Bichromatic Wave Selection for Validation of the Difference-Frequency Transfer Function for the OC6 Validation Campaign

Abstract

The focus of the Offshore Code Comparison Collaboration, Continuation, with Correlation and unCertainity (OC6) project, which operates under the International Energy Agency Wind Task 30, is to refine the accuracy of engineering tools used to design offshore wind turbines. In support of this work, a new validation campaign is being developed that seeks to better understand the nonlinear wave loading that excites floating wind systems at their low-frequency, rigid-body modes in surge and pitch. The validation data will be employed in a three-way validation between simplified engineering tools and higher-fidelity tools, such as computational fluid dynamics (CFD). Irregular wave spectrums, which are traditionally used to examine the nonlinear wave interaction with offshore structures, are too computationally expensive to be simulated in CFD tools, and so we will employ bichromatic wave cases instead. This paper reviews the process used to choose the bichromatic wave pairs to be applied in the campaign to validate the second-order difference-frequency quadratic and potential loads at the surge and pitch natural frequencies of a floating semisubmersible.

OC6 Phase Ib: Validation of the CFD predictions of difference-frequency wave excitation on a FOWT semisubmersible

“…The upper cylinder has a diameter of 12 m and a draft of 14 m (to the top face of the heave plate), whereas the heave plate has a diameter of 24 m and a height of 6 m, resulting in a total draft of 20 m. All dimensions given here are at full scale; however, all CFD simulations and wave-basin experiments were conducted at a 1:50 model scale. Five different bichromatic wave pairs, named B1 through B5, have been selected for the present analysis (Tom et al, 2019). The properties of the selected waves-including period, 𝑇 𝑚 ; frequency, 𝑓 𝑚 ; and the prescribed wave amplitude, |𝐴 𝑚 | (magnitude)-are listed in Table 1.…”

“…A subgroup in OC6 involving various academic and research institutions and industry partners worldwide modeled the captive semisubmersible using a variety of CFD tools. For this study, a set of five bichromatic wave conditions were carefully selected (Tom et al, 2019), with each OC6 participant carrying out independent simulations of some or all five wave conditions. The nonlinear, differencefrequency wave excitations in surge and pitch on the fixed semisubmersible were extracted from the simulation results.…”

OC6 Phase Ib: Floating Wind Component Experiment for Difference-Frequency Hydrodynamic Load Validation

“…Bichromatic waves are simply the addition of two regular waves, allowing one to directly examine the difference-frequency loads from one pair of waves rather than the many pairs that contribute to it in an irregular wave spectrum. If the frequencies are chosen well, bichromatic waves can be made repeatable over a short time window (see [31] for more information on the development of these bichromatic-wave cases), and the CFD simulations only need to be run for a few repeat periods (on the order of a few hundred seconds full scale) to capture the difference-frequency loads. Five different bichromatic-wave cases were run in this validation campaign (B1-B5).…”

Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

“…Such comparisons can help us better understand the limitations of the potential-flow QTFs. In [10], a set of bichromatic-wave pairs were selected for CFD investigation, and the present paper focuses on one key wave case. The two wave frequencies of the bichromatic wave pair investigated in this paper, f 1 and f 2 , are marked in Figure 1 using vertical lines.…”