7 - Planning of an Offshore Wind Farm in the Mediterranean Sea - Gkoumas

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Vladimiro Rotisciani1

Salvatore Miliziano2

Franco Bontempi2

Konstantinos Gkoumas2

1ICARIA srl 2University of Rome “La Sapienza”

12th International ConferenceON ENGINEERING, SCIENCE, CONSTRUCTION AND OPERATIONS IN CHALLENGING ENVIRONMENTS

EARTH&SPACE 2010MARCH 14-17, 2010 Honolulu, HI

“Planning of an Offshore Wind Farm in the Mediterranean Sea”

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1

Objective

• To expose the main aspects influencing the basis of the design of offshore wind turbine (OWT) structures for an OWF in Central Italy

§ the essential role of the structural analysis supporting the decisional process is enlightened

• The project under consideration is characterized by elevated complexity due to the harsh environment, and the innovating characteristics, being, among else, the support structures placed in elevated depth, in the range of 20-35 meters.EARTH & SPACE 2010, MARCH 14-17, 2010 Honolulu, HI Konstantinos Gkoumas, PhD, PE

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Presentation outline

• Offshore Wind Farms

• Advantages and trends

• Recent realizations

• Proposed OWF in the Mediterranean Sea: key facts

• Site location

• Energy production

• Geologic and geophysical characterization

• Geotechnical characterization

• Seismic response

• Meteo-marine conditions

• Structural analyses

• Additional aspects

• Conclusions and overview

2EARTH & SPACE 2010, MARCH 14-17, 2010 Honolulu, HI Konstantinos Gkoumas, PhD, PE

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Offshore Wind FarmsAdvantages and trends

• Advantages

• Considerably stronger, more consistent and less turbulent wind speeds offshore

• increased power production.

• Reduced visual impact as they are placed far away from the coast

• Future trends*

• Cumulative Offshore Europe: 1.9 GW-40 GW (2008-2020)

• Offshore wind share: 3.9% (2008), 25% (2020)

• It can be expected that total offshore wind capacity will exceed onshore capacity at some point beyond 2030

*Pure Power: Wind energy targets for 2020 and 2030:

A report by the European Wind Energy Association - 2009 update


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Recent realizations of OWF’s


Commissioned:September 2009

Location:15-30 km / 10-20 miles off the westernmost point of Denmark, Blåvands Huk

Installed capacity:209 MW

Turbines:91 Siemens SWP 2.3-93

Water depth:9-17 meters

Foundation:monopile

Horns Rev 2

Photograph: MEDVIND/BENT SOERENSEN

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Recent realizations of OWF’s (2)


Operation:2010/11

Location:100 kilometres (62 mi) northwest of the isle Borkum, in Germany

Capacity:400 MW

Turbines:80 BARD 5.0

Water depth:about 40 meters

Foundation:BARDTM triple

Demonstration turbine at Hooksiel

Bard Offshore 1

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Nominal power of a single turbine 3.0÷5.0 MW

Number of turbines 105

Hub height 100 ÷ a.s.l.

Nominal power of a the farm 315 ÷ 525 MW

Minimum distance from the shore 10 Km

Surface of the farm area 67.20 Km2

Water depth 20-35 m

Life span 29 years

Offshore wind farm: key facts


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Offshore wind farm site location(Google Map).


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Offshore wind farm site location


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Site location

• Choosing the OWF site location was the result of a series of technical, financial and environmental considerations

– the seabed morphology (seabed depth of 35 meters or lower);

– the large minimum distance from the coast (10 km), something that influences positively the visual impact;

– the absence of environmental, territory or archeological issues;

– the anemometric conditions;

• In the vast area other (inshore) wind farms are sited, a factor that identifies it as a key area for wind energy production


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Energy production estimation

• On the basis of the anemometric survey and the performed simulations, it was possible to estimate the energy production of the OWF.

• For this aim the utilization of Vestas V90 wind turbines with a 3.0 MW nominal power has been

• The energy produced of each of these turbines results being between 7000 and 9100 MWh (the maximum value corresponds to the maximum distance from the shore).

• A layout in 7 rows of turbines, with 15 turbines at each row, for a total 105 turbines has been considered. The orientation is on the South-East/North-Ovest axis (prevalent wind direction)

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Emin [GWh] Emax [GWh] Etot [GWh] Range [Km2]

8.66 8.94 919.99 67.20


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Wake effect losses

• Wake effect: “the aggregated influence on the energy production of the wind farm, which results from the changes in wind speed caused by the impact of the turbines on each other”.

• On the basis of models found in literature, the wake effect losses have been estimated for three different turbine types:

– Vestas V90 (3.00 MW)

– GE Energy 3.6r (3.6 MW)

– Repower 5M (5.00 MW)

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Turbine Ø (m) ØP ØO PL estimation (%)

Vestas V90 90 ØP = 11 ØO = 9 ≤ 8.7 %

GE Energy 3.6r 111 9 < ØP < 10 7 < ØO < 8 ≤ 8.7 %

Repower 5M 126 ØP ~ 8 6 < ØO < 7 8.7% ≤ PL ≤ 10.0%


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Meteo-marine conditions

• Available historical data of direct measurements of the surface waves have been gathered, and following analysis and numerical elaboration, the boundary conditions forming the basis for the wake propagation modeling have been set. On the basis of such propagation model, the maximum wave heights have been calculated.

• In order to obtain the proper combination of wind and wave characteristics, a prognostic model has been applied on a finer mesh (local area model), on the basis of wind profiles on the points of a coarse grid in the area of interest obtained from a global forecasting model.

– Manenti, S. and Leuzzi, G. (2010). “Wind-wave hindcasting on offshore wind turbine through combined atmospheric and spectral models”.


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Geologic and geophysical characterization

• The OWF site is part to the eastern margin of the Apulia Platform, consisting in a succession of substrates of carbon-rich facies and fine-grained sea limestone

• From the surface towards the inner substrates, the following stratigraphic sections can be identified:

– A depository of holocene clay soil, for a substrate of 20-40 meters;

– A depository of Plio-Pleistocene clay with sand intercalation, for a substrate of 300-500 meters;

– An Oligo-Miocenic depository, consisting in marl, clay and white limestone, for a substrate of 800-1000 meters.

– A mesozoic carbon depository

• The OWF site is situated near the Gondola Fault, essentially inactive since the Pliocene; the sea area is characterized by anearthquake activity of mild intensitythe, something also documented on the national earthquake map.


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Geotechnical characterization

• In order to design the foundations of the OWT and of the serviceplatform, a geotechnical model has been implemented, with the following stratigraphy:

– Substrate 1- Holocen depositories, normally consolidated (from the seabed and up to -30.00 meters below): soil of fine granulometry, essentially normally consolidated clay of recent depository;

– Substrate 2- Pleistocene depositories, over consolidated (beginning at -30.00 meters below the seabed): over-consolidated layer of slimy clay of better mechanic resistance.

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Substrate z (m) γγγγ (kN/m3) suk (kPa) Eu (MPa) Es (MPa)

1 0-30 17 0 ÷ 42 0.45 ÷ 12.5 0.84÷ 25

2 > 30 17 90 ÷ 145 27 ÷ 44 54 ÷ 88

Where, γ is the unit weight of the soil, suk the undrained shear strength, Eu is the undrained Young’s Modulus and Es the secant operative Young’s Modulus


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Geotechnical characterization

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Where, OCR is the over consolidation ratio, suk the undrained shear strength, sud

the design shear strength, Eu the undrained Young’s Modulus and Es the secant operative Young’s Modulus

1 1.5 2 2.5 3

OCR

70

65

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z (m

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0 50 100 150 200

suk/sud

(kPa)

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z (m

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suk (hp.OC)

(characteristic values)

suk (hp.NC)

(characteristic values)

sud (hp.OC)

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0 20 40 60 80 100

Eu/Es

(MPa)

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Eu=300suk

Es_working


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Seismic response

• The principal seismotectonic features and the information obtained from the geological and geophysical features of the site, lead to the characterization of the area seismicity

• The significant seismogenetic areas (in terms of magnitude-distance relationships) were defined, leading to the selection of the appropriate natural accelerograms to be used as an input forthe local seismic response

• Results obtained by the geotechnical model, allowed defining theseismic action to be implemented in the design process, in termsof time histories of the acceleration and of response spectra.

• For the reference seismic action, the prescriptions from the Italian building code have been implemented (NTC, 2008).

– Specifically, the design seismic action has been identified with reference to a return period of 475 years, in accordance with international codes and standards (DNV, 2004). This return period corresponds to a “rare” seismic event, defined as an event with a 10 % probability of exceedance in 50 years.


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Design meteo-marine conditions

• A Meteo-marine study has been carried out, aiming at extreme event characterization and definition of the off-shore wave conditions.

• The extreme event statistic analysis performed, lead to the individuation of the design waves in two sectors for a return period of 100 years.

• Successively, the design waves were propagated in the area, and four points of observation (corresponding to the vertices of the area) were considered.

• For these points, the maximum wave height, direction, and length have been calculated.• Manenti, S. and Petrini, F. (2010). “Dynamic Analysis of an Offshore Wind Turbine:

Wind-Waves Nonlinear Interaction”.


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Structural analysesSupport structure (1)

• Structural analyses have been performed, after a structural system decomposition, implementing different structural schemes and FEM models of increasing complexity

– Bontempi, F. (2010). “Advanced Topics in Offshore Wind Turbines Design”

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Foundation

Transition

Immersed

Emergent


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Structural analysesSupport structure (2)

• Numerical analyses carried out on different support structures and their variations, led to the adoption of a jacket structure


Motivation:

• Vibrations

• Displacements

• Weight (similar)

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Structural analysesSupport structure (3): extended modeling


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Structural analysesSupport structure and foundations

• On the basis of specific performed preliminary analyses, and considering the stratigraphy of the seabed, the foundation type chosen is a four-leg jacket structure.

– This is also in accordance with literature for similar cases (for a sea depth of 15-35 meters).

• Numerical analyses have been performed for both the 3 and the 5 MW turbines.

– The four-leg jacket is founded on four tubular steel piles, each having adiameter of 2 meters, and with a variable thickness

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Depth (m) d (mm)

< 24 26 (3 MW) - 28 (5 MW)

24-36 18

36 -48 16

>48 14


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Structural analysesFoundations – stress and displacements

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Turbine type N (compression) N (traction) T M

[kN] [kN] [kN] [kN*m]

3 MW 17500 9250 2170 6440

5 MW 23800 14350 2580 7840

Turbine type N (compression) N (traction) T M

[kN] [kN] [kN] [kN*m]

3 MW 13450 7115 1670 4950

5 MW 18300 11040 1990 6030

Turbine type Vertical. displ. Pile length 1,2Assumed vertical load

[cm] [m] [kN]

3 MW 3.01 57 13713

5 MW 4.52 68 18288

Maximum stress (singe pile – SLS)

Maximum stress (singe pile – ULS)

Vertical displacement and pile length - SLS


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Additional aspects

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• Electrical installations

• Among the aspects addressed are the connection between the offshore wind turbines and the offshore substation, the connection between the offshore substations, and the transfer ofenergy to the onshore substation, with successive connection to the national energy transfer network.

• Navigation safety in the confining area

• The marine traffic of interest for the nearby ports has been assessed and the possible routes that could intercept the site have been designated. The impacting force of the vessels has been evaluated.

• Environmental impact

• The visual impact of the OWF to the landscape, the acoustic impact, the electromagnetic interference and the impact of the farm to the marine biology have been assessed.


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Conclusions and overview

• In this presentation, fundamental aspects from the design procedure adopted for an offshore wind farm in the Mediterranean Sea are exposed in a crisp manner.

• The wind farm presents unique characteristics, being, due to the particular environmental conditions and the seabed level, the first of its kind in the Southern Europe.

• The adopted design choices, the approach used, and the performed analyses, can be of particular interest for future similar projects.

• The project will be further refined with the advancement from the “design” to the “construction” status


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Thank you for your attention

1 [email protected] [email protected] 3 [email protected] [email protected]

The scientific contribution in the design process of the offshore wind farm of University Professors Marcello Bernabini, Giuseppe Lanzo, Alberto Noli, Corrado Ratto and Maurizio Sciotti is greatly acknowledged.

V. Rotisciani 1, S. Miliziano 2, F. Bontempi 3 and K. Gkoumas 4
