8
Articles DOI: 10.1002/stco.201310029 178 © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Steel Construction 6 (2013), No. 3 Offshore wind energy development is lagging far behind the ambitious targets that the German government set at a time when an energy turnaround was propounded. Interest in using monopile foundations in a wider field is therefore growing because this type of foundation has proved to be the most economical solution. Their benefits lie in their sim- ple design, favourable fabrication conditions with a considerable automation potential, and short installation times. Concepts and technologies are being developed to be able to produce and install monopile foundations for water depths down to at least 40 m. The necessary design concepts have to be adapted and extended for these requirements. Approaches are presented in this paper. 1 Introduction According to targets that the German government has set for renewable en- ergy, renewables are to account for at least 20 % of the total electrical power supply by the year 2020 (roughly twice as much as in 2011). Wind energy is to provide a permanently growing por- tion, with a special focus on offshore wind energy. By 2030 the percentage of wind energy is to rise to at least 25 % from the present 5 % share in electrical power generation (10 % on- shore, 15 % offshore). This means that 20 000–25 000 MW will have to be installed offshore, most of which is earmarked for installation in the North Sea. To date, approval has been granted for 29 wind parks with 1894 individ- ual units altogether, which will pro- vide a total output of 9 000–9 300 MW (depending on the completion level). For the German Exclusive Economic Zone (EEZ), applications have been filed with the German approval au- thority, the Federal Maritime and Hy- drographic Agency (BSH), for another 68 wind parks with a total of 5178 individual units. This corresponds to an approved and/or planned output of more than 29 000 MW. Neverthe- less, the total power that has been in- stalled so far (about 285 MW) remains clearly behind expectations [1]. This can be explained in part by the problematic installation condi- tions at locations far off the coast, the inadequate dimensions of the instal- lation ships available and the envi- ronmental conditions that are prov- ing to be more difficult than expected. For these reasons, offshore wind park projects are not being implemented, or attempts are being made to find more favourable construction meth- ods, even though approval has been granted, the detailed engineering has been completed, construction phases have been planned and financing is available. The costs of foundation produc- tion and installation account for about 25–30 % of the total investment for an offshore wind park, which is where potential savings can be found by op- timising the foundation solution and installation processes. There is con- siderable interest in foundation struc- tures that can be easily implemented, are easy to handle and not prone to problems. Monopile foundations, con- sisting of a pile and transition piece, are widely used for offshore wind tur- bines. Monopile foundations already in place, such as those in the Riffgat wind park with a mean water depth of 20 m, show that this type of founda- tion is unproblematic and can be in- stalled quickly. Installation times for pile driving, mounting and grouting of the transition piece have been as short as approx. 24 hours. Pushing the limits – mega monopile foundations for off- shore wind turbines Rüdiger Scharff Michael Siems Fig. 1. Development of offshore wind energy in Germany (source: [1]), last updated 31 Dec 2012

Pushing the limits - mega monopile foundations for offshore wind turbines

  • Upload
    michael

  • View
    221

  • Download
    0

Embed Size (px)

Citation preview

Page 1: Pushing the limits - mega monopile foundations for offshore wind turbines

Articles

DOI: 10.1002/stco.201310029

178 © Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Steel Construction 6 (2013), No. 3

Offshore wind energy development is lagging far behind the ambitious targets that the German government set at a time when an energy turnaround was propounded. Interest in using monopile foundations in a wider field is therefore growing because this type of foundation has proved to be the most economical solution. Their benefits lie in their sim-ple design, favourable fabrication conditions with a considerable automation potential, and short installation times. Concepts and technologies are being developed to be able to produce and install monopile foundations for water depths down to at least 40 m. The necessary design concepts have to be adapted and extended for these requirements. Approaches are presented in this paper.

1 Introduction

According to targets that the German government has set for renewable en-ergy, renewables are to account for at least 20 % of the total electrical power supply by the year 2020 (roughly twice as much as in 2011). Wind energy is to provide a permanently growing por-tion, with a special focus on offshore wind energy. By 2030 the percentage of wind energy is to rise to at least 25 % from the present 5 % share in electrical power generation (10 % on-shore, 15 % offshore). This means that 20 000–25 000 MW will have to be installed offshore, most of which is earmarked for installation in the North Sea.

To date, approval has been granted for 29 wind parks with 1894 individ-ual units altogether, which will pro-vide a total output of 9 000–9 300 MW (depending on the completion level). For the German Exclusive Economic Zone (EEZ), applications have been filed with the German approval au-thority, the Federal Maritime and Hy-drographic Agency (BSH), for another 68 wind parks with a total of 5178 individual units. This corresponds to an approved and/or planned output of more than 29 000 MW. Neverthe-less, the total power that has been in-stalled so far (about 285 MW) remains clearly behind expectations [1].

This can be explained in part by the problematic installation condi-tions at locations far off the coast, the inadequate dimensions of the instal-lation ships available and the envi-ronmental conditions that are prov-ing to be more difficult than expected. For these reasons, offshore wind park projects are not being implemented, or attempts are being made to find more favourable construction meth-ods, even though approval has been granted, the detailed engineering has been completed, construction phases

have been planned and financing is available.

The costs of foundation produc-tion and installation account for about 25–30 % of the total investment for an offshore wind park, which is where potential savings can be found by op-timising the foundation solution and installation processes. There is con-siderable interest in foundation struc-tures that can be easily implemented, are easy to handle and not prone to problems. Monopile foundations, con-sisting of a pile and transition piece, are widely used for offshore wind tur-bines. Monopile foundations already in place, such as those in the Riffgat wind park with a mean water depth of 20 m, show that this type of founda-tion is unproblematic and can be in-stalled quickly. Installation times for pile driving, mounting and grouting of the transition piece have been as short as approx. 24 hours.

Pushing the limits – mega monopile foundations for off-shore wind turbines

Rüdiger ScharffMichael Siems

Fig. 1. Development of offshore wind energy in Germany (source: [1]), last updated 31 Dec 2012

Page 2: Pushing the limits - mega monopile foundations for offshore wind turbines

179

R. Scharff/M. Siems · Pushing the limits – mega monopile foundations for offshore wind turbines

Steel Construction 6 (2013), No. 3

As a next step, a monopile con-cept has to be developed that should provide for automated series produc-tion similar to that for onshore towers and include optimised connection op-

given preference over the monopile version (Fig. 3).

Monopiles for much greater wa-ter depths are examined and potential applications discussed in [6].

2 Trends in selecting foundation solutions

2.1 Mega monopile – new options

Almost half of the offshore wind parks planned in Germany is located in the EEZ of the North Sea, with water depths between 30 m and 40 m. In particular, 74 % of the projects where consent has been authorized and 42 % of the parks for which a consent application has been submitted are entirely or partly located in the afore-mentioned range of water depths (see Fig. 2).

This fact clearly shows that there is a considerable need for foundation options that can be easily adapted to the given water depths. The monopile foundations installed so far were de-signed for water depths of up to 25 m. At greater depths, other options, e.g. jackets, tripiles or tripods, have been

Fig. 3. Foundation options for offshore wind turbines: monopile, tripod, tripile and jacket

Fig. 2. German EEZ in the North Sea – overview of offshore wind parks in sea areas with water depths between 30 m and 40 m (source: www.bsh.de)

Page 3: Pushing the limits - mega monopile foundations for offshore wind turbines

R. Scharff/M. Siems · Pushing the limits – mega monopile foundations for offshore wind turbines

180 Steel Construction 6 (2013), No. 3

below the maximum of 160 dB at a distance of 750 m that the approv-ing authority accepts on the seabed without sound-absorbing measures such as bubble curtains.

– The wall thicknesses can remain below the minimum thicknesses ac-cording to [3], which brings eco-nomic benefits (reduced produc-tion costs).

– The drilling method can be used without any problems in difficult geological conditions, such as hard rock strata and strata with glacial erratics.

With due consideration given to the aspects discussed above, various mono-pile design parameters, such as posi-tion and configuration of the conical transition and reduced wall thick-nesses down to the theoretical stabil-ity failure limits, are systematically varied below in order to arrive at an optimum weight at great water depths of up to approx. 40 m.

3 Determining the monopile contour3.1 Design procedure

The foundation structure must com-ply with ultimate limit state (ULS) and fatigue limit state (FLS) boundary con-ditions, while its dynamic properties must also take account of the require-ments of the wind turbine. The design

method are presented in [11]. Fig. 5 shows a vertical shaft drilling ma-chine on an onshore site.

This drilling method has a num-ber of advantages over conventional driving and vibration techniques: – The predicted sound emissions [10]

during installation remain clearly

tions. An essential connection element that should be mentioned here is the grouted joint, for which individual ap-provals must be obtained for building permission, since the high-strength concrete used is a non-regulated ma-terial (in the meaning of German standards). In addition, this construc-tion method has come in for criticism because of the restrictions that must be complied with (max. significant wave height during grouting and hardening phases), the costly installa-tion this implies and the damage that has been reported for installations in Denmark, the UK and the Netherlands (see [9]). The question is whether this type of connection really has to be used. The latest projects in which large piles had to be driven show, af-ter all, that the required pile vertical-ity after driving is normally complied with. As a consequence of this experi-ence, a version with conventional structural steelwork connections in-stead of the grouted joints should be considered, which can, in addition, help reduce both the amount of mate-rial and the installation times.

2.2 Developments in installation methods

Another option for sinking large mono-piles to the required depth is the OFD®-LD drilling technique that Her-renknecht Solutions AG has devel-oped for this purpose in collaboration with HOCHTIEF Solutions AG. The offshore foundation drilling (OFD) concept provides for anchoring mono-pile structures for offshore wind tur-bines in the seabed with the aid of a vertical drilling method.

Since OFD®-LD (LD = large di-ameter) is a highly flexible method, it allows monopiles with diameters of up to 10 m to be produced with vary-ing diameters along the length of the pile (conicity). This drilling method has been developed on the basis of the vertical shaft sinking machine of Her-renknecht AG. It uses the partial-face excavation principle with a pivoted drilling arm (Fig. 4) in which the head of the machine mills off the ground within a circle that is concentric with the monopile axis in order to produce the pile base (see [10]).

The machine has two main com-ponents: the sinking unit and the ex-cavating machine. Details of the

Fig. 4. Schematic illustration of OFD®-LD machine (source: [10])

Fig. 5. Vertical shaft drilling machine being placed in a shaft (source: [11])

Page 4: Pushing the limits - mega monopile foundations for offshore wind turbines

181

R. Scharff/M. Siems · Pushing the limits – mega monopile foundations for offshore wind turbines

Steel Construction 6 (2013), No. 3

in the graph in Fig. 6. Different diam-eter conditions produce very similar results. If the middle of the cone is located at 0.0 mLAT, which is near the waterline, the damage increases continuously with an increasing cone angle because a much larger projected area is exposed to the waves in the critical region just below the surface of the water. With the middle of the cone located at –5.0 mLAT, this trend is reversed because the steeper cone now reduces the area that is exposed to the waves near the waterline. This effect becomes very pronounced with the middle of the cone located be-tween –10.0 and –15.0 mLAT, and – as expected – it decreases again at even greater depths.

Obviously, the increased stress concentration factor, which is in the order of approx. 1.3–1.5 at the point where the cylindrical and conical shell sections meet, must be considered when designing the conical transition. Owing to the smaller effective diame-ter, only the top end of the cone is a critical part in practice. If necessary, the local damage extreme can be effec-tively reduced by state-of-the-art post-weld treatment, which helps to im-prove the theoretical fatigue strength by two FAT classes, limited to the maximum FAT class 112 (see [5]). The additional stress concentration can therefore largely be compensated for.

As a result, the increase in the cone angle can produce a considera-

shape, more options are available for positioning the transition. At increas-ing water depths, attention must in particular focus on minimised wave loads.

There are several fabrication-spe-cific restrictions that apply to the cone angle, which can be varied in addition to the position of the conical transi-tion. At the moment, cone angles are normally between 1.5° and 2.0°. Im-proved machining methods will in the future allow cones to be fabricated with an angle of up to 5.0°. Analyses have therefore been made to deter-mine to what extent the fatigue-rele-vant wave loads can be reduced by varying the cone angle.

In a parameter study, the cone angle was varied in steps between 1.5° and 5.0° for different monopile and tower diameters. By keeping the cen-tral point of the cone at the same po-sition, the important lowest system eigenfrequency remained almost con-stant (variation < 0.5 %). Calculations were made for a water depth of 40 mLAT. For comparison, the dam-age resulting from the bending mo-ment at the seabed as a result of wave action was evaluated and related to the value of the damage done with a cone angle of 2°. Loads resulting from the wind turbine were not considered.

The results of the parameter study for a monopile diameter DMP = 8 000 mm and a tower diame-ter DTower = 6 000 mm are summarised

process is normally a two-phase ap-proach, with load calculation and subsequent steel structure and geo-technical design. If the results of the design process deviate too far from the first estimate for the structural pa-rameters, an iterative approach has to be applied, which is why a reliable pre-assessment of the dimensions of the global support structure is desira-ble in order to reduce the number of calculation loops.

Using a monopile for offshore wind turbines with a high RNA (rotor–nacelle assembly) mass, eigenfrequen-cies have, as a general rule, proved to be the decisive design factor. At in-creasing water depths, compliance with the lower limit of the required interval for the soft-stiff design is a major challenge. Since the wall thick-nesses selected are of lesser signifi-cance for the overall system stiffness, defining the outer contour of the sup-port structure becomes the first im-portant step in the design process.

The adjustment of the desired natural vibration characteristics of the support structure is described in de-tail in [6], which also provides graphs based on a parameter study for pre-as-sessing the lowest eigenfrequency. When the dimensions of the tubular tower are defined by the turbine man-ufacturer, the pile diameter at the mud-line remains as the essential design parameter for the integral foundation stiffness. The position of the necessary conical transition to the smaller tower diameter not only affects the eigenfre-quency, but also the wave loads that occur. The conflicting targets of mini-mized fatigue-relevant wave load and adequate system stiffness when select-ing the position of this conical transi-tion are also discussed in [6]. The pos-sibilities for increasing the diameter in the waterline region are limited be-cause of the more-than-linear increase in the wave loads.

3.2 Designing the conical transition

If a grouted joint is used, it should preferably be placed in the region of the conical transition because this will reliably prevent axial sliding. The transition is then located at the lower end of the transition piece, roughly at the level of the waterline. If a flanged connection is used, or if the grouted connection is not given a conical

Fig. 6. Relative changes in monopile fatique damage at the seabed when varying the cone angle

Page 5: Pushing the limits - mega monopile foundations for offshore wind turbines

R. Scharff/M. Siems · Pushing the limits – mega monopile foundations for offshore wind turbines

182 Steel Construction 6 (2013), No. 3

the slender pile shell in a rotationally symmetric manner. A monopile with a graded wall thickness cannot be classified in accordance with the boundary conditions of [2] without conflict, which is why the ideal buck-ling stress cannot be determined with a closed-form solution in this case. An adequate extended model is required for determining the hoop stress, too, which occurs in addition to the longi-tudinal stress. This is why a 3D model of the monopile was created at this point. Using shell elements in the cen-tral plane provides for easy develop-ment of the geometry, whereas the plate thicknesses can be flexibly con-trolled with constants.

Appropriate assumptions have to be made for pressures acting on the pile, which result from the lateral sub-grade reaction and have to be consid-ered in the shell model in a simple manner and must at the same time be sufficiently conservative. Owing to the substantial modelling effort, mod-elling the non-linear 3D pile-soil inter-action is not feasible for practical de-sign purposes. In addition, such mod-els are inherently non-linear, whereas numerical buckling analyses have to start from a linear model for the ei-genvalue analysis.

occur during the pile driving phase and applies to pile diameters D up to approx. 3 000 mm. If the pile is not driven into the ground, this restriction can be neglected. A detailed buckling analysis has to be carried out in ac-cordance with DIN EN 1993-1-6 [2].

The main loadbearing effect pro-duces a dominant stress in the direc-tion of the meridian. A stress-based buckling analysis can be carried out in this context. It should be noted, however, that the ideal buckling stress in Annex D of [2] does not apply to free boundary conditions and presup-poses that the wall thickness is uniax-ially graded. This is why an engineer-ing error estimate has to be made when this analysis format is used. The buckling reduction factors thus ob-tained are normally 0.90 and higher. The eigenvalue analyses that were car-ried out later also show that the sus-ceptibility to buckling along the direc-tion of the meridian is very low (see Fig. 7).

What is most relevant in the de-sign process is buckling due to com-pressive hoop stresses, which results from pile-soil interaction. A high ra-dial compressive stress is set up in ar-eas with the highest subgrade reac-tion, and this stress does not act on

ble economic benefit, as the fatigue stress is reduced over a large area of the wave-affected zone down to the maximum bending moment below the seabed.

4 Structural pile capacity4.1 Modelling in engineering

Stick models are widely used for dy-namic load simulation and they have also proved to be a useful method when designing steel structures for foundations. This is primarily due to the fact that the p-y method has be-come the state of the art for the mod-ulus of subgrade reaction approach with non-linear spring characteristics for modelling the geotechnical pile ca-pacity. The alternative strain-wedge model (see [8]), too, is used for reduc-ing the complex three-dimensional interaction to a stick model.

The effect that large pile diame-ters have on the subgrade stiffness must in any case be adequately ac-counted for in the analysis. Since the p-y method is not calibrated for mono-piles with a large diameter, some au-thors have developed 3D models to be able to consider the pile-soil inter-action with a higher degree of accu-racy ([7], [8]). It appeared that the p-y method tends to overestimate the sub-grade stiffness moderately for increas-ing pile diameters. To counteract this effect, correction factors can be ap-plied to the p-y curves, which will, however, not be dealt with at this point.

4.2 Defining the pile wall thickness

Based on the stress resultants derived from the stick model, the steel struc-ture is designed in compliance with the elementary rules of elastostatics. The monopiles are designed with a varying wall thickness that is adjusted to the moment line, with FLS or ULS criteria becoming alternately critical over the height of the foundation structure.

In defining the wall thickness t, a minimum wall thickness is normally complied with in order to prevent lo-cal buckling. A commonly applied rule is based on API ([3] and [4]) and re-quires a minimum plate thickness of t  =  6.35 mm + D/100. It should be noted that this requirement is specifi-cally aimed at the higher stresses that Fig. 7. Transferring loads from stick model to 3D shell model

Page 6: Pushing the limits - mega monopile foundations for offshore wind turbines

183

R. Scharff/M. Siems · Pushing the limits – mega monopile foundations for offshore wind turbines

Steel Construction 6 (2013), No. 3

medium quality that are typically en-countered in the German Bight, with alternating cohesive and non-cohe-sive strata, and with dense sand strata at greater depths. The pile diameter at the mudline would then be 8 000 mm, so the lowest system eigenfrequency will be 0.265 Hz. The diameter at the interface level is 6 000 mm.

Monopile version A is shown in Fig. 10 as an initial version in which a minimum wall thickness of t = D/100 is maintained. There is a section ap-prox. 30 m long at the bottom end of the penetration length where the de-sign is determined by the utilisation level at the ULS. Abandoning the re-quirement for a minimum wall thick-ness allows the structure in this region to be effectively optimised. A shorter region with similar characteristics is located above the cable entry hole, extending to the point where the con-ical transition starts.

In these two regions, the structure was optimised with the following steps: – Reducing the wall thickness, the

reduction having to be made grad-ually with a view to fatigue strength in order to prevent extreme stress concentrations from occurring as a result of eccentricities.

– Linear elastic calculation of the in-plane stress resultants using the shell model and idealised soil pres-sure distribution.

– Eigenvalue analysis for determin-ing the specific ideal elastic buck-ling resistance.

does not cause any shell deflection, was neglected.

The next step is to perform a nu-merical buckling analysis in accord-ance with [2] section 8.6 with the aid of the 3D shell model. Vertical actions were added to the applied substitute forces from the soil reaction, and so the complete stress condition in this model corresponded to that of a stick model. Fig. 9 shows the lowest eigen-values of a linear buckling analysis (LBA). It is evident from all three ei-genforms that buckling results exclu-sively from hoop stress. Buckling due to axial compression only was not de-tected, even for higher eigenvalues. A material non-linear analysis (MNA) was not carried out, but the plastic ref-erence resistance was extracted from the in-plane stress resultants as an “on the safe side” value.

4.3 Design example with savings potential

The design example given below is to demonstrate how the buckling analy-sis is to be carried out and what sav-ings can be achieved with the mono-pile selected. The foundation in the example has to support a 5 MW class wind turbine with a hub height of 95 m. For a water depth of 40 m and typical MetOcean data at relevant lo-cations in the North Sea, the extreme wave height would be Hmax50 = 20.0 m. The subsoil conditions were assumed to correspond to the soil profiles of

Instead, the pressure distribution can be idealised in each horizontal section, whereas the pressure distribu-tion along the pile axis can be taken directly from the stick model in ac-cordance with the p-y method. The subgrade pressures, with their spatial distribution, are then applied to the shell model as approximated nodal forces. When the corresponding wave forces are applied in addition, all ac-tions, together with the stress result-ants at the top end of the model, form an equilibrium group. Using this ap-proach, complete modelling of the soil is no longer necessary (see Fig. 7).

A 2D, physical non-linear model was prepared with a plane strain con-dition for determining the pressure distribution around the circumfer-ence. Fig. 8 shows the contact status for the applied loads. The pressure distribution produced in a parameter study, in which the shear parameters of the soil were varied, was very sim-ilar in nature and could be adequately approximated by a cosine-shaped dis-tribution. In this model, and when applying the forces to the shell model, several conservative assump-tions were made. The soil inside the pile is not included in the model, which means that its supporting ac-tion is neglected. In addition, the to-tal of the subgrade reaction in each horizontal section was applied to the pile shell with radial pressures and any tangential transmission of forces as a result of skin friction, which

Fig. 8. 2D model for determining the soil pressure distribu-tion

Fig. 9. Buckling mode shapes of a monopile in a water depth of 40 m

Page 7: Pushing the limits - mega monopile foundations for offshore wind turbines

R. Scharff/M. Siems · Pushing the limits – mega monopile foundations for offshore wind turbines

184 Steel Construction 6 (2013), No. 3

length, the pile tip should be rein-forced. This reinforcement will resist buckling at the free edge and has to take the high subgrade reaction in this zone.

Fig. 11 shows the result for the design example selected. Material sav-ings of 160 t could be achieved, which corresponds to about 12 % of the ver-sion A monopile. When checking the dynamic behaviour, it was found that the reduction in the first eigenfre-quency is a mere 1.5 % in the opti-mised version; with higher eigenval-ues, the changes are even smaller. Table 1 lists the key data of the two versions.

5 Summary

Owing to the substantial benefits in terms of construction and logistics, there is a trend towards using mono-piles also for great water depths in the German Bight. Because of the large dia meters required for these conditions the pile driving technolo-gies available are reaching their limits. An alternative could be a drilling method that has been developed by Herrenknecht AG, which also provides for a number of structural changes. The thickness of the monopile walls can be reduced to the limits defined by shell buckling because the diame-ter of this type of foundation and at great water depths is generally deter-mined by the frequency range that has to be complied with.

In addition, monopile designs with a steep cone angle offer advan-tages for fabrication and optimisation of the wave action that are relevant to fatigue behaviour. It could be demon-strated for a decisive parameter field that increasing the cone angle from 2° to 5° allows the damage that results from wave action to be reduced by up to 30 %.

Thinner walls make particular sense in the bottom pile penetration region, where the ULS governs the design. Since the compressive hoop stress, which results from the pile-soil interaction, is high in this region, the stability analysis must take account of this stress component with sufficient accuracy. A stress-based buckling analysis is not possible because the boundary conditions at the monopile cannot be classified without conflicts in accordance with DIN EN 1993-1-6.

This process is repeated iteratively un-til the stability criterion does not per-mit any further reductions, and the remaining design criteria of ULS, FLS and SLS are complied with. As the wall thicknesses are clearly reduced near the lower end of the penetration

– Checking the buckling resistance for the foundation structure on the basis of the provisions of [2].

– Using a stick model to check the distribution of stress resultants and the natural vibration characteris-tics of the complete system.

Initial version A Optimized version B

Diameter of monopile (top) 6000 mm 6000 mm

Diameter of monopile (bottom) 8000 mm 8000 mm

Pile penetration length 45 m 45 m

Total mass of monopile 1348.5 t 1189.2 t

Minimum wall thickness 70 mm 40 mm

Maximum wall thickness 85 mm 100 mm

1st eigenfrequency 0.266 Hz 0.262 Hz

Table 1. Results for the design example for a water depth of 40 m

Fig. 10. Wall thicknesses and utilization levels for monopile version A

Fig. 11. Wall thicknesses and utilization levels for monopile version B

Page 8: Pushing the limits - mega monopile foundations for offshore wind turbines

185

R. Scharff/M. Siems · Pushing the limits – mega monopile foundations for offshore wind turbines

Steel Construction 6 (2013), No. 3

[10] Els, W., Rosenberger, F., Peters, M., Eglinger, K., Budach, C.: Innovatives Verfahren zur Herstellung von Offshore-Gründungsstrukturen, Ernst & Sohn-Spezial, Mar 2012, pp. 35–40.

[11] Peters, M., Jung, B., Budach, C., Mel-zer, J.: Offshore Foundation Drilling – OFD®, Umweltfreundliche Baumethode für Infrastruktur und erneuerbare Ener-gieprojekte. Presentation, Fachtagung Baumaschinentechnik 2012, TU Dres-den.

Keywords: monopile; offshore founda-tion; wind turbine; eigenfrequency; cone angle; wall thickness; stability; shell buck-ling

Authors:Dipl.-Ing. Rüdiger Scharff, [email protected]. Michael Siems, [email protected] Peil, Ummenhofer mbHDaimlerstr. 1838112 BraunschweigTel: +49 (0)531 12331-00www.ipu-ing.de

API-RP 2A-WSD, 21st ed., Dec 2000, with Errata & Supplement 3, Oct 2007.

[5] International Institute of Welding: IIW Recommendations on Post Weld Fatigue Life Improvement of Steel and Aluminium Structures. XIII-2200r7-07, 6 Jul 2010.

[6] Scharff, R., Siems, M.: Monopile Foun-dations for Offshore Wind Turbines – Solutions for Greater Water Depths. Steel Construction – Design & Re-search, vol. 6, Feb 2013, pp. 47–53.

[7] Achmus, M.: Bemessung von Mon-opiles für die Gründung von Offshore-Windenergieanlagen. Bautechnik 88 (2011), pp. 602–616.

[8] Lesny, K., Paikowsky, S., Gurbuz, A.: Scale Effects in Lateral Load Response of Large Diameter Monopiles. In: Con-temporary Issues in Deep Foundations – Proc. of the ASCE Conference Geo, Denver, 2007.

[9] Schaumann, P., Lochte-Holtgreven, St., Lohaus, L., Lindschulte, N.: Durchrut-schende Grout-Verbindungen in OWEA – Tragverhalten, Instandsetzung und Op-timierung. Stahlbau, vol. 79, Sept 2010, pp. 637–647.

This paper discusses approaches for modelling in the numerical buckling analysis which can be easily applied in practical design calculations. A de-sign for a water depth of 40 m is used as an example to demonstrate the pro-cedure, and feasible savings potentials are quantified.

References

[1] Deutsche WindGuard GmbH: Status des Windenergieausbaus in Deutschland 2012, pp. 1–10, www.windguard.de.

[2] DIN EN 1993-1-6: Design of steel structures – Part 1-6: Strength and sta-bility of shell structures (Dec 2010).

[3] American Petroleum Institute: Recom-mended Practice for Planning, Design-ing and Constructing Fixed Offshore Platforms – Load and Resistance Fac-tor Design. API-RP 2A-LRFD, 1st ed., 1 Jul 1993.

[4] American Petroleum Institute: Recom-mended Practice for Planning, Design-ing and Constructing Fixed Offshore Platforms – Working Stress Design.