Wave direction is an important design element of offshore structures, while it is difficult to measure accurate wave direction in practical sea areas. Based on the cross-spectrum between various wave elevation time series, the down-looking radar can provide a contact-free solution for near-field wave direction measuring of offshore structures. A numerical simulation is conducted to study the applicable conditions and accuracy of the wave direction estimate considering wave energy variation for down-looking radar. The results show that the estimated wave direction consistent with the target direction in all directions. The maximum error is 0.9° for unidirectional irregular waves and 2.3° for multidirectional irregular waves.
Waves are essential element in ocean engineering design and construction (Tucker, 2001; Holthuijsen, 2007). With the development of electronic technology and sonar technology, a variety of wave instruments for marine wave measurement have emerged (Massel, 2015). According to the working principle, ocean wave measuring instruments are divided into sight distance measuring type, wave measuring rod type, pressure type, acoustic type, gravity type and remote sensing wave measuring instruments. Remote sensing wave measuring instruments are based on the measurement of radar wave irradiation on the sea surface, which can realize the contact-free measurement and provide a maintenance-free solution to obtain wave characteristics (Gronlie, 2004).
An actual wave is a multi-directional non-stationary wave field. Therefore, in addition to the wave characteristics such as wave height and period, the wave direction also has an important impact on the design, construction and stability of offshore structures. (Steele et al., 1992) The commonly used measurement methods of wave direction can be divided into contact sensor method and contact-free sensor method.
The contact sensor directly measures the relevant parameters of the wave system, such as wave fluctuation, water quality point acceleration and so on (Smith, 2002; Vlachos and Tsabaris, 2008). A wave buoy is one of the most common methods to measure wave direction. One kind of buoy uses the inclined direction in wave buoys at the zero point of the upper span represents the wave direction, that is, when the wave buoy moves with the seawater in the wave, the inclined direction of the wave buoy movement within a wave cycle represents the wave direction of the wave (Tang and Wang, 2008; Zhang, 2010). The other kind of wave direction identification method in wave buoys is mainly based on the cross-spectrum between the azimuth and the instantaneous displacement in the horizontal and vertical directions (Burdette, et al., 1978; Benoit, 1993; Steele, et al., 2004). The underwater acoustic wave and current measuring instrument mainly use the vector synthesis method to measure wave direction according to the Doppler principle (Herbers, 1991; Strong, et al., 2003; Tomasicchio, 2006).
