Process and Safety considerations while designing Floating Liquefaction Natural Gas (FLNG) facilities

There are many FLNG projects currently under consideration, at different stages of design or in construction. Shell’s Prelude will be the first facility to start production, which is expected sometime during 2016. Other ships under construction belong to Petronas, Murphy and Exmar. Companies who are perfoming engineering studies are Exxonmobil, Noble Energy, Eni, GDF, Chevron and Petrobras among others.
Some key process engineering considerations in an FLNG project are liquefaction technology to use, process flexibility, motion effects in process, weight and space considerations and safety issues. Just like in LNG, the design of FLNG processes are heavily influenced by safety considerations and regulations, so they need to be taken into consideration right at the start of the project.
1.Liquefaction Technology
The Dual Mixed Refrigerant (DMR) process is going to be used in a lot of the FLNG projects close to be implemented as it offers more flexibility and compression efficiency than the C3MR process. The DMR process uses two loops of mixed refrigerants, which run on axial compressors. These compressors have a higher efficiency than centrifugal compressors, the compressors used in the C3MR propane loop. The DMR cooling curve is adjusted to closely match the cooling curve of natural gas, therefore maximizes LNG production for a certain driver and minimizes specific energy requirements. If the composition of the feed changes, so the mixed refrigerant composition. In addition, the DMR process does not use pure propane, a strong improvement from a safety perspective. This technology is available from Air Products and Chemicals (APCI) and Shell Global Solutions.
Figure 1. Cooling curve of natural gas with different liquefaction processes A) Single refrigerant curve with three compression stages B) single mixed refrigerant curve C)Dual Mixed Refrigerant Curve D) C3MR process curve
Mixed refrigerants, the refrigerants used in the DMR process,  require less equipment than single refrigerants in each compression loop, which is beneficial in FLNG projects because of economic, weight and space reasons.
Three stage single refrigerant loop                                                          Cooling curve with single refrigerant
Mixed refrigerant loop                                                                  Cooling curve with mixed refrigerant
Figure 2. Comparison between compression loop and cooling curves of single and mixed refrigerants

2.Process Flexibility
While designing an FLNG facility, it is important to design it under different feed compositions, as the ship will move from time to time to exploit different reservoirs. Changes in gas composition affect the entire process. CO2 removal, Heavy Recovery Unit (HRU) and feed and mixed refrigerant compressors should be designed accordingly.
3.Motion Effects in Process
Motion is an important factor to be taken into consideration while designing units. Sloshing, distillation, separators, process flow direction and structures are all affected by motion.
3.1          Sloshing
Sloshing is the irregular move of the LNG splashing inside the LNG storage vessels.  These vessels need to be designed to contain from little to a full load of LNG in them and be able to withstand the impact of their inner waves, as they affect their thermal insulation. Currently Gaztransport & Technigaz (GTT) has a membrane based design that is favored by the industry because it is effective and cost efficient. The system has two redundant membranes made of a 36% nickel steel alloy, 0.7mm thick. The redundancy is required to prevent leaks. The insulation is made of expanded perlite, which is contained in plywood boxes. Perlite is a common material used for insulation in cold boxes.
3.2          Distillation
Distillation is a key process that is strongly affect by motion as channeling occurs. Both amine towers and the HRU need to be designed taking tilting and motion into consideration. No more than 50 ppmv of CO2 should leave the amine units in order to prevent a stable operation of the cryogenic processes. Less than 0.1% mole of C5+ should leave the HRU in order for the liquefaction process to run smoothly.
As the CO2 removal system also needs to be flexible to handle CO2 concentrations of up to 10% or more, the current industry consensus is to have first a dual membrane system that will reduce the CO2 concentration down to 2% and then use a typical amine tower system to take down the concentration to 50 ppmv.  Membrane systems are not influenced by motion, unfortunately current commercial systems are economical only down to 2%  mol output. Several vendors are working to offer a full membrane system, but the systems are not yet commercial.

Tilting of towers can reduce their performance from 10 to 60% due channeling of liquid and gas. Tim Cullinane et al (7) recommend to have redistributors every 2 diameters or less, these should have sufficient residence time to keep enough volume of liquid to distribute within all the diameter once the tower is tilted. Structured packing has been preferred over random packing, trays are not recommended.
3.3          Separators
Sloshing affects in a negative manner the performance of separators. Computational Fluid Dynamics (CFD) is currently used to design compartmentalizing and dampening baffles within the separators. These baffles help to decrease the inner waves formed in the separators.
3.4          Process Flow Direction
Check valves, layout and process control should enforce the flow direction within the process. Simple small slopes do not ensure flow direction in a wavy offshore environment.
3.5          Structural Issues
Load assessments and momentum forces need to be taken into consideration while setting up the layout and supports. A motion environment may cause a lot of stresses in towers and heavy equipment. Centers of gravity need also to be checked.
4.Weight and space considerations
Weight and space in offshore facilities always need to be taken into consideration as they are expensive. Processes with less equipment, being everything else the same, should be preferred.
Compressors and turbines are key in weight reduction efforts as they have considerable weight. In many offshore projects, aeroderivative turbines are used due their lower weight and smaller dimensions when compared to industrial turbines. In addition, they are 10-15% more efficient.
Core-in-kettle heat exchangers occupy about 50% of the space and have about 25% of the weight of shell and tube exchangers.
Coil Wound Heat Exchangers (CWHE) in the precooling cycle occupy less plot space than conventional propane kettle evaporators, a plus for the DMR process.
5.Safety Issues
As in onshore LNG projects, safety concerns strongly influence layout of facilities and processes to be used. It is important to have both process and safety engineers working together right from the start of the project in order to come up with feasible, cost and time effective solutions.

Both ENI and Shell report that spacing between the different modules of the plant is important to enhance ventilation between them in case of refrigerant or product leaks. Good ventilation reduces asphyxiation and explosion hazards. It also prevents fires from expanding from one area to another. They both keep living quarters away from the liquefaction and flare areas. Persund (15) came out with general guidelines which affect the general layout such as: Higher risk areas should be distant from accommodation, gaps between process modules for mitigation of escalation by jet fires and explosions, minimizing walls to promote ventilation, no gas turbines above LNG or LPG tanks, utility area to separate accommodation from process, use of submerged hydrocarbon pumps as much as possible, no use of reciprocating equipment and others.

Management of rapid phase transition issues should be reviewed. Rapid phase transitions occurs when a liquefied gas is heated up and suddenly expands, creating a physical, not chemical, explosion. It usually happens when it gets in contact and mixed with water.

Cryogenic spills need to be thoroughly planned, they should be minimized as they embrittle steel structures (module structures, hull, other carbon steel structures and equipment) and are an explosion hazard. 
A general guideline to decrease the hazards effects of cryogenic spills would be:
Use of 49 CFR-193 and NFPA 59A
Even though these standards were made for onshore, a lot of their principles apply to offshore LNG processing as well.

Plated and grated process decks
Plated process decks will keep vapors from migrating to other areas. Grated process decks will promote ventilation when compared with plated decks. They can be chosen depending on the purpose. A CFD model could be used to review the end results.
Limitation of module congestion level
The more congested a module is, the more stagnant vapors will contain.
Minimizing LPG inventories
LPG inventories are usually held under pressure, which mean that BLEVE may be present.
Minimizing leak points (flanges)
Leaks may be collected locally and directed overboard, drip trays should be of suitable materials such like stainless steel.
Use of insulation to avoid contact with metal structures
Polyurethane, wood or concrete may be used
Use of Spray guards
Spray guards may be installed where high pressure cryogenic spills may be present.
Use of Computational Fluid Dynamics (CFD) in layout design
The use of CFD is quite useful while designing layouts that promote ventilation. Studies generated by Gexcon using FLACS (14) show that the rate of dispersion of vapors can be increased by having spaces between different modules, using grated decks and decreasing congestion within modules. Spacing between modules help to keep fires located to a single module.
Comply with the American Bureau of Shipping (ABS) requirements
The ABS requires an extensive risk evaluation methodology that should include a Hazard and Operability Study (HAZOP), a Failure Mode and Effects Analysis (FMEA), Failure Mode, Effects and Criticality Analysis (FMECA), Process Hazard Analysis (PHA), Safety Reviews, checklists and experience from previous analysis.

The risk assessment needs to identify and ameliorate the following hazards: Fire and explosion, hydrocarbon release, blow out, structural failure, loss of stability, loss of station keeping/mooring, loss of electrical power, toxicity, extreme weather, environmental factors, dropped objects and ship and helicopter collision.

Current Technologies Selected

The following table was published by Dominique Pelloux Prayer from GDF Suez LNG in 2013, which shows the different technologies to be used in several FLNG projects.

Project / Equipment

Prelude

Kanowit

Santos Basin

Scarborough

Bonaparte

Capacity

 (3.6 MTPA + liquids)

(1.2 MTPA)

(2.7 MTPA + liquids)

(6/7 MTPA)

 (2.4 MTPA)

Owner

Shell/ Inpex / Kogas /CPC

Petronas

Petrobras /BG

Exxonmobil/ BHP

GDF SUEZ / Santos

Engineering / Shipyard

Technip / Samsung

Technip / DSME

Technip / JGC / Modec

?

Technip / KBR
SBM/ Chiyoda / SAIPEM

Liquefaction process

DMR

N2 Expansion (AP-N)

DMR

Mix Refrig

DMR

Mechanical Drivers

redesigned steam turbines

gas turbines

gas turbines

gas turbines

gas turbines

Containment System

Mark III membranes

No 96 membranes

SPM

?

membranes

LNG Offloading

side by side

side by side

tandem

tandem

side by side

Figure 3. The DMR Liquefaction Process                                Figure 4. The C3MR Liquefaction Process

References

Dominique Pelloux Prayer, Delivering Floating LNG in the Timor Sea

Journees Annuelles des Hydrocarbures

23 et 24 October 2013 au Palais des Congress, Paris

Sultan Seif Pwaga, Sensitivity Analysis of proposed LNG liquefaction processes for LNG FPSO, Norwegina University of Science and Technology, master thesis, July 2011
49 CFR 193
NFPA 59A 2001
Kiminori Takahasi et Al, JGC Corporation, Advanced Numerial Simulation of Gas Explosions for Assessing the Safety of LNG Plants, Poster PO-29
RR Bowen et al, LNG Technology advances and challenges, Exxonmobil development company, International Petroleum Technology Conference, SPE, IPTC 12111
Tim Cullinane et al, Exxon, Effects of Tower Motion on Packing Efficiency, SPE 143766, Brasil Offshore Conference and Exhibition, Macae Brazil 14-17 June 2011
Zainab Kayat, Petronas, Mark Schott, UOP,

Pretreatment of Acid Gas in feed for Petronas floating LNG facility

Thierry Miletic, Saipem, Design Challenges and Option for Large Scale FLNG Facilities

AOG Conference Perth, February 20, 2014

Floating Liquefied Gas Terminals, Offshore Technical Guidance OTG-02 March 2011, DNV Veritas
Justin Bukowski et al, Innovations in Natural Gas Liquefaction Technology for Future LNG Plants and Floating LNG Facilities, Air Products and Chemicals, International Gas Union Research Conference, 2011
Wilm Dam et al, Shell, Unusual design considerations drive selection of Sakhalin LNG plant facilities, Oil and Gas Journal, 10/01/2001
Adam Bradley et al, Innovation in the LNG Industry :Shell’s approach
Scott Davis et al, Gexcon, CFD Based probabilistic Explosion Hazard analysis as an early tool to improve FLNG Design, 5/2/2013 31th topical conference on gas utilization, AIChE
Safety drivers in the lay out of floating LNG plants, AIChE pub 176, third topical conference on natural gas utilization, 2003, M.A. Persund. 359-372

​Type youProcess and Safety considerations while designing Floating Liquefaction Natural Gas (FLNG) facilities
There are many FLNG projects currently under consideration, at different stages of design or in construction. Shell’s Prelude will
be the first facility to start production, which is expected sometime during 2016. Other ships under construction belong to Petronas, Murphy and Exmar. Companies who are perfoming engineering studies are Exxonmobil, Noble Energy, Eni, GDF, Chevron and Petrobras among others.
Some key process engineering considerations in an FLNG project are liquefaction technology to use, process lexibility, motion effects in process, weight and space considerations and safety issues. Just like in LNG, the design of FLNG processes are heavily influenced by safety considerations and regulations, so they need to be taken into consideration right at the start of the project.
1.Liquefaction Technology
The Dual Mixed Refrigerant (DMR) process is going to be used in a lot of the FLNG projects close to be implemented as it offers more flexibility and compression efficiency than the C3MR process. The DMR process uses two loops of mixed refrigerants, which run on axial compressors. These compressors have a higher efficiency than centrifugal compressors, the compressors used in the C3MR propane loop. The DMR cooling curve is adjusted to closely match the cooling curve of natural gas, therefore maximizes LNG production for a certain driver and minimizes specific energy requirements. If the composition of the feed changes, so the mixed refrigerant composition. In addition, the DMR process does not use pure propane, a strong improvement from a safety perspective. This technology is available from Air Products and Chemicals (APCI) and Shell Global Solutions Figure 1. Cooling curve of natural gas with different liquefaction processes A) Single refrigerant curve with three compression stages B) single mixed refrigerant curve C)Dual Mixed Refrigerant Curve D) C3MR process curve
Mixed refrigerants, the refrigerants used in the DMR process,  require less equipment than single refrigerants in each compression loop, which is beneficial in FLNG projects because of economic, weight and space reasons.
Three stage single refrigerant loop                                                          Cooling curve with single refrigerantMixed refrigerant loop                                                                  Cooling curve with mixed refrigerantFigure 2. Comparison between compression loop and cooling curves of single and mixed refrigerants
2.Process Flexibility While designing an FLNG facility, it is important to design it under different feed compositions, as the ship will move from time to time to exploit different reservoirs. Changes in gas composition affect the entire process. CO2 removal, Heavy Recovery Unit (HRU) and feed and mixed refrigerant compressors should be designed accordingly.3.Motion Effects in Process
Motion is an important factor to be taken into consideration while designing units. Sloshing, distillation, separators, process flow direction and structures are all affected by motion.
3.1          Sloshing
Sloshing is the irregular move of the LNG splashing inside the LNG storage vessels.  These vessels need to be designed to contain from little to a full load of LNG in them and be able to withstand the impact of their inner waves, as they affect their thermal insulation. Currently Gaztransport & Technigaz (GTT) has a membrane based design that is favored by the industry because it is effective and cost efficient. The system has two redundant membranes made of a 36% nickel steel alloy, 0.7mm thick. The redundancy is required to prevent leaks. The insulation is made of expanded perlite, which is contained in plywood boxes. Perlite is a common material used for insulation in cold bo
xes.
3.2          Distillation Distillation is a key process that is strongly affect by motion as channeling occurs. Both amine towers and the HRU need to be designed taking tilting and motion into consideration. No more than 50 ppmv of CO2 should leave the amine units in order to prevent a stable operation of the cryogenic processes. Less than 0.1% mole of C5+ should leave the HRU in order for the liquefaction process to run smoothly.
As the CO2 removal system also needs to be flexible to handle CO2 concentrations of up to 10% or more, the current industry consensus is to have first a dual membrane system that will reduce the CO2 concentration down to 2% and then use a typical amine tower system to take down the concentration to 50 ppmv.  Membrane systems are not influenced by motion, unfortunately current commercial systems are economical only down to 2%  mol output. Several vendors are working to offer a full membrane system, but the systems are not yet commercial.Tilting of towers can reduce their performance from 10 to 60% due channeling of liquid and gas. Tim Cullinane et al (7) recommend to have redistributors every 2 diameters or less, these should have sufficient residence time to keep enough volume of liquid to distribute within all the diameter once the tower is tilted. Structured packing has been preferred over random packing, trays are not recommended.
3.3          Separators Sloshing affects in a negative manner the performance of separators. Computational Fluid Dynamics (CFD) is currently used to design compartmentalizing and dampening baffles within the separators. These baffles help to decrease the inner waves formed in the separators.
3.4          Process Flow DirectionCheck valves, layout and process control should enforce the flow direction within the process. Simple small slopes do not ensure flow direction in a wavy offshore environment.
3.5          Structural IssuesLoad assessments and momentum forces need to be taken into consideration while setting up the layout and supports. A motion environment may cause a lot of stresses in towers and heavy equipment. Centers of gravity need also to be checked.
4.Weight and space considerationsWeight and space in offshore facilities always need to be taken into consideration as they are expensive. Processes with less equipment, being everything else the same, should be preferred.
Compressors and turbines are key in weight reduction efforts as they have considerable weight. In many offshore projects, aeroderivative turbines are used due their lower weight and smaller dimensions when compared to industrial turbines. In addition, they are 10-15% more efficient.
Core-in-kettle heat exchangers occupy about 50% of the space and have about 25% of the weight of shell and tube exchangers.
Coil Wound Heat Exchangers (CWHE) in the precooling cycle occupy less plot space than conventional propane kettle evaporators, a plus for the DMR process.
5.Safety Issues
As in onshore LNG projects, safety concerns strongly influence layout of facilities and processes to be used. It is important to have both process and safety engineers working together right from the start of the project in order to come up with feasible, cost and time effective solutions.
Both ENI and Shell report that spacing between the different modules of the plant is important to enhance ventilation between them in case of refrigerant or product leaks. Good ventilation reduces asphyxiation and explosion hazards. It also prevents fires from expanding from one area to another. They both keep living quarters away from the liquefaction and flare areas. Persund (15) came out with general guidelines which affect the general layout such as: Higher risk areas should be distant from accommodation, gaps between process modules for mitigation of escalation by jet fires and explosions, minimizing walls to promote ventilation, no gas turbines above LNG or LPG tanks, utility area to separate accommodation from process, use of submerged hydrocarbon pumps as much as possible, no use of reciprocating equipment and others.
Management of rapid phase transition issues should be reviewed. Rapid phase transitions occurs when a liquefied gas is heated up and suddenly expands, creating a physical, not chemical, explosion. It usually happens when it gets in contact and mixed with water.
Cryogenic spills need to be thoroughly planned, they should be minimized as they embrittle steel structures (module structures, hull, other carbon steel structures and equipment) and are an explosion hazard. 
A general guideline to decrease the hazards effects of cryogenic spills would be:
Use of 49 CFR-193 and NFPA 59A
Even though these standards were made for onshore, a lot of their principles apply to offshore LNG processing as well.
Plated and grated process decks
Plated process decks will keep vapors from migrating to other areas. Grated process decks will promote ventilation when compared with plated decks. They can be chosen depending on the purpose. A CFD model could be used to review the end results.
Limitation of module congestion level
The more congested a module is, the more stagnant vapors will contain.
Minimizing LPG inventories
LPG inventories are usually held under pressure, which mean that BLEVE may be present.
Minimizing leak points (flanges)
Leaks may be collected locally and directed overboard, drip trays should be of suitable materials such like stainless steel.
Use of insulation to avoid contact with metal structures
Polyurethane, wood or concrete may be used
Use of Spray guards
Spray guards may be installed where high pressure cryogenic spills may be present.
Use of Computational Fluid Dynamics (CFD) in layout design
The use of CFD is quite useful while designing layouts that promote ventilation. Studies generated by Gexcon using FLACS (14) show that the rate of dispersion of vapors can be increased by having spaces between different modules, using grated decks and decreasing congestion within modules. Spacing between modules help to keep fires located to a single module.Comply with the
American Bureau of Shipping (ABS) requirements

Process Design

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