Towing Characteristics of Large-Scale Concrete Caission for Offshore Modularised LNG Terminal

Peng, Yanjian (CNOOC Gas & Power Group Ltd.) | Chen, Feng (CNOOC Gas & Power Group Ltd.) | Zhang, Chao (CNOOC Gas & Power Group Ltd.)


Abstract Objectives/Scope

The innovations of marine LNG terminal concepts are a hot area of research for several years. In order to study the towing and sinking hydrodynamic properties of the large-scale concrete LNG terminals, the basin test and numerical simulation was carried out to simulate the dynamic motion of the concrete LNG terminals towed to the site and sinking on the seabed.

When the caission is placed, the caission generates motion under the action of the waves, and the caission is restrained by the control cable during the movement. Based on the stability and safety considerations of caission sinking construction under the marine environment conditions, it is necessary to know the motion state of the entire sinking process of caission. Therefore, it is necessary to numerically predict the motion response of caission.

The force anlysis of the caission structure in water is a viscous wave-making problem a bluff body in a restricted area. In the viscous medium with infinite domain, the flow around the object can only be solved satisfactorily when the Reynolds number is small. The caisson involves not only the blunt body but also the large Reynolds number of the medium and the influence of restricted boundary and free surface. Therefore, it is generally believed that the most reliable method is experimental research. As the caisson section is square and blunt, the flow separation point is stable, which creates conditions for self-similarity in experimental study. Therefore, in theory, the experimental study can obtain quite satisfactory results. The main purpose of the towing tests is to investigate the dynamic behavior and characteristics of the caisson and the line tensions during towing out from the dry dock to the sea area in waves and currents.

Related work on towing of Large-scale structure. Kyozuka, Y. et al.[1] conduct numerical simulation of the tidal flow and ecosystem in the sea around a very large floating structure or a Mega-Float, and simulation results with/without a Mega-Float of 4.75 km length, 1.5 km breadth and 1.2 m draft in Tokyo Bay are described. Zhang, Puyang et al. [2]When the wave height increases from 1 to 1.5m, the accelerations in six directions increase, especially surging acceleration. Besides, if the wave height is up to 5 m, the bucket foundation loses stability and may overturn in the water. Xiao, L F et al.[3] carried out mooring,towing and installing tests on immersed tunnel caissons in basin to investigate the motions and line tensions, and present that the maximum tension exists in the case that half caisson is exposed to waves and currents in the towing operation. Based on the frequency domain analysis through hydrodynamic software AQWA-Drift module, Zhu W L et al.[4] decomposed wave frequency motion and low frequency motion using the computed motion response, and calculate the full frequency response energy spectrum. Furthermore, introduced kinds of multi- ship towing solution and analyzing its application range and the tug configuration. After simulation and verification, Cheng, Y G et al. [5] [6] propose a simple anti-motion device, which is the perforated-impermeable-plate combination attached to the foreend and back-end of the VLFS(very large floating structure). Cheung, K.F et al. [7] carried out the laboratory experiments and parametric study to verify the numerical results. The pneumatic platform is composed of an array of open-bottom vertical cylinders trapping pressurized air that displaces the water. The cylinder diameter is assumed to be small compared to the wavelength and the water inside each cylinder oscillates as a piston. These assumptions simplify the mathematical formulation. To analyze the load and flow characteristics. Cho, S., et al. [8] comparied wind tunnel tests and CFD, analyzed flow physics regarding the characteristics of the flow.