Session: 02-04-01 Fatigue and Fracture Reliability 1

Paper Number: 81275

81275 - Fatigue Assessment of Pump Tower in LNG Membrane Tanks 

Membrane tanks are used for marine transport and storage of Liquid Natural Gas (LNG). LNG fuel tanks onboard ships is an emerging application. LNG carriers operate with almost full or almost empty membrane tanks. Storage tanks on floating storage units (FSUs) experience all filling levels. Partly filling may give rise to increased sloshing responses on the pump tower structure inside.

DNV has published new rules for alternative survey arrangement, ASP notation, for FSRU/FSUs, which opens for longer tank inspection intervals. This is 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: 1) utilizing modern technology on ship tracking with global hind cast data 2) Performing hydrodynamic analysis of a ship 3) Apply ship motions to calculate sloshing 4) Apply sloshing and inertia from ship motions to a detailed pump tower arrangement 5) Carry out fatigue analysis of time series of responses for a large number of potential critical locations 6) Combine damages from a large number of sea states and several filling levels and finally 7) Visualize where critical hot spots are located. 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. The complex multi-step methodology is made feasible by using specialized tools developed for the wind turbine industry. 

Presenting Author: Gaute Storhaug DNV

Authors:

Gaute Storhaug DNV
Hui Sun DNV
Yuelong Zang DNV
Olav Rognebakke DNV