Gasch R., Twele J. (Eds.) Wind Power Plants: Fundamentals, Design, Construction and Operation

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522 16.1 Offshore environment

ambient turbulence

wake turbulence

ship impact

(breaking) ice

marine growth

salinity, humidity

& temperature

currents tides

(breaking) waves

earthquake

operational

& accidental loads

icing

scour

foundation

behaviour

lightning

grid interaction

ambient turbulence

wake turbulence

ship impact

(breaking) ice

marine growth

salinity, humidity

& temperature

currents tides

(breaking) waves

earthquake

operational

& accidental loads

icing

scour

foundation

behaviour

lightning

grid interaction

Fig. 16-3 Environmental impact on an offshore wind farm

Perhaps the most important consequence of the harsh offshore environment is the

high design requirements. Turbines must be able to function unmanned over a

long repayment period (up to 20 years) at a remote location that is difficult for

maintenance personnel to access.

Using an example, we illustrate some fundamentals of wind-induced wave

loading. Generally, one has to distinguish between sea conditions generated by

local winds and swells originating from waves that have travelled a long distance.

The topography of the North Sea and the Baltic Sea, for instance, leads to less sig-

nificant swells.

The periods of energy-rich waves range between 2 s and 20 s. The energy of a

sea state is proportional to the square of the significant wave height which itself is

proportional to at least the second power of the zero-crossing period. Extreme

waves have long periods, typically 7 to 13 s. Nonetheless, waves of moderate

height are most important for dynamic wave loading and associated fatigue since

their occurrence is high - 1 to 5 million waves per annum - and their periods are

closer to the fundamental natural period of the structure.

The water surface profile of a wind-sea is random. For design calculations,

such a sea state is decomposed into many elementary, regular waves. When con-

sidering extreme loading, one dominant, regular wave is often isolated. Assuming

a wave height H, which is small compared to the water depth d, the linear, Airy-

wave theory describes the water surface profile Ș as a sinusoidal function of the

circular wave frequency Ȧ and time t (Fig. 16-4).

)cos(

)( t

 (16.1)

16 Offshore wind farms 523

Depth d

Wave length L

Pile diameter D

Wave

hight

Depth d

Wave length L

Pile diameter D

Wave

hight

Fig. 16-4 Surface profile and water particle velocities of a small amplitude wave and wave

forces on a slender vertical pile

T/2







Lk /2





(16.2)

The propagation of the wave profile results from the motion of the water particles

in stationary, elliptical orbits. The extension of the paths, and the associated parti-

cle velocities, accelerations and forces, decay exponentially with depth (Fig.

16-4).

Waves approaching the shore become steeper as wave height increases and

wave length decreases. The wave 'feels' the sea bottom and breaks when the height

is equal to 78% of the water depth under ideal conditions. The transcendental dis-

persion equation expresses the relation between wave length or, more precisely,

the wave number k and the wave frequency ω for a certain water depth d. To find

the wave number, an iterative process may be started using the deep water case,

k = ω

/ g , where g equals the gravity acceleration.

)(tan

dkhkg



(16.3)

Wave loading consists of:

 Drag loading, caused by vortices generated in the flow as it passes the struc-

ture. The associated forces are proportional to the square of the relative particle

velocity both in steady currents and in waves.

It is more convenient to use the circular wave frequency ω instead of the wave

T. Analogously, the so-called “wave number” k is defined in respect to the

wave length L.

period

524 16.1 Offshore environment

x Inertia loading, generated by the pressure gradient in an accelerating fluid mov-

ing around a member. The inertia force is proportional to the acceleration of the

water particles.

x Diffraction loading, a part of the inertia force on a structure which is so large

that the wave kinematics are altered significantly by its presence, i.e. when the

diameter of the member D is larger than one fifth of the wave length L.

x Water added mass, an inertia force experienced by a member accelerated rela-

tive to the fluid.

x Slam and slap loading, an inertia force proportional to the square of the relative

velocity when a member passes through the water surface e.g. resulting from

the impact of a breaking wave.

In most cases, diffraction is not significant and the Morison wave force equa-

tion with the empirical drag and inertia coefficient C

| 0.7 and C

| 2.0 can be

applied [4]. For a cylindrical, vertical pile with diameter D the amplitudes of the

drag and inertia wave force can be integrated analytically over the water depth.

1)2(cosh

2)2(sinh





dkdk

HDC

(16.4)

HDC

ZUS

(16.5)

Here

denotes the water density. The drag force varies with a cos

-function while

the inertia force has a sinusoidal relation on time. The maximum of the total hori-

zontal wave force can be found by numerically evaluating the quarter of the wave

period in which both components reach their extremes.













tFttFF

ZZZ





sin

coscos

max

(16.6)

16 Offshore wind farms 525

Fig. 16-5 Kinematics and forces of an inertia dominated fatigue wave calculated with linear

wave theory

The particle kinematics and associated forces for two waves are shown in relation

to a monopile with a 3 m diameter and in a water depth of 10 m. For a typical

fatigue wave with a height of 2 m, the inertia force dominates and the behaviour is

linear (Fig. 16-5).

In contrast, both the water kinematics and the loading become non-linear in ex-

treme conditions. Fig. 16.6 shows the kinematics and loads associated with a wave

of 6.4 m height computed with a non-linear wave theory. The wave crest of 4.95

m is more than three times higher than the trough. This relation yields high parti-

cle velocities in the crest and a drag force that is almost the same magnitude as the

inertia force. The maximum horizontal wave force has a lever arm approx. equal

to the water depth and the resulting overturning moment at the seabed contains

several higher harmonics with significant excitation energy.

526 16.2 Offshore design requirements

Fig. 16-6 Kinematics and forces of an extreme wave in shallow water calculated with the non-

linear, 10th order stream function wave theory

16.2 Offshore design requirements

What are the design drivers for offshore wind turbines? It is often argued that the

economic viability of offshore projects will depend on the use of very large (i.e.

5 MW or larger) turbines. This certainly holds for exposed sites with water depths

above 25 m and a large distance to shore which is both typical for German off-

shore sites. Easily, the reduction of the weight of the wind turbine, which

increases disproportionately with R

2,5

to R

(see Table 7.1), has the most influence

on the dimension of the structure. Therefore, the use of new control concepts, in-

novative drive train designs and innovative materials is often required. High infra-

structure costs, e.g. for the high-voltage grid connection, result in a minimum

profitable project size of about 100 MW and larger.

First of all, the high-quality requirements for offshore wind turbines have to be

met before large projects with large wind turbines can become viable. Therefore,

offshore-specific design requirements are often integrated into conventional wind

16 Offshore wind farms 527

turbine concepts. Such an approach would emphasise a reduction in operation and

maintenance (O&M) costs, overall risk control, and the balance of investment

costs over the entire wind farm.

Four qualitative rather than quantitative offshore design requirements can be

formulated:

- Integrated design of offshore wind farms

A more cost-efficient and reliable design can be achieved through an inte-

grated design approach that considers the entire offshore wind farm as one

system (Fig. 16-2). The optimum offshore design will differ from land-

based designs since the investment will be differently distributed among

the sub-systems and since O&M costs will be higher. Dynamics and sup-

port structure design should be adjusted according to the particular site

conditions and the turbine design.

- Design for RAMS (Reliability, Availability, Maintainability & Serviceabil-

ity)

Operation and maintenance aspects are a main design driver for offshore

wind farms. Remote locations and poor weather conditions can postpone

maintenance and result in longer downtime and greater losses of produc-

tion. Therefore, the standard for offshore turbine reliability must be even

higher than the already rigorous standards for land-based turbines. A

commitment to reliability, in conjunction with dedicated machine design,

comprehensive O&M strategies and specific maintenance hardware results

in acceptable availability and reasonable operating costs.

- Smooth grid integration and controllability

Future large-scale offshore wind farms will be operated as power plants,

and will be a part of national, and possibly international, energy systems.

Offshore wind farms must therefore be smoothly integrated into the grid

and turbines must be controllable under normal operation and in case of

grid disturbances. When combined with other considerations such as dy-

namic loading, these requirements rule out traditional stall control systems

in favour of variable-speed, variable-pitch designs.

- Optimised logistics and offshore installation

Sea-based wind farms afford the opportunity for considerable logistical

optimisation despite high cost. Manufacturing, transport and installation

can all be optimised when the number of heavy components to be lifted is

reduced and pre-commissioning can be done in the harbour.

528 16.3 Wind turbines

16.3 Wind turbines

It is clear that the offshore environment places demands on wind turbines that are

not present in an onshore context. It is also evident, however, that offshore wind

turbines, above the height of the wave impact, are not very different from onshore

machines. Two technical approaches have been taken in developing offshore wind

turbines: the marinisation of robust and proven onshore solutions and the integra-

tion of offshore-specific concepts into radical new designs.

In the marinisation process, modifications are made to onshore designs when

they are transferred to offshore sites: maintenance aids that enable on-site repair of

some components are added, climate and corrosion protection for the nacelle is

improved, control capabilities are enhanced, and the specific rating, i.e. installed

capacity per swept rotor area, is partially increased. An air-tight nacelle is some-

times proposed but this obliges to employ external heat exchangers for the gear-

box, the generator and the power electronics. In addition, an air-tight nacelle re-

quires an expensive and energy-consuming cooling system for the other smaller

sources of radiated heat. For these reasons, most designs rely on a high standard of

corrosion protection and the capsulation of individual components instead of an

air-tight nacelle. The REpower 5M, which was first built in 2004, is an example

for a marinized turbine using a conventional design concept in the 5 MW class.

Again two different trends have emerged from the integration of offshore re-

quirements into the design process: a focus on reliability and serviceability and the

development of advanced lightweight, high-tip-speed designs. Another example of

new design concepts is the Multibrid M5000 turbine which was first installed in

2004 as well. The weight of the nacelle is reduced by using a highly integrated

drive train. Moreover, it features the avoidance of high-speed stages of the gear-

box by matching a one-and-a-half-stage planetary gear with an intermediate-speed

synchronous generator with permanent magnets (Fig. 3.13). So this drive train

concept is situated between the traditional Direct-Drive design (e.g. Enercon) and

the commonly used designs with a multi-stage gearbox with a high-speed genera-

tor. The nacelle of the Multibrid M5000 is enclosed against the maritime environ-

ment and cooled by heat exchangers.

The ongoing rapid growth in turbine size and the need to base production on

proven technology favours the horizontal axis, upwind-oriented turbine design that

is used in land-based solutions. Though all offshore turbines are currently three-

bladed designs, advanced two-bladed concepts might be applied at a later, matured

stage. Two-bladed turbines have several advantages over three-bladed designs.

First of all, two-bladed turbines are comparatively simple to install and maintain.

Also, relaxed noise limitations offshore enable the use of a higher rotor speed,

which reduces both the dimensioning torque for the drive train and the transmis-

sion ratio of the gearbox. Lower blade solidity is optimal for higher speeds and

this can be achieved by omitting the third blade. Despite these advantages, how-

ever, the complicated dynamic behaviour and the alternation of high loads are

16 Offshore wind farms 529

major challenges for two-bladed machines. Furthermore, the offshore market must

be large enough at the time of the marketability of the wind turbine in order to

make the development of a product purely designed for offshore conditions profit-

able.

16.4 Support structures and marine installation

The turbine support structures, i.e. the integrity of tower and marine foundation,

are the most obvious differences between offshore and land-based wind turbines.

Ferguson classifies offshore bottom-mounted support structures according to three

basic properties: structural configuration, foundation type and installation princi-

ple [5]. Any structural concept, including monotowers, braced towers and lattice

towers, can be reasonably combined with either a piled or a gravity foundation.

From a logistical point of view, however, it is beneficial to match piled designs

with lifted installation and gravity foundations with floated installation (Fig. 16-7).

530 16.4 Support structures and marine installation

Configuration

Mono Tower Braced Tower / Lattice Tower

Piled & Lifted

Monopile

Piled tripod or lattice tower

Foundation & Installation

Gravity & Floated

Gravity base

mono tower

Gravity base tripod or

gravity base lattice tower

Fig. 16-7 Classification of bottom-mounted support structure types by structural configuration,

foundation and installation principle

16 Offshore wind farms 531

Fig. 16-8 Installation of an offshore wind turbine on a jacket support structure in 45 m water

depth [REpower Systems AG]

Fig. 16-9 Rotor installation at the Utgrunden wind farm with two jack-up barges and two tug

boats [Photo Uffe Kongsted]

Driven or drilled, steel monopiles are currently the preferred solution. They are

economically attractive for use with the 2 to 3 MW class in water depths of

approx. 20 m, and for use with 5 MW designs in depths up to 15 m. For larger

depths, tripod, quadropod or lattice structures are needed. In 2006 the first jacket

foundations for offshore turbines have been installed in 45 m water depth in Scot-

land (Fig. 16-8). Gravity foundations have been applied mainly at shallow Danish