FLNG Rock and Roll

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:

[email protected]

+1-702-643-4900

Nikkiso Cryo, Inc.

4661 Eaker Street

North Las Vegas, NV, 89081

FLNG Rock and Roll


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

FLNG Rock and Roll


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.

FLNG Rock and Roll


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

FLNG Rock and Roll


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.

FLNG Rock and Roll


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.

FLNG Rock and Roll

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