An Import LNG Terminal's experience with LNG weathering and unloading a cargo of fresh LNG into weathered LNG.

摘要

In the Unites States, for the last several years, a relatively abundant supply and low prices have characterized natural gas markets. This has led to low demand for imported LNG and LNG import facilities have optimized operating cost by minimizing boil off gas and preserving LNG inventory. The purpose of this paper is to share the experience of one LNG import terminal with LNG weathering to the point of minimal methane remaining in the terminal's LNG and then successfully unloading an LNG cargo into the weathered LNG. As the methane weathers off the stored LNG over time, the composition and physical characteristics of the remaining LNG change, which can have an impact on facility operation if not properly managed. These changes include but are not limited to LNG temperature increase, density increase, an increase in boil off gas heating value, increase in LNG heating value and an increase in boil off gas hydrocarbon dewpoint. Unloading fresh cargo LNG into the weathered LNG also presents several issues that must be managed to prevent geysering, prevent excessive vapor generation, control and monitor piping / equipment cooldown and provide proper mixing of the weathered LNG with the cargo LNG. Another issue which must be managed is the possibility of freezing heavy hydrocarbons when cold, fresh LNG is introduced into a line or piece of equipment that has experienced exaggerated boiloff. The exaggerated boiloff is due to reduced insulation, such as uninsulated top head of a submerged motor pump vessel, a poorly insulated valve, or a poorly insulated pipeline. Throughout the LNG weathering and subsequent cargo unloading, the facilities' metrics for success were to have no process conditions that would result in safety, environmental, or, process safety incidents. And all operators should coordinate, as necessary with all applicable regulatory bodies as such work is undertaken.