Time Domain Simulations of Side-by-Side Moored Vessels: Lessons Learnt From a Benchmark Test

Abstract

Time domain simulations are used extensively to assess the availability of turret moored terminals for offloading operations. The availability analysis provides a key input when assessing the economical viability of a project. In this context, it was deemed important to perform a benchmark study. Three programs have been used: AQWA-DRIFT - an ANSYS product maintained by Century Dynamics and used at SBM, LIFSIM and aNySIM both developed in MARIN. Five environments including wind, waves and current have been selected one of which with all components in-line and the remaining four with oblique wind and current. Simulations have been performed with identical input for the three programs. Results are compared statistically, but also in the time domain. Lessons learned will be discussed and areas of improvement identified.

Rankine time-domain method with application to side-by-side gap flow modeling

“…The analysis of multi-body hydrodynamic interactions have also being studied in time domain [21,2,22,23]. This approach is often used when the fully-coupled analysis involving not only the bodies, but also the mooring lines and fenders must be assessed, a situation in which a time domain approach must be necessarily applied.…”

Analysis of wave resonant effects in-between offshore vessels arranged side-by-side

“…As an exercise, if one considers a 1:150 scale factor, then the length of the geosim in full-scale would be 300 m, which may be considered a reasonable value for LNG ship vessels and the S 1 gap (0.05 m) would be come 7.5m, similar to the ones reported in literature for full scale operations (Naciri et al, 2007)). In addition, as will be further presented in this thesis, the computational results indicated that, for the multibody configurations here studied, the gap resonant frequencies are expected to occur around 9.0 s (in a 1:150 full scale) which may be considered as a typical operational peak period for sea waves.…”

“…Stiffness and pretension values of the moorings, that can be used as initial reference values, were reported in Naciri et al (2007) considering mooring lines loaded at 15 tons in the neutral position in calm water. The safe working load (SWL) of the 44 mm steel wires was assumed in their analysis to be 681 kN (55 % of MBL).…”

“…An estimation of the frequencies for the resonant modes were provided. Naciri et al (2007) used AQWA (ANSYS), including a damping lid in the gap between a FSRU and a LNGC in order to attenuate the unrealistic wave oscillations. A time domain analysis was carried out, with investigating data for designes, but numerical resonant frequencies were not commented.…”

“…In STS operating for LNG offloading in open seas, the fact that the tanker has to operate very close to the FLNG introduces the need to carefully assess the hydrodynamic interaction between these floating structures, and to accurately calculate the resulting motions. Specific elements of connection (mooring lines and fenders) must be applied to control the relative motion between both vessels (Naciri et al (2007)). The moorings enable us to control the small differential distancing whereas the fenders are applied to control the approximation between the bodies.…”