66 Oil & Gas Journal | Dec. 4, 2017
decrease again. That tank pressure on the LNGC represents
a balance point where vapor return can be maximized for
Locating this pressure balance point can be attempted
slightly before the FSRU tanks reach their upper operating-pressure limit if it is apparent that BOG return flow is too
low for the typical values expected for specific STS transfer.
The key focus of this balancing effort is to maintain CTMS
pressure if MARVS of the FSRU and LNGC are the same
or maintain CTMS + 10-30 mbar in the discharging LNGC’s
tanks if an FSRU with MARVS >250 mbarg is involved.
These levels must be achieved by adjusting the vapor return
valve. It would be fully open in case of 250-mbarg MARVS and throttled in cases where STS transfer involves FSRU
with 400-700 mbarg MARVS tanks.
LNGC typically arrive at the FSRU site with tank pressures of 110-130 mbarg, a safe and acceptable pressure for
the duration of the STS transfer. If the discharging LNGC
decides to lower tank pressure to 70-80 mbarg and attempts
to maintain its tank pressures at that level then a portion of
the LNG volume discharged from its tanks will be replaced
by vapor resulting from the evaporation above NBOR in its
tanks when pressure dropped below the LNG’s SVP. Under
such circumstances the vapor return valve will be throttled
and the receiving vessel will be forced to retain more vapor
in its tanks. That retained vapor may eventually need to be
disposed of by running the GCU-SD.
The vapor return rate will vary during STS transfer. As
the receiving ship’s tank pressure rises vapor return flow is
boosted toward the discharging LNGC. If the discharging
LNGC attempts to maintain its tank pressure significantly
below CTMS-registered value, LNG liquid in its tanks cools.
That cooling effect may be observed only during last third of
STS transfer at the receiving vessel.
The cooling of the cargo in this manner is insignificant (<
0.5° C.) and its beneficial impacts for the receiving ship negligible. Only the last quarter to one-third of a cargo transfer
might enter FSRU tanks at this slightly lowered temperature.
Gains provided by discharging LNGC while able to receive
significant return vapor by maintaining tank pressures closer
to CTMS-pressure far outweigh this cooling effect’s benefits
In general, applying this measure in case of STS transfer between LNGCs would suggest broadly the same tank
pressure reference point for the discharging LNGC (as recommended for STS transfers involving 250-mbarg MARVS
FSRU) even when both LNGCs must use GCU-SD to keep
their cargoes as cold as possible and deal with increasing
Doing so, however, depends on the prevailing conditions
in the two ships. When the discharging LNGC also runs
its GCU-SD (the typical situation in this type of STS transfer) and promotes conditions that result in the vapor return
valve being only partially open, the LNGC generates additional vapor in its own tanks, above NBOR, which it burns
immediately in its own GCU-SD. Its cargo is not significantly cooled before transfer to the receiving LNGC, with only
minor practical benefit to the receiving ship.
Alternatively, the receiving LNGC may be inhibited from
using its upper operating pressure limit, and instead target
maximum cooling of the received cargo. The resulting lower pressure differential between the ships’ tanks inherently
leads to reduced vapor return flow to the discharging LNGC.
The best solution in this instance is for the discharging
LNGC to fully open the vapor return valve and maintain tank
pressure close to the CTMS reference point by running its
LNGC-TO-FSRU TRANSFER, VAPOR RETURN FLOW RATES
LNG STS transfer Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Case 9
400-mbarg FSRU with recondensers
Recorded STS transfer rate, cu m/hr 5,711 5,799 5,978 5,791 6,011 5,750 5,750 6,782 6,802
Recorded vapor return fow rate, kg/hr < 1,000 < 1,000 2,432 3,835 5,009 6,640 5,230 8,811 9,019
LNGC vapor return valve open, 7 6 9 15 14 28 21 49 100
LNGC vapor return temperature, °C. - 15 -109 - 28 -108 -113 -116 -99 -123 -126
LNGC discharging tank pressure, mbara 1,116 1,121 1,101 1,096 1,161 1,112 1,103 1,120 1,128
Initial CTMS-LNGC cargo tank pressure, mbara — — — — 1,175 1,156 1,156 — —
FSRUtankpressure,mbara 1,237 1,203 1,174 1,186 1,239 1,155 1,165 1,141 1,141
LNG grade, origin, specifc gravity at -159.7° C. Medium, Medium, Heavy, Medium, Light, Heavy, Medium Heavy Heavy
Ras Laffan, Ras Laffan, Nigerian, Ras Laffan, Trinidad, Rotterdam, Ras Laffan, Ras Laffan, Ras Laffan,
0.439 0.439 0.450 0.439 0.428 0.450 0.439 0.458 0.458
Data comparison at common reference points
Difference in normal BOG due to LNG 250-450
composition, 1 kg/hr
Vapor return mass to STS rate, ratio 0.18 0.17 0.41 0.66 0.83 1. 15 0.91 1. 30 1. 33
Vapor return fow rate, of STS fow rate, 2 12 12 28 46 58 81 63 91 93
vapor density = 1.4324 kg/cu m
Ideal case vapor return volume equal to 8,178 8,304 8,560 8,293 8,608 8,234 8,234 9,712 9,740
pumped LNG volume, 2 kg/hr
Registered vapor return fow, kg/hr 1,000 1,000 2,432 3,835 5,009 6,640 5,230 8,811 9,019
Vapor return mass, ideal vapor mass 12. 2 12.0 28. 4 46. 2 58. 2 80. 6 63. 5 90.7 92.6
Vapor remaining in FSRU tanks, 3 88 88 72 54 42 19 36 9 7
Related vapor mass excluded from vapor 7,178 7,304 6,128 4,458 3,599 1,594 3,004 901 721
return, 3 kg/hr
1NBOR = 0.15%/day. 2At -120° C., 1,100 mbara. 3Referenced to ideal vapor return case.