Breaking wave impact on a ‘deformable’ truncated vertical wall (CCP-WSI Comparative Study 1)

Description

These test cases consider offshore breaking wave impacts on a ‘deformable’ truncated vertical wall (designed to represent a hull section of an FPSO at laboratory scale). The test cases described here are part of the experimental campaign reported in Mai et al. (2015 & 2020), and numerically work of Hu et al. (2017). In this study/test series, one of the incident wave groups (focused wave), from the campaign, is considered (the ‘slightly breaking’ case from Mai et al. 2020 and Hu et al. 2017). The wave is generated in the flume and it’s interaction with the truncated wall recorded. Three different cases are considered: with the wall considered rigid (1CS1) and with the wall able to move horizontally, compressing springs of two different spring rates (2CS1 and 3CS1).

Experimental Set-up

Flume and structural geometry

The experiments were conducted in the long, sediment flume in the COAST Laboratory at the University of Plymouth. The flume is 35m long, 0.6m wide and 1.2m tall. The flume is equipped with a single, wet-backed, piston-type wave maker (designed by EDL). The water depth in these cases was 0.7m. The truncated vertical wall (Plate 1) is an aluminium plate 0.56m wide, 0.6m high and 0.012m thick, connected to a rigid part (Plate 2 and 3) by four springs. Plate 2 and 3 are mounted on a support frame via a low profile load cell and Plate 4 (see Figure 1a). There were 0.02m gaps on both sides of the tested model to ensure no friction between the model and the flume side walls (see Figure 1b).

Figure 1: Side view of the tested model (a), and front view of the tested model (b) [all measurements in mm]

Spring system and mass properties

The spring system, responsible for the elastic behaviour of the wall, consists of four springs attached to the impact plate (Plate 1) and can incorporate springs of different stiffness or be locked to obtain a rigid wall impact model (Mai et al 2014; Mai 2017; Mai et al. 2019). Figure 2 and Table 1 give the details of the horizontal mass distribution of the tested wall configurations and spring characteristics for the different spring systems considered here. The four springs are mounted to plates 1 and 2 with mounts that have 5mm thickness, hence the distance between plates 1 and 2 is equal to the rest length of the spring plus 2 x 5mm = 10mm. Figure 3 shows the locations at which the springs attached to the Plate 1.

 

Figure 2: Horizontal mass distribution of the wall with springs CL51x102 (a), and with springs CL51x254 (b) [all measurements in mm]

Table 1: Spring parameters and mass properties of the structure/system

CCP-WSI ID

Original ID

Spring type

Stiffness

(single spring) (N/m)

Rest length (single spring) (m)

Mass

(single spring) (kg)

Mass

(four springs) (kg)

Mass of four springs + impact plate (kg)

1CS1

Rigid_FT

Locked/rigid

N/A

N/A

N/A

N/A

2CS1

Elastic1_CL51x102

CL51x102

98493

0.102

0.475

1.9

15.6

3CS1

Elastic2_CL51x254

CL51x254

37702

0.254

1.15

4.6

18.3

Pressure, force, deflection and acceleration measurement

Pressures on the impact plate were measured by an array of seven FGP XPM10 pressure sensors (P in Figure 3). The total force on the wall was measured, using a low profile load cell (Model 140), with an inline DC amplifier (see Figure 1a). Accelerometers (Model 4610) were mounted on Plates 1 (acc1 in Figure 3) and Plate 2 to measure vibration of the structure. A Linear Variable Differential Transformer (LVDT) was also mounted to the centre of Plate 1 to measure its displacement (see Figures 1 and 3). The configuration of the instrumentation is shown in Figures 1b and 3. The pressure, force, deflection and acceleration data were all sampled at 35 kHz.

 

Figure 3: Configuration of instrumentation on the impact wall [all measurements in mm]

Surface elevation measurement

Thirteen resistive wave probes were used to measure the surface elevation along the flume and in front of the model. The wave probes were positioned along the centre line of flume at locations according to those in Table 2. The wave gauge data was sampled at 128 Hz frequency.

Table 2: Wave probe positions along the centreline of the flume

 

WG1

WG2

WG3

WG4

WG5

WG6

WG7

WG8

WG9

WG10

WG11

WG12

WG13

Plate 1

Distance to wave paddle (m)

1

6

11

16

21.15

22.11

22.9

26.05

26.57

26.66

26.75

26.835

26.885

26.9

Experimental Test Program

Wave parameters/generation

A single incident wave group (focused wave) was used for all three of the cases – the ‘slightly breaking’ case reported in Mai et al. 2020 and Hu et al. 2017. The incident waves were generated in the flume using the EDL paddle control software. N = 116 wave fronts, with frequencies evenly spaced between 0.203125Hz and 2Hz, are linearly superposed and their phases assigned, at the wave maker, based on linear wave theory, a theoretical focus time, Tf = 42s, and a theoretical focus location, Xf = 31.90m downstream. The wave is a 'focused' wave with the theoretical phase of all components, φf, at the theoretical focus location, Xf, given a value of φf = π/2 rad. The amplitudes of the frequency components are derived using NewWave theory based on an underlying JONSWAP spectrum (γ = 3.3) with significant wave height, Hs = 0.163m, peak wave period, Tp = 1.601s, and crest amplitude, An = 0.1914m. The amplitudes of the frequency components, input to the paddle control software, are given in amplitudeSpectrum.txt (available as part of the supporting documentation in the Resources section of this description). 1st-order wave maker theory is applied to give the paddle displacement (NOTE: The software/paddle motion is calibrated according to laboratory-specific inconsistencies).

Wall/spring arrangement

The three cases considered are assumed to differ only by the elasticity of the wall/spring arrangement. In the first case (1CS1) the wall arrangement is considered to be rigid, in the second and third cases (2CS1 and 3CS1) the wall arrangement is free to move horizontally with the CL51x102 and CL51x254 spring arrangements respectively.

Summary

Table 3 summarises the parameters for the three test cases.

Table 3: Summary of the parameters for the three test cases

 

 

Wave parameters

Spring system

CCP-WSI ID

Original ID

An

(m)

Hs(m)

Tp

(s)

γ

Tf

(s)

Xf

(m)

φf (rad)

 

1CS1

BW3_G079_X3190_Rigid_02

0.1914

0.163

1.601

3.3

42

31.90

π/2

Rigid

2CS1

BW3_G079_X3190_tn22s_d700_05

0.1914

0.163

1.601

3.3

42

31.90

π/2

CL51x102

3CS1

BW3_X3190_EL2_06

0.1914

0.163

1.601

3.3

42

31.90

π/2

CL51x254

Physical Measurement Data

The CCP-WSI Comparative Study 1 is effective an ‘open’ comparison of numerical reproductions of a physical experiment as the physical data is already available in the literature (Hence ‘Comparative Study’ as opposed to ‘Blind Test’). However, only the time series data for the surface elevation measurements, from the three cases, is released initially (the time series data for the load, pressure and displacement measurements is to be released at a later time, when the participants have submitted their numerical solutions). It is believed that, along with this description document and the supporting files, the surface elevation measurements should provide sufficient information to reproduce the physical experiments and perform any necessary ‘calibration’ to the hydrodynamics in the numerical models. 

Submission Procedure and Timeline

If you would like to participate in the CCP-WSI Comparative Study 1, please email your expression of interest (EoI) to edward.ransley@plymouth.ac.uk.

There will be an interim webinar, on Tuesday 26th April 2022, at which preliminary comparisons will be presented (details to be confirmed).

If you would like your data submission to be included in the preliminary comparisons (at the interim webinar) please submit you data before Thursday 31st March 2022. Please submit (email to edward.ransley@plymouth.ac.uk) your results as a single tab delimited .txt file with the time series data according to Table 4 below (the sampling frequency can be irregular but please use the same time vector for all variables – submissions will be interpolated post-submission for quantitative comparison with one another and the physical data). Please use the following naming convention for the submission files:

<CCP-WSI ID>_<InstitutionalAcronym>.txt

Following the interim webinar, participants will have the opportunity to revise their submissions before the Final Workshop on Tuesday 28th June 2022. The deadline for final submissions to the Comparative Study is Tuesday 31st May 2022 (please ensure your submission has been made, as requested above, before this date or it may not be possible to include it in the final workshop/publication).

 

e.g. for case 1CS1 submitted by participants affiliated with the University of Plymouth – 1CS1_UOP.txt

Table 4

Column(s)

Data

1

Time (in seconds) coincident with the time in the physical experiments

2-14

Surface elevation (in metres) at each of the wave probes, WG1-13

15-21

Total pressure (in Pa) at each of the pressure probes, P1-7

22

Streamwise horizontal force (in Newtons) at load cell between Plate 3 and 4

23

Streamwise horizontal displacement of Plate 1 (in metres)

Relevant References

Mai, T., Mai, C., Raby, A., Greaves, D., Hydroelasticity effects on water-structure impacts, Experiments in Fluids, 61 (2020): 191, DOI

Mai, T., Mai, C., Alison, R., Greaves, D. M., Aeration effects on water-structure impacts: part 1 Drop plate impacts, Ocean Engineering, 193 (2019): 106600, DOI

Mai, T., On the role of aeration, elasticity and wave-structure interaction on hydrodynamic impact loading, PhD thesis, University of Plymouth (2017), PEARL

Hu, Z. Z., Mai, T., Greaves, D., Raby, A., Investigations of offshore breaking wave impacts on a large offshore structure, Journal of Fluids and Structures, 75 (2017): 99-116, DOI

Mai, T., Hu, Z. Z., Greaves, D., Raby, A., Investigation of hydroelasticity: wave impact on a truncated vertical wall, in Proceedings of the 25th International Ocean and Polar Engineering Conference, 21-26 June 2015, Kona, Hawaii, USA

Mai, T., Greaves, D., Raby, A., Aeration effects on impact: drop test of a flat plate, in Proceedings of the 24th International Ocean and Polar Engineering Conference, 15-20 June 2014, Busan, Korea

Resources

Accompanying documents:

filename

Description

amplitudeSpectrum.txt

Frequency component amplitudes input into the paddle control software to generate the wave in all three cases (1CS1, 2CS1 and 3CS1); tab-delimited text file (line 1 - header; column 1 – wave component frequency (Hz); column 2 – wave component amplitude (m)

1CS1_SEData.txt

Surface elevation data for 1CS1 case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-14 – surface elevation at wave gauges WG1-WG13 (m))

2CS1_SEData.txt

Surface elevation data for 2CS1 case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-14 – surface elevation at wave gauges WG1-WG13 (m))

3CS1_SEData.txt

Surface elevation data for 3CS1 case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-14 – surface elevation at wave gauges WG1-WG13 (m))