In this paper a comparison between model basin experiments and results of diffraction computations on side-by-side moored LNG carriers is presented. The computations are based on a new lid method in diffraction codes to suppress non-realistic high wave elevations between the two floating objects. This lid method was originally formulated by Chen (2005). In this method a damping value is added to the free surface by means of a damping parameter. Since no theoretical solution can be found to establish the required value of the damping parameter, model basin experiments have been performed to determine this value. However from the results of the model basin experiments it is shown that it is difficult to obtain one unique value of the lid damping (for the 4m or small gap). The way of tuning the damping value of the lid is crucial. Tuning the damping based on first order results, like motions or wave height RAO’s will lead to a much larger variation in the estimate of the second order sway wave drift force transfer function.

Modeling shallow-water waves in a basin with a finite length and width introduces challenges related to low-frequency (LF) waves, especially for testing of moored vessels with long natural periods. Waves in this frequency range are also present in reality, as for instance bound set-down waves and unbound free waves formed by the geometry bathymetry. In model basins, additional unwanted LF wave components will be formed as a side-product of the wave generation and due to the basin geometry though. Standing waves over the basin length and width (basin modes) can generally be identified, which are difficult to dampen using beaches. This is the case for every wave basin, as they all have finite dimensions. Moored structures generally have natural frequencies in the LF range, which may be excited by basin modes with similar frequencies. It is therefore important to understand the natural modes of a basin before tests with moored structures in shallow water are done. The energy of these basin modes increases and their natural frequency decreases with decreasing water depth (waves travel slower in shallow water). Generally, it can be said that the issues with basin modes are present on very shallow water (typically ∼15–30 m water depth full-scale for structures with a length around 200 m at a scale around 1 to 40). The smaller the basin for the same water depth, the higher the basin mode frequencies and the higher the likelihood of resonance problems. The energy and frequencies of the basin modes and their relevance for specific tests depend on the effective length and width of the basin, the water depth, wave conditions and the (mooring stiffness of) the structure under consideration. The influence of these variables is evaluated in the current study. Tests were done in MARIN’s Offshore Basin (OB), but most of the results are also expected to be applicable to other basins. The observed basin mode frequencies during these tests were compared to the theoretical values, and an overview of the unwanted LF wave content as a function of water depth, wave height and period is presented. The energy and shape of individual basin modes is also discussed. Considering these results, a practical approach for future basin projects on shallow water is described.

For model tests, the correct generation of the most realistic representation of the natural wave field is of greatest importance as the environmental conditions denote the starting point for all following analyses of any behaviour of a marine structure. Thus, it has to be defined first what “reality” is, followed by a thorough analysis of the inherent limitations of basin wave fields. Furthermore, all realistic aspects of the wave field have to be modelled at sufficient accuracy involving the wave maker control, flap geometries and appropriate analysis techniques. This paper gives an overview of the most recent developments in advanced basin wave modelling including a wide range of aspects as realistic wave spreading, deterministic wave generation, focusing waves, directional wave analysis, spurious waves and shallow water wave generation.

The demand for hydrodynamic research facilities to correctly model shallow-water effects associated with low-frequency (LF) waves increases. Modelling such shallow-water waves in a basin introduces extra challenges, on top of the issues such projects are associated with at full scale. Various unwanted LF wave contributions are introduced by the geometry of the basin and the wave generators. Measurements show significantly more LF wave energy than expected based on second-order bound wave theory, where certain basin modes can generally be identified. This paper describes a method to minimize the unwanted part of this LF wave energy, by creating an ‘anti-wave’ and choosing an appropriate location in the basin. A wave-splitting tool was used to separate the original wave field into bound and LF free wave components in incident and reflected directions. The resulting LF free wave travelling from the wave generator was used to define a deterministic ‘anti-wave’, which was designed to cancel this LF free wave. Some first tests of this theory in MARIN’s Offshore Basin showed that it is possible to reduce the amount of unwanted LF spurious wave energy in shallow water by generating an anti-wave at the wave generator. This is promising for further development, although phasing of the anti-wave is difficult for basin oscillation modes. A procedure to follow for future shallow-water basin projects was identified.

Operators who want to im- or export bulk (coal, iron ore, scrap, etc.) by means of offshore bulk transshipment often want to determine the operability of their bulk transshipment configuration in an early stage to assess if offshore floating bulk transshipment is a feasible option for im- or export of the commodity. The downtime of such offshore (un)loading operations may be unacceptably large. The operational windows may become limited. This implies insufficient im- or export volumes, which may result in large and expensive storage capacity onshore. In this paper a methodology is proposed to assess the operability of offshore floating bulk transshipment. Offshore bulk transshipment is a multi-phase operation. For offshore operations consisting of multiple phases, persistency of the environmental conditions has to be considered. Persistency analysis allows for multiple operational phases, individual phase duration and shift of sea states during the operation. The proposed methodology shows that using persistency data of the waves (wind and current are not considered yet), the downtime of an offshore floating bulk transshipment operation can be estimated more accurately. Compared to persistency analysis, scatter analysis (using wave scatter diagrams) resulted in an optimistic estimate of the downtime. Persistency analysis is better in demonstrating the influence due to each consecutive step on the total operability. This methodology could be further extended to other multi-phase offshore operations.