cycle loading for offshore turbin.pdf

FUGRO CONSULTANTS, INC.

EFFECTS EFFECTS OF CYCLIC LOADING L OADING ON SUCTION BUCKET FOUNDATIONS FOR OFFSHORE WIND TURBINES

Prepared for: Bureau of Safety and Environmental Enforcement

August 2016 Project No. 04.76160003

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FUGRO CONSULTANTS, INC.

August 26, 2016 Project No. 04.76160003 Bureau of Safety and Environmental Enforcement Acquisition Operations Branch 45600 Woodland Road, VAE-AMD Sterling, Virginia 20166-9216 Attention: Subject:

Mr. Altan Aydin Effects of Cyclic Loading on Suction Bucket Foundations for Offshore Wind Turbines

Dear Mr. Aydin: Fugro Consultants, Inc. (Fugro) is pleased to present this report documenting the methodology, results, and recommendations from the study performed to assess the effects of cyclic loading on suction bucket bucket foundations for offshore offshore wind turbines. Our services are being provided provided in accordance with Contract No. E16PC00004, dated dated February 8, 2016. We thank you for the opportunity to work on this project and are looking forward to continuing providing our services in the future. Sincerely, FUGRO CONSULTANTS, INC.

Connor Hayden, Ph.D., P.E. Senior Staff Engineer

Weiyu Chen, Ph.D., P.E. G.E. Principal Geotechnical Engineer

Dan Dolan, P.E. Principal Structural Engineer (Moffatt & Nichol)

Thaleia Travasarou, Ph.D., P.E., G.E. Principal Geotechnical Engineer



























Bureau of Safety and Environmental Enforcement Project No. 04.76160003

CONTENTS Page INTRODUCTION...................................................................................................................... 1-1 1.1 1.2 1.3 1.4 1.5

Project Description and Background ................................................................................ 1-1 Work Authorization .......................................................................................................... 1-2 Scope and Organization .................................................................................................. 1-2 Summary and Findings .................................................................................................... 1-2 Limitations of this Study ................................................................................................... 1-4

LITERATURE REVIEW ............................................................................................................ 2-1 2.1 2.2 2.3 2.4 2.5 2.6

Design Practice and Guidelines ....................................................................................... 2-1 Laboratory Soil Element Experiments .............................................................................. 2-2 Small Scale Laboratory Model Experiments .................................................................... 2-3 Large Scale Field experiments ........................................................................................ 2-5 Numerical Simulations of Suction Bucket Foundations .................................................... 2-5 Wind Turbine Projects Using Suction Bucket Foundations .............................................. 2-7

ADVANCED NUMERICAL MODELING MODELING METHODOLOGY ....................................................... 3-1 3.1 3.2

3.3

3.4 3.5 3.6

Introduction...................................................................................................................... 3-1 Loading Conditions .......................................................................................................... 3-2 3.2.1 Background .......................................................................................................... 3-2 3.2.2 Foundations ......................................................................................................... 3-3 3.2.3 Assumptions ........................................................................................................ 3-3 3.2.4 Loads ................................................................................................................... 3-3 Soil Degradation Models.................................................................................................. 3-4 3.3.1 Clay ..................................................................................................................... 3-4 3.3.2 Dense Sand ......................................................................................................... 3-6 Stepping Procedure ......................................................................................................... 3-6 Strength Degradation ...................................................................................................... 3-7 Estimation of System Fundamental Frequency................................................................ 3-8

ADVANCED NUMERICAL MODELING MODELING ANALYSES RESULTS............................................... 4-1 4.1 4.2

Introduction...................................................................................................................... 4-1 Cyclic Loading on Mono-bucket Foundation .................................................................... 4-1 4.2.1 Uniform Clay (soft and stiff) .................................................................................. 4-1 4.2.2 Stiff Clay .............................................................................................................. 4-2




























4.4

4.3.4 Interlayered .......................................................................................................... 4-5 Summary ......................................................................................................................... 4-5

DEVELOPMENT OF PRELIMINARY DESIGN GUIDELINES .................................................. 5-1 5.1

5.2

Cyclic Design Considerations .......................................................................................... 5-1 5.1.1 Subsurface Conditions and Soil Properties .......................................................... 5-1 5.1.2 Loading Conditions .............................................................................................. 5-2 5.1.3 Foundation Type T ype .................................................................................................. 5-2 Foundation Performance Checklist .................................................................................. 5-3 5.2.1 Cyclic Stability of Suction Bucket Foundation ....................................................... 5-3 5.2.2 Cyclic Settlement and Displacement .................................................................... 5-3 5.2.3 Cyclic Foundation Stiffness and Frequency ......................................................... 5-3 5.2.4 Cyclic Soil Reactions on Skirt and Top Plate........................................................ 5-4 5.2.5 Design Considerations ......................................................................................... 5-4

RECOMMENDATIONS FOR LABORATORY TESTING AND FUTURE STUDIES .................. 6-1 6.1 6.2

6.3

Limitations of Existing Studies and Data Gaps ........................................ ........................ 6-1 Recommendations for Future Laboratory Testing ............................................................ 6-1 6.2.1 Elemental Cyclic Soil Behavior ............................................................................. 6-2 6.2.2 Laboratory Model Experiments ............................................................................ 6-3 Recommendations for Additional Analytical Studi es ........................................................ 6-4 6.3.1 Design Soil-Load Combinations ........................................................................... 6-4 6.3.2 Definition of “Threshold” Limits............................................................................. 6-4 6.3.3 Assessment of Degradation for Actual Foundations Foundations ............................................. 6-4 6.3.4 Assessment of Cyclic Cyclic Wind Loads ....................................................................... 6-4

REFERENCES......................................................................................................................... 7-1

TABLES Page Table 2.1-1: Table 2.2-1: Table 2.3-1: Table 2.4-1: Table 2.6-1: Table 3.1-1.

Available Design Guidance for Offshore Wind Turbine ................................................. 2-1 Element Level Laboratory Cyclic Soil Tests ................................................................. 2-3 Small-scale Laboratory Experiments for Offshore Foundations .................................... 2-4 Suction Bucket Field Experiments ................................................................................ 2-5 OWT Projects with Suction Bucket Bucket Foundation ............................................................ 2-7 Foundation System Dimensions ................................................................................... 3-1




























Table 4.4-3. Table 5.2-1: Table 6.2-1: Table 6.2-2: Table 6.2-3:

Percent Change in Key Response Values during during Normal Sea State Loading............... 4-7 Cyclic Suction Bucket Foundation Design Considerations............................................ Considerations............................................ 5-4 Soil Classification Properties for US East Coast Offshore Wind Turbine Projects ........ 6-2 Recommendation for Laboratory Cyclic Cyclic Soil Elemental Tests ...................................... 6-3 Recommendation for Laboratory Model Experiments ................................................... 6-3

FIGURES

Dimensions of Caisson Foundation Systems and FLAC3D Model ................................................. 3.1-1 Monocaisson Foundation Loading Time T ime History ............................................................................. 3.2-1 Tetrapod Foundation Loading Time History ................................................................................... 3.2-2 Comparison of Vucetic and Dobry (1988) to Degradation Data ...................................................... 3.3-1 Degradation Parameter Form Used in this Study ........................................................................... 3.3-2 Example Degradation Index Curves for Varying Shear Strains ...................................................... 3.3-3 Example Approach to Variable Shear Strains ................................................................................ 3.3-4 Example of Finn/Byrne Model for Three Strain Levels ................................................................... 3.3-5 Example of Stepping Procedure..................................................................................................... 3.4-1 Validation of Stepping Approach – Contours After 900 Cycles ....................................................... 3.4-2 Validation of Stepping Approach – Rotation After 900 Cycles ........................................................ 3.4-3 Illustration of Stiffness Reduction Impact on Global Fundamental Frequencies ............................. 3.6-1 Degradation Index Contours – Monocassion, Soft Clay ................................................................. 4.2-1 Rotational Stiffness vs. Time – Monocaisson, Soft Clay ................................................................. 4.2-2 Global Fundamental Frequency vs. Time – Monocaisson, Soft Clay .............................................. 4.2-3 Maximum Total Rotation vs. Time – Monocaisson, Soft Clay ......................................................... 4.2-4 Degradation Index Contours – Monocaisson, Stiff Clay ................................................................. 4.2-5 Rotational Stiffness vs. Time – Monocaisson, Stiff Clay ................................................................. 4.2-6 Global Fundamental Frequency vs. Time – Monocaisson, Stiff Clay .............................................. 4.2-7 Maximum Total Rotation vs. Time – Monocaisson, Stiff Clay ......................................................... 4.2-8 Degradation Index Contours – Monocaisson, Dense Sand ............................................................ 4.2-9 Rotational Stiffness vs. Time – Monocaisson, Dense Sand .......................................................... 4.2-10 Global Fundamental Frequency vs. Time – Monocaisson, Dense Sand....................................... 4.2-11 Maximum Total Rotation vs. Time – Monocaisson, Dense Sand .................................................. 4.2-12 Degradation Index Contours – Monocaisson, Interlayered ........................................................... 4.2-13 Rotational Stiffness vs. Time – Monocaisson, Interlayered .......................................................... 4.2-14




























Degradation Index Contours – Tetrapod, Stiff Clay ........................................................................ 4.3-5 Rotational Stiffness vs. Time – Tetrapod, Stiff Clay........................................................................ 4.3-6 Global Fundamental Frequency vs. Time – T etrapod, Stiff Clay .................................................... 4.3-7 Maximum Total Rotation vs. Time – Tetrapod, Stiff Clay................................................................ 4.3-8 Degradation Index Contours – Tetrapod, T etrapod, Dense Sand ................................................................... 4.3-9 Rotational Stiffness vs. Time – Tetrapod, Dense Sand ................................................................ 4.3-10 Global Fundamental Frequency vs. Time – T etrapod, Dense Sand ............................................. 4.3-11 Maximum Total Rotation vs. Time – Tetrapod, Dense Sand ........................................................ 4.3-12 Loading Mechanism – Monocaisson vs. Multicaisson .................................................................... 5.1-1





























SECTION 1.0




























INTRODUCTION 1.1

PROJECT DESCRIPTION DESCRIPTION AND BACKGROUND BA CKGROUND

Suction buckets have widely been used as foundations for offshore oil and gas platforms; however, there have been limited applications applications in Offshore Wind Turbine (OWT) projects. For the OWT foundation, in addition to the design requirements on capacity and displacement, the foundation and overall system frequencies can only operate within a narrow range, which is different from the typical oil and gas platform. Due to limited data and case case histories, this research study was requested requested by the Bureau of Safety and Environmental Enforcement (BSEE) to assist in assessing the feasibility of suction bucket foundations for OWT projects. projects. This report presents the results from advanced numerical numerical analyses performed to assess the effect of cyclic loading on the response of Offshore Wind Turbines (OWT) founded on suction bucket foundations. In particular, the primary goal of this study study is to provide insight with respect to the effect of soil stiffness and strength degradation on the response of OWT founded on suction bucket foundations when when subjected to a large number of loading cycles. Of primary interest is the assessment of the effect of the soil stiffness and strength degradation on the foundation stiffness and associated fundamental period of the OWT system to ensure appropriate foundation design and avoidance of potential resonance phenomena. Throughout the design life of an offshore wind turbine (typically 20 – 30 years), the foundation will be subjected to permanent, cyclic, and extreme loads including, but not limited to: 1) environmental loads (wind, wave, and current), 2) structure-associated loads, and 3) other loads (accidental and operating loads). These different loading mechanisms excite excite the system with a wide wide range of frequencies adding complexity to the foundation response. For the cyclic foundation design, environmental loads, including wind and wave loads, and their impacts towards foundation performance are of the highest importance importance and are the main focus of this study. When subjected to cyclic loads, cyclic displacement of the suction bucket foundation would affect adjacent soils, which in return affect the cyclic foundation behavior through soil-foundation interaction effects. Under cyclic loads, soil strength can degrade, which is an issue under both operating and extreme loading conditions. Additionally, soil stiffness can degrade and result in excess deformation and change in natural




























OWT projects are currently founded on suction bucket foundations (Table ( Table 2.6-1). 2.6-1). The acceptable performance of this foundation concept for OWT has not yet been fully demonstrated due to limited number of studies (both numerical and experimental) and case histories. To make suction suction bucket foundations attractive for offshore wind applications, additional information is required to demonstrate their feasibility, particularly with respect to the effect of cyclic loading and potential soil stiffness and strength degradation on the dynamic response of these systems. systems. To this end, this study presents the results from advanced numerical analyses performed for a selected idealized cases to provide insight regarding the cyclic response of OWT founded on suction bucket foundations. foundations. Based on the analytical results, recommendations for design considerations and future laboratory testing were also developed and presented here. 1.2

WORK AUTHORIZATION

Fugro is providing engineering consulting services to the Bureau of Safety and Environmental Enforcement (BSEE) on the effect of cyclic loading on suction suction bucket foundations for OWT. The work was performed in general accordance with the scope of work outlined in Fugro’s proposal titled “Cyclic Loading of Suction Bucket Foundations”, dated September 3, 2015 which was submitted in response to Broad Agency Announcement Announcement (BAA) E15PS00085 “Research “Research for Renewable Renewable Energy” Topic 4. The work was performed in accordance with the contract terms and conditions under BSEE Award/Contract No. E16PC00004. 1.3

SCOPE AND ORGANIZATION The scope of work for this study included the following tasks: • • • • •

Task 1 – Review of Available Available Research Research and State of Practice; Practice; Task 2 – Advanced Numerical Analyses; Task 3 – Development of Preliminary Design Guidance for Suction Suction Bucket Bucket Foundation; Task 4 – Development of Recommendations for Laboratory Laboratory Testing Program; Task 5 – Meetings and Teleconferences; and Task 6 Reporting.




























•

Foundation concept: concept: a) a single single bucket with 16-meter diameter diameter and 12-meter penetration, and b) a tetrapod foundation with 10-m diameter and 6-meter penetration for each bucket. These geometries were generally based on design information from actual projects (e.g., Icebreaker);

•

Subsurface conditions: four idealized profiles were analyzed: a) soft clay, clay, b) stiff stiff clay, c) dense sand, and d) interlayered of dense sand overlying stiff clay. These idealized conditions were generally based on typical subsurface conditions from an overview of Fugro’s database of OWT projects in the US; and

•

Environmental loads: a) 1-year return period storm (wind (wind and wave), and b) 30-year return period storm were considered.

Analyses were performed considering both stiffness and strength degradation within soils due to cyclic loading and their effects on the system response. Qualitative observations from the analytical analytical results are summarized as follows: •

Soil stiffness and strength degradation is more prominent in cohesive cohesive soils (clays) than cohesionless soils (sands);

•

For clays, soil stiffness stiffness degradation results in a decrease in the system fundamental fundamental frequency and increase in cyclic foundation displacements (horizontal and vertical displacement and rotation) over time;

•

Strength degradation in clays clays does not appear to significantly significantly affect the rotational rotational stiffness stiffness and hence fundamental frequency of the system; however, it contributes to larger permanent foundation deformations, which may result in instability;

•

The amount of potential potential stiffness stiffness degradation in sands depends primarily on the sand relative relative density and drainage conditions;

•

For dense sands under fully fully drained conditions, cyclically cyclically induced soil densification results in a slight increase in the system fundamental frequency; and

•

The effect of soil and hence foundation stiffness degradation on the system system frequency is




























•

For the tetrapod buckets, the change in foundation peak rotations and fundamental frequencies are generally less compared to the mono-bucket. mono-bucket. In addition, for the tetrapod in soft clay (with strength degradation), the peak foundation rotation does not increase during the design life; and

•

From the performance standpoint, the tetrapod system system seemed to provide provide more stable performance compared to the mono-bucket. mono-bucket. However, related cost for fabrication and installation are also higher.

The above quantitative observations are based on the particular foundation geometries, idealized soil profiles, and loading combinations analyzed and the findings above should not be generalized. Also, the focus of this study study is to evaluate the effect effect of cyclic loading; foundation installation installation and static capacity/dimension analyses analyses are not part of the work scope. Additional analytical studies are recommended to examine a wider range of cases and allow for generalization and formulation of design recommendations. 1.5

LIMITATIONS OF THIS STUDY

This report has been prepared for the exclusive use of the Bureau of Safety and Environmental Enforcement and its agents for the specific application of the effects of cyclic loading on suction bucket foundation on offshore wind turbines. In our opinion, the findings, findings, conclusions, professional professional opinions and recommendations presented herein were prepared in accordance with generally accepted state of practice. This study examined a limited number of cases to explore the effect of cyclic soil stiffness and strength degradation on foundation stiffness, stiffness, stability and system response. The results of this study are applicable for conditions and assumptions similar to the cases analyzed and should not be generalized. Although information contained in this this report may be of some use for other purposes, it may not contain sufficient information information for other parties parties or uses. If any changes are made to the project project as described in this report, the conclusions and recommendations in this report shall not be considered





























SECTION 2.0




























LITERATURE REVIEW A review of available published pu blished journal and conference papers, research reports, and in-house projects related to offshore suction bucket bucket foundations is presented in the following sections. In particular, we have reviewed available literature on design guidelines, laboratory, and f ield model tests, large scale tests, existing and future future wind farm projects, and analytical studies. studies. The findings from this review were used to formulate the state of knowledge and practice in relation to the key attributes and response characteristics of suction bucket foundations for Offshore Wind Turbines (OWT). 2.1

DESIGN PRACTICE AND GUIDELINES

Design standards and guidelines for OWT have been developed by different organizations; however, no standard has yet been officially adopted for use by U.S. U.S. regulatory agencies. The majority of the guidelines were developed for conventional driven piles, shallow gravity base foundations (i.e., footings), and anchors (for floating structures), with very little or no design guidance on suction bucket foundations. The suction bucket is mentioned in the DNV-OS-J101 guideline (2014); however, itit suggests suction buckets be designed with gravity base specifications. Table 2.1-1 lists 2.1-1 lists the documents that make mention of cyclic degradation among all documents reviewed. However, none of these documents currently currently provide specific guidance on how to incorporate soil degradation phenomena in design.




























the assessment of permanent foundation rotations. In the SLS design condition, condition, soil’s shear modulus degradation due to cyclic loading shall be accounted for when calculating the foundation’s performance (fundamental vibration frequency, settlement, and lateral displacement). The DNVGL-ST-0126 design standard (Section 7.7) states that the suction bucket foundation concept is not covered by current available codes, thus a case-to-case evaluation is required. Thorough literature study should be performed prior to the design, and information based on former experience should be exploited. Soil strength degradation degradation from cyclic cyclic loads shall be considered. There are potential favorable and unfavorable consequences of the rate effect found from cyclic tests, therefore the loading mechanism should be well well understood. Last, this standard standard suggests that finite element analyses or or scale model tests be performed to better assess the possible failure modes, drainage mechanism, and soil reactions on the bucket due to cyclic loading. Overall, current design guidelines exist for more conventional gravity and pile foundations, which generally have very very different aspect ratios and response than suction buckets. buckets. DNVGL-ST-0126 recognized the difference between suction bucket with respect to those foundations as well as the limitation of understanding. Overall, detailed methodology and recommendations on how to incorporate cyclic loading effects in the design of OW T founded on suction bucket foundations are not provided due to limited knowledge in relation to these effects. This highlights the limited state of knowledge regarding the cyclic response of these systems and need for additional studies to enhance the industry’s level of understanding.




























Index (PI). In general, both higher OCR and PI values are associated with with smaller degradation compared to lower OCR and/or PI. For dense sands (Andersen & Berre 1999), the number of cycles to failure, the cyclic shear strength, and the failure mode (i.e., combination of average and cyclic shear strains at failure) of dense sand under cyclic loading was governed by the stress path, combination of average and cyclic shear stresses, and drainage conditions. For medium dense sands (Wichtmann 2010), the laboratory cyclic triaxial tests showed that excess pore pressure (if undrained) and/or volumetric strain (if drained) can occur when sand samples were subjected to cyclic cyclic loading. Magnitudes of the pore pressures pressures and densification were were dependent on the number of cycles cycles and confining pressure. Therefore, it is important to “replicate” the actual insitu stress state when conducting laboratory sand experiments. Table Table 2.2-1 2.2-1:: Element Level Level Laborator y Cyclic Soil Tests

Reference

Soil Type

D/U

PI

Relative Density (Rd)

Cyclic Test Type

Stress/Strain Control

# of cycles

Loading Frequency (Hz)

Andersen 1988

Clay

U

27

-

CTX, CSS

stress

10,000

0.1

Guo et al. 2013

Clay

U

36

-

CTX

stress

50,000

1




























Table Table 2.32.3-1: 1: Small-scale Laborator y Experim ents for Offs Offs hor e Foundatio ns Foundation Reference

Cox et al. 2014

Kim et al. 2014

Dyvik et al. 1989

Zhu et al.

Test

Centrifug e

Centrifug e

1-g

Soil

Dry Sand

Sat. Sand

Sat. Clay

Dry

Relative Density

85%

60% ~ 70%

-

Cyclic Loading

Type

Dimension (prototype)

Type

# cycles

MonoBucket

Dia. 16 m

H, M

12,000

Mono-;

Dia. 15.5 m;

Tripod

Dia. 6.5 m

Gravity Foundati on Mono-

Dia. 0.4 m

H, M

H, M

50

200

100,00

General Observation Observation

•

Rotational stiffness ↑ with # of cycles

•

Saturated sand appeared to be fully drained

•

Accumulated uplift uplift ↑ with # of cycles

•

Cyclic stiffness ↑ with # of cycles

•

Cyclic stiffness ↓ with # of cycles

•

Cyclic stiffness ↓ with load magnitude






























Permanent displacements (vertical and horizontal) and rotations were observed at the end of cyclic loading. For the suction bucket founded in dry sand, the laboratory results generally showed small increase in stiffness, primarily due to the densification effect attributed attributed to cyclic loading. However, the experiment performed by Bhattachar et al. (2013) showed that foundation stiffness could decrease after large number of cycles, probably due to irreversible damages in soil fabric (Andersen 2015). The experiments performed in saturated sands suggested that excess pore pressure can develop due the large number of of cyclic loading. As a result, foundation foundation stiffness decreased with with larger number of cycles (Byrne et al 2004). However, if drainage was allowed allowed to occur (Kelly et al. 2006, Kim et al. 2014), the foundation behaved more similar to to foundations in dry sand. Bye et al. (1995) discuss




























Compared to the conventional limit-equilibrium analyses, the numerical simulations can provide better details on time-dependent model behavior behavior rather than just the foundation capacity. However, applying the cyclic loading time histories in finite element or finite difference software with a large number of cycles can be very very time consuming and therefore not practical. An equivalent cycle (Neq) approach was proposed (Andersen 2015, Erbrich et al. 2016) to estimate the degraded soil properties and then used for design. The approach consists of three steps: In the first step, dynamic analyses are conducted with the initial soil properties to calculate the dynamic soil stress time histories in each soil element. In the second step, the irregular soil stress time histories are transformed to a number of cyclic load parcels. Each load parcel constitutes a uniform cyclic stress with different number of cycles using the rain flow counting method (Andersen 2015). The load parcels are then “plotted” on the cyclic stress vs. vs. N of cycles plot to estimate the accumulated pore pressures or shear strains. A degraded stress-strain soil




























2.6

WIND TURBINE PROJECTS USING SUCTION BUCKET FOUNDATIONS

Table 2.6-1 2.6-1 provides a list of offshore wind turbine projects with suction bucket foundations. Mono-bucket foundation was typically adopted for shallow water (<20 m) and multi-bucket foundation was used at deeper waters. The diameters for mono-bucket ranged between between 12 m to 18 m with the skirt length from 6 m to 15 m. For the multi-bucket foundation in the Borkum Riffgrund 1 wind wind farm in Germany, the diameter was 8 m with the skirt length of 8 m. The majority of the projects are located in Europe (i.e., Germany, Denmark and the UK), whereas there is only one project in the United States, which will be constructed in a lake rather than a sea or ocean environment. Table Table 2.62.6-1: 1: OWT OWT Projects wi th Suctio n Bucket Foundation






















































ADVANCED NUMERICAL MODELING METHODOLOGY METHODOLOGY 3.1

INTRODUCTION

Analyses were conducted in FLAC3D (Itasca, 2015), a finite difference software developed for geotechnical/structural analyses with with strong capabilities in non-linear, dynamic problems. problems. The soil and suction bucket were combined in an integrated 3-D model allowing analysis of the soil-bucket interaction. The wind and wave loads acting on the superstructure were converted converted into equivalent loads acting at the top of the bucket. Hence, the superstructure was not modeled in FLAC 3D. The FISH scripting language incorporated into FLAC3D allowed several custom functions to model clay and sand degradation to be developed.






























The idealized soil conditions selected represent the general soil conditions encountered in the




























approach was intended to establish the effects of degradation for the two regimes of wave load which




























The wave, current, and turbine loads were resolved into a force and overturning moment (or




























expression, which relates the degradation parameter (  ) to the number of cycles ( ) and the cyclic




























3.3.2

Dense Dens e Sand



































































































































































































































































































































































































































































































































































































































































































































































































1.66E+08

1.65E+08

1.64E+08

) 1.63E+08 d a r / m N1.62E+08 k ( s s e n1.61E+08 f f i t S l a n1.60E+08 o i t a t o R1.59E+08

‐

1.58E+08

1.57E+08

Storm Loading 1.56E+08 0.0001

0.01

1

Time (days)

Normal Sea State 100

10000



























0.492

0.491

0.49 ) z H ( y c n0.489 e u q e r F l a t 0.488 n e m a d n u F0.487 l a b o l G 0.486

0.485

Storm Loading 0.484 0.0001

0.01

Normal Sea State

1

Time (days)

100

10000



























0.24

0.23

) . g0.22 e d ( n o i t a t o R l 0.21 a t o T m u m i x a 0.2 M

0.19

Storm Loading 0.18 0.0001

0.01

1

Time (days)


10000



























G30yr/Gi



























4.60E+08

) d a r / m 4.55E+08 N k ( s s e n f f i t S l a n4.50E+08 o i t a t o R

‐

4.45E+08

Storm Loading 4.40E+08 0.0001

0.01

1

Time (days)


10000





























0.586

0.5855

) z H ( y 0.585 c n e u q e r F l a t 0.5845 n e m a d n u F l a b 0.584 o l G

0.5835



























0.1 No Strength Degradation

0.09

0.08

) . 0.07 g e d ( n0.06 o i t a t o R l 0.05 a t o T m u0.04 m i x a M0.03

0.02

Strength Degradation



























G30yr/Gi



























1.32E+09

1.32E+09

) d1.31E+09 a r / m N k ( s s e n1.31E+09 f f i t S l a n o i t a t 1.30E+09 o R

‐

1.30E+09



























0.6372

0.6371

) 0.637 z H ( y c n e u q 0.6369 e r F l a t n e m a d0.6368 n u F l a b o l G0.6367





























0.03

0.025

) . g 0.02 e d ( n o i t a t o R l 0.015 a t o T m





















































8.665E+08

8.660E+08

) d a r / m 8.655E+08 N k ( s s e n f f i t S l a n8.650E+08

‐



























0.62142

0.6214

) z 0.62138 H ( y c n e u q e r 0.62136 F l a t n e m a0.62134 d n



























0.03

0.025

) . g 0.02 e d ( n o i t a t o R l 0.015 a t o T m u























































4.50E+08

4.48E+08

) d a4.46E+08



























0.5839

0.5837 ) z H ( y 0.5835



























0.04

0.035

0.03 ) . g e
























































DEVELOPMENT OF PRELIMINARY DESIGN GUIDELINES




























For clays, cyclic stiffness degradation results in an overall reduction in foundation stiffness and system frequency with an associated increase in both the total and cyclic foundation displacements.




























is more important. important. The results from numerical analyses presented in Section 4.0 4.0 suggest suggest that cyclic foundation performance is highly dependent on the type of loads applied – overturning moments (for




























If the ratio of structure-to-foundation stiffness is larger than about one order of magnitude, then the effect of soil stiffness degradation on system frequency is likely negligible.































































































































































































































cycle loading for offshore turbin.pdf

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