# Horizontal Loading Performance of Offshore Wind Turbine Pile Foundation Based on DPP-BOTDA
## 1 Introduction  
Offshore wind power, with the advantages of high wind speed, low wind edge cutting, low turbulence, and high output, has gradually become a new field of wind power development. However, the foundation of offshore wind turbines has a special engineering attribute. It is located in the marine environment and subjected to the repeated impact of seawater. Also, it bears a huge overturning moment and is affected by the natural vibration load of wind turbines. In addition, the equipment of a wind turbine has higher requirements for bearing capacity and deformation of foundation. Therefore, it is necessary to evaluate the bearing capacity of the foundation. At present, pile foundations are widely used in offshore wind turbine foundations, and piles are large in diameter, deep in soil, and high in bearing capacity. In order to obtain the actual bearing capacity of the pile foundation, thewidely used method is to carry out the horizontal bearing capacity test in the engineering sea area. The pile test can determine not only the bearing capacity of the whole pile but also the friction of the soil and evaluate the settlement characteristics of the pile foundation [1]. In the pile test, traditional instrumentation includes vibrating wire strain gauges (VWSGs), resistance strain gauges, axial load cells, and extensometers. However, the arrangement of sensors is limited by the number of test points, connecting lines, acquisition instruments, and costs. Also, the quality of the sensor installation cannot be guaranteed [2]. In this way, the result is a limited number of sensors on tens-of-meters-long piles. This situation will lead to test results only being able to reflect the local deformation characteristics of the pile. Moreover, the effective bearing capacity of the pile cannot be determined. In recent years, distributed optical fiber sensing technology has developed rapidly. It has been well applied in the civil engineering field, such as tunnels [3,4], bridges [5,6], railways [7], and foundations [8]. Compared with traditional electrical sensors, the distributed optical fiber sensors (DOFS) have unique advantages such as distributed measurement, chemical resistance, electromagnetic interference, and easy installation. Based on the above advantages, DOFS are very suitable for the marine environment. At present, in the field of pile foundation inspection, the application of DOFS mainly includes Brillouin optical time-domain reflectometry (BOTDR) [9,10,11,12,13], Brillouin optical time-domain analysis (BOTDA) [14,15], and optical frequency domain reflectometry (OFDR) [16,17]. The first two categories are Brillouin-based sensors, whereas the last one is Rayleigh-based sensors. Until now, much attention has been paid to improving the spatial resolution of Brillouin-based DOFS. The spatial resolution is determined by the pulse width in the BOTDA system. However, the short pulse will broaden the Brillouin spectrum and then reduce the fitting accuracy, when it is shorter than the phonon lifetime (\(\sim\)10 ns) [18]. Several impressive techniques have been proposed to realize sub-metric spatial resolution, such as the pulse pre-pump method [19], dark-pulse scheme [20], Brillouin echoes [21], and differential pulse pair (DPP) technique [22]. The first two methods pre-excite the acoustic field and then utilize an ultra-short pulse or dark pulse to develop a high spatial resolution BOTDA system. However, the sensing range is limited to several kilometers due to pump depletion. The last two techniques are implemented with two long different pulses (phase or width) and the differential signal is obtained via the subtraction between two Brillouin signals. Both techniques have shown the capabilities of high spatial resolution and long-range sensing. In this paper, differential pulse pair Brillouin optical time-domain analysis (DPP-BOTDA) technique is used in the horizontal-loading performance test of wind turbine pile foundations for the first time, to the best of our knowledge.