LNG alliance closing on technology goals for stranded gas floaters
Work to qualify floating liquefiednatural gas (FLNG) technology isbeing performed by an alliance comprising Statoil, Aker Kværner, and Linde. Set up under a three-year agreement last August, the study is building on previous work to refine and optimize an FLNG concept.
FLNG has long been recognized as a possible solution to the development of stranded gas reserves offshore. In addition, recent developments in the gas market have indicated the growing importance of LNG, driven not least by the gas import needs of countries such as the US, Japan, and South Korea. These needs cannot be met economically by sources within pipeline distance of these countries, so the solution lies in LNG. To serve the growing trade, new receiving terminals will be built in the importing countries, some of which will be offshore regasification plants.
Each of the triple alliance partners has its reasons for a commitment to FLNG. For Statoil it represents a natural extension of its production and marketing capabilities. While best known for supplying European customers with piped gas, the company is implementing its first LNG project in northern Norway, in which gas from the Snøhvit field will be processed at an onshore liquefaction plant. The LNG will be shipped to the US via the Cove Point reception terminal in Maryland, which is part-owned by Statoil.
Experiments studying the effect of motions on the behavior of fluids in the liquefaction process have been carried out by Linde using a tilting apparatus.
“We are working our way into LNG via Snøhvit and Cove Point. FLNG complements these activities, and has the upstream benefit of providing a solution for stranded gas,” says Eli Aamot, Statoil’s vice president in the technology business area and chair of the triple alliance steering committee.
MFC process
For Snøhvit, Statoil developed its own LNG process, the mixed fluid cascade (MFC) process. The development took place through a technology alliance set up in 1996 between Statoil and Linde, a German company specializing in low-temperature industrial processes. A second commercial product of this alliance was the spiral-wound heat exchanger (SWHE), which will also be used at Snøhvit. Linde is responsible for marketing both the MFC and the SWHE technologies. The two companies’ collaboration on LNG technology development has been extended to 2008.
Aker Kværner has been developing its expertise in LNG for 15 years. In 1997 it developed the barge concept for the Snøhvit LNG plant in association with Statoil, and it has also performed some of the engineering for the plant. The company has worked with Statoil for nearly 10 years on stranded gas solutions. Separately it is involved in various offshore LNG regasification schemes. These include a project in the north Adriatic Sea for ExxonMobil, QatarGas, and Edison, for which it is now performing front-end engineering design and several projects for ChevronTexaco under a frame agreement.
“LNG is one of the main strategic areas for Aker Kværner,” says Trygve Lund, senior project manager in gas and onshore solutions at Aker Kværner Engineering and Technology and head of Aker Kværner’s activities for the triple alliance.
The three companies have already worked together on a pioneering project to marinize LNG technology. This was a feasibility study of an FLNG concept for NnwaDoro, a deepwater gas discovery in two blocks off Nigeria operated by Statoil and Shell (see box).
No technology gaps
The work of the triple alliance builds on the solid base provided by the NnwaDoro study. In broad terms the study concluded that the technology was applicable in a floating environment, but that the costs needed to be reduced to make it economically viable.
“Basically we don’t see any big issues to be resolved around operating an LNG plant in calm waters. All the equipment is proven, and there are no technology gaps,” says Lund.
One of the alliance’s first activities was to take a new look at the FLNG concept. Whereas the NnwaDoro study posited both a concrete hull and a steel hull, the new study focused only on the steel hull alternative. Even though concrete has the benefit of offering a significant potential for local content the cost was not considered competitive.
If the FLNG floater were built with a steel hull, this would have to be done at an existing shipbuilding facility. And since only a few docks in the world could accommodate a steel hull sharing the same dimensions as the concrete hull, the floater would have to be redesigned in that case as a longer, slimmer structure, which also meant rearranging the topsides layout.
When work started last September, the alliance also undertook a gap analysis to pinpoint areas where the existing concept could be strengthened or alternative options developed. In January the steering committee met and agreed to institute two main lines of inquiry.
The first, led by Linde, focuses on how to make the energy concept more efficient. The main issue to be tackled here is the high cost of energy production for driving the compressors involved in the liquefaction process. In the NnwaDoro study, compressors driven by electric motors were specified as there are no gas turbines qualified for directly driving a compressor in a floating environment. In principle, a significant cost reduction could be achieved through using a gas turbine to directly drive the compressor. Part of the work therefore involves qualifying a gas turbine for this service at sea, and that will be undertaken in collaboration with a compressor manufacturer.
Another set of initiatives, led by Aker Kværner, is looking at issues to do with the floater concept itself. One important issue is whether all the facilities should be placed on a single barge or split between two. A number of split concepts are therefore being examined.
Effect of motions
The effect of sea motions on the plant operation is also a concern. As part of the NnwaDoro study, Linde used a tilting apparatus to study the effects of motions on the behavior of the fluids in the heat exchangers and found nothing that constituted a ‘show-stopper.’ Aker Kværner will lead further work to examine the effects of motions on rotating equipment - turbines, compressors, separation facilities, etc. Given that such equipment is used every day on floating production units, both shipshape and semisubmersible, it seems unlikely that any major problems will be identified.
A further project to be led by Aker Kværner will look into the issue of spill protection. Concrete presents a number of advantages over carbon steel for LNG spills. It maintains its strength in contact with LNG while carbon steel does not. For an LNG barge with a steel hull, protection would therefore be needed in areas exposed to spillage.
One significant advantage of operating an LNG plant in deepwater is the ready availability of cold water for cooling the refrigerants. The efficiency of the liquefaction process is increased by 1% for every additional 1° C of coldness of the water, a fact which can substantially affect production. For NnwaDoro the plan would be to take water from a depth of 1,000 m, where the temperature is 5° C compared with 25° C at the surface. In the case of NnwaDoro, this represents a 20% increase in production for the same power input, Lund says.
The nature of the MFC process is itself beneficial in an offshore context. The fact that it is based on three cooling cycles, in each of which different refrigerants can be used, enables it to be adapted for applications in differing circumstances.
The focus of the triple alliance is very much on qualifying the technology for use in calm waters, Lund says. Extending it for application in a harsh offshore environment would be a relatively large step. For example, much larger motions would have to be taken into account, ruling out side-by-side loading of LNG and obliging the development and qualification of tandem loading methods.
The current set of projects will be completed by year-end, at which point a further re-appraisal will take place and a new work program will be defined.
Full process floating concept for NnwaDoro
NnwaDoro is a large gas field straddling two blocks, one operated by Statoil and one by Shell, and located in waters about 1,300 m deep off Nigeria. Under a memorandum of understanding between the two licenses, Nigeria National Petroleum Corp. and FGN carried out a study of a floating LNG concept for developing the structure, which was completed in 2003.
The reference case for the NnwaDoro study was a full processing scheme with production capacity of 5.8 million tonnes per year from a single processing train - considerably larger than the 4.2 mtpa of the Snøhvit plant - plus production of liquefied petroleum gas (LPG) and condensate. Storage capacity for LNG would be 240,000 cu m, LPG 80,000 cu m, and condensate 120,000 cu m.
The facilities would be placed on a huge barge built either of concrete or steel. The barge would have to support a topsides weight of around 60,000 tons. It would be moored by means of an internal turret.
Gas from subsea wells would be delivered to the barge via risers routed through the turret. The topsides facilities would include an inlet module, pre-treatment module, fractionation module, liquefaction module, powergen facilities, MEG unit, utilities modules, loading facilities, workshops, flare, living quarters, and helideck. The maximum module weight would be around 12,000 tons. Six gas turbines would provide some 210 MW of power.
The layout of the topsides facilities would mainly be governed by safety considerations. The living quarters, workshop, power generation, and utilities module would be located in the stern. Next to them would be the offloading and MEG module. At the front would be the turret and flare and in the middle the inlet, pretreatment, liquefaction, and fractionation facilities. The location of the various facilities is determined taking into account the degree of potential hazard each represents. Products would be offloaded to to a tanker stationed alongside the barge by means of loading arms.
This image is a 3D computer graphic of the NnwaDoro LNG floater.
