# The Lateral Behavior of Large-Diameter Monopiles for Offshore Wind Turbines Based on the p-y Curve and Solid FEM Methods

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## Abstract

**:**

## 1. Introduction

## 2. Offshore Full-Scale In Situ Test of Pile

#### 2.1. Testing Pile Arrangement and Soil Seabed Parameters

#### 2.2. Testing Instrument Arrangement and Loading Method

#### 2.3. Testing Result and Analysis

## 3. Testing Pile Analysis Based on the P-Y Curve and Solid Finite Element Method

#### 3.1. The P-Y Curve Method

#### 3.2. The Solid Finite Element Method

_{0}are set separately. After the initial stress field is completely balanced, the horizontal loading is applied on the pile head, and the imposed position of horizontal loading in the numerical simulation is consistent with the field test. To avoid the stress concentration caused by concentrated loading on the pile head, horizontal loading is adopted in the form of surface traction, that is, the horizontal loading is uniformly distributed around the circumference of the pile head, and there are no rigid restraints.

## 4. Result Analysis and Discussion

## 5. Conclusions

- (1)
- The testing pile under horizontal loads presents the typical response of a flexible pile, with the influence range of horizontal loading limited to 5.7 D below the mud surface. The loaded indicator of the testing pile shows a significant linear response with the increase in the loading, i.e., the pile head displacement, bending moment, and deformation of the pile. In addition, the maximum bending moment of the pile gradually moves down with the increase in horizontal loading and tends to be a stable value up to 1.1 D below the mud surface.
- (2)
- There are differences in design results for monopile foundations obtained by the p-y curve and the solid finite element method. The p-y curve recommended by the API specification tends to be flexible on the whole, resulting in a conservative calculation result. Under the condition of small displacement, the calculation result by the solid finite element method is close to the measured result, however, there is a risk of overestimating the ultimate bearing capacity under the condition of large displacement.
- (3)
- The reason for the significant difference between the p-y curve method recommended by the API specification and the solid finite element method is that the stiffness of the p-y curve method recommended by the API specification is significantly lower than that of the finite element method, and its displacement corresponding to the ultimate soil reaction (over 0.65 D) is much larger than that of the solid finite element method (about 0.03 D). Moreover, the stable value of the ultimate reaction (${P}_{ult}$) calculated by the solid finite element method is close to 12${s}_{u}D$, which is significantly higher than the recommended value of 9${s}_{u}D$ in the API specification.
- (4)
- The design result obtained by the solid finite element method depends heavily on the values of relevant parameters. For an offshore wind farm dominated by clay soil, the upper and lower values of the ultimate bearing capacity of the pile differ by 1.7 times if different modulus ratios are selected. Under the condition that there is no reliable basis, it is suggested that the relatively conservative results obtained by parameter analysis should be used as the design reference in a reasonable range. Without a reliable basis and data, sensitivity and parameter analysis within a reasonable range is suggested to be conducted to select relatively conservative calculation results as the design basis.
- (5)
- The influence of the pile–soil gap must be considered with the solid FEM adopted in the design of the monopile foundation, and bonding between pile and soil can significantly overestimate the stiffness and bearing capacity of the pile, resulting in unsafe design results.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 1.**Layout of in situ test. (

**a**) Arrangement schematic diagram of offshore testing piles. (

**b**) Site operation of offshore testing pile.

**Figure 5.**Distribution of pile deformations and bending moments subjected to different lateral loadings. (

**a**) Horizontal deformation of pile. (

**b**) Section bending moment of pile.

Parameter | E/MPa | $\mathsf{\nu}$ | c/kPa | $\mathit{\phi}$$/\xb0$ |
---|---|---|---|---|

Steel pile | 210,000 | 0.3 | - | - |

Mucky silty clay | 16.8 | 0.49 | 33.6 | 0.1 |

Sand | 23.75 | 0.49 | 1 | 32 |

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**MDPI and ACS Style**

Li, T.; Yu, X.; He, B.; Dai, S.
The Lateral Behavior of Large-Diameter Monopiles for Offshore Wind Turbines Based on the p-y Curve and Solid FEM Methods. *J. Mar. Sci. Eng.* **2023**, *11*, 2354.
https://doi.org/10.3390/jmse11122354

**AMA Style**

Li T, Yu X, He B, Dai S.
The Lateral Behavior of Large-Diameter Monopiles for Offshore Wind Turbines Based on the p-y Curve and Solid FEM Methods. *Journal of Marine Science and Engineering*. 2023; 11(12):2354.
https://doi.org/10.3390/jmse11122354

**Chicago/Turabian Style**

Li, Tao, Xinran Yu, Ben He, and Song Dai.
2023. "The Lateral Behavior of Large-Diameter Monopiles for Offshore Wind Turbines Based on the p-y Curve and Solid FEM Methods" *Journal of Marine Science and Engineering* 11, no. 12: 2354.
https://doi.org/10.3390/jmse11122354