Focused wave interaction with a floating structure (incl. heave decay tests)

Description

In this test case a floating, taut-moored, hemispherical-bottomed buoy is subjected to a focused wave event (the same wave event as detailed in CCP-WSI Test Case 1 “A uni-directional focused wave event”) following a pair of heave decay tests (one ‘free’, i.e. unmoored, and one with the mooring attached). As part of the wider X-MED project, the purpose of the experiment was to ascertain the response of a simplified wave energy converter to extreme wave events in order to understand the ‘survivability’ of wave energy converters.

Experimental Set-up

As in CCP-WSI Test Case 1, the experiments were performed in the COAST Laboratory Ocean Basin (35m long X 15.5m wide). The water depth was 2.8m. The structure was positioned with its centre coincident with the position of WG13 in Test Case 1, i.e. 18.54m from the physical wave makers.

The structure is a floating, hemispherical-bottomed, cylindrical buoy with a linearly-elastic mooring. The buoy is 0.5 m in diameter with a 0.25 m tall cylindrical section above its hemispherical bottom (Figure 1). It is constructed from 2 mm thick mild steel and has ballast weight secured within the hemispherical part. The total mass is 43.2 kg centred 0.181 m above the bottom mooring fixing. The moment of inertia of the buoy is (1.61 1.61 0.5) N m. Restrained only by a single-point mooring, which attaches at the bottom of the buoy where the symmetry axis intersects the surface of the hemisphere, the structure in this case is able to move in all six degrees of freedom (6DOF). The mooring has a stiffness of 67 N m−1 and a rest length of 2.18 m.

Figure 1: Dimensions of the hemispherical-bottomed, cylindrical buoy (left), and; a photograph from the COAST laboratory (right) showing the single-point, linear mooring and experimental setup.

Experimental Test Program

For the heave decay test without the mooring, the buoy is release from an elevated position with its centre of mass 0.0638486m above the still water level.

For the heave decay test with the mooring, the buoy is release from a position where its centre of mass is 0.09436m above the still water level.

For the case with the focused wave, the wave event is exactly as described in the CCP-WSI Test Case 1 “A uni-directional focused wave event”, please refer to this for a description, and for the data, relating to the incident wave. The buoy is position coincident with WG13, i.e. 18.54m from the physical wave makers and is initially at rest (centre of mass 0.133m below the still water level.

Physical Measurement Data

In the heave decay tests the 6DoF motion of the buoy was recorded at 200Hz, the heave displacement data can be found here. The data is arranged as follows:

  • The physical heave displacement time series, for each decay case, is given in a separate file located in physicalData > decayCases. The case without the mooring is in the file CM1_unmoored; the moored case is in the file CM3_moored; in both files, 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 second column is the heave displacement of the centre of mass of the buoy (initially, i.e. before release, the heave displacement is given as the distance between the centre of mass and the still water level, i.e. 0.0638486m and 0.09436m for the umoored and moored cases respectively). Positive heave displacement is vertically upwards.

For the focused wave case, the 6DoF motion and mooring load data was recorded at 200Hz and 128Hz respectively and can be found here. The data is arranged as follows:

  • The physical data for the wave structure interaction case is located in physicalData > waveCase. The body motion data is all in one file (S1_motionData); 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 the surge displacement (in metres) of the centre of mass, the heave displacement (in metres) of the centre of mass and the pitch angle (in radians) of the buoy (positive surge is in the direction of wave propagation, positive heave displacement is vertically upwards and positive pitch is given using the conventional 'right-hand rule') . The mooring load data is in the file S1_mooringLoadData; again 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 second column is the mooring load in Newtons.

PLEASE NOTE: When using this any of this data please be sure to cite Ransley et al. 2017 as the source of the data (see full citation in the 'Relevant References' section below) and state that 'the physical data is from the CCP-WSI Test Case 2' (referring the readers to this webpage (http://www.ccp-wsi.ac.uk/data_repository/test_cases/test_case_002).

Numerical Benchmarks

In this case we have both heave decay (both free and moored) and motion (incl. mooring load), when subject to a focused wave event (CCP-WSI Test Case 1), of a taut-moored hemispherical-bottomed buoy. The key physical phenomenon present in this case (excluding the nonlinear evolution of the surface elevation) is the motion (and mooring load) of the buoy. For the heave decay cases, a true reproduction requires only the heave displacement of the buoy to be reproduced. However, for the wave structure interaction (WSI) case, a true reproduction requires all three degrees of freedom (heave, surge and pitch) to be reproduced. For this benchmarking case, therefore, we propose the heave displacement and the heave, surge and pitch motion (plus the mooring load), for the decay cases and the WSI case respectively, as defining the key phenomenon present.

This benchmarking case has been reproduced numerically by:

  • Ransley et al. (2017) - using OpenFOAM-2.3.0 and waves2Foam (RANS-VOF) [data available to download here ].
  • Ransley (2015) - using OpenFOAM-2.2.1 and waves2Foam (r1984) (RANS-VOF) .

PLEASE NOTE: When using this data please be sure to cite the original source of the data appropriately (see full citations in the 'Relevant References' section below).

Figure 2: Heave displacements for the heave decay case: unmoored (left), and; moored (right).

Figure 3: Surge displacement, heave displacement and pitch angle of the buoy, as well as mooring load, when subject to the focused wave event.

Relevant References

Ransley, E., Greaves, D., Raby, A., Simmonds, D., Hann, M., 2017. "Survivability of Wave Energy Converters using CFD", Renewable Energy, 109: 235-247; doi; PEARL.

Ransley, E., 2015. "Survivability of Wave Energy Converter and Mooring Coupled System using CFD", PhD thesis, Plymouth University, UK; PEARL.

Hann, M., Greaves, D. and Raby, A., 2015. "Snatch loading of a single taut moored floating wave energy converter due to focussed wave groups", Ocean Engineering, 96: 258-271; doi; PEARL.

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