Cooldown of LNG Loading Systems – An Integrated Approach. Part 1: Piping Stress Analysis

Abstract A common understanding amongst designers and operators of LNG terminals is that cooling down large-bore stainless steel piping systems using Liquid Natural Gas (LNG) or Liquid Nitrogen (LN2) can lead to steep temperature gradients generating high stresses / strains and the potential for low-cycle fatigue failure. The normal approach for cooling down LNG piping systems is that they are cooled down over a long period of time during the commissioning or Start-up phase and then kept cold through continuous recirculation of the LNG to minimize or avoid the need to cooldown again during normal operation. The slow cooldown approach is typically achieved by using a sufficient volume of boil-off gas generated from LNG or LN2. Cold gas ensures uniform cooldown of large-bore stainless steel piping systems to avoid excessive thermal stresses. However, using boil-off gas for the slow cooldown of LNG loading systems is not practical in all facilities, particularly in substantial piping systems with significant thermal mass where generating sufficient volumes of cold gas is impractical. This paper presents the first part of a comprehensive study on a hybrid cooldown approach using liquid and boil-off gas to cool the LNG piping system. A multiphysics approach is adopted which includes CFD analysis to model the heat transfer from the liquid and gas to the pipe. The temperature gradients developed are then mapped to an FEA of the piping system to evaluate the impact of circumferential temperature gradient on local and global thermal stresses and thereby determine the fatigue life of the system. This paper is Part 1 of a series of papers. This paper focusses on the results of investigations carried out to better understand the theory of how the temperature gradients and stresses will develop in a piping system and their sensitivity to the rate at which the cryogenic liquid enters the pipework. From this a strategy is developed which will ultimately provide the basis for a cooldown procedure. Subsequent papers will cover the application of the cooldown procedure to the actual piping system. This will include a full CFD and FEA of the piping system combined with validation of the simulation from the commissioning of the live system. It is anticipated that Part 2 will be published in 2025.