Hydronamics of a moored LNG carrier behind a detached breakwater

Abstract

As the worldwide gas market continues to grow and environmental concerns with respect to in-port unloading of gas have increased, there has been a boom of interest in new liquefied natural gas (LNG) import terminals in the past five years. For these terminals, which are more and more located in areas with hostile sea conditions, dedicated provisions are required to create sufficient shelter for the carriers. Proposals have been made to construct a marginal low crested breakwater parallel to the coast protecting a ship moored at a jetty close to the shore. For an optimal economic design of such an LNG marine terminal, the dimensions and orientation of the detached breakwater have to be optimized as a function of the weather related downtime of the moored LNG carrier. Doing so requires adequate simulation tools. However, for the combination of wave and ship motion, a link between an efficient wave simulation tool and a program for ship response calculations is not available at present. The research on ship behaviour has resulted in the development of various so-called six degrees of freedom (SDF) computer programs. These programs solve the equations of motion of a moored vessel for all six degrees of freedom. As a consequence of the non-linear characteristics of the mooring system the equation of motion is solved in the time domain. The wave force time series are calculated from a homogeneous wave field of irregular, long-crested waves. In case of an open jetty configuration these assumptions are valid. However, considering a carrier behind a detached breakwater, the wave field is not homogeneous, but the wave height varies over the ship length. Consequently the influence of the detached breakwater on the ship motions must be considered. In addition, the reflection of the waves at the coast also has to be taken into account. This thesis deals with a methodology to predict the hydrodynamics of a moored LNG carrier behind a detached breakwater. A rapid assessment tool has been developed in order to assess the optimum breakwater dimensions in the preliminary design stage of an LNG marine terminal. In particular the effects of the breakwater dimensions on the hydrodynamic behaviour of the moored LNG carrier are considered. The computational approach for the calculation of ship motions from a given offshore wave field is described. In addition results are presented for different terminal layouts.