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Determining reliable cable ampacities for marine high voltage cables.

Tuesday 18th June 2024 | Blog written by Jon Duell, OiQ Route Engineer

Determining reliable cable ampacities for marine High Voltage Cables is currently the subject of significant industry and academic reassessment in order to optimize the operation (ampacity), design (cross-sectional area) and subsequent efficiency, longevity, CAPEX and profitability of both submarine power cables and their dependencies (Enescu et al, 2020). Ampacity models can be elaborate, and inaccuracies are increasingly based on the uncertainty in environmental inputs. A stark example is the role of ambient temperature at cable depth, which, due to the scale of cables and the inaccessibility of the seafloor, is commonly estimated at a static 15 °C. Static estimates neglect significant spatial and temporal differences anticipated along a cable’s length (> 80km AC, >300km DC) over time, especially on longer (25 yr) timescales. They also neglect the attenuation of the ambient temperature signal to the cable burial depth, which moderates the ambient temperature dynamics depending on the depth and thermal properties of the sediment (Muller et al, 2016; Worzky, 2009; Meisner, 2015).

Fortunately, oceanographic models incorporating ocean bottom temperature are increasingly available in developed areas and achieve more representative coverage and spatiotemporal resolutions for cable applications without the requirement for project specific site measurements. Figure 1 shows that mean monthly ocean bottom temperature varies significantly across transects approximating the typical locations of transmission infrastructure on the Northwest European Shelf. For three case-study transects Figure 2A demonstrates how unrepresentative the 15°C assumption can be in addition to the importance of bathymetric depth control on annual ocean bottom temperature variability.

Figure 1)  The AMM7 (a,b) and AMM15 (c,d) mean monthly modelled ocean bottom temperature for January (Jan) (a,c) and August (Aug) (b,d) annual end members across the North West European Shelf. Transects approximating the locations of typical windfarm Export Cable Routs (ECR) and Interconnectors (IC) demonstrate significant spatio-temporal ocean bottom temperature heterogeneity can be anticipated at cable length and time scales. Cable are labelled (clockwise); Moray Firth (MF), Firth of Forth (FoF), North Sea  (NS1-3), Skagorrak (SK), English Channel (EC1-3), Celtic Sea (CS1-2) and Irish sea (IS1-2). From (Duell et al, 2023).

Figure 2 AMM15 Mean Monthly Modelled Ocean bottom temperature (left axis) and EmodNet bathymetric depth (right axis) along the English Channel 2 (EC2), North Sea 1 (NS1) and Skagorrak (SK) case-study transects demonstrated in (Figure 1). From (Duell et al, 2023).

Independent validation of the AMM15 and AMM7 (Graham et al, 2018) mean monthly ocean bottom temperature models for the NW European Shelf indicates encouraging accuracies (MBE ? 1.48 °C; RMSE ? 2.2 °C) (Duell et al, 2023). Modelled ocean bottom temperatures along a series of cable case studies have been used to demonstrate that cable ampacity ratings can change between ?4.1% and +7.8% relative to ratings based on a common static (15 °C) ambient temperature value (Duell et al, 2023). Negative rating changes indicate the standard approach is unconservative, or risky, while positive changes indicate the standard approach is over-conservative or un-economic. Magnitudes of 5% change are deemed significant (Pilgrim, 2012). Consideration of such variations can result in both significant ratings gains (and hence reduced capital expenditure and operating costs) and/or the avoidance of cable overheating. Consequently, validated modelled ocean bottom temperatures are deemed sufficiently accurate, providing incomparable coverage and spatiotemporal resolutions of the whole annual temperature signal, thereby facilitating much more robust ambient temperatures and drastically improving ampacity estimates compared to current practices.

Duell, J., Dix, J., Callender, G., Henstock, T., Porter, H. Opportunities to Improve Marine Power Cable Ratings with Ocean Bottom Temperature Models. Energies. (2023); 16(14):5454.

Enescu, D., Colella, P., Russo, A. Thermal Assessment of Power Cables and Impacts on Cable Current Rating: An Overview. Energies. (2020); 13(20):5319.

Graham, J.A., O’Dea, E., Holt, J., Polton, J., Hewitt, H.T., Furner, R., Guihou, K., Brereton, A., Arnold, A., Wakelin, S. and Castillo Sanchez, J.M. AMM15: a new high-resolution NEMO configuration for operational simulation of the European north-west shelf. Geoscientific Model Development. (2018). 11(2), pp.681-696.

Müller, C., Usbeck, R. and Miesner, F. Temperatures in shallow marine sediments: Influence of thermal properties, seasonal forcing, and man-made heat sources. Applied Thermal Engineering. (2016). 108, pp.20-29.

Pilgrim, J., De Wild, F., Anders, G., Bascom, R., Cray, S., Joo, J., Kamara, W., Kvarts, T., Lesur, F., Lofti, A. and Moutassem, W. Overview of CIGRÉ WG B1. 56 regarding the verification of cable current ratings. (2019).

Worzyk, T., Submarine Power Cables: Design, Installation, Repair, Environmental Aspects; (2009). Springer: Berlin/Heidelberg, Germany.

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Author: Jon Duell

Jon Duell

OiQ Route Engineer