Focused wave interactions with floating structures (CCP-WSI Blind Test Series 2)

The incident waves were generated in the COAST Laboratory Ocean Basin (Figure 1) using the EDL paddle control software. The software is designed to reproduce the desired freesurface elevation by applying various corrections to account for the change in water depth in front of the wave paddles and the nonlinear propagation of the wave fronts. In this case, each wave was create using linear superposition of 244 wave fronts with frequencies evenly spaced between 0.101563Hz and 2Hz and a theoretical focus location, x0. Each wave front is then transformed back to the position of the wave paddles by the control software. In each case, the theoretical focus location, x0, was calibrated to produce the desired wave shape (symmetric troughs either side of the main crest) at the rest position of the buoy(s), i.e. the position of wave probe 5. The amplitudes of the frequency components were derived by applying the NewWave theory to a Pierson-Moskowitz spectrum with the parameters in the Table 4. All waves in this Blind Test Series are crest focused waves, i.e. have zero phase angle at the focus location (Figure 6). 

Figure 6: Surface elevation measurements from the empty tank tests, at wave gauge 5 (buoy position), for each of test cases (NOTE: peaks artificially aligned at time = 0s). 

The CCP-WSI Blind Test Series 2 is a ‘blind’ validation of numerical WSI codes (The CCP-WSI Blind Test Series 2 is now closed). Consequently the only physical measurement data released at this time is the surface elevation data from the wave gauges in the empty tank tests (see Figure 2 for the wave gauge positions). This data should be sufficient to reproduce the incident waves in each of the cases which are the same as those used in the cases with the structure(s) in place. The surface elevation data from the empty tank tests are supplied in the supporting set of text documents (one for each wave case), e.g. 1BT2.txt corresponds to the empty tank tests for the 1BT2 wave case.

For each case, the CCP-WSI requests time series data from simulations during the period of time between 35.3s and 50.3s (relative to the empty tank data released). We do not wish to restrict participants to this window of time but request that the submitted data and execution times correspond to this period only. In addition to time series data from the simulations, a set of assessment criteria, requiring some basic analysis from participants, have been selected and correspond to the submission data requested for participation in the CCP-WSI Blind Test Workshop – Series 2. The validity assessment criteria, for all cases, are as follows: 

• Maximums (see Figure 7) in:
  • Surface elevation at wave gauge 5, ηmax (m) (empty tank tests)
  • Heave displacement of CoM, zmax (m)
  • Surge displacement of CoM, xmax (m)
  • Pitch angle (according to the left-hand rule*), θmax (degrees)
  • Mooring load, Fmax (N)
• Preceding trough depth (see Figure 7) in:
  • Surface elevation at wave gauge 5, ηtrough (m) (empty tank tests)
  • Heave displacement of CoM, ztrough (m) 
  • Surge displacement of CoM, xtrough (m)
  • Pitch angle (according to the left-hand rule*), θtrough (degrees)
  • Mooring load, Ftough (N)
• Rising time (see Figure 7) in:
  • Surface elevation at wave gauge 5, τη (m) (empty tank tests)
  • Heave displacement of CoM, τz (s)
  • Surge displacement of CoM, τx (s)
  • Pitch angle (according to the left-hand rule*), τθ (s)
  • Mooring load, τF (s)
• Peak frequency, fp, of the single-sided variance density spectrum** (see Figure 8) of:
  • Surface elevation at wave gauge 5, fp, η (Hz) (empty tank tests)
  • Heave displacement of CoM, fp, z (Hz)
  • Surge displacement of CoM, fp, x (Hz)
  • Pitch angle (according to the left-hand rule*), fp, θ (Hz)
  • Mooring load, fp, F (Hz)
• Single-sided variance density** at fp (see Figure 8) of:
  • Surface elevation at wave gauge 5, Ψη (m2/Hz) (empty tank tests)
  • Heave displacement of CoM, Ψz(fp) (m2/Hz)
  • Surge displacement of CoM, Ψx(fp) (m2/Hz)
  • Pitch angle (according to the left-hand rule*), Ψθ(fp) (degrees2/Hz)
  • Mooring load, ΨF(fp) (N2/Hz)
• Spectral bandwidth, b, of the single-sided variance density spectrum** (see Figure 8) of:
  • Surface elevation at wave gauge 5, bη (Hz) (empty tank tests)
  • Heave displacement of CoM, bz (Hz)
  • Surge displacement of CoM, bx (Hz)
  • Pitch angle (according to the left-hand rule*), bθ (Hz)
  • Mooring load, bF (Hz)
• Execution time of simulation between 35.3s and 50.3s (relative to empty tank data) (s) 

*Pitch angle: For the purpose of the CCP-WSI Blind Tests Series 2 & 3, the pitch angle is defined using the 'left-hand rule', i.e. positive pitch is in the counter-clockwise direction when looking down the positive y-axis (NOTE: this is the opposite to the conventional 'right-hand rule').

**Single-sided variance density spectrum: The variance density is a spectral parameter that is independent of both the length and the sampling rate of the time series being analysed. It is obtained according to, (|Y|/f_s)², where Y is the array of complex outputs obtained from a Fourier transform of the time series and f_s is the sampling frequency of the time series. It is requested that the Fourier transform be performed only on data within the specified time period, i.e. from 35.3s to 50.3s (relative to the experimental data), and also that the data is zeroed relative to the equilibrium conditions and padded with zeroes to 10 times the length (this is necessary to ensure a suitably well resolved result in frequency space). An example MATLAB script for calculating the single-sided variance density spectrum of the parameters required is provided in the 'Resources' section below.

Figure 7: Explanation of validity assessment criteria. Definition of assesment criteria in the case of: (a) surface elevation, heave displacement and mooring load measurements; (b) surge displacement, and (c) pitch angle.

Figure 8: Explanation of validity assessment criteria. 

The CCP-WSI Blind Test Series 2 is now closed. The following is a record of the submission procedure.

6.1 Submission of abstracts to EWTEC2019

The Series 2 Blind Tests will be held in conjunction with EWTEC 2019, Napoli, Italy. The CCP-WSI hopes to have the CCP-WSI Blind Test Series 2 as its own track at the conference for which we require at least 10 relevant contributions to the conference proceedings. Therefore, in order to facilitate this, if you intend on submitting an abstract to EWTEC2019 considering the Blind Test, please follow the following steps when submitting your abstract:

• Ensure 'CCP-WSI Blind Test Series 2' is in the title of your submitted abstract
• When submitting your abstract, under 'select track' select 'wave hydrodynamic modelling'
• In the 'comments for Conference Director' section (at the bottom of the page) please write 'This work represents the authors participation in the CCP-WSI Blind Test Series 2'.
• Please also let me know (edward.ransley at plymouth.ac.uk) that you have submitted an abstract.

Those who make a contribution to the EWTEC proceedings will be invited to submit a revised version of their conference paper (once the blind test results are released) to be published alongside a joint paper, including all participants of the Test, in a special issue of the Proceedings of the Institution of Civil Engineers - Journal of Engineering and Computational Mechanics.

6.2 Empty tank simulations

It is requested that, for each test case performed, a corresponding empty tank simulation is also conducted and data from this submitted as part of the Blind Test. 
 
6.3 Test case priority list

The CCP-WSI Working Group realises that participation in the Blind Test Series 2 is wholly voluntary and that completing all of the test cases may not be possible for all participants. All contributions are welcomed but it is requested that the test cases are performed in priority order to ensure a meaningful comparison can be drawn between all of the submissions. The requested order is as follows: 
 
12BT2, 11BT2, 13BT2, 22BT2, 21BT2, 23BT2
 
where 12BT2 corresponds to Geometry 1 (hemispherical-bottomed buoy) with incident wave 2BT2 and 22BT2 corresponds to Geometry 2 with the same incident wave. 

6.4 Submission format

The assessment criteria listed in Section 5 should be inputted into the spreadsheet template (template_BT2_submission) supplied in the accompanying documents below. The spreadsheet should then be completed and renamed according to the convention <institution>_BT2_submission (e.g. Plymouth_BT2_submission). Please fill out the individual columns for each of the test cases [NOTE: For items where the details are the same across all six cases you need only complete the first column and we will assume this is the same for all cases]. The spreadsheet and the documents detailed below should then be sent to Ed Ransley (email: edward.ransley at plymouth.ac.uk) in a single .zip directory named according to the convention <institution>_BT2_submission (e.g. Plymouth_BT2_submission). 
 
For the empty tank submissions it is requested that time series data be submitted for surface elevation recorded at wave gauges 1, 3, 5 and 8. For each empty tank case please submit:

• A single, tab-delimited text file:
  • filename: <institution>_<caseID>_empty (e.g. Plymouth_11BT2_empty)
  • column 1: Time (in secs relative to beginning of empty tank data, i.e. 35.3 – 50.3s)
  • columns 2-5: Surface elevation measurements (in metres)
    • WG 1, 3, 5, 8 

 For the cases with structures it is requested that time series data be submitted as follows:

• A single, tab-delimited text file:
  • filename: <institution>_<caseID> (e.g. Plymouth_11BT2)
  • column 1: Time (in secs relative to beginning of empty tank data, i.e. 35.3 – 50.3s)
  • column 2: Heave displacement of CoM (in metres)
  • column 3: Surge displacement of CoM (in metres)
  • column 4: Pitch angle of buoy relative to the vertical (in degrees)
  • column 5: Mooring load (in Newtons) 

 [NOTE: please retain the order of the requested data as above and please do not submit addition data in these files] 

The CCP-WSI Blind Test Series 2 is now closed. The experimental data for the test cases (with the structure included) is now available and can be found here. The data is arranged as follows:

• For each of the test cases (e.g. 11BT2) there is one text file containing time series data for the structures motion and the mooring load (11BT2.txt).
• The measurements are unfiltered; the first row contains the column headers and begins with a '#' for post-processing purposes; the first column is the time in seconds and the proceeding columns are heave displacement of the centre of mass (in metres), surge displacement of the centre of mass (in metres), pitch angle relative to vertical (in degrees) and mooring load (in Newtons), corresponding to the submission format defined above.

NOTE: When using this data please state that 'the physical data is from the CCP-WSI Test Case 4 (originally the CCP-WSI Blind Test Series 2)' and be sure to cite Ransley, Brown et al. 2020 as the source of this data (see full citation in the 'Relevant References' section below).

Ransley, E., Brown, S., Hann, M., Greaves, D., Windt, C., Ringwood, J., Davidson, J., Schmitt, P., Yan, S., Wang, J. X., Wang, J. H., Ma, Q., Xie, Z., Giorgi, G., Hughes, J., Williams, A., Masters, I., Lin, Z., Chen, H., Qian, L., Ma, Z., Chen, Q., Ding, H., Zang, J., van Rij, J., Yu, Y., Li, Z., Bouscasse, B., Ducrozet, G., Bingham, H., Focused wave interactions with floating structures: A blind comparative study, Proceedings of the Institution of Civil Engineers - Engineering and Computational Mechanics, 174(1) (2020): 46-61, https://doi.org/10.1680/jencm.20.00006

Ransley, E., Yan, S., Brown, S., Hann, M., Graham, D., Windt, C., Schmitt, P., Davidson, J., Ringwood, Musiedlak, P.-H., J., Wang, J., Wang, J., Ma, Q., Xie, Z., Zhang, N., Zheng, X., Giorgi, G., Chen, H., Lin, Z., Qian, L., Ma, Z., Bai, W., Chen, Q., Zang, J., Ding, H., Cheng, L., Zheng, J., Gu, H., Gong, X., Liu, Z., Zhuang, Y., Wan, D., Bingham, H., Greaves, D., A blind comparative study of focused wave interactions with floating structures (CCP-WSI Blind Test Series 3), International Journal of Offshore and Polar Engineering, 30(1) (2020): 1-10, https://doi.org/10.17736/ijope.2020.jc774

Ransley, E., Greaves, D., Raby, A., Simmonds, D., Hann, M., Survivability of Wave Energy Converters using CFD, Renewable Energy, 109 (2017): 235-247, http://dx.doi.org/10.1016/j.renene.2017.03.003

Ransley, E. J., Survivability of Wave Energy Converter and Mooring Coupled System using CFD, Ph.D. Thesis, University of Plymouth (2015), http://dx.doi.org/10.24382/1289

Hann, H., Greaves, D., Raby, A., Snatch loading of a single taut moored floating wave energy converter due to focussed wave groups, Ocean Engineering, 96 (2015): 258-271,https://doi.org/10.1016/j.oceaneng.2014.11.011

Accompanying documents: