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

Description

The CCP-WSI Blind Test Series 2 involves two different floating, surface-piercing structures (moored with a simple linear spring mooring) meant to represent simplified wave energy convertors (WECs). The two geometries are: 1) a hemispherical-bottomed cylinder, and; 2) a cylinder with a moon-pool, thus increasing the complexity in the latter case by introducing an ‘internal’ body of water. The Blind Test is, therefore, split into two parts: 

  • Part 1 - corresponding to the (more simple) hemispherical-bottom cylinder 
  • Part 2 - corresponding to the cylinder with moon-pool  

Each structure is individually subjected to the same set of incident wave cases consisting of 3 focused wave events with a range of steepness, kA (0.129 – 0.193). The steepness of the waves is varied parametrically by altering the peak frequency (whilst maintaining the same crest height). The purpose of these experiments is to assess the effect of wave steepness and float geometry on the motion of the buoy and the load in the mooring line, as well as the accuracy of the numerical methods as a function of these parameters. 
 
In each experiment, the 6DoF motion of the buoy is recorded as well as the load in the single point, linear mooring line. The free surface elevation in the vicinity of the buoy is also recorded by an array of resistive wave gauges. 

Experimental Set-up

2.1 Basin geometry

The experiments were performed in the COAST Laboratory Ocean Basin (35m long X 15.5m width) at Plymouth University, UK. The basin has 24 flap-type, force feedback controlled wave makers with a hinge depth of 2m. The water depth at the wave makers is 4m and there is a linear slope to the working area where the water depth, h, was set to 3.0m. At the far end of the basin there is a parabolic absorbing beach (Figure 1). 

Figure 1: COAST Laboratory Ocean Basin dimensions (reproduced from Ransley (2015)). 

NOTE: The coordinate system used throughout this document has been defined with: the z axis running vertically (positive z upwards) with the z = 0 corresponding to the still water level, and; the x axis running in the direction from the wave makers to the beach. The y axis is then defined according to the right-hand-rule. 

2.2 Wave probe layout

For all cases, 13 wave gauge positions were used according to Figure 2. Position 5 corresponds to the rest position of the buoy(s) (with the structure in place this wave gauge was removed but the same number system maintained). 
 

Figure 2: Wave probe layout (all dimensions in mm). 

2.3 Buoy geometries and mass properties

As well as providing an increase in geometric complexity and a greater challenge due to the ‘internal’ fluid in the moon-pool, the two buoys have also been designed and ballasted to have similar drafts (it is hoped this will provide some information regarding the effect of the moon-pool on the dynamics of the system and that this can be related back to the predictive capability of the numerical models included in the tests). 
 
2.3.1 Hemispherical-bottomed buoy (Geometry 1)

Geometry 1 is a hemispherical-bottomed buoy with both the radius of the hemisphere and the cylinder equal to 0.25m. The height of the cylindrical section is also 0.25m (see Figure 3). The mooring attachment is located at the bottom-most point of the hemisphere on the axial line

Figure 3: Geometry 1 (all dimensions in mm) 

The mass of the hemispherical-bottomed buoy is 43.67 kg (including ballast). The mass properties of the buoy and the position of the centre of mass (CoM) can be found in Table 1 where zCom,rel is the axial (vertical) distance to the position of the CoM relative to the bottom of the buoy/mooring position and Izz is the moment of inertia about the vertical (z) axis, i.e. yaw.

Table 1: Mass properties of Geometry 1
m zCom,rel Ixx Iyy Izz
(kg) (m) (kgm2) (kgm2) (kgm2)
43.674 0.181 2.219 2.219 1.143

2.3.2 Cylinder with moon-pool (Geometry 2)

Geometry 2 is a cylinder with a cylindrical moon-pool running axially through the centre. The outer diameter is 0.577m and the inner diameter is 0.289m. The height of the cylinder is 0.5m (see Figure 4). An additional frame allows the mooring attachment to be located level with the bottom of the cylinder on the axial line. 
 

Figure 4: Geometry 2 (all dimensions in mm) 

The mass of the moon-pool buoy is 61.46 kg (including ballast). The mass properties of the buoy and the position of the centre of mass (CoM) can be found in Table 2 where, again, zCom,rel is the axial (vertical) distance to the position of the CoM from the bottom of the buoy/mooring position and Izz is the moment of inertia about the vertical (z) axis. 

Table 2: Mass properties of Geometry 2
m zCom,rel Ixx Iyy Izz
(kg) (m) (kgm2) (kgm2) (kgm2)
61.459 0.099 1.790 1.790 3.298

2.4 Mooring and rest positions of buoys

In both cases, the mooring used is a linear spring with a stiffness of 67 N/m and a rest length of 2.224m (Figure 5). Table 3 gives some key parameters with the buoy at rest (where z = 0 is assumed to be the still water level). 

Figure 5: Details of the mooring (all dimensions in mm) 

Table 3: Key values when buoys are at rest
Geometry draft Pretension in mooring Z position of CoM
  (m) (N) (m)
1 0.322 32.07 -0.141
2 0.330 31.55 -0.231

 

Experimental Test Program

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 3. All waves in this Blind Test Series are crest focused waves, i.e. have zero phase angle at the focus location (Figure 6). 

Table 3: Wave parameters for each of the test cases
CCP-WSI ID Original ID An fp h Hs kA
    (m) (Hz) (m) (m) -
1BT2 Test5 0.25 0.3578 3.0 0.274 0.128778 
2BT2 Test2 0.25 0.4 3.0 0.274 0.160972
3BT2 Test6 0.25 0.4382 3.0 0.274 0.193167

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). 

Physical Measurement Data

The CCP-WSI Blind Test Series 2 is a ‘blind’ validation of numerical WSI codes. 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. The remaining physical measurements will be released shortly after completion of the CCP-WSI Blind Test Series 2 through the CCP-WSI Website (http://www.ccp-wsi.ac.uk/).

 Assessment Criteria 

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, zmax (m)
    • Surge displacement, 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, ztrough (m) 
    • Surge displacement, 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, τz (s)
    • Surge displacement, 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, fp, z (Hz)
    • Surge displacement, 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, Ψz(fp) (m2/Hz)
    • Surge displacement, 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, bz (Hz)
    • Surge displacement, 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|/fs)2, where Y is the array of complex outputs obtained from a Fourier transform of the time series and fs 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. 

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]. 
 
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] 
 

Relevant References

Ransley, E. J., Survivability of Wave Energy Converter and Mooring Coupled System using CFD, Ph.D. Thesis, University of Plymouth (2015), doi: https://pearl.plymouth.ac.uk/handle/10026.1/3503 

Ransley, E., Greaves, D., Raby, A., Simmonds, D., Hann, M., Survivability of Wave Energy Converters using CFD, Renewable Energy, 109 (2017): 235-247, doi: http://dx.doi.org/10.1016/j.renene.2017.03.003 
 
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, doi: https://doi.org/10.1016/j.oceaneng.2014.11.011 
 
 

Resources

Accompanying documents: 

filename

Description

template_BT2_submission

 

Spreadsheet template for recording submission details and assessment criteria.

1BT2.txt

 

Empty tank test surface elevation data for 1BT2 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-14 – surface elevation at wave gauges WG1-WG13 (m)) 

2BT2.txt

 

Empty tank test surface elevation data for 2BT2 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-14 – surface elevation at wave gauges WG1-WG13 (m)) 

3BT2.txt

 

Empty tank test surface elevation data for 3BT2 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-14 – surface elevation at wave gauges WG1-WG13 (m)) 

2BT2.txt

 

Empty tank test surface elevation data for 2BT2 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-14 – surface elevation at wave gauges WG1-WG13 (m)) 

varianceDensity.m

Matlab script