(OK) Offshore LNG Production

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NORWEGIAN UNIVERSITY OG SCIENCE AND TECHONOLGY

DEPARTMENT OF PETROLEUM ENGINEERING AND APPLIED GEOPHYSICS

Offshore LNG,floating LNG facilities, LNG-

FPSO

⋅ Aida Kheradmand

⋅ EhsanMarashi

⋅ MasoudGhorbaniyan

Offshore LNG Production Natural gas

Aida Kheradmand Seid Ehsan Marashi Masoud Ghorbanian

NTNU Trondheim

November 2010

2010

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Abstract One-third of the gas reserves in the world are located in offshore fields, BP (2009). Development

of offshore natural gas fields have been delayed until recent years because of the lack of

appropriate technology and inadequate investments.

Considering uncertain gas prices and difficulties in estimation of project costs, enhance the use

of the commercial cases for floating LNG. Although, the cost of FPSO is massively greater than

land base LNG and also the technical part of FPSO is more difficult than onshore LNG plant.

But FPSO is essentially the only option to extract the natural recourses for many fields.

Inspecting the effect and limitation for acceptance in commercial of floating LNG production—

safety, overall cost, performance, availability and delivery schedule—have led to selection of the

nitrogen expander liquefaction process, Finn (2009).

This study discusses about the LNG chain process, and then FPSO is going to be introduced and

explained in detail. The equipments and comparison between the systems is evaluated. Safety as

one of the key parameters is also studied.

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List of Contents

1 Introduction ............................................................................................................................. 1

2 Natural gas reserves ................................................................................................................. 2

3 What is LNG? .......................................................................................................................... 4

3.1 LNG chain ........................................................................................................................ 5

4 Liquefaction processes for natural gas .................................................................................... 9

5 FPSO ...................................................................................................................................... 12

6 Floating LNG Liquefaction Process ...................................................................................... 12

6.1 Pre-treating of natural gas .............................................................................................. 13

6.2 Liquefaction process ...................................................................................................... 13

7 FPSO Offloading ................................................................................................................... 16

8 FPSO design .......................................................................................................................... 17

8.1 Suitable equipment for FPSO unit ................................................................................. 17

8.2 Layout............................................................................................................................. 18

9 Floating LNG potential .......................................................................................................... 19

10 Storage systems ..................................................................................................................... 20

11 Storage safety problems ......................................................................................................... 22

12 Discussion .............................................................................................................................. 24

13 Conclusions ........................................................................................................................... 24

14 References ............................................................................................................................. 25

List of Tables

Table 1: World’s largest gas fields in TCF, Ndrfo et al (2007) ..................................................... 3

List of figures Figure 1: Proved Natural gas reserves in TCF, BP (2009) ........................................................... 3Figure 2: natural gas consumption in the worl in Tonnes oil equivalentd, BP (2009) .................. 3Figure 3: Trade flows of Natural gas in the world, BP (2009) ...................................................... 5Figure 4: LNG value chain, Sempra energy company (2008) ........................................................ 6Figure 5: Liquefaction process description, Michelle (2007) ........................................................ 7

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Figure 6: simplified cascade process for production of LNG, Venkatarathnam (2008). ............ 10Figure 7: shows the mixed refrigerant process, Venkatarathnam (2008). ................................... 11Figure 8: expander cycle, Venkatarathnam (2008). ..................................................................... 11Figure 9: Schematic diagram of LNG offloading system, Yan and Gu (2010). ............................ 16Figure 10: Design of BHP, Dubar et al (2001) ............................................................................ 19Figure 11: Design of ABB, Yongluin and Gu (2008) .................................................................... 19Figure 12: LNG-FPSO conceptual design of ABB, Annon (2005) ............................................... 19Figure 13: Moss sphere containment, Green (2009) .................................................................... 21Figure 14: membrane containment, Green (2009) ....................................................................... 21Figure 15: SPB containment, Green (2009) ................................................................................. 22Figure 16: Sloshing of LNG, Green (2009) .................................................................................. 23

University of Science and Technology November2010

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1 Introduction

Demand for oil & gas will not abate in the near future. Peak oil is a fast-approaching reality. Oil

prices may be rise again and unstabilize the oil market. On the other hand, the demand of fossil

fuels are increasing exponentially nowadays. It will lead the countries and oil companies to be

eager to explore new reserves. Smaller more, difficult oil & gas fields, which were previously

uneconomic, are looking more attractive as the alternatives to produce fossil fuels. Offshore

Floating Liquefied Natural Gas Production is the key differential that may ensure some of these

fields can be developed.

Liquefied natural gas is one of the introduced method to transport natural gas in long distances.

There are so many projects and researches undergoing on LNG issues. The aim of these studies

is to find new efficient methods to produce and transport LNG. One of the conductive topics is

FPSO1

. Technical risk, equipment design and availability, topsides design, ease of

modularization, plant performance and operation, delivery schedule, and safety and

environmental impact has been evaluated for offshore areas in this process .These engineering

studies have further proved that this liquefaction technology is an outstanding candidate for

offshore LNG projects, Michelle (2007).

Critically, FPSO is still untested commercially anywhere in the world, the cost of FPSO is

massively greater than land based LNG units. In addition, technical challenges of FPSO are

difficult to overcome. But, FPSO is essentially the only option to extract the natural resources for

many fields. As the price for oil & gas grows the investment required for FPSO looks more

attractive. Mokhateb et al (2008)

This study has analyzed Floating LNG Production Storage and offloading units(FPSO). Its

potential is investigated to be one of the main methods to produce and transport LNG.

Equipment design and processes is explained in detail, while safety issues is discussed as one of

the critical topics in the FPSO units.

1 FPSO: Floating, production, storage and offloading


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2 Natural gas reserves In the 19th century, most of the produced natural gas have been gained as the by-product of oil

production and burnt it on the field. Due to the lack of technology, marginal gas fields or non-

associated gas field had not been developed yet at those days. While nowadays, natural gas is

commercially produced from oil and gas fields. The gas which is extracted directly from the gas

reservoirs is called non-associated gas. And the produced gas from the oil fields is called

associated gas. Most of the produced associated gas are injected back to the oil reservoirs to

maintain the pressure and implemented as one of the Enhanced oil recovery methods, BP (2009).

Huge and giant dry or non-associated gas fields had been discovered and most of them are now

producing. According to BP’s investigation statistics, which is presented in Figure 1.Most of the

proven natural gas fields are located in the Middle East, while Russia and European countries

gain the second stage (Figure 1) . US and Canada are producing large volumes of natural gas, too

,. Despite of the giant resources and production in these areas, they are also the main end users of

natural gas, according to BP and it is shows in Figure 2 ,BP (2009). The largest gas field is

located in Russia; other major proven reserves are in Iran, Qatar, Saudi Arabia and United Arabs

of Emirates .They are listed in Table 1 , Ndefo et al (2007).

Since natural gas is considered as a cleaner source of energy, it is likely that the market for

natural gas has a huge potential in the future. Natural gas exists in different size and shape which

often make it difficult to exploit all the gas. Making the natural gas feasible to transport, different

method have been introduced including: CNG, LPG, LNG, and NGL, Hyne (1991).

Although, new natural gas reserves are explored each year, but also the consumption rate of

natural is also increasing exponentially. This yields to work out to find new ways to produce

more gas easily and try to reduce the cost of production and transportation. This is the main key

of most of the natural gas researches. As a result there is an increasing activity in developing new

technologies that can get as much as possible out of the fields.


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Figure 1: Proved Natural gas reserves in TCF, BP (2009)

Figure 2: natural gas consumption in the worl in Tonnes oil equivalentd, BP (2009)

Table 1: World’s largest gas fields in TCF, Ndrfo et al (2007)


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3 What is LNG? Liquefied natural gas (LNG) is a result of super cooling of natural gas to -161C. This

temperature form the liquid phase of natural gas because of condensation. When it goes into the

liquid form, the volume reduces- approximately, the new generated liquid phase can take up to

600 times less space than in its gaseous state. The aim of this process is to make the natural gas

feasible to transport over long distances, where the pipeline networks does not exist there. It is

possible to be shipped to where it is in demand, La’o (2008).

Liquefied natural gas (LNG) was proven viable in 1917, when the first LNG plant went into

operation in West Virginia. The first commercial liquefaction plant was built in Cleveland, Ohio

in 1941. In January 1959, the world's first LNG tanker carried LNG cargo from Lake Charles,

Louisiana to Canvey Island, United Kingdom. This event demonstrated that large quantities of

LNG could be transported safely across the ocean at that time. The first liquefaction plant in the

world was commissioned at Arzew in Algeria to supply this contract with gas production coming

from huge gas reserves found in the Sahara. However, demand for LNG in Asia continued to rise

and Malaysia entered the LNG market in 1983, followed by Australia in 1989 later on. Qatar

became the second Middle Eastern LNG producer with the delivery of its first cargo of LNG

from the Qatargas LNG plant in January 1997. More recently several plants have come on line:

Trinidad; RasLaffan; Nigeria and Oman, Sempra energy company (No date).

Due to specific and unique characterization of LNG, it is using worldwide. Basically, LNG is

odorless, colorless, non-corrosive and nontoxic. Its density is near to gasoline and diesel fuels.

But it produces less pollution comparing to gasoline and diesel fuels. Liquefied Natural gas is an

energy source that has much lower air emissions those other fossil fuels, Michelle (2007).

It is shown in Figure 3 that the trades of the gas pipelines and the routes of the LNG carriers in

the world. Most of the natural gas pipelines are going through small distances on the land. For

instance, Russia‘s gas is exporting the gas to Europe by the pipelines. There are pipelines to

transport Norway’s gas down to the southern Europe. However, the gas exporting from the

Australia to Japan is transporting by the LNG ships. Also, Ships are carrying LNG from Middle

East to East Asia, and Europe. Europe is also sending natural gas in the LNG form to the Africa

and America , BP (2009).


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A majority of the world's LNG supply is exported from countries with large natural gas reserves

(15 countries contain a total of 22 natural gas liquefaction plants at the beginning of

2008)..These countries include Algeria, Australia, Indonesia, Malaysia, Nigeria, Qatar, Trinidad,

Brunei, Norway, UAE, Egypt, and Russia with Yemen opening its first operational LNG plant

during 2009. Other countries may produce natural gas for domestic use, like the US, but lack

adequate supply to export on a large scale. In situations in which domestic gas supply is

Inadequate to meet intra-country demand, LNG is imported, BP (2009).

Figure 3: Trade flows of Natural gas in the world, BP (2009)

3.1 LNG chain

The liquefied natural gas (LNG) value chain needs investment from start to finish. It begins with

natural gas extracted from underground reservoirs and is sent through a pipeline to a liquefaction

facility. The stages include: gas production, liquefaction plant, shipping, regasification terminal,

pipeline delivery. It is demonstrated in Figure 4. Sempra energy company (no date).


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Figure 4: LNG value chain, Sempra energy company (No date)

Once a potential natural gas field has been located by a team of exploration geologists and

geophysicists, a team of specialists drill down to where the natural gas is thought to be existed.

After a well has been drilled and the presence of commercially-viable quantities of gas has been

verified, then the gas is extracting out of the ground regarding to reservoir and production

engineering concepts. These concepts are related to the maximum efficiency of the tools and

methods which are involving the procedure of extracting and producing natural gas, Finn et al

(2010)

At this point, the natural gas which is extracted from the ground is called “feed” gas. The feed

gas is entering the platform to be treated. At the liquefaction facility, impurities are removed

from the gas, the impurities include:

• Sulfur, carbon dioxide and mercury which are corrosive to LNG equipment

• Water, which could freeze and make the hydrates and cause equipment blockage

• Heavier hydrocarbons which could also freeze like water and abundant the pipeline

As it is shown in Figure 5, depending on the natural gas origin it may also be required to remove

acid gases, mercury and sulfur. For example, most of Middle East extracted natural gas contains

high amount of sulfur. Consequently, the natural gas needs to be treated to remove sulfur during

the liquefaction process. After impurities removal process, it is sent through three cooling

processes. The main component of natural gas is methane, so the methane’s cooling temperature

is required, minus 161ºC, to produce and keep natural gas in a liquid state at standard condition,

Michelle (2007).


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Figure 5: Liquefaction process description, Michelle (2007)

The liquefied natural gas is prepared through the liquefaction process. Then it is loaded onto

specially designed tanker ships where it will be kept chilled for the whole duration of the voyage,

which may last anywhere from four to thirty days, depending on the destination port. In order for

LNG to remain a liquid, its extremely cold temperature must be maintained. The temperature is

maintained by heavily insulating the tanks to keep heat out and by removing the vapor that forms

in the tank. LNG's low temperature requires that it should be stored in specially designed tanks

that can withstand extreme coldness, Michelle (2007)

After crossing the routes, the ships reached the terminals on the land. LNG import terminals are

the link between the world’s natural gas reserves and the growing need for economical and

environmentally friendly energy. In regasification terminal, the ultimate destination of LNG

carriers, the liquefied natural gas is returned to its initial, gaseous state, and then fed into

transmission and distribution networks. Once the ship arrives at a regasification terminal, the

LNG is offloaded into large storage tanks. These tanks built with full-containment walls and

systems to keep the LNG cold until it is turned back into a gaseous state, Finn et al (2010).


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When the LNG has been warmed back to its natural state, the gas is moved into pipelines which

will deliver the natural gas to consumers, power plants and industrial customers across the

country.


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4 Liquefaction processes for natural gas The aim of this part is to bring out the summary of processes which are used for liquefaction of

natural gas. LNG production processes can be broadly classified into three groups: Cascade

liquefaction process, mixed-refrigerant process, and turbine-based process. It is represented in

Figure 6 the schematic of a simplified cascade refrigeration cycle process for production of LNG.

This process was used in some of base load and peak shaving plants. In the classical cascade

process, the natural gas feed is cooled and liquefied and sub cooled using three different pure

refrigerants: propane, ethylene, and methane which are evaporated at three or four pressures to

provide refrigeration at nine or ten temperature levels. For improving the efficiency of the

process, mixed refrigerant is used instead of pure refrigerant since the temperature approach in

length of heat exchanger is closer between natural gas feed and refrigerants. As it is clear in the

figure 6, a large number of heat exchanger is necessary for this process, Prible et al (2005).

The mixed refrigerant liquefaction process is illustrated in Figure 7. ‘’The composition of the

refrigerant used in the process is a strong function of the feed composition, feed pressure,

ambient temperature, and operating pressures used ‘’, Venkatarathnam (2008). Since the

approach temperature of hot stream and cold stream is small in the length of heat exchanger, the

efficiency of this process is higher than others.

The expander cycle is widely use in peak shaving units and proposed for offshore liquefaction

plant and FPSO. The simplest expander cycle is demonstrated in Figure 8. The cold low pressure

refrigerant cooled down the natural gas feed and high pressure refrigerant. After warming up the

low pressure refrigerant in LNG heat exchanger, it will be compressed to more than 100 bar and

pre cooled in the heat exchanger. The high pressure stream is cooled more in LNG heat

exchanger and after that it will be expanded in turbine to about 6 bars. Nitrogen and mixture of

methane and nitrogen is used as refrigerant. ‘’A small temperature approach all along the length

of the heat exchanger can never be achieved when a single-component refrigerant is used for pre

cooling, condensation and subcooling of the natural gas feed since the specific heat is not the

same in all three regions’’, Dubar et al (2001). So, The efficiency of this process is less than the

mixed refrigerant process, as the approach temperature between cold and hot stream in heat

exchanger with single component as refrigerant is large.


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Figure 6: simplified cascade process for production of LNG, Venkatarathnam (2008).


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Figure 7: shows the mixed refrigerant process, Venkatarathnam (2008).

Figure 8: expander cycle, Venkatarathnam (2008).


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5 FPSO2

The floating production, storage, and offloading system for liquefied natural gas (FPSO), is a

new conceptual unit nowadays that so many research studies are undergoing on it. This floating

plant can produce, store, and transfer LNG through offloading facilities to the LNG carriers to

the world market.

FPSO is movable and can be reused in other offshore fields. FPSO is an effective and realistic

way for exploitation, recovery, storage, transportation, and end-use applications of marginal gas

fields and offshore associated-gas resources. Nowadays, this system is cost competitive with the

onshore LNG production because of rising costs for onshore LNG facilities. Lots of creative

studies have been completed all around the world, but all of the studies are conceptual and have

not been built yet. Nevertheless, some breakthroughs in many key technologies must be

produced before building a real FPSO unit. FPSO has many unique characteristics that some of

them are the most rigorous factors to be taken into account during design. These factors include

restricted space, platform motion, LNG sloshing in inner storage tank and offloading system,

Yonglin and Gu (2008).

These factors have not been considered in the onshore projects, previously. For instance, the

variable wind and wave behavior will induce the FPSO to sway periodically. In addition, LNG in

the tank sloshes which will cause serious safety problems in LNG storage and offloading system.

The effects of the motion and LNG sloshing on storage and offloading of cryogenic liquid from

FPSO to the LNG carriers are very complicated. Therefore, detailed thermodynamic and

hydrodynamic analyses need to be conducted to gain quantitative information for different

situations. In addition, the restricted space on the platform of FPSO is critical, requiring all the

equipment to be compact and simple.

6 Floating LNG Liquefaction Process For many years, LNG plant licensors and engineering firms tried applying onshore technology

and plant design concepts to prospective offshore projects—with little success. Offshore

processing presents different engineering, project management and installation challenges

2 FPSO: Floating, production, storage and offloading system


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compared to an onshore plant. These issues must be addressed to determine the optimal process

technology and plant design.

For offshore LNG to be commercialized, it is essential to gain the confidence of potential

investors. Onshore LNG production is mature with well-established design concepts, engineering

procedures and hazard mitigation practices. This experience is important for FLNG production

but must be aligned with the unique requirements of an FPSO. Fundamental to ensuring the

viability and acceptance of LNG FPSOs is selecting the best process technology, Finn (2009).

6.1 Pre-treating of natural gas The pre-treating of natural gas includes de-acidification, dehydration, and de-mercury. Based on

the composition of the gas, some or all of the preprocess sections will be built. The technologies

of de-acidification and dehydration are quite developed on land, but they will not work as

predicted, in the sea.

The main methods of de-acidification and dehydration are absorption and adsorption. In the

absorption process, the impurities are removed from raw gas when it contacts with the absorbent

in the tower. The liquid absorbent will flow asymmetrically with the motion of the platform.

Thus, the absorption is poor. For solving this problem, more absorbent or bigger towers are

needed. Due to flexibility and simplicity, adsorption is considered to be more appropriate for

FPSO. It requires little space and it starts up and stops quickly, Yonglin and Gu (2008).

6.2 Liquefaction process Liquefaction process is the key section on FPSO. A floating liquefaction process should be

simple and compact. It should be efficient, reliable, safe, insensitive to the motion of FPSO, and

adaptable to natural gas with different components since FPSOs will process gas from different

fields. Apparently, the process should also be low-cost and easy to maintain, Phalen (2008).

The Cascade cycle is not the option onboard because it has many types of equipment. Some

experts and designers selected mixed refrigerant process for floating platform, while some

recommended expander cycle. Q.Y.Li and Y.L.Ju compared propane pre-cooled mixed

refrigerant cycle (C3/MRC), mixed refrigerant cycle (MRC) and nitrogen expander cycle (N2

expander), Li and Ju (2002). This study shows that the power consumption of the N2 expander is


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the highest, which is higher than that of C3/MRC and MRC by 68% and 52%, respectively. The

load of water-cooling of the N2 expander is also the largest, Li and Ju (2002).

The economic performance has been compared and analyzed for these three liquefaction

processes. The results show that the capital costs of the equipments in N2 expander are lower

than that of C3/MRC and MRC, while the operating costs of the N2 expander are the highest. By

assuming the same equipment lifetimes for the three kinds of liquefaction processes, it can be

drawn a conclusion that the economic performance of N2 expander is poorer than that of the

other two liquefaction processes. Furthermore, they designed layout for these three processes and

concluded that the plot area for N2 expander is much smaller than the other two processes.

Therefore, the N2 expander is the most simplest and compact process.

FPSO is often operated under irregular motion due to high intensity waves. Therefore, the

employed liquefaction process should be insensitive to the motion of the floating platform. In the

N2 expander process, the refrigerant is single component nitrogen which is always in the gaseous

phase, the impact of FPSO motion on the thermal performance of the heat exchangers is

negligible. In another word, FPSO motion has almost no effect on the performance of N2

expander process. However, the platform motion will cause the fluid of liquid phase or gas/liquid

two-phase distribution in equipments and pipelines, which will impact the performance of heat

transfer. The study results revealed that the refrigerants are in two phase in some stages of

C3/MRC and MRC. Therefore, FPSO motion impacts the performance of the whole liquefaction

processes of C3/MRC and MRC. Obviously, N2 expander is less sensitive to the FPSO motion

than the other two liquefaction processes, Phalen (2008).

Furthermore, with the purpose of minimizing the energy consumption, no changes need to be

done in N2 expander except regulating the parameters of the process due to its single component

refrigerant nitrogen. However, the proportion of the components for the mixed refrigerants in the

processes of C3/MRC and MRC must be matched repeatedly in order to gain the minimum

energy consumption and this is a heavy work. It can be seen that N2 expander has better

suitability for different gas resources than the other two processes, Mokhateb (2008).

The refrigerants of N2 expander are nitrogen while that of C3/MRC and MRC are the mixtures of

most hydrocarbons and a small amount of nitrogen. It is well known that nitrogen is non-


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flammable gas but hydrocarbon is flammable and is possible to explode in air. The nitrogen

refrigerant in N2 expander avoids the requirement of storing hydrocarbon refrigerant. The

hydrocarbons have the possibility to explode in air once leaking from the storage vessels and this

increases the potential safety hazards of the processes. It is evident that the safety of N2 expander

is higher than that of C3/MRC and MRC, Li and Ju (2002).

The refrigerant of N2 expander process is the single gaseous nitrogen and no refrigerant match

problem. The operation and maintenance of this process is quite simple. The mixed refrigerants

in C3/MRC and MRC processes must be matched exactly in the process of operation, while the

proportion of each component contained in the refrigerants is difficult to control and thus leading

the two processes more difficult to operate and maintain than N2 expander. In addition, N2

expander process has quicker start-up/ stop than the other two liquefaction processes. Therefore,

N2 expander process has higher operability than that of C3/MRC and MRC, Li and Ju (2002).

In general, high efficiency is the main advantage of the mixed refrigerant. But it has some

disadvantages. Its refrigerant is flammable and dangerous. It works at two-phase flow and needs

a place for storage and requires safe management. In addition, it takes long time to stabilize after

start up. Furthermore, the refrigerant should be mixed proportionally before operation.

In conclusion, expander technology was proposed for offshore LNG due to:

• Insensitivity to vessel motion since the refrigerant is in the gas phase.

• Inherent safety by avoiding liquid hydrocarbon refrigerants (and their storage), and the

potential for fire and explosion hazards

• Fast start-up and shut-down in a safe and controlled manner

• Flexibility to changes in feed gas conditions and ease of operation due to process simplicity

• A small number of equipments that has relatively low topsides weight.

Subsequent engineering studies demonstrated three further significant advantages for expander

technology:

• By conventional well-proven cryogenic equipment, competition among equipment suppliers

will be maximized and plant cost and project schedule will become minimum.

• Ease of modularization and construction due to process simplicity and low equipment count


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• Turbo-expanders are very reliable with minimal maintenance requirements, Yonglin and Gu

(2008).

7 FPSO Offloading

The offloading of LNG from FPSO to LNG carrier is quite different from the conventional

onshore LNG offloading or offshore oil offloading. It has unique features that the offloading are

carried out at unstable environment. In addition, the transported fluid is cryogenic liquid which is

sensitive to changes of parameters. The offloading system of LNG from FPSO to LNG carrier is

one of the most important parts in FPSO. Commonly the offloading system is mounted in the

stern or middle of FPSO, which consists of the supporting structure, joints and pipeline. The

conceptual designs of offloading system can be divided into two categories by the mooring way

between FPSO and LNG carrier: tandem and parallel. The tandem way can withstand severe sea

state, while the parallel way is fit to the mild environment. The schematic diagram of the

offloading system model is presented in Figure 9. The system consists of tanks on FPSO and

LNG carrier respectively, pipeline, and pump. The pipes include several pipe segments in

different angle and length, in order to approximating the pipeline between FPSO and LNG

carrier. The LNG production flows into the tank on FPSO and then is sucked into the submerged

pump in the tank at the subcooled state. Through the rigid insulated pipe on the FPSO, the

flexible pipeline suspended between the LNG-FPSO and LNG carrier, and rigid pipe on the LNG

carrier. LNG is transported into the tank on the LNG carrier by the pump, Yan and Gu (2010).

Figure 9: Schematic diagram of LNG offloading system, Yan and Gu (2010).


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8 FPSO design On any FPSO, space is restricted since process facilities must be located away from the flare,

heli deck and buildings. An integrated approach between topsides designer and vessel designer

helps establish appropriate and optimal plant design and layout strategies.

An important concern with an FLNG is vessel response to wave motions and the plant and

equipment design requirements to mitigate motion effects. There needs to be an understanding

how the vessel movement influences the effective equipment weight and vessel deck flexing.

Experience in designing cryogenic plants provides capability in pipe work stress analysis and

allowance for pipe work contraction. Piping design for the liquefaction section (and including

hull flexing) is an important activity in generating an optimized plant layout, Miyake (1998).

Process equipment influenced by vessel movement due to wave motion should be located on the

vessel centerline. All separators and columns on vapor/liquid service are potentially a concern.

The most significant are the acid gas removal unit (AGRU) contactor and the amine regeneration

column. as maloperation can lead to CO2 freezing in the liquefaction section.

Satisfactory performance, to maintain the treated gas CO2 level to 50 ppm, Finn (2009), requires

multiple beds of structured packing. And regular liquid redistribution to keep the down flowing

liquid from tending to the column wall. If, during operation, the treated gas CO2 content is

excessive, the molecular sieve dehydration system may be overloaded if this was not considered

in the design. The sensitivity dehydration system sensitivity should be evaluated for high CO2 to

ensure a robust and optimal design, Finn (2010).

8.1 Suitable equipment for FPSO unit

Aero-derivative gas turbines for running compressor have long been proposed for offshore LNG

and have a number of important advantages over their industrial counterparts that include:

• Smaller footprint and much lower weight—around half of an industrial unit with comparable

power output. These factors are especially important offshore.

• High availability and reliability (with a lower duration for planned maintenance and less than

0.5% unscheduled downtime). Engine sections are modular and light and can be replaced in less

than 24 hours without specialist technical support


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• Higher thermal efficiency – over 40% compared to 30% for an industrial unit saving on fuel

and reducing carbon emissions. However, aero-derivative gas turbines have not been used often,

even onshore, Yonglin and Gu ( 2008).

Aluminum plate-fin heat exchangers, conventional in cryogenic natural gas processing onshore,

are ideal for floating liquefaction by virtue of being light, compact and highly efficient for multi

stream duties. Air-cooling would require a prohibitive amount of deck space and cannot be

justified. Seawater cooling is conventional in offshore hydrocarbon processing but the cooling

duty on an LNG FPSO is much greater compared to oil processing and associated gas

compression, Hoff et al (1998).

8.2 Layout There is a paradox between the limited area on LNG-FPSO and safety. Firstly, the feasible

layout should be set to optimize the space on the platform. In the1990s, a small scale FPSO with

Moss tank was developed by National Oil Corporation in Japan, Miyake et al (1998). The ship is

divided into several sections based on distances to the flare tower.

The overall layout of FPSO should satisfy the requirements of compactness, safety, the motion of

the boat, the effect of radiation from flare tower and LNG sloshing in the tank. To stabilize the

boat and minimize the effect of the boat’s motion, it is better to arrange the facilities

symmetrically or in array. A better choice to get the precise dimension and compact structure is

to modularize the equipment onboard. To minimize the risks, the release sources must be far

away from the fire, the safe distances between the equipment should be guaranteed, heavier

equipments should be mounted as low as possible and the higher ones should be mounted along

the center line.

BHP Petroleum, Dubar et al (2001), designed a fixed exploitation platform to preprocess the raw

gas, liquefy, store ,and offload LNG. ABB Company in U. S. designed a FPSO by combining

two ships without shipping ability, Annon (2005). As depicted in Figure 10 , one is responsible

for all the gas state, and the other is for LNG.

ABB proposed a new floating LNG and FLPG-FPSO in 2005, Annon (2005), which is illustrated

in Figure 10 . The offloading arm is located at the bow, followed by the flare tower, the


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preprocess module, the LNG liquefaction module, the LPG extraction module, the

accommodation area, the utilities module and turret mooring orderly, Annon (2005).

Figure 10: Design of BHP, Dubar et al (2001)

Figure 11: Design of ABB, Yongluin and Gu (2008)

Figure 12: LNG-FPSO conceptual design of ABB, Annon (2005)

9 Floating LNG potential The difficulties with onshore LNG projects have increased interest in offshore LNG production.

Studies over the last 30 years have identified the main technology developments necessary to

make offshore LNG production feasible.


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Advances in offshore LNG transfer and storage have been essential to the viability of offshore

LNG, as well as process technology and plant design issues. Developments in LNG transfer at

sea have advanced to where several suppliers have commercial systems available. A decade ago,

only one LNG storage system was proven for partially full operation at sea. Today, several LNG

storage systems are certified and LNG shipbuilders can provide approved designs, Pribe et al

(2005).

Floating production, storage and offloading (FPSO) is conventional for development of

“stranded” oil reserves, with well over 100 FPSOs now in operation. Several vessel lease and

LNG shipping companies have the capability and know-how to consider FLNG FPSO projects.

Engineering firms have also developed the skills to see offshore projects to completion and

successful operation, Finn (2009).

10 Storage systems

The main function of the storage tanks on LNG-FPSO is storing LNG temporarily before LNG

carriers arrive. The capacity of the tanks depends on several factors such as the yield of gas field,

the period and capacity of LNG carriers, and the space available on LNG-FPSO, Yonglin and Gu

(2008).

The types of containment are the same as those LNG carriers: membrane containment (GTT),

Moss sphere containment and SPB containment. The Moss sphere containment is, in essence, a

spherical, Aluminum tank supported within the vessel structure but isolated from it. The

aluminum tank is insulated and then protected from the weather by a steel cover, Green (2009).

There are three different membrane systems in use, all designed and licensed by GTT. All of

them follow the same basic principle in that is provide two liquid tight layers, membranes, and

two layers of insulation, which line an LNG carriers hold spaces.SPB containment is a separate,

insulated aluminum tank within a ships hold supported on blocks top, bottom and side to prevent

movement of the tank, Green (2009).

The first two types (GTT&Moss sphere containment) are ordinarily used on the LNG carrier.

Moss sphere containment can be manufactured alone, and then fabricated on the FPSO. The


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period of shipbuilding will be shorter and the structure weakens the liquid sloshing and

withstands a given pressure, but the special shape restricts the upper deck space. This is not

allowed on FPSO due to the high number of facilities on the deck, Yonglin and Gu (2008).

As mentioned before, membrane containment (GTT) and SPB containment can offer wide upper

deck areas. The SPB containment is subdivided into four spaces by a centerline liquid tight

bulkhead and a swash bulkhead. So, the natural frequency of the cargo is far from that of the

ship’s motion, and as a result, the possibilities of resonance of the liquid cargo and ship motions

are eliminated. But the compartments of SPB containment is smaller than those of membrane

containment, thereby, more cryogenic pumps are needed to deliver LNG, Zhao (2004).

Figure 13: Moss sphere containment, Green (2009)

Figure 14: membrane containment, Green (2009)


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Figure 15: SPB containment, Green (2009)

11 Storage safety problems

One of the important factors for offshore liquefaction process is safety. Several safety problems

should be noted. The first one is that the induced sloshing3

MARINTEK of SINTEF conducted experiments in which a membrane tank swayed in three

dimensions, and the sloshing effect was observed by a high velocity camera and lots of pressure

of LNG which caused strike the tank

wall in a frequency and its impact will be great and it resonates with the ship motion. The loads

will move to the hull, and will do harm to the boat structure. In addition, the insulation stuff will

sink with the impacts on the tank and it causes the insulation to become poor on top. The

sloshing free surface of LNG makes the filling measuring ratio difficult. So it is of important to

minimize the sloshing effect by doing effective measures like the buffer room in SPB

containment , Zhao (2004).

Moreover, the loads induced by sloshing are highly nonlinear. Hence it is hard to predict the

irregular motion of the tank and inner cargo and the induced sloshing load exactly by the existing

fluid dynamic CFD software. The reason for this is that the sloshing turns up frequently in time

and space, while CFD resolves the problem by discretization in the time and space. Experiments

are, therefore, the best way to confirm safety, Wesley (2005).

3Sloshing is liquid movement within the cargo tanks caused by vessel motions .Liquid movement takes the form of

“waves” within the cargo tank (Figure 15).


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sensors in the ceiling and corners of the tank. Based on the sloshing loads, their LNG tank was

improved, Zhao (2004).

The treatment of the boiled off gas (BOG) in the LNG tank is the second safety problem. By

vaporizing the LNG, the pressure increases. Generally, there are two ways to manage BOG. The

first one is to re-liquefy BOG, and then send it back to the tank and the second way is using the

BOG as fuel in the power system, Wesley (2005).

The protection of the equipment such as the valves and the manometers on the tank from

moisture, salinity erosion and being frozen by cryogenic leak fluid is the last safety problem,

Yonglin and Gu (2008).

Figure 16: Sloshing of LNG, Green (2009)


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12 Discussion

Since performance of equipments like heat exchangers and adsorption columns are affected by

wave motions, every single part of LNG-FPSO should be carefully designed and tested before

being used onboard. Furthermore theoretical and economical analysis, experiments and

computation simulations should be conducted to gain complete information and forecast the

equipments of LNG-FPSO.

In comparison with existing LNG plants that use turbo-expanders, FLNG plants will be much

larger and this introduces remarkable new technical, engineering and safety considerations. The

refrigerant compression system configurations and the associated compressor drivers is

particularly a key area. The need for marinization and topsides interfacing with the hull are also

novel aspects of liquefaction plant design.

The offloading of LNG from FPSO to LNG carrier is quite different from the conventional

onshore LNG offloading or offshore oil offloading. It needs new design which is suitable to new

condition. Sloshing of LNG should be minimized since it resonates with the ship motion. To

prevent sloshing, anti slosh apparatus should be designed.

13 Conclusions • LNG-FPSO is an effective solution to exploit and utilize offshore natural gas while there

are many natural gas resources at sea.

• Expander cycles are better suited to offshore liquefaction on FPSOs than traditional

liquid refrigerant processes like cascade or mixed refrigerant process.

• Aluminum plate-fin heat exchangers are ideal for floating liquefaction heat exchangers.

• Aero-derivative gas turbines are the best option for running compressor.

• Adsorption is considered to be more appropriate for FPSO for de-acidification and

dehydration.

• The use of membrane containment concept has been increasingly selected for recent

project.


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14 References [1] Anon .N., “ LNG FPSO concept announced” , Naval Architect, 15(1): 20–27, 2005

[2] BRENDENG E., HETLAND J., “STATE OF THE ART IN LIQUEFACTIONTECHNOLOGIES FOR NATURAL

GAS”, SINTEF Energy Research, Dept. of Energy Processes, 2004

[3] Donald P., Huebel R., “LNG floating production, storage and offloading scheme”, US6889522, US patent, 2005

[4] Dubar C A T, LehMT O., “Liquefaction apparatus”. US patent 6250244, 2001

[5] Finn.A.J., “Are floating LNG facilities viable options? , Here’s how to evaluate technological and commercial issues

of these units”, Costain Oil, Gas & Process Ltd., Manchester, United Kingdom, 2009

[6]Finn.A.M., H.J.I., ”The LNG process decision and control of the LNG process”., Journal of Natural gas science and

engineering., Volume 2, Issue 4, September 2010

[7] Fjeld, P. E., “Analyzing the market potential for FLNG and adopting a flexible and creative approach,” IBC FLNG

2008 Conference, London, February 20–21, 2008.

[8] Geoff Green., “RISKS ASSOCIATED WITH THE MARINE ASPECTS OF LNG FPSOs (FLOATING PRODUCTION,

STORAGE AND OFFLOAD) UNITS “, Wavespec Limited, February 2009

[9] G.YAN. Y.GU.,’Effect of performance of LNG-FPSO offloading system in offshore associated gas fields”., Applied

energy., 87 (2010)

[10]Hoff G C, Yost K, Naklie M, et al. , ”Mobil’s floating LNG plant”., International offshore and polar engineering

conference. Montreal, Canada, 1998

[11]Hyne, Norman J., “Dictionary of petroleum exploration, drilling & production”, PennWell Books.

ISBN 0878143521., 1991

[12] Kennett, A. J., D. I. Limb and B. A. Czarnecki, “Offshore Liquefaction of Associated Gas—A Suitable Process for

the North Sea,” 13th Annual Offshore Technology Conference, Houston, 1981.

[13] La’o Hamutuk., “Sunrise LNG in Timor-Leste: Dreams, Realities and Challenges”., Timor-Leste Institute for

Reconstruction Monitoring and Analysis.,2008

[14] Li Q.Y., Ju Y.L., “Design and analysis of liquefaction process for offshore associated gas resources”,. Shanghai Jiao

Tong University, Institute of Refrigeration and Cryogenics, Shanghai , China., 2002

[15]Michelle M. Foss., “An overview on liquefied natural gas, its properties, organization of LNG in industry”., School

of geosciences, university of Austin, Texas, Energy economic research, 2007

[16] Miyake H, Kishimoto N, Kakutani Y., “Small scales LNG FPSO for marginal gas fields”. LNG-12. Perth, Australia,

1998

[17] Mokhateb, S., A. J. Finn and K. Shah, “Offshore LNG industry developments,” Petroleum Technology Quarterly, p.

105, Q4 2008.

[18] Ndefo E.O., Lasker G.s., Tawofaing L., “Acritical evaluation of Russia natural gas resources”,

http://www.energytribune.com/articles.cfm?aid=379, 2007

[19] Phalen, T., “Will LNG liquefaction project development prosper?” HydrocarbonProcessing, Vol. 87, No. 7, p. 15,

2008.

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[20] Prible D, Huebel R R, Foglietta J H. , “LNG floating production, storage, and offloading scheme”. US Patent

6889522, 2005

[21]Venkatarathnam G., ‘’CRYOGENIC MIXED REFRIGERANT PROCESSES’’, Springer. 2008

[22] Yonglin JU, Yan GU., “LNG-FPSO: Offshore LNG solution”., Higher Education Press and Springer-Verlag 2008,

Energy Power Eng. China 2008,

[23] Wesley R. Qualls. D. E.,,” BENEFITS OF INTEGRATING NGL EXTRACTION AND LNG LIQUEFACTION

TECHNOLOGY”., ConocoPhilips Company., 2005

[24] Zhao R, Rognebakke., “ The safe and efficient LNG sea transport”., Marintek Review,14(4):2–4., 2004

[25] http://www.bp.com/productlanding.do?categoryId=6929&contentId=7044622 , BP company

[26]http://www.bp.com/sectiongenericarticle.do?categoryId=9023783&contentId=7044475, BP company

[27]http://www.sempralng.com/pages/About/History.htm, history of LNG, Sempra LNG, LNG- the North American

Solution

[28]http://www.sempralng.com/pages/About/ValueChain.htm, LNG value chain, Sempra LNG, LNG- the north American

solution

[29]http://www.lngplants.com/KryProc.htm, LNG process description

http://www.bp.com/productlanding.do?categoryId=6929&contentId=7044622�

http://www.bp.com/sectiongenericarticle.do?categoryId=9023783&contentId=7044475�

http://www.sempralng.com/pages/About/History.htm�

http://www.sempralng.com/pages/About/ValueChain.htm�

http://www.lngplants.com/KryProc.htm�

(OK) Offshore LNG Production

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