Geotechnical earthquake design of foundations for OWTs

Transcription

1 Geotechnical earthquake design of foundations for OWTs Geotechnical Engineering for Offshore Wind Infrastructure Workshop organized by HDEC and NGI Shanghai, China, 31 May, 2018 Amir M. Kaynia Technical Expert, Vibration and Earthquake Engineering, NGI Adjunct Professor, Norwegian University of Science and Technology, NTNU Adjunct Professor, Zhejiang University (ZJU)

2 Content Introduction Earthquake hazard and definitions Earthquake design criteria Estimating accelerations and loads Limit States and performance criteria Performance of different foundation types Liquefaction Issues in response under horizontal and vertical earthquake excitation

3 Type of support structures addressed Mono-piles (most common type): Water depths 30 m, D 6 m, L/D 5 (next generation: water depth 50 m, D 11 m) Gravity-based foundation Monopod: D 15 m Steel jacket on piles Tripods on suction caissons Floating turbines with anchors Foundations constitute 25% of costs

4 Basic concepts in seismic hazard Ground Acceleration, Magnitude, Intensity Key parameters in earthquake design are peak ground acceleration (PGA) and response spectra Intensity is a qualitative means to express the level of shaking at a given location. It cannot be used in design unless it is converted to acceleration Magnitude (M) is used by seismologists to characterize earthquake energy at source. Again, to be useful, it should be used to estimate the acceleration at the site. Seismic Hazard Information about accelerations are obtained from a Probabilistic Seismic Hazard Analysis (PSHA) which gives probability of exceeding PGA in a given time period. In most design codes it is probability of 10% exceedance in 50 years in simple terms: earthquakes with Return Period 500 years. Typical values of PGA for 500-yr return period: Oslo: 5% g; Japan 40% g, Shanghai: 8% g PGA

5 Overview of PSHA a) Identify all relevant earthquake sources. b) Characterize rates at which earthquakes of various magnitudes (M) are expected to occur for each source. c) Characterize distribution of source-tosite distances (R) for each source. d) Predict the chosen intensity measure for all combinations of magnitude, distance and ε (number of standard deviations of ground motion model used to estimate the intensity measure) for each source. e) Calculate the hazard curve at each spectral period.

6 Tectonic setting (example) From Wang et al. (2014) From Carlton et al. (2018)

7 Ground Motion Prediction Equation (GMPE) Models that predict the expected range of an earthquake intensity measure (IM) at a site from a given earthquake scenario based on source, path, and site effects. Simplest models represent source effects by moment magnitude (M w ), path effects by the distance from the rupture zone to the site (R RUP ) and site effects by the average shear wave velocity over the top 30 meters (Vs 30 ) Local GMPEs are best, otherwise have to use GMPEs for similar seismic regions: Shallow crustal in active tectonic regions (e.g. California, Italy, Turkey, Greece) Shallow crustal in stable continental regions (e.g. Northern Europe, Eastern N.A.) Subduction zone (e.g. Japan, Chile, New Zealand, Alaska)

8 Results of PSHA: Hazard curve De-aggregation Uniform Hazard Spectrum (UHS) Conditional Mean Spectrum (CMS) Provide input to: Selection of ground motion time histories and matching Seismic site response analysis Liquefaction analysis Response Acceleleration (mm/s 2 ) ( a p g 5%, o o ta ot o, s /s 5 0) 4975 year 3500 year 2475 year 1000 year 475 year 300 year 100 year 1g Dynamic slope stability g Period (secs)

9 Site response (amplification of ground motion) Soil response changes frequency content of earthquake motion Generally soft soils amplify ground motion => Case of Mexico City (M = 8.1) Large amplification ground motion around natural period of site (2 sec. here)

10 Global Earthquake Hazard onshore Global map of PGA on bedrock for return period of 500 years with typical soil amplification of ~2 on soft soil in Shanghai area, one could expect PGA ~ 0.15 g on seabed

11 Earthquake Hazard: offshore (ISO) PGA = 0.1 g on bedrock for 1000 year return period, East China Sea Using amplification factor 2 for soft soil, and converting to 500 year return period, gives PGA 0.15 g on seabed (this is not a low value!)

12 Loads on OWTs Main loads: Wind (mean and turbulent) Wave loads Harmonic load in connection with rotor rotation, 1P load Harmonic load due to blade passing/shadowing, 3P load Other loads, like earthquake, ship impact, ice,.. Earthquake loads are in most regions not governing due to their long natural period, however other aspects such as liquefaction of loose-medium dense sands and vertical earthquake motions are important issues Mean and turbulent wind profiles

13 Load frequencies and dynamic criteria Natural frequency of OWTs is typically in the range Hz The loads have different frequency bands Wind about 0.01 Hz (100 s.) Wave typically Hz, 1P: depending on turbine, Hz 3P: three times above, Hz Earthquake: horizontal Hz Earthquake: vertical Hz To avoid resonance, natural frequency of turbines should lie in the 1P 3P band f 1 = 0.31 Hz Vestas s V MN turbine

14 Some design differences to other structures For design earthquake (475-yr), foundation should not experience permanent tilt (more than 0.50 ) due to strict performance criteria of turbines. Earthquake is considered simultaneously with other environmental loads (wind and wave) representing operational conditions. There is very little damping in tower structure (as low as 0.5%) in side-side direction, and equally low in fore-aft direction in stand-still condition. Mono-piles are much larger than traditional piles used in other structures; therefore, classical solutions, such as p-y curves are not valid. The response of piles to liquefaction has not been adequately studied and it is not well understood. Kinematic pile interaction will result in larger rotations at pile head than in traditional (smaller diameter) piles.

15 Limit States and performance requirements Four limit states are to be satisfied Ultimate Limit State (ULS): structural strength and stability of members and joints Serviceability Limit State (SLS): maximum deformations of structure during operation Accidental Limit State (ALS): for example, effect of impacts due to ship collision Fatigue Limit State (FLS): structure/pile to withstand accumulated damage in design life Four acceptance/performance criteria for monopoles (DNV) vertical tangent criterion or zero-toe-kick criterion at monopile s deflection curve maximum lateral deflection at mudline 120 mm maximum lateral deflection at pile toe 20 mm maximum rotation at mudline of 0.50 (this includes installation imperfection of 0.25 ) At present, there is no additional criteria for earthquake loading. It is expected that new requirements will appear in standards, especially for vertical acceleration of turbine.

16 Load combinations for foundation design DNV 5 load combinations for ULS limit sate Most codes do not specify how to combine earthquake loads with ULS loads. Germanischer Lloyd (GL, 2010) suggests using earthquake load from Eurocode 8 or API with return period of 475 years. Resulting earthquake load with a load factor of 1.0 to be combined with design wind load cases during operation (normal wind profile and turbulent model) + an additional load case in which earthquake load is combined with 80% of reference wind speed for parked (standstill) turbine.

17 Earthquake return periods There appears to be consensus on use of earthquakes with return period 500 years under ULS limit sate for OWTs. This is consistent with the two-tier design approach from ISO for earthquake analysis of offshore structures. However, collapse of accommodation platforms could involve loss of life, and should therefore also be designed for earthquakes with higher return periods (typically 2500 years) under ALS conditions following ISO. Alfa Ventus Horns Rev 2 DanTysk (Vattenfall)

18 Influence of soil/foundation on earthquake response Response of OWT is strongly dependent on soil/foundation behavior Often advanced foundation models are necessary to predict dynamic and nonlinear response of OWTs => performance-based design The following are cases where moderate-strong earthquakes could impact design. Two examples are presented to highlight this: Determination of permanent tilt, especially jackets, monopods and tripods NB: permanent foundation tilt due to loads 0.25 => performance-based design Liquefaction effect on large-diameter piles Liquefaction effect on anchors of floating OWTs Vertical shaking

19 Liquefaction and consequences Niigata, 1964 Liquefaction is the condition of pore pressure reaching total stress due to cyclic loading. 1. Assessment 2. Consequences (sinking/tilting): important to estimate in platform design 3. Mitigation Turkey 1999 NZ, 2011

20 Assessment of liquefaction susceptibility Use of empirical methods, for example by Seed, Robertson, etc. for silica sand (for example, using CPT data) based on CRR and earthquake-induced CSR (Cyclic Stress Ratio). CSR is computed by simple formula from ground surface PGA: ah CSR=0,65τmax σ τ v0 max r dσv0 g = r = ( z) Use of plasticity models - a few promising models have recently been proposed and implemented in FE/FD codes. For special soil, like carbonate sand where empirical methods do not exist, use soil test data to estimate pore pressure and assess its impact on structure d cos 0, 04

21 Liquefaction mitigation (land applications) Compaction with vibratory probes Installation of Field Drains 21

22 Liquefaction mitigation (land applications) Dynamic Compaction Method Explosives Compaction 22

23 Impact of liquefaction on foundations Pile foundations They have the advantage that the pile can still be designed safely by ignoring the pile segment in liquefiable soil. If liquefaction is close to surface, the impact on design is not major, because the soil support at shallow depth is relatively small. In thick liquefiable layer, there is the additional potential of sinking (leading to tilt in jackets). Suction piles and anchors Same principle as in piles, except that due to shorter length of anchors, the anchor might miss a large percentage of its capacity Gravity based foundation The main problem is tilt during liquefaction. If base is stiff enough, it could even out the pressure under the base and minimize the tilt.

24 Permanent tilt of foundations (demonstration of mechanism) Rated power 3.5 MW H 0 = 90 m, Rotor diameter = 20 m, mass = 220 tons Two caisson configurations in uniform soil profile with s u = 120 kpa & G max /s u = 600 D=20 m & L/D =0.5 ; D=25 m & L/D=0.2 Wind and wave loads treated as static loads, 1 MN and 2 MN, respectively Von Mises failure criterion with kinematic hardening model in Abaqus Excitation: Takatori, Kobe, scaled to PGA = 0.35 g

25 Permanent tilt of foundation Simultaneous action of static lateral loads and earthquake shaking could lead to permanent lateral tilt. Therefore, accurate prediction of nonlinear response is critical. Computed tilt is 0.12 ( half of allowed 0.25 ). Sensitivity analyses on soil parameters and earthquake record could result in larger tilt. Simpler models, like foundation macro-elements, could also be used.

26 Vertical earthquake shaking Modern, large OWTs have relatively high natural periods in lateral direction - typically s; therefore, they are not expected to be very vulnerable to horizontal earthquake shaking in areas with minor to moderate seismicity. On the other hand, they have low natural periods in axial direction which could result in large vertical response under vertical earthquake shaking. Presently, the are no standards for earthquake analysis requirements and performance of OWTs, and engineers often ignore earthquake loading on the basis of above argument. Eurocode 8 for different ground

27 Vertical earthquake shaking Following traditional earthquake structural analysis (no SSI), one should expect large vertical acceleration and stresses in tower and turbine due to vertical earthquake shaking. Recent study (Kjørlaug and Kaynia, 2015) has confirmed this, and has also pointed out importance of considering SSI, including radiation damping, in reducing earthquake response. Factor of 3 Factor of 2 No radiation damping

28 Summary and conclusions Offshore wind industry seems to have a bright future and major growth. OWTs use different foundation supports each with their own challenges and large impact on design/cost. SSI (stiffness and damping) important for design and cost. Small allowable permanent foundation tilt (as low as 0.25%) calls for detailed nonlinear dynamic analyses => Performance-Based Design. Liquefaction is important for foundation design, especially anchors for floating OWTs. Some traditional soil-foundation analyses, such as p-y curves, are not applicable for large-diameter piles in OWTs. Kinematic interaction is also more critical due to small aspect ratio of foundations.

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