A sea water hydrostatic test system for LNG storage tanks on the ground that can be completed in a short period of less than one month and whose feature is the cathodic protection of the internal wetted surfaces using Zinc-rich paints has been developed, without any risk of localized corrosion and water pollution. The galvanic corrosion and stress corrosion cracking (SCC) due to welding of different metals of 9 % Ni steel and Ni base weld metal in the internal tank vessel can be prevented by applying inorganic zinc or zinc primer and zinc tape. Their corrosion prevention performance has been confirmed by the sea water immersion test and the salt spray test. By the SCC test and the slow strain rate test (SSRT), the water quality criteria such as H2S concentration, pH value that SCC is not generated in the sea water have been also verified. It has been shown that the corrosion prevention method using zinc-rich paints does not cause any effluent pollution of zinc and this method is an improvement over the conventional cathodic protection method using sacrificial zinc anodes or impressed current cathodic protection.
Recently, reflecting the global situation of reduction of CO2 and steep rise of crude oil price, the demand for LNG as clean fuel has increased and the number of planning and construction projects of LNG shipping and receiving terminals is increasing accordingly. The LNG storage tank on the ground is mainly adopted for the LNG terminals, and the hydrostatic test is conducted for the storage tank when the construction is completed. However, as recent trends, because the tanks are very often constructed in areas such as Central and South America, the Middle East, Africa, Southeast Asia, etc. where obtaining clean water or industrial water is difficult, the hydrostatic test by using available sea water is needed.1) If the sea water is introduced into the tank, sufficient corrosion prevention measures are required to be taken, as the corrosion of LNG internal tank vessel materials or the occurrence of SCC is feared. Accordingly, in this study, the sea water hydrostatic test (SWHT) system to avoid the corrosion risk by adopting cathodic protection using zinc-rich paints (ZnPs), has been developed and verified its corrosion prevention performance. Furthermore, it is considered necessary to prevent occurrence of microbiologically induced corrosion (MIC) due to growth of sulfide reduction bacteria (SRB) as a lot of marine organisms such as planktons, etc. live in the sea water in comparison with fresh water and the O2-poor conditions are formed due to adherence and deposition of these organisms. Moreover, in case ZnPs are used, it is required to respect the sea water pollution criteria of the effluent as Zn is eluted in accordance with the degree of the corrosion environment. In order to avoid the risks of corrosion and pollution, the safety of the system was evaluated by conducting various tests under the conditions simulating the actual system. This report describes the results of the above-mentioned tests and evaluations.
SEA WATER HYDROSTATIC TEST SYSTEM
Fig. 1 shows the explanatory diagram of the SWHT system in the LNG tank on the ground. Furthermore, Fig. 2 shows the flow diagram of the SWHT execution and regulation of introduced and discharging sea water quality. The features of corrosion and water pollution prevention of this SWHT system are described below.
The sea water used for the hydrostatic test is normally collected from the sea in the vicinity of the construction site, and the water quality of the introduced sea water has been specified to prevent the occurrence of SCC as shown in Fig. 2, in accordance with the existing data. These water q