CIRED 2021 Conference 20 – 23 September 2021

Paper 1118

Thermal Analysis and Debottlenecking of HVAC Export

Cables for Offshore Windfarms
Syed Hamza H. Kazmi1*, Rogvi Østerø1, Joachim Holbøll2, Thomas H. Olesen1,
Troels S. Sørensen1
1
Ørsted Offshore Wind A/S, Copenhagen, Denmark
2
Technical University of Denmark, Lyngby
*syeka@orsted.dk

Keywords: DISTRIBUTED TEMPERATURE SENSING (DTS), OFFSHORE WINDFARMS, REAL-TIME

CONDITION MONITORING, THERMAL BOTTLENECKS, CABLE HOTSPOTS

Abstract
The substantial costs of HV export cables connecting offshore windfarms to the grid combined with their influence on energy
availability requires extensive thermal evaluation of these cables. The analysis in this paper reveals that the offshore windfarm
cables can be divided into onshore (underground), landfall, offshore (subsea) and J-tube sections based on their unique
thermal behaviour. The utilization of long-term Distributed Temperature Sensing (DTS) data from a specific windfarm export
cable in UK reveals that the J-tube and landfall sections are the major thermal bottlenecks, along with the cable
joints/transition-points at onshore and offshore. Landfall is found to comprise of the most thermally strenuous environment
due to large burial depths. While J-tube is influenced significantly by the ambient conditions in its air-section, thereby making
it most prone to be affected by the local weather. Whereas, the thermal instability of the lengthy offshore section comes from
the diffusion of thermal properties of surrounding material due to seabed geology and variation of cable burial depth over time
due to seabed movement and sedimentation. Onshore cable section is found to be thermally stable and free from hotspots not
only because of the difference in cable design and utilization of ducts for installation but also because the fibre optic cable for
DTS is not embedded inside the cable. The temperature offset therefore needs to be established for the land and submarine
cables respectively in order to obtain a more accurate account of the conductor temperature. In conclusion, employment of
modern techniques combined with thermal analysis of each section can improve the existing design and operation practices.

1. Introduction export cable for a specific offshore windfarm in UK is

used as test case. Moreover, the entire cable route of the
The cost of energy from Offshore Wind Power Plants analysed test case is examined and a comprehensive
(OWPPs) has decreased by more than 60% over the last portrayal of the thermal development across the cable’s
decade. The export cables that are used to connect OWPPs length is provided for two different years to evaluate the
to the grid on land are known to be the primary contributor impact of varying ambient conditions and wind energy
to the CAPEX of the electrical export system. Therefore, production. Moreover, the necessity and methodology of
the ratings of these export cables are statistically optimized employing dynamic rating for export cables during design
to reduce the cost of energy even further [1] [2]. Therefore, and operation is also discussed
the export cables are not only critical for ensuring energy
availability but also to optimize the system’s capital and
operation expenditures. Thermal monitoring of cables in
2. Offshore Windfarm Cable Layout
real-time is possible using fibre-optics based Distributed The OWPP export system based on HVAC technology
Temperature Sensing (DTS) equipment. This equipment consists of two or more substations: an offshore one close
allows the system operators to assess the variation of cable to the wind turbines in the sea and an onshore one on land
temperature over time along the entire cable route with an which serves as an interface to the transmission network,
acceptable degree of accuracy. Such an equipment can be as shown in Figure 1. The HV components that constitute
used for extensive thermal analysis of the inherent this system include HV/MV cables, power transformers,
bottlenecks in the export cable system. shunt reactors, HV filters, gas insulated switchgears and
compensators (incl. STATCOM, FACTS etc.). The HVAC
In this paper, debottlenecking of the OWPP export cable is solution is suitable for offshore windfarms up to 70 km
performed by identifying cable sections which are not only from the shore; which can further be increased by using
influenced by the variation in operating conditions (energy intermediate HVAC reactors in the export system. There
production, ambient temperatures, sea-bed movement, are usually multiple parallel circuits to improve the system
sedimentation etc.) but are also highly dependent on the reliability. One such setup with two export cable circuits is
installation and commissioning practices. These sections presented in Figure 1. A choice is made by the system
include onshore (buried in duct), Horizontal Directional designers to either use multiple circuits with lower
Drilling (HDD) for landfall, offshore (subsea, directly transmission voltage or single circuit with high voltage,
buried), Cable Protection System (CPS) and J-tube. The each solution differing significantly in CAPEX and OPEX.

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 CIRED 2021 Conference 20 – 23 September 2021

Paper 1118

quite often, it does not sustain long enough for the red line
to ever touch the 100% mark. Moreover, the improved
cooling of exposed cable sections during high wind
periods and unavailability of some wind turbines at any
given time further facilitates this process.

Figure 1. Simplified layout of HVAC-based OWPP export

system with 2 parallel export cable circuits

The typical layout of export cables for OWPP transmission

system based on HVAC technology is provided in Figure
2. One 3-core subsea cable per circuit is shallow buried
directly in the seabed. At the offshore substation end, the
fibre-optics embedded HV cable passes through the Cable
protection System (CPS) which protects it from
mechanical stress during installation while maintaining an
appropriate bending radius during operation. The cable
also goes through the J-tube section before entering the
offshore platform hang-off where the cable armour is
terminated. At the other end, the 3-core cable goes through Figure 3. Feasibility assessment of production and cable design
a Horizontal Directional Drilling (HDD) pipe before for an OWPP site in UK. Top: Instantaneous and 7-day averaged
transitioning to an underground cable connecting to the windspeeds for 15 years. Bottom: Instantaneous and 7-day
averaged energy production for 15 years. The difference b/w red
onshore substation. For underground transmission, single-
and blue lines exhibit intermittency of wind generation
core cables in ducts are commonly used, which requires
the presence of transition joints between HDD and the
onshore sections. 3. Analysis of Prominent Sections for
Offshore Windfarm Export Cables
In section 2, it was revealed that from the design and
commissioning point of view, export cables can be
sectionalized into onshore (underground), HDD, offshore
(subsea), CPS and J-tube sections. In this chapter,
individual thermal analysis of these sections (except CPS)
is performed for the test case of a windfarm in UK.

The temperature measurements from DTS can be used to

identify and localize the hotspots (and thermal bottlenecks)
in the export cable system. The temporal evolution of the
Figure 2. Overview of main components/sections for the HVAC- cable temperature distribution along the cable route over
based export cable system for OWPPs long periods, as shown in Figure 4 for the test windfarm,
can be examined to avoid misidentification of these
As a result of the high costs of long HVAC cables, bottlenecks. From Figure 4, it can be deduced that there
offshore windfarms are usually overplanted, which allows are regions (or pinch points) along the cable route that
the continuous transmission capacity of export cables to be exhibit uniform thermal behaviour over time and are
less than the windfarm size. This practice is appropriate for always warmer than their neighbouring sections. While it
OWPP design because of a number of reasons. The is possible to have inherent ‘pinch-points’ in OWPP cable
intermittent nature of wind energy production circuits for all periods, it is also common for the thermal
accompanied with the large thermal inertia of cables bottlenecks to move from one section to another because
(particularly for deeply buried sections) which slows down of change in seasons, tidal movement etc. The increase in
the cable heating up process suggests that the cable doesn’t rating of J-tube cable section from summer to winter is the
heat up to its maximum temperature limit because high most common example of this behaviour.
wind energy production periods do not sustain for long
enough. This is demonstrated in Figure 3 for a windfarm
site in UK over the period of 15 years, where Moving
Average (MA) method with 7-day MA window is used to
identify the intermittency of wind speed and windfarm
production. This technique imitates the slow thermal
development phenomena in cables and it can be seen that Figure 4. DTS measurements over eight days for the test OWPP
even though the instantaneous wind production is 100% export cable. The thermal behaviour is consistent along the route.

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 CIRED 2021 Conference 20 – 23 September 2021

Paper 1118

3.1. Offshore Section: solar heating during the day, which when combined with
The directly buried, 3-core, armoured, subsea cables the poor thermal dissipation properties of the surrounding
usually constitute the largest section for OWPP export air and the influence of air ambient temperature makes this
cables in terms of length. The burial depth of this section subsection one of the main thermal bottlenecks for export
can ideally vary between 1 and 3 metres below the seabed cables. This, however, is found to be less problematic
surface, depending on a range of reasons (ease of burial, during night time and during winter season. The
threat of anchor impact etc.) [2]. Over time, the burial longitudinal heat flow between the water and air
depth can change significantly due to sedimentation and subsections results in cable temperature gradient near the
seabed movement because of phenomena like scouring, boundary. The temperature reaches a plateau as the
migrating sand waves, storms etc. This can cause the longitudinal heat flow diminishes and starts to decrease as
thermal behaviour of localized offshore sub-sections to the cable approaches the air and hang-off boundary [5].
change with time, which may result in development of new The empirical method of [6] and analytical method of [7]
thermal bottlenecks. have been extensively used for thermal analysis of HV
cables in J-tubes, but the analytical inclusion of
On the other hand, the thermal resistivity and heat capacity longitudinal heat flow in the method proposed in [5]
of the seabed greatly influence the temperature variation makes it more accurate.
for these cables. The composition of the materials
surrounding the offshore cable can generally vary along its
route, as shown in Figure 5 for an OWPP along with the
instantaneous cable DTS measurement. In Figure 5, the
change in cable temperature is notable to the trained eye
for subsections with different thermal properties. The
thermal resistivity of soft mud (clay) can range between
0.56-2.5 m·K/W and has the tendency to be significantly
higher than that of gravel (0.33-0.55 m·K/W), which can
explain the sudden drop in temperature around the 28-km
mark. On the other hand, heat capacity (dependent on
material porosity) is fairly similar for both. Therefore, it Figure 6. Left: Typical J-tube layout [5]. Right: Temp.
can be concluded that seabed material composition distribution in subsections of the test windfarm J-tube.
influences the rating of offshore cable section significantly
[4]. 3.3. Onshore and HDD Sections:
In comparison with the directly buried offshore cables, the
onshore cables are laid in a much stable environment. As
shown in Figure 7, the DTS measurements are
significantly lower and relatively stable for the onshore
section, the reasons for which are touched upon in Section
5. As the 3-core offshore cable reaches land, it passes
through the landfall section (commonly HDD) before
being connected to 3 x 1-core cables at the onshore
transition joint.

Besides having significant burial depth compared to the

subsea cable, the HDD section is known to have the most
thermally strenuous environment as compared to the
Figure 5. Top: Seabed geology of the offshore cable section. remaining sections which makes it the most critical
Bottom: Cable temperature distribution (based on DTS thermal bottleneck in the export cable system [8]. The
measurements) for the same windfarm at an instant in time
situation is worsened during summer due to ambient
temperatures higher than offshore and also because of
3.2. J-tube Section: drying of the local soil due to possible migration of
The cable that rises from the sea to the offshore platform moisture. However, there are methods of resolving these
needs additional protection from waves and tides. Using J- thermal pinch points which includes filling up the HDD
tubes for this transition is one of the solutions commonly pipes with material with superior thermal properties like
used in the OWPP industry. The general layout of J-tubes bentonite and utilization of cables with larger conductor
is illustrated in Figure 6, along with the instantaneous DTS cross-section etc. Consequently, the cable temperature for
measurements for the export cable’s J-tube section for the HDD is significantly reduced but it remains to be the
test windfarm. The three main subsections of the J-tube thermal bottleneck as compared to its surrounding section,
highlighted in the layout are: water, air and armour which can be seen in Figure 7.
termination (hang-off). The air subsection is exposed to

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 CIRED 2021 Conference 20 – 23 September 2021

Paper 1118

observation around the 16-km mark, where the higher

rating cable of HDD is connected to the offshore cable of
smaller cross-section. The temperature cable variation for
the offshore section is significantly larger beyond this
point as compared to the relatively stable distribution
before.

5. Discussion on Existing Monitoring

Figure 7. Temperature distribution in different sections for the
Practices and Future Design Challenges
test windfarm at an instant in 2014.
In section 2 and 3, it was identified that long-term
historical wind generation data of around 12-15 years is
4. Temporal Variation of Thermal usually used for site assessment and windfarm cable
Properties for Export Cables design. Such an approach allows to account for any
Based on the discussion so far, it can be deduced that aberrations that can lead to wind energy curtailment if
although there are sections in the OWPP export cable dynamic rating is used for cable design due to overheating
system that constitute inherent thermal bottlenecks, the of the cable. During the operation phase, the only method
instantaneous temperature that each cable section (and to prevent cable overheating is to monitor its temperature
subsection) heats up to depends on a number of factors in real-time. The utilization of numerical thermal models
including local ambient temperature, sustained cable load for cables has widely been used for this purpose until
and variation of thermal properties of cable surroundings. recently. The parametrization of these models is arduous
To illustrate this further, the maximum annual temperature and requires explicit information about the cable design
measurements along the entire cable route for the test and its surroundings, which includes temporal progression
windfarm is provided in Figure 8, two years apart for 2014 of soil thermal resistivity, ambient temperature for ground
and 2016. Throughout 2016, maximum wind energy and seabed, solar radiation (for J-tubes) and other thermal
production did not sustain long enough for the cable issues identified in section 4. Consequently, these thermal
temperature to rise as high as it did in 2014. This was models must be validated periodically using the DTS
further facilitated by colder ambient temperatures which measurements.
were 2.6 C less on average for 2016 compared to 2014.
The influence of variation of the identified parameters on
As already identified in Section 4, it is observed in Figure the accuracy of numerical models’ temperature predictions
8, that the maximum DTS measurements for 3 single-core results in operators turning to DTS instead. Even though
cables for the onshore section are less than the temperature the DTS suppliers affirm high accuracy with exemplary
of the 3-core submarine cable in the offshore section for spatial resolution for their equipment, it is inadequate to
both the years. This difference appears because of different rely completely on the DTS measurements for curtailment
cable designs and also because for the onshore section, the of wind energy because of the significant economic impact
optical fiber cable is placed in a duct and outside the on the business case of offshore windfarms. Besides the
individual pipes containing the 3 phase conductor cables, issues related to calibration and reliability of
while the single 3-core cable in the offshore section is measurements, there are some inherent risks with the
embedded with the optical fibre. The temperature offset technology which primarily include uncertainty of fibre
therefore needs to be established for the land and location in the cable core, particularly in the case of 3-core
submarine cables respectively in order to obtain a more undersea cables where fibre core is lined inside the export
accurate account of the conductor temperature. It is further cable core. However, these risks can be mitigated by
reinstated in Figure 8 (bottom) that the cable temperature reducing the influence of human intervention and by
in the offshore section (7-46.8 km) is highly dependent on relying more upon the computationally intensive but
the burial depth of the cable with respect to the sea-bed adaptable machine-learning algorithms because of their
and also on the type of material surrounding the cable in ability to identify underlying principles that are not always
the back-filled region. visible even to the trained human eye [9].

There are two distinct temperature peaks: one due to the For the offshore section, the foremost challenge in the
onshore transition joint between HDD and onshore, while variation of cable burial depth over time. This is ensured
the other in the air section of J-tube for both the years. by pushing windfarm developers to perform periodic
Further analysis reveals that J-tube’s air section exhibits surveys of the cable route which are expensive. The
max temperature during summer (when production is not frequency of these surveys is high in the initial years but
maximum) in contrast to other sections which are hottest can typically be reduced with time. Adequate analysis of
during sustained high load conditions. This proves the fact thermal properties in combination with thermal models
that J-tube section is influenced by ambient conditions and long-term DTS measurements can reduce the
more than other sections. There is another interesting frequency of these surveys considerably.

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 CIRED 2021 Conference 20 – 23 September 2021

Paper 1118

Figure 8. Top: Maximum measured temperatures in years 2014 and 2016 along the entire length for the test windfarm export
cable. Bottom: Closer look of different sections of interest

6. Conclusion heating during the day and the poor thermal dissipation
properties of air as compared to sea water. Whereas, the
The debottlenecking of offshore windfarm export cables
increased depth of burial is the main reason for cable
has successfully shown that there is a need to address the
hotspots in the HDD section. For both these sections, the
different design concepts and commissioning practices for
situation is worst during summer due to higher ambient
dynamic rating-based cable operation and design. The five
temperatures and solar radiation. Therefore, both these
identified cable sections including onshore (underground),
sections can have seasonal bottlenecks which disappear in
landfall (HDD), offshore (subsea), cable protection system
winter, particularly J-tube. The risks involved with thermal
and J-tube have their unique thermal behaviour. This
estimation using DTS measurements and numerical
behaviour needs to be accounted for, in order to prevent
models can, however, be resolved by using modern data
revenue loss due to wind energy curtailment during
manipulation techniques.
sustained high production periods, particularly for cables
that are optimally rated using dynamic loading to reduce 7. References
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Temperature Sensing (DTS) equipment, the operators can Experience”, B1-303, CIGRE 2016.
control cable load and optimize wind energy transmission [2] Working Group B1.40, “Offshore Generation Cable
Connections”, CIGRE TB 610, Feb 2015.
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