Numerical Simulations of the Monotonic and Cyclic Behaviour of Offshore Wind Turbine Monopile Foundations in Clayey Soils
Abstract
:1. Introduction
- 1.
- the possibility of using large areas allows for major projects, which may introduce super large turbines, with higher energy production;
- 2.
- the offshore wind speed is usually higher than under onshore conditions, which means that more energy can be generated in a given period (i.e., higher efficiency);
- 3.
- less wind turbulence is usually found for offshore conditions; therefore, the energy can be produced more efficiently and the risk of wind turbine structure fatigue can be also reduced.
- easy and fast installation: monopiles can be driven in by a hydraulic hammer and no prior soil reinforcement is required for the seabed [8];
- monopiles are easy to manufacture, which is critical for large wind farm projects that may require the production of hundreds of them [13];
- the jack-up rig that employed for oil and gas platform foundations can be compatible with monopiles; therefore, no special jack-ups are needed for monopile foundations [13];
- their analysis and design are relatively simple to undertake due to its regular geometry and aerodynamic behaviour under loads such as sea waves and wind. That makes these foundations easy to be modelled.
- establishing a finite element model for offshore wind turbine monopile foundations;
- validating the model with the corresponding centrifuge test under monotonic loads;
- validating the finite element model with a centrifuge test under cyclic loads;
- developing large diameter monopile models based on the previous validations;
- improving the understanding of the non-linear cyclic behaviour of monopiles in clays;
- developing a new monopile design procedure based on the numerical simulation results.
2. Methodology
2.1. Model Geometry and Boundary Conditions
2.2. Pile Modelling
2.3. Soil Modelling
2.4. Soil-Pile Interface
2.5. Input Loads
2.6. Mesh Density and Sensitivity Analysis
3. Validation of the Numerical Model
3.1. Centrifuge Tests
3.2. Input Parameters and Results of the Validations
4. Results
4.1. Time History of the Pile Rotation
4.2. Pile Rotation in the First Cycle
4.3. Cumulative Rate of Rotation
5. Procedure to Design Monopiles in Clay
- 1.
- determine the capacity of the offshore wind turbine; estimate the corresponding design horizontal load;
- 2.
- measure the undrained shear strength (Cu) of the clay in the seabed;
- 3.
- estimate the initial pile geometry that included the pile diameter (D) and the embedded depth (L);
- 4.
- calculate the pile wall thickness (through Equation (1));
- 5.
- 6.
- estimate the number of storm cycles that the offshore wind turbine may experience;
- 7.
- calculate the pile rotation under the number of storm cycles using Equation (7);
- 8.
- if the rotation was larger than the design requirement, then we need to modify the dimensions of the pile and repeat the procedure from step 3.
5.1. Design Example
5.1.1. 5MW Offshore Wind Turbine
5.1.2. 3.5 MW Offshore Wind Turbine
6. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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D—Pile Diameter (m) | tp—Pile Thickness (mm) | H—Total Pile-Structure Length (m) | L—Embedded Length (m) |
---|---|---|---|
5.00 | 57 | 55 | 25–30 |
6.25 | 69 | 55 | 25 |
7.50 | 82 | 55 | 25 |
Material | Density (kg/m3) | Young’s Modulus (kPa) | Poisson’s Ratio |
---|---|---|---|
Steel | 7850 | 200,000 | 0.3 |
Clay Type | Unit Weight (kN/m3) | Plasticity Index (%) | Undrained Shear Strength (kPa) | Correction Factor, Kc | Young’s Modulus (kPa) |
---|---|---|---|---|---|
1 | 44 | 50 | 383 | 19,150 | |
2 | 40 | 75 | 447 | 33,525 | |
3 | 18 | 38 | 100 | 479 | 47,900 |
4 (0–10 m depth) | 44 | 50 | 383 | 19,150 | |
4 (10–20 m depth) | 40 | 75 | 447 | 33,525 |
Clay Type | Plasticity Index (%) | Coefficient of Friction |
---|---|---|
1 | 44 | 0.240 |
2 | 40 | 0.247 |
3 | 38 | 0.250 |
4 (0–10 m depth) | 44 | 0.240 |
4 (⩾10 m depth) | 40 | 0.250 |
Site | Layer 1 | Layer 2 | ||
---|---|---|---|---|
Depth a (m) | Consolidation Pressure (kPa) | Depth b (m) | Consolidation Pressure (kPa) | |
A | 13.5 | 500 | 13.5 | 500 |
B | 13.5 | 300 | 13.5 | 300 |
C | 13.5 | 180 | 13.5 | 180 |
D | 5.0 | 180 | 22.0 | 500 |
E | 10.0 | 180 | 17.0 | 500 |
Phase | Test Number | Test Nature | Pile Diameter (m) | Vertical Load (MN) | Site Specification |
---|---|---|---|---|---|
OWF-01 | 6.5 | B | |||
I | OWF-02 | Monotonic | 3.83 | 6.5 | A |
OWF-03 | 4.0 | C | |||
OWF-04 | 4.0 | C | |||
OWF-05 | 6.5 | B | |||
II | OWF-06 | Cyclic | 7.62 | 6.1 | A |
OWF-07 | 6.0 | D | |||
OWF-08 | 6.1 | E | |||
III | OWF-09 | Cyclic | 7.62 | 12.0 | A |
Stage | OWF-06 | OWF-07 | OWF-08 | OWF-09 | ||||
---|---|---|---|---|---|---|---|---|
Min, Max Load (N) | No. of Cycles | Min, Max Load (N) | No. of Cycles | Min, Max Load (N) | No. of Cycles | Min, Max Load (N) | No. of Cycles | |
1 | 0, 3 | 1000 | 0, 3 | 1000 | 0, 3 | 1000 | 0, 6 | 1000 |
2 | −3.2, 36 | 1000 | 2, 32.5 | 1000 | 0.2, 31.2 | 1000 | −1, 60 | 1000 |
3 | −2.5, 79 | 1000 | −19.5, 56.5 | 500 | 3.6, 79 | 1000 | 5, 150 | 1000 |
4 | 0, 100 | 100 | −0.6, 100 | 500 | 10.6, 111 | 500 | 17.3, 223 | 1000 |
5 | NA | NA | −30, 123 | 479 | −30.6, 121 | 500 | −65, 238 | 500 |
6 | NA | NA | 14, 167.5 | 500 | −12.5, 140 | 500 | −86, 217 | 500 |
7 | Monotonic Push |
Layer | Monotonic Behaviour Validation | Cyclic Behaviour Validation | |||||||
---|---|---|---|---|---|---|---|---|---|
Depth (m) | OCR | Kc | Cu (kPa) | E (kPa) | OCR | Kc | Cu (kPa) | E (kPa) | |
1 | 1 | 14.7 | 147.0 | 7.7 | 1126.4 | 14.7 | 147.0 | 24.0 | 3520.8 |
2 | 3 | 9.8 | 152.6 | 12.3 | 1879.2 | 9.8 | 152.6 | 27.1 | 4137.3 |
3 | 5 | 5.8 | 278.3 | 14.4 | 3998.4 | 5.8 | 278.3 | 35.9 | 9999.7 |
4 | 7 | 4.0 | 397.0 | 15.9 | 6301.1 | 10.4 | 147.0 | 44.6 | 6547.5 |
5 | 9 | 3.1 | 483.0 | 17.2 | 8326.7 | 8.0 | 197.7 | 46.9 | 9264.0 |
6 | 11 | 2.5 | 527.7 | 18.1 | 9530.9 | 6.5 | 251.0 | 50.2 | 12,607.2 |
7 | 13 | 2.1 | 561.7 | 18.6 | 10,453.4 | 5.5 | 293.0 | 54.4 | 15,948.2 |
8 | 15 | 1.8 | 579.6 | 19.2 | 11,103.3 | 4.7 | 341.0 | 56.3 | 19,205.9 |
9 | 17 | 1.6 | 589.2 | 19.7 | 11,608.8 | 4.0 | 394.0 | 60.9 | 24,012.6 |
10 | 19 | 1.3 | 597.3 | 21.2 | 12,668.1 | 3.5 | 441.0 | 65.4 | 28,823.3 |
11 | 20 | 1.3 | 599.5 | 22.0 | 13,206.5 | 3.3 | 454.0 | 67.9 | 30,817.9 |
Model No. | Pile Diameter, D (m) | Embedded Pile Length, L (m) | Undrained Shear Strength, Cu (kPa) | Number of Cycles, N |
---|---|---|---|---|
1 | 5.00 | |||
2 | 6.25 | 25 | 50 | 100 |
3 | 7.50 | |||
4 | 5.00 | |||
5 | 6.25 | 25 | 75 | 100 |
6 | 7.50 | |||
7 | 5.00 | |||
8 | 6.25 | 25 | 100 | 100 |
9 | 7.50 | |||
10 | 5.00 | |||
11 | 6.25 | 25 | 50 & 75 | 100 |
12 | 7.50 | |||
13 | 26 | |||
14 | 27 | |||
15 | 5.00 | 28 | 50 | 100 |
16 | 29 | |||
17 | 30 |
Model No. | Pile Diameter, D (m) | Embedded Pile Length, L (m) | Undrained Shear Strength, Cu (kPa) | D × L × ln(Cu) (m2 ln(kPa)) | Rotation 0 (Degree) |
---|---|---|---|---|---|
1 | 5.00 | 489 | 1.65 | ||
2 | 6.25 | 25 | 50 | 611 | 0.72 |
3 | 7.50 | 734 | 0.45 | ||
4 | 5.00 | 540 | 0.76 | ||
5 | 6.25 | 25 | 75 | 675 | 0.42 |
6 | 7.50 | 810 | 0.28 | ||
7 | 5.00 | 576 | 0.61 | ||
8 | 6.25 | 25 | 100 | 720 | 0.33 |
9 | 7.50 | 863 | 0.22 | ||
10 | 5.00 | 517 | 1.07 | ||
11 | 6.25 | 25 | 62.5 | 646 | 0.49 |
12 | 7.50 | 775 | 0.33 | ||
13 | 26 | 509 | 1.24 | ||
14 | 27 | 528 | 1.08 | ||
15 | 5.00 | 28 | 50 | 548 | 0.96 |
16 | 29 | 567 | 0.89 | ||
17 | 30 | 587 | 0.85 |
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Xie, M.; Lopez-Querol, S. Numerical Simulations of the Monotonic and Cyclic Behaviour of Offshore Wind Turbine Monopile Foundations in Clayey Soils. J. Mar. Sci. Eng. 2021, 9, 1036. https://doi.org/10.3390/jmse9091036
Xie M, Lopez-Querol S. Numerical Simulations of the Monotonic and Cyclic Behaviour of Offshore Wind Turbine Monopile Foundations in Clayey Soils. Journal of Marine Science and Engineering. 2021; 9(9):1036. https://doi.org/10.3390/jmse9091036
Chicago/Turabian StyleXie, Mian, and Susana Lopez-Querol. 2021. "Numerical Simulations of the Monotonic and Cyclic Behaviour of Offshore Wind Turbine Monopile Foundations in Clayey Soils" Journal of Marine Science and Engineering 9, no. 9: 1036. https://doi.org/10.3390/jmse9091036