Model Tests and Numerical Prediction of the Low Frequency Motions of a Moored Ship in Shallow Water With a Wave Splitting Method

Abstract

Prediction of shallow water low frequency (LF) motions of vessels in the context of mooring analysis is challenging. Model tests are often performed to calibrate and validate numerical models and, in this way, reduce the uncertainty. Model tests are part of the positioning system qualifying process. However, model tests also present challenges and uncertainties related to parasitic low frequency wave systems which are unavoidable in shallow water ocean basin conditions. The paper presents model tests with a ship moored in shallow water (20 m), the analysis and discussion of the test data and comparisons with numerical predictions. The focus is on the low frequency motions and related wave drift forces. The tests have been performed in harmonic waves, bi-harmonic waves and irregular seastates, including conditions with and without current. The first part of the study consists of analysing the wave field measured by a long array of wave sensors distributed along the ocean basin. The analysis provides split wave systems, namely the low frequency components including the bound wave, the incoming free parasitic wave, the reflected component and additional very long waves. The second part proposes a method to calibrate and validate mooring analysis numerical models, based on comparisons with model test data which includes the unavoidable effects from parasitic waves. Simulations of LF motions with the calibrated model show a good agreement with the measurements.

Qualification Criteria for the Verification of Numerical Waves – Part 1: Potential-Based Numerical Wave Tank (PNWT)

“…The LF-spectral shape must be the same for all positions in each realization. If not, this may suggest the presence of spurious free-waves or LF-reflection (see [2], [3] and [5]).…”

“…The mechanical generation of water waves in conventional experimental facilities can be hampered by the intrinsic boundary conditions of the laboratory. Wave reflection [1], spurious waves ( [2], [3], [4], [5]) and eigenmodes [6] are typical issues researchers must deal with when assessing model responses during test campaigns. However, in such facilities, the complex nonlinear physics of wave propagation is simply enforced by the laws of nature.…”

Validation of Numerical Wave Tank Simulations Using Reef3d With Jonswap Spectra in Intermediate Water Depth

“…The second reason for the better agreement is the similar wave reflection behavior between the model test and the numerical simulation with active absorption. Because the location of the free surface elevation is known for each time step at all locations, it is possible to split the free surface elevation into incoming and reflected wave by use of two-dimensional Fouriertransformation from space-time representation to wave number and wave frequency representation [29]. Estimation of the reflected wave time series at wave gauge location 4 shows (Figure 6) that the significant wave height of the reflected wave time series is about 11% of the incident wave time series, i.e.…”