Fatigue Assessment of Pump Tower in LNG Membrane Tanks

Abstract

Abstract Membrane tanks are used for marine transport and storage of LNG, while LNG fuel tanks is an emerging application. LNG carriers operate with almost full or almost empty membrane tanks, while storage tanks on FSRUs and fuel tanks experience all filling levels. Partly filling may give rise to increased sloshing responses on the pump tower structure inside. DNV has published rules for alternative survey arrangement for FSRU/FSUs, which opens for longer tank inspection intervals. This alternative survey arrangement can be applied provided that the damage risk is less or equivalent to regular tank inspections intervals as an LNG carrier. The challenge is how to assess this equivalence. A definition of the reference vessel and trade is required. This paper addresses a new proposed methodology to quantify the relative pump tower fatigue. It is built up of several steps that will be explained including hydrodynamics, satellite tracking, CFD, load combination, fatigue analysis and visualization. This is first done for a representative LNG ship and trade. The methodology is then applied for an FSRU. Combinations of filling levels relevant for the FSRU and artificial wave environments are studied to quantify when the resulting fatigue may exceed the LNG carrier reference case. Uncertainties and challenges are discussed.