The gas market is growing strongly with demand for cleaner fuels around the world coinciding with a decline in indigenous production by some larger consumers, particularly in the US and the UK. Large-volume gas supplies are remote from the main consumption centres and liquefaction provides a means of transporting the gas economically over large distances. With these drivers, the future for liquefied natural gas (LNG) is positive, especially in the US which provides good examples of the challenges and opportunities facing the LNG industry. Recent and future developments in LNG storage tank technology are reviewed in this context.
Current LNG Tank Design
Refrigerated LNG storage tanks have been operational for over 60 years and in that time there have been advances in materials, design and construction methods. Currently there are two basic tank types, below ground (largest 200,000m3) and above ground (largest 180,000m3).
Below-ground tanks have been used extensively in Japan, where the scarcity of suitable land and proximity to population centres call for a solution that provides a high level of inherent safety, closer tank spacing and reduced visual impact. This, however, comes at a cost approaching twice that of an above ground tank.
Above-ground tanks can be divided into single- and full-containment designs. The single-containment LNG tank is the simplest and cheapest tank solution on the market today. A secondary containment is provided by a low-height, open-topped bund around the tank. However, tank spacing and distances to the site boundary are greater than for a full-containment tank, which locates the secondary containment in close proximity to the primary container. The full-containment tank design is very compact and integrated and has become the standard solution for most projects where land availability, location and security do not permit the use of a single-containment design. Alternative designs use either 9% Ni-steel or pre-stressed concrete for primary containment. Stainless-steel membranes supported by the secondary pre-stressed concrete container have also been classified as fullcontainment tanks.
LNG Tank Costs and Schedule
Current full-containment LNG tanks are procured on an EPC basis at a cost of approximately US$300 to US$400 per m3 of LNG stored. Hence, for two 160,000m3 tanks, the cost is over US$100m for a 1bcfd (7MMTPA) regasification terminal. This represents over 20% of the total cost of the regasification terminal or up to 40% if additional storage is required for security of supply. With the demand for steel, particularly by China, having a major impact on commodity prices, these price increases are now feeding through into higher tank costs and longer delivery times. Current fullcontainment tanks take over 36 months to design, construct and commission. As such, the tank is on the critical path for all regasification terminals.
Recent Developments Onshore
Recent developments in LNG storage tank design and construction have concentrated on the traditional 9% Ni-steel full-containment solution. Advances in materials have permitted shell plate thicknesses to increase by up to 50mm, which allows above-ground tank sizes to increase towards 200,000m3. Further increases to above-ground tank size are limited by the available plate thicknesses, welding technology and the cost-effective design and construction of the tank roof. Fundamentally, however, the basic design has not changed and therefore the opportunities to make a step change in cost and schedule are limited. Nevertheless, changes and advances in construction methods are challenging the normal schedule times, in particular:
- improved welding methods being used on Japanese tanks are reducing the erection time of the primary container from 10 months to 7 months;
- secondary container walls are now being slipformed with recent examples in Portugal and Norway. However, the schedule advantages cannot be fully realised unless the method of roof construction is also reconsidered; and
- in conjunction with slipforming the walls, the roof can be erected at its final location rather than airlifted – this further reduces the construction schedule, but only at the expense of increased roof structural steel weight.
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