Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Numerical Verification on Wave Direction Estimate Considering Wave Energy Variation
Hong Song, Shuang Chang, Fushun Liu*

Shandong Province Key Laboratory of Ocean Engineering, Ocean University of China, Qingdao, China

ABSTRACTand so on (Smith, 2002; Vlachos and Tsabaris, 2008). A wave buoy is
one of the most common methods to measure wave direction. One kindof 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 thewave buoy movement within a wave cycle represents the wave
direction of the wave (Tang and Wang, 2008; Zhang, 2010). The otherkind 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).

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 estimatedwave direction consistent with the target direction in all directions. The
maximum error is 0.9° for unidirectional irregular waves and 2.3° formultidirectional irregular waves.

KEY WORDS: wave direction; down looking radar; energy variation;measuring point
concentration.

distance;

mainwavedirection;

Contact-free sensors are mainly used for remote sensing observationthrough the interaction between electromagnetic waves and sea surface,
including imaging and non-imaging technologies. X-band radar mainly
determines the condition of sea surface waves by the strength of echo
signals (Gangeskar, 2002; Dankert, et al., 2006). The sea surface echo
signal will form a fringe image with light and dark distribution.
Through the mathematical analysis of the image sequence, the relevant
parameters of sea surface waves can be obtained. Through thecombination of different antennas, the distance-threshold PulseDoppler radar can be used to measure the velocity spectrum of waves,and the velocity spectrum can be established through the linear wave
theoretical model and wave height to obtain the relevant wave
parameters (Borge, et al., 1999). The down-looking radar verticallytransmits a narrow wave signal to the sea surface through the sensor,
then calculates the arrival time delay of the echo, calculates thedistance from the equipment to the sea surface, and then converts thetime series of sea surface fluctuation into the time series of wave heave.
A combination of three down-looking radars can also be used toaccurately measure ocean wave height and wave direction.

directional

INTRODUCTION
Waves are essential element in ocean engineering design andconstruction (Tucker, 2001; Holthuijsen, 2007). With the developmentof 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 andremote 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-freemeasurement 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 heightand period, the wave direction also has an important impact on thedesign, 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 down-looking radar has the advantages of contact-free and longterm operation and is particularly suited for extreme conditions
offshore. It can provide a good solution for near-field wave monitoringof offshore structures. The method of wave direction identification of
down-looking radar is mainly based on the cross-spectrum between
various wave elevation time series (Benoit, 1993; Krogstad et al.,1999). Traditional wave elevation time series decomposition
technology is Fourier-based, which decomposes the time series into the

The contact sensor directly measures the relevant parameters of the
wave system, such as wave fluctuation, water quality point acceleration

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