Applying tandem pressures between discharging and receiving ships in an LNG transfer boosts vapor return, cutting cargo losses and reducing cost. Part 1 of this article described
tank pressure and vapor and LNG temperatures’ influence
on ship-to-ship (STS) transfers (OGJ, Nov. 6, 2017, pp 74-81).
Part 2, presented here, will focus on applying more advanced
mitigative measures such as exploiting pressure differences
between tanks to minimize cargo loss during STS transfers.
BOG, vapor return
The cargo-discharging LNG carrier (LNGC) can assist the
receiving ship in efficiently handling its tank pressures and
minimizing natural gas consumption in the gas combustion
unit (GCU) or steam dump (SD). Not all natural gas vapor in
the receiving ship’s tanks can be relocated to the discharging
ship while replacing the liquid LNG volume it discharges,
but there are steps that can be taken to maximize this displacement.
LNGC operators often unintentionally create conditions
that reduce vapor return flow. By encouraging declining
pressure in its tanks during an STS transfer, the discharging
ship will generate vapor in its tanks beyond design specifications for normal boil-off rate (NBOR). The fact that tank
pressure cannot stabilize much below the LNG’s saturated
vapor pressure (SVP) places a floor on pressure reduction
and causes excessive vapor generation within the tanks of
the discharging LNGC when intentional reduction is attempted. These vapors replace a large part of the reduced
LNG volume, lowering vapor return flow from the receiving
vessel. In some cases vapor returns have been reduced to 2-4
tons/hr instead of 9 tons/hr (see accompanying table).
The optimal safe and efficient balance is to maintain tank
pressures in the discharging ship at or just above (+ 10-30
mbar) the pressure registered during the initial cargo survey
when the custody transfer measuring system (CTMS) opened.
This reference point will cause no complications for the dis-
charging LNGC. The discharging vessel already has this tank
pressure and the receiving floating storage and regasification
unit (FSRU) legally cannot dictate the pressure that the dis-
charging LNGC should maintain, its authority being limited to
recommending that the discharging LNGC maintain CTMS-
registered pressure during STS transfer. This will limit vapor
generation on the discharging vessel to not more than NBOR
and assure the best-possible tank pressure conditions to allow
maximum vapor return flow from the receiving ship, the dis-
charging LNGC’s tank pressure matching its LNG’s SVP.
To maintain CTMS-registered tank pressure, the discharging LNGC must use its vapor return valve. The correct
application involving a 250-mbarg maximum allowable relief valve setting (MARVS) FSRU requires the vapor return
valve to be at least mostly open and tank pressure to be very
close to (ideally equal to), but generally not above, the CTMS-registered value.
Low differential-pressure availability in certain rare cases will cause the LNGC to have a cargo with high but still
safe SVP and associated tank pressure. LNGC tank pressure during the initial cargo survey may be so high that,
even with the FSRU tanks at their upper operating-pressure
limit, the differential pressure will be too low to allow high
BOG return flow. In this case, LNGC tank pressure must
be slowly lowered in steps and vapor return flow observed.
Vapor return flow increases as LNGC tank pressure lowers
progressively. But at a certain LNGC tank pressure during
the pressure-lowering process vapor return flow will start to
David A. Wood
DWA Energy Ltd.
Tandem pressures between ships
during STS transfer cuts gas losses
LNG CARGO HANDLING–Conclusion