China's offshore wind power earthquake prevention!

China is located between the world's two major seismic belts-the Pacific Rim seismic belt and the Eurasian seismic belt. Seismic fault zones are very developed. China's seismic belts are mainly distributed in five regions: 23 seismic belts in Taiwan, Southwest, Northwest, North China, and Southeast Coast.

Figure1 China Seismic Zone

Offshore wind power is thedevelopment direction of the wind power industry and an important support for the transformation of my country's energy structure in the future. But at present, a large proportion of offshore wind farm sites are located just near these seismic zones, and there is a potential high risk of seismic damage. Therefore, on the premise of ensuring the safety of offshore wind power equipment and supporting structures and avoiding a significant increase in the cost of supporting structure design, reasonable seismic design is very important and cannot be ignored.

At present,there are no clear design codes and industry standards for offshore wind power seismic resistance at home and abroad. Therefore, the onshore "Code for Seismic Design of Buildings"—GB5001 is taken as one of the main reference codes for seismic design of offshore wind power projects.

Figure2 GB50011-2010 Code for seismic design of buildings
(2016 version)▲

1. The natural cycle of offshore wind power support structure is generally between 3s-5S, while the GB50011 reaction spectrum is within 0.2s to 1s for onshore buildings, which does not match the natural cycle of offshore wind power complete machine support structure;

2. In the absence of seismic safety assessment for offshore wind power project sites, the design ground motion input parameters used to determine the response spectrum are generally taken from the "China Earth Motion Parameter Zoning Map"-GB18306 in accordance with the requirements of GB50011. However, GB18306 currently only covers the mainland and is far away from the offshore wind power project site. The designed ground motion parameters for offshore wind power project sites are not accurate;

3. The standard seismic response spectrum in GB50011 is based on the response spectrum of Class II soft rock sites. Although GB50011 provides a simple conversion relationship from Class II sites to other Class III and IV sites, the conversion relationship cannot reflect the actual offshore Site characteristics of the wind power project site;

4. The standard seismic response spectrum in GB50011 is for building structures with a damping ratio of 5%, while the damping ratio of offshore wind power supporting structures takes into account all steel material damping, soil damping and hydrodynamic damping, generally 0.5% to 1.2 %range, how to convert 5% damping ratio response spectrum into 0.5% damping ratio response spectrum is also problematic. At present, the industry still lacks a recognized conversion relationship.

Advanced practices in international seismic design

Especially in the United States, in the field of seismic engineering design, site-related ground motion analysis methods have been widely used in the energy industry to assess the seismic safety of related structures.
Since offshore wind power is located on the sea, damage to the equipment and supporting structure of the wind turbine will affect the power supply, so you can learn from the international advanced site-related ground motion analysis methods. The method is generally divided into the following steps:

1. Earthquake safety assessment.

   Based on the seismic mechanism, seismic frequency, magnitude and distance from the project site in the seismic fault zone within 200 to 300 km of the perimeter of the site, combined with the attenuation relationship between seismic waves and the long-distance transmission of seismic waves along the bedrock, combined with the probability design concept, the seismic response spectrum of the site is calculated.

2. Slect the seismic acceleration time history seed
   

According to the similarity of earthquake conditions, and: similar magnitude, similar epicenter distance, similar earthquake occurrence mechanism and similar conditions of the soil cover of the engineering site, select the ground motion time history seeds from the measured ground motion time history library and pass the response spectrum-earthquake Dynamic time history fitting technology, transform the time history seeds, and fit the site-related seismic response spectrum;

  3. Analysis of the nonlinear response of the soil layer of the site
Based on geotechnical engineering parameters, model the soil cover of the project site, and analyze the nonlinear response of the soil layer of the ground motion site to simulate the propagation process of the time history of ground motion acceleration in the soil cover. Various weaknesses can be considered The magnification or attenuation of the ground motion acceleration time history by the soil layer can obtain the ground motion acceleration time history of different soil layers. Generally, the analysis can be calculated by the shake91 software;

  4. Nonlinear time-domain seismic load calculation considering pile-soil coupling
Model the entire pile foundation and load different ground motion time histories at different soil depths, and consider the pile-soil interaction, perform nonlinear time history calculations, and obtain seismic loads for seismic design of supporting structures.

The above seismic load calculation method can fully consider the seismic fault structure and the historical activity of ground motion around the specific offshore wind power project site, and consider the influence of the soil cover of the project site on the time history propagation of ground motion acceleration. The design ground motion parameters of the wind power project site, and then the site-related seismic response spectrum is obtained for seismic load calculation.

Security correction

In order to ensure the safety of engineering design, the following safety modifications should be made to the site correlation response spectrum based on the application experience of the field correlation ground motion analysis method in engineering in the United States:

1. The spectrum value at any period shall not be lower than 80% of the standard spectrum;
2. The response spectrum value of 0.1s (the starting point of the horizontal section) can take the value of the site-related response spectrum at 0.1s, but it cannot be less than 90% of the value of the response spectrum at any period greater than 0.1s;
3. The natural period of the offshore wind turbine support structure is usually not less than 3s, and the spectrum value at the natural period cannot be lower than a percentage of the spectrum value at 1s. The percentage should be in accordance with the JTS 146-2012 standard response spectrum at the natural period and 1s response spectrum The ratio of values determines the response spectrum value at 1s, which should be the greater of the calculated site-related response spectrum at 1s and 2s;

4. The response spectrum value from 0.1s to the characteristic period of the site shall be enforced in accordance with the specification and shall not be less than 100% of the specification response spectrum.

Prospects for seismic design of offshore wind power in China

With the rapid development of offshore wind power generation in China, offshore wind power generation will play an increasingly important role in my country’s power supply.

Therefore, it is imperative to form a reasonable and unified seismic design method in the industry for offshore project sites located in areas where earthquakes are high. For this seismic design method, it is necessary to ensure the safety of seismic design, and because earthquakes are rare events, it cannot increase the construction cost of offshore wind power generation and hinder the development of offshore wind power generation.
This requires the use of more advanced and refined seismic design methods. The use of internationally advanced site-related ground motion analysis methods points out the direction for solving this problem.
Manufacturers of wind turbines should complement each other's strengths in the United Nations Seismological Research Institute, integrate international offshore wind power design standards, domestic existing seismic design standards for onshore structures, and fully learn and absorb international advanced site-related ground motion analysis methods. Form a unified seismic design standard practice in the industry as soon as possible to protect China's offshore wind power projects facing the threat of earthquake damage