reduction in gas consumption and avoids cargo loss (Fig. 1).
The most disadvantageous FSRU cargo operator’s action
is unjustified excessive recirculation of the regasification
plant in-tank feed pumps (Fig. 3).
International Group of Liquefied Natural Gas Importers (
GI-IGNL), “Rollover in LNG Storage Tanks,” 2nd Ed.: 2012-2015,
Hashemi, H. T. and Wesson, H.R., “Cut LNG Storage Costs,”
Hydrocarbon Processing, August 1971, pp. 117-120.
Kulitsa, M. and Wood, D.A., “Rigorous monitoring reduces
FSRU cargo-rollover risks,” Oil and Gas Journal, Vol. 115, No. 6,
June 5, 2017, pp. 74-81.
Zellouf, Y. and Portannier, B., “First step in optimizing LNG storage for offshore terminals,” International Gas Union Research
Conference, Seoul, Oct. 19-21, 2011.
a rate sufficient to maintain tank pressure at this point but
not necessarily reduce it (Fig. 1). Tank pressure will decline
naturally during STS ramp down by at least 30-40 mbar as
the transfer rate slows. The GCU and other safety equipment
also may slow to minimum flow, remaining on standby for
unexpected events, or shut down completely at the end of
the STS transfer. It is unwise to run GCU-SD for cargo-cool-ing purposes on any FSRU after an STS transfer is complete.
Doing so if FSRU tank pressure is within its permitted operating range loses cargo without producing any benefits.
In the case of an LNGC-to-LNGC STS transfer, the receiving LNGC must be prepared to allow its tank pressures
to rise to near their upper operating pressure limit during
this stage of the STS transfer, including flow rampdown. It
is generally not necessary for the receiving LNGC to prevent tank pressure increases, even when it has cool-cargo
obligations and its GCU-steam dump is already activated.
Tank pressure would typically only remain elevated for up
to about 1 hr, insufficient for the entire cargo of LNG to
warm perceptibly. Indeed, cargo heating would be so small it
would be unlikely to register more than +0.1° C. (+ 10 mbar).
Adopting this approach is likely to save only 5-10 tons of
BOG from being burned in the GCU during each STS transfer, equivalent to a cargo value of about $1,900.
During normal operations, adopting the proposed cargo
management measures on FSRU with MARVS more than
250 mbarg (e.g., 400 or 700 mbarg) and a recondenser or
latent heat capture system (LHCS) running should reduce
consumption of gas by the GCU-SD to zero. Any gas consumption in the GCU of such vessels is likely to be due to
On 250-mbarg MARVS FSRU, adopting the proposed
cargo management measures, with or without a recondenser
or LHCS installed, should cut gas consumption in GCU-SD
operations by one-third to one-half (Fig. 4). Exact benefits,
however, depend on many factors and vary from one STS to
another. For instance, if the discharging LNGC arrived with
a cold or very cold cargo or the gas send-out rate from the
FSRU was high, more flexibility would be possible in tank
pressure management, resulting in lower gas consumption
For LNGC-to-LNGC STS transfers, loss reductions by
adopting the proposed cargo management measures are less
than for LNGC-to-FSRU and are likely to vary significantly
from one STS transfer to another. But savings valued in the
thousands or tens of thousands of US dollars per STS transfer can still be achieved.
The tandem-pressure approach (Measure 9) achieves the
most significant savings. This measure can be applied with
FSRU of any specification. Applying the upper operating
pressure limit on FSRU tanks for starting safety equipment
such as the GCU-SD (Measure 7) also leads to substantial
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