FLNG Rock and Roll · To meet the needs of floating LNG (FLNG), Nikkiso Cryo has developed a method...
Transcript of FLNG Rock and Roll · To meet the needs of floating LNG (FLNG), Nikkiso Cryo has developed a method...
FLNG Rock and Roll
Technical advances for
LNG pumps aboard FSRUS
Fabien A. Wahl, NIKKISO Cryo, Inc.
FLNG Rock and Roll
© LEWA GmbH, Fabien Wahl, NIKKISO Cryo, Inc., 03/2012 01
Technical advances for LNG pumps aboard FSRUS
By Fabien Wahl
Director of Business Development
Nikkiso Cryo, Inc.
Contact:
+1-702-643-4900
Nikkiso Cryo, Inc.
4661 Eaker Street
North Las Vegas, NV, 89081
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Table of content
1. Introduction _____________________________________________________________ 3
2. LNG pumps for SRV ______________________________________________________ 3
3. Technical advances ______________________________________________________ 5
3.1. In-tank pumps _______________________________________________________ 5
3.2. Send out pumps ______________________________________________________ 5
4. Line packing pumps ______________________________________________________ 9
5. Validations ______________________________________________________________ 9
6. Conclusion ____________________________________________________________ 10
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1. Introduction
Lower capital costs, shorter on-stream time, lower coastal environment impact and flexibility –
these are some reasons that led to the development of the floating storage and regasification
unit (FSRU). Typically these units are like large barges that are semi-permanently moored
offshore from their regasified LNG delivery sites and are fed by standard LNG carriers. A
special subset of the FSRU concept is the LNG carrier, which is fitted with full regasification
capability. These ships transport LNG from the liquefaction plant to the customer and then
regasify the LNG offshore. These special vessels are known as LNG shuttle and regas vessels
(SRVs). As the SRV is repeatedly subjected to seagoing vertical and lateral accelerations
exceeding those of moored FSRUs, these unique vessels require regasification pumping
systems designed and tested to perform during and after exposure to differing levels of motion.
Along the various phases of the SRV delivery rotation, the pumps are exposed to a spectrum of
accelerations, as shown in Table 1.
Accelerations (G) Regasification operation Transit seagoing
Vertical 0.4 0.8
Transversal 0.2 0.5
Longitudinal 0.1 0.6
Table 1: Operation environment
2. LNG pumps for SRV
Figure 1 shows the Nikkiso Cryo regasification ship pump set, which has been used on board
the Excelerate Energy fleet of vessels since 2004. Standing on the left, is the emergency/in-tank
pump, which on an SRV must perform the duty of being the primary feeder to the deck mounted
high pressure send out pumps and to the line packing pumps. Lying on the Horizontal Assembly
Stand© is the send out pump. The line packing pump is positioned on the far right.
The in-tank pumps are located inside the emergency wells found on a traditional LNG carrier.
Unlike the original emergency pumps that are part of a classical ship set, these primary pumps
are permanently lowered in the column. These pumps feed the suction drum, and are rated for
a flow of 620 m3/hr, with a differential head of 115 m. The pump rated power is 200 kW.
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Fig. 1: Nikkiso Cryo regasification ship set
The send out pumps are located on the bow of the ship. They are mounted in their own suction
vessel, themselves integrated onto skids. These pumps feed the vaporizers at a flow of 205
m3/hr, and with a differential head of 2,370 m. The pump rated power is 1100 kW.
Line packing pumps are operated during the start-up of the regasification unit to pressurize the
vaporizers and the downstream pipeline smoothly without any liquid hammer. These 200 kW
pumps deliver the same differential head as the send out pumps (2,370 m), but at a lower flow
rate (20 m3/hr). The line packing pumps are vessel mounted on the bow of the ship along with
the send out pumps.
For more recent projects, such as Golar – Winter and Höegh LNG – Neptune, larger send out
pumps have been developed, reaching 15 stages and 1,500 kW.
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3. Technical advances
3.1. In-tank pumps
On a conventional LNG carrier, the emergency pump, if ever needed, is lowered with cables
and rests on the foot valve unsecured. If the pump remained in position while underway, it
would be free to tilt or rebound vertically on the foot valve seat, especially during severe sea
conditions. Therefore, emergency pumps are stored on deck.
To meet the needs of floating LNG (FLNG), Nikkiso Cryo has developed a method to replace
lifting cables with a segmented lifting pole. This engineered system not only functions to lower
or remove the pump from the column but also holds the pump on the foot valve seat and
prevents any movement of the pump in the column. Additionally, the design of the foot valve
seat has been enhanced to a conical shape, which offers a self-centering and optimized contact
surface and eliminates any possibility of the pump sliding on a flat seat.
3.2. Send out pumps
The mass and the size of the send out pump, combined with the motion of the ship, induce high
mechanical stress issues. In traditional land based applications, the send out pump is
suspended from its discharge manifold inside the suction vessel. When the ship is pitching and
rolling, the mass-inertia-induced lateral displacement of the pump would generate high stress
loads on the bolt circle connecting the pump to the suction vessel. In order to prevent any
relative motion of the pump with respect to the vessel, Nikkiso Cryo has added a stabilizing pin
underneath the suction manifold of the pump. This brass male pin engages in the stainless steel
female receiver at the bottom of the vessel.
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Fig. 2: Send out performance curve with and without motion
The design enables longitudinal translation to account for thermal contraction differences
between the aluminum pump and the stainless steel vessel. The pin also prevents lateral
displacement, suppresses any bending moment and also ensures the pump housings will not
flex or cause interference between the rotating elements of the pump and the close fitting
bushings and bearings.
The same flexing might occur between the suction vessel and the deck of the ship. If this were
to occur, the pump mounted stabilizing pin could transmit the flexure to the pump housing
through the above mentioned receiver mounted at the bottom of the suction vessel.
Additionally, the vessel supporting pods would be subject to an increased bending stress.
To eliminate these effects, Nikkiso Cryo has added a similar male/female pin arrangement on
the external bottom of the vessel and the deck of the ship. This dual male/female pin
configuration between the pump and the vessel on one hand, and between the vessel and the
deck on the other hand, creates a rigid design with no lateral degree of freedom.
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Fig. 3: Line packing pump high speed validation
In a usual land based send out pump design, when the pump is not operating, the dead weight
of the entire rotation assembly (rotor, shaft, all impellers, and their retainers) is supported by
only one radial bearing. The vertical acceleration experienced on a ship while underway would
significantly multiply the vertical load imparted to this radial bearing. The balls would brinnel the
races and the bearing could be destroyed with very little, if any, pump operation.
Nikkiso Cryo has added a stowage piston located on the suction vessel head plate. This piston
reaches through the discharge manifold to the end of the shaft and is actuated either with
compressed nitrogen or by a pneumatic motor. When the pump is not operating, the stowage
piston lifts the shaft and relieves the rotor weight from the supporting bearing. This stowage
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position totally unloads the bearing, and preserves the pump indefinitely. Just before pump
operation, the rotor is lowered smoothly onto the support bearing.
The pump stabilization method and the stowage piston have been patented by Nikkiso under
United States Patent No. US 7,063,512 B2.
Fig. 4: Articulated platform
Send out pumps are classically long multistage pumps. In the case of the Neptune project,
Nikkiso Cryo supplied a 15-stage pump. The lateral displacement of the shaft induced by the
motion of the ship must be carefully addressed to prevent a rotor/stator contact during operation
in rough weather conditions. Nikkiso Cryo has added three radial ball bearings along the pump
shaft line. This unique construction yields a stiff rotor, perfectly guided on multiple points.
Bushings disposed in all non-bearing carrying stages, provide additional damping. This stiff and
damped rotor dynamics enables a safe pump operation while the ship is pitching and rolling.
During pump operation, the weight of the rotor and the hydraulic forces are compensated by the
Nikkiso balance drum assembly. In motion less design, the balance drum has to account for the
changes in hydraulic forces. In case of vertical acceleration changes, the weight force is also
changing. The Nikkiso balance drum design is a separate function in the pump, and is
independent of the design of any other component of the pump. Therefore, the balance drum
assembly is designed in order to meet the requirement of the axial thrust balancing, regardless
of the main flow, head specifications and from the vertical accelerations environment.
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4. Line packing pumps
The line packing pump specification presents a more complicated task in that the duty requires
the combination of low flow and high head. Traditional centrifugal pump selection based on a 60
Hz two-pole motor would have led to a slim 20-stage, 3.4 m-long pump. With respect to the
vertical and longitudinal vertical accelerations environment expressed previously, such a long
and thin pump appears as a poor fit. Nikkiso has specifically developed a Small High Pressure
pump, which takes advantage of a higher speed of rotation to reconcile low flow and high head.
Nikkiso’s line packing pump uses a VFD driven motor at 120 Hz. Advantageously, this Small
High Pressure pump has only eight stages and measures only 1.9 m in length. The high speed
design was supported by a detailed rotor dynamics analysis and impeller stress calculations.
5. Validations
The compliance of the design of the large send out pump with the motion of the ship has been
demonstrated during a full size test on an articulated platform. The platform was set up near the
peak shaver in Trussville, Alabama in 2004, by El Paso. An entire regasification unit was fitted
on the platform: suction drum, send out pump and vaporizer. The platform simulated the motion
of the bow of the ship in 8 m waves. The regasification test was run at full capacity with LNG
supplied by the peak shaver. Comparisons of data between the steady and articulated
operations show that the send out pump service is insensitive to the motion.
The Small High Pressure line packing pump was successfully tested on the Nikkiso test stand of
North Las Vegas, Nevada. The pump was tested at 110 Hz and 120 Hz. The operational speed
was then determined to be 116 Hz. For this pump, no impeller trim was necessary, because the
final tune up was performed by speed adjustment.
Following the validation program, the Nikkiso regasification ship set was certified by Bureau
Veritas Marine and Det Norkse Veritas Maritime. With the eye of Hurricane Katrina only 60
miles away, the FSRU Excellence was sending out gas at 100% capacity in the Gulf of Mexico.
Regasification operations were performed in 4 m waves. This validation by Mother Nature
proved the robustness of the entire floating regasification concept.
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6. Conclusion
Nikkiso Cryo is dedicated to the LNG industry and has heavily invested in technologies to
advance FLNG including FSRU, FPSO and SRV operations. The company's packages
include the full range of in-tank, send out and line packing pumps proven in the field. Original
technologies addressing the specificity of operation during ship motion have been developed,
patented and successfully validated. This line of product is presently in service or will be
installed on board all Excelerate Energy/Exmar ships, Höegh ships, and Golar ships. Further,
this floating pumping technology has been extended to floating production facilities, like
Shell/Prelude FPSO, which uses Nikkiso Cryo in-tank pumps derived from ones presented in
this article.