Gap Effects at Side-by-Side LNG-Transfer Operations

Abstract

The current demand of liquefied natural gas (LNG) from remote marine locations pushes the design of floating LNG (FLNG) liquefaction or regasification facilities, where LNG is transferred between shuttle carrier (LNGC) and terminal. Even if the tandem configuration is the primary choice for LNG transfer at rough offshore locations, side-by-side configurations would be the preferred option because of existing midship coupling manifolds on the present carrier fleet (no need for manifold modifications) as well as standard mooring systems and transfer-process-chains similar to oil-transfer. Therefore, the operation conditions at rough seas have to be improved to allow side-by-side LNG-transfer and to reduce offloading downtime. Within the SOTLL-project, side-by-side LNG transfer up to HS = 3 m is reached as a transfer limit using a new flexible pipe design, the advantages of sheltered areas at the leeside of the terminal barge and an optimized ship transfer position due to a flexible longitudinal offloading position. In addition to the evaluation of the hydrodynamic characteristics of this multibody system, one key aspect is the analysis of the exciting forces and motions due to wave amplification between the ships. In the gap between the hulls, the incoming wave field is amplified and changes dramatically. Depending on gap width, longitudinal offset, wave heading and length, large wave amplifications, standing waves and other resonance phenomena are observed which may result in high relative motions and increased forces of the entire mooring system. In this paper, the gap effects are investigated in detail with numerical approaches in frequency domain, validated by model tests at TU Berlin. A typical offloading scenario with barge and carrier is investigated for different gap sizes to identify suitable transfer configurations and ensure safe LNG offshore transfer up to HS = 3 m.

Topographic effects on wave resonance in the narrow gap between fixed box and vertical wall

“…Similarly, Tan et al (2014) conducted two-dimensional physical experiments to consider the fluid resonance and energy dissipation in a narrow gap formed by a fixed rectangular box in front of a vertical wall. To study the free-surface resonance between the FSPO terminal and the LNG shuttle carrier or between two barges, some three-dimensional physical tests were successively carried out by Clauss et al (2013), Xu et al (2014), and Zhao et al (2017).…”

Gap resonance and higher harmonics driven by focused transient wave groups

“…There is interest in the behaviour of the fluid in the gap in transient wave groups, as considered by Eatock Taylor et al (2008) who numerically studied the gap resonant response under focused wave groups, producing time series of the response using linear theory. Experimentally, wave groups have been used to test gap resonances by Perić & Swan (2015) and Clauss et al (2013), though in these cases the emphasis was on deriving regular wave response coefficients from the results rather than on the gap behaviour. To the authors' knowledge, there are no experimental studies on the transient gap resonance problem which present time histories similar to those produced numerically by Eatock Taylor et al (2008).…”

Current practice and research directions in hydrodynamics for FLNG-side-by-side offloading

“…These points have led to interest in the behaviour of the fluid in the gap in transient wave groups as considered by Eatock Taylor et al (2008), who numerically studied the gap resonant response under focused wave groups. Experimentally, wave groups have been used to examine gap resonances by Clauss et al (2013) and Peric & Swan (2015), although in these cases the emphasis was on deriving regular wave response coefficients from the results rather than on the transient gap behaviour. Focusing on the transient behaviour of the gap, as was done numerically by Eatock Taylor et al (2008), Zhao et al (2017 carried out a series of large scale experimental studies, where the two fixed models had round bilges.…”