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FLNG heaves into view

15th April 2013

The transfer of technology from the LNG and FPSO sectors raises several concerns when it comes to picking the right liquid containment system

FLNG heaves into view
The industry’s accumulated offshore experience along with the steady growth witnessed in the LNG market over the past decades and the rising cost of onshore projects have all given FLNG the necessary push. Copyright: Shell

After a decade of unfulfilled expectations, the outline of a future floating liquefied natural gas (FLNG) industry heaves into view, as the conjunction of several contributing factors gives innovation the necessary push.


The first of these concerns the industry’s accumulated offshore experience, employing floating production, storage and offloading (FPSO) systems in several oil field projects, as well as the steady growth witnessed in the LNG market over the past decades. In addition, the cost of onshore projects has risen rapidly over the past years in comparison with the offshore sector, which, as the latter continues to grow, makes FLNG a viable prospect.


This will allow companies to open up opportunities to unlock gas resources which they might otherwise not be able to reach, as some are too distant or too difficult to explore.  Oil and gas operators can dispense both with costly offshore production platforms and long pipelines to shore. Finally, when a field has been completed, the vessel can easily be moved to develop another site.


Looking ahead, some of the challenges which the FLNG industry must go at lengths to overcome include: harsh sea states which affect all aspects of operations; getting the gas onto the ship; the implications which treating and liquefying the gas have on process design; the distance between bits of kit to maintain the overall safety and integrity of the facility, a tall order when the strategy is to reduce space at sea; and of course offloading.


Shell and Petronas lead race on FLNG


While little is known about the actual costs of a real life FLNG facility as one has yet to surface, a growing list of key projects from pioneering oil and gas companies are set to cut the uncharted and challenging territory of floating LNG.


Shell’s Prelude project unsurprisingly claims the big headline, laying the ground for the world’s first ever FLNG facility. The 488 metres long and 75 m wide structure – bigger than Malaysia’s Petronas Towers – will be deployed in the Prelude gas field, 200 km off the coast of Western Australia and is expected to produce at least 3.6 million tonnes per annum (mtpa) of LNG for the next 25 years. Prelude FLNG is scheduled for completion in 2015.


However, it is quite possible that Malaysian oil and gas company Petronas will beat Shell to the chase and launch the first large scale project with its Petronas FLNG 1, which was announced at the World Gas Conference last June.


Petronas has said the facility, smaller than Shell’s, will be able to produce 1.2 mtpa of LNG, which will increase Malaysia’s total LNG production by 5 per cent to 26.9 mtpa. The facility will be located 180 km off the coast of Bintulu, Sarawak, according to Petronas, and is scheduled to launch also in 2015.


Other projects include: the Pacific Rubiales Energy and Exmar joint venture for the construction of a floating LNG facility to operate off Colombia; Tamar FLNG, offshore Israel; GDF Suez’ Bonaparte project in the Timor Sea; and the Indonesian Abadi project.


The need to address sloshing


The industry’s experience in LNG and the FPSO sector has clearly encouraged FLNG initiatives. However, combining technologies from both areas is complicated and faces, as we have seen, various technical challenges.


In terms of containment, while the storage tanks of future FLNG facilities will be akin to those found on current LNG vessels, additional obstacles arise when this technology is transferred from the latter to the former. These include the difficulty in entering a dry dock for repairs, as these facilities are extremely large; choosing the right topside arrangement; and avoiding sloshing issues, which affect the efficiency of oil-gas separators.


Whereas LNG vessels are usually either fully loaded on laden voyages or near empty on their return journeys – which reduces sloshing effects on the ship’s hull –, FLNG facilities are expected to operate in intermediate loaded conditions (i.e. known as the “barred fill range”) in different maritime environments for more than 20 years, according to project estimates such as Shell’s Prelude. These conditions may induce resonance/sloshing in the LNG fluid in medium filled tanks, which leads to high impact pressures on the thermal insulation. As such, it is essential to use designs which adequately endure sloshing loads and allow for operations to continue without interruption.


Three main types of tanks have been identified by the industry as suitable for FLNG containment systems. These are self-supporting prismatic tanks (SPB) which prevent sloshing in partial filling conditions but are expensive and subject to fatigue; two-row membrane tanks that withstand sloshing up to a certain sea state; and spherical moss-type tanks which leave less space on the deck for the topsides and as such have only been considered suitable for small scale FLNG projects.


Self-supporting prismatic tanks


SBP containment has emerged as the industry’s most popular choice for FLNG facilities, a technology which may secure the world’s second largest shipbuilder Daewoo Shipbuilding & Marine Engineering (DSME) a prime position as a service provider in the sector.


Daewoo Shipbuilding has developed a gas technology called Aluminium Cargo Tank Independent Type B (ACTIB). ACTIB tanks are large prismatic independent cargo tanks which reduce sloshing and are suitable for widespread use for LNG containment.


The prismatic IMO Type B independent cargo tanks are made of insulated aluminium 5083 and boast “the world’s largest storage capacity” in an independent, insulated LNG tank system, the firm says, adding that it is nearly three times larger than comparable types currently in operation.


The tank is prepared to withstand sloshing and is designed for wider use in LNG shuttle tankers, LNG FPSOs and LNG-as-fuel tanks where partial fill is an operational issue. ACTIB will initially be used in the DSME-designed 210,000 m3 volume LNG carrier.


“This new IMO type B independent cargo tank with insulation system developed with our own technology for extremely low temperature LNG containment systems can be immediately incorporated into very large LNG carriers and/or LNG-FPSOs,” says vice president and head of hull design at DSME Man Soo Kim.


Daewoo Shipbuilding’s ACTIB technology may in fact be used in FLNG projects currently under development. Malaysian giant Petronas recently revealed it contracted a consortium of companies – including DSME – for the engineering, procurement, construction, installation and engineering (EPCIC) for its FLNG 1 facility.


Two-row membrane tanks


A two-row membrane tank configuration, such as the one provided by French engineering company Gaztransport & Technigaz (GTT) is also capable of handling the wave impacts expected from sloshing.


Any membrane-type containment system has two membranes which work as complementary barriers for load-bearing insulation. The first membrane contains the LNG cargo, while the second comes into play if the former fails to hold LNG safely during offloading procedures. Further isolation is provided between the secondary barrier and the ship’s inner hull.


Developing solutions for its own LNG FPSO, international shipping company Höegh LNG opted for GTT’s No. 96 membrane in double row configuration. The length of the vessel is thus equipped with not one but two parallel rows of tanks, both rows separated along the ship by a longitudinal cofferdam. The two-row system provides for reduced sloshing in the tanks along the entire filling range.


Höegh LNG is also asserting its position as a leading provider of floating LNG services to major operators, having been contracted for the front-end engineering design (FEED) of an LNG FPSO for Israel’s Tamar FLNG project. The project is set to tap an estimated 9 trillion cubic feet (tcf) of gas in 1,700 m deep waters in the Tamar gas field located 80 km west of Haifa.