OTC 17781
Commercialization of Stranded Gas With a Combined Oil and GTL FPSO
A. van Loenhout, L. Zeelenberg, and A. Gerritse, Bluewater Energy Services B.V., and G. Roth, E. Sheehan, and
N. Jannasch, Syntroleum Corp.
Copyright 2006, Offshore Technology Conference
This paper was prepared for presentation at the 2006 Offshore Technology Conference held in
Houston, Texas, U.S.A., 1–4 May 2006.
This paper was selected for presentation by an OTC Program Committee following review of
information contained in an abstract submitted by the author(s). Contents of the paper, as
presented, have not been reviewed by the Offshore Technology Conference and are subject to
correction by the author(s). The material, as presented, does not necessarily reflect any
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Abstract
Oil prices have been driven to very high levels by ever
increasing global energy demands. Gas-to-liquids (GTL)
technology, which converts natural gas into liquid products, is
set to play a major role in meeting these demands. GTL
technology has already been utilized at several onshore
locations and it won’t be long before this important
technology is applied offshore. It is particularly applicable for
realizing the potential of stranded gas reserves, where physical
or economic factors have hindered development.
The lack of infrastructure normally prevents development of
stranded gas offshore. Offshore GTL conversion offers a
solution, as it enables produced hydrocarbons to be
transported using existing infrastructures, such as trading
tankers and tanker terminals worldwide.
As most stranded fields are located in deep water, a floating
GTL production platform with storage capacity is required.
From this specification, a concept evolved that merged GTL
technology with a Floating Production, Storage and
Offloading Vessel (FPSO) capable of oil and condensate
production. This coupling of conventional and GTL
technology provides a solution to monetize a gas source that
might have been flared or reinjected otherwise. Since most
utility systems are common to oil producing FPSOs and
offshore GTL plants, the shared infrastructure and additional
revenues from oil production could greatly improve potential
project economics. A feasibility study conducted by Bluewater
Energy Services B.V. and Syntroleum i, supported by Amec
Oil & Gas, has determined the viability of combining these
two technologies.
The synergy between FPSO and GTL utilities has been given
much attention and resulted in a steam-dominated plant with
maximized heating and cooling integration. Virtually all GTL
by-products – heat, steam, low-BTU gas and process water –
are used by the FPSO systems, requiring only minor additional
fuel gas consumption.
Figure 1 – GTL FPSO
1
Introduction
GTL technology development has reached a stage where its
marinisation may be considered for FPSO application. The
Syntroleum GTL technology is the most suitable for an FPSO.
This is because the majority of GTL processes use pure
oxygen for the production of syngas, requiring a costly oxygen
plant. However, Syntroleum proprietary GTL technology uses
air, eliminating this need.
Feed gas for the GTL process will be produced from oil and
gas wells connected to the FPSO, rather than imported.
FPSOs typically receive only oil well fluids, so associated gas
is either reinjected or exported. Therefore, to maximize
production levels, the GTL FPSO will be equipped with an oil
separation train and a gas/condensate separation train.
Integrating a GTL plant with both oil and gas production
facilities is a further design challenge.
Bluewater Energy Services BV, based in Hoofddorp, in The
Netherlands, together with Syntroleum Corporation, based in
Tulsa, Oklahoma, USA and supported by Amec Oil & Gas, in
London, UK, have conducted an economic and technical
feasibility study of an FPSO equipped with a conventional oilgas-water separation plant and a gas-to-liquid (GTL) plant.
Considerations were given to the feasibility of installing and
supporting the large components needed for a GTL plant
onboard an FPSO, utilizing the synergy between oil
production and GTL plant utilities and overall safety and
operability. Well exploration data from the Aje field, located
offshore Nigeria, was used as the basis of this feasibility study.
2
OTC 17781
In this Offshore Technology Conference (OTC) paper, the
optimized integration of a GTL plant and floating oil
production facility is detailed. It takes into account the utilities
synergy, as well as new issues such as catalyst handling and
multiple product storage and offloading.
2
Potential Field Locations
The GTL FPSO is ideal for the development of stranded
offshore gas reserves between 0.5 and 3 Tcf and which are
remote from gas exporting infrastructure via subsea pipelines.
The GTL FPSO’s viability is enhanced if small oil fields are
nearby, as this generates additional income. Likely locations
have been identified as deep water (500 – 3000 m) gas and oil
fields in West Africa. This concept, however, does not exclude
other remote locations.
3
Design Basis
The well exploration data for the Aje field, located offshore
Nigeria, has been used to project for a typical deep-water West
African oil and gas/condensate field. This has set the
following design parameters:
Oil Production
Oil
Associated gas
Produced water
Total liquids
Water Injection Requirement
Produced water + GTL process water
Gas/Condensate Production
Gas
Condensate
Gas-to-Liquids Products
Light Fischer-Tropsch liquids (LFTL)
Heavy Fischer-Tropsch liquids (wax)
GTL process water
Riser Data
Oil production
Oil test / hot oil circulation
Water injection
Gas production
Gas test
Umbilicals (chem. inj. / hydr. power)
40,000
18
35,000 (max.)
60,000
bopd
MMscfd
bwpd
bfpd
60,000
bwpd
160
10,000
MMscfd
bcpd
11,000
5,300
35,000
bpd
bpd
bpd
Number
2
2
2
1
1
3
Size
6 inch
6 inch
8 inch
12 inch
5 inch
Ambient Air Temperature
Minimum
Median
Maximum
Temperature [°C]
16
26
34
Average Water Temperature
At surface
At water intake
– 500 m depth
At seabed
– 900 m depth
Temperature [°C]
29
7
5
Environmental Data
Water depth
Operational conditions (1-y return)
Significant swell height
Associated wind wave height
Associated wind
Current
Survival conditions (100-y return)
Significant swell height
Associated wind wave height
Associated wind
Current
4
900 m
2.6 m
1.2 m
5.6 m/s
0.85 m/s
3.6 m
1.2 m
5.6 m/s
1.35 m/s
Combining Oil Production with GTL Production
The GTL FPSO concept incorporates a gas/condensate
separation train to produce the feed gas required for the GTL
plant. The interaction between the gas/condensate separation
train and the GTL plant is critical when considering the
design. Gas production is restricted to the amount that the
GTL plant and utility users can process. The GTL plant is
designed to allow adequate turndown and feed gas flexibility,
according to upstream production fluctuations. For the
production of 1,000 MMscfd syngas, which forms the design
basis of the GTL plant (see section 5.1), some 160 MMscfd
feed gas is required. This, in turn, results in some 16,300 bpd
of GTL hydrocarbons. Hydrocarbon condensate is produced as
a by-product from the gas-condensate wells. Based on a
typical West-Africa field gas-oil ratio (GOR), the associated
condensate production level will be 10,000 bpd. This means
the GTL FPSO has a total daily production of some 26,300
barrels of hydrocarbon liquids from the gas/condensate wells.
Increasing hydrocarbon liquids production enhances
economics, so, naturally, tying in oil wells to the FPSO is the
next logical step. While oil production levels are relatively
high, the additional costs and footprint required for an oil
separation train and associated utility systems are small
compared to the rest of the gas/condensate separation and
GTL systems. Associated gas from the oil wells can be used as
fuel gas for supplementary power generation or be fed to the
GTL train. For the design case study, based on an oil field in
West Africa, the oil production level would be around 40,000
bpd. Adding on such relatively low capital investment would
boost the GTL FPSO’s profitability, bringing the total
hydrocarbon liquids production level to 16,300 + 10,000 +
40,000 = 66,300 bpd.
Combining GTL and oil production offers other opportunities
besides maximizing the overall production levels and offering
enhanced return on investment. The synergy between utility
systems (see section 6) supports parallel operations, so if the
GTL plant is down, crude oil can still be produced, and vice
versa. This makes the facility independent of GTL production.
The idea of combining oil and GTL production is driven by
the economics, as well as the synergy between the utility
systems.
OTC 17781
5
GTL FPSO Concept
5.1 GTL Technology
The basics of GTL technology has existed since the 1920s and
variations, using both coal and natural gas as feed stocks, have
been practiced for a number of years. The basic technology
consists of two steps (see Figure 2). The first is the reformer
where syngas is generated, and the second is Fischer-Tropsch
(FT) synthesis. For converting FT liquids into naphtha and
finished premium diesel, there is an optional refining step
(shown below).
Figure 2 - Simplified flow diagram for GTL process
The hydrocarbon refining section, normally included for an
onshore GTL plant, has been omitted from the GTL FPSO
concept. The refining of FT liquids into naphtha and premium
diesel options will be examined once the base case is
economically proven and if the FPSO can accommodate
additional refining equipment.
The economical GTL FPSO design aims to minimize capital
and maximize simplification. This has been achieved by:
1. Including ethane and LPG in the feed gas to the GTL plant,
instead of recovering them as separate products.
2. Simplifying the product slate into just two products: light
Fischer-Tropsch liquids (LFTL) and FT wax.
3. Configuring the GTL plant to a single train with one
Reformer and two stages of Fischer-Tropsch reactors
(FTRs). Even with this simplification, the overall FTRs’
carbon monoxide conversion is maintained at more than
90%, making optimum use of feed gas and maximizing
liquid production.
4. Simplifying the FT catalyst supply system so reduction and
regeneration processing are not located on the FPSO.
5. Using air as feed instead of pure oxygen - used by other
GTL technologies – makes the Syntroleum’s GTL process
inherently simpler.
3
The GTL FPSO process feed gas rate is 160 MMscfd and
produces 16,300 bpd of FT liquids. The yield of the two liquid
products is broken into 11,000 bpd of LFTL and 5,300 bpd of
FT wax.
Syntroleum’s technology can be used for all types feed gas
compositions, ranging from 0 mol% to nearly 16 mol% ethane
and heavier. The air, feed gas and steam rates are adjustable
online to optimize process performance if feed gas quality
changes due to variations in upstream production.
To optimize plant performance and increase operational
flexibility, an additional fixed bed reactor, called a prereformer, is located upstream of the reformer. The prereformer processes the ethane and heavier gases into methane,
primarily, giving the reformer a very consistent feed. The
reformer operates at low pressure. The feed gas supply
pressure to the GTL plant is less than 20 barg.
The reformer converts steam, air and feed gas in a fixed bed of
catalyst into syngas, consisting primarily of carbon monoxide,
hydrogen and nitrogen. Carbon monoxide and hydrogen are
the preferred reactants for the FTRs. The overall GTL
production capacity is dictated by the reformer’s capacity. The
processing train currently produces 1,000 MMscfd of syngas
from one reformer. Feed air is supplied to the reformer by two
50% capacity compressors, driven by a single steam turbine.
Two 50% capacity compressors, also driven by a single steam
turbine, compress the produced syngas. HP steam (47 barg) is
a by-product of the reformer - hot reformer effluentis, cooled
by BFW in two 50% capacity waste heat boilers, produces HP
steam. The 773 t/h of HP steam produced is sufficient to
power the air and syngas compressors, leaving excess HP
steam for the FPSO utilities.
The compressed syngas is then fed to two steps of FTRs in
series. The FTRs are three-phase slurry bubble column
reactors containing solid cobalt catalyst, FT liquid and gas.
Carbon monoxide and hydrogen are reacted to produce long
chain paraffins with carbon numbers ranging from two to
more than a 100. The reaction is exothermic and cooling coils
in the reactors produce 330 t/h of MP steam (8 barg). The MP
steam is used in the GTL plant for process heating and the
excess is exported to the FPSO’s utility steam system. The
first stage reactor is 11 m in diameter and has an operating
weight of 3,250 t. The second stage reactor is 10 m in diameter
and has an operating weight of 1,310 t. The reactors are both
within the capacity of numerous fabricators. As a by-product
of the FT reactions 35,000 bpd of relatively clean water is
produced, used as a feed for the water injection system in the
oil production section.
4
OTC 17781
5.2 Oil and Gas Production
The oil and gas production plant is set up in two independent
trains: one for oil separation and one for gas-condensate
separation and gas treatment. This enables production from
either the oil or gas reservoir if the other is down or its wells
are depleted.
Gas-Condensate Separation
GAS
DEHYDRATION
Gas Wells
GAS/CONDENSATE
SEPARATION
LOW
TEMPERATURE
SEPARATION
CONDENSATE
STABILISATION
Flash Gas Compressor
1 st STAGE OIL
SEPARATION
Condensate
HP Fuel Gas
Oil Separation
Oil Wells
Feed Gas
2nd STAGE OIL
SEPARATION
LP Fuel Gas
Crude Oil
Water
Figure 3 - Block diagram primary separation section
Oil Separation
The oil separation section separates associated gas and
produced water from the incoming oil well fluids to produce a
stabilised crude oil product stream.
The separation train is arranged in a two-stage configuration,
with a 1st and 2nd stage separator operating at 26 and 1.7 bara
respectively. The 1st stage separator receives the 40 °C well
fluids and separates most of the gas and produced water from
the well fluids. Associated gas from the 1st stage separator is
used as fuel gas or sent directly as feed gas to the GTL plant.
Crude oil leaving the 1st stage separator is heated to 74°C to
achieve the 2nd stage separator’s oil Reid Vapour Pressure
(RVP) specification. The basic sediment and water (BS&W)
specification is achieved in an electrostatic coalescer. The
stabilised crude oil product is cooled to 40 °C before entering
the FPSO storage tanks. All gas from the 2nd stage separator is
compressed to the desired fuel gas pressure. If the gas from the
2nd stage separator exceeds the fuel gas requirements, an
optional 2nd stage flash gas compressor can be installed to send
excess gas via the condensate stabiliser to the GTL plant.
Gas-condensate Separation
The gas-condensate separation section separates gas, light
hydrocarbon condensate and produced water from the
incoming gas-well fluids. It also further conditions the
separated gas stream to the required feed gas specifications for
the GTL plant.
A three-phase gas/condensate separator takes hydrocarbon
condensate and water from the gas stream. Produced water is
combined with the oil separation section’s produced water in
the produced water treatment plant for injection into the oil
reservoir or overboard disposal. Hydrocarbon condensate is
directed to a condensate stabiliser.
Gas leaving the gas/condensate separator is treated to produce
a combined GTL-feed gas (i.e. including associated gas from
the oil separation) to the desired C5+ specification. Therefore
the principal gas stream is chilled to approximately -30 °C to
remove the heavy components in a low-temperature separation
unit. Possible contaminants, such as mercury, arsenic and
halides, are removed from the gas to avoid damage to the GTL
catalyst. The gas is then cooled to 40 °C by the closed-loop
cooling water system and dehydrated in a Tri-Ethylene Glycol
(TEG) contactor to allow further chilling to -30 °C, via a
gas/gas exchanger and Joule-Thomson valve. The principal
gas stream is then mixed with associated gas from the oil
separation section and overhead vapours from the condensate
stabiliser, and sent as feed gas to the GTL plant.
Hydrocarbon condensate from the gas/condensate separation
and significant quantities of extracted C5+ components from
the low-temperature separation are stabilised in the condensate
stabiliser. This is a distillation column with reboiler (heated by
GTL generated HP steam) and produces a stabilised
condensate and an off-gas stream. The off-gas is processed in
the GTL plant and the condensate is blended with the crude oil
product in the FPSO’s cargo tanks. Dehydrated gas
downstream from the TEG contactor is also used as a fuel gas
source for the FPSO utility systems. When the oil separation
train is out of operation, this will be the primary fuel gas
source.
5.3 Produced Water and Water Injection
Produced water from the crude oil separation train and gascondensate separation train is degassed in a degasser/buffer
tank and de-oiled in a hydrocyclone. This is then combined
with process water from the GTL process and injected back
into the oil reservoir to maintain oil reservoir pressure. Any
surplus water not required for injection is treated to meet local
regulations before being disposed overboard.
Injecting process water from the GTL process and produced
water from the primary separation section eliminates the need
for any seawater injection and associated treatment facilities,
such as seawater deaeration.
6
Utilities
A wide variety of utilities are required to operate the GTL
FPSO, as with any FPSO facility. Typical FPSO utility
systems supplied by the hull, which are also required for the
GTL plant, include: instrument air, plant air, nitrogen,
seawater cooling and closed-loop fresh water cooling. Other
common utility systems that are relevant for the GTL plant
are: electric power generation, fuel gas system, flare systems,
drain systems, boiler feed water (BFW) supply and steam
generation - all of which are located on the topsides. The GTL
FPSO design requires new specific utilities, such as hydrogen
generation and propane refrigeration.
The GTL FPSO is an integrated facility which takes advantage
of the synergy between the various utility systems. The FPSO
supplies major utilities to the GTL plant, such as boiler feed
water (BFW), seawater cooling, closed-loop fresh water
cooling, electrical power and a small amount of hydrogen.
OTC 17781
5
The GTL plant generates a large amount of tail gas, used to
generate heat for the GTL process and produce superheated
HP steam. The GTL plant is self-sufficient in steam and
exports 23 t/hr of HP and some 123 t/hr of MP steam for use
on other FPSO systems. The FPSO uses superheated HP steam
for the steam turbine driven electric power generator and MP
steam for heating within the oil and gas separation plant, cargo
tank heating systems and fresh water generation plant.
The following examines the utility systems and their effect on
an integrated topsides design in closer detail.
6.1 Energy Utilization and Steam Generation
The Syntroleum GTL process, like all other GTL processes, is
very energy intensive. The GTL FPSO design balances and
shares heating and cooling loads between the GTL plant and
the oil and condensate systems.
A GTL plant produces high-grade energy (HP steam) and
lower grade energy (MP steam, low-BTU tail gas and a small
amount of FT off-gas).
Energy Production
HP steam
MP steam
Tail gas
FT off-gas
t/h
t/h
MMscfd
MMscfd
by GTL
Plant
773
331
570
1.5
by FPSO
Utilities
88
0
0
0
Total
861
331
570
1.5
The GTL FPSO design also balances and shares utility loads
between the GTL plant and the oil and gas production and
utility systems.
Energy Consumption
HP steam
MP steam
Tail gas
LP fuel gas
HP fuel gas
t/h
t/h
MMscfd
MMscfd
MMscfd
GTL
plant
750
208
492
0.6
0.0
Oil & Gas
plant
13
18
-
Power&
Utilities
98
105
78
1.6
5.2
Total
861
331
570
2.2
5.2
HP steam generated by the GTL process is used as feed steam
for the reformer and the steam turbine driven compressors.
Excess HP steam is sent to the FPSO utility systems
(consumed by the steam turbine driven power generator). All
the tail gas is combusted for process heating requirements and
additional HP steam generation - all HP steam and tail gas
generated are integrated into the utility demands of the GTL
plant and FPSO. Dump coolers can provide balance for the
MP steam system if required. Some additional fuel gas is
required for electric power generation, as (make-up) fuel gas
for the low-BTU combustors and fuel gas for a direct-fired
heater in the GTL plant.
By utilizing the excess energy from the GTL process, plant
efficiency is increased and less additional fuel gas is required.
An optimum must be found to take advantage of this excess
energy. When evaluating the options, capital costs, reliability,
availability and sparing requirements, maintainability and
proven equipment technology were taken into account.
Compressor Driver Selection
The process air and syngas compressors are the main rotating
equipment found on the GTL plant, with nominal power
demands of 80 MW and 90 MW, respectively. With the large
surplus of heat and tail gas from the GTL process, the choice
for drivers was between steam turbines and low-BTU gas
turbines. The choice determines the configuration of the GTL
plant’s utility systems and level of heat and cooling integration
and, subsequently, its fuel consumption.
When considering the evaluation criteria, steam turbines were
selected for their higher reliability and better heat and cooling
integration possibilities within this set-up.
Start up
When fully operational, the GTL plant generates its own steam
but this is not available at start up. While steam demand is less
during start up, some 470 t/h of high pressure steam is
required. This will be provided using a dedicated start up
boiler, capable of burning normal fuel gas and low-BTU tail
gas. This boiler runs during normal operation to provide HP
steam for the steam-driven power generator and to balance the
HP steam system if required.
6.2 Power Generation
This selection of direct steam-driven compressors for the GTL
plant has accommodated the main power consumers of the
GTL. However, there is a (peak) power demand of some 35
MW for the rest of the FPSO facilities, of which 5.5 MW is
essential power. An alternative back-up power source shall be
provided (i.e. a diesel engine) for essential power generation.
Summary of main electrical load requirements:
System
Crude oil production
Gas-condensate production
GTL production (including GTL utilities)
Water injection
Offloading (crude oil and GTL products)
FPSO and general utilities
Total
Electric Load [MW]
1.1
0.6
11.4
4.2
5.9
11.8
35.0
Although steam turbine drivers could also be used for some
other large power consumers - such as water injection pumps
and flash gas compressor - central electric power generation
was preferred. A central power plant offers more flexibility via
possible load shedding of non-essential consumers and load
sharing between spare generators, giving main processes
higher availability.
Several options - steam turbines, gas turbines, low-BTU gas
turbines and combinations - were evaluated. A 24 MW steam
turbine, combined with an 11 MW gas turbine, was selected
for the central power plant. Spare power generation capacity is
provided by two 5.5 MW diesel engine generators. This
configuration enables the gas turbine, together with the two
diesel engines, to serve as back up for the steam turbine. The
two diesel engines also serve as back up for the gas turbine
and also serve as an essential power source. The steam
required for the steam turbine is provided by the start-up boiler
steam generation. This configuration offers high reliability and
flexibility, with minimum maintenance and capital costs.
6
OTC 17781
6.3 Cooling and Chilling
Seawater Intake for Cooling
The GTL plant, including related utilities, requires 1,100 MW
of process cooling (excluding steam generation). At onshore
GTL plants, water supplies for cooling towers are often
limited, so air coolers provide a large share of the cooling
requirements. Compared to water coolers, air coolers require
considerably more plot space. Their size and location on top of
the GTL FPSO pipe-rack can partially enclose the process area
and when they are out of operation, limit ventilation of
enclosed areas and increase the risk of explosion. As seawater
cooling is readily available, air coolers are not usually applied
on an FPSO.
Water coolers have a smaller footprint and are easier to
incorporate within FPSO space limitations. Water cooling
offers additional advantages to air cooling, such as achieving
lower cooling temperatures.
Typical closed-loop cooling water systems on an FPSO are
used to cool gas and liquid streams up to about 150 – 160 °C.
The GTL process, however, requires much higher temperature
cooling, up to approximately 400 °C. High-temperature water
cooling is possible by using a combination of high-pressure
BFW (51 barg) heat recovery and closed-loop cooling water.
Using BFW for process cooling increases plant efficiency, as
it eliminates other heating requirements for the BFW, before it
can be used for HP steam generation at the reformer outlet.
Process and utility cooling requirements:
Air cooling
BFW process cooling
Fresh water cooling
Direct seawater cooling
Total
GTL
Process
[MW]
47
119
549
386
1,101
Oil & Gas
Process
[MW]
5
14
19
Power &
Utilities
[MW]
19
58
77
For high-temperature gas cooling, heat integration with the
available BFW steam is used as much as possible (although
some air coolers have been applied in the process due to BFW
deficiency). All low temperature process cooling is done with
closed-loop fresh water coolers.
Cold seawater is used to cool the closed-loop cooling water
and the FPSO’s steam condensers.
Using the relatively cold closed-loop cooling water of 12 °C,
process streams in the GTL plant are cooled as low as 21 °C,
which is colder than found at typical onshore GTL plants. This
provides specific GTL process benefits, such as improved
contaminate removal from syngas, increased FT catalyst
activity, increased product recovery, lower process air and
syngas compression power requirements, smaller process
piping sizes and smaller FT reactor cooling coils.
For the large cooling duties, large quantities of seawater are
taken onboard: 23,000 m3/h for closed-loop cooling water
cooling and 18,000 m3/h for direct seawater cooling (total
41,000 m3/h).
To achieve a closed-loop cooling water supply temperature of
12 °C, the cooling water return is cooled by cold seawater of 7
°C, taken from a depth of 500 m below the sea surface via an
intake riser suspended below the FPSO hull.
Temperature (°C)
0
5
10
15
20
25
30
0
-100
-200
Depth (m)
Air vs. Water Cooling
-300
-400
-500
-600
Sea Water
-700
-800
Cold Seawater Supply
-900
Figure 4 - Cold seawater intake
Chilling
Downstream of the Fischer-Tropsch section, the liquid
Fischer-Tropsch product recovery can be enhanced by cooling
to lower temperatures unachievable with closed-loop cooling
water or direct cold seawater cooling. Subsequently an
additional propane refrigerant chilling system is used to cool
the tail gas to temperatures of 4.4 °C.
The propane refrigeration system is an integrated part of the
GTL plant as it performs specific chilling duties in the GTL
product recovery section.
6.4 Hydrogen Generation
The GTL process requires hydrogen for the feed gas
desulphurization section. Hydrogen is produced as part of the
syngas production in the GTL process’ reformer. While
hydrogen could be recovered by a membrane separation unit,
in combination with a Pressure Swing Absorber (PSA), it is
also required for start-up of the GTL syngas production.
Therefore a hydrogen storage system would be required to
allow for start up. Storing large volumes of hydrogen is
uneconomical. Consequently hydrogen shall be produced
independent of the GTL process.
High-purity hydrogen is supplied typically from a Steam
Methane Reformer (SMR), in combination with a Pressure
Swing Absorber (PSA). In order to save space and weight, the
PSA, which requires large absorbent filled vessels, was
deleted and the GTL plant was designed to use hydrogen from
a SMR unit, with 75% hydrogen purity.
OTC 17781
7
7
Multiple Product Storage and Offloading
7.1 FPSO Hull
The FPSO hull is the platform for the oil and gas separation
and GTL plants (the topsides). The hull also provides the
storage capacity for the produced products. The topsides’
footprint, weight, and required storage capacity determines the
hull size.
In the case of the West Africa field under consideration, a
topsides footprint of 13,725 m2, weighing around 45,000 t and
a cargo storage capacity of 2 million barrels are required. The
main hull dimensions are:
Length overall
Breadth moulded
Depth
Draught
Dead weight
:
:
:
:
:
310
65
29
22
310,000
m
m
m
m
t
The FPSO hull size falls into the Very Large Crude Carrier
(VLCC) class and is similar to hulls used for deep-water
FPSOs in West Africa. The hull can be extended with a 25-m
section to accommodate product upgrade.
7.4 Cargo Storage Capacity
The storage capacity shall, at a minimum, cater for the
offloading parcel sizes. It will also accommodate some spare
storage to avoid production loss due to late arrival of the
offloading tanker and/or weather preventing the offloading
operation.
Production rate
Required storage capacity
Export parcel
Total storage capacity
[bpd]
[bbl]
[bbl]
[bbl]
Crude blend
Wax
61,000
5,300
1,600,000
401,300
1,000,000
350,000
2,001,300
7.5 Storage Conditions
The storage conditions for the oil, condensate and LFTL,
whether blended or not, are similar to what is normally
encountered on FPSOs, i.e. a maximum storage temperature of
40 °C. Crude tanks will be provided with low pressure steam
heating coils.
The wax storage temperature is 70 °C, which prevents solidification. This temperature is sufficient to keep the wax liquid.
The benign environmental conditions in West Africa allow the
FPSO hull to be spread moored. A riser balcony is positioned
on one side of the hull so the risers can be connected to the
FPSO manifold.
The wax tanks, arranged in the center tanks, are enclosed by
crude tanks to provide insulation and slow down the
solidification process. Wax tanks will be provided with low
pressure steam heating coils to keep the wax at the required
temperature for storage and offloading.
7.2 Products
7.6 Offloading
In order to simplify the design, the GTL FPSO only produces
two products. This minimalistic base case was chosen to test
the concept’s viability. Production of other product streams,
such as LPG, diesel, naphtha, could be investigated once the
base case is proven.
Tug-assisted offloading tankers will moor in a tandem
configuration to the hawser provided at the FPSO stern,
keeping more or less in line with the FPSO.
This FPSO case study produces 61,000 bpd of crudecondensate-LFTL blend (40,000 bpd oil, 10,000 bpd
condensate and 11,000 bpd LFTL) and 5,300 bpd of FT-wax.
For the base case design, only the FT-wax will be stored and
offloaded separately. The FPSO, however, enables separate
storage of each product. For flushing transfer headers,
facilities are provided to store condensate separately in one
tank.
7.3 Off-spec Products
Crude oil and condensate will be tested on-line and, if on-spec,
directly pumped into the cargo tanks. If off-spec, they will be
fed back directly into the primary oil and gas separation
system.
LFTL and wax will be pumped directly to their dedicated
rundown tanks. Once the quality is confirmed, the contents
from the rundown tanks will be pumped to the appropriate
cargo tanks. To allow for continuous production, a set of two
rundown tanks for each product is required. Off-spec product
will be transferred into the appropriate off-spec tank so that it
can be re-processed in the topsides facilities if required.
Two segregated offloading arrangements are provided, one for
the crude blend and one for the wax, to avoid any
contamination of the wax. In addition, separate deck piping
runs from the wax and the crude tanks to their dedicated
manifold at the FPSO offloading station. A heat-traced
floating wax offloading hose string connects to the wax
manifold, and a floating crude offloading hose string connects
to the crude manifold.
If crude, condensate and LFTL are stored separately, they can
be offloaded sequentially through the same offloading system.
In that case the most pure product (LFTL) will be offloaded
first, followed by the lesser pure products.
7.7 Offloading Tankers
Different tankers will be used for the offloading of the two
products: oil-condensate-LFTL blend and wax. VLCC-sized
trade tankers will offload 1,000,000 barrels of crude blend;
chemical tankers will transfer 350,000 barrels of wax.
8
OTC 17781
8
Catalyst Handling
It is anticipated that an average of one ISO-container per week
will be required to maintain the catalyst activity in both
reactors.
8.1 Catalyst Overview
The GTL plant requires several types of catalyst. Both fixed
bed and slurry bed reactors are included in the design. The
catalyst and support materials range in size from 50 micron to
40 mm. The fixed bed reactors are designed to be changed out
on a typical four/five-year turnaround cycle.
The FTRs are three-phase slurry reactors, which constantly
circulate catalyst. This circulation allows for easy catalyst
addition and removal during operations. The FTRs require
periodic catalyst removal and addition to maintain required
activity and production. Each catalyst addition or removal
involves a very small percentage of the total catalyst volume
in the reactor, resulting in no change to production.
8.2 Logistics
Catalysts for fixed bed reactors are typically delivered in
drums or supersacks, the latter weigh 1.5 – 2.2 t depending on
catalyst type. Supersacks are planned for the FPSO GTL plant
to minimize the number of parcels that need to be handled.
Fixed bed catalysts will only be handled during the initial
loading and at subsequent planned turnarounds.
The FT catalyst is very small, approximately 50 microns, and
shipped in insulated, traced ISO-containers, which weigh ~28 t
when full. For shipping, the FT catalyst is in an active state
and encapsulated in wax to prevent air from deactivating it.
The mixture of catalyst and wax is referred to as slurry. Cranes
are required to move the ISO-container from the supply boat
to the laydown area allocated on the FPSO. To transfer the
catalyst, steam will be used to heat the ISO-container.
Nitrogen will be used to pressure transfer the catalyst to an
intermediate slurry transfer vessel, which is capable of the
high pressure required, as for the final transfer of catalyst into
the online FTRs. The higher pressure slurry transfer vessel is
required since the typical ISO-container design pressure is
only 4.0 barg and the normal operation pressure of the FT
reactor is several times higher. Figure 5 summarizes the
system of catalyst addition.
Steam
Vent
LP Nitrogen
1st
Stage
FTR
F-T Catalyst
ISO-Container
Laydown Area
Slot 1
Slurry Vessel
8.3 Onboard Handling
Cranes will be used to lift the catalyst-filled supersacks to the
top of the fixed bed reactors to top-load the catalyst into the
reactor. A vacuum system will be used to remove catalyst
from the fixed bed reactors. Once removed, it will be loaded
into supersacks or drums for shipment for catalyst recovery or
disposal. These techniques are very common in onshore
refineries and chemical plants. It is assumed that catalyst
service companies will provide the necessary loading and
unloading equipment during the turnaround periods.
Typically there would be no process reason to remove the
FTRs’ entire catalyst contents, as the catalyst is added to and
removed from the reactor on a continuous basis to maintain
production. However, the slurry may need to be removed from
the reactor to allow for internal mechanical inspection. In this
event, a storage tank in the hull will handle the contents of
both reactors simultaneously. The tank has a recirculation
system and heat exchange to maintain the 70 °C storage
temperature and a nitrogen supply and spargers to keep the
slurry well mixed.
9
The footprint occupied by a land-based GTL facility is many
times greater than the space available onboard an FPSO. The
selected VLCC class hull has a breadth of 65 m. Subtracting 2
m either side for escape ways leaves a width of 61 m for the
topsides facilities. After allowing space for the
accommodation and machinery rooms, a deck length of 225 m
is free for the topsides facilities. Therefore the complete
topsides facility will be built on an area of just 13,725 m2. A
GTL plant with utilities, flare, storage tanks, laydown, roads,
and buildings onshore would occupy 237,000 m2 – 17 times
larger than the space of a GTL FPSO. With the storage tanks
located below the topsides on an FPSO, the real challenge was
to design a more compact facility without compromising
safety. This has been accomplished by adopting a series of
measures:
•
•
•
Vent
Slot 2
Safety and Layout
•
Multiple layers
Selecting compact equipment providing the same function
Optimizing module positions relative to each other to
avoid long pipe runs
Using fire walls when segregation results in large void
areas between equipment or modules
Slot 3
2nd
Stage
FTR
HP Nitrogen
Slot 4
Slot 5
Figure 5 - FT catalyst slurry loading and unloading
The plant is arranged in convenient modules for the various
process steps, such as oil separation, gas treatment, water
treatment and injection, process air compression, syngas
generation etc. These modules need to be arranged in a
sequence corresponding with the process flow, as shown in the
diagram below. This is very important for the GTL plant, with
its large gaseous flows and where the main layout objective is
to minimize the length of the very large diameter piping (up to
84-inch) between the different sections, especially for those
lines that impact the process air compressor and syngas
compressor duties.
OTC 17781
9
Feedgas
Conditioning
Pre-Reformer
Feedgas
Feedgas + H2
Feedgas
Combustor
#2
PAC
Air
Feedgas
Air
BFW
ATR
+ Steam Drum
Air
HP
Steam
SG
SG
Cooling
MP Steam
SH MP Steam
Combustor
#1
SH HP Steam
BFW
Combustor#3
+ Steam Drum
The Syntroleum GTL process has a great advantage in the use
of air instead of pure oxygen for the generation of syngas.
Still, the reformer requires about the same quantity of oxygen
for the syngas conversion. As air only contains approximately
20% oxygen, a large air flow needs to feed the ATR. The
nitrogen is not consumed in the ATR, but will flow along with
the syngas flow throughout the GTL process to end up in lowBTU tail gas. For this, large diameter piping is required, which
is easy for a land-based GTL plant. On an FPSO, with its
restricted space, the manipulation of large diameter piping is
cumbersome as is illustrated in Figure 7.
SH HP Steam
HP Steam
SG
SG
Compressor
SG
SG
Clean-up
SG
MP
Steam
MP
Steam
FTR #1
+ steam drum
FTR #2
+ steam drum
+ TG Exchanger
TG
TG
Refrig. Unit
SO
TG
SO Column
SO
TG to
Combustors
TG
SO
FT Fractionator
Figure 6 – Main flows between GTL sections
When direct-fired heaters or combustors are located in a
hazardous area, gas-tight fire walls will enclose them, leaving
the outboard side open only. Such gas-tight fire walls would
be difficult to achieve for the large diameter piping connected
to the low-BTU combustors. As a result, all combustors are
located to the stern of the FPSO, away from the hazardous
equipment, but maintaining the sequencing of the GTL
modules (see Figure 8). A minimum separation of 15 m was
reserved between the combustors and rest of the GTL plant,
based the Industrial Risk Insurers (IRI) IM.2.5.2 guidelines for
piping layout. This results, however, in longer pipe runs but
compared to the complexity of a gas tight fire wall, this
outcome is the better solution.
Figure 7 – Large piping in GTL module
The layout of oil and gas condensate plants for FPSOs is
driven by the equipment requirements. The GTL plant requires
a different mindset. With pipe sizes equivalent or larger than
the connecting equipment, the layout is very much dominated
by the piping arrangement, rather than the equipment.
Figure 8 - FPSO layout
10
10
OTC 17781
Conclusions and Recommendations
The feasibility study has proven the feasibility of applying the
Syntroleum GTL process to an FPSO. The study also revealed
areas for further study including:
•
Field development and operations philosophy
An important aspect for a combined GTL and oil
production FPSO is the field development plan and
operations philosophy. On traditional FPSOs the
production is driven from a well management perspective
to maximize oil production. For the GTL FPSO the
constant supply of natural gas to the GTL plant may be of
higher priority. The interaction between wells and GTL
feed gas supply will be investigated further in the next
study phase.
•
The oil and gas production plant
The oil and gas production plant presented below is a
typical configuration, based on the reservoir compositions
of a typical West African oil and gas field. The oil and gas
production plant could also be optimized to a more
generic format suitable for a wide range of gas
compositions in other locations. It goes beyond the
objective of this feasibility study to optimize this
configuration.
•
Additional process facilities to be considered are:
o
o
o
o
Gas lift and associated gas compression requirements
2nd stage flash gas compression
LPG production facilities
GTL product upgrading (refining) section
i
Portions of data required to prepare this paper were developed with U.S.
Government support under Contract W56HZV-05-C-0435 awarded by U.S.
Tank-automotive and Armaments Command (TACOM). The Government has
royalty-free permission to reproduce all or portions of the paper and to
authorize others to do so for official U.S. Government purposes only.