Performance Evaluation / Downtime Analysis of a Floating Liquefied Natural Gas Facility
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Numerical modeling of a particular Floating Liquefied Natural Gas (FLNG) facility, targeting on Prelude FLNG, was done to find an appropriate design of the FLNG including downtime in the specific field conditions. Hull/mooring coupled FLNG models with different vertical mass distributions (represented by the center of gravity’s vertical coordinates, ZCGs) are tested under complex environmental conditions of sea-wave, wind, current, and swell. WAMIT is used for the hydrodynamic analysis in frequency domain, and the time-series of motions under swell-excluded conditions are obtained by using CHARM3D. The time-series of linear swell-induced motions are separately obtained from the Response Amplitude Operators (RAOs, results form WAMIT). The consequential time-series of motions for the combined conditions are obtained by linearly superposing the two separate results. Each of the two split environmental conditions shows very distinct effects on roll motion about the varied ZCGs. Thus, the test results enable to estimate the proper natural periods for the floating system, which provides a reference model for further analyses of several modifications under practical assumptions. One scenario assumes a design modification with reduced weight in the topside facilities, which roughly reflects a conceptual design of the FLNG introduced by Shell. The other scenario is more practical, assuming an operational stage of less stored volume, by simply reducing the hull weight. The motional characteristics differentiated by these two scenarios are investigated through their annual performances based on two heading strategies. Performance evaluation can be done by estimating the downtime of the FLNG. In this study, the typical operation limits for topside facilities of FLNGs, 2° of roll/pitch, are compared with the maximum magnitudes of the roll/pitch angles during 3-hour time series of motion. From a sample of metocean data, only dominant waves that can cause downtime are employed to check the possibility of downtime, while the minor wave system’s joint conditions are excluded. Once the excessive weather conditions that induce downtime are defined by the combinations of Tp (peak period of waves) and Hs (significant wave height), the total downtime periods including recovery hours can be estimated throughout a year. As a result, the operability of the FLNG was very sensitive to the variation of topside weights, whereas the lighter-hull assumption brought negligible differences. These outcomes consequently demonstrate that the natural periods of an offshore system should be properly tuned based on the local sea states, and any modification of topside designs that includes nontrivial weight changes requires careful analysis. Moreover, the FLNG heading strategy by Dynamic Positioning (DP) system may have great impact on the operability, and seasonally-varied heading strategies are recommended for the FLNG to minimize downtime.
Sea-Swell Combined Condition
Cho, Yong Suk (2015). Performance Evaluation / Downtime Analysis of a Floating Liquefied Natural Gas Facility. Master's thesis, Texas A & M University. Available electronically from