Methodology for the Design of LNG Terminals in a Nearshore Environment

Nowadays, more and more nearshore LNG terminals are being built as it offers easy access to vessels coming from deep water and mitigates the risk by isolating regasification units from the cities. However, designing these terminals can be challenging in shallow water, as it is exposed to low-frequency waves which can excite the moored vessels at their natural periods. By lack of knowledge and adequate numerical simulation techniques, the effect of these low-frequency waves on the motions of moored vessels are unfortunately often ignored in the design. This is likely to result in an underestimation of the vessel motions and terminal downtime. In this paper, a methodology for the design of terminals in a nearshore wave climate is presented. The methodology consists of six steps which guide the engineer from the definition of the deep-water sea states to the calculation of the vessel motions and terminal downtime. In an initial stage, computational efficient tools are used, with the limitation that several approximations need to be made. In a later stage, more detailed but expensive methods are applied. The objective of this paper is to show how the developed methodology can give insight in the expected downtime due to the low-frequency waves in any nearshore mooring location. As an example, the methodology is applied on a fictive but realistic case, for which the motion response of a LNG carrier moored to a jetty on a sloping bottom is calculated. From seven years of deep-water sea states, the terminal downtime is estimated. The application of the methodology to the design case confirms that the terminal downtime can be significantly underestimated if shallow water effects are not taken into account. So the influence of the water depth, bathymetry, wave directionality and low-frequency waves on the vessel motions should be investigated with care. However, the results obtained in the design case also show that the spectral shape of the low-frequency waves predicted by the wave models are sensitive to the tuning of numerical parameters. Tuning the wave models against model tests or full scale data is therefore highly recommended, because the motion response of a low-damped moored vessel can be dominated by the amount of low-frequency free wave energy at its natural periods.