A recent theory of Bragg scattering of surface waves by sinusoidal sandbars on a seabed is applied to three cases not examined heretofore: (1) oblique incidence on a strip of infinitely long bars, (2) oblique incidence on the corner of a bar field and (3) seabed with a mean slope. While the Bragg mechanism has been studied previously for sandbars present on many shorelines, it can be a basis for breakwaters where the soil is not strong enough to support a single massive breakwater.

To facilitate the theoretical prediction of the evolution of a short gravity wave on a long wave of finite amplitude, we consider a model where the long wave is represented by Gerstner's exact but rotational solution in Lagrangian coordinates. Analytical formulae for the modulation of an infinitesimal irrotational short wave are shown to be qualitatively accurate in comparison with the numerical results by Longuet-Higgins (1987) and with the analytical results by Henyey et al. (1988) for irrotational long waves. Discrepancies are generally of second order in the long-wave steepness, consistent with the vorticity in Gerstner's solution. Weakly nonlinear short waves are shown to be parametrically excited by the long wave over a long time. In particular, multiple bands of modulational instability appear in the parameter space. Numerical calculations of the nonlinear evolution equation show the onset of chaos for sufficiently large parameter $\alpha = \epsilon (k\overline{A})^2/2\Omega/\sigma $, where $\epsilon k\overline{A} $ is the short-wave steepness and (εΩ, σ) the frequency of the (long, short) wave. Furthermore, if the short-wave amplitude A is approximated by a two-mode truncated Fourier series, the evolution equation reduces to a non-autonomous Hamiltonian system. The numerical solutions confirm that the onset of chaos is an inherent feature of the parametrically excited nonlinear system.

Time domain simulations are used extensively to assess the availability of turret moored terminals for offloading operations. The availability analysis provides a key input when assessing the economical viability of a project. In this context, it was deemed important to perform a benchmark study. Three programs have been used: AQWA-DRIFT - an ANSYS product maintained by Century Dynamics and used at SBM, LIFSIM and aNySIM both developed in MARIN. Five environments including wind, waves and current have been selected one of which with all components in-line and the remaining four with oblique wind and current. Simulations have been performed with identical input for the three programs. Results are compared statistically, but also in the time domain. Lessons learned will be discussed and areas of improvement identified.

With the LNG market booming, the need for reliable and safe means of transferring LNG from a producing, floating facility to an LNG carrier and from this carrier to a near-shore terminal is becoming acute. The Soft Yoke Mooring and Offloading (SYMO©) system has recently been model tested in MARIN’s offshore basin. Results of these tests are presented. Insight has been gained, from these model tests and from the calibration of numerical tools performed thereafter, on the following issues: • The inherent weakly damped nature of a moored LNG carrier, • Shallow water effects in wave drift forces, • The effect of current on drift forces, • The structure of low frequency long waves in a shallow water basin. These issues will be discussed and guidance regarding their importance will be provided. Consequences in terms of system design, mooring analysis methodology and model test program will be discussed.