to subsea power cables, it should be clearly communicated.
  • Fish/animal movement through the EMFs was deemed to be important to determine their likely exposure and could be informed through the 3D EMFs modelling approach in conjunction with animal movement models. The conductivity of the animal may be influential so should be taken into consideration.

Post workshop there have been specific activities that have provided outputs from the valuable discussions that took place. There is a simple model that can be applied for calculating magnetic fields associated with AC power cables showing how the helical twist of the cable affects the propagation distance of the magnetic field from the cable axis. The EMFs modelling recommended through the EMF technical workshop will feed directly into another Offshore Wind and Evidence Change (OWEC) funded project, FLOWERS – Floating Offshore Wind Environmental Response to Stressors (2022-24). The FLOWERS project has a work package which builds on the EMF modelling and measurement approach(es) developed through the workshop reported here. Finally, communication and knowledge transfer from the expert workshop are important to ensure the agreed approaches and recommendations (considering the natural environmental influences) are known about and referred to by the Offshore Wind (OSW) industry, the wider cable sector, environmental consultancies and also regulatory and advisory bodies. A dedicated webinar will be made available via appropriate media.

Improving the knowledge of EMFs from subsea power cables is integral to the better understanding of the potential effects and impact of EMFs on marine species. Such knowledge is required to support environmental considerations for the sustainable development of offshore wind and the global push for green energy. Better characterisation of EMFs in terms of the component parts (magnetic field, induced electric field) and how they are influenced by the marine environment is foundational to understanding how best to assess species responses to them. This workshop aimed to provide a standardised approach to estimating cable EMFs via agreeing the fundamental aspects for calculating, modelling and measuring EMFs (AC and DC) in 2D and 3D, and consider these in the context of the marine environment. Understanding the EMF interactions in the marine environment needs to take account of the natural electromagnetic field sources and relevant oceanographic considerations, which will influence the EMFs that species will encounter.