Boil-Off Gas Precooling Process for Subsea Low Temperature Lng Pipelines

Abstract

Moving liquefied natural gas (LNG) ports offshore and transporting LNG to the coast through submarine pipelines is proposed as an effective method to meet the required water depth for the LNG tankers and prevent potential safety hazards of ports in crowded waterways. At ultra-low LNG temperatures (−160 °C), the pipelines should be precooled by LNG boil-off gas (−120 °C) before normal operation to avoid instantaneous LNG gasification and sharp pipe contraction. In the present study, a transient mathematical model was developed for the precooling process in a subsea multilayered LNG pipeline. The heat conductivity varied with temperature, and the governing equations were solved using the finite difference method. Additionally, the effects of pipe size, insulation layer thickness, and insulation material were investigated. The precooling time was found to significantly reduce with increasing pipe size, and the pipe diameter in the range of 30” to 40” was suitable. In terms of economy and practical applications, the thickness of the insulation layer was suggested to be should be 30–60 mm. Moreover, compared with the glass form insulating material, rigid polyurethane foam exhibited better thermal performance during the precooling process. The results can assist in determining a safe precooling process for subsea LNG pipelines to transport LNG, which is a suitable green alternative to environmentally polluting fossil fuels.

Keywords: Multilayered LNG pipeline, Boil-off gas (BOG), Precooling, thermal analysis, Ultra-low temperature