As FLNG starts to take off on a large scale, a consortium of companies is looking to make it a more cost effective and compact proposition.
A consortium of companies has designed a new floating LNG (FLNG) system specifically aimed at small to mid-size gas fields.
LiBro FLNG is a system which hopes to use an alternative cooling process making use of waste heat from gas turbine exhausts instead of using flammable substances. This in turn allows for more conventional and smaller vessels to be used for FLNG.
It also proposes use of existing tank systems, which could be retrofitted to vessels, further opening up use of standard vessels, increasing the number of yards able to complete such projects, and potentially making conversions of existing vessels a practical possibility.
The concept has been developed by ship classification society ClassNK, Tokyo-based floating production supplier/operator Modec and plant manufacturer Toyo Engineering – and it has already secured its first front end engineering design (FEED) contract with Malaysia’s Petronas.
The consortium says that while many FLNG systems are primarily attempts to take onshore LNG technology and adapt it to offshore uses, the LiBro FLNG system has been developed specifically for use offshore.
The core technology behind the system is a combination of Modec’s LiBro technology with Air Products’ Triple N2 Expander Liquefaction technology.
While the safety of N2 refrigerants is widely accepted, the limited efficiency of such systems has prevented their widespread use in FLNG systems, says the group. The technology addresses these concerns by using Lithium Bromide absorption refrigeration technology thatproduces chilled water using waste heat from gas turbine exhaust. This process is already used in centralized mega air conditioning systems onshore and to date in excess of 10,000 such units have been delivered, with the largest in operation for over five years.
The system integrates the LiBro unit with Air Products AP-N LNG Process so that the chilled water produced in the unit pre-cools the natural gas and N2 refrigerant, resulting in improved LNG train efficiency. Chilled water is also used for cooling the gas turbine combustion air, maximizing output of the gas turbine driver and improving the overall efficiency of the LNG train.
In addition, the cooling system uses non-flammable nitrogen instead of flammable hydrocarbons such as methane, ethane, or propane, used in most FLNG systems, to cool and liquefy the gas. The latter increase both environmental and safety concerns and mean accommodation space must be kept at a safe distance – increasing hull-size specifications. The system is currently being used in the FEED of FLNG projects under development, says the group.
This includes a FEED contract of an FLNG unit from Malaysian National Oil Company Petroliam Nasional Berhad (Petronas). This will be Petronas’ second FLNG unit and would be designed to produce 1.5 million tons per annum of LNG at offshore Sabah, Malaysia. The FEED and cost estimate are expected to be completed by the middle of 2013.
“As a design which has the potential to greatly increase the scope of offshore gas development, especially for small and mid-size fields, we were particularly excited about the LiBro system and began a joint research and development project with MODEC/TEC for 10 months from September 2011 to June 2012,” said Hayato Suga, general manager, ClassNK Natural Resources and Energy Department.
“During this time, we had weekly meetings concerning the marine and topside design; we also witnessed the LiBro absorption chiller dynamic motion testing and were very impressed by the result.
“One of our main contributions to the project was risk and safety assessment of the LiBro FLNG design as a third party. As part of this process we carried out the hazard identification (HAZID) meetings to identify risks and potential areas for improvement. Through such whole process, we confirmed satisfactory integrity, feasibility and safety of the LiBro FLNG design and eventually issued Approval in Principle (AIP) to the design.”
Another difference in the system is its storage tanks – it uses the Self- Supporting Prismatic-Shape IMO Type B SPB tank design developed by Japan’s IHI (now JMU) in the 1980s. SPB tanks have been used previously in gas carriers, but are increasingly being chosen for use in FLNG designs due to their resistance to sloshing and the flat, open deck space that use of the tanks allows.
As an independent tank design, it can also be retro-fitted onto existing hulls, allowing the tank system to be built separately from the hull (conceivably even by different shipyards) and opening the way to converting existing merchant vessels to FLNG, potentially reducing capex. Modec says that the system will also allow for either side-by-side or tandem LNG offloading.
For the consortium, all these innovations mean the opportunity for much smaller vessels with less capex, potentially making smaller marginal fields economically viable where they would not make the mark otherwise.
As an example, the group say Shell’s Prelude FLNG system is nearly 500m long. It says the LiBro FLNG design could be installed in its entirety on a standard Capesize bulk carrier hull. For a typical offshore gas field, a standard 300m-long and 50m-wide, Setouchi-max, Capesize carrier could accommodate topside facilities with an LNG production capacity of two million tons per annum (MTPA) and LNG storage capacity of 160,000cu m.
The group does admit a disadvantage - typical production capacity is less than that on larger FLNG designs. A traditional large scale FLNG system typically has a process capacity of more than 3.5MTPA, compared with the 2.0 MTPA offered by the LiBro FLNG. But it says because it is aiming at the smaller fields this makes reduced capacity a practical possibility. OE
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