Borgen (Sway) Copy_Outsourcing_book_temp.qxd 24/02/2010 10:49 Page 49
Floating Wind Power in Deep Water –
Competitive with Shallow-water Wind Farms?
a report by
Eystein Borgen
Sway AS
Pioneering projects with a few bottom-mounted wind turbines in factor to understand, in addition to the water depth at the location in
shallow water were first installed over a decade ago. In the last few question, for the following reasons:
years several large-scale commercial offshore wind farms have been
installed in the North Sea, with more to come. The European Wind larger turbines offshore result in lower grid connection costs due to
Energy Association (EWEA) expects that 40GW will be installed having fewer units to connect with subsea cables;
offshore by 2020, a challenging task given the large number of larger turbines offshore result in fewer foundations and turbines to
turbines, the short time-frame and the harsh offshore environment to install and therefore normally lower installation costs than for more
be dealt with. and smaller turbines; and
larger turbines mean fewer units to visit during maintenance.
The enormous potential energy resource offshore, covering more than
70% of the Earth’s surface, can only be tapped into by those countries The problem of ever-larger turbines is the so-called square–cube law,
blessed with access to large shallow-water areas with good wind meaning that if the rotor diameter is doubled (and therefore the swept
resources, simply because it is too expensive to install wind turbines in area and power production is increased four-fold), the weight and
water more than 30–40m deep. ‘Unfortunate’ countries such as Spain, cost of most of the turbine and tower/foundation components
most of the US, Japan, Korea, Norway and many more therefore cannot increase eight-fold due to the length increases in all three dimensions
expect to play a major role in the offshore renewable rally that has now (we live in a 3D world). This is why the development of new structural
emerged in northern Europe. However, is this an accurate observation? concepts for both the turbines and the tower/foundations is required
in order to considerably reduce the cost of energy by increasing the
It is true that the cost of wind turbines per MW installed increases with turbine size and taking advantage of the three major cost-saving
greater depth for fixed bottom-mounted turbines. This is because the factors listed above.
loads (overturning moments) on the foundations increase with greater
depths. The foundations therefore need to be both longer and Most of the planned large offshore windfarms in Europe will be located
stronger, resulting in increasing capital expenditure (CAPEX). Most in fairly deep water of 20–30m or more. In general, monopile
observers will therefore be tempted to draw the conclusion that the foundations have been very successful in shallow water for smaller
cost of producing the energy will always be higher if the water depth turbines of 2–3MW due to the simple construction and corresponding
increases. However, this is not necessarily the case. low cost of €2–3/kg steel fabricated. Jacket and tripod structures are
more cumbersome to weld together and the cost of such structures is
There are two potential reasons for this. The first is easy enough to grasp: typically €5–6/kg. However, in water depths of 20–30m and with larger
if the wind blows more strongly and more steadily further out turbines, conventional monopile foundations become prohibitive due
to sea in deeper water, the additional power production will positively to the insufficient strength and stiffness they can offer. Monopiles are
influence the economy and potentially offset higher investment costs. The simply too soft, and the natural frequency of the tower may coincide
second potential reason will be discussed below and is simply the answer with the natural frequency of the 1-per revolution (1P) rotor frequency,
to the following question: can the total costs of a windfarm actually be resulting in large and unacceptable vibrations. Therefore, new concepts
kept unchanged, or even reduced, when moving out to deeper water by such as jacket and tripod solutions are now being used for the largest
changing the technology from fixed to floating foundations? To answer offshore turbines. The latest 5MW installations have shown that for
this question it is necessary to understand both the cost drivers involved water depths of 30m or more, the jacket solution seems to offer a
and the means of increasing power production from the wind farms. lower weight and cost compared with the tripod.
The net present value of an offshore windfarm and the cost of energy When shifting from a monopile foundation to a jacket, the tower/
depend on several factors, such as the cost of the turbines, towers and foundation stiffness increases considerably, and another rotor
foundations, the cost of installation, the cost of grid connection, frequency potentially triggers tower vibrations, the so called 3-per
life-cycle maintenance costs, annual power production and the price of revolution (3P) rotor frequency of a single blade passing the tower. The
electricity, including public subsidies and finally the risk level, which steel tower on top of a jacket is therefore often narrower to give
strongly influence the required rate of return on the CAPEX. All of the necessary flexibility to avoid 3P vibrations, but with much thicker
these factors except the price of electricity and in some cases the public wall thickness to maintain sufficient strength, resulting in an increased
subsidy levels differ between offshore and onshore windfarms. tower weight. In such a situation, slightly deeper water with a
corresponding longer and more flexible jacket may in some cases
For offshore wind farms, where the turbine size is not limited by actually not considerably increase the total weight and cost of the
transportation on roads, the effect of the turbine size is an important tower and foundation.
© TOUCH BRIEFINGS 2010
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