LNG FSRU and FPSO Pump Considerations for Ship Motion

LNG marine pumps installed in FSRU and FPSO applications are exposed to three degrees of ship movement as a result of wave motion.  There is concern that adding ship motion to a traditional stationary pump technology can cause issues such as unseating, free spinning, and bearing damage during non operation. The in tank LNG pumps are column mounted submerged pump and motor vertical centrifugal type based on conventional LNG land based in tank and rigid mounted marine pumps.  During operation the Ebara thrust balancing system eliminates load on the bearings and the fluid pressure holds the pump onto the seating surface with large amount of force negating the effects of ship motion. In non-operational mode they have gravity dependent seating and free moving rotating elements.  An Ebara rigid retraction system and shaft locking device are available but demonstrated to not be necessary. For this presentation, ship motion is based on 50 to 80 foot sinusoidal wave motion over typical 8 to 20 seconds producing 0.13 to 0.05 Hz frequencies.  The resulting maximum vertical acceleration value of 15 ft/sec² is insufficient to unseat or harm the Ebara pump in any way during the most vulnerable non operational condition.  The Ebara Low Center of Gravity Anti Pitch type suction valve, a minor modification from the typical land based design, allows the ship to list over 90 degrees while maintaining pump seating without a rigid type retraction system. The wave motions produce low frequency, normal direction type ship movement and will not induce a spin on the rotating mass which has been proven with several thousand marine carrier pumps in service. This presentation provides confirmation it is not necessary to install extra system hardware to LNG pumps in FSRU and FPSO units that adds complexity, weight, and costs with no value added.