IEEE Power & Energy Magazine - July/August 2019 - 74

sites that use an offshore base may use workboats to ferry technicians between an offshore base and turbines.
Current workboats are stable catamaran vessels 12-24 m
long overall with a passenger capacity of approximately 12.
They can typically transfer technicians in up to 1.2 m of significant wave height and carry 5-20 t of deck cargo. They are usually
classified as Maritime and Coastguard Agency category 2, limiting them to 60 nmi (110 km) from base. Alternatively, some
vessels follow the DNV GL Windfarm Service 1 R1 category,
which are suitable for up to 150 nmi (280 km) from shore.
The cruising speed of the current fleet of workboats is in the
range of 20-25 kn (37-46.3 km/h). For turbine maintenance, it
is standard practice for the supply of workboats, skippers, and
mates to be contracted to a wind farm operator on a long-term
basis. For maintenance of the OSS, visits will be less frequent.
Workboat-only strategies are expected to be typical at up
to 12 nmi (22 km). The superior speed and stability of smallwaterplane-area twin hull vessels and other new designs may
increase the range of workboat-only operation and maintenance strategies where regulatory, commercial, or other factors limit the use of helicopter support. Helicopters can allow
access in otherwise inaccessible sea conditions with faster
transit times, although they are inherently more expensive
and have lower load-carrying capacity compared with workboats. Nevertheless, helicopters can be economical in providing access for fault repair in harsh weather, especially at the
farther locations, and they can reduce lost production from a
lack of turbine or equipment availability at a substation.
To date, helicopters supporting maintenance for OWFs
mainly fly to and from the wind farm and only rarely land
on the OSS. (The provision of a helideck on a platform is
reviewed at the design stage in line with the developer's O&M
strategy and a review of its distance from shore.) For this type
of work, small twin-engine four- to six-passenger helicopters
have been used, such as the Eurocopter EC135, ferrying one
or two crews each of two to three technicians between the
onshore helicopter base and the OWF. This will change as
more OSSs have helipads.

Decommissioning Considerations
Decommissioning of offshore electrical systems should be relatively straightforward in that the topside on which the electrical systems are located will be dismantled in a controlled environment, such as a shipyard. This will generally encompass
dismantling the electrical equipment and removing it from the
shipyard for disposal or recycling, as applicable.
The electrical equipment is manufactured from materials
that can be readily recycled, such as steel, aluminum, and
copper. This is usually undertaken by specialist contractors
because, in some circumstances, dangerous materials can
remain present. Care must be taken, as the equipment contains substances that will require special handling and disposal. These items include
✔ SF6 gas (HV switchgear)
✔ battery acids from the uninterruptable power supplies
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✔ diesel and oil from the emergency generator
✔ transformer oil and oil-impregnated papers.

All hazardous substances will need to be disposed of in line with
regulatory requirements in place at time of decommissioning.

Summary
As the offshore wind sector grows and matures, so will the
need for the industry to develop new design approaches for
offshore transmission assets. Key considerations for future
transmission assets include developing new technology and
balancing capital and operating expenses. With the market
moving to turbines that generate more than 10 MW, we are
seeing larger developments sited farther offshore. New offshore developments will pose transmission challenges and
require more efficient O&M techniques.
Several new technologies are being developed for OWFs,
and some are in more advanced stages than others. New design
approaches for the coming wave of projects will drive reductions in cost, transmission losses, and risk. With the build-out of
more wind farms in closer proximity, we are likely to see developers use interlink models to improve economies of scale and
further reduce risk profiles. Approaches to O&M are likely to
evolve in the coming years. Remote monitoring systems and
automated maintenance and fault identification systems could
greatly reduce the required frequency of offshore inspections.

For Further Reading
E. Olsen, U. Axelsson, A. Canelhas, and S. Karamitsos,
"Low-frequency ac transmission on large scale offshore
wind power plants," in Proc. 13th Wind Integration Workshop, Berlin, Germany, 2014. doi: 10.1016/j.rser.2015.11.033.
DNV GL, "Power frequency optimisation for offshore
wind farms," in Proc. EWEA Offshore 2015, Copenhagen,
Denmark, 2015. [Online]. Available: http://www.ewea.org/
offshore2015/conference/allposters/PO053.pdf
P. B. Wyllie, Y. Tang, L. Ran, T. Yang, and J. Yu, "Low
frequency ac transmission: Elements of a design for wind
farm connection," in Proc. 11th IET Int. Conf. ac and dc Power
Transmission, 2015, pp. 367-371.
Offshore Wind Programme Board, "Lightweight offshore substation designs," ORE Catapult, 2016. [Online].
Available: https://ore.catapult.org.uk/app/uploads/2018/02/
Lightweight-Offshore-Substation-Designs.pdf
R. A. Walling and T. Ruddy, "Economic optimization of
offshore windfarm substations and collection systems," Ofgem, 2005. [Online]. Available: https://www.ofgem.gov.uk/
ofgem-publications/51782/14680-economic-optimizationoffshore-windfarms.pdf

Biographies
Vandad Hamadi is with Atkins, Birmingham, United Kingdom.
Úna Brosnan is with Atkins, Glasgow, United Kingdom.
Ingar Loftus is with Atkins, Epsom, United Kingdom.
Gavin Montgomery is with Atkins, Glasgow, United Kingdom.
p&e
july/august 2019

http://www.ewea.org/offshore2015/conference/allposters/PO053.pdf http://www.ewea.org/offshore2015/conference/allposters/PO053.pdf https://ore.catapult.org.uk/app/uploads/2018/02/Lightweight-Offshore-Substation-Designs.pdf https://ore.catapult.org.uk/app/uploads/2018/02/Lightweight-Offshore-Substation-Designs.pdf https://www.ofgem.gov.uk/ofgem-publications/51782/14680-economic-optimization-offshore-windfarms.pdf https://www.ofgem.gov.uk/ofgem-publications/51782/14680-economic-optimization-offshore-windfarms.pdf https://www.ofgem.gov.uk/ofgem-publications/51782/14680-economic-optimization-offshore-windfarms.pdf

IEEE Power & Energy Magazine - July/August 2019

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