Focused wave impact with a fixed FPSO (CCP-WSI Blind Test Series 1)

Description

In these test cases a fixed scale-model FPSO (M3) is subjected to 6 irregular (focused) wave events with a range of steepness, kA (0.13 – 0.21) and a range of incident wave angles (α = 0, 10 & 20°). The purpose of these experiments is to assess factors, such as wave steepness and direction, effecting the forces and run-up on FPSO hulls due to extreme wave events.

The structure (M3) has vertical sides and each end is semi-circular with the same radius (0.15m). The full height of the structure is 0.303m and the draft is 0.153m.

The test cases are split into two parts:

  • Part 1 – considers waves of increasing steepness but constant (0°) angle of incidence

  • Part 2 – considers waves with increasing angle of incident but constant steepness

In each experiment, measurements of the pressure on the hull were recorded by an array of pressure transducers. The run-up at various positions on the hull surface and the free-surface elevation in the vicinity of the structures were recorded by an array of resistive wave gauges.

NOTE: The pressure and wave gauge layout differs between Parts 1 and 2 (see below).

Experimental Set-up

2.1 Basin geometry – same for Part 1 & 2

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 2.93m. 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)).

2.2 Pressure sensor layout – Part 1

For the cases in Part 1, an array of 6 pressure transducers were positioned on the bow of the FPSO on the centreline (P1, P2 & P3) and at 45° to the port (P4, P5 & P6) side; at the still water level and at depths of ±0.05m (Figure 2). The sample frequency of the pressure probes was 1024Hz.

Figure 2: Pressure sensor layout on the bow of the M3 FPSO model (Part 1).

2.3. Structure geometry & position and Wave Gauge Layout – Part 1

Resistive wave gauges were positioned in the wave basin to measure the run-up at various positions on the hull surface and the free-surface elevation in the vicinity of the structures. In Part 1 the same wave gauge layout (Figure 3) was utilised for both the empty tank tests and those with the structure in place. The sample frequency for the wave gauge measurements was 128Hz.

Figure 3: Wave gauge layout for Part 1 (both with and without the model in place).

2.4 Pressure sensor layout – Part 2

In part 2, an array of 9 pressure transducers were positioned on the bow of the FPSO on the centreline (P1, P2 & P3) and at 45° to the port (P7, P8 & P9) and starboard (P4, P5 & P6) sides; at the still water level and at depths of ±0.05m (Figure 2). The sample frequency of the pressure probes was 1024Hz.

Figure 4: Pressure sensor layout on the bow of the M3 FPSO model (Part 2).

2.5. Structure geometry & position and Wave Gauge Layout – Part 2

In Part 2, two different arrays of wave gauges were used for: 1) the empty tank tests (Figure 5) and, 2) tests with the structure present (Figure 6). The sample frequency for the wave gauge measurements was 128Hz.

Figure 5: Wave gauge layout for Part 2 (for the empty tank tests)

Figure 6: Wave gauge layout for Part 2 (with the structure in place)

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 free-surface 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, 13.886m from the wave paddles. Each wave front is then transformed back to the position of the wave paddles by the control software. The amplitudes of the frequency components were derived by applying the NewWave theory to a JONSWAP spectrum with the parameters in the Table 1 where Alpha is the angle of propagation relative to the centre-line of the basin and Phi is the phase of the components at the focus location. All waves in this Blind Test Series have a Phi of π (trough focused).

 

3.1. Wave parameters - Part 1

In Part 1, the 3 waves have all been generated at 0° incident wave angle. The waves are based on the NewWave theory formed from spectra with Tp = 1.456s or 1.362s and significant wave height, Hs = 0.103 or 0.077, i.e. two wave cases (FW2 & FW3) differ by Hs and are essentially steeper versions of one another with the same relative frequency contributions; FW2 and FW6 differ only by peak frequency and are essentially steeper versions of one another with the same Hs (Table 1).

Table 1: Wave parameters for each of the test cases in Part 1

CCP-WSI ID

Original ID

A

Tp

h

Hs

kA

Alpha

Phi

 

 

(m)

(s)

(m)

(m)

 

(rad)

(rads)

11BT1

FW3A00P180

0.06914

1.456

2.93

0.077

0.13

0

π

12BT1

FW2A00P180

0.09128

1.456

2.93

0.103

0.18

0

π

13BT1

FW6A00P180

0.09363

1.362

2.93

0.103

0.21

0

π

 

 

3.2. Wave parameters - Part 2

In Part 2, all 3 waves have the same steepness (kA = 0.17). The waves differ only by incident angle (Table 2).

Table 2: Wave parameters for each of the test cases in Part 2

CCP-WSI ID

Original ID

A

Tp

h

Hs

kA

Alpha

Phi

 

 

(m)

(s)

(m)

(m)

 

(rad)

(rads)

21BT1

FW2A00P180

0.08930

1.456

2.93

0.103

0.17

0

π

22BT1

FW2A10P180

0.08930

1.456

2.93

0.103

0.17

0.174533

π

23BT1

FW2A20P180

0.08930

1.456

2.93

0.103

0.17

0.349066

π

Empty Tank Data (Physical)

The CCP-WSI Blind Test Workshop - Series 1 was a ‘blind’ validation of numerical WSI codes (The CCP-WSI Blind Test Series 1 is now closed). Consequently the only physical measurement data released at the time of participation was the surface elevation data from the wave gauges in the empty tank tests (see Figures 3 & 5 for the wave gauge positions). This data should be sufficient to reproduce the incident waves in each of the cases which are identical to those used in the cases with the structure 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. 11BT1.txt corresponds to the empty tank tests for the 11BT1 wave case.

Validity Assessment Critera

A set of criteria have been selected to ascertain the validity of codes and these correspond to the submission data requested for participation in the CCP-WSI Blind Test Workshop – Series 1. The Validity assessment criteria are as follows:

PART 1:

  • Time series of the surface elevation (run-up) at WG 16, 18, 17, 24 and 7 (Figure 3)

  • Time series of pressure at P1, 2, 3, 4, 5, 6 (Figure 2).

PART 2:

  • Time series of the surface elevation (run-up) at WG 15, 16, 17, 24 and 7 (Figure 6)

  • Time series of pressure at P1, 2, 3, 4, 6, 8 (Figure 4).

Submission Procedure

The CCP-WSI Blind Test Series 1 is now closed. The individual participants results, the final report and the experimental data for the test cases (with the structure included) will be released shortly. Please join the combined CCP-WSI & SIG-WSI community mailing list on the 'Contact Us' page for updates.

Physical Data

The CCP-WSI Blind Test Series 1 is now closed. The experimental data for the test cases (with the structure included) is now avialable and can be found here. The data is arranged as follows:

  • For each of the test cases (e.g. 11BT1) there are two text files, one for the surface elevation measurements (11BT1_surfaceElevation.txt) and one for the pressure measurements (11BT1_pressure.txt).
  • The surface elevation measurements are unfiltered measurements; 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 surface elevation in metres at each of the probes corresponding to the validation assessment criteria defined above.
  • The pressure data has been filtered and treated for thermal shock (as documented in Ransley et al. 2019); 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 pressure measurements in Pascals at each of the probes corresponding to the validation assessment criteria defined above.

NOTE: When using this data please state that 'the physical data is from the CCP-WSI Test Case 3 (originally the CCP-WSI Blind Test Series 1)' and be sure to cite Ransley et al. 2019 as the source of this data (see full citation in the 'Relevant References' section below).

Relevant References

Ransley, E., Yan, S., Brown, S., Mai, T., Graham, D., Ma, Q., Musiedlak, P.-H., Engsig-Karup, A. P., Eskilsson, C., Li, Q., Wang, J., Xie, Z., Sriram, V., Stoesser, T., Zhuang, Y., Li, Q., Wan, D., Chen, G., Chen, H., Qian, L., Ma, Z., Mingham, C., Causon, D., Gatin, I., Jasak, H., Vukčević, V., Downie, S., Higuera, P., Buldakov, E., Stagonas, D., Chen, Q., Zang, J., Greaves, D., A blind comparative study of focused wave interactions with a fixed FPSO-like structure (CCP-WSI Blind Test Series 1), International Journal of Offshore and Polar Engineering, 29(2) (2019): 1-15, doiPEARL

Ransley, E. J., Survivability of Wave Energy Converter and Mooring Coupled System using CFD, Ph.D. Thesis, University of Plymouth (2015), PEARL 

Hu, Z., Mai, T. Greaves, D. and Raby, A., A Numerical and Experimental Study of a Simplified FPSO in Extreme Free Surface Waves, in Proceedings of the 26th International Ocean and Polar Engineering Conference, 26 June - 2 July 2016, Rhodes (Rodos), Greece.

Mai, T., Greaves, D., Raby, A. and Taylor, P.H., Physical Modelling of Wave Scattering Around Fixed FPSO-Shaped Bodies, Applied Ocean Research, Vol. 61 (2016), pp. 115-129.

Ma, Q., Yan, S., Greaves, D., Mai, T., Raby, A., Numerical and Experimental Studies of Interaction between FPSO and Focusing Waves, in Proceedings of 25th ISOPE conference, 21-26 June 2015: Hawaii, USA, Vol.3: 655-662.

Resources

Accompanying documents:

filename

Description

11BT1.txt

 

Empty tank test surface elevation data for 11BT1 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-25 – surface elevation at wave gauges WG1-WG24 (m))

12BT1.txt

 

Empty tank test surface elevation data for 12BT1 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-25 – surface elevation at wave gauges WG1-WG24 (m))

13BT1.txt

 

Empty tank test surface elevation data for 13BT1 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-25 – surface elevation at wave gauges WG1-WG24 (m))

21BT1.txt

 

Empty tank test surface elevation data for 21BT1 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-18 – surface elevation at wave gauges WG1-WG17 (m))

22BT1.txt

 

Empty tank test surface elevation data for 22BT1 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-18 – surface elevation at wave gauges WG1-WG17 (m))

23BT1.txt

 

Empty tank test surface elevation data for 23BT1 wave case; tab-delimited text file (line 1 - header; column 1 – Time (s); columns 2-18 – surface elevation at wave gauges WG1-WG17 (m))