This paper presents the effect of cyclic loading on the lateral behavior of monopiles in terms of load-displacement curves, deflection curves, and
Monopiles have been frequently used to resist offshore environmental loads such as wind, wave, tidal, and ice loads. Unlike onshore loading conditions, the offshore environmental loads have two main differentiated characteristics; the magnitude of lateral load is significant and the load is cyclic. Cyclic loading may cause serious problems in offshore structures and surrounding soils depending on the soil type and properties. During the design lifetime, the offshore structure is known to undergo about 108 lateral cyclic loading events of varying amplitude [
The
As the aforementioned
The numerical analyses were often supported by small or full scale test results. Roesen et al. [
This paper investigates the lateral behavior of a monopile subjected to a lateral cyclic loading, focusing particularly on the influences of the number of cycles and magnitude of cyclic lateral load. The effect of cyclic loading was assessed in terms of accumulated strains, which were later modified into the input strains used in the SWM [
Figure
Material properties of soil and pile, and loading conditions.
The information of cyclic lateral load is difficult to quantify because the load is site specific and variable in direction and magnitude. Therefore, the static lateral capacity
The Strain Wedge Model was initially suggested by Norris [
Basic strain wedge in uniform soil (redrawn from [
Figure
Flow chart of SWM analysis accounting for cyclic loading.
Figure
Accumulated strain after
The static strain (
Relation between static strain (
The accumulated strains as a result of a large number of cycles were calculated based on test data from cyclic drained triaxial tests in compression and could be assessed using the following proposed equation [
Summary of the partial functions
Function | Material constants | |
---|---|---|
|
|
10−4 |
|
||
|
|
3.4 |
|
0.55 | |
|
6.0 | |
|
||
|
|
0.43 |
|
100 kPa | |
|
||
|
|
2.0 |
|
||
|
|
0.54 |
|
0.874 | |
|
||
|
Figure
Stress path in
The coefficient for the amplitude (
Development of strain amplitude with number of cycles for different deviatoric stress amplitudes (redrawn from [
Using the obtained deviatoric stress amplitudes along the pile, the strain amplitudes were estimated using Figure
Table
Strain amplitudes along the pile derived from Figure
Depth (m) |
| |||||
---|---|---|---|---|---|---|
25% |
50% |
75% | ||||
|
|
|
|
|
|
|
5 | 2.08 | 2.05 | 4.16 | 3.67 | 5.49 | 4.55 |
15 | 1.87 | 1.63 | 3.49 | 2.97 | 5.11 | 4.27 |
25 | 0.84 | 0.83 | 2.00 | 1.77 | 2.68 | 2.28 |
35 | 0.30 | 0.31 | 0.97 | 0.85 | 2.01 | 1.67 |
The coefficient for the number of cycles (
Tables
Calculated coefficients
Depth (m) |
| |||||
---|---|---|---|---|---|---|
25% |
50% |
75% | ||||
|
|
|
|
|
|
|
5 | 4.32 | 4.21 | 17.29 | 13.50 | 30.13 | 20.68 |
15 | 3.49 | 2.65 | 12.18 | 8.81 | 26.16 | 18.25 |
25 | 0.71 | 0.69 | 4.00 | 3.12 | 7.20 | 5.21 |
35 | 0.09 | 0.10 | 0.94 | 0.72 | 4.06 | 2.78 |
Calculated coefficients for number of cycles, average stress ratio, void ratio, polarization changes, and average mean stress.
Coefficient | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Depth |
|
|
|
|
|
|
|
| ||||
|
|
|
|
|
||||||||
5 | 36.67 | 27.50 | 1.40 | |||||||||
15 |
110.00 |
82.50 |
0.75 | 1.49 | 6.37 | 13.71 | 21.66 | 57.51 | 1.53 | 0.13 | 1 | 0.96 |
35 | 256.67 | 192.50 | 0.51 | |||||||||
|
||||||||||||
Independent of depth |
Constant values |
Independent of |
Accumulated strains and input strains at
Depth (m) | 25% |
50% |
75% | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|
|
|
|
| |||||||
|
|
|
|
|
|
|
|
|
|
|
|
|
5 | 0.17 | 6.52 | 4.59 | 10.93 | 0.69 | 20.90 | 5.11 | 25.31 | 1.21 | 32.02 | 5.62 | 36.43 |
15 | 0.10 | 2.81 | 5.59 | 8.31 | 0.33 | 9.35 | 5.83 | 14.84 | 0.72 | 19.36 | 6.21 | 24.85 |
25 | 0.01 | 0.54 | 6.10 | 6.62 | 0.08 | 2.42 | 6.17 | 8.50 | 0.14 | 4.03 | 6.23 | 10.12 |
35 | 0.00 | 0.05 | 6.51 | 6.57 | 0.01 | 0.40 | 6.52 | 6.91 | 0.06 | 1.57 | 6.57 | 8.08 |
Achmus et al. [
Comparison of accumulation rates of monopile displacement at seabed.
Figure
Load-displacement curves at pile head for 3 cyclic lateral loads: (a) 25%, (b) 50%, and (c) 75% of the static capacity (
25% of
50% of
75% of
Figure
Effect of number of cycles on the displacement at pile head.
Figure
Pile deflection curves for 3 cyclic lateral loads: (a) 25%, (b) 50%, and (c) 75% of the static capacity (
25% of
50% of
75% of
Figures
25% of
50% of
75% of
25% of
50% of
75% of
In this study, the lateral behavior of a monopile for an offshore wind turbine subject to cyclic loading was investigated using the SWM analysis in combination with the data from drained cyclic triaxial compression test results on quartz sand. From the SWM analyses, the load-displacement curves, deflection curves, and A new approach was proposed to investigate the effect of cyclic lateral loads on the lateral behavior of monopiles. The approach employed the input strains ( The displacement at pile head, representing the sum of accumulated displacements due to a cyclic loading and a subsequent monotonic loading towards a maximum load larger than the amplitude of the cycles, was found to increase exponentially as the logarithm of the number of cycles increased. For a given lateral load of 22.04 MN (lateral static capacity of the pile under consideration), the increase in lateral displacement at pile head was 111% if 105 cycles with an amplitude corresponding to 25% of the static capacity had been previously applied. The increase of the lateral displacement due to preceding cycles becomes as much as 140% at an amplitude corresponding to 75% of the static capacity. The The application of existing
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was partially supported by National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning (NRF-2013R1A1A1011983), and Korea Institute of Ocean Science and Technology (20113010020010-11-3-510).