Introduction
Douglas-Westwood has been tracking the FLNG sector, its vessel designs and concepts for three years and presents forecast capital expenditure for the next seven years in the latest edition of its The World FLNG Market Report. Douglas-Westwood forecasts capital expenditure over the period of 2012 to 2018 will total $29 billion.
There are two main sectors that form the floating LNG industry:
- Floating liquefaction – a specialized floating production, storage and offloading vessel with LNG liquefaction topsides (LNG FPSO). Other hull types, such as semi-submersibles and spars, have been suggested
- Floating regasification – this can takes place on a vessel located either offshore or alongside. There are two main types of floating regasification vessels – floating storage and regasification units (FSRU), which remain stationary on location, or regasification vessels (RV), which can also act as LNG carriers It is important to note that at present there is no industry standard definition of FLNG. Many sources that refer to FLNG simply mean LNG FPSOs, while others consider FSRU as an umbrella term to include all types of floating regasification vessels. Parties looking to progress FLNG developments include both the vertically integrated majors such as Shell and smaller independent service providers, including Flex LNG, Höegh LNG, and SBM Offshore.
Floating liquefaction
The key drivers of the floating liquefaction sector are the desire to monetize stranded offshore gas fields and the relative costs of an onshore liquidation terminal. A modular design allows the FLNG vessel to be built in lower cost environments then towed to location. Positioning the liquefaction facility on field reduces the requirements for costly upstream facilities and long pipelines to shore which would be required for an onshore development.
While principally aimed at offshore gas reserves, floating liquefaction has also been considered for onshore fields, with projects in Papua New Guinea and Western Canada in development.
Design challenges
The vast majority of systems on a floating liquefaction vessel will be the same or similar to those used on conventional oil producing FPSOs. There are, however, various equipment designed or adapted specifically for these vessels:
a) Insulated storage tanks, which need to utilize a specialized LNG containment system that is sloshing resistant.
b) Topsides modules that include gas pre-treatment and liquefaction processing equipment.
c) Offloading cryogenic liquid offshore in difficult sea conditions is a potentially hazardous task. Much research has been put into the development of safe and efficient offloading systems for LNG FPSOs.
Prelude development
Shell is currently developing two floating liquefaction design concepts: a large-scale generic facility which is expected to be able to produce around 3.6 million tonnes per annum of LNG and a smaller facility of around 2 million tonnes per annum. In July 2009 it was announced that Samsung Heavy and Technip had won the contract to design and construct up to ten of Shell’s 3.6 million tonnes per annum units, the first of which will be used Australia from 2017 on the Prelude development. Other possible locations for both sized vessels include Egypt, West or East Africa, Indonesia, Iraq and Venezuela.
Containment system
Shell will utilize an adapted version of GTT’s membrane design which is used on a large number of LNG carriers and regasification vessels. In this membrane system, prismatic shaped LNG tanks are fully integrated into the hull and effectively form an inner hull, within which the containment system fits. By introducing two rows of tanks, the liquid motions in the tanks are significantly reduced and resonance between the liquid motion and ship motion avoided. This reduces the risk of sloshing related damage to an absolute minimum.
Liquefaction processing technology
The Prelude vessel, measuring 488 by 74 meters, be the world’s largest offshore floating structure. Prelude’s liquefaction process trains will use Shell’s dual mixed refrigerant (DMR) technology. The design makes use of two compressor strings, which ensures that if one compressor fails, the whole train does not stop – it can continue running at a reduced capacity. The first application of this technology in a baseload LNG terminal was the Sakhalin II project in Russia.
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