![]() ![]() In order to capture the directional information in stochastic waves, researchers spend efforts on improving both measurement techniques and estimation methods. A three-dimension wave spectrum could provide energy and directional distributions while it is usually more difficult to obtain. ![]() Stochastic waves have an inherent nature of multidirections. A two-dimension wave spectrum only contains energy distribution in the frequency domain and is easily estimated by using measurement data from single-point gauges. ![]() Wave spectra are usually present in either two-dimension or three-dimension forms. However, limited literature focuses on this issue and still lacks in-depth investigation. Accurate estimation of the actually encountering directional wave spectrum for offshore structures in the incident and diffracted wave field could benefit the scientific and engineering practice including real-time load evaluation, structural monitoring, motion control, etc., and therefore, is essential to study. Some previous studies that investigated the disturbed wave field due to wave reflection effect in the laboratory shows that the disturbed wave components may impose significant deviations in the spectral density and direction results and hence the conventional methods for undisturbed wave field are not applicable. However, the disturbed wave field, containing the incident and diffracted wave components, brings challenges to the estimation of directional wave spectrum because the relationship between diffracted and incident waves is synchronized and coupled. Conventional methods using measurement data such as array wave elevations or array wave pressures to estimate the directional wave spectrum are based on the assumption that the wave field is an undisturbed condition, and therefore all wave components are independent and random. Wave diffraction refers to a particular phenomenon when propagating waves encounter obstacles, and the obstacle surface may, depending on the obstacle size, induce diffracted waves. However, after the structure is built in place, the local wave field is disturbed by the presence of structure due to the wave diffraction effect. Conventionally, estimation of directional wave spectrum is usually performed using measurement data collected by spatial array or pitch-and-roll buoys. Measurement and determination of the directional wave spectrum for the offshore structures is an engineering premise of multiple applications, including the evaluation of hydrodynamic loads, wave-induced vibration, etc., and hence becomes a primary task in the health monitoring and maintenance of offshore structures. The three-dimensional wave spectrum, i.e., directional wave spectrum, providing the basic information including energy and direction distributions for stochastic waves, is one of the most fundamental properties in offshore engineering. In general, the presented approach can reasonably estimate the directional wave spectrum and show advantages over the conventional approach in which the diffraction effect is excluded. The presented approach is also deployed into an in-situ measurement application on a marine structure and compared with wave observation data to test its feasibility in engineering practice. Considering the performance of the presented approach under scenarios with various gauge arrays, different directions, and spreading coefficients, multiple levels of background noise are evaluated and discussed, respectively. In this study, the diffraction wave theory is introduced into the estimation of directional wave spectrum to consider the effect of diffracted waves using array pressure data from existing pressure gauges on structures. Estimating directional wave spectrum in diffracted wave field significantly differs from the occasion in undisturbed waves since the amplitude and phase relationship between the incident and diffracted waves are coupled, and therefore making the conventional approach not applicable. Due to the wave diffraction effect, wave field around an offshore structure is the mixture of incident wave components and diffracted wave components. Determination of the directional wave spectrum which offshore structures actually encounter is essential for multiple applications including wave-induced load and vibration evaluation, and hence becomes a fundamental task in ocean engineering. ![]()
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