Focused wave impact with an FPSO/cylinder (fixed)


In this test case a series of fixed structures are subject to focused wave events based on 2 different sea states (FW2 and FW3). The purpose of these experiments was to assess factors, such as wave steepness and direction, effecting the forces and run-up on FPSO hulls due to extreme wave events.

There are three geometries under consideration;

  • a truncated cylinder (M1)
  • a short FPSO hull (M2)
  • a long FPSO hull (M3)

Both FPSOs have semi-cylindrical ends with the same radius as M1.

The two focused waves are based on NewWave and are formed from spectra with the same Tp value (1.456s). The two spectra differ only by the significant wave height, Hs, which is 0.103m and 0.77m for FW2 and FW3 respectively. This means that FW2 is essentially a steeper version of FW3 with the same relative frequency contributions.
Experiments were performed, with and without the structures present, with:

  • the waves focused on the bow and head-on to the structures (FW2A00P000 & FW3A00P000);
  • the FW2 wave was also generated at angles of 10 (FW2A10P000) and 20 (FW2A20P000) degrees to the centre-line of the structures, and;
  • the FW3 wave was also trough-focused on the bow of the structures (FW3A00P180).

In each experiment, measurements of the pressure on the hulls 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 was recorded by an array of resistive wave gauges.

Experimental Set-up

The experiments were performed in the COAST Laboratory Ocean Basin (35m long X 15.5m width). The water depth was 2.93m. Please find the pressure sensor layout and Structure geometry & position and Wave Gauge Layout below.

Experimental Test Program

The incident waves were generated in the COAST Laboratory Ocean Basin 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 below where Aplha 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.

Wave Tz Tp h Lz Hs Hs/Lz Alpha Phi
  (s) (s) (m) (m) (m)   (rad) (rad)
FW2A00P000 1.132 1.456 2.93 1.999 0.103 0.052 0 0
FW2A10P000 1.132 1.456 2.93 1.999 0.103 0.052 0.174533 0
FW2A20P000 1.132 1.456 2.93 1.999 0.103 0.052 0.349066 0
FW3A00P000 1.132 1.456 2.93 1.999 0.077 0.039 0 0
FW3A00P180 1.132 1.456 2.93 1.999 0.077 0.039 0 4.712389

The amplitudes of each of the wave components in each wave case can be found in the attachment below.

Physical Measurement Data

Sampling rate: wave data (128Hz), pressure data (1024Hz)

Numerical Benchmarks

Relevant References

  • Hu, Z., Mai, T. Greaves, D. and Raby, A., 2016. "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.