Experimental studies of cargo tank cooldown in an LNG carrier

Abstract

PURPOSE: The paper aims to show and analyze the construction of LNG tanks, systems related to temperature measurement and to describe the operation of the system of cargo tank cooldown, including the design of pressure compensation between tank membranes.

DESIGN/METHODOLOGY/APPROACH: This paper presents an experimental study on practical cooling measurements of LNG ship cargo tanks. The measures were aimed at comparing the cooling procedures recommended by the manufacturer with the actual temperature changes occurring during the investigated process.

FINDINGS: Based on the own experience of one of the co-authors, the sequence of operations performed after the LNG carrier enters the loading terminal was refined. Procedures and recommendations, important from the point of view of cargo operation and crew safety, were preserved.

PRACTICAL IMPLICATIONS: An analysis was made on safety procedures and potential problems that could arise during LNG trans-shipment. Tank cooldown takes place by delivering liquid methane to the spray pipelines embedded in the tanks. The authors' survey indicates that by lowering the tank temperature to -130°C (and below), excessive vapours can be avoided during the first critical minutes of loading. In some extraordinary circumstances ('warm' membranes), the number of produced vapours may be so large that shipboard facilities will not be able to carry away them to the terminal, pressure in the tanks will start to increase, and, in extreme cases, the pressure may get so high that emergency release of gas will be necessary.

ORIGINALITY/VALUE: Each ship has its cooldown tables, used for guidance in this operation. For tanks described in this article, the average value of cargo tanks temperature was used to calculate the time and quantity of liquid methane required for the cooling. The authors compared absolute temperatures recorded during the tank cooldown in the examined ship to temperatures specified in the manufacturer's tables. One should remember that the prerequisite for starting tank cooldown is to create or have a good atmosphere in tank inverting. The analysis presented in this article was performed on an LNG ship in continual operation, so the proper atmosphere was satisfied.