Ship motion-sloshing interaction with forward speed in oblique waves

Abstract

Seakeeping properties of large LNG carriers with partially filled tanks are known to be influenced by sloshing inside their tanks, by wave conditions and wave heading, and by their forward speed. An efficient hybrid numerical method was developed to study wave-induced ship response coupled with sloshing-induced compressible fluid response inside the tanks of a 138,000 m3 LNG carrier at zero forward speed and at the constant forward speeds of 12 and 20 kn. Wave conditions considered were oblique regular waves at heading angles of 0, 30, 60, 90, 120, 150, and 180 deg. The numerical approach combined a nonlinear boundary element Rankine source code to solve the weakly nonlinear seakeeping problem with a Reynolds-averaged Navier-Stokes equations solver to compute the compressible sloshing flow inside the tanks. Coupling effects of the six degrees-of-freedom ship motions in beam waves were validated against comparable model test measurements of ship responses and against free surface elevations in tanks. Results demonstrated that sloshing affected the simulated seakeeping behavior of the subject LNG carrier advancing at forward speeds in oblique waves. As sloshing had the greatest influence on the ship's roll motions, a key aspect was the derivation of roll damping from model decay tests.