An Experimental and Numerical Assessment of Moss-Type LNG Carrier Seakeeping Moored to PETRONAS FLNG in Partially Filled Condition

Abstract

Moss-type LNG carriers (LNGC) — equipped with spherical LNG tanks — have successfully performed multiple side-by-side (SBS) offloading operations from the PETRONAS FLNG (PFLNG1) terminal since first gas in late 2016. Such LNGC are well suited for these operations as by essence their tanks mitigate the liquid impact risk, for any filling ratio. However, regardless of the LNG tanks shape, the accurate assessment of the relative motion between PFLNG1 and a SBS-moored LNGC was critical for designing the mooring and offloading systems — in particular the LNG Marine Loading Arms (MLA) — and subsequently assessing the terminal operability. While the issue has been extensively studied for LNGC equipped with prismatic tanks (SALT I & II JIPs [4]) and a good agreement achieved with numerical models using potential theory, literature available on coupling with LNG sloshing in spherical tanks in a SBS mooring configuration is rare. Compared with prismatic tanks, the axisymmetric tank geometry is not restricting the main sloshing modes direction: these modes are therefore free to rotate due to LNGC wave-induced motions. An experimental wave basin model test campaign has been held at the MARIN facility to assess the behavior of a Moss-type LNGC with partially filled tanks and provide a good quality database for numerical model calibration and relative motion prediction. This carrier, equipped with Plexiglas spherical tanks, has been tested in free-standing conditions and SBS-moored to PFLNG1. A variety of loading conditions was covered for the LNGC including ballast, 20% and 50% filling. Sloshing modes have been captured with dedicated cameras and wave probes implemented in the LNGC tanks. In this paper, the model test set-up is presented and a comparison is made between the experiments and numerical simulations, using the potential theory software Diodore™, accounting for the hydrodynamic interaction between the vessels and the coupling with the liquid motion in tanks. Particular attention is paid to the coupling effects between longitudinal and transverse motions induced by the longitudinal asymmetry, and their consequences on the design of the offloading system.