In this paper, the nonlinear corrosion model under the combined action of the anticorrosion system and corrosive environment is chosen as the mathematical model of homogeneous corrosion of steel pipe pile foundation for the offshore wind turbine. Based on the mathematical model, a three-dimensional finite element model was established for the steel pipe pile foundation of the offshore wind turbine. And the homogeneous corrosion action of the steel pipe piles was calculated, and the reduction rules of the strength and stability of the steel pipe piles for wind turbines under different corrosion patterns are analyzed. According to the calculation results, the mathematical model can be used in the analysis of corrosion for steel pipe pile in the wind turbine. Under the normal operation conditions, the reduction rules of the strength and stability of the steel pipe piles contain three stages: no influence stage, negative exponential decrease stage, and stable stage. But under the extreme load conditions, the effect of corrosion is enormous for the strength and stability of the steel pipe pile.

Offshore wind power resources serve as renewable green energy resources. Rapid development of the offshore wind power industry is the main direction for development of clean energy. However, offshore wind power plants are in the marine environment and the hydrological, meteorological, and submarine geological conditions are complex making it difficult to construct offshore wind power plants. Particularly, the wind turbine foundation as the major structure is in the marine corrosive environment for a long period of time. Thus, anticorrosion of the wind turbine foundation is one of the problems to be first solved in construction of wind turbines.

European countries started research on offshore wind power generation in as early as 1970s. To solve the highly corrosive effect of the marine environment on the wind turbine foundation, many scholars have been studying the corrosion resistance of the offshore steel structure. For example, Jeffrey and Melchers [

At present, there is some research on corrosion of various traditional maritime steel structures but there is little research on corrosion and protection of the offshore wind turbine foundation. And there is nearly no research on the rules of effect of corrosion on the wind turbine foundation.

The types of corrosion of the steel pipe piles for the wind turbine in the marine environment can be divided into homogeneous corrosion and local corrosion. Homogeneous corrosion is one of the major causes of damage to offshore steel pipe pile for the wind turbine. Local corrosion includes pitting corrosion, crevice corrosion, impingement corrosion, cavitation corrosion, and galvanic corrosion. Occurrence of such corrosion behaviors is often associated with structural design or steel smelting.

Homogeneous corrosion refers to the corrosion occurring at almost the same rate on the surface of the steel pipe pile for the wind turbine, which is one of the major causes of structural corrosion damage. It is different from the general corrosion of arbitrary pattern produced on the surface of the steel pipe pile structure and that on the metal surface. Homogeneous corrosion generally occurs in an area where the anode region and the cathode region are difficult to be distinguished at macro level. In other words, this is a corrosion pattern with service life easily predictable.

At present, there is focus on homogeneous corrosion in terms of the effect of corrosion on the structural strength. In most of the traditional research on corrosion, the corrosion rate is simplified as a random variable with a constant mean value; that is, it is assumed that the thickness of the steel structure is decreasing linearly with time. As a permanent construction, the offshore wind turbine foundation must be designed in such a manner that anticorrosion is considered before it is put into use. Therefore, when it comes to the corrosion characteristics of the offshore steel pipe pile structure for the wind turbine, the effective function of the corrosion protection system (CPS) must be taken into consideration.

Qin et al. [

Based on the integral of time

In fact, the corrosion protection system for the offshore steel pipe pile structure for the wind turbine is a gradual failure process, and the corrosion of the steel pipe piles has started before complete failure.

Based on the data of partial corrosion of the low alloy steel in the marine environment at Qingdao Corrosion Test Station, it is assumed that the corrosion data of the low alloy steel in the marine environment is as shown in Table

Corrosion data.

Time (annual) | Mean value (mm) | Standard deviation |
---|---|---|

2 | 0.02 | 0.01 |

3 | 0.05 | 0.01 |

3.5 | 0.09 | 0.01 |

4 | 0.28 | 0.15 |

4.5 | 0.34 | 0.15 |

5 | 0.68 | 0.15 |

6 | 1.02 | 0.20 |

7 | 1.43 | 0.20 |

8 | 2.08 | 0.20 |

9 | 2.82 | 0.5 |

10 | 2.63 | 0.5 |

11 | 4.82 | 0.5 |

12 | 5.50 | 0.5 |

13 | 5.96 | 0.5 |

14 | 6.48 | 0.80 |

15 | 7.08 | 0.80 |

16 | 7.24 | 0.80 |

17 | 7.36 | 0.80 |

18 | 7.43 | 0.80 |

19 | 7.47 | 0.80 |

20 | 7.50 | 0.80 |

It is assumed that the corrosion data in Table

If it is defined that the parameter

Corrosion rate curve for offshore steel pipe piles for wind turbines.

Based on the model, the development regularity of the homogeneous corrosion rate of the offshore steel pipe pile for the wind turbine can be divided into four stages.

A three-dimensional finite element model is established for the steel pipe pile foundation of the offshore wind turbine under the condition of homogeneous corrosion. A numerical simulation calculation is performed for the homogeneous corrosion of the steel pipe piles for wind turbines. Then, the strength and stability of the steel pipe piles for wind turbines under the condition of homogeneous corrosion are calculated on the basis of the mathematical model of homogeneous corrosion of offshore steel pipe pile for the wind turbine.

The single steel pipe pile foundation for the wind turbine in Donghai Bridge offshore wind power plant is taken as an example. The large scale general finite element analysis software ANSYS 10.0 is used to establish a three-dimensional finite element model for the wind turbine foundation and groundwork. As shown in Figure

59 pipe units are used as the steel pipe piles above the sea mud level. 16 pipe units are used as the steel pipe piles below the sea mud level. For the foundation soil body, the COMBIN39 nonlinear spring element is used to express the

Finite element model of single steel pipe pile foundation for wind turbines.

Single pile foundation for offshore wind turbines

The grids of single steel pipe pile foundation for wind turbines

^{3}, wind speed is 38 m/s at 8 m above the seal level, significant wave height is 5.8 m, significant wave period is 7.4 s, flow rate on the sea surface is 1.73 m/s, middle flow rate is 1.37 m/s, bottom flow rate is 0.95 m/s, and soft clay is at 10 m below the mud surface.

^{5} MPa, Poisson’s ratio is 0.3, and low alloy steel density is 7.85 × 10^{3} kg/m^{3}.

In accordance with Design Specification for Groundsill Foundation for Wind Turbine Generators (FD 003-2007) and Shanghai Authentication Specification for Wind Turbine Generators (China Classification Society, 2012), accidental seismic load conditions, extreme load conditions, and normal load conditions should be considered in calculation of the groundwork design load for the offshore wind power. In consideration of the minor combined effect of the accidental seismic load and corrosion of the wind turbine foundation, the paper is intended to study the effect of corrosion on strength and stability of the pile foundation structure under the conditions of extreme loads and normal operation. The extreme load condition is the combined effect of the extreme loads transferred from the upper structure plus other extreme loads applied on the foundation structure. The normal operation load condition is the combined effect of normal operation loads transferred from the upper structure plus other normal operation loads applied on the foundation structure. The load conditions are as shown in Table

Load conditions.

Load on pile top | The horizontal force/KN | The vertical force/KN | Bending moment/KN⋅m |
---|---|---|---|

Normal operation condition | 208 | 4700 | 13324 |

Extreme load condition | 2005 | 4700 | 112332 |

As the maintenance period of the corrosion protection system for the offshore wind turbine foundation generally does not exceed 20 years, analog computations are performed for corrosion in 20 years of operation of the steel pipe pile for the wind turbine to obtain the deflection and equivalent stress of the pile foundation at different point in time. The deflection and equivalent stress of the pile foundation under the normal operation condition when

Deformation and stress of pile foundation under normal operation condition when

The deflection

The equivalent stress

Curve of development of maximum deflection of pile foundation under normal operation condition.

Curve of development of maximum stress of pile foundation under normal operation condition.

Curve of development of maximum deflection of pile foundation under the condition of extreme load.

Curve of development of maximum stress of pile foundation under the condition of extreme load.

Under the normal operation condition, when

According to the calculation results, under the normal load conditions, the strength of the steel pipe pile for the wind turbine meets the safety requirements after homogeneous corrosion.

Based on computation, the structure stability coefficient is

It is known from the calculation result that the maximum deflection of the steel pipe pile for the wind turbine increases from 7.02 cm to 11.0 cm and increases by 2.98 cm, and the maximum stress increases from 50.6 MPa to 82.6 MPa; it increases by 32 Mpa within a recovery phase of the corrosion protection system. Over the course of time, the reduction in the strength and stability of the steel pipe pile for the wind turbine arising from corrosion is nonlinear development. The strength reduction coefficient is

Effect of corrosion on reduction in pile foundation strength.

Effect of corrosion on reduction in pile foundation stability.

Thus, we can obtain the strength and stability reduction regularity of the offshore steel pipe pile with the effect of the homogeneous corrosion under the normal operation conditions.

Its development regularity basically complies with the mathematical model of corrosion of steel pipe pile for the wind turbine. Thus, it can be used to verify the correctness of the mathematical model of corrosion of steel pipe pile for the wind turbine.

Under the extreme load conditions, the allowable stress of the structure can increase by 20% in accordance with the specification. Then, the allowable stress is

Based on the calculation of the corrosion rate for the offshore steel pipe pile, when

The strength and stability reduction regularities of the offshore steel pipe pile with the effect of homogeneous corrosion are shown in Figures

Effect of corrosion on reduction in pile foundation strength.

Effect of corrosion on reduction in pile foundation stability.

It is thus clear that the effect of corrosion is enormous for the strength and stability of the steel pipe pile under the extreme load conditions. Therefore, this stage must be avoided in design of the corrosion protection for the offshore steel pipe pile foundation of the wind turbine. Otherwise, there may be serious engineering accidents occurring and leading to huge economic loss.

There are no competing interests related to this paper.

The authors gratefully acknowledge the financial supports from the Natural Science Foundation Project of Chongqing under Grant no. cstc2013jjB0003 and the Specialized Research for the Doctoral Program of the Ministry of Education under Grant no. 20125522110004.