OFFSHORE WIND ENERGY, THE RELIABILITY DILEMMA.
Dr. G.J.W. van Bussel
Section Wind Energy, Faculty Civil Engineering and Geosciences
Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands
Tel (31) 15 278 51 78
Fax (31) 278 53 47
e-mail: g.van.bussel@citg.tudelft.nl
ABSTRACT: Future offshore wind farms will be of a much larger size than those currently in operation. But the reliability of
the wind turbines used until now is not sufficient for such future large -scale wind farms at sites significantly farther away
from shore. Hence the requirement, often posed by bankers, investors and insurance companies, to rely on proven
technology is not a very sound proposition. Development of new, larger machines can lead to more profitable exploitation of
the offshore wind potential. But this can only be achieved when scaling up the wind turbines to the 5 MW size more is not
the only target of the offshore wind turbine manufacturer. The efforts to realize a significant increase in reliability of the wind
turbine and the other components are at least of equal importance. Integrated design of the complete wind farm, covering all
major design aspects simultaneously, and in their mutual dependence in alls phases of the design process will lead to the most
economical solution.
Keywords: Offshore wind farms, Reliability, Operation and Maintenance.
1. INTRODUCTION
At present a limited number of wind turbines have been
erected in an offshore environment. Offshore wind energy
experience started with the location of a 220kW wind
turbine just off the coast in Sweden. The projects that
followed afterwards have all been realised at rather benign
sites, with the exception of two 2 MW wind turbines
located in the North Sea 2 km in front of Blyth Harbour
(U.K).
Future offshore wind farms will be realised both at
inshore sites, mainly around Denmark, in the Baltic, at
North Sea locations and at a number of sites around the
UK and Ireland. All the offshore wind farms that are
planned or that are currently being realised are larger than
the largest farm realised so far, the 20 x 2 MW wind farm
at Middelgrunden, just in front of Copenhagen.
Increasing the size of offshore wind farms from the
present 10 to 40 MW installed capacity per wind farm to
values of 150 and probably as large as 500 MW is not a
straightforward procedure. Of course it is fairly easy to
double or triple the number of turbines in a wind farm,
extending the present 10 to 20 up to maybe 40 or 50 and
hence increase its capacity proportionally. But this does
require an equivalent increase in the installation time of the
wind farm. Assuming a typical value of 3 to 5 days per
turbine this would already require more than half a year of
carefully planned continuous offshore activity. Weather
conditions need to be favourable for installation activities,
and thus they are usually planned in the summer season.
Evidently a significant amount of time can be gained by
parallel operations. This is implemented in the realisation of
the Horns Rev wind farm, consisting of 80 2 MW wind
farms in front of the Danish North Sea coast near Esbjerg.
But installing a larger number of turbines within one year
will be virtually impossible.
Thus realising a 500 MW wind farm using present
state-of-the-art 2 to 2.5 MW wind turbines will take
several years with inherent loss of investment capital.
The challenge to explore the enormous wind potential
of the North Sea drives manufacturers towards the
development of multi megawatt wind turbines. At the
moment there are at least 5 manufacturers active in the
design and construction of wind turbines with an installed
capacity of 4.5 Megawatt and above. These wind turbines
will be equipped with rotors of around 120 m. diameter, and
will become the largest mass produced revolving structures
on earth. Wind plants with a capacity of several hundreds
of megawatts can then be realised.
Experience in the design and operation of wind turbines
however is gained with sizes that are significantly smaller.
Design tools have been validated for 500-600 kW
machines, having a typical diameter of around 40 m. It is
therefore very difficult to guarantee the reliability of the
machines. But that is exactly what investors and insurance
companies want: they require proven technology!! Is there
a way out of this dilemma??
2. RELIABILITY OF PRESENT WIND TURBINES
Over the past years an important amount of
information has become available regarding the operational
behaviour of wind turbines. Especially the wind turbines in
the 500 to 600 kW class have been the topic of surveys
and monitoring. Public information regarding the
maintenance demand of such machines can e.g. be found
in references [1] to [3]. The 500 to 600 kW class is of
special interest since this the first class that is massproduced. Thus a significant amount of identical machines
could be monitored, and fairly reliable statistics can be
developed. Unfortunately the above-mentioned sources do
not always distinguish the different makes. Furthermore
their system of classification and data ga thering differs,
In figure 1 below the failure frequency information
from the four different databases has been summarised
and compared. Because of the different classifications
used in the databases this can only be done over a limited
number of component groups. Despite that a fairly good
agreement can be seen in the failure levels. The most
striking difference can be seen in the data from WindStats.
There is a very significant difference between the
registered failure frequency of the Danish and of the
German wind turbines. But when the data is clustered on a
more global level, as is done by the author in figure 1, one
can conclude that the Danish results in WindStats seem to
under predict the actual number of failures significantly.
Yearly Wind Turbine Failure Frequencies
0.60
0.50
0.40
LWK
MWEP
WindStats DK
WindStats GE
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Figure 1: Monitored yearly failure frequencies from
different public databases. LWK: data from SchleswigHolstein [1], MWEP: data from the German monitoring
programme [2] and data from the WindStats journal [3]
The general picture is that accumulated failure
frequency is in the order of 2.5 events/year. Furthermore
regular (scheduled) maintenance of these wind turbines
takes place twice a year, which brings the total number of
visits to at least 4 per year.
3. OFFSHORE AVAILAIBLITY LEVELS
For a 150 unit wind farm consisting of 2 MW wind
turbines at least 600 visits have to be paid each year to
keep it in full operation. For land based wind turbines and
wind farms this is not a problem. Modern wind turbines and
wind farms reach availability levels of 98% or more, but
once these wind turbines are placed offshore the restricted
accessibility of the site can make things much worse. This
can be seen from figure 2 In this figure the availability of
an offshore wind farm is depicted as a
function of the accessibility of the site. This is dependent
upon the wave and wind conditions of the location of the
offshore wind farm, but also upon the way in which access
is obtained to the wind turbines.
100%
Availability
which makes it very difficult to compare and synthesize the
different sources. The database developed by Eggersglüß,
ref [1], turns out to be the most suitable for the
characterisation of the reliability profile of the machines in
this class. The main reasons are that a sufficient amount of
turbines have been monitored, and that the database
distinguishes the turbines of different makes. A small
drawback is that the ages of the machines are averaged
per type, which makes it more difficult to find possible
ageing trends.
90%
70%
Offshore
designed
heli
80%
vessel
Offshore
adapted
vessel
60%
50%
100 %
(onshore)
80 %
(near shore)
60 %
(offshore)
40 %
(remote offshore)
Accessibility
Tuno & Vindeby
(DK inshore)
Horns Rev
(North Sea)
Figure 2: Availability of an offshore wind farm as a
function of the accessibility of the site. Results obtained
from an expert system calculation, see ref [].
As can be seen the calculated availability of the Danish
Tunø Knob and Vindeby wind farms, both having an
averaged accessibility of around 85% are still at the level
of onshore wind farms. The large North Sea wind farm at
Horns Rev, would however have an availability of around
90%, according to the calculations performed with the
expert system, ref [4], which are on their turn based upon
the Monte-Carlo O&M simulation tool, ref [5]. The
estimated accessibility percentage for vessel access at
Horns Rev is around 65%, and this is significantly lower
than for the two other farms situated in the Danish inner
seas. However when a maintenance strategy is adopted in
which the maintenance crew can access the wind turbines
though a helicopter, the availability goes up again, and the
targeted level of 95% can be achieved.
Visiting wind farms by helicopter is expensive and is
certainly not a straightforward procedure. All the Horns
Rev wind turbines had to be equipped with a platform on
the nacelle, in order to enable maintenance crew to land
down from a hovering helicopter. Again an adaptation of,
what is in principle an onshore machine.
Until now all wind farms have been built which
adapted onshore wind turbines. Extra protective coatings,
sealing of bearings and nacelles against salt spray etcetera
have evidently been applied, but for future wind farms,
certainly when they are realised in the harsh North Sea
conditions, and further from shore, this is not sufficient. For
such sites a significant step has to be taken in terms of
reducing the maintenance demand of future offshore wind
farm.
There are two ways in which this can be achieved. At
first the wind turbines have to be designed with more
reliable and/or redundant components and systems together
with a reduction of the regular (scheduled) maintenance
interval to once a year. But it can also be achieved in the
design of larger machines, hence aiming at fewer
components per MW installed in the wind farm. The
maintenance costs of series produced 2.5 MW machines
will most probably be around three times higher than the
O&M costs of a 500 kW machine in the onshore case. For
offshore application the advantage will be larger, due to the
higher costs for offshore working activities and transport.
Hence operational costs, apart from investment costs
require the use of the largest machines available for true
offshore sites.
4 THE ROUTE TOWARDS 500 MW OFFSHORE
WIND POWER PLANTS
What will be the most probable route towards future
large-scale offshore wind farms? When this question is
posed to different people in the wind energy business the
answer will not be consistent. Within the wind turbine
technology world there is a strong drive towards the
development of very large machines. The tools and skills
developed for the design of these machines have been
validated with 500 to 600 kW machines from which
experience has been gathered over a number of years.
Experimental data from the (commercial) wind turbines in
the 1 to 2 MW class becomes available at present. But
designing machines of a scale five to ten times larger than
the scale at which the tools have been validated implies a
significant development risk.
This is the primary reason for the investors, bankers,
and insurance companies to demand “proven technology”
for the wind turbines to be used in (future) offshore wind
farms. Probably they are willing to accept the current
largest land based machines (2.5 MW) as “proven
technology” at the time the offshore projects of the size of
a power plant (typically 500 MW) will be built. But it is
very questionable that they will be willing to invest and/or
insure a GigaEuro project incorporating new 5 MW
dedicated offshore wind turbines. In the figure below this
dilemma is visualised:
5 THE INTEGRAL DESIGN APPROACH FOR
FUTURE OFFSHORE WIND POWER PLANTS
Only when treated as one entire system a 500MW
scale offshore wind power plant will provide a considerable
amount of electric power produced in a reliable and costefficient way over its projected lifetime. Therefore a
number of objectives for an optimum wind farm design can
be stated:
• optimum distribution of investment and operation and
maintenance (O&M) costs over the entire windfarm
and over its lifetime
• design optimisation of sub-systems with respect to
‘global’ goals, e.g. minimum energy price, windfarm
availability, overall structural dynamics, etc
• sufficiently high reliability of the system as a whole and
of essential sub-systems e.g. grid connection system,
wind turbines, etc
• adaptation to economy-of-scale: solutions developed
for plants comprising about 100 units or more
• a true symbioses of present experience from the wind
energy with the offshore technology
High investment costs for the fixed cost elements, i.e.
support structures and grid connection, favour large, multimegawatt converter units and large windfarms. The cost
breakdown between the major subsystems (wind turbine,
support structure and grid connection) is nearly equally
shared. Above that the operation and maintenance costs
are an equally important part of the break down of the cost
of energy of the electricity produced in an offshore wind
power plant. Thus optimisation has to consider all of these
elements simultaneously and contradictory goals have to be
balanced with respect to production costs and revenue over
the entire life.
(onshore)
Increased farm size
500
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50
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ign
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Tuno & Vindeby
(DK inshore)
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1- 2 MW units
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Increased reliability
2010
offshore
Figure 3: Development lines for a 500 MW offshore wind
power plant
Evidently the “proven technology” road incorporates less
risk than a road along the development of very large
machines. On the other hand the development of very large
dedicated offshore wind turbines, i.e. wind turbines that
have been designed specifically for operation and
maintenance in the harsh marine environment, will
eventually result into the lowest cost of energy of a 500
MW scale offshore wind power plant. However this can
only be established if the development of such large
machines takes place within the framework of an integral
design approach of a large -scale offshore wind farm.
So reliability as such is not a goal, it is a means to achieve a
certain availability level necessary to comply with the
primary goals of the wind farm.
Without a thorough analysis it is difficult to determine the
availability level that can be reached for a future offshore
wind farm. The reason is that achieved availability just as
strongly depends upon the reliability of the system as upon
the maintenance strategy that is applied for the system.
Key factors in such an analysis are:
• the accessibility of the site, which is on its term
determined by the weather conditions (wave height,
wind speed and visibility)
• the availability of lifting equipment
• the ease of maintenance
• the required maintenance and service level of the
machines and its components
Reliability and O&M strategy together determine the wind
farm availability ands thus its energy yield. Whenever an
optimal trade-off is achieved between reliability and O&M
strategy for one site this needs certainly not to be optimal
for another case. One of the complicating factors is that
harsher weather conditions will simultaneously lead to
decreased availability and to increased gross energy yield
for a given level of reliability under a given O&M strategy.
Therefore the operation and maintenance (O&M) aspects
of the complete offshore windfarm has to be analysed for
each specific site in a comprehensive way in order to
determine the optimal reliability level of the wind turbine
6 IMPLEMETATION OF THE INTEGRAL DESIGN
APPROACH
What can be observed, when following the current
developments in the wind turbine industry, is that the
experience so far with offshore wind turbines indeed has
lead to the awareness of the importance of reliability and
maintainability of the design future offshore wind turbines.
But as far as the merge with the knowledge and
experience of the offshore technology is concerned there is
still a long way to go. This can be seen in e.g. a separate
design of the wind turbine and the supports structure, with
only a fairly straightforward interface in the sense of
common design specifications without regular feedback in
the design process.
Another example is the move of most manufacturers
towards more complicated wind turbines: from constant
speed stall controlled towards variable speed (individually)
pitch controlled wind turbines with doubly fed generators
and power electronics. Arguments in favour of such move
can be found in terms of better controllability of the power,
the reduction of dynamic loads and an increase in
production. But for offshore applications robustness might
be a better guideline in the design. An increase of a few
percent in energy production onshore through such
modifications might in an offshore situation lead to a
significant reduction of availability (and thus production)
due to the increased maintenance demand and the inability
to perform repair actions in harsh weather conditions.
Furthermore offshore O&M activities are far more
expensive than equivalent onshore maintenance, thus
influencing the cost of energy in a negative way.
The results from a DOWEC concepts study (ref. [4])
show that indeed an advanced wind turbine design is not
always an economic design for offshore application. In that
case a robust, yet cheap design (two bladed stall controlled
constant speed) turned out to give the highest yearly yield
for remote offshore locations.
A third example is the observation that many
manufactures still approach the issue of more severe repair
activities in an onshore fashion. Although internal cranes
are added in the offshore designs, to enable the remove of
heavier components from the nacelle, the handling of such
components outside the nacelle is still a problem. Onshore
the assistance of a hoisted component is usually not a
problem, under certain restrictions with respect to
maximum wind speed. But a similar procedure offshore is
not possible, and the receiving ship or barge is moving as
well. A true offshore solution needs to be found, maybe in
the direction of a wind turbine design that enables an easy
integral exchange operation using a stable, wave
independent jack-up barge.
6 CONCLUSIONS
Future offshore wind farms will be significantly larger
than exiting and currently developed projects. With respect
to the installed capacity the wind farms will become
comparable to conventional power plants. Hence one can
speak of the development of future offshore wind power
plants.
The reliability of present machines is sufficient for land
based wind turbines, where access is in general not a
problem. Offshore wind turbines however need to be
designed with significant higher reliability specs.
Furthermore the regular maintenance demand needs to be
reduced from twice a year to at most once a year. This
enables the planning of regular maintenance in the summer
season.
The use of proven technology is often heard as a
requirement for the technology to be used offshore.
Referring to the current maintenance demand however this
is not a very clever approach. It will lead to offshore wind
farms with high O&M expenses, and is hence not very
economical.
The current trend to develop very large machines for
offshore applications is usually driven by the awareness
that it needs to be done to obtain the most profitable
investment cost for future offshore wind power plants.
Within this process the urgency to focus as much on
reliable and robust design is not always seen as of equal
importance.
6 REFERENCES
[1] W. Eggersglüß; Wind Energie IX-XI, PraxisErgebnisse 1995-2000, Landwirtschaftskammer
Schleswig-Holstein, Osterrönfeld, Germany.
[2] MWEP Jahresauswertung 1996-2000, ISET, Kassel,
Germany, ISBN 3-9805896- 6-8/5-X/n-n.
[3] WindStats Newsletters 1998 –2001, Vorlaget Vistoft,
Denmark, ISSN 0903-5648.
[4] G.J.W. van Bussel; The Development of an Expert
System for the determination of Availability and O&M
Costs for Offshore Wind Farms, Proceedings of the
1999 European Wind Energy Conference, Nice,
France, March 1999, pp. 402-405
[5] G.J.W. van Bussel; Chr. Schöntag, Operation and
Maintenance Aspects of Large Offshore Wind farms,
Proceedings of the 1997 European Wind Energy
Conference, Dublin, Ireland, October 1997, pp. 272279.
[6] G.J.W. van Bussel, M.B. Zaaijer; DOWECconcepts
study, reliability, availability and maintenance aspects,
Proceedings of the 2001 European Wind Energy
Conference, Copenhagen, Denmark, June 2001, pp.
557-560.