Applying tandem pressures
Tandem pressures are created when the receiving ship can
push maximum vapor return flow and the discharging ship
can accept maximum vapor return flow. This involves applying Measures 7 and 8 (see accompanying box) in tandem.
This tandem-pressures approach is critical for STS transfers
involving FSRU with 250-400 mbarg MARVS tank design,
but less important for 700 mbarg vessels. Applying the tandem-pressures approach in LNGC-to-LNGC transfers is
only possible if warmer cargoes are acceptable by the next
port of discharge. Low differential pressure between tanks
prevents tandem pressures from developing maximum flow,
the upper operating pressure limit (Measure 7) not applying
if a cold cargo is required.
Allowing tank pressures to naturally rise to the upper operating tank pressure reference point
for the receiving ship initiates the tandem-pressures approach, higher tank
pressures on the receiving ship pushing more vapor return to the discharging LNGC tanks during STS transfers.
Tandem pressure also results in minimum generation of boil-off on the discharging ship (Measure 8) and maximum differential pressure between the
ships, stimulating significant vapor return flow. Returning more vapor to the
discharging ship enhances Measure 7’s
benefits. Measure 9 provides a double
benefit: those achieved by Measures 7
GCU-SD. Doing so creates the ideal pressure differential between the ships, indirectly connecting the discharging ships
GCU-SD to cooling the cargo on the receiving LNGC while
also running its own GCU-SD. Higher LNG transfer rates are
an additional benefit of applying this measure to STS transfer.
When the receiving LNGC can maintain upper operating pressure limits in its tanks (e.g. the received cargo will
be discharged to a 700-mbarg MARVS FSRU, not 250, or a
shore-based terminal which has confirmed its agreement to
receive a warmer cargo), application of this measure would
be the same as for transfer to an FSRU. It is, however, still
prudent for the discharging LNGC to maintain its tank pressure reference close to and not above CTMS-registered pressure, best achieved by opening the vapor return valve as
fully as possible.
FIG. 1 RECEIVING-TANK CONDITION CHANGES DURING STS TRANSFER*
FSRU tank pressure variations
due to adjustments in vapor
return fow made by LNGC
Elapsed time, hr
0 8 16 24 32 40
*Tank pressure and BOG fow for STS transfer to dual fuel diesel-electric FSRU with enhanced vapor returns. S TS transfer at 6,000 cu m/hr with constant regasifcation for sendout and constant DFDE engine consumption.
STS stop STS start
BOG fow to DFDE engine
Estimated LNG SVP
Tank pressure BOG fow to GCU
LNG TRANSFER OPTIMIZATION MEASURES ON RECEIVING VESSEL
Measure 1: Ensure cargo to be delivered is as cold as possible before transfer.
Measure 2: Ensure lowest operating tank pressure on receiving vessel at transfer start.
Measure 3: Do not run GCU-steam dump safety equipment to cool heel cargo before transfer.
Measure 4: Ensure coldest attainable temperature around receiving-tank vapor space before transfer.
Measure 5: Minimize receiving-tank pressure increases during transfer lines cooldown.
Measure 6: Keep receiving-tank vapor space, surrounding structure as cool as possible during transfer.
Measure 7: Use tank-design upper operating pressure control limits to determine GCU-steam dump start.
Measure 8: Minimize BOG generation on discharging LNGC and enhance vapor return from FSRU.
Measure 9: Apply tandem pressures between discharging and receiving ships to boost vapor return.
Measure 10: Avoid extra heating of LNG by excessive recirculation of regasifcation feed pumps in the
Measure 11: Remove ballast water surround LNG tanks to minimize natural heat ingress.
Measure 12: Manage reduced vapor return towards discharging LNGC as STS nears end.
*Measures 1-7 described in full in Part 1 of this article (OGJ, Nov. 6, 2017, pp. 74-81).