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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

Integrated analysis of wind turbine and wind farm

Ir. M.B. Zaaijer [email protected] Tel. +31 15 27 86426 Dr. ir. G.J.W. van Bussel [email protected] Tel. +31 15 27 85178 Section Wind Energy1 Fax. +31 15 27 85347 Faculty of Civil Engineering and Geosciences, Delft University of Technology Stevinweg 1, 2628 CN Delft, The Netherlands

1

The Section Wind Energy participates in DUWIND, the interdisciplinary wind energy institute of Delft University

1 Different design approaches

1.1 Traditional (robust) design

In the traditional design process the problem of realising an offshore wind turbine is organised in two main clusters according to the parent technologies, i.e. wind energy technology (comprising wind turbine, tower, farm layout, grid connection design) and offshore technology (related to foundation, marine installation and submarine cables) (Figure 1). The treatment is denoted as robust since conventional design solutions are used (e.g. standard wind turbine with small modifications, stiff foundation) and traditional because of the direct application of experience from the normal scope of work of the parties involved.

System

Demonstration plant

Study of environmental effects Limited risk

Political order / client Project manager

Targets

Prototype

Subsystems Elements

Wind energy technology

Wind turbine (modified) Tower (onshore design) Farm layout Installation (turbine and tower) Grid connection design O&M (turbine and tower)

Offshore technology

Foundation design and construction Ancillary design Foundation installation Submarine cable laying Foundation and cable maintenance

Clusterleaders Engineers

Figure 1 Traditional (robust) design approach

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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Delft University of Technology

2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

Good examples of a traditional design approach are the two early Danish wind farms at Vindeby and Tunø Knob (Table 1 in the next section). These two small wind farms have been realised with fairly standard wind turbines (450 to 500 kW turbines) mounted on gravity base foundations. Almost without exaggeration it can be said that these wind turbines were placed on small artificial islands in a fashion identical to an onshore operation. So it started with the "foundation preparation", which consists of seabed preparation, construction of the gravity base foundation and its installation at site. The subsequent standard operations for onshore wind turbine erection were more or less copied at sea. Thus, placement of tower, attachment of nacelle, assembly of the complete rotor (at land) transport to site (on a floating barge) and finally attachment at the main shaft in the nacelle were performed. The adaptation to operation in the offshore environment was limited to marinising the wind turbines. This means that extra efforts were made with respect to sealing of bearings, avoiding salt water or salt spray access to the cooling system of generator and gearbox, and corrosion protection.

1.2

Parallel design

In a parallel design approach the implications that offshore conditions have on a wind farm are indeed recognised. Still they are considered individually in the main sub-systems as wind turbine, support structure, grid connection, etc (Figure 2). Thus the overall performance is limited to the sum of the separate optimisations of the sub-systems.

System

Demonstration plant or (near) commercial plant

Cost of energy Energy yield Life time & risk

Order

Targets

Customer

Aspectsystems

Marinisation Costs O&M

Costs

Strength & stiffness Installation

Costs Reliability

Projectmanager

Subsystems Elements

Wind turbine

Support structure

Grid connection & farm layout

Clusterleaders Engineers

Figure 2 Parallel design approach Two examples of early adaptation of a parallel design approach are the Nogersund project in the Swedish part of the Baltic Sea and the Lely wind farm in the Dutch IJsselmeer. The first case regards a single 220 kW machine on a fairly short distance from shore. The parallel design issue in this project can be seen in its installation procedure. The whole structure, including a standard wind turbine and a tripod foundation was assembled in a harbour surroundings, commissioned, transported to its offshore site and installed in one operation. So here a real offshore working method was adopted for the whole structure, and not for the offshore part (the tripod) alone. A second example of a parallel approach in realising an "offshore" wind farm can be recognised in the way wind park Lely was realised. A new issue at that time was the use of a steel monopiles as a foundation. This is a rather uncommon design solution for offshore

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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Delft University of Technology

2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

structures, but a logical extension of wind turbine technology practice (tubular towers) into offshore technology. The Lely wind turbines themselves are again slightly modified onshore machines. No attempt however was made to design the dynamics of wind turbine and offshore structure simultaneously. Installation and maintenance procedures are also modifications of the standard onshore practice. Another typical "parallel design" aspect of the project is the way the towers are fixed on the monopiles. On top of the pile a plug was grouted having a ring with standard bolts that fit to the bottom flange of the (standard) wind turbine tower. Table 1 below shows an overview of the offshore wind farms that have been realised up to the year 2000. Almost all these projects are realised along either the robust route or the parallel approach. The most advanced in terms of integrated design is probably the Utgrunden project. Table 1 First offshore wind farms, up to the year 2000

Location Nogersund, Baltic (Sw) Vindeby (Dk) Lely, Medemblik, IJsselmeer (Nl) Tunø Knob (Dk) Dronten, IJsselmeer (Nl) Bockstigen Valar, Baltic (Sw) Middelgrunden (Dk) Utgrunden (Sw) Blyth Harbour (UK) Year 1991 (abandoned in 98) 1991 1994 1995 1996 1998 2000 2000 2000 Installed Power (MW) 1 x 0.22 = 0.22 Windworld (NEG Micon) 11 x 0.45 = 4.95 Bonus 4 x 0.5 = 5 NedWind (NEG Micon) 10 x 0.5 = 5 Vestas 28 x 0.6 = 16.8 Nordtank (NEG Micon) 5 x 0.5 = 2.5 WindWorld (NEG Micon) 20 x 2 = 40 Bonus 7 x 1.425 = 10 Tacke 2x2=4 Vestas Location in 7 m water, 250 m from shore in 3-5 m water, 1.5 km from shore in 5-10 m water, 750 m from shore in 3-5 m water, 6 km from shore in 5 m water, 20 m from shore in 5.5-6.5 m water, 3 km from shore in 3-6 m water, 3 km from shore in 7-10 m water 8 km from shore in 8 m water, 800 m from shore Foundation details Tripod on solid rock; Box caisson on sandy soil, Steel tower driven in sandy soil, corrosion lifetime 50 years, in fresh water. Box caisson on sandy soil Turbines located on monopiles a few meters from the dike, in fresh water 2.25 meter steel monopiles secured by grouting in 10-m deep holes drilled in limestone rock. Concrete caissons resting on sea bed First soft-soft monopile foundation (low eigenfrequency of support structure) The first North Sea wind farm, monopile foundation

1.3

Integrated design

Design practice applied so far for the design of offshore wind farms fitted the objectives of demonstration plants. Such practice will however not be suitable to arrive at commercial, large-scale offshore wind farms to be erected in the coming decennium, and a highly integrated design approach had to be adopted. This refers not only to the interaction between the turbine and the support structure but also to an integrated treatment of all major wind farm parameters in all the design phases. Moving wind turbines further offshore will require major changes in the design, far beyond the obvious marinisation of the components open to the environment, if the optimum solution is to be used. Greater freedom in the design is also possible, regarding aspects such as the noise, visual impact and electrical aspects. A framework for an integrated design methodology recently developed for large, complex civil engineering projects can be applied as well for the design of future large-scale offshore wind farms. For this approach it is at first important to determine the client. Once the client is clearly identified his goals are known and can be translated into quantifiable criteria at the system level, the aspect-systems. Such quantifiable criteria facilitate the control of the design process in order to achieve the goals of the client (Figure 3).

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

Society / Political order Life time & risk Customer

System

Delivery of substantial electricity

Energyquality and quantity

Targets

Profit

Aspectsystems

Costs per kWh

Location

Dynamics

Installation

Availability

Projectmanager

Subsystems Elements

Wind turbine

Support struct. & installation

Grid connection & farm layout

Operation & maintenance (O&M)

Clusterleaders Engineers

Figure 3 Integrated design approach An example of a set of such aspect-systems is: · · · · · · levelised (sometimes called lifetime) production costs adaptation to the actual site conditions, dynamics of the entire system, structural integrity of the design, installation and commissioning efforts availability of the offshore wind farm.

From the aspect system level the total system is furthermore split into sub-systems and elements for efficient control of the design steps. Interactions between sub-systems have to be considered in a complete though manageable form as possible so that the selection of the final design solution is governed by overall criteria. Although a genuine integrated approach will most likely produce an innovative design solution, the evolutionary nature of technical progress and the commercial risks inherent to innovative solutions must be kept in mind. In particular it would be a serious mistake to interpret it as a justification for the application of unproven wind turbine design to the harsh offshore conditions

2 Examples of analysing aspect systems

2.1 Cost aspects

2.1.1 LPC cost model

The Levelised Production Cost (LPC) is the cost price of production per unit of energy, expressed in actualised nominal money. This means that the levelised production cost expresses the production cost in terms of current purchasing power, allowing comparison with other current costs of energy. The LPC is one of the main assessment criteria of a wind farm and recommended practises for its calculation are given by IEA.

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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

The following expression is used to calculate the LPC:

LPC =

where: Ct Rt Et T r

(C

t =0 T

T

t

- Rt )(1 + r )

-t t

-t

E (1 + r )

t =0

,

= total ongoing costs for year t, = total ongoing revenues for year t, = total energy production in year t, = economic lifetime, = real interest rate, defined by:

1+ r =

with: i v

1+ i 1+ v ,

= annual interest rate on debts, = annual inflation rate.

In the LPC calculation the costs and energy production are weighted with the effect of interest and inflation on the cash flow during the lifetime of the wind farm. Common values to apply are a 20 year economic lifetime and 5% real interest rate. The strength of the LPC as an assessment criterion is the simplicity of the model for economic effects. However, this simplicity also implies that more complicated economic and market effects are not presented. Furthermore, the wind farm design with the lowest LPC is not necessarily the most desirable. The LPC has to be weighted against other targets and constrictions, such as energy yield per square kilometre, predictability and continuity of energy production and liquidity.

2.1.2 Cash flow

The cost of energy of a wind farm can be decomposed into three major contributions, with a typical division over time. The largest contribution is the investment cost at the beginning of the wind farm life. The second largest contribution are operation and maintenance costs, that are distributed over the entire lifetime and finally the decommissioning costs contribute at the end of the wind farm's life. A straightforward expression of these contributions is obtained under the following assumptions: · · · · energy production starts after the construction period, annual O&M costs are constant in terms of purchasing power, annual energy production is constant. decommissioning takes place immediately after the last year of production.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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Delft University of Technology

2. Symposium offshore-windenergie The resulting LPC equals:

Integrated analysis of wind turbine and wind farm

C invest + LPC =

t = t ' +1

C

T

O&M T

(1 + r )-t + C D (1 + r )-T

-t y

t = t ' +1

E (1 + r )

' C C C (1 + r ) = invest + O & M + D aE y Ey aE y

-T '

with: Cinvest C'invest CO&M CD a = investment costs, = investment costs including interest during construction, =annual operation and maintenance costs, = decommissioning costs, = annuity factor, defined by:

a = (1 + r )

t =1

T'

-t

1 1- 1+ r . = r

T'

The annuity factor in the contribution of investment costs reflects a constant annual amortisation in terms of purchasing power. The investment costs are slightly increased due to interest. The effect of interest causes a large reduction of the impact of decommissioning costs. Operation and maintenance costs are not affected by interest, because they are paid from the revenues of the same year.

2.1.3 Cost components

In the wind farm design process the LPC must be broken down to a sufficient level of detail. The following main components can be identified as contributors to the costs of the wind farm: · · Design and acquisition Construction · Turbine · Support structure · Electrical infrastructure · Transportation and installation · Project management Operation and maintenance · Maintenance · Retrofit of turbines · Other recurring costs, such as social-, insurance- and administrative costs Decommissioning · Decommissioning activities · Revenues from salvaged materials and components

·

·

The level of detail of the breakdown depends on the desired accuracy of the results. Assessment of this accuracy is difficult, but insight may be gained through parameter variations.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

2.1.4 Energy yield and losses

Several aspects contribute to the energy production of the wind farm. As for the cost components, the breakdown and modelling of energy production depends on the required accuracy. When the turbine has a technical availability of 100% for energy production its annual energy production, E, can be estimated from:

E = P (Vi ) f (Vi ) ,

i

where: P(Vi) f(Vi) = power of the turbine at wind speed Vi, = time per year in wind speed band with reference Vi.

The power of the turbine can be further specified by:

P(Vi ) = ½ AVi 3 ARotor Rotor Drive train Generator ,

where: A = air density, ARotor = surface of rotor plane, Rotor = aerodynamic rotor efficiency, Drivetrain = drive train efficiency, Generator = generator efficiency. The efficiencies are different for different wind speeds. Several aspects that relate to the design and operation of the wind farm reduce this technically achievable energy production per wind turbine. The four most common and important ones are: · · · · technical availability of the turbine, technical availability of the farm electrical infrastructure, reduced wind speed in wakes of upwind turbines, transformation and transmission losses in the farm electrical infrastructure.

The technical availability of the turbine depends on the wind turbine design through failure behaviour and on wind farm operation through the O&M strategy. The technical availability of the farm electrical infrastructure affects single turbines, clusters of turbines or even the entire wind farm. The reduced wind speed in wakes is commonly expressed in an array efficiency that integrates the effect for the entire wind farm for different wind directions and wind speeds. Transformation and transmission losses differ for full load and partial load of the electrical infrastructure. Several additional issues could be considered for special types of wind farm design and operation. For instance the phased construction of the farm, with intermediate start-up of clusters of turbines could be beneficial for large wind farms. Intentional operation of the wind farm at less than maximum power may be profitable, because of better power control and use of the farm as a peak load plant.

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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

2.2

RAMS aspects

2.2.1 Introduction

RAMS regards the complete issue of Reliability Availability, Maintainability and Serviceability in all phases of the design process. Commercial onshore wind turbines show very high availability levels nowadays. With a proper service organisation and by ensuring that regular maintenance and repair actions are quick and can be performed in a minimal time the operators of modern wind turbines achieve an availability level of 98% or sometimes even beyond. It must be stressed however that this is achieved through visiting a wind turbine about three to four times a year, either for regular service of for curing (repair) actions. In situations where both limited access and limited availability of maintenance equipment are at stake, such as for the offshore environment, this may easily lead to an unacceptable down time level. This makes it inevitable to assess the O&M demand of an offshore wind farm in conjunction with the other design parameters in order to achieve the required availability level against optimal cost expenditure. The latter being a trade-off between investment costs in order to increase the reliability and the cost and time of maintenance actions to boost the availability to a high level. A number of observations can be made when extrapolating the current onshore situation: · · · The availability of wind farms at real offshore sites employing commercial wind turbines without significantly improved reliability and without optimised operation and maintenance solution may be unacceptably low, e.g. 70% or even less. Reliability of offshore wind energy converters and operation and maintenance solutions should be optimised with respect to the LPC rather than either to capital or to operation and maintenance costs. Operation and maintenance costs mainly related to the wind turbine can account for up to 30% and more of the energy costs.

2.2.2 The reliability dilemma

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 also 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 this 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 service level

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 wind farm have to be analysed for each specific site in a comprehensive way in order to determine the optimal reliability level of the wind turbine.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

2.2.3 O&M strategy

Of the possible maintenance strategies there is a number that may turn out to be adequate for future offshore wind farms: · · · Preventive and Corrective maintenance Opportunity based maintenance Condition based maintenance

The first possibility is more or less a continuation of the onshore service and repair practice. It might be feasible under the assumption that the reliability of the wind turbines is improved significantly, and that the service demand is reduced to say one every 12 to 18 months. Opportunity based maintenance may come up as a good offshore practice. Preventive maintenance is only performed on the occasions that a failure has occurred. The repair and service actions are then combined and the number of (expensive) visits to the turbines is reduced. Such an approach is only possible when the wind turbine is designed for a very flexible servicing period. Condition based maintenance assumes a certain level of condition monitoring in the wind farm. The maintenance crew only carries out service or maintenance when the condition of components indicates the necessity of a visit. This is a rather ultimate form of reducing the number of visits, but requires an extra investment in monitoring systems. An extreme strategy to provide no maintenance at all is evidently the least demanding option with respect to the number if visits. Another advantage is that cranage operations can be done batchwise, which reduces costs significantly. The big disadvantage is loss of production capacity. Simulations showed that a reduction of the (total) failure rate to a value significantly lower then 0.2 failures/year would be necessary to obtain a reasonable availability without undertaking any corrective maintenance over a period of 3 to 5 years. This would mean a factor of about 10 with respect to the current reliability level. It is clear, in this light, that offshore wind farms that adopt such a maintenance strategy are not feasible options for the near future.

2.2.4 Installation and O&M hardware

Since offshore work is between 5 to 10 times more expensive than on land, reduction of installation and maintenance effort is essential. For instance, gravity base support structures that can be floating fully equipped to their final destination offer reduced installation effort in comparison to lifted configurations. Another example of particular importance is the solution of the so-called cranage problem. Apart from wind turbines that are designed such that they can entirely rely on internal cranes or on add-up cranes a heavy crane is required about every five years. The day rate for general purpose equipment from the offshore oil and gas industry can be such that it is better to purchase cranage together with the installation of the farm and use it also for maintenance. A good proposal might be a permanently available modified jack-up platform for a large scale offshore wind farm (typically of the order of a few hundred megawatts). It can be used during installation and it would be a solution, which reduces the maintenance cost, ensures quick repair work leading to high availability and can serve as a permanent basis for maintenance crew. When the design of the wind turbine is sufficiently tuned it might be even possible to use such a jack up as a maintenance base where complete nacelles including blades can be overhauled and prepared for replacement of a failed turbine.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

3 Integrated approach in the design process

3.1 Wind farm optimisation procedure with respect to primary targets

A future offshore wind farm will comprise a large number (about 100) of offshore wind energy converters (i.e. wind turbine and support structure), a grid connection system and an integrated operation and maintenance strategy. Only as one entire system the offshore wind farm can provide a considerable amount of electric power in a reliable and cost-efficient 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 wind farm and over its lifetime design optimisation of sub-systems with respect to `global' goals, e.g. minimum energy price, wind farm availability, overall dynamics, etc high reliability system as a whole and of essential sub-systems e.g. grid connection system, wind turbines, etc adaptation to economy of scale, e.g. wind farms with about 100 units or more symbioses of experience from wind energy and offshore technology

High investment costs for the fixed cost elements, i.e. support structures and grid connection, favour large, multi-megawatt converter units and large wind farms with a capacity of at least 100 to 200 MW. The latest commercial wind turbines in the so-called `megawatt class', i.e. ratings between 1,5 MW and 2 MW, offer a good starting point for large OWECS. However, even greater turbines of 3 to 5 MW or even more might be promising for offshore sitting in the future. Under such conditions the cost breakdown between the major subsystems (wind turbine, support structure and grid connection) is nearly equally shared. Thus cost reduction 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 The following stepwise approach can be identified in a wind farm optimisation procedure: · Project identification The entire process starts with the project identification: a potential site and an initial wind farm size, as well as identification of the stakeholders an operator and an initial statement of the project objectives Problem statement It defines the objectives of the design (e.g. delivery of a certain amount of electricity from offshore wind energy at a certain location and over a certain time span), the acceptable price (e.g. investment costs, costs of energy) and the demanded quality (e.g. power variation, grid influence, etc). The primary targets are thus clearly defined in this phase of the process. Overall realisation plan Based upon the characterisation of the design problem a feasibility study is carried out which obtains a functional description of a solution. The requirements are refined with respect to criteria as function, technology, construction and economics Preliminary or conceptual design The overall realisation plan forms the input to the conceptual design. The definition of main dimensions and site result in the preliminary design and the final requirements on function, technology, construction and economics Structural pre-design This design phase yields the solution for the problem of the client based on the concept and the site chosen at the end of the conceptual design phase. Again the design work of

·

·

·

·

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

·

·

the sub-systems is done in parallel. However, now the interactions between the subsystems are taken into account as complete as possible. For instance, between wind turbine and support structure design a number of iterations might be necessary until the design solution has converged. In a similar fashion reliability options and maintenance strategies are iterated mutually to comply with the primary targets and with the secondary goals defined in the conceptual design phase. Final structural design Next materials and components are chosen, dimensions are fixed, structure i.e. relations between sub-systems is generated and the final design is documented as result of the structural design. System description and detailed specification. In the last step of the design process detailed engineering or elaboration of the final design in the design reports, specifications and drawings for the realisation (i.e. construction, installation and commissioning) and for the exploitation are obtained. Further the use of resources and the data flow is planned.

3.2

Optimisation aspects of wind farms

3.2.1 Identification of stakeholders

The entire design process starts with the project identification, an identification of the stakeholders and the statement of the project objectives. Stakeholders for offshore projects can be utilities, public organisations, private companies, banks etc. Stakeholders will define some pre-conditions or demands that have to be considered in the project definition. These conditions will mainly regard the economical viability and/or the operational goals of the operational wind farm. Stakeholders can also be external parties that will become involved in the project in some phase. It is of a paramount important that the important external stakeholders are identified as well in the very early phases of the project.

3.2.2 Definition of primary targets

Often the operator of the offshore wind farm, which can be a utility, a private company or public organisation) will also define some goals. One of the most important objectives is the primary target of the offshore wind farm. Is it e.g. a demonstration or a commercial plant; does it aim at the lowest energy costs, the maximum amount of profit, or a certain minimum amount of power. Are there limitations of short term and long term power variations, etc. etc. All these objectives will have a large impact on the optimum design. After approval of the primary targets of the stakeholders and the operator (which can be the same body (bodies)) these should be documented as a primary reference document for the further development of the design process.

3.2.3 Position of manufacturer and the offshore contractor.

At the development of present onshore wind farms the position of the wind turbine manufacturer is a rather central one. Often wind turbine manufacturing companies are operating together with sister companies that perform the project development, and not seldom they operate the wind farms themselves and sell the electricity to a utility. With the development of future offshore wind farms the prominent position of the manufacturer will be quite different. A utility, a private company or a public organisation will perform the project co-ordination, and both wind turbine manufacturers and offshore construction- and operations companies will act as subcontractors. This may have major consequences to the way the wind turbine manufacturers organise their design and production process. In stead of a few standard products that can be sold to several projects, the client will demand dedicated design, in conformity with the primary targets of the stakeholders and the operator of the wind farm.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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Integrated analysis of wind turbine and wind farm

3.2.4 Wind farm site and lay-out selection

Some of the choices for the wind farm layout may be restricted by external conditions or by the targets set for the wind farm. The optimisation of several of the other issues has interactions with other parts of the design. Most aspects of the wind farm site selection have a major interaction with the design of the electrical infrastructure. Evidently, distances between turbines, distance to shore and the definition of clusters affects cable lengths and routing of the cables. Indirectly, this may affect optimal voltage levels and the selection between AC and DC transmissions. The optimisation of the layout of the farm depends largely on the costs and losses of the electrical transmission balanced against the aerodynamic losses caused by wakes. The reduced average wind speed in the wake leads to power losses with respect to the undisturbed average wind speed. Since the ambient turbulence in the offshore environment is relatively mild mixing of the wake with the ambient wind is slower than onshore. With a small spacing the effect of wakes on turbine loading may also become important. The wake vortex, leading to increased turbulence, gives higher fatigue loading on downwind rotors. When the site is not fixed, its selection has a major interaction with the reliability of the turbine and the support structure design. The reliability of the turbine will become more important for remote and exposed sites, with worse accessibility. Due to variation in the loading environment, water depth and soil conditions, the support structure design is very dependent on site selection and different concepts may be selected at different sites. If soil conditions and water depth vary significantly within one site the support structure design may even vary within the farm. The four wind turbines of the Lely wind farm near Medemblik are stall controlled, constant speed machines. So there is a "forbidden" area of resonance frequencies for the monopiles. But soil conditions varied a lot so that there are three different monopiles for the four wind turbines. Site selection has a minor interaction with power performance of the turbine. For instance, different sites will have a different optimal rated wind speed.

3.2.5 Wind turbine and support structure selection

The OWEC consists of many components for which different concepts can be selected. Several of these concept choices interact with other parts of the OWEC, or with other aspects of the wind farm. Some examples are given, although much more interactions could be identified. Exclusive interactions are often very straightforward, but very essential for the concept selection. For instance, the number of blades and the hub concept have an exclusive interaction since the teetering hub can only be combined with a two-bladed rotor. The jacket or tower and the foundation also have an exclusive interaction. The monopile only combines with the monotower and suction piles are virtually impossible as foundation for a monotower. The generator has a major interaction with the gearbox. The transfer ratio of the gearbox determines the rotational speed of the high-speed shaft in the generator, which is an important design parameter. The direct drive generator exemplifies this interaction. A related minor interaction exists between generator, optimum tip speed ratio and rotor diameter. The support structure interacts in many ways with turbine concepts. In the first place, the loading of the turbine on the tower top is a major interaction. This relates both to the magnitude and the characteristic excitation frequencies of the load spectrum. Important aspects of the turbine are in this case the number of blades, pitch or stall control and speed control. The operation of the rotor in downwind position has a major interaction with the support structure concept, because truss towers have less wake in the downstream airflow.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

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Integrated analysis of wind turbine and wind farm

Required tower clearance for the blades gives a minor interaction between tower dimensions, blade flexibility and nacelle tilt and overhang. This interaction is most important for truss towers that have larger cross-sectional dimensions. The support structure interacts with operation and maintenance activities, because it has to provide access to the turbine and because it may provide facilities for lifting of larger components or a self-elevating nacelle.

3.2.6 Electrical infrastructure and Power control

The main target of the design of the electrical infrastructure with respect to the cost of energy is the collection and transportation of electrical energy to shore with minimum costs and minimum electrical losses. There is a close interaction with the design of the electrical infrastructure in the turbine. For instance, voltage level and type of generator are important aspects for the design of the farm electrical infrastructure. A more involved interaction between the electrical infrastructure and the turbine is found in power control. The turbines have individual power control by constant or variable speed operation and by pitch or stall control. Variable speed control may also be implemented for clusters of turbines that are connected to a sub-grid operating at its own frequency. This saves inverters at the individual turbines. There may also be a central power control for the wind farm, for instance to reduce the total power output. This could be implemented to operate the wind farm as a peak load plant or to obtain better power quality. The central power control can regulate shut-down or power reduction of individual turbines or clusters. These types of integrated solutions of power control require careful consideration of the effect of the cluster or farm control on the dynamics of the OWEC. Particularly, cluster control affects short-term dynamic behaviour and stability and farm control affects long-term fatigue of the OWEC. The latter effect occurs, because the fatigue of the OWEC depends on the operational status of the turbine.

3.2.7 Assembly, transport, installation and decommissioning procedures

Methods applied in current offshore wind energy projects build on the experience of offshore industry and onshore wind energy as separate disciplines. A consequence of this approach is an expensive installation method, particularly due to many lifting activities. In order to reduce costs of transportation and installation, procedures have to be developed that are tailored for offshore wind farms. This includes an integral approach of assembly, transportation and installation, but also parts of the OWEC design are involved. There is an exclusive interaction between the support structure concept and the installation procedures. For instance, gravity base foundations enable assembly of the entire OWEC in a dry-dock, after which it is towed to the site. This is not possible for a monopile foundation. Also, two-bladed and three-bladed rotors have different implications for transportation and installation.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

13

Delft University of Technology

2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

A summary of procedures to be considered for the topside of the structure is given in Table 2 below: Table 2 Summary of possible procedures for installation of tower, nacelle and rotor Assembly Onshore (land or dock) Offshore Transportation Self-floating Self-floating with auxiliary floating elements (may be purpose built) On pontoon Installation With large external crane, lifting tower top Tower With smaller external crane lifting above mass centre With smaller external crane erecting tower with hinge connection at tower base Nacelle* With large external crane With internal crane at tower top Self-elevating along tower Hub / Blades* With large external crane With internal crane at tower top

*The internal crane and self-elevating nacelle may be beneficial for maintainability

An important difference between installation of a wind farm and installation of a conventional offshore structure is the large number of identical structures that need to be installed. The procedures for offshore activities must aim at an optimal use of equipment and short installation times per turbine. For large wind farms this includes the optimisation of (parallel) batched procedures to balance the costs of equipment and the duration of the total installation of the farm. Adaptations to existing equipment or even development of special purpose equipment may prove profitable, because of the repeated use. This equipment may also be used during maintenance or in the installation of other wind farms. The cost reduction is expected particularly for the many high lifting activities of components with relatively low masses. This might be performed with lighter vessels and in shorter times.

Acknowledgements

This paper is based to a large extend on notes for the lecture "Integrated wind farm design", which is part of the course "Technology of offshore wind energy", provided by DUWIND. However, much of the ideas presented here were developed during the Opti-OWECS project and the thesis work of Mr. M. Kühn, which is gratefully acknowledged. Documentation of both investigations can be found in the bibliography.

Ir. M.B. Zaaijer - Dr. ir. G.J.W van Bussel

14

Delft University of Technology

2. Symposium offshore-windenergie

Integrated analysis of wind turbine and wind farm

Bibliography

[1] Barltrop, N.D.P., Adams, A.J., Dynamics of Fixed Marine Structures, Butterworth-Heinemann Ltd, Oxford, 1991. [2] Cockerill, T.T., Opti-OWECS Final Report Vol. 5: User Guide OWECS Cost Model, Institute for Wind Energy, Delft, 1998. [3] Cockerill, T.T.; Harrison, R.; Kühn, M.; Bussel, G.J.W. van, Opti-OWECS Final Report Vol. 3: Comparison of Cost of Offshore Wind Energy at European Sites, Institute for Wind Energy, Delft, 1998. [4] Ferguson, M.C., Structural and Economic Optimisation of OWEC Support Structure, In: Proceedings of the European Seminar on Offshore Windenergy in Mediterranean and Other European Seas, Held in La Maddalena, ENEA, Italy, April 1997. [5] Ferguson, M.C. (editor); Kühn, M.; Bussel, G.J.W. van; Bierbooms, W.A.A.M.; Cockerill, T.T.; Göransson, B.; Harland, L.A.; Vugts, J.H.; Hes, R., Opti-OWECS Final Report Vol. 4: A Typical Design Solution for an Offshore Wind Energy Conversion System, Institute for Wind Energy, Delft, 1998. [6] Hendriks, H.B., e.a., DOWEC Concept Study, Evaluation of Wind Turbine Concepts for Large Scale Offshore Application, In: Proceedings of the European Seminar on Offshore Windenergy in Mediterranean and Other European Seas, Held in Siracusa, ENEA, Italy, April 2000. [7] Kühn, M., Dynamics and Design Optimisation of Offshore Wind Wind Energy Conversion Systems, PhD thesis, Delft University of Technology, Delft, 2001. [8] Kühn, M.; Bierbooms, W.A.A.M.; Bussel, G.J.W. van; Ferguson, M.C.; Göransson, B.; Cockerill, T.T.; Harland, L.A.; Vugts, J.H., Opti-OWECS Final Report Vol. 1: Integrated Design Methodology for Offshore Wind Energy Conversion Systems, Institute for Wind Energy, Delft, 1998. [9] Kühn, M.; Bierbooms, W.A.A.M.; Bussel, G.J.W. van; Ferguson, M.C.; Göransson, B.; Cockerill, T.T.; Harrison, R.; Harland, L.A.; Vugts, J.H.; Wiecherink, R., Opti-OWECS Final Report Vol. 0: Structural and Economic Optimisation of Bottom-Mounted Offshore Wind Energy Converters, Executive Summary, Institute for Wind Energy, Delft, 1998. [10] Kühn, M. (editor); Bussel, G.J.W. van; Schöntag, C.; Cockerill, T.T.; Harrison, R.; Harland, L.A.; Vugts, J.H., Opti-OWECS Final Report Vol. 2: Methods Assisting the Design of Offshore Wind Energy Conversion Systems, Institute for Wind Energy, Delft, 1998. [11] Nitteberg, J. (editor); Boer, A.A. de; Simpson P.B., Expert Group Study on Recommended Practises for Wind Turbine Testing and Evaluation, 2. Estimation of Cost of Energy from Wind Energy Conversion Systems, IEA, 1983. [12] Zaaijer, M.B., e.a., Starting-point and Methodology of Cost Optimisation for the Conceptual Design of DOWEC, Section Wind Energy, Delft, March 2000.

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Delft University of Technology

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