Boil-off gas precooling process for subsea low temperature LNG pipelines

Moving liquefied natural gas (LNG) ports offshore and transporting LNG to the coast through submarine pipelines can address the required water depth for LNG tankers and prevent potential safety hazards at ports in crowded waterways. At ultra-low LNG temperatures (−160 °C), 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. 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 decreased significantly with increasing pipe size, and a 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 30–60 mm. Moreover, compared with glass foam insulating material, rigid polyurethane foam exhibited better thermal performance during the precooling process. These results can assist in determining a safe precooling process for subsea LNG pipelines, providing a suitable green alternative to fossil fuels.