New promising generation of twin-gondola LNG carriers optimised with the aid of CFD calculations

The world has enormous quantities of natural gas, but much of it is located in areas far from where the gas is needed.
As natural gas is becoming an important energy source a large fleet is needed to transport it in liquefied form across
the oceans in specially designed LNG carriers from mainly the Middle East towards the Far East. During the overall
design process of such a vessel the shape of the hull form and its propulsors play an important role from a hydrodynamic point of view. This paper describes the design of a twin-gondola LNG carrier for Navantia. The twingondola aft body has proven to be an adequate design concept, but due to the complexity of the flow around the aft body the design should be carried out with great care. Computational Fluid Dynamics (CFD) tools are extremely
valuable in the hydrodynamic optimization process of the hull. In this design both potential flow codes and viscous
flow codes have been used to obtain the optimum hull form. Special interest has been paid to the shape and the
orientation of the gondolas. The performance of the resulting design has been verified by model tests in MARIN’s
Deep Water Towing Tank.
The hull form optimization process started with analysis of the initial lines supplied by Navantia, applying MARIN’s
non-linear potential flow code RAPID. In this phase the bulbous bow and the shape of the hull have been optimized.
Furthermore, for a few variants the pressure distributions around the gondolas have been studied in order to obtain
a first impression of the optimum gondola orientation. Viscous effects, however, play an important role in the flow
around the aft body and therefore the shape of the aft body and the orientation of the gondolas have been further
improved by using MARIN’s viscous flow code PARNASSOS. With the results of the PARNASSOS calculations it was
possible to make decisions with regard to the horizontal angle and the inclination of the gondolas, and the slope of
the buttocks in the area between the gondolas. Special attention has been paid to avoid flow separation around the
aft body. The wake field in the propeller plane has been improved by optimizing the shape of the gondolas. The gondolas have been oriented in such a way that maximum efficiency is achieved. The effect of a working propeller has been investigated by applying an axial force field in the propeller plane. Scale effects have been studied by conducting calculations for both model scale and full scale. After the optimum hull form was considered to be found, a model was built to verify the findings of the calculations.
Given the very promising results of this new generation of LNG carriers, achieving besides the excellent propulsive properties also a higher payload target, the yard became more competitive and is expecting quite some orders for this particular ship type.