SP2 Soil Structure Interaction for Offshore Wind Towers. Soil Structure Interaction for OWT monopile foundation

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1 SP2 Soil Structure Interaction for Offshore Wind Towers Soil Structure Interaction for OWT monopile foundation R 7 December 2011

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3 Project Project: SP2 Soil Structure Interaction for Offshore Wind Towers Document No.: R Document title: Soil Structure Interaction for OWT monopile foundation Date: 7 December 2011 Client Client: Client s contact person: Contract reference: Norges forskningsråd SP2 250 knok For NGI Project manager: Prepared by: Reviewed by: Jörgen Johansson Jörgen Johansson Christian Madshus Summary We have looked into dynamic stiffness and damping for a monopile foundation for an OWT with help of Piles and Comsol FEA. Results indicate that cutoff frequencies are above most loading frequencies and therefore radiation damping does not contribute to the global damping. Global stiffness reduces a considerable amount up to the cutoff frequency. Furthermore the non-linear behavior of soil will affect the global stiffness and damping and further study of this is recommended.

4 Contents Document No.: R Page: 4 1 Introduction 5 2 Literature 5 3 Description of Models Soil profiles Structure Used computational models 9 4 Results and discussion Piles/Comsol results Effect of shear strain on stiffness and damping Numerical issues with Comsol Axial pile stiffness damping Suction anchor foundation 13 5 Conclusions and ideas for future research 15 6 References 16 Appendix A Laysac Analysis Review and reference page

5 Page: 5 1 Introduction Offshore wind turbines are subjected to wave and wind forces of loading frequencies mainly below 1 Hz. Installation by pile-driving and breaking of waves are impulsive type of loading that would lead to response at higher frequencies than wind and wave loads. Driving of pile creates waves in both water and the seabed, which can be disturbing to marine life. An emergency shutdown may induced dynamic loads. For such loading the cyclic and dynamic properties of the soil-foundation are important. In current design of offshore wind turbine monopile foundations the effects of soil-structure interaction is not considered to its full extent. For most load cases (LC) the soil is responding cyclically and cyclic stiffness and damping are important to consider. For other design load cases the dynamic behaviour become important and then the foundation stiffness and damping varies considerably with frequency. Above a so called cut off frequency the foundation damping increases considerably and the stiffness reduces. We have computed stiffness and damping for a monopile foundation for a typical soil profile at an OWT installation in the North Sea. We have also looked into cut off frequencies for different soil profiles. 2 Literature Several papers, reports, and presentation were collected during the project. Due to time constraints a documentation of these has not been done. 3 Description of Models Data files used for this report are given in Table 1.

6 Page: 6 Table 1 Data files used for this report Content File Calculation Comsol P:\2011\01\ \Beregninger\Comsol\Mon Horizontal load input opel3vsprofile1.mph Comsol input Input data Scripts for reading output from Laysac and Piles Literature P:\2011\01\ \Beregninger\Comsol\Mon opel3vsprofile1vertload.mph P:\2011\01\ \Beregninger\ Soi lprofileinputdatahandcalcs.xlsx P:\2011\01\ \Beregninger\MatlabScript s P:\2011\01\ \Grunnlagsmateriale\Litter atur Vertical load Hand calculations Creating figures Soil profiles The shear- and pressure-wave profiles shown with blue and red dots in Figure 1 were based on a profile from a location in the North sea. From 6.5 m down to 45m the shear and p-wave velocities are based PS-logging results and from mudline down to 6 meter they are based on laboratory bender element experiment. Depth beneath mudline Velocity (Vs, Vp) vs PS-log vp PS-log vs Piles Profile 1 vp Piles Profile 1 vs Piles Profile 2 vp Piles Profile 2 vs Piles Profile 3 vp Piles Profile Figure 1 Soil profiles used in piles calculations

7 Page: 7 Computations have been performed in the frequency domain and therefore an hysteretic damping has been used. The hysteretic damping factor, η is related to the viscous damping ratio, ξ with the following formula. η= 2 ωωn ζ Eq. (1) ω and ω n are the loading and eigenfrequency. In the Piles computations three different soil profiles were used as shown with solid lines in Figure 1. A damping factor of 0.02 constant with depth was used in piles. The comsol shear and p-wave velocity profiles are given in Figure 2. In comsol an isotropic lossfactor of 0.04, corresponding to a damping factor of 0.02 was used. Figure 2 Soil Model used in Comsol 3.2 Structure The parameters of the monopile modeled with Piles and Comsol are given in Table 2. The embedded pile length is 30m as shown in Figure 1 and water depth is 20m. Water is not included in the computations.

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9 Page: Used computational models Initial computations were performed with Laysac [3], which is an NGI in-house code for wave propagation based on Green s functions. With this we could evaluate cut-off frequencies for vertically and horizontally loaded plates on semiinfinite media. Results are shown in Appendix A. Piles is an elastodynamic software for computing stiffness and damping of pilegroups based on discrete Greens functions [2]. Piles was used to obtain stiffness and damping for a single large diameter pile. However for wavelengths shorter (or frequencies over a certain value) than a few pile (e.g. 4) diameters, the assumptions in Piles are not longer valid. E.g. a wave speed of 100 m/s would give limiting frequency, f=100/4*d=5hz. Comsol multiphysics [4] a general FE program has been used to visualize the effects of a cut-off frequency and also to compare stiffness and damping with Piles Comsol FE model The linear elastic FE model shown in Figure 4 consists of solid elements for the soil and the pile is modeled with shell elements. Full contact between soil and pile is assumed. The domain outside blue ring are absorbing domain (so called PML). The lowest 10 m of the soil domaing beneat the pile is also and absorbing boundary. The loading frequency is varied from 0.1 Hz to 5 Hz. The larges element size set to be lower than 1/5 of the wavelength of shear waves at the highest frequency of 5 Hz. (100/5/5=4 m). The element size of the pile is set to 2 meter. (This may be course.) The number of degrees of freedom are some Computations were performed for 0.1Hz step between 0.1 and 4 Hz. The calculation time on the Z PC with 24 processor an 48 GB ram took about 10 hours. Horizontal or vertical point load was applied to the upper perimeter of the pile shown in red in the inset in Figure 4.

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11 Page: x Stiffness [N/m] Piles Kxx Comsol Kxx Piles Dxx Comsol Dxx Frequency [Hz] Figure 5 Comparison of horizontal stiffness and damping computed with Piles and Comsol Figure 6 show Mudline bending stress response spectra for a Blyth wind turbine (after [1]). The largest loads occur at frequencies from 0Hz to just above 1 Hz. Figure 6 Mudline bending stress response spectra (after Jan Van Der Tempel [1])

12 Page: Effect of shear strain on stiffness and damping As seen in Figure 7 the shear strain is is larger closer to the pile. For a typical nonlinear soil behaviour this would result in a lower stiffness and higher material damping in the vicinity of the pile. Figure 7 Logrithm of shear strains for 0.5Hz loading. Red color 1% strain and blue color 1x10-3 % Figure 8 Logrithm of shear strains for 2.4Hz loading. Red color 1% strain and blue color 1x10-3 % Figure 9 Logrithm of shear strains for 2.5Hz loading. Red color 1% strain and blue color 1x10-3 %

13 Page: Numerical issues with Comsol As shown in Figure 10 shear strains in the vicinity of the PMLs and the variation in stiffness and damping with frequency, indicate that there are possibly some reflections at the boundary and that the PML is not absorbing the waves as expected. Figure 10 Top view of pile moving in x-direction, logarithm of maximum shear strain in x-y plane 4.4 Axial pile stiffness damping Due to time constraints this figures have not been prepared. (Low frequency figure High frequency figure of shear strains, figure of stiffness comparison with Piles.) 4.5 Suction anchor foundation Some preliminary results for stiffness and damping for suction anchors were established with Piles (see Figure 11 and Figure 12) for the soft soil profile 3. These results need to be verified with other methods, because the Piles program results may not be valid for such large diameter and short piles.

14 Page: 14 Stiffness and damping 2 x kxx φ 10m h=5m Bucket Profile 3 dxx φ 10m h=5m Bucket Profile 3 kxx φ 5.2m Monopile Profile 3 dxx φ 5.2m Monopile Profile Frequency [Hz] Figure 11 Suction (Bucket) anchor horizontal stiffness and damping 14 x 1010 Moment Stiffness Real and Imaginary Stiffness and damping kmm φ 10m h=5 m Bucket Profile 3 kmm φ 10m h=10m Bucket Profile 3 kmm φ 5.2m Monopile Profile 3 dmm φ 10m h=5 m Bucket Profile 3 dmm φ 10m h=10m Bucket Profile 3 dmm φ 5.2m Monopile Profile Frequency [Hz] Figure 12 Suction (bucket) anchor moment stiffness and damping

15 Page: 15 5 Conclusions and ideas for future research Many load cases for OWT foundations are likely to have loads of frequencies below the cutoff frequency of soil deposits, e.g. see Figure 6, i.e. wave propagation will not contribute to damping. For some types of loading, slamming waves, pile-driving, dynamic effects do become important. This should be looked into further. For loading frequencies there is a spatial variation of shear strain, large near the pile and smaller further away. With increasing shear strain the shear modulus reduces and damping increases. The effect of this on the overall stiffness and damping should be evaluated in detail. To do this the absolute level of strain induced by different load cases has to be considered. Most likely non-linear effects are important and therefore a way to account for it in design should be developed. The stiffness varies with frequency also for frequencies below the cutoff frequency. Examples above show up to a 30% reduction compared to the static stiffness. This reduction of stiffness could possibly be modelled with an added mass in horizontal direction. Plots of shear strains around the pile (Figure 10) and the variation in stiffness and damping with frequency, indicate that there are some reflections at the boundary and that the PML is not absorbing the waves as expected. This should be further looked in to e.g. if Comsol is to be used further. Modelling soil in 3D is computationally expensive and the SSI method used for e.g. in the Maxlab IV project could be used also for obtaining global stiffness and damping Tools for computing stiffness of suction anchors should be looked into further.

16 Page: 16 6 References [1] Jan Van Der Tempel, Design Of Support Structures For Offshore Wind Turbines, Ph.D. Thesis, ISBN , [2] Kaynia A.M., and Kausel, E., 1982, Dynamic behaviour of pile groups, Proceedings of the 2nd International Conference on Numerical Methods in Offshore Piling, University of Texas at Austin, April 29-30, 1982 [3] Kaynia, A.M., Green's Functions in Layered Media under Fluid. NGI Research Report, , Dec. 18, 1996 [4] Comsol multiphysics 4.2,

17 Kontroll- og referanseside/ Review and reference page Dokumentinformasjon/Document information Dokumenttittel/Document title Soil Structure Interaction for OWT monopile foundation Dokument nr/document No R Dokumenttype/Type of document Rapport/Report Teknisk notat/technical Note Oppdragsgiver/Client SP2 (NFR) Distribusjon/Distribution Fri/Unlimited Begrenset/Limited Ingen/None Dato/Date Rev.nr./Rev.No. 0 Emneord/Keywords Piles, Laysac, Comsol, Stiffness, damping, cutoff frequency. Stedfesting/Geographical information Land, fylke/country, County Havområde/Offshore area Kommune/Municipality Feltnavn/Field name Sted/Location Sted/Location Kartblad/Map Felt, blokknr./field, Block No. UTM-koordinater/UTM-coordinates Dokumentkontroll/Document control Kvalitetssikring i henhold til/quality assurance according to NS-EN ISO9001 Rev./ Rev. Revisjonsgrunnlag/Reason for revision Egenkontroll/ Self review av/by: 0 Original document JJo CM Sidemannskontroll/ Colleague review av/by: Uavhengig kontroll/ Independent review av/by: Tverrfaglig kontroll/ Interdisciplinary review av/by: Dokument godkjent for utsendelse/ Document approved for release Dato/Date Sign. Prosjektleder/Project Manager Jörgen Johansson Skj.nr. 043

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